Spintronics, or the functional control and pull stringsing the negatron spin grade of freedom into electronic devices, is a new and promising research field that attract the scientific and technology involvement [ 1 ] . The possibility of executing electronics with the spin of the negatron has been cardinal for possible chances devices with high treating velocity and integrating, low power ingestion, non-volatility, multifunctionality and their suitableness for quantum calculating [ 2 ] . In the last decennaries, the explosive growing of digital informations storage based on spintronics has been occurred.
Presently, hard-disk thrusts read-heads are the maximal exponent [ 3 ] . In fact, these devices are scientifically based merely on the tunnel magnetoresistance consequence TMR which is defined as the alteration in electrical opposition of a device in presence of an external magnetic field [ 3 ] . A tunnel magnetoresistive perpendicular spin valve is the rule spintronics device and composes of two ferromagnetic beds separated by a thin insulating bed. The two ferromagnetic electrodes exhibit two different exchanging Fieldss, and the opposition can be switched between a parallel and antiparallel magnetisation constellation upon the application of a magnetic field [ 1,3 ] .Recently, the usage of organic stuffs has garnered much attending for advanced spintronics applications. Chiefly owing to their easiness, low-weight, little cost of processing and mechanical flexibleness. Besides, the highly long spin lifetimes found in organic stuffs propose considerable advantages comparing to other stuffs.
This promising belongings is related to the little spin orbit interaction and atomic hyperfine interaction [ 2 ] . Using thin beds of organic stuff have been performed successfully as spin tunnel junctions and important magnetoresistance ( MR ) values have been obtained at low temperatures. Basically, the room temperature is the basic operational demand for most applications of spin conveyance in organic. Up to now, the surveies on thin organic spin tunnel junction have exposed important MR consequence at room temperature.
While the devices with organic bed thicker than 15nm indicates a lessening in MR below room temperature scope [ 3 ] . Then the survey of organic spintronics extends to include C60 due to its considerable belongingss among other organic stuffs. C60 exhibit lower hyperfine matching due to the absence of H in the molecular construction in add-on to little natural copiousness of the 13C atomic spin [ 1,3,4 ] .
The lowest unoccupied molecular orbital LUMO of C60 is good matched with the Fermi energy of common ferromagnetic stuffs. This duplicate allows easy current injection and maintains a moderate energy injection barrier at the same clip [ 3 ] .
Overview of current undertaking
As suggested by the rubric, the chief focal point of my undertaking is to understand the cardinal rules behind spin polarized tunneling devices.
The tunneling in my survey will be through molecules positioned between ferromagnetic electrodes. In inside informations, C60 is inserted into ferromagnet/insulator/ferromagnet constructions to organize perpendicular spin conveyance devices. This allows an experimental survey the tunneling and perchance inter-molecular conveyance regulation of spin-polarized bearers in C60 in a perpendicular conveyance geometry based on magnetic tunnel junctions ( MTJs ) .
C60 is an attractive pick due its several belongingss that make it ideal for spintronics devices, presented in 3.1.3. This survey accomplish with a Meservey-Tedrow technique, which ideally provides direct finding of spin-polarized current tunneling through an OS utilizing a superconductor [ 5 ] . In kernel, it depends on the fact that a really thin superconducting aluminium movies in an applied magnetic field, demoing Zeeman splitting of the quasi-particle denseness of provinces into spin-up and spin-down parts. The ensuing spin densenesss of provinces in the superconductor are so similar, but shifted from their original energy by i‚±i?H, where U is matching to the magnetic minute of negatron [ 6 ] . Surely, Meservey-Tedrow technique is straightforward to determine the quality of the C60 barrier junction and mensurating the spin polarisation of the ferromagnetic electrode.
Indeed, the survey involves samples fiction and word picture utilizing varies devices such as spatter deposition system, MR rig, He flow cryostat and vibrating sample magnetometery ( VSM ) .This study shows scientific advancements have been made in burrowing magnetoresistance field utilizing organic stuffs. Followed by deep account of experimental techniques. A brief reappraisal of the early phase obtained consequences is given later. Finally, a sum-up of the survey is provided and some suggestions for future work.
This subdivision includes description of the surveies conducted in the field of organic spintronics followed by surveies accompanied on the usage of C60 in peculiar.
The corresponding analyses of experimental consequences are discussed clearly. Furthermore, the chief facets and different factors that influence the magnetoresistance are presented.Many scientific advancements have been made in tunnel magnetoresistance field. The system of FM/I/FM has been studied by experimentation and theoretically.
In 1975 Julliere [ 7 ] stated a theoretical account to gauge the magnitude this phenomenon. His theoretical account is a combination of Mott ‘s current theoretical account in ferrmomagnetic metals besides the theoretical account proposed by Mesrevey-Tedrow of the effectual denseness of provinces estimation utilizing spin-polarized tunneling. Julliere theoretical account gave a good penetration nevertheless it was missing important effects including temperature, electromotive force dependance and belongingss of the tunnel barrier such as the stuff, tallness and the width [ 8, 9 ] . This survey altered by Slonczwski [ 10 ] to account the permeableness of barriers that consequences in an convergence of the moving ridge maps inside barriers. However, the temperature and electromotive force dependance of the TMR ratio has non justified in this Slonczwski theoretical account.
Two-step tunneling has been put frontward to explicate the temperature and electromotive force dependance in which the pervious theoretical account was unable to construe [ 9 ] . This followed by extended surveies whereby series of experiments were performed in this field.At get downing of this century, the field of spintronics has been spread outing by presenting organic semiconducting material ( OSC ) stuffs taking to a new research country called organic spintronics. Different types of OSCs such as polymer and little molecule have been applied as spin conveyance beds in organic spintronics devices. OSCs have long spin diffusion clip due to hebdomad hyperfine interaction and low spin orbit yoke. Besides, it presents as a good campaigner for spin injection unlike inorganic semiconducting material spintronics where this characteristic considers as one of the major challenges. The later can be explained in term of mismatch conduction where spin injection from ferromagnetic electrodes straight to inorganic semiconducting material is inefficient in contrast to OSCs [ 11 ] .The first presentation of an organic spintronic device was in 2002 by Dediu et Al.
[ 12 ] . The writers designed a sidelong device holding two La0.7Sr0.3MnO3 ( LSMO ) electrodes patterned by negatron beam lithography.
These electrodes were separated by 70-500 nanometers and bridged by a narrow channel of sexithienyl ( T6 ) ( Fig 1a ) . A strong magnetoresistance ( MR ) response up to 30 % was observed at room temperature for 100-200 nm T6 channel lengths ( Fig. 1b ) . Beside that, an appraisal of a spin diffusion length of 200 nanometers in the organic bed room temperature was reported.
However, this work did non supply a straightforward presentation that the ascertained MR was related to the magnetisation of the electrodes and accordingly to its spin polarisation. This is because the antiparallel constellation of two LSMO electrodes magnetisation could non be set. Alternatively, a opposition alteration was measured between nothing filed and a perpendicular magnetic field of 3.4 kOe, matching to a random and parallel magnetisation alliance severally [ 12 ] .Figure 1: ( a ) Schematic position of intercrossed junction LSMO/T6/LSMO and dc four-probe electrical strategy and ( B ) Magnetoresistance ( H = 0.3 T, where H is the magnetic field ) of the intercrossed junction device depicted in ( a ) as a map of the channel length ( [ 13 ] , reproduced in [ 12 ] ) .A twosome of old ages subsequently, Xiong et Al. reported the first perpendicular organic spin valve device dwelling of LSMO and Co electrodes, with a midst ( 130-250 nanometer ) bed of Alq3 in between [ 14 ] ( Fig.
2a ) . A ample opposite MR of 40 % was obtained in an LSMO/Alq3/Co spin valve device with a 130 nm thick bed at 11 K. The MR exhibited a rapid lessening when the Alq3 movie thickness was increased or the sample temperature was increased ( Fig.
2b ) . Subsequently, the MR vanishes when the temperature above 200K. Following this encouraging study, the same group performed a elaborate survey utilizing assorted organic stuffs as a spacer bed between LSMO and varies metallic electrodes in 2005 [ 15 ] .
Their devices exhibit big negative high-field MR responses. They attributed this high-field MR to the anomalous field-dependent Fermi degree displacement in LSMO that leads to heighten bearer injection at the LSMO-organic interface. Since so, different groups carried out many surveies on the devices with the same architecture utilizing both Alq3 and other OSC Martial as spacer bed.
However, the ascertained MR is varied in size and subscribe even with the same LSMO/Alq3/Co construction. All these differences give clear grounds on the consequence of ferromagnetic electrode/OSC interface belongingss. The latter can be induced by interface interactions, intrinsic belongingss of the stuff in add-on to different fiction conditions [ 14 ] .
Figure 2: ( a ) Conventional representation of a typical LSMO/Alq3/Co device ( B ) GMR cringle of a LSMO ( 100 nanometer ) /Alq3 ( 130 nanometer ) /Co ( 3.5 nm ) spin-valve device measured at 11 K.
The insets show the anti-parallel ( AP ) and parallel ( P ) constellations of the FM magnetisation orientations at low and high H, severally [ 14 ]An cardinal measure frontward for organic spin tunnelling was the fiction of devices by direct in situ UHV organic vapour deposition with shadow cover in 2007 by Santos et Al. [ 16 ] . Furthermore, a thin Al2O3 bed of 0.6nm was introduced at interface between the Co electrode and the Alq3 barrier by plasma oxidization to organize the perpendicular tunneling device of Co/Al2O3/Alq3/NiFe. In fact, utilizing a intercrossed inorganic-organic ultrathin barrier produced positive ( non-inverse ) TMR when the electrode magnetisations were switched from analogue to antiparallel.
This sort of device showed a few per centum TMR at room temperature. Basically, the survey demonstrated the of import function of infixing an ultrathin Al2O3 tunnel barrier to forestall incursion and chemical reaction [ 17 ] of the Co atoms into the Alq3. In inside informations, Al2O3 interfacial bed suppresses the formation of trap provinces that are concentrated chiefly at the metal-organic interface. As a consequence, higher spin polarisation can be obtained hence improves spin injection efficiency a cross the FM/OSC interface.
Recently, elephantine positive TMR up to 300 % was detected at 2 K in Alq3-based nanojunction using a LSMO and Co electrodes [ 18 ] . However, the ascertained value undergoes a crisp lessening in temperature and vanished below the noise degree at 180 K. Barraud et Al. proposed a new mechanism to explicate different mark and amplitude of MR in devices with thin and thick Alq3 tunnel barrier [ 19 ] .
The mechanism attributed to the formation of spin- hybridization-induced polarized provinces in the first monolayer at the electrode interface which can take to a complete alteration in organic valve magnetoresistance. This in bend means an addition of the effectual spin polarisation and a alteration in mark of the electrodes as pointed above. By and large, although many surveies took topographic point in this new uprising field, the mechanism underlying the spin injection into the OSCs are still to be unraveled and remain one of the cardinal challenges.A singular survey on C60-based perpendicular spin valves has been demonstrated by Gobbi et Al. [ 3 ] . The devices were fabricated in-situ in a UHV double chamber evaporator. Through deposition procedure utilizing shadow masks a construction of Co/AlOx/C60/Py was obtained.
Where AlOx bed was achieved by lodging a thin Al bed ( 0.9 nanometer ) followed by plasma oxidization. Additionally, a C60 bed was thermally evaporated through shadow mask with thickness between 5 and 28 nanometer. The devices were designed in such manner that the fullerene merely covered some of the bottom lines whereas the staying electrodes were left merely with the thin AlOx bed and are used as mention junctions. This allows detecting the tunnelling effects due to the C60 bed strictly. A study of the devices is shown in figure 3a. It is of import to detect that the bed of AlOx was a “ leaky ” movie alternatively of a full functional tunnel barrier. The thought of this pick is to guarantee that the mensural electronic belongingss in the intercrossed ferromagnetic-organic junctions are matching to the C60 molecules.
Therefore extinguishing the consequence of inorganic from that of the organic barrier. Nevertheless, the AlOx bed is significantly required to forestall the C60 responding with the Co underside electrode, since Co is really reactive with varies organic molecules as proved by many surveies [ 3 ] . A important room temperature magnetoresistance in surplus of 5 % was achieved for different thickness of the C60 interlayer ( from 5 nanometers to 28 nanometer ) . Explanation of both electronic and spin coherency conveyance was based in term of a multistep burrowing theoretical account. In this theoretical account the negatrons undergo a figure of burrowing stairss from molecule to molecule through the interface barrier until they reach the 2nd electrode [ 20 ] .
Figure 3: ( a ) Schematic of a C60 based tunnel junction with 3 of the Co underside electrodes covered with C60 go forthing 2 electrodes with merely AlOx as mention junctions. ( B ) A cross-section representation for the Co/AlOx/C60/Py device [ 3 ] .
Lin et Al. have been accomplished a survey utilizing C60-based perpendicular spin valves. In the devices, a C60 bed was thermic vaporization as the organic semiconducting material spacer bed on LSMO, covered by a 15 nanometer midst Co movie on top [ 1 ] . All fiction stairss were performed inside the vacuity vaporization chamber. Basically, the intent of this survey was to potentially get the better of the restrictions imposed by hyperfine yoke. Since the ulterior paly an of import function limit the spin-transport length in organic semiconducting materials. C60 has been proposed because it has orders of magnitude lower hyperfine matching than most other organic stuffs [ 21 ] . The lowest hyperfine yoke of C60 turned out because it does non incorporate H atoms, which considered as the beginning of such sort of yoke ( or hyperfine matching originate from these atoms ) .
In contrary most other organic stuffs exhibit strong hyperfine matching due to the presence of H in the molecular construction [ 21, 22 ] . Despite the above-named fact, Lin et Al. survey did non uncover a important betterment of the spin-diffusion length. As a effect, three possible readings have been suggested to reflect this observation.
First, a mechanism other than hyperfine yoke is responsible for the loss of spin-polarization. Second, the ascertained magnetoresistance is matching to burrowing magnetoresistance which vanishes beyond the tunneling scope for C60 thicknesses. Finally, for thick devices the mismatch in conduction bounds spin-injection.
Fabrication and word picture
This subdivision will depict the experimental techniques used throughout this survey. The experimental techniques were carried out in two chief stairss. The first measure is the sample fiction utilizing sputter deposition technique, plasma oxidization and thermic vaporization. The 2nd measure is deal with the word picture and electrical measurings used during this work. This includes structural and magnetic word picture beside magnetoresistance measurings at room temperature and low temperature conditions.
Besides, a general history of electrical contacts constellations has been briefly considered before set uping the measurings.Additionally, this chapter outlines some of the basic rules behind these techniques. This is to supply an apprehension of the consequences and restrictions in theoretical and practical facets of the analysis. A brief description of the stuffs used is given besides at the beginning.
Ferromagnetic stuffs: Cobalt ( Co ) and Cobalt-Iron-Boron ( CoFeB )
A ample tunnel magnetoresistance ( TMR ) values were attained at room temperature since 1995, the point has attracted extended attending on spin dependent stuffs. In rule these stuffs provides two key advantages comparing to other magnetoresistive stuffs, a high field sensitiveness and orders-of-magnitude higher opposition which can be easy accomplishable even with little sizes. Since that a particular attractive has been taken toward these stuffs for low field/low power device applications due to the sophisticated belongingss antecedently mentioned. In applications point of position and specifically in term of devices, such as MRAM, read caputs and magnetic detectors, a high TMR value is required because it leads to larger signal degree, lower power ingestion and greater velocity in add-on to flexible design border.
[ 23 ] Thus, a considerable involvement has been half metallic ferromagnetic electrodes, such as the manganite perovskites and CrO2, due to higher the burrowing spin polarisation ( TSP ) hence higher the TMR they have. However, it is of import to detect that the highest TMR values in tunnel junctions with electrodes formed from the perovskite manganites have been obtained merely at low temperatures. Room temperature measurings contributed really little effects. This has been interpreted in term of the low Curie temperature of these ferromagnets. On other manus, ferromagnetic metals formed from Fe, Co and Ni have much higher Curie temperatures which are above room temperature [ 24 ] For this ground, most surveies are based on these sort of stuffs.In the present work, ferromagnetic electrode of CoFeB is chosen in tunnel junctions due to a high tunnel magnetoresistance ( TMR ) , at room temperature, associated with high spin polarisation.
In add-on, it has low coercivity ( Hc ) which is desirable for detector applications [ 26, 27, 28 ] . Most interestingly, formless CoFeB electrodes addition of import advantage of being homogeneous in the really little graduated tables, which can non accomplish with polycrystalline electrodes. The 1 is needed for high denseness entering media [ 29 ] . Furthermore, TMR values of over 55 % have been obtained utilizing formless CoFeB as a magnetically soft electrode in MTJs. It have been pointed out through varies surveies that CoFeB / AlOx interfaces are characterized by a high microstructural quality to which the big TM ascription can be considered [ 30 ] .
Basically, D. wang et Al. investigated spin dependent tunneling ( SDT ) junctions with a stack construction of Si/Si3N4/Ru/CoFeB/Al2O3/CoFeB/CrMnPt.
The SDT wafers were deposited utilizing District of Columbia magnetron spatter and photolithography techniques were used for the intent pattering and connexion with one bed of metal lines. It has been revealed by high-resolution transmittal microscopy that the CoFeB has an formless construction in add-on to a smooth interface with the Al2O3 tunnel barrier. High TMR value of 70.2 % has been obtained with a spin polarisation of 51 % for CoFeB at room temperature status. Further survey has been demonstrated one twelvemonth subsequently by, D. Djayaprawira et Al. where magnetoresistance ratio up 230 % at room temperatures was observed utilizing MgO tunnel barrier sandwiched with formless CoFeB ferromagnetic electrodes prepared by magnetron sputtering [ 31 ] .
In most recent MgO- based MTJs, a really big tunnel magnetoresistance ( TMR ) ratio of 604 % at 300K was observed [ 32 ] . These surveies provide interesting consequences about formless CoFeB electrodes and let farther probe in present work.
Tunnel barrier – Aluminum Oxide ( Al2O3 )
Insulators such as AlN, Al2O3, and MgO have been proposed as a barrier for many magnetic tunnel junction devices. Of these, Al2O3 has several first-class belongingss in add-on to insulating belongingss, make it a desirable stuff for such sort of devices. Aluminium oxide is transparence, high abradant and corrosion opposition, every bit good as good optical belongingss. It has outstanding belongingss in footings of chemical inertness, mechanical strength and hardness [ 30 ] .
The most common signifier of aluminum oxide is the formless signifier. Crystalline aluminum oxide exists in different agreements and is formed when aluminum is oxidized at really high temperatures exceed 4008 C [ 31 ] .A high quality barrier is required to better junction belongingss harmonizing to its stableness and the prejudice dependance. The barrier thickness is a critical parametric quantity should be considered since the electrical electric resistance of the barrier depends exponentially on the thickness of the barrier. In peculiar, a thin barrier is preferred to cut down the opposition of the junction [ 33 ] . The present technique to organize Al2O3 barrier is by oxidising a thin Al bed deposited on the ferromagnetic stuff.
The oxidising procedure involves sputtering aluminium in an Ar ambiance which contains O where a homogenous and stoichiometric Al2O3 bed can be obtained. Koski et Al. gave a comprehensive survey about this technique [ 31 ] . Extensive surveies have been done to optimise Al movie thickness required for the barrier formation. Depending on the type of FM electrodes, a unvarying coverage of Al movie thickness varied between 7 A to 18 A. In general, the Al bed should be thick plenty to avoid oxidising the FM surface during the barrier formation. On the other manus, it should non be really thick so that an surplus of Al metal will be left behind unoxidized. The presence of nonmagnetic metal at the interface leads to a decrease in polarisation and the JMR consquentely [ 34 ] .
Buckminsterfullerene – C60
The C60 molecule is a fullerene coop consists of 12 Pentagons and 20 hexagons, arranged as in the polyhedron known as the abbreviated icosahedron, with a C atom at each vertex. C60 was discovered by chance by H.W. Kroto, R.
E. Smalley and colleagues in 1985. In fact, they evaporated black lead by optical maser irradiation and a singular stable bunch dwelling of C60 has been produced. Their find leads to a Nobel Prize in 1996. The C60 molecule known as Buckminster-fullerenes, it was named for R. Buckminster Fuller, an American designer noted who created the geodetic dome [ 35, 36 ] .
C60 a symmetric construction, sublime at moderate temperatures of 350oC. The electrical electric resistance of undoped solid movie of C60/70 has been found to be i?? i‚»1014 ( i?- centimeter ) -1 at room temperature [ 37 ] . For a extremely crystalline C60 thin movie, the room temperature electric resistance reaches i?? i‚»1010 ( i?- centimeter ) -1 similar to C60 individual crystal measuring [ 38, 39 ] . A series of different techniques chiefly the spectroscopic were utilized to set up the bandgap of C60 movies. A scope of 1.
5eV to 2.3 electron volt has been indicated for the cardinal spread under these probes, which basically gives qualitative understanding with theoretical anticipations [ 40 ] . The electronic construction of polycrystalline C60 thin movies has been shown to exhibit n-type semiconducting behaviour based conduction measurings and surface photovoltage spectrometry ( SPS ) [ 41 ] .Due to several belongingss that C60 attain, it is considered as ideal pick for organic spintronic devices. First, C60 molecules can be grown by sublimation under ultra-high vacuity ( UHV ) and are really robust. Consequently they can be flawlessly between ferromagnetic metallic thin movies and sustain without being damaged [ Gobbi ] .
Second, C60 exhibit lower hyperfine matching due to the absence of H in the molecular construction in add-on to little natural abudance of the 13C atomic spin [ 1, 3, 4 ] . Finally, the lowest unoccupied molecular orbital LUMO of C60 is good matched with the Fermi energy of common ferromagnetic stuffs. This duplicate allows easy current injection and maintains a moderate energy injection barrier at the same clip [ 3 ] .
Direct Fabrication “ Sputter deposition ”
Sputter deposition is a physical vapor deposition procedure that is extensively used to lodge high quality movie of atoms on semiconducting material wafer, on caput surface and magnetic media. Specifically, sputtering is carried out by agencies of plasmas, which in bend generate charged atoms that can be accelerated towards a surface electrically [ 42 ] In rule, sputter deposition procedure is fundamentally involved taking the surface atoms by energetic atoms, such as accelerated ions, and later roll uping of these atoms onto a substrate to organize a thin movie [ 43, 44 ] . There are several sputtering methods including DC rectifying tube, RF rectifying tube, and magnetron spatter.DC sputtering system is considered to be the simplest theoretical account among other sputtering systems.
This system consists of a brace of electrodes: cathode and anode. The cathode is biased at negative 2~5 kilovolt and covered with mark stuffs in the front surface to be deposited subsequently. On the anode, the substrates are placed and normally grounded. The sputtering chamber is filled with an inert gas ( typically Argon gas ) , which is introduced at low force per unit area.
In sputter deposition, positive Ar ions are created in plasma under the application of dc electromotive force between the two electrodes. Then argon ions accelerated towards the surface of the mark since it is negatively biased. Due to the impact, atoms of the mark stuff are ejected as a effect of the emerging hit Cascadess.
Subsequently, the sputtered atoms condense on the surface of the substrate and organize the turning thin movie [ 44, 45, 46 ] .In magnetron spatter, a lasting magnet is located behind the mark surface in order to increase the rate of ionisation. Chiefly, the ensuing magnetic field forms a closed cringle way that acts as an negatron trap so that the ejected negatrons from the mark move in cycloid curves instantly above the mark. Magnetic field traps the negatrons nearby the mark and triggers them to gyrate around in the parallel plane right above the mark.
This negatron pin downing consequence strongly increases the chance of ionisation of the spatter gas ( i.e. enhances the hit rate between the negatrons and sputtering gas molecules and make ions ) that is eventually increases deposition rate efficaciously. Furthermore, the way length travelled by negatrons is increased and the magnetic field in this manner moving as the gas force per unit area had been increased. The above procedure enables to take down the gas force per unit areas used in magnetron sputtering down to 0.5 mTorr [ 42,43, 44, 47 ] . A conventional diagram illustrates the basic rule of sputter deposition of a typical District of Columbia magnetron sputtering system is shown in the figure below.Figure 4: The basic rule of spatter deposition where ions are produced in plasma and so accelerated by using dc electromotive force between the electrodes towards a mark.
Whereby, the mark atoms will be removed due to hit between the negatrons and sputtering gas molecules. Finally, the atoms of the mark will be condensed at the substrate [ 48 ] .In this research, A spatter deposition system viz. IVOR is used for samples fiction. It consists of a sum of eight sputtering guns ( two magnetic and six nonmagnetic ) . The operational procedure of the spatter system is run under high vacuity conditions utilizing roughing and cryo pumps. The first 1 is used to pump the chamber down from room temperature to about 25 mTorr. Then the cryo-pump can pump down the chief chamber to a basal force per unit area of about 10-6 mTorr.
Indeed, the base force per unit area in the chief chamber is maintained at about 10-8 mTorr through liquid N, which flows in tubing system located inside upper portion of the chamber. This measure lowers the force per unit area by promoting soaking up of molecules into the walls.The substrates are cut to allow sizes and cleaned with propanone and isopropyl alcohol later. Then, they are mounted on the sample wheel with a shutter system to command the growth of multilayers construction. After lading the sample in the chief chamber and cut downing the chamber force per unit area following the above-named process, the fiction procedure is carried out. Taking into history, all marks are ever subjected to a pre-sputtering cleansing procedure before existent deposition by energetic Ar ion barrage to take contaminated surface beds from the marks. The fiction of tunnel junctions ‘ devices is processed with three-shadow masks procedure. The masks are designed to bring forth a cross construction with a round form at crossed point ( see figure 5 ) .
Initially, the bottom electrode is deposited through the first mask to obtain a specific alliance of tunnel junction. After that, a thin insulating barrier is achieved utilizing plasma oxidization. Specifically, a thin bed of aluminium, in scope of 1nm, is deposited and followed by plasma oxidization. During oxidization, both Ar and O are introduced to the chamber with flow rate of 16 sccm and 76 sccm severally while the magnetron gun is kept enkindled.
After that, C60 bed is formed with thermic vaporization, which is considered the simplest of all the thin-film growing techniques. In item, with a basal force per unit area of 10a?’8 Torr, C60 is heated to its boiling point interior of a vacuum chamber from a pulverization beginning at significantly low power [ 49 ] . Vapor from the beginning travels to the substrate, where some of it condenses in the signifier of a thin-film. In current experiment, current of 21.5 A is applied to heat a fibril and turn it evaporates C60 molecules. The ulterior condense on the substrate surface under the high vacuity fortunes.
Finally, the top electrode is deposited following the same processs as the top electrode with different mark stuffs.For protection intent or on other word to cut down oxidization in ambient air, the complete stack was topped of, over whole construction, by a screen bed of aluminium of 1.5nm. This measure showed a important betterment to try opposition. The whole multilayer layout of the samples is Si-substrate, SiO2, Ta ( 7.
5 ) , Cu ( 5 ) , Co ( 3 ) , Al ( 1 ) plus oxidization for 40sec, C60 ( 5 ) , Co60Fe20B20 ( 5 ) , Cu ( 5 ) , Al ( 5 ) . The Numberss in parentheses give the bed thickness in nanometer. For Mesevey-Tedrow samples the construction consists of SiO2, AlSi ( 6 ) , oxidization for 20sec, Py ( 6 ) , Ta ( 5 ) . The full procedure in the spatter system is programmed so that multilayer deposition, oxidization stairss and vaporization procedure are SubstrateTop electrodeBottom electrodeInsulating layer/ C60performed automatically.
Figure 5: ( a ) A diagram illustrates the construction of tunnel junction samples in cross hair constellation. ( B ) The same construction in stack signifier, colors here to as indicant of construction in a and B.
Word picture and Measurements
Structural Word picture
X raies are comparatively short wavelength, high-energy electromagnetic radiation. They characterized by an electric field vibrating perpendicular to the way of motion at changeless frequence.
This fluctuation of the electric field provides negatrons a sinusoidal oscillation with clip at the same frequence. Hence, X raies are generated as a effect of periodic acceleration and slowing of the negatron [ 50, 51 ] . The conventional method of bring forthing X raies in a research lab is to use a vacuity tubing. This tubing contains a tungsten cathode fibril, which is heated by an AC electromotive force, and so negatrons are produced through thermionic emanation. Electrons are accelerated in vacuity under high potencies runing from 5 to 80 kVs toward a metal mark, the anode, making the X-rays [ 50 ] .When an X-ray beam is subjected onto a crystalline stuff whose atomic agreement shows the long scope cyclicity, a physical phenomenon called diffraction is occurred [ Yoshio ] .
It has been found singular characteristic forms of reflected X ray on crystalline stuffs where intense extremums have been observed for certain aggressively defined wavelengths of incident waies. This probe has been found by W. L.
Bragg and the extremum has been named the Bragg extremum in award of the inventor. In rule, a crystal made out of parallel planes of ions. The planes spaced by a distance d. The x-rays specularly reflected by ions arranged in one plane. A constructive intervention consequences from the reflected beams from consecutive planes.
The way difference between the two beams is 2dsini?± which obeys Bragg status where the pervious term is stand foring an built-in figure of wavelengths. Take into history i?± is the angle of incidence [ 52, 53 ] .Based on the rule mentioned-above, the measurings are carried out by changing the angle of incidence i?± and the corresponding strengths of the end point diffracted extremums produce a good defined form. The form is used to examine out the thickness of the samples where the Bragg ‘s Law status is satisfied. In fact, X-ray diffraction ( XRD ) is a versatile, non-destructive technique that reveals specific inside informations of the sample studied. For case, structural, physical and chemical information about the stuff investigated can be obtained.
However, samples thickness and finding of growing rate is the chief intent of utilizing XRD.
Magnetic measuring in the survey was done with a vibrating sample gaussmeter ( VSM ) . The VSM is a simple but effectual technique for qualifying belongingss of magnetic stuffs established by S. Foner ( 1956 ) . It has straightforward design allows probe of a common experimental technique for mensurating magnetic stuff belongingss such as hysteresis, impregnation and coercivity.
The VSM relies on the sensing of the voltage induced in a spiral of wire based on Faraday ‘s jurisprudence of initiation [ 54, 55 ] . The rule of operation is comprised two constituents, the sample vibrator mechanism and the initiation signal sensing spirals. The latter is brace of indistinguishable pick-up spirals lesion in opposite waies being situated in close propinquity to the sample which is besides located in an external magnetic field. It is required that the sample oscillates in a periodic and stable mode typically through the usage of electromechanical or piezoelectric transducers. Chiefly, when the applied magnetic field, coil place, or sample place is changed ; the electromotive force is induced in a sensing spiral by a flux alteration. The induced electromotive force in the pickup spiral is relative to the sample ‘s magnetic minute and gives a direct step of the magnetisation. Many different experimental agreements can be employed to accommodate peculiar probes of the magnetic initiation measurings affecting elaboration and lock-in sensing of the signal.
Whereby, the full process is running on the Personal computer and control through the VSM package application [ 55, 56, 57 ] .Unlike other techniques, The VSM has highly high sensitiveness whereas alterations of 5 Ten 10-5 to 5 X 10-6 electromagnetic unit can be detected. It is besides simple, in- expensive, and versatile and at the same clip provides precise magnetic minute measurings can be carried out as a map of temperature, magnetising field, and crystallographic orientation [ 57 ] .In this survey, the samples under probe are vibrated sheer to the applied field ( up to 6T ) and the oscillatory magnetic field of the vibrating sample induces a electromotive force in the stationary sensing spirals. From electromotive force measurings the magnetic belongingss of the sample are deduced in temperature of about 1.
7 K which is important to the present survey.
Electrical Conveyance Measurements
The tunnel junction samples were chiefly characterized with magneto-current measurings at RT. Magneto-transport measurings were performed utilizing the four-point investigation technique which was originally proposed to mensurate earth electric resistance by Wenner in 1916. Clearly, it is provides considerable practical method to mensurate material electric resistance therefore the magnetoresistance of the system in a simple and accurate manner. Whereby, the influence of the lead oppositions was removed from the measuring.
In rule, the current is applied thorough one electrode to the other whereas the electromotive force bead is measured. From this, the opposition can be found utilizing Ohm ‘s jurisprudence [ 58, 59, 60, 61, 62 ] . Basically, this method enables to mensurate the opposition alteration as a consequence of changing magnetic field in term of magnetoresistance measurings.
Important point sing this method is that the measurings can be carried out for each electrode individually. Therefore, the electric conveyance can be measured particularly for superconducting electrode in instance of Meservey-Tedrow samples as will be explained subsequently in this study.Throughout this undertaking, the room temperature measuring of magnetic junction samples is carried out in MR Rig in which sample is placed in the centre of two magnetic poles. A field runing from -600 Oe to 600 Oe was varied in order to exchange the two electrodes of the MTJs between the analogue and antiparallel constellations. Correspondingly, the current-voltage curves can be measured at fixed values of magnetic field.Low temperature measurings, particularly for Meservey-Tedrow, are performed with variable temperature He cryostat, which equipped with 8T magnet. The magnet consists of a figure of coaxal solenoid subdivisions wound utilizing multifilamentary superconducting wire. A variable temperature insert is used to let a uninterrupted accommodation of sample temperature over a broad scope.
The sample temperature can typically be controlled over the scope from 1.4 K to 300 K. This is done by equilibrating the liquid He with the warmer and the rotary pump. Temperatures below 4.2 K, the normal boiling point of He, are obtained by cut downing the vapour force per unit area of liquid He in the sample infinite. The later can be filled with liquid He continuously by commanding the needle valve. A changeless liquid degree can be maintained if the flow rate set right so this can merely replace the evaporating liquid.
The sample temperature can be set to about 1.4K, if a big plenty rotary pump is used to cut down the vapour force per unit area of the liquid. The rotary pump is besides used to pull out air so it does non come in and stop dead inside [ 63, 64 ] . Similar to room temperature measuring, four- point investigation technique. The current is varied and the electromotive force bead cross the sample is recorded. In the same attack, conductivity-voltage measurings were built by utilizing the lock-in amplifier end product. For mensurating the burrowing magneto opposition ( TMR ) the magnetic field was swept, between negative and positive Fieldss, while the sample current was kept at a fixed. The full system is controlled by computing machine leting different sort of measurings such as current, temperature and field expanses.
Consequences and treatment
To show, two different sets of samples have been prepared. The first 1 is magnetic tunnel junction samples with a construction consists of, from the substrate side, Ta ( 7.5 ) /Cu ( 5 ) /Co ( 3 ) /AlOx-C60 ( 5 ) /Co60 Fe20 B20 ( 5 ) /Cu ( 5 ) /Al ( 5 ) ( Numberss are nominal thicknesses in nanometres ) .
The 2nd set is the Meservey-Tedrow samples with a superconducting electrode of AlSi.Given the importance of finding the thickness of the beds in the samples, XRD measurings have been conducted for this intent. The deposition rate for each stuff so can be set for sputtering procedure. Basically, standardization samples, thin movies samples, are exposed to X raies at an angle i?± with regard to the bed surface, and at an angle 2i?± the reflected X raies are detected. As explained in subdivision 3.3.1, constructive intervention occurs when the Bragg status is satisfied.
These extremums provide information on the crystal construction, lattice invariables, bed raggedness ‘s and bed thicknesses when proper analysis is done. [ 65 ] .Figure 6 shows the x-ray contemplation strength as a map of i?± for C60 movie on Si/SiO2 sputtered for 200s utilizing a current of 22A. By analysing the cyclicity of the intervention peripheries ( Bragg extremums ) , a thickness of 238.8i‚± 5.1nm is found, matching to the deposition rate of 12.
0nm/s. The current used to turn C60 in present work is 21.5 A where the deposition rate of 9.0 nm/s has satisfied following the same process.
Figure 6: ( a ) The XRD form for a C60 sample ( B ) The information tantrum of Bragg extremums in ( a ) and the corresponding thickness of the sample.A four-point measuring technique is used to mensurate the magnetic field dependance of the junction opposition.
The current sourced through two contacts of the top and bottom electrodes while the electromotive force bead a cross the other two contacts was measured as a map of the external magnetic field. Following same technique, IV characteristic curves were measured at different magnetic Fieldss. Consequently, The electronic conveyance belongingss of the MTJs, in which C60 is deposited between the aluminum oxide barrier and the ferromagnetic electrode, have been investigated utilizing Al2O3 and C60 barriers 5 and 1 nm midst severally. The I/V features of all devices are nonlinear and symmetric. Figure 7a shows a typical I/V curve for MTJ sample at 10K.
The inset shows the conductance versus electromotive force V of the same junction. There are two standards to foreground: foremost, the I/V is nonlinear and 2nd parabolic form of the differential conductance as a map of electromotive force are implicative of burrowing as proposed by Rowell [ 66 ] .
Figure 7: a ) Current electromotive force informations for MTJ sample at 10K. The inset the tunnel conductance G is plotted against the applied electromotive force across the junction at same temperature. B ) TMR was measured at 1.4K as a map of magnetic field.
The alteration in the junction opposition ( at 1uA ) is plotted as an external magnetic field, which goes from 600 to -600 Oe and so reversed from -600 to 600 Oe with the samples at 1.4. 10, 20 and 50 K is shown in Fig 7b. The field dependance of opposition in FM/I/FM junction can be explained briefly as follow. At high Fieldss, the magnetisations of the two FM electrodes are to the full saturated and have alignment in the way of the applied field. The burrowing chance and therefore the current is high in this instance. As the applied field decreases toward zero, alterations marks and make a value of Hc of one of the electrode, so the magnetisation of the electrode with lower Hc reverses whereas for the electrode with higher Hc the magnetisation remains the same.
Two electrodes exhibits opposite magnetisation, antiparallel to each other, in this field scope therefore the tunneling chance and the current is low. The constellation of both electrodes becomes parallel upon a farther raising the field to make Hc for the 2nd electrode. The burrowing chance and the current once more attain higher value. Principally, The difference of Hc in the FM electrodes causes the alteration in tunnel junction and tunnel opposition in effect [ 67 ] . Therefore, the tunnel magnetoresistive consequence can be calculated utilizing the relation:Where Rp and Rap are the oppositions of junction at analogue and antiparallel magnetisation of two ferromagnetic beds, severally [ 67 ] .Figure 1b shows the magnetoresistance ( MR ) for junctions with FeCoB top electrodes plotted versus the external magnetic field. One can clearly see the two stable opposition provinces as the applied field is varied.
The junction magnetoresistance ( JMR ) seen in this instance ( defined with regard to the peak opposition ) is 0.62 % at 1.4K matching to 1.67Ki?- . These values varied somewhat with increasing the temperature. Furthermore, the full work shows a lessening in the opposition as the temperature increases from 1.4 to 50Kwhich represent a typical of burrowing behaviour ( see figure 8a ) .A common feature for CoFeB junctions is that the MR is non maximal at the lowest temperature ( 1.
4 K ) , but increases with higher temperatures to make a maximal around 10-20 K ( as shown in figure 8b ) . This could be due to a alteration in the hopping procedure in C60 and the relationship between the thermic energy available with the activation energy. Below 10-20 K, this will ensue in shorter burrowing stairss and higher spin somersault sprinkling. At 10-20 K the temperature is sufficient to bring forth phonon assisted burrowing with longer skiping distances.
For temperatures above 10-20 K, the MR is reduced due to the lessening in polarisation of the magnet and the increased vibron/phonon denseness.
Figure 8: Temperature dependance of ( a ) the opposition and ( B ) the magnetoresistance.One of the ideal investigations to analyze spin conveyance is the spin-polarized tunneling using the Meservey-Tedrow method of observing spin-polarized tunnel current utilizing a superconductor. Here samples with construction of AlSi ( 6 ) / barrier/Py ( 6 ) /Ta ( 5 ) have been prepared with changing C60 thicknesses alumina barrier oxidization clip. The critical temperature, current and field of the superconducting electrode has been characterized foremost so the junction measurings have been carried out.
The undermentioned figures show the measurings of AlSi electrode of one of the sample.
Figure 9: ( a ) The I-V charatuerstic curve of AlSi electrode measured at 1.4K. ( B ) The opposition measurings as a map of the applied magnetic Fieldss which revealed a critical field value between 2.
2 T and 4.3 T.However, the barrier measurings have non shown Zeeman splitting of the quasi-particle denseness of provinces under application of the magnetic field, as it is clear in figure 10. In kernel, this phenomenon has no ideally optimized in our survey in which the direct finding of spin-polarized current tunneling through an OS has non achieved. This can be interpreted by two grounds. The first one, may the sample need to chill down below 1.4 K so the consequence can be clearly observed. The 2nd 1 is the electronic since such sort of measuring for low electromotive forces ( below 150mV ) really noisy for that an electronic circuit demand to be satisfied.
Figure 10: ( a ) Tunnel conductance versus prejudice at 1.4 K with and without an applied magnetic Fieldss for a junction consists of: 6 nanometers AlSi/barriers/ 6 nanometer Py/ 5 nanometer Ta with 1 nanometers Al2O3/ 2 nm C60 as barrier. ( B ) Normalized conductance of an Al-Al2O3-Ni junction measured as a map of electromotive force for different magnetic Fieldss [ 68 ] .
Spintronics is a new considerable research field has a great potency for usage in a assortment of engineerings that involve extremely developed magnetic field detectors, such as difficult disc thrusts, read caputs and digital informations storage. Recently, an of import measure has been developed uniting the potency of spintronics and organics electronics to uncover a promising field called molecular spintronics. It offers flexibleness, chemical technology and low production costs. These are peculiarly of import indispensable advantages comparing to inorganic devices [ 19, 69 ] .
In the visible radiation of the above advantages and practical applications, nowadays survey introduced a considerable experimental work to analyze burrowing magnetoreistance ( TMR ) of magnetic tunnel junctions ( MTJ ) with organic molecules as an of import phase in the development of spintronics. Current survey trades with two types of samples based on burrowing junction construct. The first set includes magnetic tunnel junctions in which and insulating bed of aluminium oxides is sandwiched between two ferromagnetic electrodes.
A bed of organic stuff of C60 involves as spacer bed is considered to analyze spin conveyance through this type of organic stuff which provides first-class belongingss comparing to other organic stuffs. Electrodes stuff is selected based on specific characteristic as explained antecedently. The 2nd set of samples comprises replacing of one of ferromagnetic electrode with superconductor electrode viz. Meservey-Terdrow samples. It gives direct finding of spin-polarized current tunneling through an organic stuff. In this survey, a chiseled experimental work has been followed including samples fiction, word picture and opposition and magnetoresistance measurings at both room temperature and low poster paint.ture up to 1.4 K.
In present work, TMR of 0.6 % has been obtained from tunnel junction samples utilizing Co and CoFeB electrodes. Further probe demand to be demonstrated with these samples.For Meservey-Terdrow samples, word pictures superconducting word pictures such as critical current, field and temperature has been achieved. However, Zeeman splitting of the quasi-particle denseness of provinces under application of the magnetic field has non obtained yet.The obtained consequences up to this phase can supply a good platform for more elaborate analysis for future survey of tunnel magnetoresistance consequence. Basically, there are some suggested betterments for future work outlined as follow. A fiction method needs to be improved whereby varies parametric quantities could be improved to acquire good samples including force per unit area, power and oxidization procedure.
Cobalt-Gadolinium could be used as option of CoFeB feromagntic electrode. Furthermore, Mesevey-Tedrow samples need to be optimized so the following measure will be a much low temperature measuring in scope of mK utilizing 3He flow cryostat. Optimizing this point let a suited ferromagnetic stuff for magnetic tunnel junctions.Indeed, more research is required to derive a complete apprehension of spin conveyance through organic stuffs. A better penetration into the conveyance through C60 demands to execute through good fiction and word picture processs. By the terminal of this undertaking, optimized C60 based magnetic tunnel junctions need to achieve.