Tunnel Magnetoresistance with Atomically Thin Two-Dimensional Hexagonal Boron Nitride Barriers
André Dankert, M. Venkata Kamalakar, Abdul Wajid, R. S. Patel, Saroj P. Dash
11 Tunnel
Magnetoresistance with
Atomically
Thin
Two ‐ Dimensional
Hexagonal
Boron
Nitride
Barriers
Andre
Dankert , M. Venkata
Kamalakar Abdul
Wajid , R. S. Patel , Saroj P. Dash Department of Microtechnology and
Nanoscience,
Chalmers
University of Technology, SE ‐ Göteborg,
Sweden Department of Physics,
Birla
Institute of Technology and
Science
Pilani – K K Birla
Goa
Campus,
Zuarinagar–
Goa,
India * [email protected]; [email protected] ‐ pilani.ac.in, † [email protected] ABSTRACT
The two ‐ dimensional atomically thin insulator hexagonal boron nitride (h ‐ BN) constitutes a new paradigm in tunnel based devices. A large band gap along with its atomically flat nature without dangling bonds or interface trap states makes it an ideal candidate for tunnel spin transport in spintronic devices. Here, we demonstrate the tunneling of spin ‐ polarized electrons through large area monolayer h ‐ BN prepared by chemical vapor deposition in magnetic tunnel junctions. In ferromagnet/h ‐ BN/ferromagnet heterostructures fabricated over a chip scale, we show tunnel magneto resistance at room temperature. Measurements at different bias voltages and on multiple devices with different ferromagnetic electrodes establish the spin polarized tunneling using h ‐ BN barriers. These results open the way for integration of monolayer insulating barriers in active spintronic devices and circuits operating at ambient temperature, and for further exploration of their properties and prospects. Keywords:
Hexagonal
Boron
Nitride, layered materials, CVD,
Spintronics,
Magnetic tunnel junction,
Tunnel magneto resistance,
Tunnel barrier. Introduction
The quantum phenomenon of electron tunneling enables novel spintronic, electronic, optoelectronic and superconducting nanodevices with enhanced efficiency and low power consumption [1]. Such tunnel devices typically require growth of insulating materials of few atomic layers thin, which is a major challenge in materials science. Mostly used conventional tunnel barrier materials are made up of metal oxides, which have non ‐ uniform thicknesses, pinholes, defects and trapped charges. These issues compromise the performance and reliability of the devices. Tunnel barriers are main building blocks of magnetic tunnel junctions (MTJs), where two ferromagnetic (FM) contacts are separated by ultra ‐ thin oxide barriers [2 ‐ Such
MTJs are currently used in read ‐ heads of hard drives and new emerging technologies including magnetic random access memory and spin ‐ transfer torque devices [3 ‐ For many of these applications, it is crucial to achieve tunnel magnetoresistance (TMR) signals with optimal junction resistances [3], where the demand for controlling thickness of metal oxide tunnel barriers with atomic level precession pose a serious challenge. The discovery of the two ‐ dimensional (2D) atomic crystals has opened up the possibility of exploring their fascinating properties [7, Recently, graphene has been explored as a barrier for spin transport, where TMR and spin filtering effects have been observed [9 ‐ including the bias dependence of the TMR [20].
However, the absence of a bandgap in semi ‐ metallic and low resistive graphene, leads to an increasing demand for an insulating crystal [7 ‐ Hexagonal boron nitride (h ‐ BN) is an insulating isomorph of graphene with a large bandgap of ~ eV, making it an ideal candidate for dielectric substrates and tunnel barriers [8, ‐ The atomically thin and inert nature of h ‐ BN can provide the ultimate control over the morphology and can minimize defects related to interface states and interfacial alloy formation. It has been theoretically proposed to use h ‐ BN as tunnel barrier in MTJs to achieve large magnetoresistance signals [26 ‐ The spin filtering nature of h ‐ BN/ferromagnet interfaces and tailoring of TMR by uniaxial strain has been proposed [26 ‐ Experimentally, h ‐ BN has been used as tunnel barrier for spin injection in lateral graphene spin transport devices [29 ‐ . The chemical vapor deposited (CVD) h ‐ BN is found to show reproducible tunneling behavior circumventing the conductivity mismatch problem between the ferromagnet and graphene for efficient spin injection [30]. In order to further establisdemons Here, wferromaCVD h ‐ BTMR at electrodeffects insulatimagnet Figure tunnelinjunctionand FM Magnetelectroda parallsh the potstrate
TMR we employ lagnetic Ni BN exhibits room tempdes and biausing h ‐ BNng tunnel toresistance Hexagon ng in a FM/h with
Py/h ‐ Belectrodes.
Experimen tic tunnel jdes are schlel orientatitential of in technololarge area C Fe and Cos quantum tperature.
Mas dependeN barriers.
Obarriers ine at ambien nal boron ni h ‐ BN/FM tunBN/Co conta tal junctions inematically pion it is pre atomically ogically releCVD grown o contacts itunneling oMeasuremenence of theOur results n magneticnt temperat itride magn nel junction.act. ( c ) Fabric ncorporatinpresented iferable tha thin h ‐ BNevant
MTJs,monolayerin a verticaof spin polants on multe TMR signademonstrac tunnel dtures. netic tunnel . ( b ) Optical cation steps ng a h ‐ BN n Fig Wt spin polarN tunnel which has r h ‐ BN as a l structure. rized electrtiple devicesal establishate the intedevices and junction. ( a micrographof magnetic tunnel barhen the marized electrbarriers, itnot been retunnel barWe show trons for thes with diffeh the spin pegration of d the obse a ) Schemati of a fabricac tunnel jun rrier betweagnetizationons will tunt is imporealized so farier in MTJsthat atomice demonstrrent ferrompolarized tuatomically ervation of ics of spin pated magnetction with C een two FMns of the FMnnel througrtant to ar. s having ally thin ration of magnetic unneling thin f tunnel polarized tic tunnel CVD h ‐ BN M metal Ms are in gh the h ‐ BN than if they are antiparallel, giving rise to a change in the resistance across the junction [32]. We fabricated magnetic tunnel devices using a h ‐ BN barrier and ferromagnetic Ni Fe (Py) and Co electrodes (Fig. The main fabrication steps of our devices are presented in Fig.
The bottom Co or Py electrodes are prepared on Si/SiO substrate by photo lithography, electron beam evaporation and lift off methods. Before deposition of FM electrodes, we deposited a thin layer of Ti in order to increase adhesion of FM to the SiO substrate. Subsequently we transferred the CVD grown layer of h ‐ BN on bottom FM electrodes. We have been able to achieve a ripple free transfer of CVD h ‐ BN over large areas on our chips by using a frame assisted process. The
CVD h ‐ BN layer used in our experiment was grown on copper substrate (purchased from Graphene supermarket).
The h ‐ BN surface is first covered with PMMA, and then isolated from the Cu substrate by etching in a H O ‐ HCl solution.
This Cu etchant produces very clean h ‐ BN in comparison to other available chemical etching procedures. The h ‐ BN/PMMA layer is washed with deionized water, and subsequently transferred onto the chip containing bottom FM electrodes in isopropyl alcohol medium. We avoided the use of DI water on FM contacts to minimize the oxidation. However, oxidation and contamination of bottom FM electrode due to air exposure could not be avoided in the present process. After drying the chip in an ambient environment, we annealed it at ̊ C for It has been observed that this annealing step improved adhesion of h ‐ BN with bottom FM electrode. The chip was then washed with acetone to remove the PMMA resulting in chip containing h ‐ BN/FM heterostructures.
The top electrodes of ferromagnetic Co and capping Au layer were prepared on the h ‐ BN/FM heterostructures by photo lithography, e ‐ beam evaporation and lift off techniques. The atomic force microscopy was performed with a Bruker
Dimension
SPM using tapping mode.
The
Raman spectrum was measured with a Horiba
XploRA system using a nm laser and a grating of lines/mm. TMR measurements were performed at room temperature with application of an in ‐ plane magnetic field B in . We sweep the B in while applying a constant bias current and measure the voltage change with a Keithley
Sourcemeter. We fabricated various devices consisting of Py/h ‐ BN/Co and
Co/h ‐ BN/Co magnetic tunnel structures on chip scale suing such CVD h ‐ BN. Figure for thsingle la(Fig. our p Figure
Atomscan shosubstrat
Tunnel geometwhile aa fixed while thplane mcorrespdifferenmagnetobservethresho Results and shows tte. Atomic he h ‐ BN layayer h ‐ BN [3c)), which nces of the res. The deprevious wo CVD
Hexa mic force micowing an efte. magnetoretry (Fig. othbias currenhe voltage magnetic fieponding to nt materialstizations cae two welold switchin d Discussion he optical mforce microyer on the The
Ramatches wh ‐ BN contatailed tunnork, where a agonal boron croscopy imaffective h ‐ BN esistance ma)), which her effects snt betweendrop acroseld (B). A mtheir resps and also thn be alignel ‐ defined dg fields of t n micrographoscope (AFMSiO /Si subman shift fowith the reacts are in teling characan effective n nitride : ( a) age of a CVD N height of measuremeenables thesuch as anis the top ans the junctmagnetizatective coerhrough diffeed either padifferent rethe two FM h of large aM) measurebstrate as sor the CVD sults of exthe range octeristics of barrier hei ) Optical mich ‐ BN layer o~5 Å. (c) T ents were e measuresotropic mand bottom fion was meion reversarcivities. Therent geomarallel or anesistance selectrodesrea CVD h ‐ ements revehown in Figh ‐ BN has bxfoliated h ‐ of – k Ω f CVD h ‐ BN ght of φ ~ crograph of on a SiO /Si sThe Raman p performedment of thagnetoresisferromagneeasured as al of the FMhe switchinmetries of thntiparallel astates due . BN transfereal an effectg (b), wheen found tBN [32]. W Ω .µm in oubarriers ha1.49 eV was CVD h ‐ BN onsubstrate. Thpeak of CVD in a fourhe tunnel jutance of thetic electroda function M electrodng field is he FM electand the systo differerred on thetive thickneich correspto be at ~13We find theur magneticave been prs extracted n a SiO /Si suhe red line isD h ‐ BN on a r probe crunction rese FMs. We des of the jof the extees occurs aachieved brodes. Hencstem enableence betwee
SiO /Si ess of ~5 ponds to cm ‐ e tunnel c tunnel resented [30]. ubstrate. s the step a SiO /Si ross bar sistance, applied junction ernal in ‐ at fields by using ce, their es us to een the Figure four proBN/Co Mfield swe
The
TMupon smagnetfor the
TMR ~0are the of the twith rebarrierscontactdifferenresults bottom TMR with obe cross baMTJ for appleep direction MR measurewitching thtization conanti ‐ paralle0.15 % was tunnel resitwo FM eleversing bias were repts. Figure widths osuggest tham FM layers, Py/h ‐ BN/Co ar geometry.lied bias voltns. ment in Fighe magnetnfiguration. el magnetizcalculated bistances meectrodes ress polarity (produced o4 shows a of Co electroat one atom which lead o MTJ at roo (b, c) TMRtages of +/ ‐ g. clearly ization of tThe sharp zation confiby using theeasured at aspectively [(shown in Fon differentTMR signaodes were mic layer ofd to observa om temperat R measureme5 mV at shows thatthe FM eleswitching oiguration ise relation Tantiparallel As expFig. [33 ‐ t devices al of fchosen to f h ‐ BN is suation of TM ture. (a) Scheents with in ‐ K. The arro t the tunneectrodes frof the FM e a strong inMR = (cid:3019) (cid:3250)(cid:3265) (cid:2879)(cid:3019) (cid:3265) and parallepected, the ‐ The oand also infor a Co/h ‐ achieve difufficient enoR signal at r ematics of a ‐ plane magnows indicate l resistancerom a paraelectrodes ndication of (cid:2879)(cid:3019) (cid:3265)(cid:3265) ×100 %,el magnetizTMR signabservation n junction ‐ BN/Co
MTfferent switough to decroom tempe Py/h ‐ BN/Conetic field B e the up and e abruptly inallel to antand the flaf TMR [33 ‐ where (cid:1844) (cid:3002)(cid:3017) ation configl also chanof TMR wiwith differTJ at Ktching fieldscouple the erature [33
MTJ in a in Py/h ‐ d down B ncreases tiparallel t region The (cid:3017) and (cid:1844) (cid:3017) guration ges sign ith h ‐ BN rent FM K, where s. These top and ‐ Figure a four pMTJ for direction The obsBN barrof the
Fof the
FrelationPy/h ‐ BNare lessgraphenh ‐ BN/grdeviceshigher vFM/h ‐ Bverticalthe air interfacpolariza41].
HigexposuMTJ lay TMR with robe cross bapplied biasns. served
TMRrier.
The reFM/h ‐ BN inFM/h ‐ BN con TMR = and Co/hs than previne in lateraraphene chs are compavalues for sBN/FM verti MTJ devicexposure ace is knowation even wgher spin pre. Therefoyer stack is r h Co/h ‐ BN/Co ar geometrys voltages of R is a conselationship bnterface canontacts areP P /(1 − P ‐ BN interfaiously reporl spin transhannel [30]arable to tspin polarizical MTJ stres to the pand h ‐ BN trwn to prodwith convenpolarization ore, the direrequired to o MTJ at roo y. (b) TMR mf at quence of sbetween thn be explain estimated P ), whereaces are assrted TSP of port device. Such spinhat obtaineation were uctures [26possible conransfer procuce strongntional metcan be exect growth demonstra om tempera measurement00 K. The arr spin ‐ depende observedned by the Jto be P ∼ P and P sumed to bP ~ % ines, where FMn polarizatioed using
Alpredicted t6 ‐ We antaminationcess [38 ‐ spin ‐ scatttal oxide tuxpected in sof h ‐ BN onate the true ature. (a) Schts with in ‐ plarows indicat dent tunne TMR and tJulliere mo ∼ P = P ∼ the tue similar, ren Co/h ‐ BN cM electrodeon obtaine O and TiOtheoreticallttribute then of the bo]. The oxidatering that nnel barriestructures gn FMs and spintronics hematics of aane magneticte the up and ling from a tunnel spin del [33]. Th0.05 – spin pespectively contacts fores are placed in grapheO tunnel bly considerie lower valuottom Py oration or coreduces trs such as Agrown in ‐ siin ‐ situ preps potential. a Co/h ‐ BN/Cc field in Co/d down B fie FM througpolarizatiohe spin pola5 % considepolarization[33]. Theser spin injected on the toene spin trbarriers [43ing lattice mue obtainedr Co surfacentaminatiothe tunneliAl O and Mtu withoutparation of Co MTJ in /h ‐ BN/Co ld sweep gh the h ‐ ns (TSP) arization ring the s of the e values tion into op of the ransport ]. Much matched d for our e during n of the ing spin MgO [38 ‐ any air vertical Figure at bias direction300 K (b Figure atomicathe factpositiveCo → Pmaximupolarizedependpropertsuch chobservefunctionin the interfacmonoladifferenspin po Bias depe voltage of ‐ ns. (b) Full bbule circles ‐ D shows therature. The te bias depTo be noteally thin h ‐ Bt that tunnee bias rangePy respec (cid:415) vum around ed tunnelingdence of TMties and maharacteristiced asymmens, and denobserved
Tce conditioayer thicknent thicknessolarization endence of T and ‐ ias dependeDev and re e bias depeTMR data opendence od we restricBN tunnel beling domine corresponvely. We obszero voltagg [35 ‐ OMR relies uagnon excitacs also excletry in TMRnsities of staTMR valuesns, which iess, with feses in laterafor higher TMR with h ‐ B mV at nce of TMR ed circles ‐ D ndence of Tof the
Dev1of
TMR signct our measbarriers.
Thnates the conds to spinsserve a decge, which is Our results upon the qations at thlude any arR with bias ates for the s for two is not knowew patches al graphenecontact re BN barrier a K. The arrowsignal for twDev2).
The lin
TMR for tw1 at ‐ mV al for both surements e low voltaonduction ms tunneling creasing treconsistent reflect the tquality andhe FM/h ‐ BNrtifacts suchcan be duetwo differedifferent dwn exactly.of bi ‐ layer e spin transesistances c at room temp ws indicate wo different Pnes are guide o devices wand ‐ mVdevices arto low bias age study ismechanism through h ‐ end of TMR with behavtypical behad height of N interfacesh as anisote to interfaent FM elecdevices canThe CVD gand thickesport devicecorrespondi perature . (a the up and Py/h ‐ BN/Co e to the eye. with Py/h ‐ BNV are showne plotted wvoltages < s also appro[35 ‐ ‐ BN barrier at higher bvior generalavior for Mthe h ‐ BN s [35, Tropic magnaces oxidatictrodes [35 ‐ be due togrown h ‐ BNer layers [30es, an enhaing to the a) TMR plots down B fieldevices mea N/Co
MTJs n in Fig. with bias vo50 mV, becopriate con. The negafrom Py → bias voltagelly observedTJs, where barrier, elThe observanetoresistanion, differe ‐ The difo slightly dN is having0, Usinancement inthickness for
Dev1 ld sweep asured at at room and the oltage in cause of sidering tive and → Co and es with a d in spin the bias ectronic ations of nce. The nt work fference different g mostly ng these n tunnel of h ‐ BN barrier has been observed previously [30]. It would be interesting to investigate such thickness dependence of TMR in h ‐ BN based MTJ structures and its proposed spin filtering attributes [26 ‐ Conclusions In conclusion, we have demonstrated the feasibility of spin ‐ dependent tunneling employing single ‐ layer insulating h ‐ BN barriers in MTJs. We measured a TMR effect of – % at room temperature corresponding to tunnel spin polarization P of – % for FM/h ‐ BN junctions. The lower values of TMR and P are attributed to oxidation and contamination of the bottom FM electrode during h ‐ BN transfer process. It is expected that much higher TMR ratios can be obtained through improvements in fabrication process with cleaner interfaces, incorporating multi ‐ layer h ‐ BN barriers [26 ‐ and use of h ‐ BN/graphene heterostructures to achieve spin filtering [26]. These results demonstrate that uses of atomically thin large area CVD h ‐ BN tunnel barrier are generic for a large class of devices requiring tunneling of spin polarized carriers and particularly interesting for technologically important magnetic tunnel junctions. Such h ‐ BN tunnel barriers can also be employed for efficient spin injection into silicon and other semiconductor materials [44, Opportunities are growing with improvements in the growth process of large area CVD h ‐ BN and h ‐ BN/graphene van der
Waals heterostructures on ferromagnets, which can be used in future spintronic devices to achieve desired contact resistances and large magnetoresistance signals [46 ‐ Acknowledgement
The authors acknowledge the support from colleagues of Quantum
Device
Physics
Laboratory and
Nanofabrication
Laboratory at Chalmers
University of Technology.
The authors would like to thank Jie
Sun and
Niclas
Lindvall for sharing the recipe for layer transfer process. This project is financially supported by the Nano
Area of the Advance program at Chalmers
University of Technology, EU FP7
Marie
Curie
Career
Integration grant and the
Swedish
Research
Council (VR)
Young
Researchers
Grant.
RSP acknowledges the financial support from the
Department of Science and
Technology,
Government of India through nanomisson project (grant
No.
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