Spin transfer torque oscillator based on asymmetric magnetic tunnel junctions
Witold Skowroński, Tomasz Stobiecki, Jerzy Wrona, Günter Reiss, Sebastiaan van Dijken
aa r X i v : . [ c ond - m a t . m t r l - s c i ] O c t Spin transfer torque oscillator based on asymmetric magnetic tunnel junctions
Witold Skowro´nski, ∗ Tomasz Stobiecki, and Jerzy Wrona
Department of Electronics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krak´ow, Poland
G¨unter Reiss
Thin Films and Physics of Nanostructures, Bielefeld University, 33615 Bielefeld, Germany
Sebastiaan van Dijken
NanoSpin, Department of Applied Physics, Aalto University School of Science, P.O.Box 15100, FI-00076 Aalto, Finland (Dated: November 7, 2018)We present a study of the spin transfer torque oscillator based on CoFeB/MgO/CoFeB asymmetricmagnetic tunnel junctions. We observe microwave precession in junctions with different thicknessof the free magnetization layer. Taking advantage of the ferromagnetic interlayer exchange couplingbetween the free and reference layer in the MTJ and perpendicular interface anisotropy in thinCoFeB electrode we demonstrate the nanometer scale device that can generate high frequency signalwithout external magnetic field applied. The amplitude of the oscillation exceeds 10 nV/ √ Hz at1.5 GHz. Magnetic tunnel junctions (MTJs) consisting of twoferromagnetic electrodes separated by a thin tunnel bar-rier has recently drawn a significant attention due to theirpotential applications as a high density memory cell and microwave electronic components . DC currents insuch structures can induce steady state precessions dueto the interaction between spin-polarized electrons andthe local magnetization of the free layer (FL). This spin-transfer-torque (STT) effect induces resistance fluctu-ations in the MTJ which in turn generate an AC signalin the GHz frequency range. Such STT-based nanometerscale oscillator can be a competitive device to the exist-ing LC-tank technologies used widely in high-frequencyelectronics. One of the key issues of the spin torque oscil-lators (STOs) is the ability to produce microwave signalwithout the a need of operating in an external magneticfield. In this work, we report on STO based on asymmet-ric magnetic tunnel junctions with the thin MgO tunnelbarrier and the FL ferromagnetically coupled to the refer-ence layer (RL) that are able to operate with no magneticfield applied. To our knowledge, such operation has notbeen published yet.The MTJ stack with a CoFeB wedged shaped electrodewas deposited in a Singulus Timaris cluster tool system.The multilayer structure consisted of the following ma-terials (thickness in nm): buffer layers / PtMn (16) /Co Fe (2) / Ru(0.9) / Co Fe B (2.3) / MgO(0.85) /Co Fe B (1 - 2.3) / capping layer. The deposition pro-cess was similar to the one used in our previous studies .After deposition, three different parts of the sample withdifferent FL thickness were selected for patterning intonanometer size pillars (later in the paper referred to asA1, A2 and A3, see Table I for details). In this paperwe focus mainly on the sample with 1.57 nm thick FL -A2. Using a three-steps electron beam lithography pro-cess, which included ion beam milling, lift-off and oxidedeposition steps, nanopillars with elliptical cross-sectionof 250 ×
150 nm were fabricated. The pillars were etcheddown to the PtMn layer. To ensure good RF performance of the device, the overlap between the top and bottomleads was about 4 µ m , which resulted in a capacitanceof less than 1 × − F. The DC measurements wereconducted at room temperature with a magnetic field ap-plied in the sample plane. The high-frequency measure-ments were carried out using a Agilent N9030A spectrumanalyzer with built preamplifier. The MTJ bonded to thehigh frequency chip carrier was connected to a bias-tee.The DC signal from a sourcemeter was fed to the samplethrough the inductive connector of the bias-tee, whereasthe spectrum was measured at the capacitive connector.In this paper, the positive voltage denotes the electronflowing from a bottom RL to the top FL favoring theparallel alignment of the magnetizations.Figure 1a shows the TMR loops for samples A1-A3,with field applied along the in-plane easy axis of the MTJ.The TMR decreases with decreasing FL thickness dueto reduced spin polarization of the tunneling electrons .Moreover, the magnetization of the FL is tilted out of thefilm plane due to the perpendicular interface anisotropyin thin CoFeB layer and therefore, at zero magneticfield a full parallel state for samples A1 and A2 is notachieved. The coercive field of about 100 Oe for sam-ple A3 is reduced to zero for sample A1 . Differentialconductance versus DC bias voltage was measured forall samples using lock-in technique. The results are pre-sented in Fig. 1b. The asymmetry between the thinFL and the RL for samples A2 and A3 is observed incomparison with symmetric A3 sample. This asymmetryarises from a different band structures in the ferromag-netic electrodes .Ferromagnetic coupling between the RL and FL stabi-lizes the low resistance state of the MTJs at zero appliedmagnetic field . All samples exhibited clear currentinduced magnetization switching measured for relativelylong current pulses of 10 ms. The absolute switchingcurrent value needed to change the MTJ state from a Pto AP, measured with no magnetic field assistance, wasfound to decrease with FL thickness due to reduced satu- Field (Oe) T M R ( % ) d I/ d V ( no r m ) Voltage (V) A3 A2 A1
FIG. 1: TMR vs. magnetic field (a) and differential conduc-tance (dI/dV) vs. DC bias voltage at low-resistance state(b) measured for samples with different FL thickness. Clearasymmetry between CoFeB electrodes is observed in dI/dVmeasurement for samples A1 and A2.TABLE I: Summary of static parameters of the prepared MTJnanopillars.Sample No. FL thickness TMR Ic P → AP(nm) (%) mAA1 1.35 50 -0.95A2 1.57 100 -1.8A3 2.3 120 -2.4 ration magnetization (volume) of the FL (Table I). Thesample spectra of A2 measured with no magnetic fieldapplied are shown in Fig. 2. Existence of the perpen-dicular interface anisotropy in the FL results in non zeroangle theta between FL and RL magnetizations in a lowresistance state at zero magnetic field (Fig. 1) and there-fore the STT precession is excited even at low DC bias.An increase of the negative DC current of the polariza-tion that favor the AP state (electrons flowing from theFL to the RL) results in increased amplitude of the os-cillations, that exceeds 10 nV/ √ Hz at 1.5 GHz and -1.7mA. Further increase of the negative current magnituderesults in switching the MTJ to the high resistance APstate, where peak of much smaller amplitude and widerlinewidth is observed.Fig. 3 presents the STO amplitude, peak frequency( f ) and linewidth ∆ f versus DC bias current with nomagnetic field applied. Clearly, the oscillations for neg-ative current favoring the AP state are more powerfulthan for the positive one, favoring P state, however peak P o w e r ( n V / H z . ) Frequency (GHz)
DC current -0.1 mA -0.5 mA -1 mA -1.5 mA -1.7 mA -1.8 mA
FIG. 2: Sample STO spectra of A2 measured at low-resistancestate and different negative current applied to the MTJ with-out external magnetic field. Peak oscillation amplitude ex-ceeds 10 nV/ √ Hz at -1.7 mA. Further increase of currentmagnitude results in switching the MTJ to the high-resistancestate. ) b ) A m p li t ude ( n V / H z . ) a ) pea k f r equen cy ( G H z ) experiment linear fit li ne w i d t h ( M H z ) Idc (mA)
FIG. 3: The DC bias current dependence of the amplitude(a), peak frequency f (b) and linewidth ∆ f (c). No magneticfield was applied during the measurements. The dashed linein (c) represents the linear fit to the experimental data forpositive I dc . Near the switching current (dotted line) both f and ∆ f increase. P ea k f r equen cy ( G H z ) FL thickness (nm)
FIG. 4: Peak frequencies for MTJs with different thicknessof the FL measured at low resistance state and I dc = -1 mAwithout magnetic field applied. at both current polarizations are visible due to a non zeroangle theta at zero field. The dependence of linewidth ofDC current is expressed as:∆ f = σ π ( I c − I dc ) (1)where σ is the spin polarization efficiency and I c is thethreshold current . Extrapolating the ∆ f at the damp-ing side (when MTJ is in P state and current favors Pstate) to zero Hz estimates the threshold current value(dotted line in Fig. 3. Moreover, a rapid change bothin f and ∆ f is observed near the switching threshold.A similar current value was measured during the staticCIMS experiment. The switching voltage is much smallerthan the breakdown voltage, therefore we can inducesteady state precession without destroying the MTJ.The peak frequency at constant I dc = -1 mA and ( H dc )= 0 was found to increase with increasing FL thickness,results are presented in Fig. 4. Increased anisotropy con-stant results in smaller precession trajectories and there- fore increased f . It should be noted, that a sample-to-sample distribution in both oscillation’s amplitude andfrequency is observed, mainly due to the size and shapedistribution during a nano-lithography process, howeverthe overall tendency is retained. For other samples withthinner FL of 1.22 nm we were not able to observe anyoscillation in the measured bandwidth even with strongmagnetic field applied in-plane of perpendicular to thesample’s easy axis. For sample A2 the oscillation’s am-plitude is of the same order than the highest reportedto date exceeding 10 nV/ √ Hz . We conclude, that tak-ing advantage of the coupling mechanisms in MTJs witha thin MgO tunnel barrier in combination with perpen-dicular interface anisotropy might enhance STOs perfor-mance without the need of an external magnetic fieldapplication.In summary we have demonstrated an STO based onan asymmetric MTJ, that is able to produce a microwavesignal without a need of the magnetic field applications.Due to the ferromagnetic interlayer exchange coupling inour system, the MTJ is in stable low resistance state at H dc = 0. Perpendicular interface anisotropy is thin FL isused, to induce the magnetization precession at small DCbias. The oscillation’s amplitude exceeds 10 nV/ √ Hz at1.5 GHz and I dc = -1.7 mA.The authors would like to thank Singulus Technolo-gies AG for consultation and technical help with MgOwedge MTJs preparation. Work supported by the SPIN-LAB POIG.02.02.00-00-020/09 project. T.S. and W.S.acknowledge Foundation for Polish Science MPD Pro-gramme co-financed by the EU European Regional De-velopment Fund. and the Polish Ministry of Scienceand Higher Education grants (IP 2010037970 and NN515544538). S.v.D. acknowledges financial support fromthe Academy of Finland for the ACTIVE-BAR project(no. 127731). ∗ Electronic address: [email protected] Yiming Huai, Frank Albert, Paul Nguyen, MahendraPakala, and Thierry Valet. Observation of spin-transferswitching in deep submicron-sized and low-resistancemagnetic tunnel junctions.
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