Tunnel Magnetoresistance Signal Research Articles

Tunable High‐Temperature Tunneling Magnetoresistance in All‐van der Waals Antiferromagnet/Semiconductor/Ferromagnet Junctions

AbstractMagnetic tunnel junctions (MTJs) are widely applied in spintronic devices for efficient spin detection through the imbalance of spin polarization at the Fermi level. The van der Waals (vdW) property of 2D magnets with atomically flat surfaces and negligible surface roughness greatly facilitates the development of MTJs, primarily in ferromagnets. Here, A‐type antiferromagnetism in 2D vdW single‐crystal (Fe0.8Co0.2)3GaTe2 is reported with TN ≈ 203 K in bulk and ≈ 185 K in 9‐nm nanosheets. The metallic nature and out‐of‐plane magnetic anisotropy make it a suitable candidate for MTJ electrodes. By constructing heterostructures based on (Fe0.8Co0.2)3GaTe2/WSe2/Fe3GaTe2, a large tunneling magnetoresistance (TMR) ratio of 180% at low temperature is obtained, with the TMR signal persisting at near‐room temperature 280 K. Furthermore, the TMR is tunable by the electric field, and the MTJ device operates stably with a low applied bias down to 1 mV (≈0.6 nA), highlighting its potential for energy‐efficient spintronic devices. This work opens up new opportunities for 2D antiferromagnetic spintronics and quantum devices.

Modeling of Spin Transport in Hybrid Magnetic Tunnel Junctions for Magnetic Recording Applications

We have demonstrated modeling of phonon and defect-induced spin relaxation length (LS) in Fe3O4 and organic semiconductor (OSC) Alq3. LS of Alq3 decreases with enhanced disorder and film thickness at a low film width regime. Exponential change of LS at low width regime is found for Alq3 which is, however, absent for Fe3O4 indicating comparable spin-dependent scattering and LS in Fe3O4. LS also decreases with spin-flip probability both for Alq3 and Fe3O4. Voltage-dependent tunnel magnetoresistance (TMR) response in Fe3O4/Alq3/Co and La0.7Sr0.3MnO3 (LSMO)/Alq3/Co hybrid magnetic tunnel junction (MTJ) devices has been attributed to modified spin filter effect across magnetic/OSC junction at high bias regime. TMR reduction with Alq3 thickness for Fe3O4 device has been attributed to spin relaxation at the organic spacer layer. A low bias peak from differential TMR indicates spin-polarized injection for both MTJ devices. Enhanced in-plane spin transfer torque for both MTJ is associated with modified spin filtering at magnetic/OSC junctions. Lower TMR signal for LSMO device indicates reduced tunneling and enhanced carrier injection across the OSC, which is also supported by the band structure profile. The TMR response observed from simulation results matches well with previously reported experimental results. Higher TMR response for Fe3O4 device indicates the possibility of device employment in room temperature magnetic recording applications.

Single-shot all-optical switching of magnetization in Tb/Co multilayer-based electrodes

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

Impact of Annealing Temperature on Tunneling Magnetoresistance Multilayer Stacks

The effect of annealing temperatures on the tunnel magnetoresistance (TMR) of MgO-based magnetic tunnel junctions (MTJs) has been investigated for annealing between 190 and 370°C. The TMR shows a maximum value of 215% at an annealing temperature of 330°C. A strong sensitivity of the TMR and the exchange bias of the pinned ferromagnetic layers on the annealing temperature are observed. Depending on sensor application requirements, the MTJ can be optimized either for stability and pinning strength or for a high TMR signal by choosing the appropriate annealing temperature. The switching mechanism of the ferromagnetic layers in the MTJ and the influence of the annealing on the layer properties, including the MgO barrier, are discussed.

The Influence of Edge Inhomogeneities on Vortex Hysteresis Curves in Magnetic Tunnel Junctions

Magnetoresistive sensors using a tunnel magnetoresistance (TMR) spin valve structure with CoFe and CoFeB free layer (FL) are investigated. Lateral dimension, thickness, and magnetic properties of the circular-shaped FL energetically favor the nucleation of a magnetic vortex. In the vortex configuration, the sensors show the expected hysteresis-free transfer curve. Distinct differences between CoFe and CoFeB in the vortex typical parameters are observed. The experimental results are compared to micromagnetic simulations. Influence of saturation magnetization ${M}_{s}$ on the vortex transfer curve is studied in order to reproduce the experimental data. However, only by adjusting ${M}_{s}$ iteratively the experimental data cannot be fully reproduced: The vortex annihilation ( ${H}_{\mathrm {an}}$ ) in CoFeB occurs at smaller fields than simulated. On the other hand, the change in TMR signal at ${H}_{\mathrm {an}}$ is significantly smaller in the experiment for CoFe. Discrepancies are most pronounced at ${H}_{\mathrm {an}}$ . This investigation focuses on the magnetic and electric properties at the edge of the FL structure. Reduction in ${H}_{\mathrm {an}}$ can be explained for 1.1 $\mu \text{m}$ FL diameters by introducing a magnetically disturbed edge. Reduced change in TMR signal at ${H}_{\mathrm {an}}$ can be understood by introducing an electrically inactive edge area. The analysis shows that for CoFe and CoFeB different edge inhomogeneity effects are present or edge inhomogeneities have different impacts.

Transport signatures of surface potentials on three-dimensional topological insulators

The spin-momentum locked nature of the robust surface states of three dimensional topological insulators (3D TI) make them promising candidates for spintronics applications. Surface potentials which respect time reversal symmetry can exist at the surface between a 3D TI and the trivial vacuum. These potentials can distort the spin texture of the surface states while retaining their gapless nature. In this work, the effect of all such surface potentials on the spin textures is studied. Since, a tunnel magnetoresistance signal carries the information of the spin texture, it is proposed that spin-polarized tunneling of electrons to a 3D TI surface can be used to uniquely identify the surface potentials and quantitatively characterize them.

Spin polarized transport driven by square voltage pulses in a quantum dot system

AbstractWe calculate current, spin current and tunnel magnetoresistance (TMR) for a quantum dot coupled to ferromagnetic leads in the presence of a square wave of bias voltage. Our results are obtained via time‐dependent nonequilibrium Green function for a non‐interacting electron model. Both parallel and antiparallel lead magnetization alignments are considered. The main findings include a wave of spin accumulation and spin current that can change sign as the time evolves, spikes in the TMR signal and a TMR sign change due to an ultrafast switch from forward to reverse current in the emitter lead. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-dependent density of states in Ga1−xMnxAs probed by tunneling spectroscopy

We conducted spin-dependent tunneling spectroscopy measurements in Fe∕ZnSe∕Ga1−xMnxAs magnetic tunnel diodes that had a metal-insulator-semiconductor structure. The tunneling magnetoresistance signal in differential conductance spectra (dI∕dV−V) was found to disappear for bias voltages (V) higher than 350meV for a bias direction where electrons tunnel into Ga1−xMnxAs. This indicates that there is exchange splitting in the density of states of Ga1−xMnxAs up to 350meV above the Fermi energy.

Interface roughness effects on the performance of magnetic tunnel junctions

In-situ thermal anneal and substrate biased radio frequency plasma treatments are chosen to smooth the bottom electrode prior to the deposition of the AlO x barrier and the top electrode in the bottom pinned magnetic tunnel junctions (MTJs) stackes. Atomic force microscopy images indicate that surface roughness could be reduced by low energy plasma etching (40 W) on the in-situ heated bottom electrode. The tunnel magnetoresistance (TMR) and ferromagnetic interlayer coupling ( H in) behaviors versus ex-situ anneal temperature for treated and untreated junctions are studied. The results indicate that magnetic tunnel junctions deposited on the smoothed electrode show good thermal stability, high TMR signal, low interlayer coupling field, and sharp magnetization reversal of the free layer. It is concluded that the interface roughness is of great importance and must be well controlled in order to obtain high performance MTJs.

High magnetoresistance in Co–Fe–B-based double barrier magnetic tunnel junction

The magnetic transport properties of the double barrier magnetic tunnel junctions (DBMTJs) with amorphous Co–Fe–B electrodes, deposited by magnetron sputtering and patterned using optical lithography combined with lift-off technique, were investigated. A high tunnel magnetoresistance (TMR) ratio of 90.3% and 59.4%, resistance-area product of about 6.2 and 5.5 kΩ μm 2 at 4.2 K and room temperature, respectively, and a high bias voltage at which the TMR signal decreased to half of its maximum value, V 1/2 =1.14 V at negative bias voltage were measured experimentally for the DBMTJs. These results show that the DBMTJs are promising candidates for spin electronic devices.

Thermal stability of magnetic tunnel junctions with new amorphous ZrAl-alloy films as the under and capping layers

We studied the thermal stability of new amorphous ZrAl-based magnetic tunnel junctions (MTJs) with a ZrAl-oxide barrier replacing the Ta layers traditionally used for the under and capping layers. The MTJs were compared with similar conventional MTJs (Ta-based MTJ with Al-oxide). After annealing at various temperatures up to 450/spl deg/C, the ZrAl-based MTJs still had a significant tunnel magnetoresistance signal of nearly 21%. The thermal stability of amorphous ZrAl-based and conventional Ta-based MTJs differs dramatically, mainly because of the different microstructural evolution. The noncrystalline ZrAl-alloy film had superior surface uniformity and an induced microstructure that resisted interdiffusion, with dense, equiaxed grains making up the upper stacks' films. By contrast, the conventional Ta-based MTJ had a broad columnar structure with less dense boundaries, which act as a source of interdiffusion, resulting in barrier deformation at elevated temperatures.

Correlation between tunneling magnetoresistance and magnetization in dipolar-coupled nanoparticle arrays

The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled anisotropic magnetic nanoparticles is studied using a resistor network model and a realistic micromagnetic configuration obtained by Monte Carlo simulations. Analysis of the field-dependent TMR and the corresponding magnetization curve shows that dipolar interactions suppress the maximum TMR effect, increase or decrease the field-sensitivity depending on the direction of applied field and introduce strong dependence of the TMR on the direction of the applied magnetic field. For off-plane magnetic fields, maximum values in the TMR signal are associated with the critical field for irreversible rotation of the magnetization. This behavior is more pronounced in strongly interacting systems (magnetically soft), while for weakly interacting systems (magnetically hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in contrast to the situation for non-interacting nanoparticles or in-plane fields (Hmax=Hc). The relation of our simulations to recent TMR measurements in self-assembled Co nanoparticle arrays is discussed.

Large Magnetoresistance at High Bias Voltage in Double Magnetic Tunnel Junctions

This paper reports on the magnetic and transport properties of [IrMn/CoFe]/AlOx/SL/AlOx/[CoFe/IrMn] double magnetic tunnel junctions (DMTJ) deposited by magnetron sputtering and patterned using optical lithography. The aim is to obtain a high-quality magnetoresistive hard-soft system presenting a large magnetoresistance at a high applied bias voltage. For this purpose, two DMTJ were studied where the soft layer (SL) was CoFe/sub 1//NiFe/sub 5//CoFe/sub 1/ (nm) and NiFe/sub 7/ (nm). The similar quality of both AlOx barriers within each sample is supported by the symmetric character of the tunnel magnetoresistance (TMR) versus the bias voltage variation. The junctions having a CoFe/NiFe/CoFe (NiFe) SL show a resistance-area product (RAP) about 35 (19) k /spl Omega//spl middot//spl mu/m/sup 2/, a high TMR at room temperature of 49 (20)%, and a high bias voltage at which the TMR signal is decreased to half of its maximum value, V/sub 1/2/=1.33(1.0) V. Both hard magnetic layers are rigid in negative field up to 51.5 kA/m, while the coercive field of the SL is around 1.16 and 0.48 kA/m for the DMTJ integrating, respectively, the CoFe/NiFe/CoFe and NiFe SL. The large difference of coercive fields associated with the low dependence of the TMR versus bias voltage, recommend this systems for spin electronic devices.

He-RBS, He-ERDA and heavy ion-ERDA analysis of Si/Ta 70 Å/CoFe 35 Å/HfAlO x/CoFe 35 Å/Ta 30 Å systems

Low resistance spin-dependent tunnel junctions are investigated for read-head applications due to their large tunnelling magnetoresistance effect and low junction resistance-area product. Typical full junction structures can be Ta 70 Å/NiFe 70 Å/MnIr 80 Å/CoFe 35 Å/Hf x Al y O z 10 Å/CoFe 35 Å/NiFe 40 Å/Ta 30 Å, annealed to temperatures up to 250 °C. We analyzed simpler structures with RBS experiments at a grazing angle of incidence in order to study the junction only. Heavy ion- and He-ERDA experiments were also done. The composition of the HfAlO barrier is determined, showing that a slightly substoichiometric oxide is formed. On annealing at 240 °C, the interfaces of the barrier become sharper. TEM results confirm that the barrier is continuous after annealing. This is related to an increase in the tunnel magnetoresistance signal from 4% to 13.5% after annealing.

Low tunnel magnetoresistance dependence versus bias voltage in double barrier magnetic tunnel junction

We report on the magnetic and transport properties of [IrMn8/CoFe1.5]/AlOx1.2/[CoFe1/NiFe5/CoFe1]/AlOx1.2/[CoFe1.5/IrMn8] (nanometer) double magnetic tunnel junctions (DMTJs) deposited by magnetron sputtering and patterned using optical lithography. The tunnel magnetoresistance (TMR) versus the bias voltage presents a symmetric characteristic, which indicates a good and similar quality of both AlOx barriers. The junctions show a resistance-area product about 35 kΩ μm2, a high TMR at room temperature of 49.5%, and a high bias voltage at which the TMR signal is decreased to half of its maximum value, V1/2DMTJ=1.33 V. Both hard magnetic layers are rigid in negative field up to 51.5 kA/m, while the coercive field of the soft layer is around 1.1 kA/m. The large difference of coercive fields, combined with the large TMR and V1/2, makes these systems very promising for spin electronic devices.

Low-resistance spin-dependent tunnel junctions with HfAlO/sub x/ barriers for high-density recording-head application

Spin-dependent tunnel junctions with the structure (Ta 70 /spl Aring//NiFe 70 /spl Aring//MnIr 80 /spl Aring//CoFe 35 /spl Aring//HfAlO/sub x//CoFe 35 /spl Aring//NiFe 40 /spl Aring//TiW(N) 150 /spl Aring/) were fabricated on top of 600-/spl Aring/-thick ion-beam-smoothed low-resistance Al electrodes. HfAlO/sub x/ barriers were formed by natural oxidation (5 min at 1 torr in pure O/sub 2/) of 5-/spl Aring/-thick (2-/spl Aring/ Hf+3-/spl Aring/ Al) films or 6-/spl Aring/-thick (2-/spl Aring/ Hf+4-/spl Aring/ Al) films. Resistance/spl times/area (R/spl times/A) products of 0.65 /spl Omega//spl times//spl mu//sup 2/ and 2.1 /spl Omega//spl times//spl mu/m/sup 2/ were achieved with 9.5% and 13.5% tunnel magnetoresistance signal (TMR), respectively. Current inhomogeneity effects on the measured (R/spl times/A) products and TMR values were calculated in particular for junctions with resistance below 1 /spl Omega//spl times//spl mu/m/sup 2/. Transmission electron microscopy indicates that HfAlO/sub x/ forms a continuous amorphous barrier that follows conformally the topography of the bottom electrode. X-ray photoelectron spectroscopy analysis indicates that 2.5% metallic Hf is left inside the barrier closer to the bottom electrode. These low-resistance tunnel junctions are attractive for read-head applications at recording densities above 100 Gbit/in/sup 2/.

Micromagnetic simulation for tunnel junctions with synthetic antiferromagnetic pinned layers annealed at different external fields

Bottom-pinned tunnel junctions with synthetic antiferromagnetic (SAF) pinned layers require an annealing step near the blocking temperature under an applied magnetic field to set the exchange direction. Experimental results (anneal at 270 °C, for a MnIr/SAF structure) show that the tunnel magnetoresistance signal is strongly reduced (3%) for anneals under an applied field of 2000 Oe, when compared with large tunnel magnetoresistance signals (>20%) obtained for samples annealed either at low (250 Oe) or high (5000 Oe) fields. To clearly understand this behavior, a micromagnetic simulation was performed to demonstrate the magnetization evolution of the SAF layer during the different field anneals. The simulated magnetization configurations indicate that the SAF layer net moment aligns parallel to the applied field (high field anneal) or antiparallel to the applied field (low field anneal), leading in both cases to high TMR signal. After annealing at intermediate field, the SAF net moment rotates almost orthogonal to the annealing field direction, resulting in the measured low TMR signal.

Comparison of hard magnetic electrodes for magneto-electronics by magnetic force microscopy

Both spin valve sensor elements and magnetic tunnel junctions usually consist of a magnetically fixed hard magnetic layer and a soft magnetic counter electrode which can rotate freely in an external driving field. In order to yield reliable sensor signals, the hard magnetic electrode should be magnetically rigid against small fields, i.e., must not be influenced during switching of the sensing electrode. Furthermore, in order to provide a homogeneous switching behavior, the electrodes should show uniformity in their magnetic properties, especially if scaled and patterned down to sub-micron lateral sizes. In this paper, magnetic force microscopy is used to compare the magnetic properties of different hard magnetic layer stacks commonly used in magneto-electronic device technology, i.e. an artificial antiferrimagnet (CoFe/Ru/CoFe and Co/Cu/Co) and exchange biased stacks (MnIr/CoFe/Ru/CoFe). The domain pattern of a polycrystalline artificial ferrimagnet (AFi) shows a strong magnetization ripple, i.e., local variations of the magnetization directions, which are maintained in patterned submicron elements. These statistical local fluctuations can be quantitatively correlated with a reduction of the tunneling magnetoresistance (TMR) of 3%–5% in the minor loop. In addition for small area junctions, the ripple can cause different switching fields in neighboring elements. An additional exchange biasing by a thin natural antiferromagnet is shown to rigidly pin the local magnetization fluctuations of the AFi. The process used for the preparation of this system, however, creates 360° domain walls, which again deteriorate the TMR signal.

Low-resistance spin-dependent tunnel junctions with ZrAlOx barriers

Spin-dependent tunnel junctions with ZrAlOx barriers were formed by natural oxidation (5 min at 10 Torr) of 7-Å-thick Zr–Al films. Resistance×area products of 6 Ω μm2 were achieved with a 15.3% tunnel magnetoresistance (TMR) signal. Bottom-pinned (MnIr) junctions were deposited on top of 600-Å-thick, ion-beam-smoothed, low-resistance, Al electrodes. Effective average barrier height and thickness are 0.28 eV and 8.2 Å, respectively, and breakdown voltage is 0.41 V for 1 μm2 junctions. The TMR signal decreases by half at a bias voltage of 210 mV. Junction TMR decreases for anneals above 250 °C. High-resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low-resistance tunnel junctions are attractive for read head applications above 100 Gbit/in.2 where competitive signal-to-noise ratios imply resistance×area products of few Ω μm2 and TMR signals near or above 20%.

Field-dependent domain structure evolution in artificial ferrimagnets analyzed by spin-polarized tunnel transport in magnetic tunnel junctions

A powerful technique for investigating field-dependent micromagnetism in thin magnetic layers is presented. The technique uses the spin-polarized tunnel-transport mechanism in magnetic-tunnel junctions. We used this technique to study the micromagnetic-reversal mechanism in an artificial ferrimagnetic system, which consists of two ferromagnetic layers strongly antiferromagnetically coupled through a nonmagnetic interlayer. We show that the high sensitivity of the spin-polarized current to the fluctuations of magnetization allows to probe the magnetic-domain structure in the magnetic electrodes. As a contrast to standard M-$H$ and giant-magnetoresistance measurements, which are only able to probe the global magnetic state of this artificial ferrimagnet, we show here that the tunnel magnetoresistance discriminates the field-dependent evolution of the domain phases in selective magnetic layers. Furthermore, we demonstrate the capability of a tunnel-magnetoresistance signal to be used as a quantitative probe for investigating residual walls during the reversal process.