Tunneling Anisotropic Magnetoresistance Research Articles

Tunneling anisotropic magnetoresistance in fully epitaxial magnetic tunnel junctions with different barriers

We report the tunneling anisotropic magnetoresistance (TAMR) in fully epitaxial Fe/Barrier/Fe (001) magnetic tunnel junctions (MTJs) where the Barrier is annealed MgO, MgAlOx, MgO-MgAlOx, or as-grown MgO/MgAlOx. The TAMR was measured as the magnetization of Fe electrodes rotated from in-plane to out-of-plane. The angular dependence of TAMR for all samples exhibited superposed behavior of twofold and fourfold symmetries. The proportion of fourfold symmetry is larger in MTJs with MgO and MgO-MgAlOx than that in MTJs with MgAlOx and MgO/MgAlOx barriers. By characterizing inelastic electron tunneling spectroscopy in the antiparallel state and parallel conductance of the MTJs, we revealed diverse minority interfacial resonant states (IRSs) and different contributions from Δ1 and Δ5 symmetry states to the conductance in the MTJs. Our results illustrate that the minority IRS dominated by Δ5 symmetry can mix with majority Δ1 states and give rise to the enhanced fourfold symmetric angular dependence in MTJs with MgO and MgO-MgAlOx barriers.

Giant Tunnel Magnetoresistance with a Single Magnetic Phase-Transition Electrode

Magnetic phase transition tunnel magnetoresistance (MPT-TMR) effect with a single magnetic electrode has been investigated by first-principles calculations. The calculations show that the MPT-TMR of FeRh/MgO/Cu tunnel junction can be as high as hundreds of percent when the magnetic structure of FeRh changes from G-type antiferromagnetic (GAFM) to ferromagnetic order. This new type of MPT-TMR may be superior to the tunnel anisotropic magnetoresistance because of its huge magneto-resistance effect and similar structural simplicity. The main mechanism for the giant MPT-TMR can be attributed to the formation of interface resonant states at GAFM-FeRh/MgO interface. A direct FeRh/MgO interface is found to be necessary for achieving high MPT-TMR experimentally. Moreover, we find the FeRh/MgO interface with FeRh in ferromagnetic phase has nearly full spin-polarization due to the negligible majority transmission and significantly different Fermi surface of two spin channels. Thus, it may act as a highly efficient and tunable spin-injector. In addition, electric field driven MPT of FeRh-based hetero-magnetic nanostructures can be utilized to design various energy efficient tunnel junction structures and the corresponding lower power consumption devices. Our results will stimulate further experimental investigations of MPT-TMR and other fascinating phenomenon of FeRh-based tunnel junctions that may be promising in antiferromagnetic spintronics.

Large room-temperature tunneling anisotropic magnetoresistance and electroresistance in single ferromagnet/Nb:SrTiO3 Schottky devices

There is a large effort in research and development to realize electronic devices capable of storing information in new ways - for instance devices which simultaneously exhibit electro and magnetoresistance. However it remains a challenge to create devices in which both effects coexist. In this work we show that the well-known electroresistance in noble metal-Nb:SrTiO3 Schottky junctions can be augmented by a magnetoresistance effect in the same junction. This is realized by replacing the noble metal electrode with ferromagnetic Co. This magnetoresistance manifests as a room temperature tunneling anisotropic magnetoresistance (TAMR). The maximum room temperature TAMR (1.6%) is significantly larger and robuster with bias than observed earlier, not using Nb:SrTiO3. In a different set of devices, a thin amorphous AlOx interlayer inserted between Co and Nb:SrTiO3, reduces the TAMR by more than 2 orders of magnitude. This points to the importance of intimate contact between the Co and Nb:SrTiO3 for the TAMR effect. This is explained by electric field enhanced spin-orbit coupling of the interfacial Co layer in contact with Nb:SrTiO3. We propose that the large TAMR likely has its origin in the 3d orbital derived conduction band and large relative permittivity of Nb:SrTiO3 and discuss ways to further enhance the TAMR.

Anisotropic sensor and memory device with a ferromagnetic tunnel barrier as the only magnetic element

Multiple spin functionalities are probed on Pt/La2Co0.8Mn1.2O6/Nb:SrTiO3, a device composed by a ferromagnetic insulating barrier sandwiched between non-magnetic electrodes. Uniquely, La2Co0.8Mn1.2O6 thin films present strong perpendicular magnetic anisotropy of magnetocrystalline origin, property of major interest for spintronics. The junction has an estimated spin-filtering efficiency of 99.7% and tunneling anisotropic magnetoresistance (TAMR) values up to 30% at low temperatures. This remarkable angular dependence of the magnetoresistance is associated with the magnetic anisotropy whose origin lies in the large spin-orbit interaction of Co2+ which is additionally tuned by the strain of the crystal lattice. Furthermore, we found that the junction can operate as an electrically readable magnetic memory device. The findings of this work demonstrate that a single ferromagnetic insulating barrier with strong magnetocrystalline anisotropy is sufficient for realizing sensor and memory functionalities in a tunneling device based on TAMR.

Terahertz-Frequency Signal Source Based on an Antiferromagnetic Tunnel Junction

We propose a novel type of terahertz-frequency (TF) signal source based on an antiferromagnetic tunnel junction (ATJ), where the generated ac signal is extracted through the variations of the tunneling anisotropic magnetoresistance (TAMR) of an ATJ. The signal source comprises a layered structure consisting of a current-driven platinum (Pt) layer and a layer of an antiferromagnet (AFM) separated by an MgO spacer from an additional Pt electrode. A dc electric current flowing in the first Pt layer due to the spin Hall effect creates a perpendicular spin current that, being injected in the AFM layer, tilts the magnetizations of the AFM sublattices and, therefore, causes TF rotation of these magnetizations in a large internal exchange magnetic field of the AFM. This rotation, through the TAMR effect, causes the TF variation of the total resistance of the layered structure. We evaluate the output power and efficiency of the above-described Pt/AFM/MgO/Pt TF signal source, and demonstrate that optimization of the source's geometrical parameters allows one to obtain the output power exceeding 1 $\mu$ W at the frequency of 0.5 THz with the efficiency not smaller than 1%. Although the efficiency of the TAMR method of the TF signal extraction is reduced with the increase of the generation frequency, we believe that this method could be used for the generation of signals with frequencies of up to 10 THz due to its relative simplicity and convenience.

Determination of spin relaxation times in heavy metals via second-harmonic spin injection magnetoresistance

In tunnel junctions between ferromagnets and heavy elements with strong spin orbit coupling the magnetoresistance is often dominated by tunneling anisotropic magnetoresistance (TAMR). This makes conventional DC spin injection techniques impractical for determining the spin relaxation time ($\tau_s$). Here, we show that this obstacle for measurements of $\tau_s$ can be overcome by 2nd harmonic spin-injection-magnetoresistance (SIMR). In the 2nd harmonic signal the SIMR is comparable in magnitude to TAMR, thus enabling Hanle-induced SIMR as a powerful tool to directly determine $\tau_s$. Using this approach we determined the spin relaxation time of Pt and Ta and their temperature dependences. The spin relaxation in Pt seems to be governed by Elliott-Yafet mechanism due to a constant resistivity $\times$spin relaxation time product over a wide temperature range.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.

Hidden peculiar magnetic anisotropy at the interface in a ferromagnetic perovskite-oxide heterostructure

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the eg and t2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.

Proximity Band Structure and Spin Textures on Both Sides of Topological-Insulator/Ferromagnetic-Metal Interface and Their Charge Transport Probes.

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin textures around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi2Se3 and Co or Cu layers comprising the interface. Instead of naively assumed Dirac cone gapped by the proximity exchange field spectral function, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level EF0, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions from metallic layers and pushed, due to charge transfer from Co or Cu layers, a few tenths of an electron-volt below EF0 for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of EF0. Crucially for spin-orbit torque in TI/FM heterostructures, few monolayers of Co adjacent to Bi2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures (despite Co magnetization being out-of-plane) due to proximity spin-orbit coupling in Co induced by Bi2Se3. We predict that out-of-plane tunneling anisotropic magnetoresistance in Cu/Bi2Se3/Co vertical heterostructure can serve as a sensitive probe of the type of spin texture residing at EF0.

Spintronic materials and devices based on antiferromagnetic metals

In this paper, we review our recent experimental developments on antiferromagnet (AFM) spintronics mainly comprising Mn-based noncollinear AFM metals. IrMn-based tunnel junctions and Hall devices have been investigated to explore the manipulation of AFM moments by magnetic fields, ferromagnetic materials and electric fields. Room-temperature tunneling anisotropic magnetoresistance based on IrMn as well as FeMn has been successfully achieved, and electrical control of the AFM exchange spring is realized by adopting ionic liquid. In addition, promising spin-orbit effects in AFM as well as spin transfer via AFM spin waves reported by different groups have also been reviewed, indicating that the AFM can serve as an efficient spin current source. To explore the crucial role of AFM acting as efficient generators, transmitters, and detectors of spin currents is an emerging topic in the field of magnetism today. AFM metals are now ready to join the rapidly developing fields of basic and applied spintronics, enriching this area of solid-state physics and microelectronics.

Magnetic anisotropy and anisotropic tunneling magnetoresistance of compacted half-metal CrO2 nanopowders

Low-temperature magnetic and magnetoresistive properties of compacted ferromagnetic halfmetal CrO2 powders with nanoparticle shape anisotropy are studied. Magnetic anisotropy induced by the formation of magnetic texture during compaction is revealed. It is shown that tunneling magnetoresistance anisotropy is associated with the difference between a sample’s rates of magnetization in the longitudinal and transverse fields.

Influence of spin-orbit interaction within the insulating barrier on the electron transport in magnetic tunnel junctions

We present a theory of the anisotropy of tunneling magnetoresistance (ATMR) phenomenon in magnetic tunnel junctions (MTJs) attributed to Rashba spin-orbit interaction in the insulating barrier. ATMR represents the difference of tunnel magnetoresistance (TMR) amplitude measured with in-plane and out-of-plane magnetic configurations. It is demonstrated that within the spin-polarized free-electron model the change of conductance associated with the ATMR is exactly twice the change of conductance measured at full saturation (i.e., in parallel configuration of magnetizations) between in-plane and out-of-plane configuration, i.e., the tunneling anisotropic magnetoresistance (TAMR). Both ATMR and TAMR are closely related to the TMR amplitude and spin-orbit constant. The predicted ATMR phenomenon is confirmed experimentally, showing a few percent value in the case of the widely studied CoFeB/MgO/CoFeB based MTJ.

Large tunneling anisotropic magnetoresistance in La0.7Sr0.3MnO3/pentacene/Cu structures prepared on SrTiO3 (110) substrates

We investigated tunneling anisotropic magnetoresistance (TAMR) at the interface between pentacene and La0.7Sr0.3MnO3 (LSMO) thin films prepared on SrTiO3 (STO) (110) substrates. The dependence of the TAMR ratio on the magnetic field strength was approximately ten times larger than that of the magnetic field angle at a high magnetic field. This large difference in the TAMR ratio is explained by the interface magnetic anisotropy of strain-induced LSMO thin films on a STO (110) substrate, which has an easy axis with an out-of-plane component. We also note that the TAMR owing to out-of-plane magnetization was positive at each angle of the in-plane magnetic field. This result implies that active control of the interface magnetic anisotropy between organic materials and ferromagnetic metals should realize nonvolatile and high-efficiency TAMR devices.

Prospect for tunneling anisotropic magneto-resistance in ferrimagnets: Spin-orbit coupling effects in Mn3Ge and Mn3Ga

Magnetic anisotropic phenomena in Mn3Ge and Mn3Ga ferrimagnets are studied by first-principles density functional theory calculations. We find a large positive magnetic anisotropy energy, associated with the Mn-atoms in the 4d-crystallographic positions. Sizable anisotropy in the density of states is found in the vicinity of the Fermi energy, and suggests the promising possibility for the generation of a sizable tunneling anisotropic magneto-resistance effect (TAMR). The use of the ferrimagnetic materials for TAMR magnetic tunneling junctions is discussed as a prospective alternative for ferromagnetic and antiferromagnetic materials.

Electric field modulation of tunneling anisotropic magnetoresistance in tunnel junctions with antiferromagnetic electrodes

We present electric field modulation of tunneling anisotropic magnetoresistance (TAMR) in MnIr|MgO|Ta tunnel junctions. TAMR enables direct observation of the antiferromagnetic spin direction at the MnIr|MgO interface. We found that the shape of magnetoresistance (MR) curve can be modulated by an electric field, which can be explained by electric field modulation of the interfacial magnetic anisotropy at MnIr|MgO.

Temperature and thickness dependence of tunneling anisotropic magnetoresistance in exchange-biased Py/IrMn/MgO/Ta stacks

We investigate the thickness and temperature dependence of a series of NiFe/IrMn bilayer samples with varying thickness ratio of the ferromagnet/antiferromagnet () in order to explore the exchange coupling strengths in tunneling anisotropic magnetoresistance (TAMR) devices. Specific values of lead to four distinct scenarios with specific electric responses to moderate magnetic fields. The characteristic dependence of the measured TAMR signal on applied voltage allows us to confirm its persistence up to room temperature despite an overlapped contribution by a thermal magnetic noise.

Angular Dependence of Tunneling Magnetoresistance in Hybrid Fe/GaAlAs/GaMnAs Magnetic Tunnel Junctions

Tunneling magnetoresistance (TMR) phenomena in hybrid Fe/GaAlAs/GaMnAs magnetic tunnel junctions (MTJs) were investigated by rotating a magnetic field of constant strength in the film plane. When a strong field (e.g., 4000 G) is used, the magnetization in GaMnAs and Fe coherently rotates in both layers, resulting in a smooth angular dependence of TMR. In contrast, abrupt transition steps and plateaus are observed in TMR, when a weak field (below 100 G) is rotated. The behavior observed in strong fields is ascribed to tunneling anisotropic magnetoresistance, an effect that occurs when magnetizations in both magnetic layers in the MTJ are aligned parallel to each other. The tunneling behavior observed in weak fields, on the other hand, is caused by differences in relative magnetization alignments in the two layers that arise from differences in their magnetocrystalline anisotropies. The latter behavior provided the anisotropic TMR that involved with parallel and antiparallel alignments at specific crystallographic directions.

Gate-Voltage-Induced Magnetization Reversal and Tunneling Anisotropic Magnetoresistance in a Single Molecular Magnet with Temperature Gradient**Supported by the National Natural Science Foundation of China under Grant No 11274208.

We study the control of gate voltage over the magnetization of a single-molecule magnet (SMM) weakly coupled to a ferromagnetic and a normal metal electrode in the presence of the temperature gradient between two electrodes. It is demonstrated that the SMM's magnetization can change periodically with periodic gate voltage due to the driving of the temperature gradient. Under an appropriate matching of the electrode polarization, the temperature difference and the pulse width of gate voltage, the SMM's magnetization can be completely reversed in a period of gate voltage. The corresponding flipping time can be controlled by the system parameters. In addition, we also investigate the tunneling anisotropic magnetoresistance (TAMR) of the device in the steady state when the ferromagnetic electrode is noncollinear with the easy axis of the SMM, and show the jump characteristic of the TAMR.

Tunneling anisotropic magnetoresistance in La2/3Sr1/3MnO3/LaAlO3/Pt tunnel junctions

The magnetotransport properties of La2/3Sr1/3MnO3(LSMO)/ LaAlO3(LAO)/Pt tunneling junctions have been analyzed as a function of temperature and magnetic field. The junctions exhibit magnetoresistance (MR) values of about 37%, at H=90 kOe at low temperature. However, the temperature dependence of MR indicates a clear distinct origin than that of conventional colossal MR. In addition, tunneling anisotropic MR (TAMR) values around 4% are found at low temperature and its angular dependence reflects the expected uniaxial anisotropy. The use of TAMR response could be an alternative of much easier technological implementation than conventional MTJs since only one magnetic electrode is required, thus opening the door to the implementation of more versatile devices. However, further studies are required in order to improve the strong temperature dependence at the present stage.

Tunneling Anisotropic Magnetoresistance in Fe Nanoparticles Embedded in MgO Matrix

The tunnel magnetoresistance (TMR) effect is related to the relative orientation of the magnetizations of the two ferromagnetic electrodes in magnetic tunnel junctions (MTJs). The tunnel anisotropic magnetoresistance (TAMR) effect is related to the orientation of the magnetization with respect to the current direction or the crystallographic axes. Beyond the TMR, the TAMR is not only present in MTJs in which both electrodes are ferromagnetic but may also appear in tunnel structures with a single magnetic electrode. We investigated the magnetotransport properties in an Au/MgO/Fe nanoparticles/MgO/Cu tunnel junction. We found that both the TMR and TAMR can appear in tunnel junctions with Fe nanoparticles embedded in an MgO matrix. The TMR is attributed to spin-dependent tunneling between Fe nanoparticles, so the device resistance depends on the magnetization directions of adjacent Fe nanoparticles. The TAMR is attributed to the interfacial spin–orbit interaction, so the device resistance depends on each magnetization direction of an Fe nanoparticle. This is the first observation of the TAMR in Fe nanoparticles embedded in an MgO matrix.