Anisotropic Magnetoresistance Measurements Research Articles

Impact of Chiral Spinterfaces on Magneto-Photoluminescence Effects for Chiral Lead Halide Perovskites.

Chiral lead halide perovskites (LHPs) have been widely investigated in chiroptical spintronics due to their significant Rashba spin-orbit coupling (SOC) and chiral-induced spin selectivity (CISS). Ferromagnet/LHP spinterface stems from the orbital hybridization at the interface of the ferromagnet and the nonmagnetic semiconductor, where interfacial density of state is spin-dependent. By far, the impact of the ferromagnet/chiral LHP spinterface on magneto-photoluminescence (Magneto-PL) of chiral LHPs remains unknown. In this work, we find that the negative and tunable Magneto-PL effects for the pristine LHP bulk film can be drastically enhanced by incorporating ferromagnetic/chiral LHP interfaces. A large Magneto-PL magnitude can reach approximately -13% for the Ni/(S-MBA)2PbI4 interface at the field strengths of ±900 mT. With the assistance of circularly polarized PL spectra, anisotropic magneto-resistance, and X-ray photoelectron spectroscopy measurements, we demonstrate that the ferromagnet/chiral LHP interfaces are chirality/spin-dependent and possess ferromagnetic property due to distinct magnetic switching behavior and electronic orbit coupling at interfaces, which boost the Rashba splitting and spin mixing. The comprehensive effects of Rashba-induced exciton states and chiral-induced SOC at chiral spinterfaces with CISS are responsible for the enhanced Magneto-PL of Ni/(R/S-MBA)2PbI4. It is postulated that the chiral spinterfaces play a dominant role for achieving large and tunable magneto-optical effect of chiral LHPs. This work paves the way for chiroptical spintronic applications.

Enhancing Interfacial Ferromagnetism and Magnetic Anisotropy of CaRuO3/SrTiO3 Superlattices via Substrate Orientation.

Artificial oxide heterostructures have provided promising platforms for the exploration of emergent quantum phases with extraordinary properties. One of the most interesting phenomena is the interfacial magnetism formed between two non-magnetic compounds. Here, a robust ferromagnetic phase emerged at the (111)-oriented heterointerface between paramagnetic CaRuO3 and diamagnetic SrTiO3 is reported. The Curie temperature is as high as ≈155K and the saturation magnetization is as large as ≈1.3µB per formula unit for the (111)-CaRuO3/SrTiO3 superlattices, which are obviously superior to those of the (001)-oriented counterparts and are comparable to the typical itinerant ferromagnet SrRuO3. A strong in-plane magnetic anisotropy with six-fold symmetry is further revealed by the anisotropic magnetoresistance measurements, presenting a large in-plane anisotropic field of 3.0-3.6T. More importantly, the magnetic easy axis of the (111)-oriented superlattices can be effectively tuned from 〈 1〉 to 〈 〉 directions by increasing the layer thickness of SrTiO3. The findings demonstrate a feasible approach to enhance the interface coupling effect by varying the stacking orientation of oxide heterostructures. The tunable magnetic anisotropy also shows potential applications in low-power-consumption or exchange spring devices.

Thermodynamic and magnetotransport properties of EuAg2Ge2 single crystals

Here, we report the crystallographic, thermodynamic, and magnetotransport properties of flux grown EuAg2Ge2 single crystals using X-ray diffraction, anisotropic magnetic susceptibility, and magnetization, heat capacity, electrical resistivity, and magnetoresistance measurements. The room temperature powder X-ray diffraction pattern confirms that EuAg2Ge2 crystallizes in a body-centered tetragonal ThCr2Si2–type structure (space group I4/mmm) with lattice parameters, a = 4.401 Å and c = 10.845 Å. The magnetic susceptibility data reveal two subsequent antiferromagnetic transitions at TN1 ∼ 8.3 K, and TN2 ∼ 6.2 K. For H‖c at T = 5 K, the magnetization data show the spin-flop like metamagnetic transition around the critical field of 1.4 T. The existence of antiferromagnetic transitions is further confirmed by the heat capacity measurements, which display two anomalies around TN1 = 8.3 K and TN2 = 6.1 K. The electrical resistivity data show the metallic behavior of the compound, which is best described by the Bloch-Grüneisen model above 30 K. At low temperatures, the antiferromagnetic transition manifests through a slight upturn around 8 K, followed by a sharp drop in the resistivity data. The transverse magnetoresistance (MR) measured for H‖c below TN first increases, showing a broad maximum at critical field strength that shifts towards lower fields with increasing temperature. With further increasing fields beyond the maximum, MR decreases, and changes its value from positive to negative. The evolution of MR from positive to negative could be associated with the metamagnetic transition in EuAg2Ge2. In the paramagnetic region, MR remains negative across the entire field range.

Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co2(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co2(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey are fabricated on SiO2/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co2Mn0.5Fe0.5)1−yGey composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization.

Anisotropic magnetoresistance and the field-induced anomalous valley inversion in half-Heusler TbPtBi

We have performed detailed magnetic and anisotropic magnetoresistance (AMR) measurements on a magnetic-field-induced topological Weyl semimetal, TbPtBi, exhibiting a weak metamagnetic transition in the antiferromagnetic state. The magnetic torque and temperature-dependent AMR measurements indicate that the spin order dominates the symmetry of AMR curves at temperature below Neel temperature (TN) and low magnetic fields. Remarkably, the AMR with a twofold symmetry gradually dominates the curves as the magnetic fields increase. Moreover, sign change in the AMR effect is confirmed for both T < TN and T > TN at high magnetic fields, which may be related to the magnetic field-induced splitting of the topological band in TbPtBi.

Investigation of orthogonal spin–orbit fields in crystalline ferromagnetic films with four-fold in-plane magnetic anisotropy

We report an investigation of current-induced spin–orbit fields (SOFs) in a crystalline (Ga,Mn)As ferromagnetic film with four-fold in-plane anisotropy. By designing a Hall device with current channels along crystallographic 〈100〉 directions, we observe the effects of SOFs in all four magnetization transitions over the four 〈110〉 hard axes of the film. The SOF effects are observed in the form of opposite shifts of angles at which the magnetization of the film switches sign for opposite current polarities in anisotropic magnetoresistance (AMR) measurements carried out as the applied magnetic field was rotated. Note that the Dresselhaus- and Rashba-type SOFs generated by currents flowing along the 〈100〉 directions—as in the device designed for the present experiments—are orthogonal to one another and can thus be separated by appropriate analysis of AMR. The analysis of experimental results based on magnetic free energy, in which the effects of SOFs are included, reveals that the Dresselhaus-type SOF is much stronger than the Rashba-type SOF. The values of the two SOFs obtained independently with currents flowing either along the [100] or [010] channels are consistent with each other, showing the reliability of the above-mentioned approach for investigating SOFs in ferromagnetic films.

Efficient characteristics of exchange coupling and spin–flop transition in Py/Gd bilayer using anisotropic magnetoresistance

Interlayer antiferromagnetic coupling rare-earth/transition-metal bilayer ferrimagnet systems have attracted much attention because they present different unusual temperature- and field-dependent nontrivial magnetic states and dynamics. These properties and the implementation of their applications in spintronics highly depend on the significant temperature dependence of the magnetic exchange stiffness constant A. Here, we quantitatively determine the temperature dependence of magnetic exchange stiffness APy−Gd and AGd in the artificially layered ferrimagnet consisting of a Py/Gd bilayer, using a measurement of anisotropic magnetoresistance of the bilayer thin film at different temperatures and magnetic fields. The obtained temperature dependencies of APy−Gd and AGd exhibit a scaling power law with the magnetization of Gd. The critical field of spin–flop transition and its temperature dependence can also be directly obtained by this method. Additionally, the experimental results are well reproduced by micromagnetic simulations with the obtained parameters APy−Gd and AGd, which further confirms the reliability of this easily accessible technique.

Hysteresis and training effect in the electric control of spin current in Pt/Y3Fe5O12 heterostructures

We have reported on the hysteresis and training effect of spin current in $\mathrm{Pt}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ heterostructures during subsequent cycles of ionic liquid gate voltage ${V}_{g}$. The inverse spin Hall effect voltage in spin pumping and spin Hall magnetoresistance exhibit diode-like behaviors in the first half cycle of ${V}_{g}$ and also show hysteresis in the first cycle of ${V}_{g}$. Both the diode-like behavior and the hysteresis become weak and even vanish in the second cycle of ${V}_{g}$ due to the training effect. The above experimental results can be well explained by the screening charge doping model, in which the charge and the local magnetic moment are asymmetrically distributed in the Pt layer. The applicability of this model is further confirmed by measurements of anisotropic magnetoresistance and ferromagnetic resonance. The diode-like behavior is attributed to interplay between the asymmetrically distributed local magnetic moment and the spin current relaxation in the Pt layer. The hysteresis and the training effect arise from the incompletely reversible process between oxidation and reduction of Pt atoms and the evolution of the surface morphology at the ionic liquid/Pt interface under electric gating. This work provides new insights to improve the functional performance of electrically controlled spin current devices.

Sign reversal of planar Hall effect with temperature in La-doped Sr2IrO4 films

Electron-doped Sr2IrO4 is the best candidate for unconventional superconductivity, but direct evidence of superconductivity has not been experimentally confirmed. Therefore, it is urgent to explore the complex and rich physical properties caused by doping. The planar Hall effect (PHE) is a sensitive technique for the characterization of intrinsic magnetic properties in magnetic thin films and is applied widely in spintronic devices. In this work, the PHE for La-doped Sr2IrO4 films as a function of the magnetic field direction and temperature exhibited unique properties caused by electron doping. The amplitude of PHE is proportional to the strength of the applied magnetic field. Remarkably, as the temperature increased, a sign reversal of angle-dependent PHE occurred at 90 K, which indicated the change of magnetic anisotropy. Subsequent variable-temperature traditional Hall measurements and time-resolved optical studies eliminated different types of carrier interactions. The anisotropic magnetoresistance measurements indicated that the sign reversal can be attributed to the changes of a spin structure after electron doping, and the reversal temperature is related to the strength of ferromagnetism. These results provide a platform to study the magnetic interactions and suggest the possibility of realizing thermal controllable magnetic sensor devices in electron-doped Sr2IrO4 films.

Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport

The recent discovery of a zero-resistance state in nickel-based compounds has generated a re-excitement about the long-standing problem in condensed matter of high-critical temperature superconductivity, in light of the similarities between infinite-layer nickelates and cuprates. However, despite some formal valence and crystal symmetry analogies, the electronic properties of infinite-layer nickelates are remarkably original accounting, among other properties, of a unique Nd 5d-Ni 3d hybridization. This designates infinite-layer nickelates as a new class of oxide superconductors which should be considered on their own. Here we report about Nd1-x Sr x NiO2 (x = 0, 0.05 and 0.2) thin films synthesized with and without a SrTiO3 (STO) capping-layer, showing very smooth and step-terraced surface morphologies. Angle-dependent anisotropic magnetoresistance (AMR) measurements performed with a magnetic field rotating in-plane or out-of-plane with respect to the sample surface, rendered important information about the magnetic properties of undoped STO-capped and uncapped samples. The results point at a key role of the capping-layer in controlling the magnitude and anisotropy of the AMR. We discuss this control in terms of a combined effect between the Nd 5d-Ni 3d hybridization and an intra-atomic Nd 4f-Nd 5d exchange coupling. Further studies foresee the influence of the capping layer on infinite-layer nickelates with no magnetic rare-earth.

Frequency dependence of coercivity in nickel and Co–Fe–B thin film for DC to 100 kHz region

To investigate the magnetization process in the intermediate frequency region, the frequency dependences of coercivities in Ni and Co–Fe–B thin films were determined by anisotropic magnetoresistance measurements up to ∼160 kHz. In the low-frequency region (<5 kHz) the coercivity of Co–Fe–B was lower than that of Ni. However, the increasing rate of the coercivity in Ni was lower than that in Co–Fe–B. Consequently, above 19 kHz, the coercivity of Ni was lower than one of Co–Fe–B. Considering the basic material properties, better soft magnetic properties of Ni compared with Co–Fe–B should arise from the higher Walker breakdown field. This difference was mainly due to the Gilbert damping constant.

Anisotropic magnetoresistance in Mn4−xNixN and the change in the crystalline field

We focus on rare-earth-free anti-perovskite Mn4−xNixN epitaxial films, which can be used for ultrafast current-induced domain wall motion (CIDWM) in magnetic strips. The magneto-transport properties of these materials are very important for a deep understanding of CIDWM. In this study, we investigated the magneto-transport properties of Mn4−xNixN epitaxial films grown on SrTiO3(001) and MgO(001) substrates through anisotropic magnetoresistance (AMR) measurements at temperatures between 2 and 300 K. In samples with a small Ni composition such as x = 0.05−0.1, the AMR ratio of Mn4−xNixN drastically decreased with increasing temperature. We also analyzed the twofold and fourfold symmetries in the AMR curves. Fourfold symmetry is caused by tetragonal crystal fields and is unique to anti-perovskite 3d-metal nitrides such as Mn4N and Fe4N. Only slight fourfold symmetry was observed in Mn4−xNixN. We also performed first-principles calculations with the Vienna ab initio simulation package (VASP) to obtain the projected density of states (PDOS) of d orbitals in Mn4−xNixN, which is responsible for the magnetism of these materials. We conclude that these results are due to the Ni atoms, which function as magnetic impurities and lead to a noticeable change in PDOS, as proved by VASP calculation.

Spin Hall magnetoresistance of CoFe2O4/Pt heterostructures with interface non-collinear magnetic configurations

We have investigated the spin Hall magnetoresistance (SMR) in CoFe2O4/Pt heterostructures deposited on (001) MgO and (001) MgAl2O4 substrates, respectively. It is found that the interface non-collinear magnetic configurations should exist in both of the two films in the whole temperature range from 5 to 300 K but mainly dominate the spin transport properties at low temperature. Furthermore, the spin transport has featured epitaxial-strain-modified characteristics induced by different substrates, which has been investigated by magnetic curves, SMR, anisotropic magnetoresistance, and Hall effect measurements.

Spin-Momentum Locking Induced Anisotropic Magnetoresistance in Monolayer WTe2.

Monolayer WTe2 is predicted to be a quantum spin Hall insulator (QSHI), and its quantized edge transport has recently been demonstrated. However, one of the essential properties of a QSHI, spin-momentum locking of the helical edge states, has yet to be experimentally validated. Here, we measure and observe gate-controlled anisotropic magnetoresistance (AMR) in monolayer WTe2 devices. Electrically tuning the Fermi energy into the band gap, a large in-plane AMR is observed and the minimum of the in-plane AMR occurs when the applied magnetic field is perpendicular to the current direction. In line with the experimental observations, the theoretical predictions based on the band structure of monolayer WTe2 demonstrate that the AMR effect originates from spin-momentum locking in the helical edge states of monolayer WTe2. Our findings reveal that the spin quantization axis of the helical edge states in monolayer WTe2 can be precisely determined from AMR measurements.

Evolution of transport properties in FeSe thin flakes with thickness approaching the two-dimensional limit

Electronic properties of FeSe can be tuned by various routes. Here, we present a comprehensive study on the evolution of the superconductivity and nematicity in FeSe with thickness from bulk single crystal down to bilayer ($\sim$ 1.1 nm) through exfoliation. With decreasing flake thickness, both the structural transition temperature $T_{\rm s}$ and the superconducting transition temperature $T_{\rm c}^{\rm zero}$ are greatly suppressed. The magnetic field ($B$) dependence of Hall resistance $R_{xy}$ at 15 K changes from $B$-nonlinear to $B$-linear behavior up to 9 T, as the thickness ($d$) is reduced to 13 nm. $T_{\rm c}$ is linearly dependent on the inverse of flake thickness (1/$d$) when $d\le$ 13 nm, and a clear drop of $T_{\rm c}$ appears with thickness smaller than 27 nm. The $I$-$V$ characteristic curves in ultrathin flakes reveal the signature of Berezinskii-Kosterlitz-Thouless (BKT) transition, indicating the presence of two-dimensional superconductivity. Anisotropic magnetoresistance measurements further support 2D superconductivity in few-layer FeSe. Increase of disorder scattering, anisotropic strains and dimensionality effect with reducing the thickness of FeSe flakes, might be taken into account for understanding these behaviors. Our study provides systematic insights into the evolution of the superconducting properties, structural transition and Hall resistance of a superconductor FeSe with flakes thickness and provides an effective way to find two-dimensional superconductivity as well as other 2D novel phenomena.

Effects of the atomic order on the half-metallic electronic structure in the Co2Fe(Ga0.5Ge0.5) Heusler alloy thin film

The atomic order and its effect on the electronic structure is a key issue in the application of half-metallic Heusler alloys as spin-polarized electron sources. In this study, we investigated the atomic ordering and electronic structures of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{Fe}(\mathrm{G}{\mathrm{e}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5})$ (CFGG) Heusler alloy thin films at various annealing temperatures using anomalous x-ray diffraction (AXRD) and hard x-ray photoelectron spectroscopy (HAXPES). AXRD measurements clearly showed that the Co-Fe disorder is large in the as-deposited state, which is reduced to almost zero by annealing at ${T}_{\mathrm{an}}=\phantom{\rule{0.16em}{0ex}}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for 30 min. The reduction of the Co-Fe disorder explains the observed increase in the spin polarization of currents estimated by ordinary and anisotropic magnetoresistance measurements. The photoelectron spectra of the valence band in CFGG thin films with different ${T}_{\mathrm{an}}$ agree well with the first-principles calculations that considers the atomic disorder. We also found that the characteristic peaks appearing in the valence band HAXPES spectra shifted to higher binding energies, compared to the calculated density of states for stoichiometric $\mathrm{L}{2}_{1}$-ordered CFGG. This indicates that the Fermi level is shifted and located in the vicinity of the conduction band edge in the present CFGG thin film due to electron doping by a mildly Co-rich chemical composition. Our first-principles calculations of partial density of state of $sp$ electron predicted that the high spin-polarization is obtainable in a wider energy region near Fermi level in the $\mathrm{L}{2}_{1}$-ordered structure than the B2 structure, which explains the enhancement of spin-polarization by promoting $\mathrm{L}{2}_{1}$-ordering in the present CFGG film.

Itinerant ferromagnetism in van der Waals Fe5−xGeTe2 crystals above room temperature

Two-dimensional (2D) van der Waals (vdW) magnets have recently attracted increasing attention, as they provide a novel system for exploring 2D magnetism. However, intrinsic ferromagnetism in 2D systems has almost exclusively been observed at low temperatures, limiting their technological relevance. ${\mathrm{Fe}}_{\mathrm{N}}{\mathrm{Ge}\mathrm{Te}}_{2}$ ($N=3$, 4, and 5) systems are currently becoming the most attractive 2D vdW materials due to their relatively high Curie temperatures and large saturation magnetization. However, the nature of their complex yet intriguing magnetic behaviors is still unclear, in part due to the multiple inequivalent iron sites and iron vacancies. Here, we show evolution of magnetic ordering transitions in ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ with high Curie temperature and a strong saturation magnetization using photoemission electron microscopy and transport measurements. At 275 K, the ferromagnet transitions to a ferrimagnet, and below 110 K transitions to a state with glassy clusters. These are evidenced from temperature-dependent magnetic stripe domain evolution and anisotropic magnetoresistance measurements. Our findings show a clear magnetic ground state of ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ at room temperature which signals that ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ system is a very promising candidate for spintronic devices and provides a material design pathway to further increase the Curie temperature and saturation moments in vdW ferromagnets.

Tunable Magnetic Vortex Dynamics in Ion-Implanted Permalloy Disks.

Nanoscale, low-phase-noise, tunable transmitter-receiver links are key for enabling the progress of wireless communication. We demonstrate that vortex-based spin-torque nano-oscillators, which are intrinsically low-noise devices because of their topologically protected magnetic structure, can achieve frequency tunability when submitted to local ion implantation. In the experiments presented here, the gyrotropic mode is excited with spin-polarized alternating currents and anisotropic magnetoresistance measurements yield discrete frequencies from a single device. Indeed, chromium-implanted regions of permalloy disks exhibit different saturation magnetization than neighboring, non-irradiated areas, and thus different resonance frequency, corresponding to the specific area where the core is gyrating. Our study proves that such devices can be fabricated without the need for further lithographical steps, suggesting ion irradiation can be a viable and cost-effective fabrication method for densely packed networks of oscillators.

Multiple Transitions in Permalloy Half-Ring Wires with Finite-Size Effect.

Six permalloy (Py) half-rings with finite-size from 120 nm to 360 nm were connected in series on five corners. The magnetization reversal processes were investigated by the measurement of anisotropic magnetoresistance (AMR). The number of switching jumps in the AMR loops, from zero to five, varied with the longitudinal applied field. These discrete jumps resulted from domain wall (DW) nucleating and depinning on the corners. The larger external field had a fewer number of jumps in the magnetoresistance (MR) curve. This reproducible and particular response of the domain wall device in the half-ring wires pattern might be one of the new promising magnetoelectronic devices.

An approach to determine the easy axis of magnetic film by anisotropic magnetoresistance measurements

A series of Ta/NiCo/Ta films was grown through DC-sputtering. During deposition, a constant magnetic field was applied along the substrate in different directions to induce different orientations of the easy axis. Three different magnitudes of external fields were applied in the angle-dependent anisotropic magnetoresistance (AMR) measurements of the samples. Through an effective analysis, the angle-direction extracted via the intersection of three angle-dependent AMR curves was confirmed as the easy axis. Compared to the normal methods applied to achieve the easy axis, the results are convincing. This new method provides an effective, convenient and low cost way of studying magnetic materials and devices.