Anomalous Hall Resistivity Research Articles

Ferrimagnetic–ferromagnetic phase transition in Mn4N films favored by non-magnetic In doping

The ferrimagnet Mn4N forms a family of compounds useful in spintronics. In a compound comprising non-magnetic and magnetic elements, one basically expects the compound to become ferromagnetic when the proportion of the magnetic element increases. Conversely, one does not expect ferromagnetism when the proportion of the non-magnetic element increases. Surprisingly, Mn4N becomes ferromagnetic at room temperature when the Mn content is decreased by the addition of In atoms, a non-magnetic element. X-ray magnetic circular dichroism measurement reveals that the magnetic moment of Mn atoms at face-centered sites, Mn(II), reverses between x= 0.15 and 0.27 and aligns parallel to that of Mn atoms at corner sites, Mn(I), at x = 0.27 and 0.41. The sign of the anomalous Hall resistivity also changes between x = 0.15 and 0.27 in accordance with the reversal of the magnetic moment of the Mn(II) atoms. These results can be interpreted using first-principles calculations, showing that the magnetic moment of Mn(II) sites which are the nearest neighbors to the In atom align to that of Mn(I) sites.

Investigation of spin gapless semiconducting behaviour in quaternary CoFeMnSi Heusler alloy thin films on Si (1 0 0)

Spin gapless semiconducting (SGS) behaviour of polycrystalline CoFeMnSi (CFMS) Heusler alloy thin films was investigated through structural, magnetic, magnetotransport and electron-transport characterization. In this study, we have tailored the structural ordering in CFMS thin films (∼60 nm) grown at different substrate temperatures (TS) in 150 °C to 550 °C range on Si(100) substrates using pulsed DC magnetron sputtering. All the grown films were polycrystalline in nature, showing an enhancement in structural ordering with TS. Among all the samples, the film grown at 550 °C showed an optimal structure with L21 ordering. The magnetic measurements revealed that saturation magnetization (Ms) of the sample decreased with the an increment in measurement temperature with a value ∼ 3.42 μB/f.u. at 5 K to 3.11 μB/f.u. at 300 K. The Curie temperature (TC) of the sample was estimated to be 778 ± 13 K. The longitudinal resistivity (ρXX) measurements clearly exhibited a negative temperature coefficient of resistivity, indicating the non-metallic behaviour of these CFMS films. The fitting of resistivity curve dig out contribution of electron–phonon scattering and defect scattering to resistivity. Fitting also suggest that holes are the majority charge carriers in optimized CFMS thin film which was further confirmed through positive slope of anomalous Hall resistivity curve. The temperature-dependent anomalous Hall effect (AHE) measurements have also been carried out to identify the different scattering mechanisms responsible for AHE. The results suggested that both the intrinsic as well as extrinsic factors contribute to the AHE of CFMS deposited film with non-zero contribution of phonon-related skew scattering. The carrier concentration (n) and mobility (μ) were found to be nearly temperature-independent, revealing typical SGS characteristics. At room temperature, the n and μ values were found to be 3.91 ± 0.04 × 1021 cm−3 and 88.83 ± 1.58 cm2/Vs, respectively. The value of anomalous Hall conductivity at 5 K is found to be 53.79 S/cm..

Mimicking synaptic plasticity with a wedged Pt/Co/Pt spin–orbit torque device

We fabricated a wedge-shaped Pt/Co/Pt device with perpendicular magnetic anisotropy and manifested that the Co magnetization can be solely switched by spin–orbit torque without any magnetic field. Similar to the synaptic weight, we observed that the state of Co magnetization (presented by the anomalous Hall resistance R H) of the wedged Pt/Co/Pt device can be tuned continuously with a large number of nonvolatile levels by applied pulse currents. Furthermore, we studied the synaptic plasticity of the wedged Pt/Co/Pt device, including the excitatory postsynaptic potentials or inhibitory postsynaptic potentials and spiking-time-dependent plasticity. The work elucidates the promise of the wedged Pt/Co/Pt device as a candidate for a new type of artificial synaptic device that is induced by a spin current and paves a substantial pathway toward the combination of spintronics and synaptic devices.

Observation of large intrinsic anomalous Hall conductivity in polycrystalline Mn3Sn films

Topological antiferromagnets have a significant role to play in the modern day electronics and spintronics due to the presence of peculiar symmetries and magnetically driven large anomalous transport behaviour. We report the observation of anomalous Hall effect in Mn3Sn polycrystalline thin films deposited on Al2O3 substrate with a large anomalous Hall conductivity of 65 (Ωcm)−1 at 3 K. The Hall and magnetic measurements show a very small hysteresis owing to a weak ferromagnetic moment present in this material. The longitudinal resistivity decreases sufficiently for the thin films as compared to the polycrystalline bulk sample used as the target for the film deposition. The anomalous Hall resistivity and conductivity decreases almost linearly with the increase in the temperature. A negative magnetoresistance is observed for all the measured temperatures with the negative decrease in the magnitude with the increase in temperature.

Pressure tuning of the anomalous Hall effect in the kagome superconductor CsV3Sb5

Controlling the anomalous Hall effect (AHE) inspires potential applications of quantum materials in the next generation of electronics. The recently discovered quasi-2D kagome superconductor CsV3Sb5 exhibits large AHE accompanying with the charge-density-wave (CDW) order which provides us an ideal platform to study the interplay among nontrivial band topology, CDW, and unconventional superconductivity. Here, we systematically investigated the pressure effect of the AHE in CsV3Sb5. Our high-pressure transport measurements confirm the concurrence of AHE and CDW in the compressed CsV3Sb5. Remarkably, distinct from the negative AHE at ambient pressure, a positive anomalous Hall resistivity sets in below 35 K with pressure around 0.75 GPa, which can be attributed to the Fermi surface reconstruction and/or Fermi energy shift in the new CDW phase under pressure. Our work indicates that the anomalous Hall effect in CsV3Sb5 is tunable and highly related to the band structure.

Resonant precession of magnetization and precession—induced DC voltages in FeGaB thin films

Measurements of frequency dependent ferromagnetic resonance and spin pumping driven dc voltage (V dc) are reported for amorphous films of Fe78Ga13B9 alloy to address the phenomenon of self-induced inverse spin Hall effect (ISHE) in plain films of metallic ferromagnets (FMs). The V dc signal, which is anti-symmetric on field reversal, comprises of symmetric and asymmetric Lorentzians centered around the resonance field. Dominant role of thin film size effects is seen in setting the magnitude of static magnetization, V dc and dynamics of magnetization precession in thinner films (⩽8 nm). The film thickness dependence of magnetization parameters indicates the presence of a magnetically disordered region at the film—substrate interface, which may promote preferential flow of spins generated by the precessing magnetization towards the substrate. However, the V dc signal also draws contributions from rectification effects of a ≈0.4% anisotropic magnetoresistance and a large (≈54 nΩ m) anomalous Hall resistivity (AHR) of these films which ride over the effect of spin–orbit coupling driven spin-to-charge conversion near the film—substrate interface. We have addressed these data in the framework of the existing theories of electrodynamics of a ferromagnetic film subjected to radio-frequency field in a coplanar waveguide geometry. Our estimation of the self-induced ISHE for the sample with 54 nΩ m AHR shows that it may contribute significantly (≈90%) to the measured symmetric voltage. This study is expected to be very useful for fully understanding the spin pumping induced dc voltages in metallic FMs with disordered interfaces and large anomalous Hall effect.

Anomalous Hall effect from gapped nodal line in the Co2FeGe Heusler compound

Full Heusler compounds with Cobalt as a primary element show anomalous transport properties owing to the Weyl fermions and broken time-reversal symmetry. We present here the study of anomalous Hall effect (AHE) in Co2FeGe Heusler compound. The experiment reveals anomalous Hall conductivity (AHC) 100 S/cm at room temperature with an intrinsic contribution of 78 S/cm . The analysis of anomalous Hall resistivity suggests the scattering independent intrinsic mechanism dominates the overall behaviour of anomalous Hall resistivity. The first principles calculation reveals that the Berry curvature originated by gapped nodal line near EF is the main source of AHE in Co2FeGe Heusler compound. The theoretically calculated AHC is in agreement with the experiment.

Giant magnetoresistance and topological Hall effect in the EuGa4 antiferromagnet

We report on systematic temperature- and magnetic field-dependent studies of the EuGa4 binary compound, which crystallizes in a centrosymmetric tetragonal BaAl4-type structure with space group I4/mmm. The electronic properties of EuGa4 single crystals, with an antiferromagnetic (AFM) transition at T N ∼ 16.4 K, were characterized via electrical resistivity and magnetization measurements. A giant nonsaturating magnetoresistance was observed at low temperatures, reaching % at 2 K in a magnetic field of 9 T. In the AFM state, EuGa4 undergoes a series of metamagnetic transitions in an applied magnetic field, clearly manifested in its field-dependent electrical resistivity. Below T N, in the ∼4–7 T field range, we observe also a clear hump-like anomaly in the Hall resistivity which is part of the anomalous Hall resistivity. We attribute such a hump-like feature to the topological Hall effect, usually occurring in noncentrosymmetric materials known to host topological spin textures (as e.g., magnetic skyrmions). Therefore, the family of materials with a tetragonal BaAl4-type structure, to which EuGa4 and EuAl4 belong, seems to comprise suitable candidates on which one can study the interplay among correlated-electron phenomena (such as charge-density wave or exotic magnetism) with topological spin textures and topologically nontrivial bands.

Fully Compensated Synthetic Antiferromagnets with Pronounced Anomalous Hall and Magneto-Optical Responses

We report the realization of fully compensated synthetic antiferromagnets (SAFs) in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}/{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ multilayers with perpendicular magnetic anisotropy, pronounced anomalous Hall resistances and magneto-optical responses. In particular, the properties of SAFs are systematically investigated through optimizing the thicknesses of the $\mathrm{Co}$ layer, the $\mathrm{Ru}$ spacer, and the concentration (x) of the ferrimagnet (FIM) ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ layers. The incorporation of the FIM ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ films into SAFs accelerates the search for a magnetic multilayer with fully compensated magnetism and exhibiting pronounced anomalous Hall resistance and magneto-optical signals. These advantages enable spin-orbit-torque switching and magneto-optical Kerr effect imaging experiments on the fully compensated SAFs to be readily investigated. Through incorporating FIMs into SAFs, we believe that the present material system could provide a promising platform not only for adequately addressing the fully compensated spin dynamics of SAFs at room temperature, but also for developing intriguing SAF-based spin-orbitronic devices.

Pressure and magnetic field-induced transport effects in Ni45.4Mn40In14.6 alloy

Electrical transport properties were investigated under hydrostatic pressure in the Ni45.4Mn40In14.6 alloy. Temperature dependency of electrical resistivity was analysed to find dominant scattering mechanisms and to estimate the volume change during martensitic transformation. The main contributions into electrical resistivity in austenitic phase were found to be scattering by magnetic fluctuations and scattering due to structural disorder. The latter provides significantly higher residual resistivity in comparison with that of the stoichiometric alloy due to irregular positions occupied by excess Mn atoms. The value of volume change upon austenite to martensite transformation was −0.09%. It was found that alloy is characterised by hole conductivity and negative anomalous Hall resistivity, which doubles on application of hydrostatic pressure due to pressure-induced martensitic transformation and reaches the magnitude of 20 μΩ·cm. The unusual correlation between the Hall and the longitudinal resistivities in the martensitic phase is discussed.

Magnetism and transport behavior of Ni42Co8Mn38Sb12: Magnetization, electrical resistivity and Hall effect measurements

Magnetic and transport behavior of Ni42Co8Mn38Sb12 alloy is discussed using the combined results of magnetization, electrical resistivity and Hall effect measurements. The alloy undergoes a ferromagnetic transition at 364 K, followed by a martensitic transition at 110 K. Large positive Curie-Weiss temperature, soft-ferromagnetic behavior with negative magnetic entropy change below 50 K and a well-scaled normalized magnetic-entropy onto a single universal curve confirm the predominant ferromagnetic exchange interactions in the alloy. Normal Hall coefficients were found to be nearly constant in ferromagnetic region in the temperature range of 120–300 K. The electrical transport properties are largely governed by electrons with a carrier concentration of about 1023/cc. Quadratic dependence of anomalous Hall resistivity (ρxy) on longitudinal resistivity (ρxx) indicates the dominant intrinsic contribution. In a magnetic field of 50 kOe, magnetoresistance is found to be about 34% near the martensitic transition temperature while it is about 2.8% near the second-order transition.

Efficient current-driven magnetization switching owing to isotropic magnetism in a highly symmetric 111-oriented Mn4N epitaxial single layer

We investigated in-plane current-induced magnetization switching in a Mn4N epitaxial single layer. Efficient magnetization switching was detected via the measurement of anomalous Hall resistivity after the application of current pulses, with a duration of 1 s, to the 111-oriented Mn4N film compared with a reference 001-oriented Mn4N film. The threshold current density of magnetization switching with 0.5 s pulse durations, Jc ≈ 1 MA/cm2, was relatively low compared with that reported for magnetic tunnel junctions and/or ferromagnet/heavy metal bilayer systems. The relatively low Jc in the 111-oriented film was attributed to the low magnetic anisotropy on the (111) plane of Mn4N owing to the isotropic crystal symmetry as revealed by x-ray diffraction and transmission electron microscopy as a reduced switching barrier boosts the probability of magnetization switching. It was concluded that manipulation of the magnetic anisotropy based on the crystal orientation is one of the promising approaches to develop materials suitable for application in highly efficient switching devices with Mn4N layers.

Anomalous Hall effect of facing-target sputtered ferrimagnetic Mn4N epitaxial films with perpendicular magnetic anisotropy

Epitaxial Mn4N films with different thicknesses were fabricated by facing-target reactive sputtering and their anomalous Hall effect (AHE) is investigated systematically. The Hall resistivity shows a reversed magnetic hysteresis loop with the magnetic field. The magnitude of the anomalous Hall resistivity sharply decreases with decreasing temperature from 300 K to 150 K. The AHE scaling law in Mn4N films is influenced by the temperature-dependent magnetization, carrier concentration and interfacial scattering. Different scaling laws are used to distinguish the various contributions of AHE mechanisms. The scaling exponent γ > 2 for the conventional scaling in Mn4N films could be attributed to the residual resistivity ρ xx0. The longitudinal conductivity σxx falls into the dirty regime. The scaling of is used to separate out the temperature-independent ρ xx0 from extrinsic contribution. Moreover, the relationship between ρ AH and ρxx is fitted by the proper scaling to clarify the contributions from extrinsic and intrinsic mechanisms of AHE, which demonstrates that the dominant mechanism of AHE in the Mn4N films can be ascribed to the competition between skew scattering, side jump and the intrinsic mechanisms.

Quantum anomalous Hall edge channels survive up to the Curie temperature

Achieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Here we show, through a careful analysis of the non-local voltages on a multi-terminal Corbino geometry, that the chiral edge channels continue to exist without applied magnetic field up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator, and that thermally activated bulk conductance is responsible for this quantization breakdown. Our results offer important insights on the nature of the topological protection of these edge channels, provide an encouraging sign for potential applications, and establish the multi-terminal Corbino geometry as a powerful tool for the study of edge channel transport in topological materials.

Tunable Berry curvature and transport crossover in topological Dirac semimetal KZnBi

Topological Dirac semimetals have emerged as a platform to engineer Berry curvature with time-reversal symmetry breaking, which allows to access diverse quantum states in a single material system. It is of interest to realize such diversity in Dirac semimetals that provides insight on correlation between Berry curvature and quantum transport phenomena. Here, we report the transition between anomalous Hall and chiral fermion states in three-dimensional topological Dirac semimetal KZnBi, which is demonstrated by tuning the direction and flux of Berry curvature. Angle-dependent magneto-transport measurements show that both anomalous Hall resistance and positive magnetoresistance are maximized at 0° between net Berry curvature and rotational axis. We find that the unexpected crossover of anomalous Hall resistance and negative magnetoresistance suddenly occurs when the angle reaches to ~70°, indicating that Berry curvature strongly correlates with quantum transports of Dirac and chiral fermions. It would be interesting to tune Berry curvature within other quantum phases such as topological superconductivity.

Carrier-dependent quadratic scaling of anomalous Hall conductivity in ferromagnetic semiconductor

We report an unconventional carrier-dependent anomalous Hall effect (AHE) with a quadratic scaling relation in epitaxial films of a ferromagnetic semiconductor (ZnCo)O with a high Co concentration. We show the co-existence of AHE together with the nonlinear ordinary Hall effect (NLHE) and the separation of NLHE by using a two-conducting channels model in the expression of Hall resistivity. We found that the NLHE depends strongly on both temperature and carrier density n and dominates at low temperature when n <1.0×1020cm−3, indicating the very necessary of separation them from each other for a proper scaling of the AHE. The anomalous Hall resistivity is nearly independent on carrier density and longitudinal resistivity regardless the underlying transport mechanisms and thermal properties. Very interestingly, a quadratic scaling relation between anomalous Hall conductivity and longitudinal conductivity σxyAHE∝σxx2 is obtained. The further analysis shows that σxyAHE∝n2 with a frozen electron mobility. In addition, sign reversal of the AHE has been observed by reducing magnetization via growth controlling engineering. Our results reveal a carrier-dependent AHE in low conductivity regime and provide an experimental evidence for the itinerant ferromagnetism in ferromagnetic semiconductor (ZnCo)O.

Ferroelectric gate control of Rashba–Dresselhaus spin–orbit coupling in ferromagnetic semiconductor (Zn, Co)O

In this paper, we demonstrate the ferroelectric gate control of Rashba–Dresselhaus spin–orbit coupling (R–D SOC) in a hybrid heterostructure consisting of a ferromagnetic semiconductor channel (Zn, Co)O(0001) and a ferroelectric substrate PMN-PT(111). The R–D SOC causes a transverse spin current via the charge-spin conversion, which results in unbalanced transverse spin and charge accumulations due to the spin-polarized band in the ferromagnetic (Zn, Co)O channel. By the reversal of gated ferroelectric polarization, we observed 55% modulation of the R–D SOC correlated Hall resistivity to the magnetization correlated anomalous Hall resistivity and 70% modulation of the low-field magnetoresistance at 50 K. Our experimental results pave a way toward semiconductor-based spintronic-integrated circuits with an ultralow power consumption in ferromagnetic semiconductors.

On the anomalous low-resistance state and exceptional Hall component in hard-magnetic Weyl nanoflakes

Magnetic topological materials, which combine magnetism and topology, are expected to host emerging topological states and exotic quantum phenomena. In this study, with the aid of greatly enhanced coercive fields in high-quality nanoflakes of the magnetic Weyl semimetal Co3Sn2S2, we investigate anomalous electronic transport properties that are difficult to reveal in bulk Co3Sn2S2 or other magnetic materials. When the magnetization is antiparallel to the applied magnetic field, the low longitudinal resistance state occurs, which is in sharp contrast to the high resistance state for the parallel case. Meanwhile, an exceptional Hall component that can be up to three times larger than conventional anomalous Hall resistivity is also observed for transverse transport. These anomalous transport behaviors can be further understood by considering nonlinear magnetic textures and the chiral magnetic field associated with Weyl fermions, extending the longitudinal and transverse transport physics and providing novel degrees of freedom in the spintronic applications of emerging topological magnets.

Spin-flop led peculiar behavior of temperature-dependent anomalous Hall effect in Hf/Gd-Fe-Co

Here we investigate the temperature dependence of anomalous Hall effect in Hf/GdFeCo/MgO sheet film and Hall bar device. The magnetic compensation temperature (Tcomp) for the sheet film and device is found to be ~240 K and ~118 K, respectively. In sheet film, spin-flopping is witnessed at a considerably lower field, 0.6 T, close to Tcomp. The AHE hysteresis loops in the sheet film have a single loop whereas in the Hall bar device, hystereses consist of triple loops are observed just above the Tcomp. Moreover, the temperature-dependent anomalous Hall resistance (RAHE) responds unusually when a perpendicular magnetic field is applied while recording the RAHE. The zero-field RAHE scan suggests the Hall signal generates solely from the FeCo moment. However, the behavior of 3T-field RAHE scan in which the RAHE drops close to zero near the Tcomp seems to be following the net magnetization response of the device, is explained by considering the low field spin-flopping around the compensation temperature. The results presented here give important insight to understand the complex AHE behavior of ferrimagnets for their spintronic applications.

Magnetic critical behavior and anomalous Hall effect in 2H−Co0.22TaS2 single crystals

We report ferromagnetism in 2H-Co$_{0.22}$TaS$_2$ single crystals where Co atoms are intercalated in the van der Waals gap, and a systematic study of its magnetic critical behavior in the vicinity of $T_c \sim 28$ K. The obtained critical exponents $\beta$ = 0.43(2), $\gamma$ = 1.15(1), and $\delta = 3.54(1)$ fulfill the Widom scaling relation $\delta = 1+\gamma/\beta$ and follow the scaling equation. This indicates that the spin coupling in 2H-Co$_{0.22}$TaS$_2$ is of three-dimensional Hersenberg type coupled with long-range magnetic interaction, and that the exchange interaction decays with distance as $J(r)\approx r^{-4.69}$. 2H-Co$_{0.22}$TaS$_2$ exhibits a weak temperature-dependent metallic behavior in resistivity and negative values of thermopower with dominant electron-type carriers, in which obvious anomalies were observed below $T_c$ as well as the anomalous Hall effect (AHE). The linear scaling behavior between the modified anomalous Hall resistivity $\rho_{xy}/\mu_0H$ and longitudinal resistivity $\rho_{xx}^2M/\mu_0H$ implies that the origin of AHE in 2H-Co$_{0.22}$TaS$_2$ should be dominated by the extrinsic side-jump mechanism.