Ordinary Hall Coefficient Research Articles

Direct evidence from high-field magnetotransport for a dramatic change of quasiparticle character in van der Waals ferromagnet Fe3−xGeTe2

Magnetometry and magnetoresistance (MR) data taken on the van der Waals ferromagnet Fe3−xGeTe2 (FGT) reveal three distinct contributions to the MR: a linear negative component, a contribution from closed Fermi-surface orbits, and an enhancement proportional to the square of the applied magnetic field which is linked to a noncoplanar spin arrangement. Contrary to earlier studies on FGT, by accounting for the field dependence of the anomalous Hall effect, we find that the ordinary Hall coefficient decreases markedly below 80 K, indicating a significant change in character of the electrons and holes on the Fermi-surface at this temperature. The resulting altered ground state eventually causes the Hall coefficient to reverse sign at 35 K. Our Hall data support the proposal that Kondo-lattice behavior develops in this d-electron material below 80 K. Additional evidence comes from the negative linear component of the MR, which arises from electron-magnon scattering with an atypical temperature dependence attributable to the onset of Kondo screening. Published by the American Physical Society 2024

Spin-reorientation driven emergent phases and unconventional magnetotransport in quasi-2D vdW ferromagnet Fe4GeTe2

Non-trivial spin textures driven by strong exchange interaction, magneto-crystalline anisotropy, and electron correlation in a low-dimensional magnetic material often lead to unusual electronic transitions. Through a combination of transport experiments in exfoliated nanoflakes down to 16 layers and first principle calculations, we unravel emergent electronic phases in quasi-2D van der Waals ferromagnet, Fe4GeTe2, possessing ferromagnetic TC ~ 270 K, along with a spin-reorientation transition (TSR ~ 120 K) with the change of magnetic easy axis. Two electronic transitions are identified. The first transition near TSR exhibits a sharp fall in resistivity, followed by a sign change in the ordinary Hall coefficient (R0), together with, maximum negative magnetoresistance (MR) and anomalous Hall conductivity. Another unusual electronic transition, hitherto unknown, is observed near ~ 40–50 K (TQ), where R0 again changes sign and below which, the resistivity shows a quadratic temperature dependence, and MR becomes positive. An analysis of the experimental data further uncovers the role of competing inelastic scattering processes in anomalous magnetotransport behavior. The density-functional theory based first-principle calculations unveil two possible magnetic phases, followed by a low-energy model Hamiltonian which captures the essence of these phases as well as explains the observed magnetotransport behavior. Thus, we demonstrate an interplay between magnetism and band topology and its consequence on electron transport in Fe4GeTe2, important for spintronic applications.

Cluster spin glass in off-stoichiometric Ni2.01Mn1.58Sn0.41 alloy

The Ni2.01Mn1.58Sn0.41 alloy is a member of the Heusler material family which undergoes diffusionless structural transition from cubic austenite to orthorhombic martensite during its cooling. The transition was observed at temperature 398 K from paramagnetic austenite to martensite with complex magnetic structure. The measurement of AC magnetic susceptibility of the alloy confirmed spin-glass state at temperatures below 155 K and from the frequency dependence of the cups in AC susceptibility, a presence of clusters (11 nm3 in size) with strong interaction is deduced. The alloy shows anomalous Hall effect (AHE) of the order of 0.051 μΩ cm at 2 K in the spin-glass state of the alloy and its sign changes at around 140 K. The ordinary Hall coefficient of the metallic alloy is strikingly positive in the whole studied temperature region, and it points to complex magnetic and electronic structure of the alloy. The electrical resistance of the sample shows a flat maximum of 220 μΩ cm at spin-glass transition temperature. The magnetoresistance is of the order of 1% and is negative in the whole temperature region which points to a standard scattering of charge carriers on magnetic fluctuations. Quantum-mechanical calculations of supercells with compositions Ni32Mn25Sn7, Ni32Mn26Sn6 and Ni32Mn24Sn8 simulated different arrangement of Mn and Sn atoms. Ni32Mn25Sn7 composition is thermodynamically the most stable, although its magnetic moment is 1.67 μB/f.u., twice higher than the measured value. The combination of the latter two states, with additional Mn–Sn swaps, provides the magnetic moments matching the experimental value at a very small energy cost. Theoretical results then indicate that a scenario of local chemical variations and clustering is likely in the studied alloy.

Hall effect of RCo2 (R= Er and Lu) compounds

The Hall effect of RCo2 with R = Er and Lu has been studied. LuCo2 is an exchange-enhanced Pauli paramagnet. ErCo2 is ferrimagnetic below TC = 32 K and undergoes a first-order magnetic transition at TC. The Hall resistivity ρxy was measured as a function of the magnetic field B at various temperatures. We estimated the normal (ordinary) Hall coefficient RN and the anomalous Hall coefficient RA from the analyses of the ρxy – B curves and the magnetization curves. For LuCo2, ρxy increases linearly with increasing field and its slope decreases rapidly with increasing temperature below 70 K. This is attributable to strong temperature dependence of RN. The ρxy – B curve of ErCo2 depends on a magnetic state. In the ferrimagnetic state, both RN and RA have positive values, while RN is positive and RA is negative in the paramagnetic state. We calculated RN's of nonmagnetic and ferromagnetic LuCo2 from the energy band structures and compared them with the experimental values. The origin of a sign change in RA of ErCo2 is briefly discussed.

Hall effect of itinerant electron metamagnet Co(S1-xSex)2

We have studied the Hall effect of Co(S1-xSex)2 with 0.08 ≤ x ≤ 0.14, which shows itinerant electron metamagnetism (IEM). The Hall resistivity ρxy was measured as a function of the magnetic field B at various temperatures. It is found that ρxy decreases nearly linearly with increasing B in the paramagnetic state, while it has a positive slope in the ferromagnetic state. Abrupt drops were observed at around the metamagnetic transition field in the ρxy – B curves, which are attributable to the anomalous Hall effect. From the analyses of the ρxy – B curves and the magnetization curves, we estimated the anomalous Hall coefficient, the normal (ordinary) Hall coefficient in the ferromagnetic state, and that in the paramagnetic state. Appearance of both ferromagnetic and paramagnetic states depending on a magnetic field in IEM makes it possible to estimate these values, simultaneously. The estimated normal Hall coefficients are compared with the theoretical values calculated from the energy band structures. The present study has demonstrated that IEM provides a suitable stage to understand the electronic structures of both ferromagnetic and paramagnetic states of the itinerant electron system at the same temperature.

Magnetic and anomalous transport properties in spin-gapless semiconductor like quaternary Heusler alloy CoFeTiSn

The magnetic and anomalous transport properties have been studied for the as-melted and annealed half-metallic ferromagnet CoFeTiSn. First-principles calculations suggest that CoFeTiSn presents band structure of the near spin-gapless semiconductor. The experiments show that the alloy’s phase purity, chemical ordering and grain size have been improved by annealing, which causes a different transport property. The as-melted sample shows a temperature coefficient of resistivity sign transition from positive to negative accompanying the ferromagnetic–paramagnetic transition, while the annealed sample only shows negative temperature coefficient resistivity in the whole temperature range. The calculated positive ordinary Hall coefficient suggest that transport properties are dominated by the hole carrier. The measured saturation magnetic moment at 5 K is lower but close to 1 μB/f.u, which is probably attributed to the inhomogeneous magnetic phase and the anti-site disorder between Co-Fe. The band structure of near spin-gapless semiconductor is another possible reason. The anomalous Hall conductivity changes from 20 S/cm to 27 S/cm after annealing due to the altered chemical ordering. The positive linear magnetoresistance indicates that the alloy appears to have spin-gapless semiconductor like behavior as predicted by its band structure.

Hall Effect Study of the Metamagnetic Transition in the Sr4(Ru0.99Fe0.01)3O10 Nanosheet

Sr4(Ru0.99Fe0.01)3O10 shows a ferromagnetic (FM) transition at TC ∼ 105 K with the magnetic easy axis in the ab plane, followed by a metamagnetic transition (MMT) at low temperatures when the magnetic field H is applied along the c axis, which is in sharp contrast to that of the pure Sr4Ru3O10, where the easy axis is along the c axis and the MMT is in the ab plane. Here, we studied the MMT in the Sr4(Ru0.99Fe0.01)3O10 nanosheet by the Hall effect. It was found that the ordinary Hall coefficient of Sr4(Ru0.99Fe0.01)3O10 is almost the same as that of the pure Sr4Ru3O10, while a sudden increase in the Hall resistance Rxy is observed below ∼50 K, above which the Rxy presents the conventional anomalous Hall effect up to TC. Analysis of the results indicates that the MMT has no direct correlation to the electronic structure but closely relates to the magnetic moment locking, where the magnetic-field-induced breakdown of the locked moments is responsible for the MMT.

Electrical and thermal transport in van der Waals magnets 2H−MxTaS2 (M=Mn, Co)

We report a detailed study of electrical and thermal transport properties in 2H-M$_x$TaS$_2$ (M = Mn, Co) magnets where M atoms are intercalated in the van der Waals gap. The intercalation induces ferromagentism with an easy-plane anisotropy in 2H-Mn$_x$TaS$_2$, but ferromagnetism with a strong uniaxial anisotropy in 2H-Co$_{0.22}$TaS$_2$, which finally evolves into a three-dimensional antiferromagnetism in 2H-Co$_{0.34}$TaS$_2$. Temperature-dependent electrical resistivity shows metallic behavior for all samples. Thermopower is negative in the whole temperature range for 2H-Co$_x$TaS$_2$, whereas the sign changes from negative to positive with increasing Mn for 2H-Mn$_x$TaS$_2$. The diffusive thermoelectric response dominates in both high- and low-temperature ranges for all samples. A clear kink in electrical resistivity, a weak anomaly in thermal conductivity, as well as a slope change in thermopower were observed at the magnetic transitions for 2H-Mn$_{0.28}$TaS$_2$ ($T_\textrm{c}$ $\approx$ 82 K) and 2H-Co$_{0.34}$TaS$_2$ ($T_\textrm{N}$ $\approx$ 36 K), respectively, albeit weaker for lower $x$ crystals. Co-intercalation promoted ferromagnetic to antiferromagnetic transition is further confirmed by the Hall resistivity; the sign change of the ordinary Hall coefficient indicates a multi-band behavior in 2H-Co$_x$TaS$_2$.

Unveiling transport properties of Co2MnSi Heusler epitaxial thin films with ultra-low magnetic damping

The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 - 10-3 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5 – 300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

Large anomalous Hall effect in the kagome ferromagnet LiMn6Sn6

Kagome magnets are believed to have numerous exotic physical properties due to the possible interplay between lattice geometry, electron correlation and band topology. Here, we report the large anomalous Hall effect in the kagome ferromagnet LiMn$_6$Sn$_6$, which has a Curie temperature of 382 K and easy plane along with the kagome lattice. At low temperatures, unsaturated positive magnetoresistance and opposite signs of ordinary Hall coefficient for $\rho_{xz}$ and $\rho_{yx}$ indicate the coexistence of electrons and holes in the system. A large intrinsic anomalous Hall conductivity of 380 $\Omega^{-1}$ cm$^{-1}$, or 0.44 $e^2/h$ per Mn layer, is observed in $\sigma_{xy}^A$. This value is significantly larger than those in other $R$Mn$_6$Sn$_6$ ($R$ = rare earth elements) kagome compounds. Band structure calculations show several band crossings, including a spin-polarized Dirac point at the K point, close to the Fermi energy. The calculated intrinsic Hall conductivity agrees well with the experimental value, and shows a maximum peak near the Fermi energy. We attribute the large anomalous Hall effect in LiMn$_6$Sn$_6$ to the band crossings closely located near the Fermi energy.

Evidence of Fermi surface reconstruction at the metamagnetic transition of the strongly correlated superconductor UTe2

Thermoelectric power ($S$) and Hall effect ($R_\mathrm{H}$) measurements on the paramagnetic superconductor UTe$_2$ with magnetic field applied along the hard magnetization $b$-axis are reported. The first order nature of the metamagnetic transition at $H_\mathrm{m}=H^b_\mathrm{c2}=35$~T leads to drastic consequences on $S$ and $R_\mathrm{H}$. In contrast to the field dependence of the specific heat in the normal state through $H_\mathrm{m}$, $S(H)$ is not symmetric with respect to $H_\mathrm{m}$. This implies a strong interplay between ferromagnetic (FM) fluctuations and a Fermi-surface reconstruction at $H_\mathrm{m}$. $R_\mathrm{H}$ is very well described by incoherent skew scattering above the coherence temperature $T_\mathrm{m}$ corresponding roughly to the temperature of the maximum in the susceptibility $T_{\chi_\mathrm{max}}$ and coherent skew scattering at lower temperatures. The discontinuous field dependence of both, $S(H)$ and the ordinary Hall coefficient $R_0$, at $H_\mathrm{m}$ and at low temperature, provides evidence of a change in the band structure at the Fermi level.

Transition from intrinsic to extrinsic anomalous Hall effect in the ferromagnetic Weyl semimetal PrAlGe1−xSix

Recent reports of a large anomalous Hall effect (AHE) in ferromagnetic Weyl semimetals (FM WSMs) have led to a resurgence of interest in this enigmatic phenomenon. However, due to a lack of tunable materials, the interplay between the intrinsic mechanism caused by Berry curvature and extrinsic mechanisms due to scattering remains unclear in FM WSMs. In this contribution, we present a thorough investigation of both the extrinsic and intrinsic AHEs in a new family of FM WSMs, PrAlGe1−xSix, where x can be tuned continuously. Based on the first-principles calculations, we show that the two end members, PrAlGe and PrAlSi, have different Fermi surfaces, but similar Weyl node structures. Experimentally, we observe moderate changes in the anomalous Hall coefficient (RS), but significant changes in the ordinary Hall coefficient (R0) in PrAlGe1−xSix as a function of x. By comparing the magnitude of R0 and RS, we identify two regimes: |R0| < |RS| for x ≤ 0.5 and |R0| > |RS| for x > 0.5. Through a detailed scaling analysis, we uncover a universal anomalous Hall conductivity (AHC) from intrinsic contribution when x ≤ 0.5. Such a universal AHC is absent for x > 0.5. Our study, thus, reveals the significance of extrinsic mechanisms in FM WSMs and reports the first observation of the transition from the intrinsic to extrinsic AHE in PrAlGe1−xSix.

Anomalous Hall effect with variable-range hopping in Mn4-xAuxN (x = 0, 0.5) epitaxial films

Single-crystalline Mn4-xAuxN (x = 0, 0.5) (Mn4N and Mn3.5Au0.5N) epitaxial films were grown on MgO(001) substrates by plasma assisted molecular beam epitaxy. Mott variable-range hopping in two dimensions is the dominant transport mechanism in Mn4-xAuxN films. The linear background of anomalous Hall curves at high field corresponds to the ordinary Hall coefficient. The films show apparently different anomalous Hall behaviors from Mn4N and other anti-perovskite nitrides due to the introduction of Au atoms. Hole-like (electron-like) anomalous Hall effect is observed at low (high) temperature. The reversal of transport type appears at 155 K where distortion of curve is found because of the competition of different carriers. The slopes of Mn4N films lie between 7 × 10−8 and 3.9 × 10−7, while the slopes of Mn3.5Au0.5N films lie between 3.7 × 10−4 and 1.1 × 10−3. The nett anomalous Hall effect presents linear relation to the longitudinal resistivity, which shows that the anomalous effects are originated from the skew scattering electrons.

Short-range magnetic order and electrical behavior in epitaxial NiCo2O4 thin films

As an electrode material with reported ferrimagnetic properties, the NiCo2O4 film exhibits critical electrical transport properties under a magnetic field as well as a magnetic microstructure. In this study, epitaxial NiCo2O4 films were prepared on LaAlO3 (100) substrates, and the effects of growth temperature on the magnetic and electrical transport properties of the films were investigated. Negative magnetoresistance was observed in the films due to the local spin effect. The NiCo2O4 films demonstrated a topological Hall effect, which is closely related to short-range magnetic order and a noncoplanar magnetic structure in NiCo2O4 films. The results indicate that there are two types of spin frustration in the NiCo2O4 structure. One is a diamond lattice composed of tetrahedral site cations, while the other is a triangular lattice structure formed by the edge-shared octahedral cations. For the NiCo2O4 (100) films, the sign of the ordinary Hall coefficient reverses by decreasing the measurement temperature, indicating that the carrier type changes from p to n.

Experimental realization and magnetotransport properties of half-metallic Fe2Si

Thin films of the high Curie temperature intermetallic ferromagnet β-Fe2Si were synthesized via molecular beam epitaxy. Investigation using X-ray diffraction and atomic force microscopy shows a hexagonal crystal structure and a smooth topography. Theoretically, Fe2Si has been predicted to exhibit uniquely desirable magnetotransport properties. We report on these properties experimentally, including the ordinary Hall coefficient R0 and anomalous Hall resistivity ρxyAH. The compound is found to be a soft Heisenberg ferromagnet with temperature dependent magnetization based on the thermal excitation of spin waves. We present a detailed look into the contributions to its longitudinal resistivity, which due to the presence of a spin-flip gap ΔkB, indicating a half-metallic band structure. The correct scaling relations between these components of the resistivity tensor (ρxx and ρxy) are also discussed.

Influence of orbital two-channel Kondo effect on anomalous Hall effect in ferrimagnetic composites of LaNiO3 and CoFe2O4

In this report, we have investigated the magnetoresistance (MR) and Hall effect of the ferrimagnetic composites containing LaNiO3 and CoFe2O4 (CFO) (with CFO content 15% and 20%) which exhibit orbital two-channel Kondo (2CK) effect and therefore pronounced resistivity upturn at low temperature. Both composites manifest a negative to positive crossover in MR with increasing temperature. The MR is described by the Khosla and Fisher model of spin fluctuations scattering of conduction electrons and the two-band theory based on hybridized p-d sub-bands. The Hall resistivity of the composites consists of both ordinary and anomalous part. The negative sign of the ordinary Hall coefficient suggests electrons as the dominating charge carriers. The coefficient of anomalous Hall resistivity () follows the scaling relation () with longitudinal resistivity () at high temperature above the resistivity upturn. However, at low temperature shows non-monotonous behaviour and deviates from the scaling relation where orbital 2CK effect takes place. More detailed study below the resistivity upturn of the composite with 20% CFO reveals that this deviation occurs around the Kondo temperature. This breakdown of scaling relation around the Kondo temperature indicates the possible influence of orbital 2CK on the anomalous Hall effect.

Surface magnetization thermal fluctuation driven anomalous behaviour of ordinary Hall effect in Pt/YIG

The ordinary Hall coefficient (OHC) characterizes the electron diffusion and therefore is the essential prerequisite for rigorously acquiring the spin diffusion length and spin Hall angle in magnetic heterostructures that involves strong spin–orbit coupling. Here, OHC is investigated in Pt layers in contact with ferromagnetic insulating Y3Fe5O12 films. The OHC, being negative, abnormally increases in magnitude significantly with increasing temperature for ultrathin Pt layers whereas it shows negligible change for thick ones. Similar anomalous phenomena are observed for isoelectronic and ferromagnetic Ni films on paramagnetic Gd3Ga5O12 substrates. For Pt films on Gd3Ga5O12 substrates, however, the OHC always decreases in magnitude with increasing temperature. The anomalous behaviour of OHC in spin polarized Pt on Y3Fe5O12 and ferromagnetic Ni films is suggested to arise from significant changes of the Fermi level and the Fermi surface shape through combined effects of the magnetic proximity and the surface magnetization thermal fluctuation. The present work will facilitate the understanding of relaxation mechanism of pure spin current in Pt layers.

Giant topological Hall effect in tetragonal Heusler alloy Mn2PtSn

Giant topological Hall resistivity up to 1.53μΩcm has been observed in our tetragonal Heusler alloy Mn2PtSn at temperatures lower than 200K. The appearance of topological Hall effect is mainly attributed to the formation of non-collinear magnetism due to the competition between the antiferromagnetic coupling and the ferromagnetic interaction. Furthermore, accompanying the spin reorientation at 192K, the ordinary Hall coefficient changed from positive to negative during cooling, implying the transition from holes conduction to electrons conduction owing to the magnetic reorientation. The present study suggests that Mn2PtSn is a promising candidate for technological applications in spintronic devices.

Specific heat and Hall effect of the ferromagnetic Kondo lattice UCu0.9Sb2

We have investigated the electrical resistivity ρ, specific heat Cp and Hall coefficient RH on a single crystal of a ferromagnetic Kondo lattice UCu0.9Sb2. The experimental , Cp(T) and data evidence a bulk magnetic phase transition at K, and additionally exhibit an unexpected bump located in the temperature range TC/10–TC/3. UCu0.9Sb2 has an enhanced electronic specific heat coefficient mJ molK−2, corresponding to Kondo temperature K. An analysis of the Hall effect data for j//(a, b)-plane and H// c-axis reveals that the low-temperature ordinary Hall coefficient R0 is positive, suggesting that p-type electrical conductivity is dominant. The density of the carriers at 2 K is about 0.6 holes f.u.−1, which may categorize the studied compound into class of low carrier density compounds. Combined γ and R0 data divulge an effective mass of charge carriers 27 me. This finding together with quite low Hall mobility cm2 Vs−1 and Kadowaki–Woods ratio cm (mol K2 mJ−1)2, manifest the development of heavy-fermion state in the ferromagnetic UCu0.9Sb2 compound at low temperatures.

A low-temperature study of manganese-induced ferromagnetism and valence band convergence in tin telluride

SnTe is renowned for its promise in advancing energy-related technologies based on thermoelectricity and for its topological crystalline insulator character. Here, we demonstrate that each Mn atom introduces ∼4 μB (Bohr magneton) of magnetic moment to Sn1−xMnxTe. The Curie temperature TC reaches ∼14 K for x = 0.12, as observed in the field dependent hysteresis of magnetization and the anomalous Hall effect. In accordance with a modified two-band electronic Kane model, the light L-valence-band and the heavy Σ-valence-band gradually converge in energy with increasing Mn concentration, leading to a decreasing ordinary Hall coefficient RH and a favorably enhanced Seebeck coefficient S at the same time. With the thermal conductivity κ lowered chiefly via point defects associated with the incorporation of Mn, the strategy of Mn doping also bodes well for efficient thermoelectric applications at elevated temperatures.