Angular-dependent Magnetoresistance Measurements Research Articles

Efficient spin current generation in low-damping Mg(Al, Fe)2O4 thin films

Low-damping magnetic insulators are essential for pure spin current-based electronics as they can generate and transfer spin currents without associated charge currents. Nanometer-thick epitaxial thin films of low-damping magnetic insulators are particularly important in order to control and switch the magnetization via spin transfer torques. We have recently developed films of the ferromagnetic insulator MgAl0.5Fe1.5O4 (MAFO) with a low Gilbert damping parameter (∼0.001). In contrast to Y3Fe5O12 (YIG), MAFO films can be grown on a variety of substrates and have significant in-plane magnetic anisotropy, leading to higher spin-wave frequencies. Here, we demonstrate efficient spin current injection from MAFO into adjacent Pt and β-W layers by ferromagnetic resonance (FMR) broadening and inverse spin Hall effect measurements. Angular dependent magnetoresistance (ADMR) measurements indicate that the proximity effect magnetoresistance is small compared to the spin Hall magnetoresistance associated with spin pumping. FMR and ADMR measurements indicate that MAFO/Pt interfaces have a spin-mixing conductance of ∼2 × 1014 Ω−1 m−2, comparable to that of YIG/Pt. These measurements also show that the spin transport can be described by Dyakonov-Perel spin relaxation combined with an extrinsic spin Hall effect (from skew scattering). These results demonstrate the promise of spinel ferrites for spin current-based spintronics.

Interface morphology effect on the spin mixing conductance of Pt/Fe3O4 bilayers

Non-magnetic (NM) metals with strong spin-orbit coupling have been recently explored as a probe of interface magnetism on ferromagnetic insulators (FMI) by means of the spin Hall magnetoresistance (SMR) effect. In NM/FMI heterostructures, increasing the spin mixing conductance (SMC) at the interface comes as an important step towards devices with maximized SMR. Here we report on the study of SMR in Pt/Fe3O4 bilayers at cryogenic temperature, and identify a strong dependence of the determined real part of the complex SMC on the interface roughness. We tune the roughness of the Pt/Fe3O4 interface by controlling the growth conditions of the Fe3O4 films, namely by varying the thickness, growth technique, and post-annealing processes. Field-dependent and angular-dependent magnetoresistance measurements sustain the clear observation of SMR. The determined real part of the complex SMC of the Pt/Fe3O4 bilayers ranges from 4.96 × 1014 Ω−1 m−2 to 7.16 × 1014 Ω−1 m−2 and increases with the roughness of the Fe3O4 underlayer. We demonstrate experimentally that the interface morphology, acting as an effective interlayer potential, leads to an enhancement of the spin mixing conductance.

Spin Hall magnetoresistance in the non-collinear ferrimagnet GdIG close to the compensation temperature

We investigate the spin Hall magnetoresistance (SMR) in a gadolinium iron garnet (GdIG)/platinum (Pt) heterostructure by angular dependent magnetoresistance measurements. The magnetic structure of the ferromagnetic insulator GdIG is non-collinear near the compensation temperature, while it is collinear far from the compensation temperature. In the collinear regime, the SMR signal in GdIG is consistent with the usual relation well established in the collinear magnet yttrium iron garnet, with the angle between magnetization and spin Hall spin polarization direction. In the non-collinear regime, both an SMR signal with inverted sign and a more complex angular dependence with four maxima are observed within one sweep cycle. The number of maxima as well as the relative strength of different maxima depend strongly on temperature and field strength. Our results evidence a complex SMR behavior in the non-collinear magnetic regime that goes beyond the conventional formalism developed for collinear magnetic structures.

Extremely Large Magnetoresistance in a Topological Semimetal Candidate Pyrite PtBi_{2}.

While pyrite-type PtBi_{2} with a face-centered cubic structure has been predicted to be a three-dimensional (3D) Dirac semimetal, experimental study of its physical properties remains absent. Here we report the angular-dependent magnetoresistance measurements of a PtBi_{2} single crystal under high magnetic fields. We observed extremely large unsaturated magnetoresistance (XMR) up to (11.2×10^{6})% at T=1.8 K in a magnetic field of 33T, which is comparable to the previously reported Dirac materials, such as WTe_{2}, LaSb, and NbP. The crystals exhibit an ultrahigh mobility and significant Shubnikov-de Hass quantum oscillations with a nontrivial Berry phase. The analysis of Hall resistivity indicates that the XMR can be ascribed to the nearly compensated electron and hole. Our experimental results associated with the abinitio calculations suggest that pyrite PtBi_{2} is a topological semimetal candidate that might provide a platform for exploring topological materials with XMR in noble metal alloys.

Temperature dependence of Rashba-Edelstein magnetoresistance in Bi/Ag/CoFeB trilayer structures

We have investigated the recently discovered Rashba-Edelstein magnetoresistance (REMR) in Bi/Ag/CoFeB trilayer structures with different temperatures, Bi thicknesses, and magnetic fields via the angular-dependent magnetoresistance (ADMR) measurements. We found that the magnitude of the REMR shows a small variation with changing temperature, which is consistent with the previous work on the temperature evolution of the inverse Rashba-Edelstein effect. Furthermore, the ADMR measurements with different thicknesses of Bi layers and external magnetic fields revealed that the contribution from the field-dependent magnetoresistance is not negligibly small for thicker Bi films (≥10 nm) at low temperature.

Transport evidence for the three-dimensional Dirac semimetal phase inZrTe5

Topological Dirac semimetal is a newly discovered class of materials and has attracted intense attentions. This material can be viewed as a three-dimensional (3D) analogue of graphene and has linear energy dispersion in bulk, leading to a range of exotic transport properties. Here we report direct quantum transport evidence of 3D Dirac semimetal phase of layered material ZrTe5 by angular dependent magnetoresistance measurements under high magnetic fields up to 31 Tesla. We observed very clear negative longitudinal magnetoresistance induced by chiral anomaly under the condition of the magnetic field aligned only along the current direction. Pronounced Shubnikov-de Hass (SdH) quantum oscillations in both longitudinal magnetoresistance and transverse Hall resistance were observed, revealing anisotropic light cyclotron masses and high mobility of the system. In particular, a nontrivial {\pi}-Berry phase in the SdH gives clear evidence for 3D Dirac semimetal phase. Furthermore, we observed clear Landau Level splitting under high magnetic field, suggesting possible splitting of Dirac point into Weyl points due to broken time reversal symmetry. Our results indicate that ZrTe5 is an ideal platform to study 3D massless Dirac and Weyl fermions in a layered compound.

Dimensional crossover in ultrathin buried conductingSrVO3layers

The structure and resistive properties of buried ${\mathrm{SrVO}}_{3}$ layers between two insulating ${\mathrm{LaVO}}_{3}$ layers are investigated by varying the thickness of the ${\mathrm{SrVO}}_{3}$ layers between 3 and 35 monolayers. The thickest ${\mathrm{SrVO}}_{3}$ layer shows a bulklike metallic behavior, while in the thinnest ${\mathrm{SrVO}}_{3}$ layer, a weak localization regime is observed below 100 K manifesting a logarithmic temperature dependence. Angular-dependent magnetoresistance measurements indicate a cylindric shape of the Fermi surface, and therefore a two-dimensional transport in the thinnest buried ${\mathrm{SrVO}}_{3}$ layer. The modification of the charge carrier properties by the reduced thickness of the ${\mathrm{SrVO}}_{3}$ layer are furthermore underlined by the appearance of a relatively strong positive magnetoresistance under a magnetic field perpendicular to the sample surface. The present study therefore highlights a way to synthesize oxide electrodes with reduced dimension for future oxide electronics application.

Robust surface state transport in thin bismuth nanoribbons.

While a two-dimensional (2D) metallic surface state in bismuth has been proposed, experimental 2D evidence of quantum transport, e.g., angular dependent Shubnikov-de Haas (SdH) oscillations is still lacking. Here, we report the angular-dependent magnetoresistance measurements in single-crystal Bi nanoribbons, and found that both the low-field weak antilocalization behavior and the high-field angle-dependent SdH oscillations follow exactly the 2D character, indicative of the 2D metallic surface states which dominate the transport properties of thin Bi nanoribbons. Moreover, by controllable exposing the ribbons to ambient environment (1 atm and room temperature), the metallic surface states were found to be robust to the oxidation although the carrier density in the surface states are modified after the exposures. These results suggest that the metallic surface states in Bi nanoribbons should be topologically protected which can provide key information in understanding the surface properties of Bi in nanometer scale.

Anisotropic superconductivity in graphite intercalation compound YbC6

We report anisotropy of the upper critical field (Bc2) of an intercalated graphite superconductor YbC6 (Tc=6.5K) determined from angular dependent magnetoresistance measurements. Though the perpendicular coherence length is much longer than interlayer spacing, measured angular dependences of Bc2 are well fitted by the Lawrence–Doniach model or the Tinkham model, which are known to be applicable to quasi two-dimensional materials or thin films, rather than the effective mass model. This observation is similar to the measurements for the other intercalated graphite superconductor, CaC6, by Jobiliong et al. [E. Jobiliong, H.D. Zhou, J.A. Janik, Y.-J. Jo, L. Balicas, J.S. Brooks, C.R. Wiebe, Phys. Rev. B 76 (2007) 052511]. A possible explanation for the unexpected applicability of these models is that our YbC6 samples are synthesized as thin flakes in the host graphite.

Commensurability effects in two-dimensional electron gases with periodically arranged Ni and NiFe nanopillars

We have investigated the magnetoresistance of high mobility two-dimensional electron gases (2DEG) under the influence of periodically arranged ferromagnetic nanopillars. A huge positive magnetoresistance and commensurability oscillations (COs) indicate that the 2DEG is subjected to a surprisingly strong modulation. Angular-dependent magnetoresistance measurements as well as the phase of the COs suggest that the modulation is dominated by electrostatic rather than by magnetic contributions.

Anisotropic Vortex Plasticity in the Liquid State ofYBa2Cu3O7: Evidence for Quenchedc-Axis Vortex Correlation Length

A generalized methodology, based on anisotropic angular dependent magnetoresistance measurements, is presented to deconvolute the contributions to the vortex activation energy (i.e., plastic and pinning energies) in the vortex liquid state of high temperature superconductors. Experimental evidence is given for the appearance of a partially entangled liquid vortex state in YBa 2Cu 3O (7) when random quenched and correlated disorders compete, as in twinned melt textured YBa 2Cu 3O (7) quasisingle crystals with Y 2BaCuO (5) precipitates. The hallmark of this new phase is a quench of the c-axis vortex correla-tion length.

Magnetic field orientation dependent ground states in (TMTSF) 2PF 6

We report anisotropic and angular dependent magneto-resistance measurements in the “normal” state of TMTSF 2PF 6 under pressure at low temperature. The most striking feature of this study occurs below 5K where we observe metallic behavior (dR/dT>0) when the field is aligned along intermolecular tunneling directions (Lebed magic angles) and non-metallic behavior (dR/dT>0) for other orientations. Previous results showed this behavior along the most conducting a direction, but here we show that the b and c (interplane) directions also exhibit this metallic to non-metallic transition with angle. At higher temperatures (~15–20K) there is a resistance minimum in the conducting (a-b) plane which is absent in the interplane (c) direction. Since we expect phase transitions, or other changes in the electronic state, to be observable in all directions, this observation implies that the nature of the ground state is reflected in the 5K magic angle effects and is determined by interplane decoupling rather than in-plane dimensionality crossovers.

Solubility limit and anisotropy analysis of Mg doping in melt textured YBa{sub 2}(Cu{sub 1{minus}x}Mg{sub x}){sub 3}O{sub 7{minus}{delta}}

Strong signatures of Mg substitution in YBa{sub 2}(Cu{sub 1{minus}x}Mg{sub x}){sub 3}O{sub 7{minus}{delta}} have been obtained from single domain samples prepared by top seeding using YBa{sub 2}Cu{sub 3}O{sub 7(US){delta}} + 30%wt Y{sub 2}BaCuO{sub 5} + X%wt MgO mixtures. A drastic decrease of the superconducting transition temperature with increasing X has been obtained reaching values of {Tc} {approximately}40 K for X=16.7. The {Tc}(x) dependence and thermogravimetric results suggest that Mg atoms substitute Cu(2) sites and that a solubility limit exists. Angular dependent magnetoresistance measurements have been used to determine the anisotropy of YBa{sub 2}(Cu{sub 1{minus}x}Mg{sub x}){sub 3}O{sub 7{minus}{delta}} samples. These results are discussed on the basis of the recent controversy results reported for Zn-doped samples.