Angular-dependent Magnetoresistance Research Articles

Robust properties of the superconducting ferromagnet UCoGe

The angular dependent magnetoresistance of the superconducting ferromagnet UCoGe is studied in detail at 40 mK to fields of 18 T in a single crystal sample with a low residual resistivity ratio [R(300 K)/R(1 K)=RRR=5]. A ferro-to-ferrimagnetic transition near 9 T that depends only on the field component along the uniaxial magnetic c axis is observed in both transport and susceptibility. Reverse hysteresis in the magnetoresistance in the critical field region indicates a coupling of the superconductivity to the magnetization for fields aligned closely to the ab plane. These results corroborate and advance previous findings on higher residual resistivity ratio samples.

Stray field and the superconducting surface spin valve effect in La0.7Ca0.3MnO3/YBa2Cu3O7−δ bilayers

Electronic transport and magnetization measurements were carried out on La0.7Ca0.3MnO3/YBa2Cu3O7−δ (LCMO/YBCO) bilayers below the superconducting transition temperature in order to study the interaction between magnetism and superconductivity. This study shows that a substantial number of weakly pinned vortices are induced in the YBCO layer by the large out-of-plane stray field in the domain walls. Their motion gives rise to large dissipation peaks at the coercive field. The angular dependent magnetoresistance (MR) data reveal the interaction between the stripe domain structure present in the LCMO layer and the vortices and anti-vortices induced in the YBCO layer by the out-of-plane stray field. In addition, this study shows that a superconducting surface spin valve effect is present in these bilayers as a result of the relative orientation between the magnetization at the LCMO/YBCO interface and the magnetization in the interior of the LCMO layer that can be tuned by the rotation of a small H. This latter finding will facilitate the development of superconductive magnetoresistive memory devices. These low-magnetic-field MR data, furthermore, suggest that triplet superconductivity is induced in the LCMO layer, which is consistent with recent reports on triplet superconductivity in LCMO/YBCO/LCMO trilayers and LCMO/YBCO bilayers.

Electrical Transport Properties of Topological Insulators

From a viewpoint of transport properties, topological insulators are the materials that are insulating in the bulk but are metallic at their edge. After briefly introducing the basic phenomena such as Dirac fermions and quantum spin Hall effect for topological insulators, we describe the details of the quantum oscillations originating from the surface state of a three-dimensional topological insulator Bi1-xSbx, which was observed for the first time by our group. Intriguingly, this material was found to exhibit a novel angular-dependent magnetoresistance oscillation phenomenon, whose origin is still unknown but probably reflects a new physics in the surface transport properties of the topological insulator.

Quasi-Two-Dimensional Fermi Surfaces and Coherent Interlayer Transport inKFe2As2

We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces (FSs) in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe2As2 with Tc ≈ 3.7 K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, correspond to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs.

Flux-closure pattern in a two-dimensional NbN–Fe superconductor-ferromagnet nanocomposite: Anisotropy of the angular magnetoresistance

The angular dependence of magnetoresistance (MR) of distributed NbN–Fe–NbN Josephson-junctions in the out-of-plane and in-plane magnetic field geometries shows a striking anisotropy on the polarity of the current (I+/I−) and its direction with respect to the applied field. The origin of this anisotropy is suggested to be the difference in the degree of spin polarization of electrons injected from Fe nanoplaquettes into the superconducting NbN for I+ and I−. Such a conclusion is based on the topography of flux-closure domains in Fe plaquettes. The anisotropy of MR is suppressed at high fields as the flux-closure domains transform into a single-domain structure.

Angular dependent magnetoresistance with twofold and fourfold symmetries in A-type antiferromagnetic Nd0.45Sr0.55MnO3 thin film

The angular dependent magnetoresistance (AMR) of Nd0.45Sr0.55MnO3 thin film epitaxially grown on SrTiO3 (001) is examined at different temperatures and magnetic fields. Twofold and fourfold symmetric AMR and a transition between them are observed under two different measurement modes and are found to be dependent on temperature and/or strength of a magnetic field. In comparison with AMR occurring in other systems, we believe that the twofold/fourfold symmetric AMR observed here corresponds to different spin-canted states induced by the magnetic field at certain temperatures below the Néel temperature.

Angular Dependence of Magnetic Quantum Oscillations and of Magnetoresistance in Quasi-2D Metals

We study analytically the angular dependence of magnetoresistance and of magnetic quantum oscillation frequencies in layered quasi-two-dimensional (Q2D) metals. The angular dependences of magnetoresistance and of Fermi-surface cross-sectional area are closely related. The harmonic expansion analytical formulas for the angular dependence of Fermi-surface cross-sectional area in an external magnetic field are obtained for various typical crystal symmetries. The simple azimuth-angle dependence of the Yamaji angles is derived for the elliptical in-plane Fermi surface. These formulas correct some previous results and allow the simple and effective interpretation of the magnetic quantum oscillations and magnetoresistance data in cuprate high-temperature superconducting materials, in organic metals and in other Q2D metals. The applicability region of some previous widely used analytical results and of the results obtained is investigated.

Oscillatory angular dependence of the magnetoresistance in a topological insulatorBi1−xSbx

The angular-dependent magnetoresistance and the Shubnikov-de Haas oscillations are studied in a topological insulator Bi_{0.91}Sb_{0.09}, where the two-dimensional (2D) surface states coexist with a three-dimensional (3D) bulk Fermi surface (FS). Two distinct types of oscillatory phenomena are discovered in the angular-dependence: The one observed at lower fields is shown to originate from the surface state, which resides on the (2\bar{1}\bar{1}) plane, giving a new way to distinguish the 2D surface state from the 3D FS. The other one, which becomes prominent at higher fields, probably comes from the (111) plane and is obviously of unknown origin, pointing to new physics in transport properties of topological insulators.

Reply to “Comment on ‘Anisotropic scattering in angular-dependent magnetoresistance oscillations of quasi-two-dimensional and quasi-one-dimensional metals: Beyond the relaxation-time approximation’ ”

In reply to the Comment by Kennett regarding our article [Phys. Rev. B 77, 235123 (2008)], we clarify the key points at which we disagree with Kennett's derivation of the interlayer magnetoconductivity in layered metals and argue that our original derivation of this quantity is valid.

Comment on “Anisotropic scattering in angular-dependent magnetoresistance oscillations of quasi-two-dimensional and quasi-one-dimensional metals: Beyond the relaxation-time approximation”

Recently, Smith and McKenzie [Phys. Rev. B 77, 235123 (2008)] used Boltzmann theory to calculate expressions for the interlayer resistivity in quasi-one-dimensional and quasi-two-dimensional metals for an arbitrary elastic collision integral. In this Comment I point out that their treatment of the equations of motion leads to an error in their expressions, and I derive corrected expressions for the interlayer conductivity in quasi-one-dimensional and quasi-two-dimensional metals in the presence of anisotropic scattering.

Structural symmetry and magnetocrystalline anisotropy ofSrRuO3films onSrTiO3

The structural, magnetic, and magnetotransport properties of ${\text{SrRuO}}_{3}$ films grown on ${\text{SrTiO}}_{3}$ (001) substrates were investigated with the aim to determine the crystalline symmetry, crystalline orientation, and magnetocrystalline anisotropy of an ultrathin (5 nm) ${\text{SrRuO}}_{3}$ film. 60- and 40-nm-thick ${\text{SrRuO}}_{3}$ films were extensively studied by transmission electron microscopy as well as magnetic and magnetotransport techniques, respectively. These studies showed orthorhombic symmetry with a slight monoclinic distortion and a well-defined long-range order of crystallographic domains with the ${[001]}_{o}$ axis parallel to terraces on the slightly vicinal ${\text{SrTiO}}_{3}$ substrate. The magnetocrystalline anisotropy is very strong. The easy axis lies in the ${(001)}_{o}$ plane under a temperature-dependent angle of about $35\ifmmode^\circ\else\textdegree\fi{}$ with respect to the ${[110]}_{o}$ direction that is along the film normal. Below 70 K indications for a spin reorientation transition were detected. The angular-dependent anisotropic magnetoresistance shows distinctive characteristics of the monoclinic symmetry that can be used as a fingerprint. Transmission electron microscopy images of the 5 nm thin ${\text{SrRuO}}_{3}$ film show a coherently strained state. Using the fingerprints from the angular magnetoresistance it could be clearly shown that this ultrathin ${\text{SrRuO}}_{3}$ film, although grown in step flow growth as well, has still monoclinic symmetry, but lacks long-range order of the crystallographic domains. A fraction of $30--40\text{ }\mathrm{%}$ of misaligned domains (rotated in plane by $90\ifmmode^\circ\else\textdegree\fi{}$) was estimated from the magnetoresistance curves.

Angular-dependent oscillations of the magnetoresistance inBi2Se3due to the three-dimensional bulk Fermi surface

We observed pronounced angular-dependent magnetoresistance (MR) oscillations in a high-quality Bi2Se3 single crystal with the carrier density of 5x10^18 cm^-3, which is a topological insulator with residual bulk carriers. We show that the observed angular-dependent oscillations can be well simulated by using the parameters obtained from the Shubnikov-de Haas oscillations, which clarifies that the oscillations are solely due to the bulk Fermi surface. By completely elucidating the bulk oscillations, this result paves the way for distinguishing the two-dimensional surface state in angular-dependent MR studies in Bi2Se3 with much lower carrier density. Besides, the present result provides a compelling demonstration of how the Landau quantization of an anisotropic three-dimensional Fermi surface can give rise to pronounced angular-dependent MR oscillations.

Spin-orbit coupling and anomalous angular-dependent magnetoresistance in the quantum transport regime of PbS

We measured magnetotransport properties of PbS single crystals which exhibit the quantum linear magnetoresistance (MR) as well as the static skin effect that creates a surface layer of additional conductivity. The Shubnikov-de Haas oscillations in the longitudinal MR signify the peculiar role of spin-orbit coupling. In the angular-dependent MR, sharp peaks are observed when the magnetic field is slightly inclined from the longitudinal configuration, which is totally unexpected for a system with nearly spherical Fermi surface and points to an intricate interplay between the spin-orbit coupling and the conducting surface layer in the quantum transport regime.

Electrodeposition of Ni/Co and Ni Magnetic Nanowires Using Allumina Template

Nickel and nickel-cobalt alloy nanowires were fabricated through template-assisted electrochemical deposition method and sandwiched between two caps of electrodeposited pure gold. Free standing porous alumina templates were prepared and used for the synthesis of the ordered nanowires. The morphology and composition of as-deposited nanowires were determined by scanning electron microscope. Nanowires were characterized by a diameter of 70 nm, as the alumina template pores diameter, and by a length in the range of 2-3 μm. Backscattered electron signal showed the presence of Au and Ni or Ni-Co sandwich, highlighting a well defined and void free interface between the gold caps and the Ni or Ni-Co layer. Particular optimization has been considered for the subsequent magnetoresistance characterization of nanowires. Angular dependence of magnetoresistance has been measured as a function of temperature from in-plane to perpendicular magnetic field configuration. Anisotropic magnetoresistive measurements suggested a magnetization process by magnetization rotation from nanowire axis towards in-plane orientation.

Role of anion size, magnetic moment, and disorder on the properties of the organic conductor κ-(BETS) 2Ga 1- xFe xCl 4- yBr y

Shubnikov–de Haas and angular dependent magnetoresistance oscillations have been used to explore the role of anion size, magnetic moment, and disorder in the organic conductors κ ‐ ( BETS ) 2 GaBr 4 and κ ‐ ( BETS ) 2 FeCl 2 Br 2 in the isomorphic class κ ‐ ( BETS ) 2 Ga 1 ‐ x Fe x Cl 4 ‐ y Br y . The results, combined with previous work, show correlations between the anion composition ( Ga 1 ‐ x Fe x Cl 4 ‐ y Br y ) and the superconducting transition temperature, effective mass, Fermi surface topology, and the mean free path.

Geometrical and orbital effects in a quasi-one-dimensional conductor

The angular dependent magnetoresistance of (Per)2[Au(mnt)2] under pressure has revealed geometrical effects associated with the crystallographic parameters. Pressure suppresses the charge density wave ground state of the material, and in the metallic state both geometrical and orbital quantum interference effects appear. Through magnetic field dependence and orientation, the orbital and geometrical effects are independently identified. We compare the results from (Per)2[Au(mnt)2] with previous studies of the well-known Bechgaard salts.

Crossover of angular dependent magnetoresistance with the metal-insulator transition in colossal magnetoresistive manganite films

The temperature and magnetic field dependence of angular dependent magnetoresistance (AMR) along two orthogonal directions ([100] and [01¯1]) was investigated in a charge-orbital-ordered Sm0.5Ca0.5MnO3 (SCMO) film grown on (011)-oriented SrTiO3 substrates. A dramatic decrease of AMR magnitude in both directions was observed with the appearance of magnetic-field-induced metal-insulator transition, which further led to a sign crossover in the AMR effect. The AMR crossover may give a direct evidence of the drastic modification of electronic structure or possible orbital reconstruction with the magnetic-destruction of charge/orbital ordering in SCMO films.

Three-valued magnetic tunnel junction for nonvolatile ternary content addressable memory application

Three-valued epitaxial Co50Fe50/MgO/Co50Fe50 magnetic tunnel junctions (MTJs) were developed for nonvolatile ternary content addressable memory (TCAM) application. Four remanent magnetization states in the single-crystalline Co50Fe50 electrode, due to cubic anisotropy with easy axes of the ⟨110⟩ directions, result in four possible angular-dependent tunnel magnetoresistance ratios. Three states selected from among the four states were separated by more than 56% at room temperature. Analysis of the asteroid curve for Co50Fe50 indicated that a magnetic field along 22.5° from the ⟨110⟩ directions made it possible to change the magnetization direction of the selected cell without disturbing those of the half-selected cells in the MTJ arrays. A nonvolatile TCAM cell using the three-valued MTJ for three-level data storage was proposed, and its operation was verified by circuit simulation. The TCAM cell reduces the device count to 5, a value 1/3 that of conventional complementary metal-oxide semiconductor-based TCAMs.

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.

Anisotropic scattering in angular-dependent magnetoresistance oscillations of quasi-two-dimensional and quasi-one-dimensional metals: Beyond the relaxation-time approximation

The electrical resistivity for a current moving perpendicular to layers (chains) in quasi-two-dimensional (2D) [quasi-one-dimensional (1D)] metals under an applied magnetic field of varying orientation is studied using Boltzmann transport theory. We consider the simplest nontrivial quasi-2D and quasi-1D Fermi surfaces but allow for an arbitrary elastic collision integral (i.e., a scattering probability with arbitrary dependence on momentum transfer) and obtain an expression for the resistivity which generalizes that previously found using a single relaxation-time approximation. The dependence of the resistivity on the angle between the magnetic field and current changes depending on the momentum dependence of the scattering probability. So, whereas zero-field intralayer transport is sensitive only to the momentum-averaged scattering probability (the transport relaxation rate), the resistivity perpendicular to layers measured in a tilted magnetic field provides detailed information about the momentum-dependence of interlayer scattering. These results help us to clarify the meaning of the relaxation rate determined from fits of angular-dependent magnetoresistance oscillations (AMROs) experimental data to theoretical expressions. Furthermore, we suggest how AMRO might be used to probe the dominant scattering mechanism.