Magnetoresistance Measurements Research Articles

Lateral Mn5Ge3 spin-valve in contact with a high-mobility Ge two-dimensional hole gas

Abstract Ge two-dimensional hole gases (2DHG) in strained modulation-doped quantum-wells represent a promising material platform for future spintronic applications due to their excellent spin transport properties and the theoretical possibility of efficient spin manipulation. Due to the continuous development of epitaxial growth recipes extreme high hole mobilities and low effective masses can be achieved, promising an efficient spin transport. Furthermore, the Ge 2DHG can be integrated in the well-established industrial complementary metal-oxide-semiconductor (CMOS) devices technology. However, efficient electrical spin injection into a Ge 2DHG—an essential prerequisite for the realization of spintronic devices—has not yet been demonstrated. In this work, we report the fabrication and low-temperature magnetoresistance (MR) measurements of a laterally structured Mn5Ge3/Ge 2DHG/ Mn5Ge3 device. The ferromagnetic Mn5Ge3 contacts are grown directly into the Ge quantum well by means of an interdiffusion process with a spacing of approximately 130 nm, forming a direct electrical contact between the ferromagnetic metal and the Ge 2DHG. Here, we report for the first time a clear MR signal for temperatures below 13 K possibly arising from successful spin injection into the high mobility Ge 2DHG. The results represent a step forward toward the realization of CMOS compatible spintronic devices based on a 2DHG.

A concise 40 T pulse magnet for condensed matter experiments

There is a growing interest in using pulsed high magnetic field as a controlling parameter of physical phenomena in various scientific disciplines, such as condensed matter physics, particle physics, plasma physics, chemistry, and biological studies. We devised a concise and portable pulsed magnetic field generator that produces a 40 T field with a pulse duration of 2 ms. It is assembled using only off-the-shelf components and a homemade coil that leverages small computers, Raspberry Pi, and Python codes. It allows for straightforward modification for general purposes. As working examples, we show representative applications in condensed matter experiments of magnetoresistance, magnetization, and magnetostriction measurements for graphite, NdNi2P2, and NdCo2P2, respectively, with the maximum magnetic field of 41 T and the lowest temperature of 4.2 K.

Separate magnetic and structural phase transitions in Mn50-xFexRh50 films grown on MgO

Abstract The investigation of the magnetic and structural characteristic of Mn50-xFexRh50 alloy is of physical importance due to the phase transition behavior. In this study, a series of highly ordered epitaxial films with varying Fe concentrations are grown on MgO (001) substrate. At low Fe concentrations (x=0, 2, 6), a separation between the structural phase transition and magnetic phase transition is observed. Different with the structural phase transition, the temperature-dependent magnetization presents a quite large temperature hysteresis behavior. In addition, the structural transition induces further tetragonal distortion and forms intermediate phase between B2 and L10 structure. This separated magnetic and structural phase transition has been further validated through XRD, anisotropic magnetoresistance and spin pumping measurements. Moreover, with the further increase in the Fe concentration, the Mn50-xFexRh50 films shows ferromagnetic behaviors due to the competitive magnetic exchange interaction, while the structural phase transition is suppressed.

Temperature-Dependent Surface Anisotropy in (110) Epitaxial Rare Earth Iron Garnet Films.

Ferrimagnetic oxide thin films are important material platforms for spintronic devices. Films grown on low symmetry orientations such as (110) exhibit complex anisotropy landscapes that can provide insight into novel phenomena such as spin-torque auto-oscillation and spin superfluidity. Using spin-Hall magnetoresistance measurements, the in-plane (IP) and out-of-plane (OOP) uniaxial anisotropy energies are determined for a thickness series (5-50nm) of europium iron garnet (EuIG) and thulium iron garnet (TmIG) films epitaxially grown on a gadolinium gallium substrate with (110) orientation and capped with Pt. Pt/EuIG/GGG exhibits an (001) easy plane of magnetization perpendicular to the substrate, whereas Pt/TmIG/GGG exhibits an (001) hard plane of magnetization perpendicular to the substrate with an IP easy axis. Both IP and OOP surface anisotropy energies comparable in magnitude to the bulk anisotropy are observed. The temperature dependence of the surface anisotropies is consistent with first-order predictions of a simplified Néel surface anisotropy model. By taking advantage of the thickness and temperature dependence demonstrated in these ferrimagnetic oxides grown on the low symmetry (110) orientations, the complex anisotropy landscapes can be tuned to act as a platform to explore rich spin textures and dynamics.

Universal correlation between H-linear magnetoresistance and T-linear resistivity in high-temperature superconductors

The signature feature of the ‘strange metal’ state of high-Tc cuprates—its linear-in-temperature resistivity—has a coefficient α1 that correlates with Tc, as expected were α1 derived from scattering off the same bosonic fluctuations that mediate pairing. Recently, an anomalous linear-in-field magnetoresistance (=γ1H) has also been observed, but only over a narrow doping range, leaving its relation to the strange metal state and to the superconductivity unclear. Here, we report in-plane magnetoresistance measurements on three hole-doped cuprate families spanning a wide range of temperatures, magnetic field strengths and doping. In contrast to expectations from Boltzmann transport theory, γ1 is found to correlate universally with α1. A phenomenological model incorporating real-space inhomogeneity is proposed to explain this correlation. Within this picture, superconductivity in hole-doped cuprates is governed not by the strength of quasiparticle interactions with a bosonic bath, but by the concentration of strange metallic carriers.

Influence of demagnetizing effects on the magnetization curves of finite-size rectangular slabs

According to some recent studies, the magnetoresistance curves of ferromagnetic strip-shaped samples can significantly differ depending on whether the in-plane external applied magnetic field H is oriented in parallel to either the long or the short edge of the strip. To address this problem, in the present work magnetization curves M(H) were measured for similarly shaped samples with both magnetic field orientations used in the magnetoresistance measurements. It was found that the M(H) curves strongly depend on the saturation magnetization and shape of the samples as well as on the magnetic field orientations. For some samples with sufficiently large saturation magnetization, the effective demagnetizing factors could be deduced from the measured M(H) curves. By considering the investigated samples as a ferromagnetic slab, and approximating them with a general ellipsoid, the demagnetizing factors were calculated from known formulae and compared to the experimental values. A fairly good matching was observed, although the latter data were systematically slightly larger, certainly due to the not completely homogeneous magnetization within the rectangular slab as opposed to the case of a general ellipsoid. The differences in the M(H) curves for the two orientations of the magnetic field could be completely attributed to demagnetizing effects.

Tilted-chiral-state-induced topological Hall effect in chiral magnetic soliton host Cr1/3TaS2

Chromium-intercalated Cr1/3TaS2 is well known for hosting a nontrivial chiral magnetic soliton lattice (CSL) with the reported highest Curie temperature (TC∼151 K) and strongest spin–orbit coupling, which has significant applications in gigahertz and high-speed spintronic devices. Herein, we thoroughly investigate the magneto-electrical transport properties of Cr1/3TaS2 single crystals. For H//ab, our magnetoresistance (MR) measurements reveal distinctive step-like behaviors, which are attributed to the formation and annihilation of chiral magnetic solitons. When under an oblique field, similar but weaker MR behaviors are observed compared to H//ab, indicating the appearance of an inclination-field-induced tilted-CSL. A topological Hall effect is observed under an oblique field, which is suggested to be induced by a nonzero topological charge density resulting from the tilted chiral states. The Cr1/3TaS2 offers an intriguing platform for studying the impact of chiral magnetic structures on magneto-electrical properties, which holds promise for both future spintronic device applications and fundamental investigation.

Anisotropic magnetoresistance of GaMnAs:Be

The effect of co-doping with Be on the magnetic anisotropy in Ga0.972Mn0.028As epitaxial layers has been studied by magnetoresistance measurements. Co-doping with Be has been shown to lead to reorientation of both easy and hard magnetic axes in GaMnAs. Measurements of the temperature dependence of the anisotropic magnetoresistance demonstrate no changes in the type of the magnetic anisotropy with the increase in temperature. The results of the study of the anisotropic magnetoresistance indicate that the parameters of the magnetic anisotropy in GaMnAs are significantly influenced by the magnitude of the compressive strain.

Weyl fermion excitations in the ideal Weyl semimetal CuTlSe2

An ideal Weyl semimetal is characterized by a dispersion in which only Weyl cones intersect the Fermi level, with low-energy behavior being governed by Weyl fermions. Although ideal Weyl semimetals have long been anticipated, only a few are realized in nonmagnetic materials. In this study, we confirm the presence of Weyl-fermion excitations in the ideal Weyl semimetal CuTlSe2 via a combination of magnetoresistance, Hall-effect, magnetic-susceptibility, nuclear magnetic resonance (NMR), and muon-spin relaxation (µSR) experiments. Magnetoresistance measurements reveal a negative longitudinal magnetoresistance (LMR), which scales as B2, while Hall-effect results indicate a predominant contribution from Weyl fermions with a hole-type charge. Magnetic susceptibility and µSR measurements indicate the lack of any intrinsic spontaneous magnetic moments down to base temperature. Finally, the NMR results can be modeled by a two-component effective Hamiltonian, which reproduces well the temperature-dependent Cu63 NMR (T1T)−1 factor, shown to scale as T2 below 100 K and as T1 above 100 K. Overall, we find that the extremely low concentration (1017cm−3) of carriers in CuTlSe2 originates from an ideal nonmagnetic Weyl semimetallic state, persisting up to a thermal excitation energy of 9 meV (100 K), above which trivial electronic bands close to EF take over. Our findings highlight CuTlSe2 as a new member of the intriguing class of Weyl semimetals. Published by the American Physical Society 2024

Evidence for spin reorientation transition in antiferromagnetic FeRh

FeRh is a unique intermetallic compound known for its temperature-induced phase transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state, making it a promising candidate for AFM memory applications. In this study, we discovered that FeRh exhibits a temperature-driven spin reorientation transition in the AFM phase. Our investigation using magnetoresistance (MR) measurements reveals that the Néel vector changes from an in-plane direction at high temperatures to an out-of-plane direction at low temperatures. This spin reorientation can be effectively detected through MR, offering a more practical approach for applications. Our findings are crucial for the emerging field of AFM memory, as controlling and identifying the Néel vector is vital for defining the memory state in AFM-based data storage technologies.

Giant Anisotropic Magnetoresistance in Few-Layer α-RuCl3 Tunnel Junctions.

The spin-orbit-assisted Mott insulator α-RuCl3 is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl3 crystals with various layer numbers to probe their magnetic, electronic, and crystal structures. We observe a giant change in resistance, as large as ∼2500%, when the magnetic field rotates either within or out of the α-RuCl3 plane, a manifestation of the strongly anisotropic spin interactions in this material. In combination with scanning transmission electron microscopy, this tunneling anisotropic magnetoresistance (TAMR) reveals that few-layer α-RuCl3 crystals remain in the high-temperature monoclinic phase at low temperatures. It also shows the presence of a zigzag antiferromagnetic order below the critical temperature TN ≃ 14 K, which is twice the one typically observed in bulk samples with rhombohedral stacking. Our work offers valuable insights into the relation between the stacking order and magnetic properties of this material, which helps lay the groundwork for creating and electrically probing exotic magnetic phases such as QSLs via van der Waals engineering.

Promoting Surface Conduction through Scalable Structure Engineering of Flexible Topological Insulator Thin Films

AbstractSpintronics, micro/nanofabrication, and the semiconductor industry are undergoing significant transformations, highlighting the need for flexible technologie. This study examines the possibility of integrating topological insulators (TIs), specifically Bi2Te3 thin films (13–191 nm), into flexible spintronic applications through scalable magneto‐sputtering techniques, and investigates how structural organization influences electrical and topological properties through post‐thermal annealing. Films annealed above 250 °C display improved polycrystalline structure, larger crystallites, fewer local defects, and higher a room‐temperature Seebeck coefficient (S) and resistivity (ρ), suggesting decreased bulk electronic contributions. A metastable transport regime is identified in the temperature‐dependent resistivity of films annealed at 300 °C between 20–100 K; in this range, the thinnest film exhibits markedly metallic behavior, signaling the presence of topological surface states (TSS). Magnetoresistance measurements of as‐grown and annealed films, between 3–20 K, demonstrate weak antilocalization. Applying the Hikami‐Larkin‐Nagaoka model, α‐coefficients are found to scale with thickness from 0.5–1, indicating thickness‐controlled coupling of the TSS. Post‐annealing at 300 °C, the phase coherence length, Lϕ, of the thinner films increases, while the temperature dephasing rate shifts from ∼0.7 (as‐grown) to ∼0.5 (post‐annealing), aligning with expected values for 2D TSS. These are encouraging findings for large‐scale manufacturing flexible TI technologies, without sacrificing tunabilit.

Effects of electronic correlation on topological properties of Kagome semimetal Ni3In2S2.

Kagome metals gain attention as they manifest a spectrum of quantum phenomena such as superconductivity, charge order, frustrated magnetism, and allied correlated states of condensed matter. With regard to electronic band structure, several of them exhibit non-trivial topological characteristics. Here, we present a thorough investigation on the growth and the physical properties of single crystals of Ni3In2S2 which is established to be a Dirac nodal line Kagome semimetal. Extensive characterization is attained through temperature and field-dependent resistivity, angle-dependent magnetoresistance and specific heat measurements. The central question we seek to address is the effect of electronic correlations in suppressing the manifestation of topological characteristics. In most metals, the Fermi liquid behaviour is restricted to a narrow range of temperatures. Here we show that Ni3In2S2 follows the Fermi-liquid behaviour up to 86K. This phenomenon is further supported by a high Kadowaki-Woods ratio obtained through specific heat analysis. Different interpretations of the magneto-transport study reveal that magnetoresistance exhibits linear behavior, suggesting the presence of Dirac semi-metallic signatures at lower temperatures. The angle-dependent magneto-transport study obeys the Voigt-Thomson formula. This, on the contrary, implies the classical origin of magnetoresistance. Thus, the effect of strong electron correlation in Ni3In2S2 manifests itself in the anisotropic magneto-transport. Furthermore, the magnetization measurement shows the presence of de-Haas van Alphen oscillations. Calculations of Berry phase provide insights into the topological features in the Kagome semimetal Ni3In2S2.&#xD.

Vortex matching at 6 T in YBa2Cu3O7−δ thin films by imprinting a 20-nm periodic pinning array with a focused helium-ion beam

Controlled engineering of vortex pinning sites in copper-oxide superconductors is a critical issue in manufacturing devices based on magnetic flux quanta. To address this, we employed a focused He-ion beam (He-FIB) to irradiate thin YBa2Cu3O7−δ films and create ultradense hexagonal arrays of defects with lattice spacings as small as 20 nm. Critical current and magnetoresistance measurements demonstrate efficient pinning by a matching field of 6 T visible in a huge temperature range from the critical temperature Tc down to 2 K. These results show that He-FIB irradiation provides excellent opportunities for the development and application of superconducting fluxonic devices based on Abrikosov vortices. In particular, our findings suggest that such devices can operate at temperatures far below Tc, where superconductivity is robust. Published by the American Physical Society 2024

Changing of the excess conductivity near the irreversibility temperature of the top seeded YBa2Cu3O7−x

A top seeded YBa2Cu3O7-x (YBCO) superconductor was grown using a bulk Nd123 seed crystal. Orientation peaks directed in the c-axis were observed from the X-ray diffraction measurement for the Y123 sample, indicating single crystal behavior. Magneto-resistivity measurements were carried out as a function of temperature under applied magnetic fields up to 4 T. The superconducting transition temperature (Tc) of the sample was obtained to be approximately 93 K and the excess conductivity curve was obtained experimentally from the resistivity measurement at zero magnetic field. The critical exponents were found to be λsfw = 2.26 ± 0.25, λ3D = 0.47 ± 0.03 and λ2D = 0.82 ± 0.07 for the short wave fluctuation (SWF) region and the mean field region characterized by 3D and 2D fluctuations, respectively. The crossover temperature determined from the crossover from 2D to 3D regime was found to be 96.37 K. The irreversible temperatures determined from the magneto-resistivity measurements and the irreversibility field line of the sample was obtained using the giant flux creep (gfc) model. The sample exhibits a behavior consistent with the gfc and the vortex glass model according to the n parameter calculated from the resistivity temperature measurements. The width enlarges with the applied magnetic field and was found to be approximately 4 K at 4 T for the Y123 sample. The sample was in the 3D regime below 96.37 K, while the sample was in the 2D regime between 96.37 K and 101.35 K. On the other hand, the reversible region was confined between 93.95 K and 93.33 K at 0 T. It was observed that 3D fluctuations are dominant in the reversible region, whereas 2D fluctuations are dominant in the irreversible region.

Characterization of induced quasi-two-dimensional transport in n-type InxGa1−xAs1 − yBiy bulk layer

The temperature-dependent transport properties of n-type InGaAsBi epitaxial alloys with various doping densities are investigated by conducting magnetoresistance (MR) and Hall Effect (HE) measurements. The electronic band structure of the alloys and free electron distribution were calculated using Finite Element Method (FEM). Analysis of the oscillations in the transverse (Hall) resistivity shows that quasi-two-dimensional electron gas (Q-2D) in the bulk InGaAsBi epitaxial layer (three-dimensional, 3D) forms at the sample surface under magnetic field even though there is no formation of the spacial two-dimensional electron gas (2DEG) at the interface between InGaAs and InP:Fe interlayer. The formation of Q-2D in the 3D epitaxial layer was verified by temperature and magnetic field dependence of the resistivity and carrier concentration. Analysis of Shubnikov-de Haas (SdH) oscillations in longitudinal (sample) resistivity reveals that the electron effective mass in InGaAsBi alloys are not affect by Bi incorporation into host InGaAs alloys, which verifies the validity of the Valence Band Anti-Crossing (VBAC) model. The Hall mobility of the nondegenerate samples shows the conventional 3D characteristics while that of the samples is independence of temperature for degenerated samples. The scattering mechanism of the electrons at low temperature is in long-range interaction regime. In addition, the effects of electron density on the transport parameters such as the effective mass, and Fermi level are elucidated considering bandgap nonparabolicity and VBAC interaction in InGaAsBi alloys.

Crystal structure, magnetic and electrical transport properties of titanium-doped half-Heusler alloys Ni1−xTixMnSb

Ni[Formula: see text]TixMnSb polycrystalline alloys in the range [Formula: see text] were synthesized employing the standard solid-phase synthesis method. The crystal, magnetic, as well as electrical properties of the alloys were investigated using neutron powder diffraction and magneto-resistance measurements. The results reveal that the substitution of Ti for Ni within the temperature range of 2.5–300 K does not induce alterations in the crystal and magnetic structures of NiMnSb. The ordered Ni magnetic moment approaches zero. The study of the electrical transport properties of the Ni[Formula: see text]TixMnSb alloys, where [Formula: see text], has demonstrated a half-metallic state at low temperatures and metallic conductivity for temperatures exceeding 160 K. A semiconductor state manifests at titanium concentrations of x = 0.2 and was observed at temperatures below 21 K. The obtained experimental results are elucidated through first-principles theoretical calculations.

A new ferromagnetic semiconductor system of Eu[formula omitted]Sr[formula omitted]AgP [formula omitted] compounds: Crystallographic, magnetic, and magneto-resistive properties

Adjusting chemical pressure through doping is a highly effective method for customizing the chemical and physical properties of materials, along with their respective phase diagrams, thereby uncovering novel quantum phenomena. Here, we successfully synthesized Sr-doped Eu1−xSrxAgP (x=0.0−0.6) and conducted a comprehensive investigation involving crystallography, magnetization, heat capacity, and magnetoresistance. Utilizing X-ray diffraction and PPMS DynaCool measurements, we studied Eu1−xSrxAgP in detail. The hexagonal structure of parent EuAgP at room temperature, with the P63/mmc space group, remains unaltered, while the lattice constants expand. The magnetic phase transition from paramagnetism to weak ferromagnetism, as temperature decreases, is suppressed through the gradual introduction of strontium doping. Heat capacity measurements reveal a shift from magnon-dominated to predominantly phonon and electron contributions near the ferromagnetic phase with increasing doping levels. The resistivity–temperature relationship displays distinct characteristics, emphasizing the impact of Sr doping on modifying charge transport. Magnetoresistance measurements uncover novel phenomena, illustrating the adjustability of magnetoresistance through Sr doping. Notably, Sr doping results in both positive magnetoresistance (up to 20%) and negative magnetoresistance (approximately -60%). The resistivity and magnetic phase diagram were established for the first time, revealing the pronounced feasibility of Sr doping in modulating EuAgP’s resistivity. This study has provided valuable insights into the intricate interplay between structural modifications and diverse physical properties. The potential for technological advancements and the exploration of novel quantum states make Sr-doped Eu1−xSrxAgP a compelling subject for continued research in the field of applied physics.

Thickness dependence of Morin transition of Ru-doped α-Fe2O3 films detected by spin Hall magnetoresistance measurements

We conducted spin Hall magnetoresistance (SMR) measurements to investigate the Ru-doping effect on the Morin transition of α-Fe2O3(0001) films, which is the transition from the low-temperature antiferromagnetic state with c-axis magnetic moments to the high-temperature weak ferromagnetic state with c-plane magnetic moments, under a thickness of 6–100 nm. We clarified that the Morin temperature of the 2.6 at. % Ru-doped α-Fe2O3 film was greater than 400 K when the thickness was 100 nm and decreased with a decrease in thickness. Our results demonstrated that SMR measurements are a valid verification method for the Morin transition of very thin α-Fe2O3 films down to several nm.

Excess conductivity and magnetoresistance analysis for (BSF)x/(Bi, Pb)-2223 composite

This study examined the impact of adding hard ferrite Ba0.5Sr0.5Fe12O19 (BSF) nanoparticles to the Bi1.8Pb0.4Sr2Ca2Cu3.2O10+δ (Bi, Pb)-2223) superconductor phase. The investigation specifically focused on evaluating the critical current density, fluctuation-induced conductivity, and magnetoresistance of nano-(BSF)x/(Bi, Pb)-2223 composite, where 0.00 ≤ x ≤ 0.20 wt.%. The results revealed that the critical current density, Jc, increased with the addition of nano-(BSF) up to x = 0.04 wt.%, reaching a value of 441.20 A/cm2. The Aslamazov and Larkin (A–L) approach has been evaluated the fluctuation-induced conductivity. Several superconducting parameters, including coherence length ζc(0), effective layer thickness d, penetration depth λpd(0), and Fermi energy EF\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${E}_{F}$$\\end{document} showed improvement as the concentration of nano-(BSF) increased up to x = 0.04 wt.%. In addition to Ginzburg–Landau critical parameters, such as the thermodynamic critical field Bc(0), lower critical magnetic field Bc1(0), upper critical magnetic field Bc2(0), and critical current density Jc(0) demonstrated an increase up to x = 0.04 wt.%, followed by a decrease for higher concentrations. The magnetoresistance measurements were performed at various applied DC magnetic fields, with values ranging from 0.29 to 4.44 kG, and were analyzed using the thermally activated flux creep (TAFC) and Ambegaokar–Halperin (AH) models. The calculated flux pinning energy (U) increased with the addition of nano-(BSF) up to x = 0.04 wt.% and then decreased for x > 0.04 wt.%. Furthermore, the transition width (ΔT), was observed to increase as the applied magnetic field values increased. Moreover, the addition of nano-(BSF) increased the field-independent critical current density, Jc,0(0), up to x = 0.04 wt.%, after which it decreased for higher concentrations.