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DOE OSTI2026

Collapse of Jahn-Teller phonons in La 1−x Sr x MnO 3 with weak magnetoresistance

Perovskite manganites are quantum materials exhibiting competing interactions inducing colossal magnetoresistance (CMR). The prevailing theory of CMR highlights the essential role of electron-phonon coupling (EPC), but mounting evidence suggests the underlying mechanism is more complicated. Here, we investigate phonons and spin-phonon coupling in ferromagnetic CMR manganites La 1−x Sr x MnO 3 (x=0.2,0.3) with relatively small CMR associated with melting of the magnetic order above room temperature. High-resolution neutron scattering experiments combined with density functional theory (DFT) show that the low-temperature ferromagnetic phase is conventional: neutron scattering from phonons agrees with DFT predictions and magnons follow sinusoidal dispersions. Fluctuating magnetic moments and low-energy phonons remain conventional in the high-temperature paramagnetic phase, indicating the Mn and La/Sr sublattices are not strongly perturbed by melting of ferromagnetism. In contrast, the Jahn–Teller-active optical oxygen vibrations collapse entirely above the Curie temperature, despite low CMR in these compositions, with some of the lost spectral weight reappearing as quasielastic scattering. We attribute this highly anomalous behavior to giant EPC in the charge and/or orbital channel. It drives cooperative diffusive motion of quasistatic carrier-trapping oxygen sublattice distortions once ferromagnetism disappears. We hypothesize the magnitude of magnetoresistance correlates with the rate of diffusion rather than with the strength of Jahn–Teller EPC.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators.

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi 2 Se 3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. Additionally, by applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (similar to 6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2026

Quadratic magnetoresistance across the insulator–metal transition in V4O7 thin films

We present a comparative magnetotransport study of V4O7 thin films grown simultaneously on single-crystal Al2O3 (r-cut) and amorphous SiO2 substrates, to examine substrate-induced effects on electronic transport across the insulator–metal transition (IMT). Temperature-dependent resistivity reveals a broad electronic ordering regime below the IMT, with both films exhibiting similar transition temperatures but differing by nearly an order of magnitude in absolute resistivity. Despite this strong substrate dependence of the resistivity scale, the magnetoresistance remains predominantly quadratic in magnetic field over wide temperature intervals for both substrates. Analysis of the quadratic magnetoresistance defines an effective mobility scale that evolves systematically across the IMT and exhibits a similar temperature dependence for the two films. Structural characterization, including atomic-resolution scanning transmission electron microscopy of the V4O7/Al2O3 interface and azimuthal X-ray diffraction measurements, reveals a sharp interface and well-defined in-plane crystallographic registry for the r-cut film, in contrast to the rotationally averaged in-plane structure expected for films grown on amorphous SiO2. These results indicate that while substrate-controlled structural registry modifies the resistivity scale, the quadratic magnetotransport response across the IMT remains largely robust, providing a consistent diagnostic of electronic transport in V4O7 thin films.

25 ENERGY STORAGE↗

DOE OSTI2024

Colossal magnetoresistance from spin-polarized polarons in an Ising system

Recent experiments suggest a new paradigm toward novel colossal magnetoresistance (CMR) in a family of materials EuM 2 X 2 (M = Cd, In, Zn; X = P, As), distinct from the traditional avenues involving Kondo–Ruderman–Kittel–Kasuya–Yosida crossovers, magnetic phase transitions with structural distortions, or topological phase transitions. Here, we use angle-resolved photoemission spectroscopy and density functional theory calculations to explore their origin, particularly focusing on EuCd 2 P 2 . While the low-energy spectral weight royally tracks that of the resistivity anomaly near the temperature with maximum magnetoresistance ( T MR ) as expected from transport-spectroscopy correspondence, the spectra are completely incoherent and strongly suppressed with no hint of a Landau quasiparticle. Using systematic material and temperature dependence investigation complemented by theory, we attribute this nonquasiparticle caricature to the strong presence of entangled magnetic and lattice interactions, a characteristic enabled by the p - f mixing. Given the known presence of ferromagnetic clusters, this naturally points to the origin of CMR being the scattering of spin-polarized polarons at the boundaries of ferromagnetic clusters. These results are not only illuminating to investigate the strong correlations and topology in EuCd 2 X 2 family, but, in a broader view, exemplify how multiple cooperative interactions can give rise to extraordinary behaviors in condensed matter systems.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Colossal transverse magnetoresistance due to nematic superconducting phase fluctuations in a copper oxide

Electronic anisotropy (“nematicity”) has been detected in cuprate superconductors by various experimental techniques. Using angle-resolved transverse resistance (ARTR) measurements, a very sensitive and background-free technique that can detect 0.5% anisotropy in transport, we have observed it also in La 2-x Sr x CuO 4 (LSCO) for 0.02 ≤ x ≤ 0.25. A central enigma in LSCO is the rotation of the nematic director (orientation of the largest longitudinal resistance) with temperature; this has not been seen before in any material. Here, we address this puzzle by measuring the angle-resolved transverse magnetoresistance (ARTMR) in LSCO. We report the discovery of colossal transverse magnetoresistance (CTMR)—an order-of-magnitude drop in the transverse resistivity in the magnetic field of 6 T. We show that the apparent rotation of the nematic director is caused by anisotropic superconducting fluctuations, which are not aligned with the normal electron fluid, consistent with coexisting bond-aligned and diagonal nematic orders. We quantify this by modeling the (magneto-)conductivity as a sum of normal (Drude) and paraconducting (Aslamazov–Larkin) channels but extended to contain anisotropic Drude and Cooper-pair effective mass tensors. Strikingly, the anisotropy of Cooper-pair stiffness is much larger than that of the normal electrons. It grows dramatically on the underdoped side, where the fluctuations become quasi-one-dimensional. Our analysis is general rather than model dependent. Still, we discuss some candidate microscopic models, including coupled strongly-correlated ladders where the transverse (interladder) phase stiffness is low compared with the longitudinal intraladder stiffness, as well as the anisotropic superconducting fluctuations expected close to the transition to a pair-density wave state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Extremely large magnetoresistance in high-mobility SrNbO 3 /SrTiO 3 heterostructures

In this report an extremely large linear magnetoresistance (LMR) is a ubiquitous phenomenon emerging from topological Dirac and Weyl semimetals. However, the connection between an LMR and a nontrivial topology is under extensive debate. In this paper, by precisely controlling the thickness of SrNbO 3 thin films grown on SrTiO 3 substrates, we observe an LMR over a large carrier density range with a magnetoresistance as high as 150000% at a carrier density n~10 21 cm -3 , far away from the quantum-limit regime. The temperature-, magnetic-field-, and carrier-density-dependent LMR in SrNbO 3 /SrTiO 3 heterostructures provides compelling evidence of a mobility-driven LMR in coherent electronic systems. Our results uncover the general principle of an LMR and shed light on proper categorization of transport properties in topological and correlated materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Magnetic Structure and Spin Fluctuation in Colossal Magnetoresistance Ferrimagnet Mn 3 Si 2 Te 6

The ferrimagnetic insulator Mn 3 Si 2 Te 6 , which features a Curie temperature T c at 78 K and a delicate yet consequential magnetic frustration, exhibits colossal magnetoresistance (CMR) when the magnetic field is applied along the magnetic hard axis, surprisingly inconsistent with existing precedents [Y. Ni, H. Zhao, Y. Zhang, B. Hu, I. Kimchi, and G. Cao, Phys. Rev. B 103, L161105 (2021)]. This discovery motivates a thorough single-crystal neutron diffraction study in order to gain insights into the magnetic structure and its hidden correlation with the new type of CMR. Here we report a noncollinear magnetic structure below the T c where the moments lie predominantly within the basal plane but tilt toward the c axis by ≈10° at ambient conditions. A substantial magnetic diffuse scattering decays slowly and persists well above the T c . The evolution of the spin correlation lengths agrees well with the electrical resistivity, underscoring the role of spin fluctuation contributing to the magnetoresistivity near the transition. Further, application of magnetic field along the c axis renders a swift occurrence of CMR but only a slow tilting of the magnetic moments toward the c axis. The unparalleled changes indicate a nonconsequential role of magnetic polarization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Magnetoresistance anomaly during the electrical triggering of a metal-insulator transition

Phase separation naturally occurs in a variety of magnetic materials and it often has a major impact on both electric and magnetotransport properties. In resistive switching systems, phase separation can be created on demand by inducing local switching, which provides an opportunity to tune the electronic and magnetic state of the device by applying voltage. Here we explore the magnetotransport properties in the ferromagnetic oxide La 0.7 ⁢Sr 0.3 ⁢MnO 3 (LSMO) during the electrical triggering of an intrinsic metal-insulator transition (MIT), which produces volatile resistive switching. This switching occurs in a characteristic spatial pattern, i.e., the formation of a high-resistance barrier perpendicular to the current flow, enabling an electrically actuated ferromagnetic-paramagnetic-ferromagnetic phase separation. At the threshold voltage of the MIT triggering, both anisotropic and colossal magnetoresistances exhibit anomalies including a large increase in magnitude and a sign flip. Computational analysis revealed that these anomalies originate from the coupling between the switching-induced phase separation state and the intrinsic magnetoresistance of LSMO. Furthermore, this work demonstrates that driving the MIT material into an out-of-equilibrium resistive switching state provides the means for electrical control of the magnetotransport phenomena.

Complex oxides↗

DOE OSTI2025

Large negative magnetoresistance in antiferromagnetic G⁡d 2 ⁢S⁢e 3

Rare earth chalcogenides provide a great platform to study exotic quantum phenomena such as superconductivity and charge density waves. Among various interesting properties, the coupling between magnetism and electronic transport has attracted significant attention. Here, we report the investigation of such coupling in α-G⁡d 2⁢ S⁢e 3 single crystals through magnetic, calorimetric, and transport property measurements. α-G⁡d 2 ⁢S⁢e 3 is found to display an antiferromagnetic ground state below 11 K with metamagnetic spin-flop transitions. The magnetic fluctuations remain strong above the transition temperature. Transport measurements reveal an overall metallic transport behavior with a large negative magnetoresistance of approximately 65% near the magnetic transition temperature, together with positive magnetoresistance near the field-induced spin-flop transitions, which can be understood in terms of the suppression of spin scattering by the magnetic field.

electrical properties↗

DOE OSTI2021

Interface roughness governed negative magnetoresistances in two-dimensional electron gases in AlGaN/GaN heterostructures

Negative magnetoresistances (NMRs) have been widely observed in two-dimensional electron gases (2DEGs). However, their origins are under debate. Here, we report on NMRs in the 2DEG in AlGaN/GaN heterostructures, aiming to uncover their origins by utilizing electric field gating. We systematically measured the magnetoresistances in magnetic fields up to 12 T and at temperatures between 3 and 260 K and observed NMRs over a wide range of temperatures from 3 to 170 K, which become more pronounced with decreasing temperature. We conducted electric field gating experiments to correlate the occurrence of NMRs with the relationship between electron mobility and density. The latter is governed by defect scattering sources in the sample and can be theoretically modeled. Comparison of the measured electron mobility and electron density relationship with theory reveals that interface roughness scattering plays a crucial role in obtaining large NMRs. Our work demonstrates that electric field gating provides a means not only to tune the values of NMRs but also to uncover their mechanisms.

2-dimensional systems↗

DOE OSTI2022

Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal films

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The Gd 3 + ion in GdSb has a high spin S=7/2 and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. Here, we present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular-beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band-structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density-functional theory (DFT) calculations. ARPES shows a hole-carrier rich, topologically trivial, semimetallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum-well states. DFT-predicted Fermi wave vectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole-carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Intergranular Spin Dependent Tunneling Dominated Magnetoresistance in Helimagnetic Manganese Phosphide Thin Films

Helical magnets are emerging as a novel class of materials for spintronics and sensor applications; however, research on their charge- and spin-transport properties in a thin film form is less explored. Herein, we report the temperature and magnetic field-dependent charge transport properties of a highly crystalline MnP nanorod thin film over a wide temperature range (2 K < T < 350 K). The MnP nanorod films of ~100 nm thickness were grown on Si substrates at 500 °C using molecular beam epitaxy. The temperature-dependent resistivity ρ(T) data exhibit a metallic behavior (dρ/dT > 0) over the entire measured temperature range. However, large negative magnetoresistance (Δρ/ρ) of up to 12% is observed below ~50 K at which the system enters a stable helical (screw) magnetic state. In this temperature regime, the Δρ(H)/ρ(0) dependence also shows a magnetic field-manipulated CONE + FAN phase coexistence. The observed magnetoresistance is dominantly governed by the intergranular spin dependent tunneling mechanism. These findings pinpoint a correlation between the transport and magnetism in this helimagnetic system.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Positive spin Hall magnetoresistance in single-crystalline Pt/CoO(001) bilayers

Here, the spin Hall magnetoresistance (SMR) effect in single-crystalline Pt/CoO(001) bilayers has been systematically investigated. X-ray magnetic linear dichroism measurements prove that CoO antiferromagnetic (AFM) spins can be switched into the direction orthogonal to the applied field. We find the SMR signal is comprised of two components related to either the switching of CoO AFM Néel order or the applied strong field effect. Both SMR components show a “positive” angular dependence with $R_{∥} > R_{⊥}$, while $R_{∥} (R_{⊥})$ is defined as the resistance with the applied in-plane field parallel (perpendicular) to the current. The observed positive SMR is mainly attributed to the uncompensated spins at the Pt/CoO interface, instead of the CoO AFM spins. Our study may attract a great deal of interest to understand the complicated SMR effect in AFM spintronics materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Electrically Controlled All-Antiferromagnetic Tunnel Junctions on Silicon with Large Room-Temperature Magnetoresistance

Antiferromagnetic (AFM) materials are a pathway to spintronic memory and computing devices with unprecedented speed, energy efficiency, and bit density. Realizing this potential requires AFM devices with simultaneous electrical writing and reading of information, which are also compatible with established silicon-based manufacturing. Recent experiments have shown tunneling magnetoresistance (TMR) readout in epitaxial AFM tunnel junctions. However, these TMR structures are not grown using a silicon-compatible deposition process, and controlling their AFM order required external magnetic fields. Here are shown three-terminal AFM tunnel junctions based on the noncollinear antiferromagnet PtMn 3 , sputter-deposited on silicon. The devices simultaneously exhibit electrical switching using electric currents, and electrical readout by a large room-temperature TMR effect. First-principles calculations explain the TMR in terms of the momentum-resolved spin-dependent tunneling conduction in tunnel junctions with noncollinear AFM electrodes.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Giant Modulation of Magnetoresistance in a Van Der Waals Magnet by In‐Plane Current Injection

Efficient magnetization control is a central issue in magnetism and spintronics. Particularly, there are increasing demands for manipulation of magnetic states in van der Waals (vdW) magnets with unconventional functionalities. However, the electrically induced phase transition between ferromagnetic-to-antiferromagnetic states without external magnetic field is yet to be demonstrated. Here, the current-induced magnetic phase transition in a vdW ferromagnet Fe 5 GeTe 2 is reported. Based on magneto-transport measurements and theoretical analysis, it is demonstrated that transition in the interlayer magnetic coupling occurs through vertical voltage drop between layers induced by current which is attributed to high anisotropy of the resistivity caused by the vdW gaps. Such magnetic phase transition results in giant modulation of the longitudinal magnetoresistance from 5% to 170%. The electrical tunability of the magnetic phase in Fe 5 GeTe 2 with current-in-plane geometry opens a path for electric control of magnetic properties, expanding the ability to use vdW magnets for spintronic applications.

2D magnet↗

DOE OSTI2022

Enhancement of the Magnetoresistance in the Mobility‐Engineered Compensated Metal Pt 5 P 2

Abstract The magnetoresistance (MR) in nonmagnetic materials continues to be a fertile research area in materials science. The search for giant, positive MR has been limited to a rather small window of materials such as high‐mobility semimetals in single‐crystalline form. Here, the observation of a very large positive MR in metallic Pt 5 P 2 in polycrystalline form is reported. The observations reveal that improvement of the crystallinity results in a significant enhancement of the positive MR, exceeding 10 000% at 9 T, comparable to high‐mobility semimetals. Based on first‐principles calculations combined with magnetotransport and thermoelectric measurements, the Fermi surface of Pt 5 P 2 is found to consist of a collection of multiple electron and hole pockets compensating one another, along with a characteristic pocket continuously connected to the adjacent Brillouin zone, together with possible topologically protected band crossings. This work extends the landscape of high MR candidate materials to polycrystalline metals, which demonstrates the importance of crystallinity and purity of the samples for the optimization of the MR.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Manipulation of the Magnetoresistance by Strain in Topological TaSe 3

1D TaSe 3 exhibits many unusual physical properties due to its distorted type-II chains. Ribbon-shaped single crystals can be easily bended along the $b-axis$, forming rings. This study investigates the magnetoresistance (MR) of TaSe 3 up to 60 T in both unbended (ribbon shape) and ring-shaped (bended ribbon) samples. Notable changes are found in the magnetotransport properties between the two different shaped samples. One is that the MR in ring-shaped samples is three orders lower than that in unbended samples under the same sample environment. In addition, linear MR is observed above ≈20 T in ring-shaped samples when the magnetic field is parallel or perpendicular to the rings. Quantum oscillations are also observed as a function of the magnetic field when the magnetic field is applied parallel to rings, possibly due to the Altshuler–Aronov–Spivak effect or the inversion of the lowest Landau level beyond the quantum limit. All these results are related to strain-induced electronic structure change in TaSe 3 , an effective way to tune physical properties in low-dimensional materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2025

Large non-saturating Nernst thermopower and magnetoresistance in compensated semimetal ScSb

Today, high-performance thermoelectric and thermomagnetic materials operating in the low-temperature regime, particularly below the boiling point of liquid nitrogen remain scarce. Most thermomagnetic materials reported to date exhibit a strong Nernst signal along specific crystallographic directions in their single-crystal form. However, their performance typically degrades significantly in the polycrystalline form. Here, we report an improved Nernst thermopower of ~ 128 μV/K at 30 K and 14 T in polycrystalline compensated semimetal ScSb, in comparison to that was observed in single crystal ScSb previously. The magnetic field dependence of Nernst thermopower shows a linear and non-saturating behavior up to 14 T. The maximum Nernst power factor reaches to ~ 240 x 10 -4 W m −1 K −2 and Nernst figure of merit reaches to ~ 11 x 10 -4 K −1 . Polycrystalline ScSb also shows a large non-saturating magnetoresistance of ~ 940% at 2 K and 14 T. These enhanced properties originate from better electron–hole compensation, as revealed by Hall resistivity measurements. In conclusion, the cubic symmetry and absence of anisotropy in ScSb allow its polycrystalline form to achieve similar enhanced thermomagnetic and electromagnetic performance comparable to that of the single crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗