Showing papers on "Ferromagnetism published in 2022"

Light-induced ferromagnetism in moiré superlattices

Abstract: Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moiré superlattices have emerged as a promising platform for quantum engineering such phenomena1-3. The power of the moiré system lies in the high tunability of its physical parameters by adjusting the layer twist angle1-3, electrical field4-6, moiré carrier filling7-11 and interlayer coupling12. Here we report that optical excitation can highly tune the spin-spin interactions between moiré-trapped carriers, resulting in ferromagnetic order in WS2 /WSe2 moiré superlattices. Near the filling factor of -1/3 (that is, one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moiré-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré-trapped holes9, and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter13-19.

Unconventional superconductivity in UTe₂

Abstract: The novel spin-triplet superconductor candidate UTe2was discovered only recently at the end of 2018 and already attracted enormous attention. We review key experimental and theoretical progress which has been achieved in different laboratories. UTe2is a heavy-fermion paramagnet, but following the discovery of superconductivity, it has been expected to be close to a ferromagnetic instability, showing many similarities to the U-based ferromagnetic superconductors, URhGe and UCoGe. This view might be too simplistic. The competition between different types of magnetic interactions and the duality between the local and itinerant character of the 5fUranium electrons, as well as the shift of the U valence appear as key parameters in the rich phase diagrams discovered recently under extreme conditions like low temperature, high magnetic field, and pressure. We discuss macroscopic and microscopic experiments at low temperature to clarify the normal phase properties at ambient pressure for field applied along the three axis of this orthorhombic structure. Special attention will be given to the occurrence of a metamagnetic transition atHm= 35 T for a magnetic field applied along the hard magnetic axisb. Adding external pressure leads to strong changes in the magnetic and electronic properties with a direct feedback on superconductivity. Attention is paid on the possible evolution of the Fermi surface as a function of magnetic field and pressure. Superconductivity in UTe2is extremely rich, exhibiting various unconventional behaviors which will be highlighted. It shows an exceptionally huge superconducting upper critical field with a re-entrant behavior under magnetic field and the occurrence of multiple superconducting phases in the temperature-field-pressure phase diagrams. There is evidence for spin-triplet pairing. Experimental indications exist for chiral superconductivity and spontaneous time reversal symmetry breaking in the superconducting state. Different theoretical approaches will be described. Notably we discuss that UTe2is a possible example for the realization of a fascinating topological superconductor. Exploring superconductivity in UTe2reemphasizes that U-based heavy fermion compounds give unique examples to study and understand the strong interplay between the normal and superconducting properties in strongly correlated electron systems.

Field-free superconducting diode effect in noncentrosymmetric superconductor/ferromagnet multilayers

Abstract: The diode effect is fundamental to electronic devices and is widely used in rectifiers and a.c.–d.c. converters. At low temperatures, however, conventional semiconductor diodes possess a high resistivity, which yields energy loss and heating during operation. The superconducting diode effect (SDE)1–8, which relies on broken inversion symmetry in a superconductor, may mitigate this obstacle: in one direction, a zero-resistance supercurrent can flow through the diode, but for the opposite direction of current flow, the device enters the normal state with ohmic resistance. The application of a magnetic field can induce SDE in Nb/V/Ta superlattices with a polar structure1,2, in superconducting devices with asymmetric patterning of pinning centres9 or in superconductor/ferromagnet hybrid devices with induced vortices10,11. The need for an external magnetic field limits their practical application. Recently, a field-free SDE was observed in a NbSe2/Nb3Br8/NbSe2 junction; it originates from asymmetric Josephson tunnelling that is induced by the Nb3Br8 barrier and the associated NbSe2/Nb3Br8 interfaces12. Here, we present another implementation of zero-field SDE using noncentrosymmetric [Nb/V/Co/V/Ta]20 multilayers. The magnetic layers provide the necessary symmetry breaking, and we can tune the SDE by adjusting the structural parameters, such as the constituent elements, film thickness, stacking order and number of repetitions. We control the polarity of the SDE through the magnetization direction of the ferromagnetic layers. Artificially stacked structures13–18, such as the one used in this work, are of particular interest as they are compatible with microfabrication techniques and can be integrated with devices such as Josephson junctions19–22. Energy-loss-free SDEs as presented in this work may therefore enable novel non-volatile memories and logic circuits with ultralow power consumption. Superconducting diodes, which can operate without dissipation losses at low temperature, usually require a magnetic field to function. A well-designed multilayer device now shows a reversible, non-volatile superconducting diode effect.

Phase engineering of Cr5Te8 with colossal anomalous Hall effect

Abstract: Two-dimensional materials that are intrinsically ferromagnetic are crucial for the development of compact spintronic devices. However, most non-layered 2D magnets with a strong ferromagnetic order are difficult to synthesize. Here we show that the flakes of trigonal and monoclinic Cr5Te8 can be grown via a chemical vapour deposition method. Using magneto-optical and magnetotransport measurements, we show that both phases exhibit robust ferromagnetism with strong perpendicular anisotropy at thicknesses of a few nanometres. A high Curie temperature of up to 200 K can be obtained by manipulating the phase structure and thickness. We also observe a colossal anomalous Hall effect in the more structurally distorted monoclinic Cr5Te8, with an anomalous Hall conductivity of 650 Ω−1 cm−1 and anomalous Hall angle of 5%. Few-nanometre-thick flakes of trigonal and monoclinic Cr5Te8 can be grown using chemical vapour deposition, with the monoclinic phase exhibiting an anomalous Hall conductivity of 650 Ω–1 cm–1 and anomalous Hall angle of 5%.

Composition and phase engineering of metal chalcogenides and phosphorous chalcogenides

Abstract: Two-dimensional (2D) materials with multiphase, multielement crystals such as transition metal chalcogenides (TMCs) (based on V, Cr, Mn, Fe, Cd, Pt and Pd) and transition metal phosphorous chalcogenides (TMPCs) offer a unique platform to explore novel physical phenomena. However, the synthesis of a single-phase/single-composition crystal of these 2D materials via chemical vapour deposition is still challenging. Here we unravel a competitive-chemical-reaction-based growth mechanism to manipulate the nucleation and growth rate. Based on the growth mechanism, 67 types of TMCs and TMPCs with a defined phase, controllable structure and tunable component can be realized. The ferromagnetism and superconductivity in FeXy can be tuned by the y value, such as superconductivity observed in FeX and ferromagnetism in FeS2 monolayers, demonstrating the high quality of as-grown 2D materials. This work paves the way for the multidisciplinary exploration of 2D TMPCs and TMCs with unique properties.

Spin-orbit–driven ferromagnetism at half moiré filling in magic-angle twisted bilayer graphene

Abstract: Strong electron correlation and spin-orbit coupling (SOC) can have a profound influence on the electronic properties of materials. We examine their combined influence on a 2-dimensional electronic system at the atomic interface between magic-angle twisted bilayer graphene and a tungsten diselenide crystal. Strong electron correlation within the moiré flatband stabilizes correlated insulating states at both quarter and half filling, and SOC transforms these Mott-like insulators into ferromagnets, evidenced by robust anomalous Hall effect with hysteretic switching behavior. The coupling between spin and valley degrees of freedom is demonstrated through the control of the magnetic order with an in-plane magnetic field, or a perpendicular electric field. Our findings establish an experimental knob to engineer topological properties of moiré bands in twisted bilayer graphene and related systems.

Excellent cryogenic magnetocaloric properties in heavy rare-earth based HRENiGa2 (HRE = Dy, Ho, or Er) compounds

Abstract: Abstract RENiX 2 compounds, where RE = rare-earth element and X = p -block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi 2 -type, NdNiGa 2 -type, or MgCuAl 2 -type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl 2 -type aluminides show larger values than those of the CeNiSi 2 -type silicides and the NdNiGa 2 -type gallides due to the favored ferromagnetic ground state. However, RENiGa 2 gallides can crystallize in either NdNiGa 2 - or MgCuAl 2 -type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa 2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl 2 -type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (∣∆ S iso max ∣) values are 6.2, 10.4, and 11.4 J kg −1 K −1 (0–5 T) for DyNiGa 2 , HoNiGa 2 , and ErNiGa 2 , respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa 2 and ErNiGa 2 compounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems.

Emerging Research Landscape of Altermagnetism

Abstract: Magnetism is one of the largest, most fundamental, and technologically most relevant fields of condensed-matter physics. Traditionally, two basic magnetic phases have been considered – ferromagnetism and antiferromagnetism. The breaking of the time-reversal symmetry and spin splitting of the electronic states by the magnetization in ferromagnets underpins a range of macroscopic responses in this extensively explored and exploited type of magnets. By comparison, antiferromagnets have vanishing net magnetization. This Perspective reflects on recent observations of materials hosting an intriguing ferromagnetic-antiferromagnetic dichotomy, in which spin-split spectra and macroscopic observables, akin to ferromagnets, are accompanied by antiparallel magnetic order with vanishing magnetization, typical of antiferromagnets. An unconventional non-relativistic symmetry-group formalism offers a resolution of this apparent contradiction by delimiting a third basic magnetic phase, dubbed altermagnetism. Our Perspective starts with an overview of the still emerging unique phenomenology of the phase, and of the wide array of altermagnetic material candidates. In the main part of the article, we illustrate how altermagnetism can enrich our understanding of overarching condensed-matter physics concepts, and have impact on prominent condensed-matter research areas.

Monolayer CeI2 : An intrinsic room-temperature ferrovalley semiconductor

Abstract: Two-dimensional ferrovalley semiconductors with robust room-temperature ferromagnetism and sizable valley polarization hold great prospects for future miniature information storage devices. As a new member of the ferroic family, however, such ferrovalley materials have rarely been reported. By first-principles calculations, we identify that monolayer $\mathrm{Ce}{\mathrm{I}}_{\text{2}}$ is an intrinsic ferromagnetic semiconductor and exhibits excellent ambient stability, strong easy in-plane magnetocrystalline anisotropy, and a high magnetic transition temperature up to 374 K. The ferromagnetism is found to arise from the hybridization of Ce-$4f/5d$ and I-$5p$ orbitals. When monolayer $\mathrm{Ce}{\mathrm{I}}_{\text{2}}$ is magnetized toward the off-plane $z$ direction, a spontaneous valley polarization as large as 208 meV in the top valence band can be achieved due to the simultaneous breaking of both inversion symmetry and time-reversal symmetry, which is further verified by the perturbation theory of spin-orbital coupling. Also, the anomalous valley Hall effect can be observed under an in-plane electrical field due to the robust valley-contrasting Berry curvature. Overall, the combination of intrinsic semiconducting ferromagnetism and spontaneous valley polarization renders monolayer $\mathrm{Ce}{\mathrm{I}}_{\text{2}}$ a compelling room-temperature ferrovalley semiconductor for potential applications in nanoscale spintronics and valleytronics.

Room-temperature magnetoresistive and magnetocaloric effect in La1−xBaxMnO3 compounds: Role of Griffiths phase with ferromagnetic metal cluster above Curie temperature

Abstract: The evolution of the Griffiths phase (GP) with a ferromagnetic metal (FMM) cluster above the Curie temperature (TC) and its effect on the magnetic properties, electrical transport, magnetoresistance (MR), and magnetocaloric effect (MCE) is studied comprehensively, using bulk compounds of La1−xBaxMnO3 (0.15 ≤ x ≤ 0.25) with different lattice distortions but with the same structural symmetry and space group. These La1−xBaxMnO3 samples show ferromagnetic transition at TC increasing from 229 K for x = 0.15–300 K for x = 0.25, in addition to the presence of GP with FMM clusters in the paramagnetic (PM) region, which have been confirmed by the combination of magnetization (susceptibility) measurements, the GP theory, and electron paramagnetic resonance technology. With increasing the Ba2+ ion doping, GP temperature (TG) and TC of La1−xBaxMnO3 are increased, and the GP regime is strengthened. The GP ratio in the PM region reached 27.7% for the sample with x = 0.20. The resistivity decreases and the FMM phase increases with increasing x from 0.15 to 0.25, which can be explained by the decrease in the bandgap (Eg) and the enhancement of the double-exchange effect. Remarkably, large room-temperature MR (∼44.7%) can be observed in the sample with x = 0.25 under 60 kOe, which is related to the presence of the GP regime. Furthermore, the MCE is also affected by the GP regime, and it is deduced that the magnetic transition is of second order. The value of magnetic entropy change ( |ΔSM|) reaches 3.04 J/kg K near room temperature for the sample with x = 0.25 under 50 kOe. This value is associated with a relative cooling power (RCP) of 248.1 J/kg. For the sample with x = 0.15, the value of RCP reaches 307.6 J/kg under 50 kOe. The discovery of the MR and MCE near room temperature is of great significance from the practical application of perovskite manganites in magnetic sensors and magnetic refrigerants.

Planar thermal Hall effect of topological bosons in the Kitaev magnet α-RuCl_3

Abstract: The honeycomb magnet α-RuCl3 has attracted considerable interest because it is proximate to the Kitaev Hamiltonian whose excitations are Majoranas and vortices. The thermal Hall conductivity κxy of Majorana fermions is predicted to be half-quantized. Half-quantization of κxy/T (T, temperature) was recently reported, but this observation has proven difficult to reproduce. Here, we report detailed measurements of κxy on α-RuCl3 with the magnetic field B ∥ a (zigzag axis). In our experiment, κxy/T is observed to be strongly temperature dependent between 0.5 and 10 K. We show that its temperature profile matches the distinct form expected for topological bosonic modes in a Chern-insulator-like model. Our analysis yields magnon band energies in agreement with spectroscopic experiments. At high B, the spin excitations evolve into magnon-like modes with a Chern number of ~1. The bosonic character is incompatible with half-quantization of κxy/T.

Spin–Orbit Torque Switching in an All‐Van der Waals Heterostructure

Abstract: Current-induced control of magnetization in ferromagnets using spin-orbit torque (SOT) has drawn attention as a new mechanism for fast and energy efficient magnetic memory devices. Energy-efficient spintronic devices require a spin-current source with a large SOT efficiency (ξ) and electrical conductivity (σ), and an efficient spin injection across a transparent interface. Herein, single crystals of the van der Waals (vdW) topological semimetal WTe2 and vdW ferromagnet Fe3 GeTe2 are used to satisfy the requirements in their all-vdW-heterostructure with an atomically sharp interface. The results exhibit values of ξ ≈ 4.6 and σ ≈ 2.25 × 105 Ω-1 m-1 for WTe2 . Moreover, the significantly reduced switching current density of 3.90 × 106 A cm-2 at 150 K is obtained, which is an order of magnitude smaller than those of conventional heavy-metal/ferromagnet thin films. These findings highlight that engineering vdW-type topological materials and magnets offers a promising route to energy-efficient magnetization control in SOT-based spintronics.

Large anomalous Nernst effect and nodal plane in an iron-based kagome ferromagnet

Abstract: Kagome ferromagnet Fe3Sn exhibits large magnetic thermoelectric effect due to Berry curvature enhanced by a nodal plane.

Ferroelectric incommensurate spin crystals

Abstract: Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices1 and skyrmions2,3 when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO3/SrTiO3, which was later shown to be a model system owing to its high depolarizing field4-8. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi)9,10 has not been experimentally observed. Here we examine a domain structure in a single PbTiO3 epitaxial layer sandwiched between SrRuO3 electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials11-13. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.