Showing papers on "Magnetization published in 2020"

Reprogrammable shape morphing of magnetic soft machines

Abstract: Shape-morphing magnetic soft machines are highly desirable for diverse applications in minimally invasive medicine, wearable devices, and soft robotics. Despite recent progress, current magnetic programming approaches are inherently coupled to sequential fabrication processes, preventing reprogrammability and high-throughput programming. Here, we report a high-throughput magnetic programming strategy based on heating magnetic soft materials above the Curie temperature of the embedded ferromagnetic particles and reorienting their magnetic domains by applying magnetic fields during cooling. We demonstrate discrete, three-dimensional, and reprogrammable magnetization with high spatial resolution (~38 μm). Using the reprogrammable magnetization capability, reconfigurable mechanical behavior of an auxetic metamaterial structure, tunable locomotion of a surface-walking soft robot, and adaptive grasping of a soft gripper are shown. Our approach further enables high-throughput magnetic programming (up to 10 samples/min) via contact transfer. Heat-assisted magnetic programming strategy described here establishes a rich design space and mass-manufacturing capability for development of multiscale and reprogrammable soft machines.

Nearly room temperature ferromagnetism in a magnetic metal-rich van der Waals metal.

Abstract: In spintronics, two-dimensional van der Waals crystals constitute a most promising material class for long-distance spin transport or effective spin manipulation at room temperature. To realize all-vdW-material–based spintronic devices, however, vdW materials with itinerant ferromagnetism at room temperature are needed for spin current generation and thereby serve as an effective spin source. We report theoretical design and experimental realization of a iron-based vdW material, Fe4GeTe2, showing a nearly room temperature ferromagnetic order, together with a large magnetization and high conductivity. These properties are well retained even in cleaved crystals down to seven layers, with notable improvement in perpendicular magnetic anisotropy. Our findings highlight Fe4GeTe2 and its nanometer-thick crystals as a promising candidate for spin source operation at nearly room temperature and hold promise to further increase Tc in vdW ferromagnets by theory-guided material discovery.

Single-shot all-optical switching of magnetization in Tb/Co multilayer-based electrodes.

Abstract: Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Abstract: Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Electrical switching of magnetic order in an orbital Chern insulator

Abstract: Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favors ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields---a longstanding technological goal in spintronics and multiferroics---can be achieved only indirectly. Here, we experimentally demonstrate direct electric field control of magnetic states in an orbital Chern insulator, a magnetic system in which non-trivial band topology favors long range order of orbital angular momentum but the spins are thought to remain disordered. We use van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer to realize narrow and topologically nontrivial valley-projected moire minibands. At fillings of one and three electrons per moire unit cell within these bands, we observe quantized anomalous Hall effects with transverse resistance approximately equal to $h/2e^2$, which is indicative of spontaneous polarization of the system into a single-valley-projected band with a Chern number equal to two. At a filling of three electrons per moire unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate nonvolatile electric field induced reversal of the magnetic state. A theoretical analysis indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favored magnetic state. Voltage control of magnetic states can be used to electrically pattern nonvolatile magnetic domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultralow power magnetic memory.

Tunable Room-Temperature Ferromagnetism in Two-Dimensional Cr2Te3.

Abstract: The manipulation of magnetism provides a unique opportunity for the development of data storage and spintronic applications. Until now, electrical control, pressure tuning, stacking structure dependence, and nanoscale engineering have been realized. However, as the dimensions are decreased, the decrease of the ferromagnetism phase transition temperature (Tc) is a universal trend in ferromagnets. Here, we make a breakthrough to realize the synthesis of 1 and 2 unit cell (UC) Cr2Te3 and discover a room-temperature ferromagnetism in two-dimensional Cr2Te3. The newly observed Tc increases strongly from 160 K in the thick flake (40.3 nm) to 280 K in 6 UC Cr2Te3 (7.1 nm). The magnetization and anomalous Hall effect measurements provided unambiguous evidence for the existence of spontaneous magnetization at room temperature. The theoretical model revealed that the reconstruction of Cr2Te3 could result in anomalous thickness-dependent Tc. This dimension tuning method opens up a new avenue for manipulation of ferromagnetism.

Tunable room-temperature spin galvanic and spin Hall effects in van der Waals heterostructures.

Abstract: Spin–orbit coupling stands as a powerful tool to interconvert charge and spin currents and to manipulate the magnetization of magnetic materials through spin-torque phenomena. However, despite the diversity of existing bulk materials and the recent advent of interfacial and low-dimensional effects, control of this interconversion at room temperature remains elusive. Here, we demonstrate strongly enhanced room-temperature spin-to-charge interconversion in graphene driven by the proximity of WS2. By performing spin precession experiments in appropriately designed Hall bars, we separate the contributions of the spin Hall and the spin galvanic effects. Remarkably, their corresponding conversion efficiencies can be tailored by electrostatic gating in magnitude and sign, peaking near the charge neutrality point with an equivalent magnitude that is comparable to the largest efficiencies reported to date. Such electric-field tunability provides a building block for spin generation free from magnetic materials and for ultra-compact magnetic memory technologies. Enhanced spin-charge conversion in graphene is realized by proximity coupling to WS2 and electric-field tunability is demonstrated.

Deterministic Magnetization Switching Using Lateral Spin–Orbit Torque

Abstract: Current-induced magnetization switching by spin-orbit torque (SOT) holds considerable promise for next generation ultralow-power memory and logic applications. In most cases, generation of spin-orbit torques has relied on an external injection of out-of-plane spin currents into the magnetic layer, while an external magnetic field along the electric current direction is generally required for realizing deterministic switching by SOT. Here, deterministic current-induced SOT full magnetization switching by lateral spin-orbit torque in zero external magnetic field is reported. The Pt/Co/Pt magnetic structure is locally annealed by a laser track along the in-plane current direction, resulting in a lateral Pt gradient within the ferromagnetic layer, as confirmed by microstructure and chemical composition analysis. In zero magnetic field, the direction of the deterministic current-induced magnetization switching depends on the location of the laser track, but shows no dependence on the net polarization of external out-of-plane spin currents. From the behavior under external magnetic fields, two independent mechanisms giving rise to SOT are identified, i.e., the lateral Pt-Co asymmetry as well as out-of-plane injected spin currents, where the polarization and the magnitude of the SOT in the former case depends on the relative location and the laser power of the annealing track.

Magnetic and spectroscopic properties of Ni–Zn–Al ferrite spinel: from the nanoscale to microscale

Abstract: This article presents the annealing effect on the structural, elastic, thermodynamic, optical, magnetic, and electric properties of Ni0.6Zn0.4Fe1.5Al0.5O4 (NZFAO) nanoparticles (NPs). The samples were successfully synthesized by the sol–gel method followed by annealing of the as-synthesized at 600, 800, 900, 1050, and 1200 °C. This approach yielded the formation of a highly crystalline structure with crystallite size ranging from 17 nm to 40 nm. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques, as well as energy disperse spectroscopy (EDS), Fourier transform infrared (FTIR) and Raman spectroscopy, were used in order to determine the structural and morphological properties of the prepared samples. Rietveld XRD refinement reveals that Ni–Zn–Al ferrite nanoparticles crystallize in inverse cubic (Fdm) spinel structure. Using FTIR spectra, the elastic and thermodynamic properties were estimated. It was observed that the particle size had a pronounced effect on elastic and thermodynamic properties. Magnetic measurements were performed up to 700 K. The prepared ferrite samples present the highest Curie temperature, which decreases with increasing particle size and which is consistent with finite-size scaling. The thickness of the surface shell of about 1 nm was estimated from size-dependent magnetization measurements using the core–shell model. Besides, spin resonance, magnetostriction, temperature coefficient of resistance (TCR), and electrical resistivity properties have been scientifically studied and appear to be different according to their size. The optical properties of synthesized NZFAO nanoparticles were investigated, and the differences caused by the particle sizes are discussed on the basis of the phonon confinement effect. This effect was also inspected by the Raman analysis. Tuning of the physical properties suggests that the Ni–Zn–Al ferrite samples may be promising for multifunctional diverse applications.

Assessing magnetic and inductive thermal properties of various surfactants functionalised Fe 3 O 4 nanoparticles for hyperthermia

Abstract: This work reports the fabrication of magnetite (Fe3O4) nanoparticles (NPs) coated with various biocompatible surfactants such as glutamic acid (GA), citric acid (CA), polyethylene glycol (PEG), polyvinylpyrrolidine (PVP), ethylene diamine (EDA) and cetyl-trimethyl ammonium bromide (CTAB) via co-precipitation method and their comparative inductive heating ability for hyperthermia (HT) applications. X-ray and electron diffraction analyses validated the formation of well crystallined inverse spinel structured Fe3O4 NPs (crystallite size of ~ 8-10 nm). Magnetic studies confirmed the superparamagnetic (SPM) behaviour for all the NPs with substantial magnetisation (63-68 emu/g) and enhanced magnetic susceptibility is attributed to the greater number of occupations of Fe2+ ions in the lattice as revealed by X-ray photoelectron spectroscopy (XPS). Moreover, distinctive heating response (specific absorption rate, SAR from 130 to 44 W/g) of NPs with similar size and magnetisation is observed. The present study was successful in establishing a direct correlation between relaxation time (~ 9.42-15.92 ns) and heating efficiency of each surface functionalised NPs. Moreover, heat dissipated in different surface grafted NPs is found to be dependent on magnetic susceptibility, magnetic anisotropy and magnetic relaxation time. These results open very promising avenues to design surface functionalised magnetite NPs for effective HT applications.

Topological Magnetic Phase in the Candidate Weyl Semimetal CeAlGe

Abstract: We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, square-coordinated multi-k[over →] magnetic phases below T_{N}. The topological properties of a phase stable at intermediate magnetic fields parallel to the c axis are suggested by observation of a topological Hall effect. Our findings highlight CeAlGe as an exceptional system for exploiting the interplay between the nontrivial topologies of the magnetization in real space and Weyl nodes in momentum space.

Topological-chiral magnetic interactions driven by emergent orbital magnetism.

Abstract: Two hundred years ago, Ampere discovered that electric loops in which currents of electrons are generated by a penetrating magnetic field can mutually interact. Here we show that Ampere's observation can be transferred to the quantum realm of interactions between triangular plaquettes of spins on a lattice, where the electrical currents at the atomic scale are associated with the orbital motion of electrons in response to the non-coplanarity of neighbouring spins playing the role of a magnetic field. The resulting topological orbital moment underlies the relation of the orbital dynamics with the topology of the spin structure. We demonstrate that the interactions of the topological orbital moments with each other and with the spins form a new class of magnetic interactions [Formula: see text] topological-chiral interactions [Formula: see text] which can dominate over the Dzyaloshinskii-Moriya interaction, thus opening a path for realizing new classes of chiral magnetic materials with three-dimensional magnetization textures such as hopfions.

Enabling room temperature ferromagnetism in monolayer MoS 2 via in situ iron-doping

Abstract: Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS2. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature. Ferromagnetism with a Curie temperature above room temperature in 2D materials is highly desirable for practical spintronics applications. Here, the authors demonstrate such phenomenon in monolayer MoS2 via in situ iron-doping and measured local magnetic field strength up to 0.5 ± 0.1 mT.

Giant orbital magnetoelectric effect and current-induced magnetization switching in twisted bilayer graphene.

Abstract: Recently, quantum anomalous Hall effect with spontaneous ferromagnetism was observed in twisted bilayer graphenes (TBG) near 3/4 filling. Importantly, it was observed that an extremely small current can switch the direction of the magnetization. This offers the prospect of realizing low energy dissipation magnetic memories. However, the mechanism of the current-driven magnetization switching is poorly understood as the charge currents in graphenes are generally believed to be non-magnetic. In this work, we demonstrate that in TBG, the twisting and substrate induced symmetry breaking allow an out of plane orbital magnetization to be generated by a charge current. Moreover, the large Berry curvatures of the flat bands give the Bloch electrons large orbital magnetic moments so that a small current can generate a large orbital magnetization. We further demonstrate how the charge current can switch the magnetization of the ferromagnetic TBG near 3/4 filling as observed in the experiments. The mechanism of current-driven magnetization switching in twisted bilayer graphene (TBG) is poorly understood. Here, He et al. show that a small current can generate a large orbital magnetization due to symmetry breaking by the twisting and substrate in TBG, leading to a giant orbital magnetoelectric effect.

Magnetic and microwave properties of SrFe12O19/MCe0.04Fe1.96O4 (M = Cu, Ni, Mn, Co and Zn) hard/soft nanocomposites

Abstract: This paper reports the synthesis, structural characteristics and magnetism of SrFe12O19/MCe0.04Fe1.96O4 (M = Cu, Ni, Mn, Co and Zn) hard/soft nanocomposites. The hard/soft compositions were manufactured via a one-pot reactions citrate sol-gel approach. The hard/soft phases formation was confirmed using XRD, SEM, TEM and HRTEM techniques. M vs. H (Magnetization measurements) were done at unbent temperature and 10 K. Smoothed M against H loops and single peaks in dM/dH vs. H curves were noticed in SrFe12O19/MnCe0.04Fe1.96O4, SrFe12O19/CuCe0.04Fe1.96O4 and SrFe12O19/ZnCe0.04Fe1.96O4 hard/soft nanocomposites. This indicated the manifestation of well exchange-coupled effect among hard and soft phases in these composites. However, SrFe12O19/CoCe0.04Fe1.96O4 and SrFe12O19/NiCe0.04Fe1.96O4 hard/soft nanocomposites showed non-well smoothed M against H loops and two peaks in dM/dH versus H plots, indicating that the dipolar interactions are unimportant compared to exchange-coupling behavior. Among all prepared nanocomposites, the SrFe12O19/MnCe0.04Fe1.96O4 hard/soft nanocomposite showed the highest exchange-coupling behavior. Microwave properties of the SrFe12O19/MCe0.04Fe1.96O4 (M = Cu, Ni, Mn, Co and Zn) hard/soft nanocomposites were investigated using coaxial method with applied frequency values fall between 2 and 18 GHz. Reflection losses were calculated from frequency dependences of the imaginary and real parts of permeability and permittivity. The correlation between the chemical composition of the spinel phase (A-cation) and microwave properties of composites. Most intensive electromagnetic absorption was observed for Ni- and Mn-spinels. This is can be a result of the differences in electron shell configuration and radii for A-site ions in the spinel phase. Change of the absorption mechanisms (transition from ionic polarization to dipole polarization) was observed.

Imaging orbital ferromagnetism in a moir\'e Chern insulator

Abstract: Electrons in moire flat band systems can spontaneously break time reversal symmetry, giving rise to a quantized anomalous Hall effect Here we use a superconducting quantum interference device to image stray magnetic fields in one such system composed of twisted bilayer graphene aligned to hexagonal boron nitride We find a magnetization of several Bohr magnetons per charge carrier, demonstrating that the magnetism is primarily orbital in nature Our measurements reveal a large change in the magnetization as the chemical potential is swept across the quantum anomalous Hall gap consistent with the expected contribution of chiral edge states to the magnetization of an orbital Chern insulator Mapping the spatial evolution of field-driven magnetic reversal, we find a series of reproducible micron scale domains whose boundaries host chiral edge states

Direct observation of 2D magnons in atomically thin CrI$_3$

Abstract: Exfoliated chromium triiodide (CrI$_3$) is a layered van der Waals (vdW) magnetic insulator that consists of ferromagnetic layers coupled through antiferromagnetic interlayer exchange. The resulting permutations of magnetic configurations combined with the underlying crystal symmetry produces tunable magneto-optical phenomena that is unique to the two-dimensional (2D) limit. Here, we report the direct observation of 2D magnons through magneto-Raman spectroscopy with optical selection rules that are strictly determined by the honeycomb lattice and magnetic states of atomically thin CrI$_3$. In monolayers, we observe an acoustic magnon mode of ~0.3 meV with cross-circularly polarized selection rules locked to the magnetization direction. These unique selection rules arise from the discrete conservation of angular momentum of photons and magnons dictated by threefold rotational symmetry in a rotational analogue to Umklapp scattering. In bilayers, by tuning between the layered antiferromagnetic and ferromagnetic-like states, we observe the switching of two magnon modes. The bilayer structure also enables Raman activity from the optical magnon mode at ~17 meV (~4.2 THz) that is otherwise Raman-silent in the monolayer. From these measurements, we quantitatively extract the spin wave gap, magnetic anisotropy, intralayer and interlayer exchange constants, and establish 2D magnets as a new system for exploring magnon physics.

Investigation of structural and physical properties of Eu3+ ions substituted Ni0.4Cu0.2Zn0.4Fe2O4 spinel ferrite nanoparticles prepared via sonochemical approach

Abstract: Green and facile process for Ni0.4Cu0.2Zn0.4Fe2−xEuxO4 (x = 0.00–0.10) spinel ferrite nanoparticles (SNPs) prepared via ultrasonic irradiation (without any post annealing process) has been deeply investigated. The influence of Eu3+ substitutions on the structure, morphological, optical, magnetic, electrical and dielectric traits of NiCuZn SNPs was assessed. Tauc plots revealed direct optical band gaps in a very tight interval of 1.86–1.90 eV. Magnetization measurements exposed a superparamagnetic behavior at room temperature and below the blocking temperature (TB) a superparamagnetic-ferromagnetic transition was noticed. The saturation magnetization (Ms) value is highest for pure Ni0.4Cu0.2Zn0.4Fe2O4 (i.e. x = 0.00) SNP with Ms ~ 58.9 emu/g at room temperature. The saturation magnetization (Ms) declines with rising Eu3+ substituting content. AC conductivity decreases as a function of exponent power base law. Maximum variation in dc conductivity is observed to be around the substitution ratio of x = 0.02. It is found that activation energy is highly dependent on both Eu ions substitution ratios and temperature ranges. The frequency dependence of dielectric functions is explained by Koop's models based on Maxwell-Wagner theory.

Layer-Resolved Magnetic Proximity Effect in van der Waals Heterostructures

Abstract: Magnetic proximity effects are crucial ingredients for engineering spintronic, superconducting, and topological phenomena in heterostructures. Such effects are highly sensitive to the interfacial electronic properties, such as electron wave function overlap and band alignment. The recent emergence of van der Waals (vdW) magnets enables the possibility of tuning proximity effects via designing heterostructures with atomically clean interfaces. In particular, atomically thin CrI3 exhibits layered antiferromagnetism, where adjacent ferromagnetic monolayers are antiferromagnetically coupled. Exploiting this magnetic structure, we uncovered a layer-resolved magnetic proximity effect in heterostructures formed by monolayer WSe2 and bi/trilayer CrI3. By controlling the individual layer magnetization in CrI3 with a magnetic field, we found that the spin-dependent charge transfer between WSe2 and CrI3 is dominated by the interfacial CrI3 layer, while the proximity exchange field is highly sensitive to the layered magnetic structure as a whole. These properties enabled us to use monolayer WSe2 as a spatially sensitive magnetic sensor to map out layered antiferromagnetic domain structures at zero magnetic field as well as antiferromagnetic/ferromagnetic domains near the spin-flip transition in bilayer CrI3. Our work reveals a new way to control proximity effects and probe interfacial magnetic order via vdW engineering.

Observation of Magnetic Proximity Effect Using Resonant Optical Spectroscopy of an Electrically Tunable MoSe2/CrBr3 Heterostructure

Abstract: van der Waals heterostructures combining two-dimensional magnetic and semiconducting layers constitute a promising platform for interfacing magnetism, electronics, and optics. Here, we use resonant optical reflection spectroscopy to observe the magnetic proximity effect in a gate-tunable ${\mathrm{MoSe}}_{2}/{\mathrm{CrBr}}_{3}$ heterostructure. The high quality of the interface leads to a giant zero-field splitting of the $K$ and ${K}^{\ensuremath{'}}$ valley excitons in ${\mathrm{MoSe}}_{2}$, equivalent to an external magnetic field of 12 T, with a weak but distinct electric field dependence that hints at potential for electrical control of magnetization. The magnetic proximity effect allows us to use resonant optical spectroscopy to fully characterize the ${\mathrm{CrBr}}_{3}$ magnet, determining the easy-axis coercive field, the magnetic anisotropy energy, and critical exponents associated with spin susceptibility and magnetization.

Controlling spin current polarization through non-collinear antiferromagnetism.

Abstract: The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.

Efficient Spin‐Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr 2 Ge 2 Te 6

Abstract: Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr2 Ge2 Te6 , is studied using the anomalous Hall effect in Cr2 Ge2 Te6 /tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out-of-plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in-plane magnetic field and a charge current flowing through Ta-a heavy metal exhibiting giant spin Hall effect-is used. In the presence of in-plane fields of 20 mT, charge current densities as low as 5 × 105 A cm-2 are sufficient to switch the out-of-plane magnetization of Cr2 Ge2 Te6 . This finding highlights that current densities required for spin-orbit torque switching of Cr2 Ge2 Te6 are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low-power memory and logic applications.

Understanding magnetic relaxation in single-ion magnets with high blocking temperature

Abstract: The recent discovery of single-ion magnets with magnetic hysteresis above liquid-nitrogen temperatures placed these compounds among the best candidates to realize high-density storage devices. Starting from a prototypical dysprosocenium molecule, showing hysteresis up to 60 K, we derive here a general recipe to design high-blocking-temperature rare-earth single-ion magnets. The complex magnetic relaxation is unraveled by combining magnetization and nuclear magnetic resonance measurements with inelastic neutron scattering experiments and ab initio calculations, thus disentangling the different mechanisms and identifying the key ingredients behind slow relaxation.

Ferromagnetic hybrid nodal loop and switchable type-I and type-II Weyl fermions in two dimensions

Abstract: As a novel type of fermionic state, the hybrid nodal loop with the coexistence of both type-I and type-II band crossings has attracted intense research interest. However, it remains a challenge to realize the hybrid nodal loop in both two-dimensional (2D) materials and in ferromagnetic (FM) materials. Here, we propose a FM hybrid nodal loop in a 2D CrN monolayer. We show that the material has a high Curie temperature ($g600$ K) FM ground state, with the out-of-plane [001] magnetization. It shows a half-metallic band structure with two bands in the spin-up channel crossing each other near the Fermi level. These bands produce both type-I and type-II band crossings, which form a fully spin-polarized hybrid nodal loop. We find the nodal loop is protected by the mirror symmetry and robust against spin-orbit coupling. An effective Hamiltonian characterizing the hybrid nodal loop is established. We further find the configuration of the nodal loop can be shifted under external perturbations such as strain. Most remarkably, we demonstrate that both type-I and type-II Weyl nodes can be realized from such FM hybrid nodal loop by simply shifting the magnetization from out of plane to in plane. Our work provides an excellent candidate to realize a FM hybrid nodal loop and Weyl fermions in 2D material, and is also promising for related topological applications with their intriguing properties.

Shape anisotropic iron oxide-based magnetic nanoparticles: synthesis and biomedical applications

Abstract: Research on iron oxide-based magnetic nanoparticles and their clinical use has been, so far, mainly focused on the spherical shape. However, efforts have been made to develop synthetic routes that produce different anisotropic shapes not only in magnetite nanoparticles, but also in other ferrites, as their magnetic behavior and biological activity can be improved by controlling the shape. Ferrite nanoparticles show several properties that arise from finite-size and surface effects, like high magnetization and superparamagnetism, which make them interesting for use in nanomedicine. Herein, we show recent developments on the synthesis of anisotropic ferrite nanoparticles and the importance of shape-dependent properties for biomedical applications, such as magnetic drug delivery, magnetic hyperthermia and magnetic resonance imaging. A brief discussion on toxicity of iron oxide nanoparticles is also included.

Cation distribution, magnetic and hyperfine interaction studies of Ni–Zn spinel ferrites: role of Jahn Teller ion (Cu2+) substitution

Abstract: This study reports cation distribution, magnetic, and hyperfine interaction studies of Cu2+-substituted mixed Ni–Zn nano-spinel ferrites prepared by combustion technique. X-ray diffraction and electron microscopy were used to study the structural and morphological aspects of all the samples. Rietveld refined diffraction patterns exhibited a cubic-spinel lattice structure with the Fd3m space group for all the samples. Morphological investigations revealed the spherical morphology of particles with some agglomeration. The magnetic properties investigated at 300 K and 5 K implied a soft ferromagnetic character of all the samples. The magnetization at 5 K progressively enhanced due to surface effects. Field-cooled and zero-field-cooled measurements indicated net irreversibility for all the samples. Hyperfine interaction studies revealed the ferrimagnetic nature of Cu2+-substituted mixed Ni–Zn spinel nano-ferrites. All the obtained results show that the prepared nanoparticles are useful for magnetic fluid hyperthermia and other bio-applications.

Exchange bias switching in an antiferromagnet/ferromagnet bilayer driven by spin–orbit torque

Abstract: The electrical manipulation of magnetization and exchange bias in antiferromagnet/ferromagnet thin films could be of use in the development of the next generation of spintronic devices. Current-controlled magnetization switching can be driven by spin–orbit torques generated in an adjacent heavy-metal layer, but these structures are difficult to integrate with exchange bias switching and tunnelling magnetoresistance measurements. Here, we report the current-induced switching of the exchange bias field in a perpendicularly magnetized IrMn/CoFeB bilayer structure using a spin–orbit torque generated in the antiferromagnetic IrMn layer. By manipulating the current direction and amplitude, independent and repeatable switching of the magnetization and exchange bias field below the blocking temperature can be achieved. The critical current density for the exchange bias switching is found to be larger than that for CoFeB magnetization reversal. X-ray magnetic circular dichroism, polarized neutron reflectometry measurements and micromagnetic simulations show that a small net magnetization within the IrMn interface plays a crucial role in these phenomena. The magnetization and exchange bias field in an IrMn/CoFeB bilayer can be independently switched using a current-controlled spin–orbit torque generated in the antiferromagnetic IrMn layer.