Hard Ferromagnets Research Articles

Giant Hall Switching by Surface-State-Mediated Spin-Orbit Torque in a Hard Ferromagnetic Topological Insulator.

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states (TSS) with ultrahigh efficiency. Here, efficient SOT switching of a hard MTI, V-doped (Bi,Sb)2Te3 (VBST), with a large coercive field that can prevent the influence of an external magnetic field, is demonstrated. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems, which makes the Hall channel a good readout and eliminates the need to fabricate complicated magnetic tunnel junction (MTJ) structures. The SOT switching current density can be reduced to 2.8×105Acm-2, indicating its high efficiency. Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to (1.56±0.12)×10-6TA-1cm2 and the interfacial charge-to-spin conversion efficiency to 3.9±0.3 nm-1. The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices.

Highly Robust Room-Temperature Interfacial Ferromagnetism in Ultrathin Co2Si Nanoplates.

The reduced dimensionality and interfacial effects in magnetic nanostructures open the feasibility to tailor magnetic ordering. Here, we report the synthesis of ultrathin metallic Co2Si nanoplates with a total thickness that is tunable to 2.2 nm. The interfacial magnetism coupled with the highly anisotropic nanoplate geometry leads to strong perpendicular magnetic anisotropy and robust hard ferromagnetism at room temperature, with a Curie temperature (TC) exceeding 950 K and a coercive field (HC) > 4.0 T at 3 K and 8750 Oe at 300 K. Theoretical calculations suggest that ferromagnetism originates from symmetry breaking and undercoordinated Co atoms at the Co2Si and SiO2 interface. With protection by the self-limiting intrinsic oxide, the interfacial ferromagnetism of the Co2Si nanoplates exhibits excellent environmental stability. The controllable growth of ambient stable Co2Si nanoplates as 2D hard ferromagnets could open exciting opportunities for fundamental studies and applications in Si-based spintronic devices.

Large-scale vibrating coil magnetometer for the magnetic characterization of bulk superconductors

This work presents the design and validation of a vibrating coil magnetometer for the characterization of the field dependence of the critical current density of centimeter-sized bulk superconductors as an alternative to the destructive methods typically used. The magnetometer is also shown to be capable of measuring the magnetic moment in an applied field of up to 5 T for diverse magnetic materials, such as soft and hard ferromagnets and high-temperature superconducting pellets. The vibrating coil magnetometer was first optimized using finite element simulations and calibrated using a commercial vibrating sample magnetometer. The vibrating coil magnetometer was benchmarked with hysteresis measurements of a Nd2Fe14B disk made with a commercial hysteresisgraph, showing good agreement between the different setups. The magnetic hysteresis of a YBa2Cu3O7−x superconducting pellet was measured at 77 K, showing a penetration field of 1 T and an irreversibility field of 4 T. The field dependent critical current density of the superconductor was then inferred from the magnetic hysteresis measurements and extrapolated at low fields. Finally, the resulting critical current density was used to successfully reproduce the measured magnetization curve of the pellet at 2 T with finite element simulations.

Template-assisted electroplating of Sm-Co composite nanowires: Issue of boric acid additive via R-D process

A considerable number of Sm-Co nanowires were synthesized in anodic aluminum oxide (AAO) by electroplating in aqueous solutions. To ensure a uniform Sm composition ratio, the electroplating was conducted using linear sweep voltammetry method. Dispersed Sm-Co nanowires were synthesized via a combination of annealing for densification and template etching. Subsequently, a low-temperature reduction and diffusion process was performed at 700 ℃ in a facile way using NaCl, KCl, and Ca granules. The one-dimensional structure morphology was maintained; however, contrary to expectations, the hysteresis loop in the VSM did not exhibit the excellent magnetic properties of the Sm2Co17 hard ferromagnet. Analysis by XPS, XRD, and TEM revealed that the addition of boric acid during electroplating suppressed the transformation of Sm-Co nanowires into the Sm2Co17 phase. The use of boric acid as an additive when synthesizing nanowires by electroplating using an AAO template has been found to be beneficial for a highly ordered morphology; however, this may lead to undesirable issues in the reduction and diffusion process, and thus should be carefully considered.

Nano-Patterned Magnetic Edges in CrGeTe3 for Quasi 1-D Spintronic Devices.

The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices.

Dynamics of curved domain walls in hard ferromagnets with nonlinear dissipative and inertial effects

This article investigates the field-driven motion of curved domain walls in ferromagnetic nanostructures under the framework of modified Landau–Lifshitz–Gilbert equation with inertial effects. The considered governing equation involves the torques arising from nonlinear dissipative (viscous and dry friction) and inertial effects. We study the most relevant dynamical features in the steady dynamical regime for the considered model by employing the reductive perturbation technique. Finally, we illustrate the results numerically for the various domain wall surfaces (plane, cylinder, and sphere) and discuss their physical significance. The results obtained here agree with the recent theoretical and experimental observations.

Hard ferromagnets as a new perspective on materials for thermomagnetic power generation cycles

We consider the ways in which magnetically hard materials can be used as the working materials in thermomagnetic power generation (TMG) cycles in order to expand the area in the magnetisation vs. applied field (M−H) plane available for energy conversion. There are 3 parts to this Perspective. First, experiments on commercially available hard ferrites reveal that, while these materials are not yet good TMG candidates, hard ferromagnets with higher thermal conductivity and a greater change of magnetization with temperature could outperform existing TMG materials. Second, computational results indicate that biasing a soft magnet with a hard ferromagnet is essentially equivalent to shifting the M−H loop by an amount proportional to the field of the biasing magnet. Work outputs under biased conditions show a substantial improvement over unbiased cycles, but experimental verification is needed. Third, we discuss the rationale for exploring artificial spin reorientation materials as novel TMG working materials.

Strain Tunability of Perpendicular Magnetic Anisotropy in van der Waals Ferromagnets VI3.

Layered ferromagnets with strong magnetic anisotropy energy (MAE) have special applications in nanoscale memory elements in electronic circuits. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements. For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis loop with a large coercivity (1.775 T at 10 K) and remanent magnetization. Furthermore, the coercivity can be enhanced to a maximum of 2.6 T under a 3.8% external in-plane tensile strain. Our DFT calculations show that the increased MAE under strain contributes to the enhancement of coercivity. Meanwhile, the strain tunability on the coercivity of CrI3, with a similar crystal structure, is limited. The main reason is the strong spin-orbit coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes highlights its potential for integration into vdW heterostructures.

Bloch point dynamics in exchange-spring heterostructures

Magnetization textures that are stabilized by topological constraints, such as skyrmions and chiral bobbers, as well as the emergent electrodynamics associated with their motion, provide a promising avenue toward novel energy-efficient nanomagnetic devices. Here, it is shown that exchange-spring-type heterostructures, where soft ferromagnets with azimuthal symmetry are exchange-coupled to a ferromagnetic layer with perpendicular magnetic anisotropy, can be used for the creation and control of skyrmion tubes and Bloch points during magnetization reversal of the soft ferromagnet, where the rapid motion of the Bloch points induces an emergent electric field with a magnitude of the order of megavolts per meter. The exchange coupling to the hard ferromagnet restores the system to its original configuration, making the process fully reversible and repeatable, and the duration of the magnetization processes and the motion of the Bloch points can be tuned by adjusting the size of the ferromagnet. Based on these numerical predictions, it is proposed that exchange-spring heterostructures could be used to generate picosecond electromagnetic pulses.

Superconducting imprint of magnetic textures in ferromagnets with perpendicular magnetic anisotropy

Research on proximity effects in superconductor/ferromagnetic hybrids has most often focused on how superconducting properties are affected—and can be controlled—by the effects of the ferromagnet’s exchange or magnetic fringe fields. The opposite, namely the possibility to craft, tailor and stabilize the magnetic texture in a ferromagnet by exploiting superconducting effects, has been more seldom explored. Here we show that the magnetic flux trapped in high-temperature superconducting YBa2Cu3O7-δ microstructures can be used to modify the magnetic reversal of a hard ferromagnet—a cobalt/platinum multilayer with perpendicular magnetic anisotropy—and to imprint unusual magnetic domain distributions in a controlled manner via the magnetic field history. The domain distributions imprinted in the superconducting state remain stable, in absence of an external magnetic field, even after increasing the temperature well above the superconducting critical temperature, at variance to what has been observed for soft ferromagnets with in-plane magnetic anisotropy. This opens the possibility of having non-trivial magnetic configuration textures at room temperature after being tailored below the superconducting transition temperature. The observed effects are well explained by micromagnetic simulations that demonstrate the role played by the magnetic field from the superconductor on the nucleation, propagation, and stabilization of magnetic domains.

Imaging Domain Reversal in an Ultrathin Van der Waals Ferromagnet

The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen-vacancy (NV) microscope is employed to directly image these processes in few-layer flakes of the magnetic semiconductor vanadium triiodide (VI3 ). Complete and abrupt switching of most flakes is observed at fields Hc ≈ 0.5-1T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (Tc ≈ 50K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness-dependent domain wall depinning fields well below Hc . These results point to ultrathin VI3 being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field- and current-driven domain wall dynamics.

Phase diagram and evolution of the magnetic ordering state driven by a field-effect transistor in (Li,Fe)OHFeS thin flakes

Manipulating collectively ordered electronic and magnetic states in a correlated system is at the core of condensed matter physics. Besides tuning the carrier density of a crystal, the latest developed field-effect transistor (FET) with a solid ion conductor (SIC) can also drive ions into the crystal and lead to structural transformations that are difficult to access with conventional methods including conventional field-effect transistors. Here, we exploit the SIC-FET gating technique to realize a metastable phase in (Li,Fe)OHFeS thin flakes, and achieve a rich phase diagram involving superconductivity, soft ferromagnetism, and hard ferromagnetism. With the injection of Li ions into the thin flake, the superconductivity is gradually suppressed and a soft ferromagnetic phase with new crystal structure sets in, evidenced by the anomalous Hall effect and magnetoresistance measurements. Further injecting Li ion drives the soft ferromagnetic state into a hard ferromagnetic state, where different types of magnetic interactions dominate due to the simultaneous tuning of carrier density and magnetic Fe ion concentration in the Li ion injection process. Our paper paves a way to control structural phase transformations as well as physical properties by the electric field. These findings demonstrate the superior performance of the SIC-FET in regulating the physical properties of layered crystals and its potential applications for multifunctional devices.

Hard ferromagnetic van-der-Waals metal (Fe,Co)3GeTe2: a new platform for the study of low-dimensional magnetic quantum criticality

The widely-studied ferromagnetic van-der-Waals (vdW) metal Fe3GeTe2 has great promise for studies of quantum criticality in the 2D limit, but is limited by a relatively high Curie temperature in excess of 200 K. To help render the quantum critical point achievable in such a system within the reach of practically possible tuning methods, we have grown single crystals of a variant of (Fe,Co)3GeTe2 with useful physical properties for both this purpose and the wider study of low-dimensional magnetism and spin transport. (Fe,Co)3GeTe2 is found through x-ray diffraction and electron microscopy to have an equivalent crystal structure to Fe3GeTe2, with a random distribution of the cobalt dopant sites. It exhibits a sharp ferromagnetic transition at a value below 40 K, a stronger anisotropy and a coercive field ten times larger than that of Fe3GeTe2. The transport properties and specific heat show the electronic properties and strong correlations of Fe3GeTe2 to be near-unchanged in this doped material. We demonstrate that (Fe,Co)3GeTe2 can be cleanly exfoliated down to monolayer thickness. This unprecedented hard metallic vdW ferromagnet is a valuable new addition to the limited range of materials available for the study of 2D magnetism.

Effect of the Semiconductor Spacer on Positive Exchange Bias in the CoNi/Si/FeNi Three-Layer Structure

Films consisting of a hard magnetic ferromagnet CoNi and a soft magnetic ferromagnet FeNi interacting through a nonmagnetic Si semiconductor spacer are experimentally studied. The temperature and field dependences of the magnetic properties of film structures with different silicon thicknesses are examined. It is found that the multilayer structure has the properties inherent in magnetic springs and exhibits positive exchange bias as a function of the silicon thickness.

Bulk properties of the van der Waals hard ferromagnet VI3

We present comprehensive measurements of the structural, magnetic and electronic properties of layered van-der-Waals ferromagnet VI$_3$ down to low temperatures. Despite belonging to a well studied family of transition metal trihalides, this material has received very little attention. We outline, from high-resolution powder x-ray diffraction measurements, a corrected room-temperature crystal structure to that previously proposed and uncover a structural transition at 79 K, also seen in the heat capacity. Magnetization measurements confirm VI$_3$ to be a hard ferromagnet (9.1 kOe coercive field at 2 K) with a high degree of anisotropy, and the pressure dependence of the magnetic properties provide evidence for the two-dimensional nature of the magnetic order. Optical and electrical transport measurements show this material to be an insulator with an optical band gap of 0.67 eV - the previous theoretical predictions of d-band metallicity then lead us to believe VI$_3$ to be a correlated Mott insulator. Our latest band structure calculations support this picture and show good agreement with the experimental data. We suggest VI$_3$ to host great potential in the thriving field of low-dimensional magnetism and functional materials, together with opportunities to study and make use of low-dimensional Mott physics.

Structural, electronic and magnetic properties of MnxGa/Co2MnSi (x\u2009=\u20091, 3) bilayers

Directly coupled hard and soft ferromagnets were popularly used as the hybridized electrodes to enhance tunnel magnetoresistance (TMR) ratio in the perpendicular magnetic tunnel junction (pMTJ). In this paper, we employ the density functional theory (DFT) with general gradient approximation (GGA) to investigate the interfacial structure and magnetic behavior of tetragonal Heusler-type MnGa (MG)/L21-Co2MnSi (CMS) Heusler alloy bilayers with the MnGa being D022-MnGa alloy (Mn3Ga) and L10-MnGa alloy (MnGa). The MM-MS_B interface with the bridge (B) connection of MnMn termination (MM) of D022- and L10-MnGa layers to MnSi termination (MS) of CMS layers is found to be most stable in the energy point of view. Also, a strong antiferromagnetic coupling and relatively higher spin polarization can be observed in the MM-MS_B interface. Further, a remarkable potential difference to derive electrons to transfer from MG layer to CMS layer appears at the interface. These theoretical results indicate that the MG/CMS bilayers are promising candidates as coupled composites, and moreover, the D022-MG/CMS bilayer is better than L10-MG/CMS bilayer due to its larger spin polarization and built-in field at the interface.

Magnetic Properties of Bilayer Thin Film Composed of Hard and Soft Ferromagnetic Prussian Blue Analogues

AbstractThe molecule‐ based bilayer system composed of hard Ni3.38[Fe(CN)6]2⋅nH2O and soft Ni3.1[Cr(CN)6]2⋅nH2O ferromagnetic Prussian blue analogues has been fabricated on a solid substrate by “layer by layer” deposition. The structure and morphology characterization as well as results of magnetic measurements are described. The thickness of the bilayer is ca. 140 nm including a 35 nm interface. This bilayer system shows anisotropic magnetic properties reflected in the shape of magnetic hysteresis measured in various film orientation with respect to the direction of external magnetic field. There is no exchange interaction between hard and soft magnetic layer and irrespective of bilayer orientation, the magnetization and demagnetization process of both Ni3.38[Fe(CN)6]2⋅nH2O and Ni3.1[Cr(CN)6]2⋅nH2O layers occurs independently.

Simulation of Field Dependence of Critical Current Densities of Bulk High Tc Superconducting Materials regarding Thermally Activated Flux Motion

In the upcoming generation, bulk high temperature superconductors (HTS) will play a crucial and a promising role in numerous industrial applications ranging from Maglev trains to magnetic resonance imaging, etc. Especially, the bulk HTS as permanent magnets are suitable due to the fact that they can trap magnetic fields being several orders of magnitude higher than those of the best hard ferromagnets. The bulk HTS LREBa2Cu3O7-δ (LREBCO or LRE-123, LRE: Y, Gd, etc.,) materials could obtain very powerful compact superconducting super-magnets, which can be operated at the cheaper liquid nitrogen temperature or below due to higher critical temperatures (i.e., ∼90 K). As a result, the new advanced technology can be utilized in a more attractive manner for a variety of technological and medical applications which have the capacity to revolutionize the field. An understanding of the magnetic field dependence of the critical current density (Jc(H)) is important to develop better adapted materials. To achieve this goal, a variety of Jc(H) behaviours of bulk LREBCO samples were modelled regarding thermally activated flux motion. In essence, the Jc(H) curves follows a certain criterion where an exponential model is applied. However, to fit the complete Jc(H) curve of the LRE-123 samples an unique model is necessary to explain the behavior at low and high fields. The modelling of the various superconducting materials could be understood in terms of the pinning mechanisms.

The effects of interfacial exchange coupling in Fe/ErFeO3 heterostructures

Exploring exchange bias in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures is vital for both fundamental magnetism and practical application. However, in the case of conventional FM/AFM systems, the essential field cooling process above the Néel temperature of AFM materials hinders their application if the Néel temperature is far higher than room-temperature. Here, we report the effects of interfacial exchange coupling in Fe/ErFeO3 heterostructures. The magnetic-field-induced switchable exchange bias, originating from the AFM exchange coupling between Fe film and Dzyaloshinskii–Moriya-interaction-induced net moment of ErFeO3 along c axis, is successfully achieved without field cooling or in-field growth process of AFM. Different from the most previous pinning layer using a hard FM or traditional AFM, ErFeO3 pinning layer has the advantages of both the magnetic field sensitivity (~780 Oe) and ultrahigh dynamic frequency. In addition, although Fe film is polycrystalline, it exhibits a strong uniaxial magnetic anisotropy resulted from the so-called ‘spin-flop-coupling effect’, i.e. the magnetic coupling between Fe film and the compensated G-type AFM spins of EFO along a axis. Interestingly, the exchange bias field and asymmetric switching field offer entirely different information about the asymmetry of magnetization reversal near hard axis. The asymmetric switching field is further proved to be an effective measure to determine the weak unidirectional magnetic anisotropy for film with nearly 180° domain wall displacement. Our experimental results provide a practical method to establish room-temperature exchange bias in FM/G-type AFM without field cooling. Furthermore, the magnetic-field-induced switchable exchange-bias, the spin-flop coupling effect and the angular dependent asymmetry of magnetization reversal in the vicinity of hard axis in Fe/ErFeO3 heterostructures may provide new insights on the interfacial exchange coupling in FM/AFM systems.

Emergent propagation modes of ferromagnetic swimmers in constrained geometries

Magnetic microswimmers, composed of hard and soft ferromagnets connected by an elastic spring, are modelled under low Reynolds number conditions in the presence of geometrical boundaries. Approaching a surface, the magneto-elastic swimmer's velocity increases and its trajectory bends parallel to the surface contour. Further confinement to form a planar channel generates new propagation modes as the channel width narrows, altering the magneto-elastic swimmer's speed, orientation, and direction of travel. Our results demonstrate that constricted geometric environments, such as occuring in microfluidic channels or blood vessels, may influence the functionality of magneto-elastic microswimmers for applications such as drug delivery.