External In-plane Magnetic Field Research Articles

Effects of Magnetostatic Interactions in FeNi-Based Multilayered Magnetoimpedance Elements.

Multilayered [Cu(3 nm)/FeNi(100 nm)]5/Cu(150 nm)/FeNi(10 nm)/Cu(150 nm)/FeNi(10 nm)/Cu(150 nm)/[Cu(3 nm)/FeNi(100 nm)]5 structures were obtained by using the magnetron sputtering technique in the external in-plane magnetic field. From these, multilayer magnetoimpedance elements were fabricated in the shape of elongated stripes using the lift-off lithographic process. In order to obtain maximum magnetoimpedance (MI) sensitivity with respect to the external magnetic field, the short side of the rectangular element was oriented along the direction of the technological magnetic field applied during the multilayered structure deposition. MI sensitivity was defined as the change of the total impedance or its real part per unit of the magnetic field. The design of the elements (multilayered structure, shape of the element, etc.) contributed to the dynamic and static magnetic properties. The magnetostatic properties of the MI elements, including analysis of the magnetic domain structure, indicated the crucial importance of magnetostatic interactions between FeNi magnetic layers in the analyzed [Cu(3 nm)/FeNi(100 nm)]5 multilayers. In addition, the uniformity of the magnetic parameters was defined by the advanced technique of the local measurements of the ferromagnetic resonance field. Dynamic methods allowed investigation of the elements at different thicknesses by varying the frequency of the electromagnetic excitation. The maximum sensitivity of 40%/Oe with respect to the applied field in the range of the fields of 3 Oe to 5 Oe is promising for different applications.

Incommensurate magnetic order in an axion insulator candidate EuIn2As2 investigated by NMR measurement

Magnetic topological insulators exhibit unique electronic states due to the interplay between the electronic topology and the spin structure. The antiferromagnetic metal EuIn2As2 is a prominent candidate material in which exotic topological phases, including an axion insulating state, are theoretically predicted depending on the magnetic structure of the Eu2+ moments. Here, we report experimental results of the nuclear magnetic resonance (NMR) measurements of all the nuclei in EuIn2As2 to investigate the coupling between the magnetic moments in the Eu ions and the conduction electrons in In2As2 layers and the magnetic structure. The 75As and 115In NMR spectra observed at zero external magnetic fields reveal the appearance of internal fields of 4.9 and 3.6 T, respectively, at the lowest temperature, suggesting a strong coupling between the conduction electrons in the In2As2 layer and the ordered magnetic moments in the Eu ions. The 75As NMR spectra under in-plane external magnetic fields show broad distributions of the internal fields produced by an incommensurate fan-like spin structure which turns into a forced ferromagnetic state above 0.7 T. We propose a spin reorientation process that an incommensurate helical state at zero external magnetic field quickly changes into a fan state by applying a slight magnetic field.

Robust Field-Free Switching Using Large Unconventional Spin-Orbit Torque in an All-Van der Waals Heterostructure.

The emerging all-van der Waals (vdW) magnetic heterostructure provides a new platform to control the magnetization by the electric field beyond the traditional spintronics devices. One promising strategy is using unconventional spin-orbit torque (SOT) exerted by the out-of-plane polarized spin current to enable deterministic magnetization switching and enhance the switching efficiency. However, in all-vdW heterostructures, large unconventional SOT remains elusive and the robustness of the field-free switching against external magnetic field has not been examined, which hinders further applications. Here, the study demonstrates the field-free switching in an all-vdW heterostructure combining a type-II Weyl semimetal TaIrTe4 and above-room-temperature ferromagnet Fe3GaTe2. The fully field-free switching can be achieved at 2.56 × 1010 Am-2 at 300K and a large SOT effective field efficiency of the out-of-plane polarized spin current generated by TaIrTe4 is determined to be 0.37. Moreover, it is found that the switching polarity cannot be changed until the external in-plane magnetic field reaches 252mT, indicating a robust switching against the magnetic field. The numerical simulation suggests the large unconventional SOT reduces the switching current density and enhances the robustness of the switching. The work shows that all-vdW heterostructures are promising candidates for future highly efficient and stable SOT-based devices.

Field-free switching of perpendicular magnetization in a noncollinear antiferromagnetic Mn3Sn/[Pt/Co]4 heterostructure

Spin-orbit torque (SOT) manipulation of perpendicular magnetization is essential for realizing spintronic storage and logic devices. When a charge current flows through a heavy metal/ferromagnetic layer structure, a spin current can be generated by the heavy metal and injects into the ferromagnetic layer with perpendicular magnetic anisotropy and switch its magnetization via the SOT. However, in order to realize the deterministic switching, an in-plane external magnetic field is needed to break the inversion symmetry, a technological requirement that hampers the practical application of SOT. Pt/Co multilayer is a good magnetic recording media and, thus, the all-electric manipulation of its perpendicular magnetic moments is of practical significance. The noncollinear spin structure in Mn3Sn allows an unconventional spin polarization direction, and the associated SOT can switch the magnetic moments in the absence of an external field. In this work, we demonstrate the field-free SOT switching of a heterostructure utilizing Mn3Sn and Pt/Co multilayer as the spin source and the ferromagnetic layer with perpendicular magnetization, respectively. Our results are important for advancing the development of spintronic devices based on noncollinear antiferromagnets.

Field-Free Spin-Orbit Torque Magnetization Switching in a Perpendicularly Magnetized Semiconductor (Ga,Mn)As Single Layer.

Current-induced spin-orbit torque (SOT) in a perpendicularly magnetized single layer has a strong potential to switch the magnetization using an extremely low current density, which is generally 2-3 orders of magnitude smaller than that required for conventional metal bilayer systems. However, an in-plane external magnetic field has to be applied to break the symmetry and achieve deterministic switching. To further enhance the high-density integration and accelerate the practical application of highly efficient SOT magnetic random-access memory (SOT-MRAM) devices, field-free SOT magnetization switching in a ferromagnetic single layer is strongly needed. In a spin-orbit ferromagnet (a ferromagnet with strong spin-orbit interaction) with crystal inversion asymmetry and a multi-domain structure, the internal Dzyaloshinskii-Moriya effective fields are considered to induce field-free switching. Here, combined with strong spin-orbit coupling and a tilted anisotropy axis induced by a nonuniform Mn distribution and a possible magnetocrystalline anisotropy resulting from a slight substrate tilting, we successfully achieve magnetization switching in a spin-orbit ferromagnet (Ga,Mn)As single layer by utilizing SOT without applying any external magnetic field. Our findings help to deeply elucidate the SOT switching mechanism and can advance the development of a highly efficient MRAM with better scalability.

Photogalvanic spectroscopy on MnBi2Te4 topological insulator thin films

We demonstrate zero-bias mid-infrared photocurrent generation in topological insulator MnBi2Te4 thin films. The symmetry breakings at the surface and interfaces lead to the coexistence of Dirac and Rashba band states, which enable two kinds of photogalvanic responses. One is the magneto-photogalvanic effect in the presence of an external in-plane magnetic field perpendicular to photocurrent direction, and the other is the light-polarization-dependent linear photogalvanic effect arising from in-plane symmetry breakings, both observed up to room temperature. We disentangle these contributions by light-polarization and temperature dependent spectroscopy under the varying magnetic field.

Control of closure domain state circulation in coupled triangular permalloy elements using MFM tip

We demonstrate experimentally and numerically the control of magnetization circulation of the closure domain state in thin permalloy squares divided diagonally into four dipolar-coupled isosceles triangles. The control over the magnetization circulation was achieved by the mutual application of a suitable external in-plane magnetic field and interaction with a magnetic force microscopy (MFM) tip. Numerical simulations confirm that the proposed switching protocol is valid for the broad range of MFM tips. Experiments performed on four diagonally cut squares show that this method is robust against slight imperfections in the fabrication process. Moreover, by varying the polarity of the applied field and the direction of the MFM scanning, proper symmetry response of the switching method is observed.

Strain-mediated voltage controlled magnetic anisotropy based switching for magnetic memory applications

Reliability and packing density concerns are the two major shortcomings of spin transfer torque and spin orbit torque based magnetic memory, respectively. Voltage controlled magnetic anisotropy (VCMA) becomes energy efficient and fast, showing transcendence for the writing mechanism in the magnetic tunnel junction. Deterministic switching cannot be achieved by VCMA alone in the out of plane nanomagnet. It requires an external in-plane magnetic field, but the use of an external field is inconvenient for on-chip applications. We exploit stress and exchange bias provided by an antiferromagnetic material to mitigate the external magnetic field requisite. We perform macro-spin simulations using the Landau–Lifshitz–Gilbert equation at room temperature. We use the VCMA effect cum stress effect to investigate field free switching performance, and this improves the write error rate (WER) to 5×10−5 against WER of 0.1 with the VCMA effect alone. We studied the effects of applied voltage (amplitude and pulse width), exchange bias field, and VCMA coefficient on the switching performance in detail. This proposed two-terminal device can be helpful in achieving high cell density to implement nonvolatile magnetic memory.

Field-Free Magnetization Switching in A1 CoPt Single-Layer Nanostructures for Neuromorphic Computing

Current-induced field-free magnetization switching in single-layer films has been widely investigated for the application of spin–orbit torque (SOT) drivers in low-power, high-density, and nano-sized memory. In this paper, we report field-free SOT switching with threefold angle dependence in A1 disordered single-layer CoPt using the MgO (111) substrate. It is found that the magnetization could not switch in the absence of an external in-plane magnetic field when the current was flowing in the direction of the mirror symmetry high-symmetry axis ([11–2]). While along the direction of the low-symmetry axis ([1–10]) which destroyed the mirror symmetry to the greatest extent, field-free switching could be performed, and the switching ratio was up to 30%. This was mainly attributed to the composition gradient-induced in-plane inversion asymmetry and the low symmetry at the interface of Co/Pt broken out-of-plane inversion. Furthermore, CoPt devices with nanoscale thickness exhibited stable multistate magnetic switching behavior without an external magnetic field, and simultaneously nonlinear synaptic characteristics were obtained. A neural network was established, in which the synaptic weights between the hidden layer and the output layer are updated by using the resistance state of SOT synaptic devices, and the network could achieve about 90.54% recognition accuracy when applied to MNIST’s handwritten digital dataset. This work confirmed that a field-free SOT switch could be realized in single-layer CoPt, which provided theoretical guidance and data support for spin devices.

High magnetic field superconductivity in a two-band superconductor

We present a mechanism allowing for superconductivity at high magnetic fields, beyond the Pauli-Chandrasekhar-Clogston limit. We consider spin splitting induced by an in-plane external magnetic field in a superconductor with two relevant bands close to the Fermi level. The magnetic field therefore controls which bands are available for Cooper pair formation. The presence of interband superconducting pairing, i.e., Cooper pairs formed by electrons with different band indices, produces high-field reentrant superconducting domains, whose critical magnetic field violates the Pauli-Chandrasekhar-Clogston limit. We analyze how the interband superconducting domains are influenced by the band parameters, and show that, for a certain range of parameters, the system presents two separate superconducting regions, for low and high magnetic field.

Asymmetry-driven reconfigurability of magnetic vortices in hemispherical shells

Inhomogeneous magnetic configurations like vortices attract tremendous appeal as an emerging candidate in understanding nanoscale spin behaviours and utilizing their spin configurations for advanced technological applications. For vortex-driven practical applications, independent control and manipulation of both the circularity and polarity of the magnetic vortex is a prerequisite. In this study, we have shown that both the circularity and polarity of the magnetic vortex in an asymmetric hemispherical shell can be controlled by changing a single parameter - the direction of the in-plane external magnetic field. Furthermore, our results demonstrate the influence of geometrical asymmetry on the characteristics of magnetic vortices in ferromagnetic permalloy shells. These findings are expected to be helpful while designing vortex-based advanced technologies.

Field-free spin-orbit torque switching of GdCo ferrimagnet with broken lateral symmetry by He ion irradiation

Current-induced magnetization switching by spin-orbit torque (SOT) is of great importance for the energy-efficient operation of spin-based memory and logic devices. However, the requirement of an external in-plane magnetic field to deterministically switch the perpendicular magnetization of a device is a bottleneck for device application. There have been many efforts to realize field-free SOT switching using interlayer/exchange coupling, the spin valve structure, or materials with lateral symmetry breaking. However, limitations of material selection or layer structure modification hinder the application of these methods in practice. Here, we demonstrate the field-free SOT switching of a GdCo ferrimagnet with lateral symmetry breaking by He ion irradiation. Local control of the magnetic property with different He ion irradiation conditions induces a lateral magnetic gradient orthogonal to the current flow direction in the ferrimagnet. We also observe out-of-plane-SOT generation due to lateral symmetry breaking, which is essential for field-free switching. Since the He ion irradiation technique is utilized for the fabrication of complementary−metal−oxide−semiconductors, and resolution can reach the nanometer level, our findings have the potential to serve as the basis for new developments in the fabrication of wafer-scale spintronic memory and logic devices with high energy efficiency and high density.

Possible quantization and half-quantization in the anomalous Hall effect caused by in-plane magnetic field

In this paper we propose that quantized and nearly half-quantized intrinsic anomalous Hall effect can be induced by in-plane external magnetic field through the Zeeman coupling in nonmagnetic two-dimensional (2D) systems with sizable spin-orbital coupling but without twofold rotational symmetry. An analytical result is derived for the 2D electron gas model with ${C}_{3v}$ symmetry. Further, based on the $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ Hamiltonian derived from first principle calculations, we find that the quantized and nearly half-quantized Hall conductance can be observed in ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ thin film in the clean limit with strong in-plane magnetic field $B>20\phantom{\rule{4pt}{0ex}}\mathrm{T}$ and low temperature $T<100\phantom{\rule{4pt}{0ex}}\mathrm{mK}$.

Current-driven magnetization switching under zero field in Pt/Ta(wedge)/CoFeB/MgO multilayers

The switching of perpendicularly magnetized ferromagnets via current-induced spin–orbit torques is of great interest because of its potential applications in memory and logic devices. However, the in-plane electric current itself is not enough to switch the magnetization. In addition to the electric current, an in-plane external magnetic field is required for magnetization switching. This limits the usage of such devices in spintronic applications. Here, we work on the current-driven perpendicular magnetization switching in the Pt/Ta(wedge)/CoFeB/MgO multilayer. The structural symmetry is broken in both z-axis and in-plane due to the wedge Ta layer, which results in a field-like spin–orbit torque. The βz value extracted from the slope of the offset field vs current density increases with Ta layer thickness (<1.0 nm) and then decreases up to <1.3 nm. Thanks to both in-plane and z-axis asymmetries that enable the current-driven magnetization switching without the need for a magnetic field. We showed switching of the magnetization with a perpendicular magnetic anisotropy, switching in a wide range of Ta layer in Pt/Ta(wedge)/CoFeB/MgO multilayer.

Spin Seebeck effect at low temperatures in the nominally paramagnetic insulating state of vanadium dioxide

The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO2 films that appears below 30 K and that increases with a decrease in temperature. The spin Seebeck response has a nonhysteretic dependence on the in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the monoclinic ground state.

Evaluation of multi-bit domain wall motion by low current density to obtain ultrafast data rate in a compensated ferrimagnetic wire

Architectures based on multi-bit magnetic domain walls (DWs) take advantage of the fast speed, high density, nonvolatility, and flexible design of DWs to process and store data bits. However, controlling multi-bit DWs driven by electric current at an ideal position remains a significant challenge for developing integrated spintronic applications with high reliability and low power consumption. We exhibit the possibility of driving fast and stable multi-bit DWs at low current density without an in-plane external magnetic field in Fe-rich GdFeCo magnetic wires. When an in-plane magnetic field is applied in the wire direction, the front edge accelerates, although the rear edge decelerates, and the recorded data are destroyed. Hence, this method is not practical. Here, the DW speed of the multi-bit DWs is 1500 m/s under a low current density of 29 × 1010 (A/m2). A straight DW shape is required to accurately read the bits of information by the tunneling magnetoresistance head in real DW memory devices. Moreover, we demonstrate that the DW position is related to the DW shape after injecting a pulse current into the magnetic wire. A straight DW shape is exhibited for 3 ns pulse duration width, while the DW shape became rounded for 30 and 50 ns pulse duration widths. Our finding provides a practical concept for multiple-bit-per-cell memory and presents a viable platform for DW memory applications.

Transition magnon modes in thin ferromagnetic nanogratings

This work presents micromagnetic simulations in ferromagnetic nanogratings for the full range of directions of an applied in-plane external magnetic field. We focus on the modification of the magnon mode characteristics when the magnetic field orientation is gradually changed between the classical Damon-Eshbach and backward-volume geometries. We found that in a specific range of field directions, the magnon mode parameters differ significantly from the parameters in the classical cases; namely, the modes are characterized by complex spatial distributions and have low group velocities. The center of this range corresponds to the direction of the external magnetic field, which gives the maximal nonuniform distribution of the static magnetization in the nanogratings.

(Invited) Unusual Superconducting Behavior in an Ultrathin Weyl Semimetal

Two dimensional materials are layered materials that may be readily exfoliated down to a single atomic layer, presenting an opportunity to noninvasively, and efficiently, control electrical and magnetic ordering, as well as topology. In this talk, I discuss how topology emerges from the bulk, down to the monolayer and how the influence of an electrostatic gate allows us to identify a unique superconducting state in few-layer Td-MoTe2. In the bulk, MoTe2 is a type II Weyl semimetal with a superconducting transition temperature of 120 mK. First, I will discuss how in the clean limit the superconducting transition temperature is enhanced by a factor of 60x, as compared to bulk, in monolayer Td-MoTe2, while still retaining a low carrier density (~1013/cm2), and a density of states that is comparable to the bulk. I will show how the crystal structure factors into the behavior of the superconducting state under external in-plane magnetic fields and how this can be used to quantify of the spin-orbit coupling, an important factor for determining topology and realizing topological superconducting states. After discussion of the monolayer, I will show that this enhancement remains in bilayer MoTe2, despite the change in symmetry. Strangely, our results indicate that superconductivity in bilayer MoTe2 is much more tunable by an electrostatic gate than that of monolayer MoTe2. In addition, for monolayer the response to high in-plane magnetic fields is distinct from that of other 2D superconductors and reflects the canted spin texture of the electron pockets. In bilayer, there is a strong deviation from this behavior, and instead an anomalous anisotropic behavior is observed with in-plane magnetic fields. These findings have profound implications on 2D superconductivity, where previously strong spin-orbit effects were only considered in the case of out-of-plane spin textures and offer an avenue for future exploration in similarly structured materials.

Controlling domain wall and field-free spin–orbit torque switching in synthetic antiferromagnets

Perpendicular magnetization switching driven by spin–orbit torques plays an increasingly important role for spintronic devices toward practical applications but is also hindered by the well-known technical challenge that an external in-plane magnetic field is required for deterministic switching. Here, we show that the deterministic switching can be achieved in synthetic antiferromagnets through the flexible domain control in the absence of external magnetic fields. Specifically, we have observed that the domain wall (DW) distorts under an applied electric current in contrast to the conventional rigid DW motion in a single ferromagnet. More importantly, the distorted DWs can be precisely controlled under zero magnetic field, leading to the deterministic switching. Our results indicate that the critical technical challenge may be addressed by employing a synthetic antiferromagnetic layer through the DW motion dominated field-free switching.

Chiral symmetry protected topological nodal superconducting phase and Majorana Fermi arc

With an external in-plane magnetic field, we show the emergence of a topological nodal superconducting phase of the two-dimensional topological surface states. This nodal superconducting phase is protected by the chiral symmetry with a non-zero magnetic field, and there are corresponding Majorana Fermi arcs (also known as flat band Andreev bound states) connecting the two Majorana nodes along the edges, similar to the case of Weyl semimetal. The topological nodal superconductor is an intermediate phase between two different chiral superconductors, and is stable against the effects of substrates. The two-dimensional effective theory of the nodal superconducting phase also captures the low energy behavior of a three-dimensional lattice model which describes the iron-based superconductor with a thin film geometry. The localizations of the Majorana nodes can be manipulated through external in-plane magnetic fields, which may introduce a non-trivial topological Berry phase between them.