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We investigate the surface oxidation in a van der Waals ferromagnet Fe3-xGeTe2, a material widely utilized for spintronic applications. While known for its relative air-insensitivity, exposure to air during the handling process (e.g. device or heterostructure fabrication) can lead to reduction or disappearance of its magnetic signal. Comparison of x-ray absorption and x-ray magnetic circular dichroism spectra between pristine and air-exposed Fe3-xGeTe2 confirm a naturally oxidized surface layer on the material. The surface oxide layer has predominantly Fe3+ content. X-ray absorption spectroscopy done on micron-sized exfoliated Fe3-xGeTe2 flakes reveal that the change in the surface chemical properties can be quite significant for thin flakes. The surface modulation of Fe3-xGeTe2 can lead to inaccuracies in characterizing its interfacial magnetic and spin transport properties, and complicate device and heterostructure fabrication processes.

The induced Pt magnetization in a Pt/Co/Pt thin film structure is studied. The normally nonmagnetic Pt acquires a magnetic moment due to the magnetic proximity effect at the Co–Pt interfaces. Element specific Pt structural and magnetic properties are characterized by synchrotron-based resonant x-ray reflectivity and x-ray resonant magnetic reflectivity measurements. An advanced analysis method based on Bayesian inference is used for model fitting of the x-ray data. Using this method, we retrieve the best fit values of material parameters (e.g., thickness, interfacial roughness) from the data. Analysis of x-ray reflectivity data of this specific system shows that the Pt magnetization and Co–Pt interfacial roughness is significantly different between the top and bottom Pt layers, with both values being larger in the top Pt. The successful application of this Bayesian method to study the magnetic and structural properties of a thin film system demonstrates its effectiveness for x-ray reflectivity data analysis.

A controversy arose over the interpretation of the recently observed hump features in Hall resistivity ρ x y from ultra-thin SrRuO3 (SRO) film; it was initially interpreted to be due to topological Hall effect but was later proposed to be from existence of regions with different anomalous Hall effect (AHE). In order to settle down the issue, we performed Hall effect as well as magneto-optic Kerr-effect measurements on 4 unit cell SRO films. Clear hump features are observed in ρ x y , whereas neither hump feature nor double hysteresis loop is seen in the Kerr rotation which should be proportional to the magnetization. In addition, magnetization measurement by superconducting quantum interference device shows no sign of multiple coercive fields. These results show that inhomogeneous AHE alone cannot explain the observed hump behavior in ρ x y data. We suggest that emergence of the hump structure in ρ x y is closely related to the growth condition.

The electric field-induced modifications of magnetic anisotropy in CoFeB/MgO systems are studied using X-ray resonant magnetic scattering and magneto-optical Kerr effect. Voltage dependent changes of the magnetic anisotropy of −12.7 fJ/Vm and −8.32 fJ/Vm are observed for Ta/CoFeB/MgO and Hf/CoFeB/MgO systems, respectively. This implies that the interfacial perpendicular magnetic anisotropy is reduced (enhanced) when electron density is increased (decreased). X-ray resonant magnetic scattering measurements reveal that the small in-plane magnetic component of the remanent state of CoFeB/MgO systems with weak magnetic anisotropy changes depending on the applied voltage leading to modification of the magnetic anisotropy at the CoFeB/MgO interface.

Understanding spin textures in magnetic systems is extremely important to the spintronics and it is vital to extrapolate the magnetic Hamiltonian parameters through the experimentally determined spin. It can provide a better complementary link between theories and experimental results. We demonstrate deep learning can quantify the magnetic Hamiltonian from magnetic domain images. To train the deep neural network, we generated domain configurations with Monte Carlo method. The errors from the estimations was analyzed with statistical methods and confirmed the network was successfully trained to relate the Hamiltonian parameters with magnetic structure characteristics. The network was applied to estimate experimentally observed domain images. The results are consistent with the reported results, which verifies the effectiveness of our methods. On the basis of our study, we anticipate that the deep learning techniques make a bridge to connect the experimental and theoretical approaches not only in magnetism but also throughout any scientific research.

Current-induced magnetic domain wall (DW) motion is an important operating principle of spintronic devices. Injected current generates spin torques (STs) on the DWs in two ways. One is the spin transfer from magnetic domains to the walls by the current flowing in the magnet. Current flow in attached heavy metals also generates another ST because of the spin-Hall effect. Both phenomena explain the wall motions well; therefore, their respective contribution is an important issue. Here, we show the simultaneous measurement of both torques by using magnetic facet domains that form mountain-shaped domains with straight walls. When the STs and the external magnetic field push the walls in opposite directions, the walls should have equilibrium angles to create balanced states. Such angles can be modulated by an additional in-plane magnetic field. Angle measurements distinguish the STs because each torque has a distinct mechanism related to the DW structure.

Merons which are topologically equivalent to one-half of skyrmions can exist only in pairs or groups in two-dimensional (2D) ferromagnetic (FM) systems. The recent discovery of meron lattice in chiral magnet Co8Zn9Mn3 raises the immediate challenging question that whether a single meron pair, which is the most fundamental topological structure in any 2D meron systems, can be created and stabilized in a continuous FM film? Utilizing winding number conservation, we develop a new method to create and stabilize a single pair of merons in a continuous Py film by local vortex imprinting from a Co disk. By observing the created meron pair directly within a magnetic field, we determine its topological structure unambiguously and explore the topological effect in its creation and annihilation processes. Our work opens a pathway towards developing and controlling topological structures in general magnetic systems without the restriction of perpendicular anisotropy and Dzyaloshinskii–Moriya interaction., A meron is one half of a skyrmion but whether a single meron pair can be created and stabilized remains a challenging question. Here, Gao et al. develop a method to create and stabilize individual pairs of merons in a continuous Py film by local vortex imprinting from Co disks.

We propose a new efficient algorithm to simulate magnetic structures numerically. It contains a generative model using a complex-valued neural network to generate k-space information. The output information is hermitized and transformed into real-space spin configurations through an inverse fast Fourier transform. The Adam version of stochastic gradient descent is used to minimize the magnetic energy, which is the cost of our algorithm. The algorithm provides the proper ground spin configurations with outstanding performance. In model cases, the algorithm was successfully applied to solve the spin configurations of magnetic chiral structures. The results also showed that a magnetic long-range order could be obtained regardless of the total simulation system size.

Using Monte-Carlo simulations and micromagnetic simulations, we reveal how the spin structural correlation and the skyrmion dynamics are affected by the interlayer coupling in a chiral magnetic bilayer system, in which the two layers have opposite chirality. The interaction through interlayer coupling between chiral magnetic structures influences the static and dynamics properties profoundly. The competition between the Dzyaloshinskii-Moriya interaction and the interlayer interaction allows multiple magnetic structures to be energetically stable, which includes sole skyrmion states (skyrmion appears in only one of the layers) and skyrmion pair states (coupled skyrmions in top and bottom layers). When current driven spin transfer torques are applied to each state, the sole skyrmion state is mainly propelled by a spin transfer torque causing the skyrmion hall effect, but the skyrmion pair state is propelled by a torque from skyrmion-skyrmion interaction and not influenced by the skyrmion hall effect. Also upon application of an external magnetic field, we found the skyrmions in a skyrmion pair state extinguish in an exclusive way, as the annihilation of a skyrmion in one of the layers stabilizes the once paired skyrmion in the other layer, i.e. the skyrmion lattice sites have only one skyrmion in either layer.

The depth-resolved magnetic configuration of a Ga0.97Mn0.03As/GaAs:Be multilayer with an antiferromagnetic interlayer exchange coupling was investigated using surface-sensitive soft x-ray resonant magnetic reflectivity (XRMR). We observed intriguing opposite increments in the XRMR intensities at the half-Bragg peak position between two different remanent states with opposite field sweep directions. A quantitative analysis shows that these opposite intensity increments result from two different anti-parallel spin configurations in such a way that the magnetization of top-most magnetic layer is pinned along the saturation magnetization direction before the remanent state. This surface pinning effect is important for understanding spin-dependent transport in semiconducting magnetic multilayers.

Magnetization, the magnetic moment per unit volume, is the basic quantity for describing a magnetic system. Generally, magnetic imaging techniques show the relative magnitude of magnetization, and its direction. Determining the $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e$ value of magnetization requires separate measurement with a reference sample, though. This study describes the stripe state of magnetization, in which the absolute value can be measured simply by magneto-optical imaging. In particular, this approach is applicable to samples so small that they cannot be measured by conventional magnetometers.

We use a Monte Carlo simulation to investigate the magnetic structures in a bilayer magnetic system containing a locally inverted interlayer coupling region. The stabilization of multiple magnetic structures, including the magnetic skyrmion, is possible in the locally inverted interlayer coupling region. Various factors such as the region area, anisotropy, interlayer coupling strength, and exchange coupling strength affect the properties of the structures, including their size and chirality. We obtain conditions for the stabilization and transitions of these magnetic structures. The Dzyaloshinskii-Moriya interaction (DMI) and dipolar interaction play prominent roles as they significantly enhance the formation and stability of the structures. An asymmetric feature can arise from the broken inversion symmetry in the structure formation, yielding an interfacial DMI, which stabilizes the skyrmion. It is demonstrated that the dipole interaction also acts as an effective interfacial DMI in the system.

How might we measure the Dzyaloshinskii-Moriya interaction (DMI), which can be so important in spintronics? When this magnetic exchange interaction is small, knowing the actual value becomes significant, since operating speed is proportional to it in spintronic devices based on domain-wall movement. The authors develop a method for measuring a weak DMI: When current and a magnetic field are applied to a perpendicularly magnetized thin film, the characteristic morphology of the magnetic domains that form can be observed, and the DMI obtained from the angles present in the unusual domain shapes. This will improve the engineering of spintronic logic and memory applications.

The structural and magnetic properties of Co/C60/pentacene and Co/C60 thin film structures were investigated. Atomic force microscopy and x-ray reflectivity analysis show that the presence or absence of a pentacene buffer layer leads to a highly textured or randomly oriented C60 layer, respectively. A Co film deposited on a randomly oriented C60 layer penetrates into the C60 layer when it is deposited at a slow deposition rate. The Co penetration can be minimized, regardless of the Co deposition rate, by growth on a highly textured and nanostructured C60/pentacene layer. Vibrating sample magnetometry measurements show that the saturation magnetization of Co/C60/pentacene is significantly reduced compared to that of Co/C60. On the other hand, the Co penetration does not seem to have an effect on the magnetic properties, suggesting that the structural properties of the Co and C60 layer, rather than the Co penetration into the organic C60 layer, are critical to the magnetic properties of the Co/C60.

Magnetic skyrmions are topologically protected spin textures with attractive properties suitable for high-density and low-power spintronic device applications. Much effort has been dedicated to understanding the dynamical behaviours of the magnetic skyrmions. However, experimental observation of the ultrafast dynamics of this chiral magnetic texture in real space, which is the hallmark of its quasiparticle nature, has so far remained elusive. Here, we report nanosecond-dynamics of a 100nm-diameter magnetic skyrmion during a current pulse application, using a time-resolved pump-probe soft X-ray imaging technique. We demonstrate that distinct dynamic excitation states of magnetic skyrmions, triggered by current-induced spin–orbit torques, can be reliably tuned by changing the magnitude of spin–orbit torques. Our findings show that the dynamics of magnetic skyrmions can be controlled by the spin–orbit torque on the nanosecond time scale, which points to exciting opportunities for ultrafast and novel skyrmionic applications in the future., Magnetic skyrmions are potentially suitable for future spintronic devices, but their dynamical behaviour in real space remains elusive. Here, Woo et al. report nanosecond-dynamics of a 100nm-size magnetic skyrmion triggered by current-induced spin-orbit torques.

The interfacial perpendicular magnetic anisotropy in W/CoFeB ( 1.2 similar to 3 nm)/MgO thin film structures is strongly dependent on temperature, and is significantly reduced at high temperature. The interfacial magnetic anisotropy is generally proportional to the third power of magnetization, but an additional factor due to thermal expansion is required to explain the temperature dependence of the magnetic anisotropy of ultrathin CoFeB films. The reduction of the magnetic anisotropy is more prominent for the thinner films; as the temperature increases from 300 K to 400 K, the anisotropy is reduced similar to 50% for the 1.2-nm-thick CoFeB, whereas the anisotropy is reduced similar to 30% for the 1.7-nm-thick CoFeB. Such a substantial reduction of magnetic anisotropy at high temperature is problematic for data retention when incorporating W/CoFeB/MgO thin film structures into magneto-resistive random access memory devices. Alternative magnetic materials and structures are required to maintain large magnetic anisotropy at elevated temperatures. (C) 2017 Author(s).

In order to utilize the spin field effect transistor in logic applications, the development of two types of complementary transistors, which play roles of the n- and p-type conventional charge transistors, is an essential prerequisite. In this research, we demonstrate complementary spin transistors consisting of two types of devices, namely parallel and antiparallel spin transistors using InAs based quantum well channels and exchange-biased ferromagnetic electrodes. In these spin transistors, the magnetization directions of the source and drain electrodes are parallel or antiparallel, respectively, depending on the exchange bias field direction. Using this scheme, we also realize a complementary logic operation purely with spin transistors controlled by the gate voltage, without any additional n- or p-channel transistor.

Spintronic devices based on magnetic skyrmions are a promising candidate for next-generation memory applications due to their nanometre-size, topologically-protected stability and efficient current-driven dynamics. Since the recent discovery of room-temperature magnetic skyrmions, there have been reports of current-driven skyrmion displacement on magnetic tracks and demonstrations of current pulse-driven skyrmion generation. However, the controlled annihilation of a single skyrmion at room temperature has remained elusive. Here we demonstrate the deterministic writing and deleting of single isolated skyrmions at room temperature in ferrimagnetic GdFeCo films with a device-compatible stripline geometry. The process is driven by the application of current pulses, which induce spin-orbit torques, and is directly observed using a time resolved nanoscale X-ray imaging technique. We provide a current-pulse profile for the efficient and deterministic writing and deleting process. Using micromagnetic simulations, we also reveal the microscopic mechanism of the topological fluctuations that occur during this process., Comment: 27 pages, 4 figures

In spintronic devices consisting of ferromagnetic/nonmagnetic systems, the ferromagnet-induced magnetic moment in the adjacent nonmagnetic material significantly influences the spin transport properties. In this study, such magnetic proximity effect in a Pd/Co/Pd trilayer system is investigated by x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity, which enables magnetic characterizations with element and depth resolution. We observe that the total Pd magnetic moments induced at the top Co/Pd interface are significantly larger than the Pd moments at the bottom Pd/Co interface, whereas transmission electron microscopy and reflectivity analysis indicate the two interfaces are nearly identical structurally. Such asymmetry in magnetic proximity effects could be important for understanding spin transport characteristics in ferromagnetic/nonmagnetic systems and its potential application to spin devices.

Artificial interface anisotropy is demonstrated in alternating Co/Pt and Co/Pd stripe patterns, providing a means of forming magnetic anisotropy using lithography. In-plane hysteresis loops measured along two principal directions are explained in depth by two competing shape and interface anisotropies, thus confirming the formation of interface anisotropy at the Co/Pt and Co/Pd interfaces of the stripe patterns. The measured interface anisotropy energies, which are in the range of 0.2–0.3 erg/cm2 for both stripes, are smaller than those observed in conventional multilayers, indicating a decrease in smoothness of the interfaces when formed by lithography. The demonstration of interface anisotropy in the Co/Pt and Co/Pd stripe patterns is of significant practical importance, because this setup makes it possible to form anisotropy using lithography and to modulate its strength by controlling the pattern width. Furthermore, this makes it possible to form more complex interface anisotropy by fabricating two-dimensional patterns. These artificial anisotropies are expected to open up new device applications such as multilevel bits using in-plane magnetoresistive thin-film structures.