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Funding Information: This work was supported by the Academy of Finland (Grants No. 295269, No. 306978, and No. 327804). K.R.E. acknowledges funding from the NSF (Grant No. NSF-MPS-2006456). E.G. was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo -FAPESP (Grant No. 18/19586-9). T.A.-N. has been supported in part by the Academy of Finland QTF CoE Grant No. 312298. Publisher Copyright: © 2022 American Physical Society. Magnetic skyrmions are topologically distinct particles whose thermally activated motion could be used to implement probabilistic computing paradigms. While solid-liquid phase transitions in skyrmion lattices have been demonstrated, the behavior of a skyrmion liquid and the effects of pinning are largely unknown. Here we demonstrate the formation of a weakly pinned skyrmion liquid in a magnetic heterostructure. By inserting a Ru wedge layer at the ferromagnet/heavy metal interface we evaluate the dependence of skyrmion dynamics on the skyrmion size and density. Our experiments demonstrate that the diffusion of skyrmions is largest in dense liquids with small skyrmions. The thermal motion of skyrmions at room temperature easily overcomes the narrow distribution of pinning site energies in the granular film structure, satisfying a key requirement of probabilistic device architectures. Micromagnetic simulations support the findings and also reveal the existence of a thermally activated high-frequency collective oscillation.

Funding Information: This work was supported by the Academy of Finland (Grant Nos. 295269, 306978, 321983, 325480, and 327804). We acknowledge the provision of computational resources provided by the Aalto Science-IT project. Publisher Copyright: © 2022 Author(s). Active control of propagating short-wavelength spin waves in perpendicularly magnetized materials is promising for designing nanoscale magnonic devices. One method of manipulating spin waves on the nanoscale is through their interaction with magnetic textures, an example of which is the magnetic skyrmion - a particle-like topological object stabilized in thin film heterostructures by the Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy. In this paper, the interaction between spin waves and skyrmions is studied using micromagnetic simulations. The magnetic parameters chosen are similar to those found experimentally, leading to a skyrmion with an extended core of reversed magnetization. The effect of a propagating spin wave on the skyrmion is to cause the emission of a secondary spin wave by the skyrmion. At low frequencies, where the incoming spin wave wavelength is much larger than the skyrmion, this leads to a nearly circular re-emitted spin wave. The pattern of emission becomes increasingly complex at higher frequencies as the wavelength becomes similar to the skyrmion size due to the complex excitation of the extended core. The emitted spin wave profile can be controlled by altering the size of the skyrmion through the magnitude of the DMI, providing a method of tuning the system.

Magnetic skyrmions are topologically-distinct swirls of magnetic moments which display particle-like behaviour, including the ability to undergo thermally-driven diffusion. In this paper we study the thermally activated motion of arrays of skyrmions using temperature dependent micromagnetic simulations where the skyrmions form spontaneously. In particular, we study the interaction of skyrmions with grain boundaries, which are a typical feature of sputtered ultrathin films used in experimental devices. We find the interactions lead to two distinct regimes. For longer lag times the grains lead to a reduction in the diffusion coefficient, which is strongest for grain sizes similar to the skyrmion diameter. At shorter lag times the presence of grains enhances the effective diffusion coefficient due to the gyrotropic motion of the skyrmions induced by their interactions with grain boundaries. For grain sizes significantly larger than the skyrmion diameter clustering of the skyrmions occurs in grains with lower magnetic anisotropy., 7 pages, 5 figures

The magnetic interfacial Dzyaloshinskii-Moriya interaction (DMI) in multi-layered thin films can lead to exotic chiral spin states, of paramount importance for future spintronic technologies. Interfacial DMI is normally manifested as an intralayer interaction, mediated via a paramagnetic heavy metal in systems lacking inversion symmetry. Here we show how, by designing synthetic antiferromagnets with canted magnetization states, it is also possible to observe interfacial interlayer-DMI at room temperature. The interlayer-DMI breaks the symmetry of the magnetic reversal process via the emergence of noncollinear spin states, which results in chiral exchange-biased hysteresis loops. This work opens up yet unexplored avenues for the development of new chiral spin textures in multi-layered thin film systems., 15 pages, 4 figures

Large changes in the magnetization of ferromagnetic films can be electrically driven by non-180° ferroelectric domain switching in underlying substrates, but the shear components of the strains that mediate these magnetoelectric effects have not been considered so far. Here we reveal the presence of these shear strains in a polycrystalline film of Ni on a 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 substrate in the pseudo-cubic (011)pc orientation. Although vibrating sample magnetometry records giant magnetoelectric effects that are consistent with the hitherto expected 90° rotations of a global magnetic easy axis, high-resolution vector maps of magnetization (constructed from photoemission electron microscopy data, with contrast from X-ray magnetic circular dichroism) reveal that the local magnetization typically rotates through smaller angles of 62–84°. This shortfall with respect to 90° is a consequence of the shear strain associated with ferroelectric domain switching. The non-orthogonality represents both a challenge and an opportunity for the development and miniaturization of magnetoelectric devices. Non-orthogonal magnetization switching is related to the shear strain associated with ferroelectric domains, with implications for magnetoelectric devices.

Voltage manipulation of skyrmions is a promising path towards low-energy spintronic devices. Here, voltage effects on skyrmions in a GdOx/Gd/Co/Pt heterostructure are observed experimentally. The results show that the skyrmion density can be both enhanced and depleted by the application of an electric field, along with the ability, at certain magnetic fields to completely switch the skyrmion state on and off. Further, a zero magnetic field skyrmion state can be stablized under a negative bias voltage using a defined voltage and magnetic field sequence. The voltage effects measured here occur on a few-second timescale, suggesting an origin in voltage-controlled magnetic anisotropy rather than ionic effects. By investigating the skyrmion nucleation rate as a function of temperature, we extract the energy barrier to skyrmion nucleation in our sample. Further, micromagnetic simulations are used to explore the effect of changing the anisotropy and Dzyaloshinskii-Moriya interaction on skyrmion density. Our work demonstrates the control of skyrmions by voltages, showing functionalities desirable for commercial devices., Comment: 5 pages, 5 figures

We present a realization of highly frustrated planar triangular antiferromagnetism achieved in a quasi-three-dimensional artificial spin system consisting of monodomain Ising-type nanomagnets lithographically arranged onto a deep-etched silicon substrate. We demonstrate how the three-dimensional spin architecture results in the first direct observation of long-range ordered planar triangular antiferromagnetism, in addition to a highly disordered phase with short-range correlations, once competing interactions are perfectly tuned. Our work demonstrates how escaping two-dimensional restrictions can lead to new types of magnetically frustrated metamaterials.

The ability to imprint a given material property to another through a proximity effect in layered 2D materials has opened the way to the creation of designer materials. Here, molecular-beam epitaxy is used for direct synthesis of a superconductor-ferromagnet heterostructure by combining superconducting niobium diselenide (NbSe

Funding Information: This work was supported by the Academy of Finland (Grant Nos. 316857 and 295269). The authors wish to acknowledge CSC – IT Center for Science, Finland, and the Aalto Science‐IT project for computational resources. Publisher Copyright: © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. The voltage control of magnetism is a promising path to the development of low-power spintronic devices. Magneto-ionics—exploiting voltage-driven ion migration to control magnetism—has attracted interest because it can generate large magnetoelectric effects at low voltage. Here, the use of the solid-state lithium-ion battery technology for reversible voltage-controlled switching between perpendicular and in-plane magnetization states in a Co–Pt bilayer is demonstrated. Due to the small size and high mobility of lithium ions, small voltages produce an exceptionally high magnetoelectric coupling efficiency of at least 7700 fJ V–1 m–1 at room temperature. The magnetic switching effect is attributed to the modulation of spin-orbit coupling at the Co–Pt interface when lithium ions migrate between a lithium storage layer (LiCoO2) and the magnetic interface across a lithium phosphorous oxynitride (LiPON) solid-state electrolyte, which is corroborated by density functional theory calculations. Voltage control of magnetism in the battery structure does not show degradation over more than 500 voltage cycles, demonstrating promise for solid-state lithium-based magneto-ionic devices.

Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.

Using photoemission electron microscopy (PEEM) to image ferromagnetism in polycrystalline Ni disks, and ferroelectricity in their single-crystal BaTiO3 substrates, we find that voltage-driven 90° ferroelectric domain switching serves to reversibly annihilate each magnetic vortex via uniaxial compressive strain, and that the orientation of the resulting bi-domain reveals the chirality of the annihilated vortex. Micromagnetic simulations reveal that only 60% of this strain is required for annihilation. Voltage control of magnetic vortices is novel, and should be energetically favourable with respect to the use of a magnetic field or an electrical current. In future, stray field from bi-domains could be exploited to read vortex chirality. Given that core polarity can already be read via stray field, our work represents a step towards four-state low-power memory applications.

The Dzyaloshinskii-Moriya interaction gives rise to a chiral exchange between neighboring spins in the technologically relevant class of perpendicularly magnetized ultrathin film materials. In this paper, we study the temperature dependence of the Dzyaloshinskii-Moriya interaction based on extensive characterization of a thin film which hosts a skyrmion state using both bulk magnetometry and x-ray magnetic circular dichroism photoemission electron microscopy. A version of the Bloch law explicitly for thin film geometries is derived to extract the exchange stiffness. The strength of the Dzyaloshinskii-Moriya interaction, $D$, is found to have a dependence on the saturation magnetization, ${M}_{s}$ of $D\ensuremath{\propto}{M}_{s}^{1.86\ifmmode\pm\else\textpm\fi{}0.16}$. Further, by extracting the uniaxial anisotropy ${K}_{u}$ and the exchange stiffness $A$, we find that $D\ensuremath{\propto}{K}_{u}^{1.02\ifmmode\pm\else\textpm\fi{}0.11}$ and $D\ensuremath{\propto}{A}^{0.95\ifmmode\pm\else\textpm\fi{}0.07}$. Skyrmion radii are also used to extract the strength of the Dzyaloshinskii-Moriya interaction which is compared to that derived from measurements of stripe domains. The origins of the correlations between material parameters are discussed and consequences of these relationships for skyrmion devices are considered.

We demonstrate a tenfold resonant enhancement of magneto-optical effects in perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots mediated by the excitation of optimized plasmon modes. Two magnetoplasmonic systems are considered: square arrays of ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots on glass and identical arrays on a $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_{2}$ bilayer. On glass, the optical and magneto-optical spectra of the nanodot arrays are dominated by the excitation of a surface lattice resonance (SLR), whereas on $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_{2}$, a narrow surface plasmon polariton (SPP) resonance tailors the spectra further. Both the SLR and SPP modes are magneto-optically active leading to an enhancement of the Kerr angle. We detail the dependence of optical and magneto-optical spectra on the number of Co/Pt bilayer repetitions, the nanodot diameter, and the array period, offering design rules on how to maximize and spectrally tune the magneto-optical response of perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots.

The discovery of magnetic skyrmions in ultrathin heterostructures has led to great interest in possible applications in memory and logic devices. The non-trivial topology of magnetic skyrmions gives rise to a gyrotropic motion, where, under an applied energy gradient a skyrmion gains a component of motion perpendicular to the applied force. So far, device proposals have largely neglected this motion or treated it as a barrier to correct operation. Here, we show that skyrmions can be efficiently moved perpendicular to an energy step created by local changes in the perpendicular magnetic anisotropy. We propose an experimentally-realizable skyrmion racetrack device which uses voltage-controlled magnetic anisotropy to induce a step in magnetic anisotropy and drive a skyrmion unidirectionally using alternating voltage pulses.

We identify room-temperature converse magnetoelectric effects (CMEs) that are non-volatile by using a single-crystal substrate of PMN–PT (001)pc (pc denotes pseudocubic) to impart voltage-driven strain to a polycrystalline film of Ni. An appropriate magnetic-field history enhances the magnetoelectric coefficient to a near-record peak of ∼10−6 s m−1 and permits electrically driven magnetization reversal of substantial net magnetization. In zero magnetic field, electrically driven ferroelectric domain switching produces large changes of in-plane magnetization that are non-volatile. Microscopically, these changes are accompanied by the creation and destruction of magnetic stripe domains, implying the electrical control of perpendicular magnetic anisotropy. Moreover, the stripe direction can be rotated by a magnetic field or an electric field, the latter yielding the first example of electrically driven rotatable magnetic anisotropy. The observed CMEs are associated with repeatable ferroelectric domain switching that yields a memory effect. This memory effect is well known for PMN–PT (110)pc but not PMN–PT (001)pc. Given that close control of the applied field is not required as for PMN–PT (110)pc, this memory effect could lead the way to magnetoelectric memories based on PMN–PT (001)pc membranes that switch at low voltage.

The thin films of Pt/Co/AlO x , showing perpendicular magnetic anisotropy, were grown by magnetron sputtering with AlO x formed by the oxidation of thin Al layers using an oxygen atom source. Films were studied as a function of Pt thickness and Al oxidation, and films that showed full remanence and sharp switching coercivity were achieved. In order to prevent further oxidation of the interface in ambient conditions, we use a double Al growth and oxidation process. The magnetooptical Kerr effect and vibrating sample magnetometry were used to analyze these films. We find an effective perpendicular anisotropy of $2 \times 10^{6}$ erg/cm3, with the majority of the perpendicular anisotropy coming from the Pt/Co interface. From the sweep rate dependence on the coercivity, we are able to extract an activation volume of 4.3 $\pm 0.5 \times 10^{-18}$ cm3, similar to other Co-based perpendicular systems.

We report on studies of magnetic domain wall (DW) movement within nanodevices containing magnetic components formed from perpendicularly magnetised CoFeB (0.6 nm) thin films. Methods for low energy and smooth DW propagation as well as controlled and reproducible pinning of DWs at geometrically defined sites along nanowires are presented. Conventional anomalous Hall effect measurements as well as an indirect method using overlying electrodes are presented and compared. The results obtained from the individual studies of DW injection, propagation, pinning and measurement are assimilated into a prototype device suitable for the controlled pinning and annihilation of a single domain wall within the structure.

Magnetic solitons in antiferromagnetic superlattices can be used for vertical data transfer of information. In this chapter, we introduce this concept and summarise recent results of our group where controlled soliton nucleation and propagation was achieved.

Electron-electron interactions in topological p-n junctions consisting of vertically stacked topological insulators are investigated. n-type Bi2Te3 and p-type Sb2Te3 of varying relative thicknesses are deposited using molecular beam epitaxy and their electronic properties measured using low-temperature transport. The screening factor is observed to decrease with increasing sample thickness, a finding which is corroborated by semi-classical Boltzmann theory. The number of two-dimensional states determined from electron-electron interactions is larger compared to the number obtained from weak-antilocalization, in line with earlier experiments using single layers., Comment: 38 pages, 5 figures, 1 table

Thin films of Fe have been epitaxially sputtered on GaAs substrates with native oxide removal prior to the deposition carried out by an Ar ion milling. Films grown at substrate temperatures above 100 °C show well-defined fourfold anisotropies. The onset of epitaxial growth is accompanied by an increase in the surface roughness with growth occurring in a distinct island-like pattern. The Fe layers show significantly reduced moments, which decrease with increasing temperature. Antiferromagnetic coupling between Fe layers with Cr spacers was measured in a multilayer with a Cr thickness of 2.7 nm, around the second antiferromagnetic peak. The magnetic properties of the films are discussed in the context of multilayer storage applications.

Devices comprised of CoFeB nanostructures with perpendicular magnetic anisotropy and non-magnetic Ta channel were operated in thermal lateral spin valve (LSV) mode and studied by magnetotransport measurements and magnetic scanning gate microscopy (SGM). Due to the short spin diffusion length of Ta, the spin diffusion signal was suppressed, allowing the study of the contribution from the anomalous Nernst (ANE) and anomalous Hall effects (AHE). The magnetotransport measurements identified the switching fields of the CoFeB nanostructures and demonstrated a combination of AHE and ANE when the devices were operated in thermally-driven spin-injection mode. Modified scanning probe microscopy probes were fabricated by placing a NdFeB magnetic bead (MB) on the apex of a commercial Si probe. The dipole magnetic field distribution around the MB was characterized by using differential phase contrast technique and direct measurement of the switching field induced by the bead in the CoFeB nanodevices. Using SGM we demonstrate the influence of localized magnetic field on the CoFeB nanostructures near the non-magnetic channel. This approach provides a promising route towards the study of thermal and spin diffusion effects using local magnetic fields.Devices comprised of CoFeB nanostructures with perpendicular magnetic anisotropy and non-magnetic Ta channel were operated in thermal lateral spin valve (LSV) mode and studied by magnetotransport measurements and magnetic scanning gate microscopy (SGM). Due to the short spin diffusion length of Ta, the spin diffusion signal was suppressed, allowing the study of the contribution from the anomalous Nernst (ANE) and anomalous Hall effects (AHE). The magnetotransport measurements identified the switching fields of the CoFeB nanostructures and demonstrated a combination of AHE and ANE when the devices were operated in thermally-driven spin-injection mode. Modified scanning probe microscopy probes were fabricated by placing a NdFeB magnetic bead (MB) on the apex of a commercial Si probe. The dipole magnetic field distribution around the MB was characterized by using differential phase contrast technique and direct measurement of the switching field induced by the bead in the CoFeB nanodevices. Using SGM we demo...

Canted magnetization is obtained in ultrathin, antiferromagnetically coupled magnetic bilayers with\ud thicknesses around the spin reorientation transition. The canting angle is controlled by both the\ud magnetic layer thickness and interlayer coupling strength, which are tuned independently. Hysteresis\ud loops are obtained, where magnetization components parallel and transverse to the applied field are\ud measured, and analyzed by comparison to micromagnetic simulations. This enables the canting angle\ud to be extracted and the behavior of the individual layers to be distinguished. Two types of canted\ud systems are obtained with either single-layer reversal or complex, coupled two-layer reversal, under\ud moderate external magnetic fields. Controlling the magnetization canting and reversal behavior of\ud ultra-thin layers is relevant for the development of magnetoresistive random-access memory and\ud spin-torque oscillator devices.

Perpendicularly magnetized microparticles offer the ability to locally apply high torques on soft matter under an applied magnetic field. These particles are engineered to have a zero remanence magnetic configuration via synthetic antiferromagnetic coupling using a Ru coupling interlayer. The flexibility offered by the top down thin film fabrication process in a CoFeB/Pt perpendicular thin film is demonstrated by using the Pt interlayer thicknesses in a Pt/Ru/Pt antiferromagnetic coupling multilayer to tune the applied magnetic field value of the easy axis spin-flip transition to saturation and hence the field value at which the magnetic particles are magnetically activated via a distinct transition to saturation. The importance of a Ta buffer layer on the magnetic behavior of the stack is shown. While Au capping layers are desirable for biotechnology applications, we demonstrate that they can drastically change the nucleation and propagation of domains in the film, thereby altering the reversal behavior of the thin film. The effect of Au underlayers on a multilayer thin film composed of repeated motifs of a synthetic antiferromagnetic building block is also investigated.

The Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between two magnetic layers leads to many important technological applications. Here, the interaction between changing antiferromagnetic RKKY coupling and domain structure is studied in a sample consisting of two 5 nm thick CoFeB layers separated by a wedge of Cu up to 4 nm thick. Magnetic reversal occurs via the propagation of a zigzag domain wall front along the wedge. The modification of domain patterns created in the reversal of a coupled layers in the presence of antiferromagnetic RKKY coupling and coupling gradients is demonstrated. Firstly, the coupling leads to a smaller amplitude of the zigzag wall, which is aligned perpendicular to the easy axis, followed by elongation of the walls at higher coupling strength. The antiferromagnetic RKKY coupling, while not strong enough to cause antiparallel alignment of the layers, is argued to lead to coupling between the spins in the domain walls in the two layers, lowering their energy and driving the reversal behavior of the film.

Magneto-optic Kerr effect (MOKE) describes the changes of reflected light from a spin-polarised surface.

In magnetic nanowires with perpendicular magnetic anisotropy (PMA) magnetic domain walls (DW) are narrow and can move rapidly driven by current induced torques. This enables important applications like high-density memories for which the precise detection of the position and motion of a propagating DW is of utmost interest. Today's DW detection tools are often limited in resolution, or acquisition speed, or can only be applied on specific materials. Here, we show that the anomalous Nernst effect provides a simple and powerful tool to precisely track the position and motion of a single DW propagating in a PMA nanowire. We detect field and current driven DW propagation in both metallic heterostructures and dilute magnetic semiconductors over a broad temperature range. The demonstrated spatial resolution below 20 nm is comparable to the DW width in typical metallic PMA systems., 26 pages including supplementary information

Uniaxial magnetic anisotropy was imposed on a CoFeB film by applying an in-plane magnetic field during growth. Electrically driven strain from a ferroelectric 0.68Pb(Mg 1/3 Nb 2/3 )O 3 -0.32PbTiO 3 (011) substrate resulted in giant magnetoelectric effects, whose coupling constant peaked at a record value of ∼8.0 × 10 -6 s m -1 . These large magnetoelectric effects arose due to non-volatile 90° rotations of the magnetic easy axis, reflecting a competition between the fixed growth anisotropy and the voltage-controlled magnetoelastic anisotropy. In contrast to previous work, our non-volatile rotations did not require the assistance of an applied magnetic field or the setting of an in-plane substrate polarization prior to deposition.

Compressively strained Ge quantum well devices have a spin-splitting in applied magnetic field that is entirely consistent with a Zeeman effect in the heavy hole valence band. The spin orientation is determined by the biaxial strain in the quantum well with the relaxed SiGe buffer layers and is quantized in the growth direction perpendicular to the conducting channel. The measured spin-splitting in the resistivity ρxx agrees with the predictions of the Zeeman Hamiltonian where the Shubnikov-deHaas effect exhibits a loss of even filling factor minima in the resistivity ρxx with hole depletion from a gate field, increasing disorder or increasing temperature. There is no measurable Rashba spin-orbit coupling irrespective of the structural inversion asymmetry of the confining potential in low p-doped or undoped Ge quantum wells from a density of 6 × 1010 cm−2 in depletion mode to 1.7 × 1011 cm−2 in enhancement.

By combining $n$-type $\mathrm{Bi_2Te_3}$ and $p$-type $\mathrm{Sb_2Te_3}$ topological insulators, vertically stacked $p$-$n$ junctions can be formed, allowing to position the Fermi level into the bulk band gap and also tune between $n$- and $p$-type surface carriers. Here we use low-temperature magnetotransport measurements to probe the surface and bulk transport modes in a range of vertical $\mathrm{Bi_2Te_3/Sb_2Te_3}$ heterostructures with varying relative thicknesses of the top and bottom layers. With increasing thickness of the $\mathrm{Sb_2Te_3}$ layer we observe a change from $n$- to $p$-type behavior via a specific thickness where the Hall signal is immeasurable. Assuming that the the bulk and surface states contribute in parallel, we can calculate and reproduce the dependence of the Hall and longitudinal components of resistivity on the film thickness. This highlights the role played by the bulk conduction channels which, importantly, cannot be probed using surface sensitive spectroscopic techniques. Our calculations are then buttressed by a semi-classical Boltzmann transport theory which rigorously shows the vanishing of the Hall signal. Our results provide crucial experimental and theoretical insights into the relative roles of the surface and bulk in the vertical topological $p$-$n$ junctions., 11 pages, 5 figures

Magnetic kink solitons are used as a probe to experimentally measure the layer-by-layer coercivity and interlayer coupling strength of an antiferromagnetically coupled perpendicularly magnetized Co multilayer. The magnetic response is well described by a nearest neighbor Ising macrospin model. By controlling the position of one, two or three solitons in the stack using globally applied magnetic fields, we successfully probe the switching of individual buried layers under different neighboring configurations, allowing us to access individual layer's characteristic parameters. We found the coercivity to increase dramatically up the multilayer, while the interlayer coupling strength decreased slightly. We corroborate these findings with scanning transmission electron microscopy images where a degrading quality of the multilayer is observed. This method provides a very powerful tool to characterize the quality of individual layers in complex multilayers, without the need for depth-sensitive magnetic characterization equipment.

We experimentally study the tunability of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interlayer exchange coupling (IEC) in Pt/CoFeB/Pt/Ru/Pt/CoFeB/Pt stacks with perpendicular magnetic anisotropy (PMA). The perpendicular magnetization of a single Pt/Co60Fe20B20/Pt (at. %) shows full remanence and square hysteresis loops for a CoFeB thickness range of 0.60–1.0 nm. By inserting a Pt layer between the Ru and CoFeB, the PMA of the ultrathin CoFeB layers is stabilized and the IEC can be tuned. In particular, we show that the IEC versus Pt thickness exhibits a simple exponential decay with a decay length of 0.16 nm.

For 1 µm-diameter Ni discs on a BaTiO3 substrate, the local magnetization direction is determined by ferroelectric domain orientation as a consequence of growth strain, such that single-domain discs lie on single ferroelectric domains. On applying a voltage across the substrate, ferroelectric domain switching yields non-volatile magnetization rotations of 90°, while piezoelectric effects that are small and continuous yield non-volatile magnetization reversals that are non-deterministic. This demonstration of magnetization reversal without ferroelectric domain switching implies reduced fatigue, and therefore represents a step towards applications.

GaAs-based spin LEDs were used to study spin injection through a Schottky barrier from a ferromagnetic contact formed by an in-situ technique. Optical measurements were performed in both oblique Hanle and Faraday geometry. In addition to direct recombination at the Gamma-point in the quantum well, emission peaks were seen due to phonon-assisted recombination from the X and L valleys in the semiconductor. The presence of the phonon replica peaks indicates that strong tunneling occurs from the Fe into the X and L minima at the interface. As measured in the oblique Hanle geometry, the satellite peaks display optical polarization, which indicates spin polarized injection into these minima. It is argued that enhanced tunneling into the X and L minima leads to a lower spin polarization in the device due to the transport properties of these valleys. This mechanism then limits the maximum effective spin polarization that can be achieved at a coherent interface.

Electrical control of magnetism has been demonstrated in multiferroic compounds and ferromagnetic semiconductors, but electrical switching of a substantial net magnetization at room temperature has not been demonstrated in these materials. This goal has instead been achieved in heterostructures comprising ferromagnetic films in which electrically driven magnetic changes arise due to strain or exchange bias from ferroic substrates, or due to charge effects induced by a gate. However, previous work focused on electrical switching of an in-plane magnetization or involved the assistance of applied magnetic fields. In heterostructures made of juxtaposed ferroelectric and ferromagnetic layers, we have shown electrical control with no applied magnetic field of the perpendicular magnetization of small features [1] and of magnetic stripe domains patterns [2]. Here we investigate Ni81Fe19 films on ferroelectric substrates with and without buffer layers of Cu, whose presence precludes charge-mediated coupling. Ni81Fe19 has virtually zero magnetostriction, but sufficiently thin films show large magnetostriction, and thus, on increasing film thickness through the threshold for zero magnetostriction, we have seeked the crossover from charge- to strain-mediated coupling. We will then show that strain associated with the motion of 90°- ferroelectric domain walls in a BaTiO3 substrate, can switch the magnetization of an array of overlying single-domain Ni dots. [1] M. Ghidini, R. Pellicelli, J. L. Prieto, X. Moya, J. Soussi, J. Briscoe, S. Dunn and N. D. Mathur, Nature Communications 4 (2013) 1453. [2] M. Ghidini, F.Maccherozzi, X. Moya, L. C. Phillips, W.Yan, J. Soussi, N. Metallier, M.Vickers, , N. -J.Steinke, R. Mansell, C. H. W. Barnes, S. S. Dhesi, and N. D. Mathur, Adv. Mater.doi: 10.1002/adma.201404799 (2015).

Magnetic particles that can be precisely controlled under a magnetic field and transduce energy from the applied field open the way for innovative cancer treatment. Although these particles represent an area of active development for drug delivery and magnetic hyperthermia, the in vivo anti-tumor effect under a low-frequency magnetic field using magnetic particles has not yet been demonstrated. To-date, induced cancer cell death via the oscillation of nanoparticles under a low frequency magnetic field has only been observed in-vitro. In this report, we demonstrate the successful use of spin-vortex, disk-shaped permalloy magnetic particles in a low-frequency, rotating magnetic field for the in vitro and in vivo destruction of glioma cells. The internalized nanomagnets align themselves to the plane of the rotating magnetic field, creating a strong mechanical force which damages the cancer cell structure inducing programmed cell death. In vivo, the magnetic field treatment successfully reduces the brain tumor size and increases the survival rate of mice bearing intracranial glioma xenografts, without adverse side effects. This study demonstrates the novel approach of controlling magnetic particles for treating malignant glioma that should be applicable to treat a wide range of cancers.

Vertical shift registers and 3-dimensional magnetic memory structures potentially provide a revolutionary increase in the areal density of magnetic data storage devices [1-3]. The Ruderman-Kit-tel-Kasuya-Yosida (RKKY) exchange coupled magnetic multilayer architecture proposed in [2] uses a localized topological soliton as a bit of information. The nucleation and injection of this bit, which is a form of excitation in the antiferromagnetic stack, has to be highly reproducible in order for this architecture to work [1, 4]. The nucleation and subsequently the propagation of a magnetic soliton are experimentally shown in [4]. This is achieved by modifying the ratio of the exchange coupling to the magnetic anisotropy strength of one of the edge layers in a synthetic ferromagnetic superlattice. Here, we further extend this to a system where the magnetic properties of both the edge layers of the synthetic ferrimagnetic superlattice are modified. The magnetic configurations of the superlattice during a hysteresis loop are investigated using macrospin simulation.

We report magnetotransport measurements of a SiGe heterostructure containing a 20 nm p-Ge quantum well with a mobility of 800 000 cm2 V−1 s−1. By dry etching arrays of wires with widths between 1.0 μm and 3.0 μm, we were able to measure the lateral depletion thickness, built-in potential, and the phase coherence length of the quantum well. Fourier analysis does not show any Rashba related spin-splitting despite clearly defined Shubnikov-de Haas oscillations being observed up to a filling factor of ν = 22. Exchange-enhanced spin-splitting is observed for filling factors below ν = 9. An analysis of boundary scattering effects indicates lateral depletion of the hole gas by 0.5 ± 0.1 μm from the etched germanium surface. The built-in potential is found to be 0.25 ± 0.04 V, presenting an energy barrier for lateral transport greater than the hole confinement energy. A large phase coherence length of 3.5 ± 0.5 μm is obtained in these wires at 1.7 K.

A sharp magnetic soliton can be created and propagated in a vertical ratchet structure based on magnetic layers with out-of-plane anisotropy using a combination of antiferromagnetic and ferromagnetic interlayer couplings. This allows the use of identical magnetic layers in the stack, which simplifies the implementation of the ratchet compared to schemes which use alternating layer thicknesses. The ratchet behavior is analyzed using an Ising-macrospin approximation and conditions are derived for the propagation of a soliton, which is demonstrated experimentally. Values extracted from the experimental data for the coercivities and interlayer couplings show significant variation, which demonstrates the robustness of the soliton propagation.

We present new studies of the characteristics and detection ability of nanoscale sensors based on domain wall nucleation within perpendicularly magnetised ultrathin CoFeB/Pt films. A combination of MFM imaging and anomalous Hall effect transport measurements were used to verify and characterise the behaviour of fabricated devices. After initial characterisation, the influence of magnetic nanoparticles on device behaviour was studied using individual iron oxide (FeOx) particles mounted on modified atomic force microscopy probes. We demonstrate the successful detection of particles ranging in diameter between 2.8 μm and 100 nm. In the case of the 2.8 μm particle, we have mapped the signal amplitude produced at a variety of distances from the sensor. We find that the particle’s influence may be detected at separations up to 700 nm. Furthermore, we demonstrate a method for resolving the location of a particle with respect to the centre of the device, providing a lateral sensing ability.

© 2017 IOP Publishing Ltd.The magnetic reversal of epitaxial Fe/Cr/Fe trilayer samples grown on GaAs is studied. In wedged samples both long and short period coupling oscillations associated with Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling in Cr are seen in the easy axis saturation fields. By using vector vibrating sample magnetometry and both longitudinal and transverse magneto-optical Kerr effect magnetometry we are able to determine the exact reversal path of both the magnetic layers. Changes in the reversal behavior are seen with sub-monolayer changes of the thickness of the Cr interlayer. The two main reversal paths are described in terms of whether the reversal is dominated by bilinear RKKY coupling, which leads to an antiparallel state at remanence or by biquadratic coupling which leads to a 90 degree alignment of layers at remanence. The changing reversal behaviour is discussed with respect to the possibility of using such systems for multilayer memory applications and, in particular, the limits on the required accuracy of the sample growth.

Structural and magnetic properties of 1–10 nm thick Fe films deposited on GaN(0001) were investigated. In-situ reflecting high energy electron diffraction images indicated a α-Fe(110)/GaN(0001) growth of the 3D Volmer-Weber type. The α-Fe(110) X-ray diffraction peak showed a 1° full-width at half-maximum, indicating ≈20 nm grain sizes. A significant reduction in Fe atomic moment from its bulk value was observed for films thinner than 4 nm. Both GaN/Fe interface roughness and Fe film coercivity increased with Fe thickness, indicating a possible deterioration of Fe crystalline quality. Magnetic anisotropy was mainly uniaxial for all films while hexagonal anisotropies appeared for thicknesses higher than 3.7 nm.

High-resolution magnetoelectric imaging is used to demonstrate electrical control of the perpendicular local magnetization associated with 125 nm-wide magnetic stripe domains in 100-nm-thick Ni films. This magnetoelectric coupling is achieved in zero magnetic field using strain from ferroelectric BaTiO3 substrates to control perpendicular anisotropy imposed by the growth stress. These findings may be exploited for perpendicular recording in nanopatterned hybrid media.

We have designed a bilayer synthetic antiferromagnet where the order of layer reversal can be selected by varying the sweep rate of the applied magnetic field. The system is formed by two ultra-thin ferromagnetic layers with different proximities to the spin reorientation transition, coupled antiferromagnetically using Ruderman-Kittel-Kasuya-Yosida interactions. The different dynamic magnetic reversal behavior of both layers produces a crossover in their switching fields for field rates in the kOe/s range. This effect is due to the different effective anisotropy of both layers, added to an appropriate asymmetric antiferromagnetic coupling between them. Field-rate controlled selective switching of perpendicular magnetic anisotropy layers as shown here can be exploited in sensing and memory applications.