Unambiguous evidence of oscillatory magnetic coupling between Co layers in ultrahigh vacuum grown Co/Au(111)/Co trilayers

Chirality-Induced Noncollinear Magnetization and Asymmetric Domain-Wall Propagation in Hydrogenated CoPd Thin Films

Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits a strong vertical and lateral antiferromagnetic coupling in the perpendicular component between the areas with and without plasma pretreatment, and asymmetric domain-wall propagation in the plasma-pretreated areas during magnetization reversal. These phenomena exhibit evident magnetic chirality and can be interpreted with the Ruderman-Kittel-Kasuya-Yosida coupling and the Dzyaloshinskii-Moriya interaction (DMI). The sample processing demonstrated in this study allows easy incorporation of lithography techniques that can define areas with or without DMI to create intricate magnetic patterns on the sample, which provides an avenue toward more sophisticated control of canted spin textures in future spintronic devices.

Interlayer exchange coupling in ferromagnetic semiconductor trilayers with out-of-plane magnetic anisotropy

We report the observation of ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange coupling (IEC) in GaMnAsP-based trilayer structures with out-of-plane magnetic anisotropy. Magnetization and anomalous Hall effect (AHE) measurements show well-resolved magnetization transitions corresponding to the two GaMnAsP layers. Minor loop measurements reveal a characteristic shift caused by IEC in all trilayer samples investigated. Interestingly, the FM IEC changes to AFM IEC for a trilayer with the thinnest (7 nm) top GaMnAsP layer as the temperature increases. The observation of temperature-induced transition of FM and AFM IEC in the same sample suggests the possibility of device applications by controlling the type of IEC in such GaMnAsP-based multilayers.

Wide-range tuning of interfacial exchange coupling between ferromagnetic Au/Co and ferrimagnetic Tb/Fe(Co) multilayers

The ability to perform wide-range tuning of the magnetic field required to switch the magnetization of ferromagnetic layers with perpendicular magnetic anisotropy is of great importance for many applications. We show that, for (Au/Co)₂₍₃₎ multilayers, this field can be changed from minus several kOe to plus several kOe because of changes to the coupling with a ferrimagnetic multilayer [either (Tb/Fe)₆ or (Tb/Co)₆] across a Au spacer (either homogeneous 1 nm thick or wedge-shaped). The adjustable parameters are the ratio of sublayer thicknesses of the ferrimagnet and the sequence of layers around the Au spacer. The change of the sequence from Co/Au/Co to Tb/Au/Co is accompanied by both the reduction of the interaction energy and the change of the magnetic field sign necessary to switch the magnetization of ferromagnetic multilayers. For a 1 nm thick Au spacer this fields change from positive (negative) to negative (positive) if the ferrimagnet is dominated by the transition metal (rare earth) as a result of its composition. The characteristic oscillatory behavior of RKKY-like coupling is demonstrated using a system with a wedge-shaped Au spacer.

XMCD studies of magnetic polarization at Mo atoms in CoMo alloy and magnetically coupled Co/Mo multilayers

Magnetic polarization of Mo atoms in Co₉₆Mo₄ alloy film and Co/Mo multilayered structures has been studied by X-ray magnetic circular dichroism. Samples with Mo spacers of two different thicknesses (0.9 nm and 1.8 nm) were investigated. Mo atoms receive a magnetic moment of -0.21μ_B in the alloy. In the multilayer with the thinner Mo spacer (d_Mo = 0.9 nm) the magnetic moment is much smaller (-0.03μ_B). In both cases the measured induced moment at the Mo site is oriented antiparallel to the moment at the Co atoms. The presence of the induced moment in the Mo spacer coincides with antiferromagnetic coupling between the Co component slabs. In contrast, neither measurable induced moment at the Mo site nor interlayer coupling between the Co layers has been found for the multilayer with the thicker Mo spacer. Possible mechanisms of the coupling associated with the induced moment are discussed in detail.

Velocity Enhancement by Synchronization of Magnetic Domain Walls

Magnetic domain walls are objects whose dynamics is inseparably connected to their structure. In this Letter, we investigate magnetic bilayers, which are engineered such that a coupled pair of domain walls, one in each layer, is stabilized by a cooperation of Dzyaloshinskii-Moriya interaction and flux-closing mechanism. The dipolar field mediating the interaction between the two domain walls links not only their position but also their structure. We show that this link has a direct impact on their magnetic-field-induced dynamics. We demonstrate that in such a system the coupling leads to an increased domain wall velocity with respect to single domain walls. Since the domain wall dynamics is observed in a precessional regime, the dynamics involves the synchronization between the two walls to preserve the flux closure during motion. Properties of these coupled oscillating walls can be tuned by an additional in-plane magnetic field enabling a rich variety of states, from perfect synchronization to complete detuning.

Current-induced skyrmion generation and dynamics in symmetric bilayers

Magnetic skyrmions are quasiparticle-like textures which are topologically different from other states. Their discovery in systems with broken inversion symmetry sparked the search for materials containing such magnetic phase at room temperature. Their topological properties combined with the chirality-related spin–orbit torques make them interesting objects to control the magnetization at nanoscale. Here we show that a pair of coupled skyrmions of opposite chiralities can be stabilized in a symmetric magnetic bilayer system by combining Dzyaloshinskii–Moriya interaction (DMI) and dipolar coupling effects. This opens a path for skyrmion stabilization with lower DMI. We demonstrate in a device with asymmetric electrodes that such skyrmions can be independently written and shifted by electric current at large velocities. The skyrmionic nature of the observed quasiparticles is confirmed by the gyrotropic force. These results set the ground for emerging spintronic technologies where issues concerning skyrmion stability, nucleation and propagation are paramount.

The creation of practical devices based on magnetic skyrmions depends on the development of methods to create and control stable individual skyrmions. Here, the authors present a bilayer device that uses dipolar interactions to stabilize skyrmions that can be manipulated using electric currents.

Tuning giant magnetoresistance in rolled-up Co-Cu nanomembranes by strain engineering

Compact rolled-up Co-Cu nanomembranes of high quality with different numbers of windings are realized by strain engineering. A profound analysis of magnetoresistance (MR) is performed for tubes with a single winding and a varied number of Co-Cu bilayers in the stack. Rolled-up nanomembranes with up to 12 Co-Cu bilayers are successfully fabricated by tailoring the strain state of the Cr bottom layer. By carrying out an angular dependent study, we ruled out the contribution from anisotropic MR and confirm that rolled-up Co-Cu multilayers exhibit giant magnetoresistance (GMR). No significant difference of MR is found for a single wound tube compared with planar devices. In contrast, MR in tubes with multiple windings is increased at low deposition rates of the Cr bottom layer, whereas the effect is not observable at higher rates, suggesting that interface roughness plays an important role in determining the GMR effect of the rolled-up nanomembranes. Furthermore, besides a linear increase of the MR with the number of windings, the self-rolling of nanomembranes substantially reduces the device footprint area.

Preliminary Results from a New Spin Spectrometer

The first preliminary results from a novel spectrometer for elementally-specific measurements of magnetic surfaces and ultrathin films are presented here. The key measurements are based upon spin-resolving and photon-dichroic photoelectron spectroscopy. True spinresolution is achieved by the use of a Mini-Mott detection scheme. The photon-dichroic measurements include the variant magnetic x-ray linear dichroism (MXLD). Both a multi-channel, energy dispersive collection scheme as well as the spin-detecting Mini-Mott apparatus are used in data collection. The [Spin Spectrometer] is based at the Spectromicroscopy Facility (Beamline7)at the Advanced Light Source.

Dynamic binding of driven interfaces in coupled ultrathin ferromagnetic layers

We demonstrate experimentally dynamic interface binding in a system consisting of two coupled ferromagnetic layers. While domain walls in each layer have different velocity-field responses, for two broad ranges of the driving field H, walls in the two layers are bound and move at a common velocity. The bound states have their own velocity-field response and arise when the isolated wall velocities in each layer are close, a condition which always occurs as H→0. Several features of the bound states are reproduced using a one-dimensional model, illustrating their general nature.

How to measure a spinon fermi surface

We propose an experiment to identify the potential existence of a spinon Fermi surface by looking for oscillatory coupling between two ferromagnets via a spin liquid spacer. Three candidate spin liquids are investigated, and it is found out that in all cases long period oscillations should be present, the period of which would identify the Fermi wave vector of the spinon surface.