The magnetic interfacial Dzyaloshinskii–Moriya interaction (DMI) in multilayered thin films can lead to chiral spin states, which are of paramount importance for future spintronic technologies1,2. Interfacial DMI typically manifests as an intralayer interaction, mediated via a paramagnetic heavy metal in systems lacking inversion symmetry3. Here we show that, by designing synthetic antiferromagnets with canted magnetization states4,5, it is also possible to observe direct evidence of the interfacial interlayer DMI at room temperature. The interlayer DMI breaks the symmetry of the magnetic reversal process via the emergence of non-collinear spin states, which results in chiral exchange-biased hysteresis loops. The spin chiral interlayer interactions reported here are expected to manifest in a range of multilayered thin-film systems, opening up as yet unexplored avenues for the development and exploitation of chiral effects in magnetic heterostructures6–8. An interlayer Dzyaloshinskii–Moriya interaction is observed in a synthetic antiferromagnet, with implications for achieving chiral spin textures in multilayered thin films.

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Symmetry-breaking interlayer Dzyaloshinskii–Moriya interactions in synthetic antiferromagnets

Magnetic anisotropy and magnetization reversal processes in Pt/Co/Pt films

An overview on the dynamics of the field-induced magnetization reversal in ultrathin ferromagnetic films and in related laterally patterned nanostructures down to the nanosecond range is presented. The associated experimental techniques are briefly described. For quasi-perfect films, spin reversal is controlled by intrinsic nanostructural nucleation and pinning centers. Nucleation is thermally activated, and its rate varies exponentially with applied field H. For ultrathin films, domain walls propagate uniformly. When H is not large enough to overcome pinning energy barriers, the domain wall motion proceeds by successive thermally activated small jumps. When H is larger than the depinning force, a viscous wall motion with velocity linear in H is observed. A simple model is proposed to reconcile low and high field behaviors. More refined theoretical random field treatments and numerical simulations are presented. An alternative way is to analyze the data in terms of the movement of a 1-D interface in a disordered medium. The dynamics of the nucleation and domain wall processes are extensively investigated in a quasi-perfect Pt/Co(0.5 nm)/Pt film structure. Until now, few results on the dynamics of magnetization reversal in isolated submicron-sized magnetic elements arranged into periodic arrays are available. In this review, we report on the local and collective magnetic dynamic behavior and noncoupled or coupled dot arrays with out-of-plane anisotropy. In noninteracting dot arrays, reversal is controlled by the large distribution of local nucleation fields already present in the unpatterned film. As predicted theoretically, the dynamics of assemblies of coupled and noncoupled dots differ strongly.

Dynamics of Magnetization Reversal: From Continuous to Patterned Ferromagnetic Films

Beautiful theories of magnetic hysteresis based on random microscopic disorder have been developed over the past ten years. Our goal was to directly compare these theories with precise experiments. We first developed and then applied coherent x-ray speckle metrology to a series of thin multilayer perpendicular magnetic materials. To directly observe the effects of disorder, we deliberately introduced increasing degrees of disorder into our films. We used coherent x-rays to generate highly speckled magnetic scattering patterns. The apparently random arrangement of the speckles is due to the exact configuration of the magnetic domains in the sample. In effect, each speckle pattern acts as a unique fingerprint for the magnetic domain configuration. Small changes in the domain structure change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed. How is the magnetic domain configuration at one point on the major hysteresis loop related to the configurations at the same point on the loop during subsequent cycles? The microscopic return-point memory(RPM) is partial and imperfect in the disordered samples, and completely absent when the disorder was not present. We found the complementary-point memory(CPM) is also partial and imperfect in the disordered samples and completely absent when the disorder was not present. We found that the RPM is always a little larger than the CPM. We also studied the correlations between the domains within a single ascending or descending loop. We developed new theoretical models that do fit our experiments.

Disorder-induced magnetic memory : Experiments and theories

The design of a magneto-optical microscope magnetometer (MOMM) for simultaneously probing local magnetic properties is described. The MOMM consists of an optical polarizing microscope capable of magneto-optical contrast that is used as a magnetometer by sweeping a magnetic field from an electromagnet. Due to full-field optical imaging, as opposed to single photodiode detection, the system is capable of simultaneous measurement of magnetic hysteresis loops and magnetization viscosity curves on 8000 individual local regions of 400×400 nm2 area in ferromagnetic materials. The most striking feature of the system is that it provides two-dimensional maps of the local magnetic properties including the coercivity, the switching time, and the activation magnetic moment from two-dimensional arrays of the hysteresis loops and the viscosity curves. We present the local magnetic properties and their correlations in Co/Pd multilayer films prepared by electron-beam evaporation.

Magneto-optical microscope magnetometer for simultaneous local probing of magnetic properties

Magnetization-reversal processes in a ferromagnetic cobalt film structure (Au/Co/Au), with perpendicular anisotropy, were investigated by magneto-optical magnetometry and microscopy. In the considered ultrathin Co film, the magnetization reversal between the two Ising-spin equilibrium states is dominated by the domain-wall motion mechanism. We focused our studies on processes initiated from a given demagnetized state. Starting from a magnetically saturated state generated under a large field ${\mathrm{H}}_{\mathrm{S}}$, applied perpendicular to the film, this demagnetized state is created through magnetic aftereffects in a field ${\mathrm{H}}_{\mathrm{d}}$ antiparallel but smaller than ${\mathrm{H}}_{\mathrm{S}}$ and applied during a selected time. Direct (${\mathrm{R}}^{\mathrm{D}}$) and indirect (${\mathrm{R}}^{\mathrm{I}}$) magnetization processes are then studied from this state for application of the field parallel and antiparallel to ${\mathrm{H}}_{\mathrm{d}}$, respectively. The dynamics of the magnetization reversal is much faster for the ${\mathrm{R}}^{\mathrm{I}}$ process since it is initiated from a quasihomogeneous 'Swiss cheese' domain state with small nonreversed regions. The magnetic accommodation phenomenon is studied, and a domain-shape memory effect evidenced. A theoretical analysis of the dynamics of magnetization processes is proposed, starting from the model of a patchy inhomogeneous media with a realistic distribution of local coercivities. The pertinent parameters for calculations are deduced from our experimental data using appropriate analytical expressions of the magnetic relaxation time and domain-wall velocity under a field. Computer simulations using these parameters reproduce well the time evolution of the magnetic domain pattern and different magnetization curves both for ${\mathrm{R}}^{\mathrm{D}}$ and ${\mathrm{R}}^{\mathrm{I}}$ magnetization processes.

M. Kisielewski

Papers

Magnetization-reversal processes in an ultrathin co/au film

Self-biasing effect based on mixed magnetic anisotropy idea

Domain structure shape memory in a magnet with magnetization induced anisotropy

A new look on magnetic aftereffect

Unusual magnetic domain structure properties in YIG + Co films☆