Real-Space Observation of Magnon Interaction with Driven Space-Time Crystals.

The concept of space-time crystals (STC), i.e., translational symmetry breaking in time and space, was recently proposed and experimentally demonstrated for quantum systems. Here, we transfer this concept to magnons and experimentally demonstrate a driven STC at room temperature. The STC is realized by strong homogeneous microwave pumping of a micron-sized permalloy (Py) stripe and is directly imaged by scanning transmission x-ray microscopy (STXM). For a fundamental understanding of the formation of the STC, micromagnetic simulations are carefully adapted to model the experimental findings. Beyond the mere generation of a STC, we observe the formation of a magnonic band structure due to back folding of modes at the STC's Brillouin zone boundaries. We show interactions of magnons with the STC that appear as lattice scattering, which results in the generation of ultrashort spin waves (SW) down to 100-nm wavelengths that cannot be described by classical dispersion relations for linear SW excitation. We expect that room-temperature STCs will be useful to investigate nonlinear wave physics, as they can be easily generated and manipulated to control their spatial and temporal band structures.

The Interaction between Surface Acoustic Waves and Spin Waves: The Role of Anisotropy and Spatial Profiles of the Modes.

The interaction between different types of wave excitation in hybrid systems is usually anisotropic. Magnetoelastic coupling between surface acoustic waves and spin waves strongly depends on the direction of the external magnetic field. However, in the present study we observe that even if the orientation of the field is supportive for the coupling, the magnetoelastic interaction can be significantly reduced for surface acoustic waves with a particular profile in the direction normal to the surface at distances much smaller than the wavelength. We use Brillouin light scattering for the investigation of thermally excited phonons and magnons in a magnetostrictive CoFeB/Au multilayer deposited on a Si substrate. The experimental data are interpreted on the basis of a linearized model of interaction between surface acoustic waves and spin waves.

Demonstration of k-vector selective microscopy for nanoscale mapping of higher order spin wave modes.

As a potential route towards beyond CMOS computing magnonic waveguides show outstanding properties regarding fundamental wave physics and data transmission. Here, we use time resolved scanning transmission X-ray microscopy to directly observe spin waves in magnonic permalloy waveguides with nanoscale resolution. Additionally, we demonstrate an approach for k-vector selective imaging to deconvolute overlapping modes in real space measurements. Thereby, we observe efficient excitation of symmetric and antisymmetric modes. The profiles of higher order modes that arise from sub-micron confinement are precisely mapped out and compared to analytical models. Thus, we lay a basis for the design of multimode spin wave transmission systems and demonstrate a general technique for k-specific microscopy that can also be used beyond the field of magnonics.

Perpendicularly magnetized Co20Fe60B20 layer sandwiched between Au with low Gilbert damping.

Nowadays, the CoFeB thin layered film is intensively studied because of its potential applications in spintronic devices, especially devices based on spin-transfer torque phenomena. Hitherto, it has been shown that CoFeB may possess perpendicular magnetic anisotropy (PMA) when it is sandwiched between different layers (e.g. MgO, Pt, Pd, Ta, W). However, there is no experimental evidence that CoFeB, sandwiched between Au layers, has strong PMA. Moreover, in comparison with other noble metals, Au-based film systems exhibit the smallest spin pumping effect, which provides the main contribution to the damping in thin films in contact with heavy metals. Therefore, Au/CoFeB/Au may be a good candidate for future applications, where perpendicular magnetic anisotropy and low damping are required. Here, we show that PMA and low damping can be achieved in a Au/CoFeB/Au system without annealing.

Impact of orthogonal exchange coupling on magnetic anisotropy in antiferromagnetic oxides/ferromagnetic systems.

The influence of interface exchange coupling on magnetic anisotropy in the antiferromagnetic oxide/Ni system is investigated. We show how interfacial exchange coupling can be employed not only to pin the magnetization of the ferromagnetic layer but also to support magnetic anisotropy to orient the easy magnetization axis perpendicular to the film plane. The fact that this effect is only observed below the Néel temperature of all investigated antiferromagnetic oxides with significantly different magnetocrystalline anisotropies gives evidence that antiferromagnetic ordering is a source of the additional contribution to the perpendicular effective magnetic anisotropy.