Parity-controlled spin-wave excitations in synthetic antiferromagnets

We report in this study the current-induced-torque excitation of acoustic and optical modes in Ta/NiFe/Ru/NiFe/Ta synthetic antiferromagnet stacks grown on SiO2/Si substrates. The two Ta layers serve as spin torque sources with the opposite polarisations both in spin currents and Oersted fields acting on their adjacent NiFe layers. This can create the odd symmetry of spatial spin torque distribution across the growth direction, allowing us to observe different spin-wave excitation efficiency from synthetic antiferromagnets excited by homogeneous torques. We analyse the torque symmetry by in-plane angular dependence of symmetric and anti-symmetric lineshape amplitudes for their resonance and confirm that the parallel (perpendicular) pumping nature for the acoustic (optical) modes in our devices, which is in stark difference from the modes excited by spatially homogeneous torques. We also present our macrospin model for this particular spin-torque excitation geometry, which excellently supports our experimental observation. Our results offer capability of controlling spin-wave excitations by local spin-torque sources and we can explore further spin-wave control schemes based on this concept.Comment: 31 pages, 12 figure

Spin injection characteristics of Py/graphene/Pt by gigahertz and terahertz magnetization dynamics driven by femtosecond laser pulse

Spin transport characteristics of graphene has been extensively studied so far. The spin transport along c-axis is however reported by rather limited number of papers. We have studied spin transport characteristics through graphene along c-axis with permalloy(Py)/graphene(Gr)/Pt by gigahertz (GHz) and terahertz (THz) magnetization dynamics driven by femtosecond laser pulses. The relatively simple sample structure does not require electrodes on the sample. The graphene layer was prepared by chemical vapor deposition and transferred on Pt film. The quality of graphene layer was characterized by Raman microscopy. Time resolved magneto-optical Kerr effect is used to characterize gigahertz magnetization dynamics. Magnetization precession is clearly observed both for Pt/Py and Pt/Gr/Py. The Gilbert damping constant of Pt/Py was 0.015, indicates spin pumping effect from Py to Pt. The Gilbert damping constant of Pt/Gr/Py is found to be 0.011, indicates spin injection is blocked by graphene layer. We have also performed the measurement of THz emission for Pt/Py and Pt/Gr/Py. While the THz emission is clearly observed for Pt/Py, a strong reduction of THz emission is observed for Pt/Gr/Py. With these two different experiments, and highly anisotropic resistivity of graphite, we conclude that the vertical spin transport is strongly suppressed by the graphene layer.Comment: Submitted to AIP adv (MMM

Tunable magnon-magnon coupling in synthetic antiferromagnets

In this work, we study magnon-magnon coupling in synthetic antiferromagnets (SyAFs) using microwave spectroscopy at room temperature. Two distinct spin-wave modes are clearly observed and are hybridised at degeneracy points. We provide a phenomenological model that captures the coupling phenomena and experimentally demonstrate that the coupling strength is controlled by the out-of-plane tilt angle as well as the interlayer exchange field. We numerically show that a spin-current mediated damping in SyAFs plays a role in influencing the coupling strength.Comment: 13 pages, 11 figures(including supplementary

Beyond a phenomenological description of magnetostriction

We use ultrafast x-ray and electron diffraction to disentangle spin-lattice coupling of granular FePt in the time domain. The reduced dimensionality of single-crystalline FePt nanoparticles leads to strong coupling of magnetic order and a highly anisotropic three-dimensional lattice motion characterized by a- and b-axis expansion and c-axis contraction. The resulting increase of the FePt lattice tetragonality, the key quantity determining the energy barrier between opposite FePt magnetization orientations, persists for tens of picoseconds. These results suggest a novel approach to laser-assisted magnetic switching in future data storage applications.Comment: 12 pages, 4 figure