Spin pumping has been studied within Ta/Ag/Ni 81 Fe 19 (0-5 nm)/Ag(6 nm)/Co 2 MnGe(5 nm)/Ag/Ta large-area spin valve structures, and the transverse spin current absorption of Ni 81 Fe 19 sink layers of different thickness has been explored. In some circumstances the spin current absorption can be inferred from the modification of the Co 2 MnGe source layer damping in vector network analyser ferromagnetic resonance (VNA-FMR) experiments. However the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thickness of less than 0.6 nm are found to be discontinuous, and super paramagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5 nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR linewidth that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.

To meet growing data storage needs, the density of data stored on hard disk drives must increase. In pursuit of this aim the magnetodynamics of the hard disk write head must be characterized and understood, particularly the process of "flux beaming". In this study, seven different configurations of perpendicular magnetic recording (PMR) write heads were imaged using time-resolved scanning Kerr microscopy, revealing their detailed dynamic magnetic state during the write process. It was found that the precise position and number of driving coils can significantly alter the formation of flux beams during the write process. These results are applicable to the design and understanding of current PMR and next-generation heat-assisted magnetic recording (HAMR) devices, as well as being relevant to other magnetic devices. a) Corresponding author. rajv202@ex.ac.uk

Four different designs of partially built hard disk write heads with a yoke comprising four repeats of NiFe (1 nm)/CoFe (50 nm) were studied by both x-ray photoemission electron microscopy (XPEEM) and time-resolved scanning Kerr microscopy (TRSKM). These techniques were used to investigate the static equilibrium domain configuration and the magnetodynamic response across the entire structure, respectively. Simulations and previous TRSKM studies have made proposals for the equilibrium domain configuration of similar structures, but no direct observation of the equilibrium state of the writers has yet been made. In this study, static XPEEM images of the equilibrium state of writer structures were acquired using x-ray magnetic circular dichroism as the contrast mechanism. These images suggest that the crystalline anisotropy dominates the equilibrium state domain configuration, but competition with shape anisotropy ultimately determines the stability of the equilibrium state. Dynamic TRSKM images were acquired from nominally identical devices. These images suggest that a longer confluence region may hinder flux conduction from the yoke into the pole tip: the shorter confluence region exhibits clear flux beaming along the symmetry axis, whereas the longer confluence region causes flux to conduct along one edge of the writer. The observed variations in dynamic response agree well with the differences in the equilibrium magnetization configuration visible in the XPEEM images, confirming that minor variations in the geometric design of the writer structure can have significant effects on the process of flux beaming.

The response of Y 3 Fe 5 O 12 /Cu/Ni 81 Fe 19 trilayer structures to excitation by a femtosecond laser pulse has been studied in optical pump-probe experiments and compared with the response of Y 3 Fe 5 O 12 (YIG) and Ni 81 Fe 19 reference samples. The optical pump induces a partial demagnetization of the Ni 81 Fe 19 , a large thermal gradient within the YIG, and temperature differences across the interfaces within the sample stack. When a moderate magnetic field is applied close to normal to the sample plane, so as to quasialign the YIG magnetization with the field and cant the Ni 81 Fe 19 magnetization from the plane, ultrafast demagnetization initiates precession of the Ni 81 Fe 19 magnetization. The transient temperature profile within the samples has been modeled using a one-dimensional finite-element computational model of heat conduction, while the magnetization dynamics are well described by a macrospin solution of the Landau-Lifshitz-Gilbert equation. The precessional response of the Ni 81 Fe 19 layers within the trilayers and the Ni 81 Fe 19 reference sample are very similar for pump fluences of up to 1.5 mJ/cm 2 , beyond which irreversible changes to the magnetic properties of the films are observed. These results suggest that the spin Seebeck effect is ineffective in modifying the precessional dynamics of the present YIG/Cu/Ni 81 Fe 19 samples when subject to ultrafast optical excitation.