No Arabic abstract
Liquid phase epitaxy of an 18 nm thick Yttrium Iron garnet (YIG) film is achieved. Its magnetic properties are investigated in the 100 -- 400 K temperature range, as well as the influence of a 3 nm thick Pt overlayer on them. The saturation magnetization and the magnetocrystalline cubic anisotropy of the bare YIG film behave similarly to bulk YIG. A damping parameter of only a few $10^{-4}$ is measured, together with a low inhomogeneous contribution to the ferromagnetic resonance linewidth. The magnetic relaxation increases upon decreasing temperature, which can be partly ascribed to impurity relaxation mechanisms. While it does not change its cubic anisotropy, the Pt capping strongly affects the uniaxial perpendicular anisotropy of the YIG film, in particular at low temperatures. The interfacial coupling in the YIG/Pt heterostructure is also revealed by an increase of the linewidth, which substantially grows by lowering the temperature.
We report the nonlocal spin Seebeck effect (nlSSE) in a lateral configuration of Pt/Y$_3$Fe$_5$O$_{12}$(YIG)/Pt systems as a function of the magnetic field $B$ (up to 10 T) at various temperatures $T$ (3 K < $T$ < 300 K). The nlSSE voltage decreases with increasing $B$ in a linear regime with respect to the input power (the applied charge-current squared $I^2$). The reduction of the nlSSE becomes substantial when the Zeeman energy exceeds thermal energy at low temperatures, which can be interpreted as freeze-out of magnons relevant for the nlSSE. Furthermore, we found the non-linear power dependence of the nlSSE with increasing $I$ at low temperatures ($T$ < 20 K), at which the $B$-induced signal reduction becomes less visible. Our experimental results suggest that in the non-linear regime high-energy magnons are over populated than those expected from the thermal energy. We also estimate the magnon spin diffusion length as functions of $B$ and $T$.
High quality nanometer-thick (20 nm, 7 nm and 4 nm) epitaxial YIG films have been grown on GGG substrates using pulsed laser deposition. The Gilbert damping coefficient for the 20 nm thick films is 2.3 x 10-4 which is the lowest value reported for sub-micrometric thick films. We demonstrate Inverse spin Hall effect (ISHE) detection of propagating spin waves using Pt. The amplitude and the lineshape of the ISHE voltage correlate well to the increase of the Gilbert damping when decreasing thickness of YIG. Spin Hall effect based loss-compensation experiments have been conducted but no change in the magnetization dynamics could be detected.
Magnetocaloric properties of an inhomogeneous magnetic system of a 7.6 nm La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ consisting of superparamagnetic (SPM) with blocking temperature ( $T_B$ = 240 K) and ferromagnetic (FM) phases ( $T_C$ = 290 K) is studied by dc magnetization measurements. Isothermal magnetization versus applied magnetic field is carried out from 100 K to 320 K in magnetic fields up to 4 kOe to determine changes in the magnetic entropy ($-Delta S_M$) and the relative cooling power (RCP). Due to the co-existence of SPM and FM phases, there are two peaks in the temperature dependence of$-Delta S_M$ in different applied magnetic fields from 1.3 kOe to 4 kOe. The peaks are at 220 K and 270 K which are close $T_B$ and $T_C$ of the film. The highest RCP occurs at 270 K (which is in $T_B$ $mathrm{<}$ $T$ $mathrm{<}$ $T_C$) in H = 4 kOe with the value of 0.19 (J/kg K). The $-Delta S_M$ vs T data are fit to the exponent power law, $-Delta S_M=a H^n$ where it shows good fits for the whole measured temperature range. This analysis reveals a deviation of n from $n$ = 2/3 which is likely due to the presence of SPM spin clusters in the dead layer for $T$ $mathrm{<}$ $T_C$. Results show that the thin film of La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ can be a good candidate for magnetic refrigeration devices with multiple RCP peaks in low and high temperatures.
Gate-induced modulation of the spin-orbit interaction (SOI) in a 1.5 nm-thick Pd thin film grown on a ferrimagnetic insulator was investigated. Efficient charge accumulation by ionic gating enables a substantial upshift in the Fermi level of the Pd film, which was corroborated by suppression of the resistivity in the Pd. Electromotive forces arising from the inverse spin Hall effect in Pd under spin pumping were substantially modulated by the gating, in consequence of the modulation of the spin Hall conductivity of Pd as in an ultrathin Pt film. The same experiment using a thin Cu film, for which the band structure is largely different from Pd and Pt and its SOI is quite small, provides further results supporting our claim. The results obtained help in developing a holistic understanding of the gate-tunable SOI in solids and confirm a previous explanation of the significant modulation of the spin Hall conductivity in an ultrathin Pt film by gating.
A range of high quality Ga1-xMnxN layers have been grown by molecular beam epitaxy with manganese concentration 0.2 < x < 10%, having the x value tuned by changing the growth temperature (Tg) between 700 and 590 {deg}C, respectively. We present a systematic structural and microstructure characterization by atomic force microscopy, secondary ion mass spectrometry, transmission electron microscopy, powder-like and high resolution X-ray diffraction, which do not reveal any crystallographic phase separation, clusters or nanocrystals, even at the lowest Tg. Our synchrotron based X-ray absorption near-edge spectroscopy supported by density functional theory modelling and superconducting quantum interference device magnetometry results point to the predominantly +3 configuration of Mn in GaN and thus the ferromagnetic phase has been observed in layers with x > 5% at 3 < T < 10 K. The main detrimental effect of Tg reduced to 590 {deg}C is formation of flat hillocks, which increase the surface root-mean-square roughness, but only to mere 3.3 nm. Fine substrates surface temperature mapping has shown that the magnitudes of both x and Curie temperature (Tc) correlate with local Tg. It has been found that a typical 10 {deg}C variation of Tg across 1 inch substrate can lead to 40% dispersion of Tc. The established here strong sensitivity of Tc on Tg turns magnetic measurements into a very efficient tool providing additional information on local Tg, an indispensable piece of information for growth mastering of ternary compounds in which metal species differ in almost every aspect of their growth related parameters determining the kinetics of the growth. We also show that the precise determination of Tc by two different methods, each sensitive to different moments of Tc distribution, may serve as a tool for quantification of spin homogeneity within the material.