We present a comprehensive study of the temperature (T ) dependence of the longitudinal spin Seebeck effect (LSSE) in Pt/EuO 1−x and Pt/La 2 NiMnO 6 (LNMO) hybrid structures across their Curie temperatures (T c ). Both systems host ferromagnetic interaction below T c , hence present optimal conditions for testing magnon spin current based theories against ferri magnetic YIG. Notably, we observe an anomalous Nernst effect (ANE) generated voltage in bare EuO 1−x , however, we find LSSE predominates the thermal signals in the bilayers with Pt. The T -dependence of the LSSE in small T -range near T c could be fitted to a power law of the form (T c − T ) P . The derived critical exponent, P, was verified for different methods of LSSE representation and sample crystallinity. The results are explained based on the magnon-driven thermal spin pumping mechanism that relate the T -dependence of LSSE to the spin mixing conductance (G mix ) at the heavy metal/ferromagnet (HM/FM) interface, which in turn is known to vary in accordance with the square of the spontaneous magnetization (M s ). Additionally, the T -dependence of the real part of G mix derived from spin Hall magnetoresistance measurements at different temperatures for the Pt/LNMO structure, further establish the interdependence.

No abstract

We report on the angular and field dependence of the magnetoresistance (MR) in bilayers of Pt/EuO1−x thin films, measured in both in-plane and out-of-plane geometries at different temperatures (T). The presence of oxygen vacancies manifested by a metal–insulator transition as well as a high-T ferromagnet-to-paramagnet transition (TP) was observed in the bilayers. The anisotropic magnetoresistance could be extracted in the entire T-range, even above TP, exhibiting two sign crossovers. We attribute its T-evolution to the rotation of the easy axis of the magnetization direction from a high-T out-of-plane to a low-T in-plane orientation. In addition, we provide direct experimental evidence of the spin Hall effect-induced spin Hall magnetoresistance, systematically considering several known MR contributions that can arise from the films' (111) texture and interface.

Single phase materials showing large Exchange Bias (EB) at elevated temperatures are highly desirable for their technological advantage. However, the fundamental interactions leading to the effect are often very complex and ill-understood. Here, we report the observation of EB in single phase (111) oriented MgFe2O4(MFO) thin films on MgO buffered Si(100). The effect is seen to persist till a significantly higher temperature of 110 K. Measurements on stoichiometric powder MFO showed the absence of EB, lower coercivity, and an enhanced saturation magnetization in comparison to thin films. This suggests the modified exchange interaction in the thin films possibly due to the presence of growth induced magnetic and structural disorder. Investigation on the nature of this EB produced some results which are akin to a conventional FM/AFM coupled system like the training effect. However, the same could not explain the observed variation in coercivity and the exchange bias field with temperature and cooling fields. We propose the existence of a disordered layer with random spin orientations at the crystalline grain boundaries of our film, promoting the AFM interaction between the ordered ferrimagnetic grains once field cooled below the spin freezing temperature. This generates a unidirectional anisotropy along the field direction which manifests as a loop shift in hysteresis measurements. A further study shows that the EB field can be varied systematically by changing the thickness of the thin films. This indicates a simple way to tune the disorder for achieving desirable magnetic properties in single phase magnetic thin films.

Orthoferrites (RFeO 3 ) containing rare-earth (R) elements are 3D antiferromagnets (AFMs) that exhibit characteristic weak ferromagnetism originating due to slight canting of the spin moments and display a rich variety of spin-reorientation transitions in the magnetic field (H )-temperature (T ) parameter space. We present spin Hall magnetoresistance (SMR) studies on a b plate (ac plane) of crystalline Ho 0.5 Dy 0.5 FeO 3 |Pt hybrid at various T in the range 11-300 K. In the room temperature 4 phase, the switching between two degenerate domains occurs at fields above a critical value, H c ≈ 713 Oe. Under H > H c , the angular dependence of SMR (α scan) yielded a highly skewed curve with a sharp change (sign-reversal) along with a rotational hysteresis around the a axis. Notably, at H < H c , the α-scan measurements on the single degenerate domain exhibited an anomalous sinusoidal signal of periodicity 360 • . Low-T evolution of the SMR curves (H = 2.4 kOe) suggests weakening of the anisotropy possibly due to the T evolution of Fe-R exchange coupling. Below 25 K, the SMR modulation showed an abrupt change around the c axis, marking the presence of a 2 phase. We have employed a simple Hamiltonian and computed SMR to examine the observed skewed SMR modulation. In summary, SMR is found to be an effective tool to probe magnetic anisotropy as well as a spin reorientation in Ho 0.5 Dy 0.5 FeO 3 . Our spin-transport study highlights the potential of Ho 0.5 Dy 0.5 FeO 3 for future AFM spintronic devices.