The onset of a Griffiths-like phase has been observed in ${\mathrm{Tb}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ ($T_C=110\mathrm{K}$) by means of magnetic susceptibility and small-angle neutron scattering experiments. We show the growth of a ferromagnetic cluster system characterized by an inverse susceptibility exponent lower than unity at $T_C<T<T_G\approx 200\mathrm{K}$. We suggest that the Griffiths-like state is originated by local disorder within the crystallographic structure, stabilized and enhanced by competing intralayer and interlayer magnetic interactions. Both factors thus promote segregation of nanometric regions with ferromagnetic interactions.

Epitaxial films of ${\mathrm{SrMnO}}_3$ and bilayers of ${\mathrm{SrMnO}}_3/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ have been deposited by pulsed laser deposition on different substrates, namely, ${\mathrm{LaAlO}}_3$ (001), $\left({\mathrm{LaAlO}}_3{)}_{0.3}\right({\mathrm{Sr}}_2{\mathrm{AlTaO}}_6{)}_{0.7}$ (001), and ${\mathrm{SrTiO}}_3$ (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures ($T_N$) of the ${\mathrm{SrMnO}}_3$ films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, $T_N$ is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around $1.6\pm 0.1\%$. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the ${\mathrm{SrMnO}}_3$ layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the ${\mathrm{SrMnO}}_3$ and ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the ${\mathrm{SrMnO}}_3$ layer.

We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe₃O₄/Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance simultaneously, therefore resulting in significant power enhancement. This result demonstrates that the multilayer structures are useful for improving the thermoelectric performance of the spin Seebeck effect.

We have studied the anomalous Nernst effect (ANE) in [Fe₃O₄/Pt]-based heterostructures grown at high temperature, by measuring the ANE-induced electric field with a magnetic field applied normal to the sample surface, in the perpendicular magnetized configuration, where only the ANEs from the ferromagnetic layers or magnetic proximity effects can be detected. An ANE voltage is observed for [Fe₃O₄/Pt]_n multilayers, and we further investigated its origin by performing measurements in [Fe₃O₄/Pt/Fe₃O₄] trilayers as a function of the Pt thickness. Our results suggest the presence of an interface-induced ANE in the metallic layer, possibly driven by a heat induced subnanometer interdiffusion which affects the nature of the Fe₃O₄/Pt interface. Despite this ANE, the spin Seebeck effect is the dominant mechanism for the transverse thermoelectric voltage in the in-plane magnetized configuration, accounting for about 70% of the voltage in the multilayers.

We report the fabrication of high crystal quality epitaxial thin films of maghemite (γ−Fe₂O₃), a classic ferrimagnetic insulating iron oxide. Spin Seebeck effect (SSE) measurements in γ−Fe₂O₃/Pt bilayers as a function of sample preparation conditions and temperature yield a SSE coefficient of 0.5(1) μV/K at room temperature. Dependence on temperature allows us to estimate the magnon diffusion length in maghemite to be in the range of tens of nanometers, in good agreement with that of conducting iron oxide magnetite (Fe₃O₄), establishing the relevance of spin currents of magnonic origin in magnetic iron oxides.

The dependence of Spin Seebeck effect (SSE) with the thickness of the magnetic materials is studied by means of incoherent thermal excitation. The SSE voltage signal in Fe₃O₄/Pt bilayer structure increases with the magnetic material thickness up to 100 nm, approximately, showing signs of saturation for larger thickness. This dependence is well described in terms of a spin current pumped in the platinum film by the magnon accumulation in the magnetic material. The spin current is generated by a gradient of temperature in the system and detected by the Pt top contact by means of inverse spin Hall effect. Calculations in the frame of the linear response theory adjust with a high degree of accuracy the experimental data, giving a thermal length scale of the magnon accumulation (Λ) of 17 ± 3 nm at 300 K and Λ = 40 ± 10 nm at 70 K.