In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Spin Hall effects

Critical phenomena and renormalization-group theory

Nanostructures have attracted huge interest as a rapidly growing class of materials for many applications. Several techniques have been used to characterize the size, crystal structure, elemental composition and a variety of other physical properties of nanoparticles. In several cases, there are physical properties that can be evaluated by more than one technique. Different strengths and limitations of each technique complicate the choice of the most suitable method, while often a combinatorial characterization approach is needed. In addition, given that the significance of nanoparticles in basic research and applications is constantly increasing, it is necessary that researchers from separate fields overcome the challenges in the reproducible and reliable characterization of nanomaterials, after their synthesis and further process (e.g. annealing) stages. The principal objective of this review is to summarize the present knowledge on the use, advances, advantages and weaknesses of a large number of experimental techniques that are available for the characterization of nanoparticles. Different characterization techniques are classified according to the concept/group of the technique used, the information they can provide, or the materials that they are destined for. We describe the main characteristics of the techniques and their operation principles and we give various examples of their use, presenting them in a comparative mode, when possible, in relation to the property studied in each case.

/pdf/characterization-techniques-for-nanoparticles-comparison-and-2ubze3ehrc.pdf

Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties

Ordered magnetic nanostructures: fabrication and properties

We review the structural, magnetic and transport properties of double perovskites (A2BB'O6) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B?O?B'?O?B exchange interaction mediated by itinerant electrons. We first focus on the BB' =?FeMo-based double perovskites, with Sr2FeMoO6 (TC = 420?K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B' = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A2FeReO6 series, with maximum TC = 520?K for Ca2FeReO6, which shows a tendency to semiconducting behaviour. Finally, we describe the Sr2(Fe1?xCrx)ReO6 series, with maximum TC = 625?K for Sr2CrReO6, which is the highest TC in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions.

https://iopscience.iop.org/article/10.1088/0953-8984/19/2/023201/pdf

Double perovskites with ferromagnetism above room temperature

We have performed electron spin resonance (ESR) and dc susceptibility measurements in Mn perovskites up to 1000 K. Assuming an effective Heisenberg-like interaction for ${\mathrm{Mn}}^{3+}\mathrm{\ensuremath{-}}{\mathrm{Mn}}^{4+}$ spin pairs, the dc susceptibility, ${\ensuremath{\chi}}_{\mathrm{dc}}(T),$ is well described in the paramagnetic regime by the constant coupling approximation. Absolute determination of the ESR intensity indicates that all Mn spins contribute to the ESR line in the temperature range studied. The ESR linewidth can be described by $\ensuremath{\Delta}{H}_{\mathrm{pp}}(T)=\ensuremath{\Delta}{H}_{\mathrm{pp}}(\ensuremath{\infty})[C/T{\ensuremath{\chi}}_{\mathrm{dc}}(T)],$ thus presenting a universal behavior in a temperature scale normalized to ${T}_{c}.$ A single relaxation mechanism, related to spin-only interactions, explains the $T$ dependence of $\ensuremath{\Delta}{H}_{\mathrm{pp}}(T)$ for all the compounds studied: ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3},$ ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3},$ ${\mathrm{Pr}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3},$ and ${\mathrm{La}}_{0.67}{\mathrm{Pb}}_{0.33}{\mathrm{MnO}}_{3}.$ The dc susceptibility and the ESR linewidth and intensity all reflect the progressive importance of magnetic clustering below $\ensuremath{\approx}{2T}_{c}.$

High-temperature spin dynamics in CMR manganites: ESR and magnetization

In this work we present a study of the magnetic and transport properties of the system LaNiO3-x (0≤x≤0.5) where the x=0 member is a well-known metallic oxide. We found that the x=0.5 compound is an antiferromagnetic insulator at room temperature. The metal-insulator transition is observed for an x value around x.25. The role played by the sample inhomogeneities on the occurrence of this transition is dis-cussed. © 1996 The American Physical Society.

Metal-insulator transition in oxygen-deficient lanio3-x perovskites

Ferromagnetic correlations and mixed Ru valence in the magnetic superconductor RuSr 2 (Eu,Gd)Cu 2 O 8

By means of X- and Q-band magnetic-resonance measurements we have investigated the magnetic interactions and derived the magnon gap in the ferromagnetic superconductor ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}.$ Two microwave absorptions are observed: first, a paramagnetic resonance of the ${\mathrm{Gd}}^{3+}$ ions, and second, appearing below the Curie temperature ${(T}_{\mathrm{Curie}}),$ a ferromagnetic resonance due to the ruthenium lattice. Located between the superconducting ${\mathrm{CuO}}_{2}$ planes, the Gd ions serve as intrinsic probes of the internal magnetic fields near the Cu sites. Below ${T}_{\mathrm{Curie}}$ the ${\mathrm{Gd}}^{3+}$ signal shifts, evidencing the appearance of a homogeneous internal field, ${H}_{\mathrm{Ru}\ensuremath{-}\mathrm{Gd}}\ensuremath{\sim}600\mathrm{Oe}.$ We show that ${H}_{\mathrm{Ru}\ensuremath{-}\mathrm{Gd}}$ is due to a Ru-Gd ferromagnetic exchange interaction. The Ru ferromagnetic resonance indicates the existence of a magnon anisotropy gap $\ensuremath{\omega}/\ensuremath{\gamma}\ensuremath{\gtrsim}4600\mathrm{Oe}.$ We estimate the out-of-plane and in-plane anisotropy fields to be, respectively, ${H}_{z}\ensuremath{\sim}110\mathrm{kOe}$ and ${H}_{x}\ensuremath{\sim}200\mathrm{Oe}.$

Magnetic interactions and magnon gap in the ferromagnetic superconductor RuSr 2 GdCu 2 O 8

Magnetic dc susceptibility $(\ensuremath{\chi})$ and electron-spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss behavior for $\ensuremath{\chi}(\mathrm{T})$ with a ferromagnetic $\ensuremath{\Theta}=446(5) \mathrm{K}$ and ${\ensuremath{\mu}}_{\mathrm{eff}}=4.72(9){\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.,$ this being lower than that expected for either ${\mathrm{Fe}}^{3+}(5.9{\ensuremath{\mu}}_{B})$ or ${\mathrm{Fe}}^{2+}(4.9{\ensuremath{\mu}}_{B})$ ions. The ESR g factor $g=2.01(2),$ is associated with ${\mathrm{Fe}}^{3+}.$ We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized ${\mathrm{Fe}}^{3+}$ ${(3d}^{5})$ cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, assuming for the itinerant electrons a bare susceptibility independent on T. We obtained ${\ensuremath{\chi}}_{e}^{0}=3.7 {10}^{\ensuremath{-}4} \mathrm{e}\mathrm{m}\mathrm{u}/\mathrm{m}\mathrm{o}\mathrm{l}.$ We show that the reduction of ${\ensuremath{\mu}}_{\mathrm{eff}}$ for ${\mathrm{Fe}}^{3+}$ arises from the strong antiferromagnetic interaction between the two sublattices. At variance with classical ferrimagnets, we found that $\ensuremath{\Theta}$ is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa-type equations. Bottleneck is evidenced by the absence of a g shift. Surprisingly, as observed in colossal magnetoresistant manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ does not originate in superexchange interactions, but from a different mechanism recently proposed for double perovskites.

Alejandro Butera

Papers

High-temperature spin dynamics in CMR manganites: ESR and magnetization

Metal-insulator transition in oxygen-deficient lanio3-x perovskites

Ferromagnetic correlations and mixed Ru valence in the magnetic superconductor RuSr 2 (Eu,Gd)Cu 2 O 8

Magnetic interactions and magnon gap in the ferromagnetic superconductor RuSr 2 GdCu 2 O 8

Evidence of strong antiferromagnetic coupling between localized and itinerant electrons in ferromagnetic Sr 2 FeMoO 6