Probing phase transition and anisotropy in magnetic insulator based heterostructures employing magnon spin currents

Abstract

Thermoelectric phenomena like the Seebeck and Peltier effects are known for more than two centuries with a wide range of applications. In 2008 Uchida et al. first demonstrated the phenomena called Spin Seebeck Effect (SSE) in which spin currents are generated in a ferromagnetic (FM) material in the presence of a magnetic field with an applied temperature gradient. These spin currents were electrically detected in an adjoining heavy metal layer (usually Pt) employing the Inverse Spin Hall effect (ISHE). The absence of charge currents in insulators meant reduced losses due to joule heating and hence the interest in ferromagnetic insulator (FMI) based spintronic devices wherein spin information gets transferred by pure magnon spin waves. In 2012 another unusual magnetoresistance effect called the Spin Hall Magnetoresistance (SMR), was discovered in similar Pt/FMI bilayers in which the resistance of the Pt layer could be modulated by the magnetization orientation of the FMI layer underneath. Subsequently, both SSE and SMR have been explored in a wide range of magnetic materials with exotic magnetic structures both for its rich physics and potential application as thermoelectric materials. In this regard, we have investigated these phenomena in different spintronic materials across their magnetic phase transition. We start with a description of the various experimental arrangements developed in-house to perform steady-state measurement of longitudinal SSE and SMR in a broad range of magnetic fields and temperatures. Next, we outline the fundamental characteristics of both phenomena revealed by our investigations on YIG single crystals in bulk and thin film form. Following the optimization of measurement conditions on YIG, we probe the competing interactions at low temperature in MgFe2O4 (MFO), which forms part of another popular class of insulating magnetic material, ferrites. Enhanced SSE was observed in Pt/MFO bilayers, hosting both Ferromagnetic and Antiferromagnetic (AFM) interactions. A simple model that considers a distribution of anisotropy energies for the AFM could corroborate our experimental findings. In the second part, we will discuss the temperature evolution of the SSE across ferromagnet to paramagnet phase transition in Pt/EuO1−x and Pt/La2NiMnO6 heterostructures, where we try to resolve a longstanding disparity between theoretical predictions and experimental evidence with regards to the nature of power law decay of the SSE while approaching the ferromagnetic phase transition temperature (TC). We conclusively demonstrate a correlation between the critical exponents of SSE and magnetization for both FMs. We substantiate our results by extracting spin mixing conductance, (Gmix) from SMR study on LNMO, which emphasize the role of Gmix in determining thermal spin pumping. At the end, we will highlight the effect of anisotropy on SMR and the related phenomena, anisotropic magnetoresistance (AMR). The nature of SMR in epitaxial LNMO films on STO (001) suggested that the magnetic structure of the LNMO surface was different from the bulk of the film. Considering a uniaxial magnetic anisotropy of the surface oriented along (110), we could reproduce most of the observed features. On the other hand, these MR phenomena in polycrystalline EuO1-x on Si (100) serve as a sensitive probe for its bandstructure.