Interplay between spin, heat, and charge currents in strongly correlated systems

Abstract

Understanding the interplay between spin, heat, and charge currents is of fundamental importance in the area of Spintronics, where the spin degree of freedom plays a crucial role. The interaction between spin and heat currents alone is classified as the Spin-Caloritronics, which is basically a newly emerging sub-field of Spintronics. Depending on the nature of interaction among spin, heat, and charge degrees of freedom, various kinds of thermally driven spintronic phenomena are observed; and among them, three distinct phenomena are now being actively investigated: (I) The Spin Seebeck effect (SSE), (II) The anomalous Nernst effect (ANE), and (III) The magnon Hall effect (MHE).

(I) The Spin Seebeck effect: This pertains to the generation of pure spin current under the application of a temperature gradient (∇T) across a magnetic material. A reliable way to detect this spin current is to use a normal metal (NM) with a large spin Hall angle, e.g., Pt, Ta, or W, on top of the magnetic material. Thermally-induced spin current is then pumped into the NM layer via the interfacial exchange interaction between itinerant and localized electrons across the interface, which further gives rise to a transverse voltage via the inverse spin Hall effect (ISHE).

(II) The anomalous Nernst effect: This pertains to the generation of a transverse electrical voltage across a magnetic system under the application of a longitudinal thermal gradient. In particular, the ANE is defined in the direction orthogonal to both magnetization and the applied temperature gradient, where the ANE-voltage stems from the interaction between the conduction electrons with the local magnons under the influence of spin-orbit interactions.

(III) The magnon Hall effect: This describes the Hall effect of magnon current. Even though the magnons are charged neutral, they exhibit Hall transport that stems from the Berry curvature of the magnon bands. Experimentally, the MHE is quantified in terms of the thermal Hall effect of magnons (THEM), which pertains to the generation of a transverse thermal gradient (∇Tzx) by the spin excitations (magnons) due to the application of a longitudinal thermal gradient across the magnetic material . This thesis is divided into six parts:

Chapter 1: This Chapter provides an introduction to the research area of SpinCaloritronics, where the possible mechanisms of the generation, detection, and characterization of various Spin-Caloritronic signals are described in brief. In particular, the issues associated with the (i) SSE, (ii) ANE, and (iii) MHE are highlighted.

Chapter 2: Here we provide the details of the experimental set-up that was designed and developed during the course of this thesis for the sensitive measurements of SSE, ANE, and MHE. The set-up comprises of a closed cycle refrigerator (CCR), a temperature controller, a nano-voltmeter, a source meter, and an electromagnet with an upper field limit of 2 kOe. Two PID controlled heaters control the temperature gradient across the sample, as well as the average temperature of the sample, where the temperatures of the opposite ends of the sample are sensed by two temperature sensors attached near the sample edges. The holder’s geometry is so designed that the applied thermal gradient is always orthogonal to the rotation plane of the magnetic field. The whole setup is kept inside a Faraday cage to reduce the noise, and signals as low as 10nV can be reliably measured.

Chapter 3: In this chapter, we discuss the detailed investigations of the longitudinal spin Seebeck effect (LSSE), as measured in the La0.7Ca0.3MnO3/Pt bilayer system. We show that the longitudinal spin Seebeck voltage (VLSSE) is proportional to T^0.5 in low temperatures, which matches well with that predicted by the magnon-driven spin current model. On the other hand, the critical exponent of VLSSE near the para-toferromagnetic transition is found to be much higher than that of the magnetization, and also depends on the thickness of the spin-to-charge conversion layer (the NM layer). These observations highlight the importance of individually ascertaining the temperature evolution of different mechanisms − especially the spin mixing conductance − which contribute to the measured spin Seebeck signal.

Chapter 4: This chapter describes detailed investigations of the Ni1.96Mn1.04Ga system through various magnetic, electronic, and thermal characterizations. Upon cooling, such a material undergoes many sequential phase transitions, like, the pre-martensitic transition (PMT) and martensitic transition (MT). The ANE is primarily investigated, and our measurements of the ANE reveal that it is very sensitive to these phase transitions. In particular, a pronounced change in the ANE signal is observed across the pre-martensitic phase of this material, whereas such a change in other transport and magnetic measurements remains faint. With the ANE being sensitive to changes at the Fermi surface, we have discussed the possible coupling between local magnetism and the Fermi surface − where, the ANE is tuned by the magnetic field-driven changes at the Fermi surface.

Chapter 5: This chapter presents detailed investigations of the THEM-signal in single-crystalline Y2V2O7 specimens. The measurements were performed in two different crystals whose planes are cut and polished into two different orientations, {100} and {111}. Unlike the previous experimental detection method [12, 15], we measure the ∇Tzx by putting a thin non-magnetic normal metal (NM), like, Pt, or W, on top of the crystal’s plane. The THEM-signal is then quantified in terms of thermo-power, generated along the length of the NM-bar (also called as the detection layer). In contrast to the prior reports, where the ∇Tzx was directly measured by utilizing thermocouples, our method significantly improves the signal to noise ratio. The detailed investigation on the Y2V2O7 crystals reveals that a significant part of the observed THEM signal is contributed by the magnons of the higher energy bands. This is in contrast to the previous report, where only the contribution of the lowest magnon-band was considered to have a role in dictating the THEM-signal [12, 15]. Additionally, the crystal with {111} planes is observed to display a significant magnon drag effect at the interface with that NM layer − which is further linked to the topologically protected chiral surface state of magnons.

Chapter 6: This chapter presents the conclusions of the work described in this thesis, along with possible future directions.