Spin-resolved microscopy and spectroscopy on magnetic semimetals

Abstract

Ferromagnetism in complex materials has emerged as a subject of great interest in many areas of research. They provide fertile testing grounds for new physics and discovery. Furthermore, they have great potential for variety of applications in spintronic devices, racetrack memory devices, information technology applications etc. In this thesis, the results of our investigation of two such complex ferromagnetic materials will be discussed in detail. Co3Sn2S2 is a ferromagnetic system that has garnered significant attention due to the existence of novel Weyl semimetallic(WSM) phase, along with the possibility of half metallicity, as indicated by several experimental and theoretical investigations. In the context of topological phase, this compound exhibits a large anomalous hall effect and a large anomalous Nernst signal that is much larger than the known fer romagnets. Both of these effects emerged due to enhanced contribution from Berry curvature near the Fermi level. Additionally, the Fermi arc surface states, a hall mark signature of Weyl semi-metallic phase was also detected and studied using an gle resolved photoemmision spectroscopy(ARPES). The combination of topological aspects along with half metallicity in Co3Sn2S2 makes it a promising candidate for spintronic applications. While the existence of half-metallicity was predicted via a number of theoretical calculations and supported via ARPES and photo emis sion spectroscopy, the transport spin polarisation of the Fermi surface has remained undetermined. We investigated the transport spin polarisation in this compound by spin resolved point contact Andreev reflection spectroscopy. Our investigation revealed that the transport spin polarisation in Co3Sn2S2 is much lower than was indicated previously. This deviation from half-metallicity was investigated via de tailed band structure calculations revealing a prominent spin depolarising effect at the Fermi energy driven by spin-orbit coupling. iv The magnetic phase diagram of Co3Sn2S2 is also very interesting. Co3Sn2S2 un dergoes a phase transition from paramagnetic to ferromagnetic phase below 177 K. However, this transition proceeds through an intermediate non-trivial magnetic phase known as the A- phase. The A-phase exhibits competing ferromagnetic and anti-ferromagnetic orders. Interestingly, several experiments have revealed the ex istence of non-trivial spin phases such as spin glass, Skyrmions, etc., just below the curie temperature. We investigated the domain structure at various tempera tures, magnetic fields and field angles by magnetic force microscopy. Our investiga tion revealed uniaxial anisotropy and high mobility of domain walls governing the anomalous phase below the Curie temperature. Additionally, the Fermi surface of Co3Sn2S2 was probed via angular magnetoresis tance (AMR) measurements in four probe geometry and also under a point contact. The four probe measurements yielded a two fold symmetric AMR arising from avail able states when the field direction is parallel to the direction of current. The point contact AMR measurements exhibited a field induced transition from two fold sym metric to four-fold symmetric AMR indicating that the Fermi surface undergoes a f ield induced reconstruction at temperatures above the critical temperature of the superconducting point contact. Furthermore, we look into one of the newly emerging ferromagnets, Co7Zn8Mn5, a topological chiral ferromagnet where we probe the ferromagnet under spin resolved Andreev reflection spectroscopy. Our investigation revealed a spin polarisation of 55% along with a small amount of particle hole asymmetry. This material crystal lizes into two chiral crystal structures both at the same energy. Our spectroscopic experiments reveals that the spin polarization in the material is 55% regardless of the crystal structure