Noncollinear magnetic materials for energy-efficient data storage devices

Abstract

Nature has given us abundant materials with multiple structures, where spin phases have been playing a vital role in spintronic devices. Recently, it has been noted that not only from typical collinear spin materials such as collinear ferromagnets and collinear antiferromagnetically coupled materials, noncollinear spintronic materials are the new hot spots of research owing to their exotic physical phenomena. In this thesis, firstly, the introduction of three different types of noncollinear spin structures, that is, the helical spin structure that offers helical spin phases and the coplanar noncollinear spin structure that could yield momentum-space Berry phases, and then move to relevant physical phenomena,including the topological Hall effect, anomalous Hall effect, multiferroic, spin-polarisedcurrent, and spin Hall effect. Afterwards, this thesis summarises and elaborate on themagnetic-field control of the noncollinear spin structure and related physical effects, which could enable ultra-low power spintronic devices. This thesis presents experimental investigations on NiBr2, Cd-Cu2OSeO3, and Mn doped SmFeO3 single crystals by dc magnetisation, ac susceptibility, small-angle neutron scattering (SANS) measurements along with X-ray absorption spectroscopy (XAS). The main focus is on NiBr2; CSO and SFMO are toward spin reorientation from commen surate to incommensurate magnetic structure and weak ferromagnetic to compensatedantiferromagnetic, respectively, at low temperatures, which have immense scientific and technological implications for next-generation energy-efficient magnetic data storage devices. The thesis consists of the following chapters: • Chapter 1: presents a literature survey of the existing work related to the formation of a helical ground state, formation of complex magnetic structures: multi-q states and magnetic skyrmions and their experimental and theoretical realization. • Chapter 2: describes the experimental techniques, structure, morphology, and composition employed in this work. Introduction of dc magnetisation and ac magnetic susceptibility measurements using a Superconducting Quantum Interference Device Small-Angle Neutron Scattering (SANS), and X-ray absorption spectroscopy (XAS) are presented. • Chapter 3: presents magnetometry to study the low-temperature magnetic phase dia gram of NiBr2 single crystals in the vicinity of TIC = 23 K and around the incommensurate phase. By covering a field range from 0.1 T to 3 T, the ac susceptibility reveals the characteristic relaxation associated with the transitions between the collinear and noncollinear phases, which shows the signature of the field-induced helimagnetic transition. The natureof field-induced transition and microscopic features were analysed in Chapter 5, with a broad range of magnetic fields and small-angle neutron scattering experiments. • Chapter 4: represents the extended range of ac & dc magnetometry with the ap plied magnetic field (1 -14 T) in a-b basal plane and out of the plane, complemented by SANS. The results demonstrate a clear view of the magnetic-field-induced non-collinear to collinear spin transition in the triangular spin-lattice of helimagnet NiBr2 single crystals. Our experimental outcomes are closely related to Okubo et al. (PRL 108, 017206 (2012)), where the formation of multi-q states and the skyrmions phase for a triangular spin-lattice has been suggested. This study searched for these states in NiBr2 single crystals, but couldnot find any signature of the skrmionic phase in the NiBr2 single crystal. Instead, a typical degeneracy occupation of the equivalent wave vector for the incommensurate state was detected. • Chapter 5: In this chapter, the structural, electronic, and magnetic properties of Cd-doped Cu2OSeO3 nanocrystallites are described. In addition, cost-effective and fast synthesis of Cu2OSeO3 nanocrystallites with sizes ranges over 50-200 nm. The physical significance of Cd doping on the Cu2OSeO3 skyrmions will carry significant technological importance, as doping-induced chemical pressure can be used to tune and control the skyrmionic phases and their various physical properties. • Chapter 6: presents the influence of Mn-doping on structural and magnetic properties of canted antiferromagnet SmFeO3 single crystals obtained by optical float zone technique. SQUID magnetometry was performed in the temperature range from 5 to 400 K. The resultsreveal a new spin reorientation from the weak ferromagnetic state to the compensatedantiferromagnetic state at nearly 180 K, which is missing in the parent compound formagnetic fields applied along with the different crystallographic directions due to Mndoping. Variations in the coercive field indicate the emergence of the exchange biasphenomenon at low temperatures. The microscopic origin of spin reorientations remain sun clear. • Chapter 7: Finally, in this chapter, concluding remarks are presented on the basis of the experimental findings and theoretical interpretation obtained in all the single crystal and nanocrystallite samples that are addressed in the thesis.