Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Hariharan Srikanth, Ph.D.

Co-Major Professor

Manh-Huong Phan, Ph.D.

Committee Member

Inna Ponomareva, Ph.D.

Committee Member

Dario Arena, Ph.D.

Committee Member

Michael Osofsky, Ph.D.

Keywords

Chiral Helimagnet, Critical Exponents, Magnetic Phase Transition, Magnetocaloric Effect, Noncentrosymmetric Magnets, Skyrmion Lattice

Abstract

Noncentrosymmetric magnetic materials have gained special attention due to their ability to stabilize topologically nontrivial magnetic states via the competition between symmetric exchange and the antisymmetric Dzyaloshinskii-Moriya (DM) interaction. The spin struc- tures in these materials have become a center of interest for spintronics applications due to their stable, particle-like properties, and high degree of tunability via control of external parameters, such as magnetic and/or electric field and temperature. Understanding how these robust magnetic structures stabilize, evolve, dynamically respond, and adhere to existing models, all in the presence of external stimuli, are topics of fundamental interest. In this dissertation, the static, dynamic and magnetocaloric properties are investigated in selected materials that host noncollinear spin textures. Three material systems are studied, all of which share the common property of broken inversion symmetry, but realize distinct spatially modulated states and topological spin structures: the chiral soliton lattice in Cr1/3NbS2, the chiral Bloch-type skyrmion material MnSi, and the polar N ́eel skyrmion lattice (SkL) material GaV4S8.

For chiral helimagnets (CHM) Cr1/3NbS2 and MnSi, the magnetization for magnetic fields corresponding to the forced ferromagnetic (FFM) or field-polarized (FP) phase is used to analyze the static critical exponents describing a paramagnetic (PM) to ferromagnetic (FM) phase transition. The monoaxial CHM, Cr1/3NbS2, exhibits 3D Heisenberg universal behavior consistent with the localized nature the of Cr3+ moments and suggest short-range ferromagnetic interactions. The temperature and field dependence of the critical exponents of MnSi are systematically studied to analyze their deviation from the tricritical mean field model. By exploiting the magnetocaloric effect, the magnetic entropy change is utilized to probe the phase transformations of complex magnetic structures in Cr1/3NbS2. The magnetic entropy change (∆SM) is systematically analyzed to resolve details of the temperature and field dependent phase evolution of the chiral soliton lattice (CSL) from the CHM ground state. Our observations in the low field region are consistent with the existence of chiral ordering in a temperature range above the Curie temperature defined by the critical exponents analysis, TC < T < T∗, where a first-order transition has been previously predicted. An analysis of the universal behavior of ∆SM (T,H) experimentally demonstrates the deviation from the universal curve approaching the chiral ordered phases from high temperature.

The linear and nonlinear ac magnetic response (Mnω) further refines the details of the metamagnetic crossovers and phase transitions in Cr1/3NbS2. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time scales with increasing temperature. The tricritical point is experimentally resolved at TTCP in a temperature regime above the Curie temperature which separates the linear and nonlinear magnetic regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarizes the features of the field and temperature dependence of the magnetic crossovers and phase transitions in Cr1/3NbS2. Using a combination of static magnetization, ac magnetic response and magnetocaloric measurements, the magnetic phase evolution and magnetization dynamics in GaV4S8 is studied as a function of temperature and magnetic field. At low temperature, the transition between the zero-field spin cycloid and the ferromagnetic ground state is ac- companied by large contributions from higher harmonics in the ac magnetic response. The observations support the picture of a harmonically-modulated cycloid spin structure which becomes distorted on approaching the ferromagnetic ground state. Positive entropy changes approaching the FM ground also support this picture.

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