Date of Award

8-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

David Mandrus

Committee Members

Takeshi Egami, Elbio Dagotto, Stephen E. Nagler

Abstract

Broken inversion symmetry in a crystal lattice allows an extra term called the Dzyaloshinskii-Moriya (DM) interaction in the magnetic Hamiltonian. The DM interaction tends to align spins in perpendicular orientation and therefore competes with the exchange interaction that favors collinear spins. This competition results in different modulated chiral magnetic structures depending on the relative strength of the two interactions. Skyrmion, soliton and magnetic blue phases are some of the anticipated structures. This dissertation extends the search for these exotic magnetic structures using various techniques to study the magnetism in the noncentrosymmetric magnets Cr11Ge19 [chromium eleven germanium nineteen], Cr1/3NbS2 [chromium one-third niobium disulphide] and K2V3O8 [fresnoite-type potassium vanadate].

Experimental investigations of magnetic, thermal, structural and elastic properties of Cr11Ge19 indicate complex itinerant ferromagnetism, evidence of spin wave excitations, and strong magnetoelastic coupling in this material. First principles calculations support the presence of itinerant ferromagnetism and suggest a noncollinear ground state may be expected. In the chiral helimagnet Cr1/3NbS2 the magnetic transition is found to strongly affect the electrical transport. Spin reorientation from the helimagnetic ground state to the commensurate ferromagnetic state is evident in the magnetoresistance. Neutron scattering is used to demonstrate the change in the periodicity of the incommensurate structure and the eventual incommensurate to commensurate transition, in accordance with the theoretical prediction of the soliton model. The tetragonal easy axis antiferromagnet K2V3O8 has been investigated by DC magnetization, AC susceptibility and heat capacity measurements. Based on the comparison of the behaviors observed in these measurements with other well-studied chiral helimagnets, the existence of two different spiral structures - one parallel and one perpendicular to the c axis - is proposed.

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