Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

Frustrated magnetic systems, where all spin interactions cannot be simultaneously satisfied, have continued to attract interest due to a plethora of novel magnetic states that emerge in them due to frustration and their potential technological applications. Spinel oxides AB2O4, where A and B are metal ions) are an excellent testing ground for the exploration of frustrated magnetism. This dissertation presents the experimental investigation of complex magnetic phenomena in the spinel oxides FeMn2O4, MnFe2O4, and NiFe2O4.

FeMn2O4 and MnFe2O4 are members of the manganese ferrite family where both manganese and iron can possess mixed oxidation states, resulting in additional spin interactions that compete with the collinear ferrimagnetic order leading to complex magnetic ground states. From our measurements, we found that FeMn2O4 undergoes one structural and two magnetic transitions. The structural transition occurs at Ts ~ 595 K from a high temperature cubic to a low temperature tetragonal phase. Below TFI-1 ~ 373 K, it becomes collinear ferrimagnetic, and below TFI-2 ~ 50 K, its O-site spins form a spin ice-like “two-in-two-out” order. Similarly, in MnFe2O4, we identified three magnetic transitions: paramagnetic to collinear ferrimagnetic transition at TFI-1 ~ 575 K, followed by spin rearrangements at TFI-2 ~ 50 K and Tx ~ 15 K. Furthermore, in both systems, we found that the different magnetic orders have significant effects on other physical properties.

Frustration and complex magnetism can also be induced at the interfaces due to the competing magnetic orders. We extend our study into frustrated magnetism by exploring the interface-induced behavior in NiFe2O4 matrix containing self-assembled NiO columns that are magnetically distinct. The bulk magnetic measurements revealed a spin glass state below TSG ~ 28 K. By combining the macroscopic measurements with the microscopic analysis, like transmission electron microscopy and magnetic force microscopy, we establish that the spin glass state occurs at the interface due to the competing magnetic orders that frustrate the interfacial spins. We demonstrate the viability of self-assembly of microstructures within a matrix as an effective way to enable emergent phenomena.

Committee Chair

Jin, Rongying

Available for download on Thursday, March 11, 2021

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