Graduation Year

2021

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

Dario Arena, Ph.D.

Committee Member

Nicholas Bingham, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Committee Member

Jing Wang, Ph.D.

Keywords

Competing Interactions, Double perovskites, Helical magnets, Magnetic phase diagram

Abstract

Materials with competing magnetic interactions provide a unique combination of intriguing physics originating from structural, magnetic, and electronic degrees of freedom, along with their technological applications. The complex oxides and helimagnets also share an interesting physics involving competitive interactions and understanding the behavior of one helps us to yield an insight into the other system. A rigorous understanding about the competing interactions in strongly correlated magnetic systems provide an opportunity to explore the related effects like spin frustration, glassiness, metamagnetism, which play an essential role in manipulating their magnetic properties and functionality. In this dissertation, we combine dc magnetization [M(H,T)], nonlinear χnw, radio frequency transverse susceptibility (TS), critical exponent analysis, and magnetocaloric effect (MCE), along with x-ray photoelectron spectroscopy (XPS) and neutron diffraction (ND) studies to account for the complex magnetic interactions in double perovskite oxides (A2BBʹO6), such as La2CoMnO6 (LCMO), Y2CoMnO6 (YCMO), Y2NiMnO6 (YNMO), and Y2Ni0.5Co0.5MnO6 (YNCMO) and employ similar techniques to unravel the magnetic complexity of a well-known helimagnet, MnP, that shares common and distinct magnetic characteristics. A systematic study is designed to investigate how the size of A-site cation (A = La and Y) will change the ground-state and the phase diagram of the double perovskite oxides, which is further extended to observe the significant changes in the magnetism caused by the substitution and co-doping at B-site (YNMO and YNCMO). The larger A-site cation results in the evolvement of a frustrated state with cluster-glass-like spin dynamics, while a metamagnetic behavior is observed in YCMO. The ac-χ revealed two regimes of glassy dynamics in LCMO due to the presence of multiple valent cations. TS tracked the local anisotropy of individual spin clusters to reveal an extended freezing mechanism with a very long time-scale, likely driven by progressive pinning of spin clusters to the FM matrix. For the YCMO system, the coexistence of AFM and FM matrix gave rise to the kinetic arrest phenomenon, verified via cooling and heating of the system in an unequal fields (CHUF) protocol. A comparative study between YCMO, YNMO, and YNCMO suggests that the AFM coupling among Y2NixCo1-xMnO6 compounds becomes stronger and is thermally favored at low temperatures with the addition/increase of Co2+ concentration. A careful examination of the temperature and field-dependent magnetic entropy change provided an understanding of coexisting magnetically ordered and disordered phases in the system from high to low temperature, leading to a comprehensive magnetic phase diagram of these multifunctional double perovskite systems. The extension of this study into the highly crystalline MnP nanorod films grown on Si (100) substrate assisted to elucidate the role of the strain in the coexisting helicity and ferromagnetism of the MnP film. The findings of TS established the impact of the anisotropy in stabilizing multiple magnetic phases in the MnP film. The comprehensive magnetic diagrams of the MnP film for two different field configurations were constructed for the first time, revealing the features that are absent in its single crystal counterpart. In general, these studies provide a new physical insight into the ground-state magnetic properties of strongly correlated systems in which the external stimuli like magnetic field, temperature, strain and composition can cause an imbalance of the competing phases leading to crossovers or an emergence of completely new magnetic phase.

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