Graduation Year

2024

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Manh-Huong Phan, Ph.D.

Co-Major Professor

Sarath Witanachchi, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Committee Member

Sanjukta Bhanja, Ph.D.

Keywords

magnetoresistance, Manganese phosphide, nanostructuring, spintronics

Abstract

The study of magnetoresistance (MR) phenomena in magnetic materials has been pivotal for advancing magnetic sensors and spintronic devices. Helimagnets, characterized by a non-collinear arrangement of magnetic moments that form a helical or spiral configuration, present an intriguing avenue for MR-based device applications. Theoretical predictions suggest that the MR magnitude in the helimagnetic (HM) regime can exceed that in the ferromagnetic (FM) regime by over an order of magnitude. However, in metallic helimagnets like manganese phosphide (MnP), the MR in the HM phase remains modest (< 10%), limiting its applicability in MR devices. This dissertation investigates the emergent magnetism and magneto-transport phenomena in nanostructured MnP helimagnets and their interfaces with ferromagnets (Fe) and ferrimagnets (Fe3O4), aiming to enhance device performance.

In the first part of the dissertation, we present a novel approach to achieve a giant low-field MR effect in nanostructured MnP films by leveraging confinement and strain effects along with spin helicity. The MnP films were grown on Si substrates using molecular beam epitaxy. Unlike the modest MR observed in bulk MnP single crystals and large-grain polycrystalline films, which exhibit a small negative MR in the FM region (~2%) that increases to ~8% in the HM region across 10–300 K, we discovered a grain size-dependent giant positive MR (~90%) near the FM-HM transition temperature (TN ~110 K), followed by a rapid decline to negative MR below ~55 K in the MnP nanocrystalline films.

In the second part of the dissertation, we explore the varying interfacial effects in the Fe/MnP bilayer system, where ferromagnetic Fe layers of varying thickness (5 and 15 nm) were sputtered onto the MnP film (grain size ~39 nm; thickness ~100 nm). We observed that the presence of a 5 nm Fe layer in the Fe(5 nm)/MnP bilayer enhances coercivity in the FM regime due to the dual magnetic proximity effect (MPE), peaking around the FM-HM transition. However, as the thickness of the Fe film increases to 15 nm, the magnetism of the Fe(15 nm)/MnP bilayer becomes predominantly influenced by the soft ferromagnetism of the 15 nm Fe layer. Notably, we observed a significant reduction in the MR ratio in the Fe(5 nm)/MnP bilayer compared to the bare MnP film, attributed to MPE-weakened spin-dependent scattering of conduction electrons.

In the final part of the dissertation, we demonstrate an innovative approach to enhance the MR effect near the FM-HM transition in MnP nanostructured films by interfacing them with an Fe3O4 layer, which undergoes a first-order structurally coupled electronic/magnetic phase transition known as the Verwey transition at ~120 K. Fe3O4 layers of varying thickness (7 nm and 58 nm) were deposited onto MnP films (grain size ~86 nm; thickness ~100 nm). Our findings indicate that the presence of the Fe3O4 layer in the Fe3O4/MnP bilayers enhances the MR around the FM-HM transition by 20% and 37% for the 7 nm and 58 nm Fe3O4 layers, respectively. The enhancement in MR arises from interface-enhanced spin-dependent scattering, with the Verwey transition of the Fe3O4 layer playing a crucial role.

In summary, the findings of this dissertation illuminate a novel strain-mediated spin helicity phenomenon in nanostructured helimagnets, presenting a promising pathway for developing high-performance MR sensors and spintronic devices through the strategic utilization of reduced dimensionality, confinement, and strain effects.

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