DGIST Scholar: Controlled Spin Structure in the Magnetic Phase Mixture Film

Department of Physics and Chemistry Theses Ph.D.

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Controlled Spin Structure in the Magnetic Phase Mixture Film

Title: Controlled Spin Structure in the Magnetic Phase Mixture Film

Author(s): Min-Seung Jung

DGIST Authors: Jeong, Soon Moon ; Jung, Min-Seung ; Hong, Jung-Il

Advisor: 홍정일

Co-Advisor(s): Soon Moon Jeong

Issued Date: 2020

Awarded Date: 2020-02

Type: Thesis

Description: Phase mixture film, Nanogranular film, FM/AFM interface, 3-dimensional internal interface, Exchange bias effect, Uniaxial alignment of AFM

Abstract: Spin structure between magnetic phases is a crucial role for understanding of magnetic phenomena such as exchange interaction, magnetic anisotropy, interfacial exchange bias effect, spin torque and so on. Research on nanoscale of complexity spin structure provides not only considerable understanding of physical phenomena but also the feasibility of spin based devices. In this Theses, we fabricated phase mixture film with 3-dimensional internal interface structure which is composed of randomly distributed FM and AFM phases within the single layer thin film structure. We revealed novel magnetic property by exchange interaction between nano-scale of parallel FM and antiparallel AFM ordering due to spin canting within the diffuse interface area of ‘hypo-oxide state’, which is opposite behavior to generally known fact same with ferromagnetism and antiferromagnetism. Also, the improved exchange bias effect was manifested with overcoming the limitations (thickness and shape anisotropy dependence of ferromagnet layer) through 3-dimensional internal interface structure within the single layer in comparison to previous studies with layered film structure. Furthermore, we confirmed that the uniaxial alignment of AFM spins in the phase mixture film can be achieved by injecting polarized spins which is generated through inserted nonmagnetic heavy metal (platinum) while the charge current flows the film, and it was reflected in exchange bias effect. Direction reversibility and repeatability of exchange bias effect was succeeded with flowing the current of the opposite direction.

Table Of Contents: Chapter 1. Introduction
Chapter 2. Theoretical Background
2.1 Magnetism
2.1.1 Ferromagnetism
2.1.2 Antiferromagnetism
2.2 Exchange interaction
2.3 Superexchange coupling
2.4 Exchange bias effect
2.5 Anisotropy
2.5.1 Magnetocrystalline anisotropy
2.5.2 Shape anisotropy
2.6 Spin orbit coupling (SOC)
2.7 Spin orbit torque (SOT)
Chapter 3. Preparation of Magnetic Thin Films
3.1 Reactive magnetron sputtering
3.2 Patterning through lithography
Chapter 4. Magnetism of the Phase Mixture Single Layer Film
4.1 Overview
4.2 Experimental details
4.3 Structural characterization of Co-Ni-O mixture films
4.4 Unusual behaviors at the interface region
4.4.1 Temperature dependence of magnetization
4.4.2 Atomic scale micromagnetic simulation
4.5 Exchange anisotropy
4.6 Compare with layered films system
4.7 Summary
Chapter 5. Exchange Bias Effect through 3-dimensional Internal Interfaces Structure
5.1 Overview
5.2 Experimental details
5.3 Lift off the limitations of exchange bias effect through phase mixture film
5.3.1 Thickness dependence
5.3.2 Shape anisotropy dependence
5.4 Improvement of the thermal stability
5.5 Exchange bias field depending on FM materials
5.6 Exchange biasing along an arbitrary direction with phase mixture bilayers
5.7 Summary
Chapter 6. The Uniaxial alignment of AFM through current-induced spin torque in phase mixture film
6.1 Overview
6.2 Experimental details
6.3 Structural characterization of Co-Ni-O-Pt films
6.4 Uniaxial alignment of AFM spin structure through spin Hall effect
6.5 Effect of spin torque depending on degree of oxidation
6.6 Directional reversibility of exchange bias effect
6.7 Summary
Chapter 7. Concluding Remarks
References
Summary (in Korean)