Magnetic and thermal properties of metallic antiferromagnetic materials

Yang, Kexin

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https://hdl.handle.net/2142/108482 Copy

Description

Title

Magnetic and thermal properties of metallic antiferromagnetic materials

Author(s)

Yang, Kexin

Issue Date

2020-07-15

Director of Research (if dissertation) or Advisor (if thesis)

Cahill, David G.

Doctoral Committee Chair(s)

Mason, Nadya

Committee Member(s)

• Schleife, Andre
• Lorenz, Virginia O.

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Magnetism
• magneto-optical effects
• magnetometry
• pump-probe techniques

Abstract

Metallic antiferromagnetic materials are of great potential in spintronics due to their insensitivity to external fields and faster dynamics compared to typical ferromagnetic materials. Although they have these advantages, studies of their order parameter is difficult to perform because of the lack of net magnetization. The linear magneto-optic Kerr effect (MOKE) is often used to probe magnetism of ferromagnetic materials, but MOKE cannot be applied to collinear antiferromagnets due to the cancellation of sublattice magnetization. Magneto-optic constants that are quadratic in magnetization, however, provide an approach for studying antiferromagnets on picosecond timescales. I combined transient measurements of linear birefringence and optical reflectivity to study the optical response of Fe2As to small ultrafast temperature excursions. We performed temperature-dependent pumpprobe measurements on crystallographically isotropic (001) and anisotropic (010) faces of Fe2As bulk crystals and found that the largest optical signals arise from changes in the index of refraction along the z-axis, perpendicular to the Neel vector. Both real and imaginary parts of the transient optical birefringence signal approximately follow the temperature dependence of the magnetic heat capacity, as expected if the changes in dielectric function are dominated by contributions of exchange interactions to the dielectric function. In spintronic devices, it is essential to determine the dynamics of magnetic precession, the frequency of spin waves, the thermal stability of magnetic domains, and the efficiency. Thus magnetocrystalline anisotropy is a fundamental property of antiferromagnetic materials. Torque magnetometry measurements of Fe2As were performed. We reported that the four-fold magnetocrystalline anisotropy K22 in the (001)-plane of Fe2As is K22 = −150 kJ/m^3 at T = 4 K, much smaller than the perpendicular magnetic anisotropy of ferromagnetic materials structure widely used in spintronics device. K22 is strongly temperature dependent and close to zero at T > 150 K. The anisotropy K1 in the (010) plane is too large to be measured by torque magnetometry and we determine K1 = −830 kJ/m^3 using first-principles density functional theory. Our simulations show that the contribution to the anisotropy from classical magnetic dipole-dipole interactions is comparable to the contribution from spin-orbit coupling. The calculated four-fold anisotropy in the (001) plane K22 ranges from −290 to 280 kJ/m^3, the same order of magnitude as the measured value. We used K1 from theory to predict the frequency and polarization of the lowest frequency antiferromagnetic resonance mode and find that the mode is linearly polarized in the (001)-plane with frequency 670 GHz. As we observed The field-dependent domain distribution and quadratic magnetization can potentially be measured with optical technique. We set up a static system for imaging in-plane magnetic domains. To test this system, I measured the quadratic MOKE coefficient of ferromagnetic cobalt and YIG thin films, and the field-dependent quadratic magneto-optical signal of Fe2As. The noise floor of the mapping system is determined to be ∼ 10^−5

Graduation Semester

2020-08

Type of Resource

Thesis

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http://hdl.handle.net/2142/108482

Copyright and License Information

Copyright 2020 Kexin Yang

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