Date of Award

2020

Document Type

Dissertation

Degree Name

Ph.D.

Department

Materials Science

First Advisor

Xin Fan

Second Advisor

Robin Sterling

Third Advisor

Barry Zink

Keywords

Ferromagnetic metals, Inverse spin hall effect, Spin Seebeck effect

Abstract

Since the discovery in 2008, the spin Seebeck effect has become one of the most active topics in the spin caloritronics research field. It opened a new way to create spin current by a combination of magnetic fields and heat. A temperature gradient in ferromagnetic (FM) metals generates a ow of spin current due to split of spin chemical potential between spin up and spin down electrons. This thermal spin current has been detected using an attached nonmagnetic heavy metal with large spin Hall angle via the inverse spin hall effect (ISHE). A voltage signal is generated since the nonmagnetic material converts the spin current into a charge current in the direction orthogonal to both the spin current and spin polarization directions.

Ferromagnetic (FM) metals can also convert the spin current to a charge current with same exact symmetry if the magnetization is collinear with the spin polarization. Interestingly, a new symmetry of spin-to-charge conversion has been observed in FM metals in which the magnetization is out of the film plane and orthogonal to the in-plane spin polarization of the spin current. In this case, the generated voltage signal is parallel to the spin polarization direction. A comprehensive study is carried out to understand this new spin-to- charge conversion effect which we referred to as the spin galvanic effect with spin rotation symmetry (SGE-SR) . The sample under study is a spin valve structure consisting of two FM layers, one of which is a free layer with an in-plane magnetization and the other is a fixed layer with perpendicular magnetic anisotropy (PML). By free-layer-thickness-dependent study, we observed two microscopic mechanisms that contribute to the SGE-SR voltage signal. The results can be fitted well to a drift-diffusion-magnetoelectronic-circuit model.

Spin current generated due to the SSE in FM metals and converted into a voltage signal via the ISHE is measured with additional anomalous Nernst effect (ANE) because they are sharing same symmetry. In an attempt to resolve this issue, we use SGE-SR in which PML is utilized as spin current detector to study SSE where the influence of ANE is absent. Here, we report an experimental study of the longitudinal spin Seebeck effect (LSSE) of spin valve structure composite of a thin ferromagnetic metals and a ferromagnetic metal with perpendicular anisotropy detection layer. Using the perpendicular magnetized layer (PML) as a spin current detector allows to generate two voltage signals in two different symmetry. A charge current with symmetry same as the inverse spin hall effect in non-magnetic metals (NM) and charge current detected perpendicular the previous one. We called them as spin galvanic effect with conventional symmetry (SGE-C) and with rotational symmetry (SGE-SR). We measure series samples with different FMs and PMLs. We use Py, Ni, and Co as a free layer, and Pt=Co; Pt=Co=Ni as a detecting layer. We find that the ratio of spin dependent seebeck coefficient for two different FMs can be estimated by taking the ratio of the SGE - SR signal for samples with different FM layer and same PML. However, inconsistent ratio of the voltage signal generated from PML1=PML2 for Co data is observed while the ratio for Py and Ni agrees with each other. We extrapolate the spin diffusion length for both Ni and Co to be 5nm and 4nm, respectively.

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Provenance

Received from ProQuest

Rights holder

Wafa Saud Aljuaid

File size

145 p.

File format

application/pdf

Language

en

Discipline

Materials science

Share

COinS