Ultrafast laser driven spin generation in metallic ferromagnets

Choi, Gyungmin

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https://hdl.handle.net/2142/78473 Copy

Description

Title

Ultrafast laser driven spin generation in metallic ferromagnets

Author(s)

Choi, Gyungmin

Issue Date

2015-04-24

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

Cahill, David G.

Doctoral Committee Chair(s)

Cahill, David G.

Committee Member(s)

• Zuo, Jian-Min
• Shoemaker, Daniel P.
• Gilbert, Matthew J.

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Spintronics

Abstract

This dissertation presents experimental studies of spin generation in metallic ferromagnets (FM) driven by ultrafast laser light using a pump-probe technique. The pump light gives a driving force for spin generation by depositing energy or spin angular momentum on FM. The probe light measures spin responses by magneto-optical Kerr effect or temperature responses by time-domain thermoreflectance. I find that ultrafast laser light generates spins in FM in three distinct mechanisms: (i) demagnetization; (ii) spin-dependent Seebeck effect (SDSE); (iii) optical helicity. The demagnetization-driven spin generation is due to energy transport between electrons and magnons of FM and conservation of angular momentum for electron-magnon coupling. Ultrafast laser light deposits its energy in electrons of metallic layers and leads to a sharp increase of the electron temperature. The excited electrons transport energy to magnons of FM by the electron-magnon coupling. The magnon excitation results in ultrafast demagnetization of FM. I find that the spin loss by magnon excitations during the demagnetization process is converted to the spin generation in electrons of FM by the conservation of angular momentum for electron-magnon coupling. The generated spins diffuse to other layers and leads to spin accumulation in nonmagnetic metals (NM) or spin transfer torque on other FMs. I measure the demagnetization-driven spin accumulation in a NM/FM1/NM structure and spin transfer torque in a NM/FM1/NM/FM2 structure. The SDSE-driven spin generation is due to a heat current at FM/NM interfaces and spin-dependent Seebeck coefficient of FM. Ultrafast laser light deposits its energy in a heat absorbing layer of a multilayer structure and leads to a heat current from the heat absorbing layer to heat sinking layer. When an FM is incorporated in the multilayer structure, the spin-dependent Seebeck coefficient of FM converts the heat current to spin generation at interfaces between FM and NM. The interfacial spin generation rate is proportional to the heat current through FM and spin-dependent Seebeck coefficient of FM. I find that the heat current and spin-dependent Seebeck coefficient can be controlled by thickness of the heat sink layer and composition of FM, respectively. The generated spins diffuse to other layers and leads to spin accumulation on NM or spin transfer torque on other FM. I measure the SDSE-driven spin accumulation in a NM/FM1/NM structure and spin transfer torque in a NM/FM1/NM/FM2 structure. The optical helicity-driven spin generation is due to angular momentum transport between light and electrons of FM and spin-orbit splitting of FM. A circularly polarized light with a wavelength of 785 nm triggers a dipolar transition from occupied 3d to unoccupied 4p bands of 3d transition FMs. The selection rule predicts a significant spin polarization for the dipolar transition from spin-orbit 3d-sub-bands (3d3/2 and 3d5/2) to 4p band. However, energy degeneracy between 3d3/2 and 3d5/2 leads to zero spin polarization. I find that a small-but-finite spin-orbit splitting of the 3d bands leads to a finite spin generation from a circularly polarized light. The generated spins in electrons can be absorbed by magnetization of FM and lead to spin transfer torque. I measure the optical helicity-driven spin transfer torque in a single FM structure.

Graduation Semester

2015-5

Type of Resource

text

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http://hdl.handle.net/2142/78473

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Copyright 2015 Gyungmin Choi

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