Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Dissertation final.pdf (3.03 MB)

ETD Abstract Container

Abstract Header

A STUDY OF SURFACE ACOUSTIC WAVE AND SPIN PRECESSION USING AN ULTRAFAST LASER FOR LOCALIZED ELASTIC AND MAGNETIC PROPERTY MEASUREMENT

Zhao, Peng
http://rave.ohiolink.edu/etdc/view?acc_num=osu1367591435

Abstract Details

2013, Doctor of Philosophy, Ohio State University, Physics.
Ultrafast laser optics is becoming a powerful tool in materials research. The interaction between femtosecond laser pulses with electrons and the subsequent relaxation process is an active research topic in recent years. In the time scale of femtoseconds to nanoseconds, several interesting physics take place. The laser pulses are short that they can be used to probe these very short time scale interactions, for example, the spin precession in GHz range. The laser can be easily focused using an objective lens, thus providing a micron-scale spatial resolution. In this dissertation, I will start by discussing the dynamics of electron, lattice and spin after a sample absorbs focused femtosecond laser pulses and the information can be used for measurement of elastic constants and saturation magnetization. The micron-scale spatial resolution and picosecond temporal resolution of our ultrafast laser pump-probe system allows us to measure elastic, magnetic and thermal properties of materials locally. By performing such measurements on diffusion couple/multiple samples with composition gradients, we can more effectively establish composition dependent property databases than conventional ways of making single uniform alloys and measuring them one at a time. Absorption of low power focused femtosecond laser pulses by sample surface leads to localized thermal expansion, which launches Surface Acoustic Waves (SAW) that can be used to measure elastic modulus. Such measurements must be supplemented by theoretical calculations since there are complications related to pseudo-SAWs and skimming longitudinal waves in addition to regular SAWs. It is a bit surprising that a mathematical solution to the surface response induced by a thermally expansion source on an arbitrary bulk surface (half space) of an isotropic crystal/solid is not available in the literature. By convolving the strain Green’s function with the thermal stress field created by an ultrafast Gaussian laser illumination, I solved the complete surface displacement using the reciprocity principle and programmed the semi-analytical solution into a MatLab code. The solution is validated by performing femtosecond laser pump-probe measurement in which the surface displacement is monitored by time-dependent probe beam deflection. This solution will be an important base for localized measurement of anisotropic elastic tensor. As a demonstration, I determined the elastic tensor of Si from a Si (111) wafer, with values close to the literature data. Localized measurement of the absolute value of saturation magnetization (magnetic moment) is useful for obtaining composition dependent magnetic moment data for discovering new magnetic materials. Existing methods for localized magnetic measurement, such as scanning Hall probe, scanning SQUID, and magnetic force microscopy (MFM) either do not have the required spatial resolution (in microns) or require stringent sample size or testing condition, or do not have the ability to obtain quantitative absolute magnetic moment values. I built a time-resolved MOKE to excite and measure spin precession in magnetic materials with a focused femtosecond laser. The precession is excited and detected within a region essentially defined by the focused laser spot (a radius of 3.6 microns with a 20X objective lens) and its frequency is used to extract the saturation magnetization (magnetic moment). It was found from my experiments on pure Fe, Ni and Co that the predominant mode of spin precession on bulk metallic samples induced by focused femtosecond laser pulses is the uniform Kittel mode. Using Kittel’s dispersion relationship which relates the Kittel mode spin frequency to the strengthen of the applied magnetic field and the saturation magnetization, the saturation magnetization can be evaluated. The process is quite straightforward for cubic phases with negligible crystal magneto-anisotropy. For pure Co and Co-rich alloys with the HCP crystal structure that usually has strong crystal magneto-anisotropy, the fitting to both saturation magnetization and anisotropic parameters are possible when variable applied field experiments are performed. TR-MOKE measurements were carried out in a Ni-Fe diffusion couple across the diffusion region and a composition dependent saturation magnetization data in Ni-Fe alloy are obtained and compared favorably with literature values.
Peter Hammel (Committee Chair)
Zhao Ji-Cheng (Advisor)
John Wilkins (Committee Member)
Yuri Kovchegov (Committee Member)
Physics
ultrafast laser; surface acoustic wave; anisotropy; spin; precession; saturation magnetization; diffusion multiple

Recommended Citations

Citations

  • Zhao, P. (2013). A STUDY OF SURFACE ACOUSTIC WAVE AND SPIN PRECESSION USING AN ULTRAFAST LASER FOR LOCALIZED ELASTIC AND MAGNETIC PROPERTY MEASUREMENT [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1367591435

    APA Style (7th edition)

  • Zhao, Peng. A STUDY OF SURFACE ACOUSTIC WAVE AND SPIN PRECESSION USING AN ULTRAFAST LASER FOR LOCALIZED ELASTIC AND MAGNETIC PROPERTY MEASUREMENT. 2013. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1367591435.

    MLA Style (8th edition)

  • Zhao, Peng. "A STUDY OF SURFACE ACOUSTIC WAVE AND SPIN PRECESSION USING AN ULTRAFAST LASER FOR LOCALIZED ELASTIC AND MAGNETIC PROPERTY MEASUREMENT." Doctoral dissertation, Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1367591435

    Chicago Manual of Style (17th edition)

osu1367591435
1,132
© 2013, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.