Theses and Dissertations

Vivek Dixit

Issuing Body

Mississippi State University

Advisor

Kim, Seong-Gon

Committee Member

Novotny, Mark A.

Committee Member

Horstemeyer, Mark F.

Committee Member

Clay, Torsten

Committee Member

Gwaltney, Steven R.

Date of Degree

12-11-2015

Document Type

Dissertation - Open Access

Major

Applied Physics

Degree Name

Doctor of Philosophy (Ph.D)

College

James Worth Bagley College of Engineering

Department

Applied Physics Program

Abstract

This dissertation investigates how the magnetic properties of strontium hexaferrite change upon the substitution of foreign atoms at the Fe sites. Strontium hexaferrite, SrFe12O19 is a commonly used hard magnetic material and is produced in large quantities (around 500,000 tons per year). For different applications of strontium hexaferrite, its magnetic properties can be tuned by a proper substitution of the foreign atoms. Experimental screening for a proper substitution is a cost-intensive and time-consuming process, whereas computationally it can be done more efficiently. We used the ‘density functional theory’ a first principles based method to study substituted strontium hexaferrite. The site occupancies of the substituted atoms were estimated by calculating the substitution energies of different configurations. The formation probabilities of configurations were used to calculate the magnetic properties of substituted strontium hexaferrite. In the first study, Al-substituted strontium hexaferrite, SrFe12-xAl x O19, with x = 0.5 and x = 1.0 were investigated. It was found that at the annealing temperature the nonmagnetic Al+3 ions preferentially replace Fe+3 ions from the 12k and 2a sites. We found that the magnetization decreases and the magnetic anisotropy field increases as the fraction, x of the Al atoms increases. In the second study, SrFe12-x Gax O19 and SrFe12-x Inx O19 with x = 0.5 and x = 1.0 were investigated. In the case of SrFe12-x Gax O19, the sites where Ga+3 ions prefer to enter are: 12k, 2a, and 4f1. For SrFe12-x Inx O19, In+3 ions most likely to occupy the 12k, 4f1, and 4f2 sites. In both cases the magnetization was found to decrease slightly as the fraction of substituted atom increases. The magnetic anisotropy field increased for SrFe12-x Gax O19, and decreased for SrFe12-x Inx O19 as the concentration of substituted atoms increased. In the third study, 23 elements (M) were screened for their possible substitution in strontium hexaferrite, SrFe12-x Mx O19 with x = 0.5. In each case the site preference of the substituted atom and the magnetic properties were calculated. We found that Bi, Ge, Sb, Sn, and Sc can effectively increase the magnetization, and Cr, P, Co, Al, Ga, and Ti can increase the anisotropy field when substituted into strontium hexaferrite.

URI

https://hdl.handle.net/11668/17380

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