Department of Physics

Department News

Physics colloquium on Friday, October 23, 2009.

Oct 20, 2009

A Physics colloquium will be held on Friday, October 23 at 4 p.m. in Currens Hall 205. Please join us for refreshments at 3:30 in Currens 210.

Speaker: Dr. Jyotirmoy Saha
Date: October 23, 2009 (Friday)
Time: 4 PM
Room: 205 Currens Hall

Induced Anisotropies in a Ferromagnet Exchange - Coupled to a Polycrystalline Antiferromagnet

Abstract: When a ferromagnet (FM) is coupled to an antiferromagnet (AF), the magnetocrystalline anisotropy of the AF induces various forms of anisotropies in the FM mediated by uncompensated spins at the AF-FM interface. The two basic yet cardinal experimental outcomes that quantify these anisotropies are: 1) Hysteresis loops (HLs) yielding exchange bias (HE) and enhanced coercivity and 2) ferromagnetic resonance (FMR) plots yielding the resonant frequency (fR) of the FM. A crucial point of distinction is that HE yields a measure of the induced unidirectional anisotropy (UA) as the FM switches between the two magnetic states whereas fR is a measure of the ground state UA field induced in the FM. To draw system independent (general) conclusions on these two classes of UA, a realistic model is presented that can produce HLs and FMR response independently based on inputs from material characteristics and avoids the use of adjustable parameters. The time evolution of the FM magnetization is established by solving the Landau-Lifshitz-Gilbert equation for each FM degree of freedom. The energetics of each AF grain is considered within the framework of a uniform rotation model and its time evolution is evaluated by its barrier switching capability under thermal excitations. Two technologically relevant systems are investigated: 1) Co35Fe65/Ni50Mn50; 2) Ni80Fe20/Ni50Mn50 and the results are compared with those obtained from experiments. A novel mechanism is presented that outlines the induction of UA in an otherwise isotropic FM/AF system just by the mere application of the first field point in the hysteresis loop evaluation.

About the speaker: Dr. Jyotirmoy Saha is a theoretical physicist studying magnetism in the broad area of condensed matter physics. He is a graduate from the University of Minnesota, Twin Cities and he did his post-doctoral research at the Commissariat à l'Énergie Atomique (CEA), Grenoble, France. Dr. Saha uses computational methods to study the properties of magnetic alloys.