2005 APS March Meeting
Monday–Friday, March 21–25, 2005;
Los Angeles, CA
Session L42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures V
2:30 PM–5:30 PM,
Tuesday, March 22, 2005
LACC
Room: 150B
Sponsoring
Units:
DMP GMAG
Chair: Jian Shen, Oak Ridge National Lab
Abstract ID: BAPS.2005.MAR.L42.1
Abstract: L42.00001 : Intergranular Exchange in Magnetic Nanostructures*
2:30 PM–3:06 PM
Preview Abstract
Abstract
Author:
Ralph Skomski
(Department of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588)
Exchange interactions determine not only atomic-scale properties such as the
Curie temperature but are also paramount to the realization of mesoscopic
magnetism. Nanoscale exchange reflect the relativistic origin of magnetism.
On an atomic scale, interatomic exchange tends to be much stronger than
magnetic interactions, but the quadratic wave-vector dependence of the
exchange energy makes magnetic interactions competitive on a nanoscale. The
corresponding characteristic length scale is $a_{o}$/\textit{$\alpha $} = 7.252 nm, where
$a_{o}$ is the Bohr radius and \textit{$\alpha $} = 1/137 is Sommerfeld's fine structure
constant. In homogeneous solids, the competing relativistic and
nonrelativistic interactions determine, for example, the thickness of domain
walls. In nanostructures, the situation is more complex, because mesoscopic
and atomic exchange effects interfere with structural length scales. This is
important in many areas of magnetism, such as permanent magnetism, soft
magnetism, spin electronics, and magnetic recording. (For a recent review,
see Skomski, J. Phys. CM, vol. 15, 2003, p. R841.) From an atomic point of
view, local magnetic moments embedded in an itinerant electron gas are
coupled by RKKY-type interactions, whose oscillatory period is determined by
the Fermi wave vector $k_{F}$. First, RKKY interaction between embedded
clusters or particles do not average to zero but actually \textit{increase} with particle
size. Second, the low carrier densities of semimetals and semiconductors
yield small Fermi wave vectors and nanoscale oscillation periodicities. From
a mesoscopic point of view, traditional random-anisotropy scaling amounts to
a dimensionless coupling constant $A$/$K_{1}R^{2}$, but this expression fails
to account for important real-structure features. For example, grain
boundaries with reduced interatomic exchange give rise to a
quasi-discontinuity of the magnetization, create a magnetization
perturbation that extends far into the bulk, and modify scaling relations
for the coercivity and other quantities. Additional complexity is due to
finite-temperature excitations. Nanostructuring has little or no effect on
the Curie temperature, but strongly affects the temperature dependence of
the coercivity and the magnetization dynamics.
*This work has been performed in close collaboration with J. Zhou and D. J. Sellmyer. Support by NSF-MRSEC, DOE, AFOSR, ONR, ARO, the W. M. Keck Foundation, and CMRA is gratefully acknowledged.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2005.MAR.L42.1