Sponsorship: National Science Foundation, DMR 9410943, DMR 01XXXX
Personnel:
 

R.C. O'Handley Elizabeth Lyons

Hans Hug Pieter van Schendel

Magnetic domains form to reduce the magnetostatic energy associated with a surface across which these exists a discontinuity in the normal component of magnetization. The energies that determine the equilibrium direction of magnetization and domain structure are very different in a thin film compared to bulk. Indeed, many ultrathin magnetic films exhibit perpendicular magnetization and in some cases, very large domains or even single-domain behavior.

In a collaboration with our colleagues at the University of Basel ( Hans Hug, H.-J. Güntherodt), we have made the first quantitative magnetic force microscope studies of domain structures and magnetization orientation in ultrathin magnetic films. Some of these domain images can be seen on the home page of the APS Division of Materials Physics (http:/www.aps.org/units/dmp/epitaxial.html). While a well-defined domain size is favored in Cu/Ni/Cu/Si (001) films having Ni thickness of order 10 nm or greater, the energy difference between domain structures of different length scales becomes very small in thinner films so the length scale is not as well defined. See Fig. 2.1.
 
 

a)

b)

The magnetization patterns in the films that give rise to the images above are not necessarily uniform through the film thickness. We are now able to describe the variations in magnetization orientation with film thickness using a phase diagram (Fig. 2.2) described independently by Thiaville and Fert, by Hu and Kawazoe, by Bertram and Paul, as well as by Thomas. The diagram describes the dependence of the magnetic structure in a thin film on the basis of its surface anisotropy and film thickness. The data points on the figure are labeled by the Ni layer thickness in our Cu/Ni/Cu sandwiches. The phase diagram suggests that the magnetization pattern in the 7 nm film is the result of a fully perpendicular magnetization vector. It also suggests that in the 15 nm film above, the MFM images is the result of a magnetization vector that varies in orientation through the film thickness, being more nearly perpendicular near the surface.

Fig. 2.2 Phase diagram for magnetization structure in films of thickness d normalized to twice the exchange length, x. The vertical axis is the surface anisotropy energy density normalized to the energy per unit area due to the surface magnetization orientation, K_c = (AK^v)^1/2.

It is more likely that the magnetization structure in these films is perpendicular in the interior wiith closure domains near each Cu/Ni iinterface. The magnetization in these closure domains is not parallel to the surface (as would usually be the case) but is canted out of the film plane due to the surface anisotropy (van Schendel and Hug). This work is of fundamental interest and increasing technical importance as magnetic thin films are used in a greater variety of sensors and other devices.

Some relevant publications:

"Magnetic Domain Structure in Ultrathin Films," G. Bochi, H.-J. Hug, D.I. Paul, B. Stiefel, A. Moser, I. Parashikov, H.-J. Güntherodt, and R.C.

Lipp, H.-J. Güntherodt, and R.C. O'Handley, J. Appl. Phys. 79, 5609 (1996).

S. Martin, H.- J. Güntherodt, S. Porthun, L. Abelmann, J.C. Lodder, G. Bochi and R.C. O'Handley, J. Appl. Phys. 83, 1 (1998)..

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