Herbert J. Hopster

Professor, Physics & Astronomy
School of Physical Sciences Phone: (949) 824-5946, 6690
Fax: (949) 824-2174
Email: hhopster@uci.edu

University of California, Irvine
2125 Frederick Reines Hall
Mail Code: 4575
Irvine, CA 92697
Research Interests
Magnetic materials, ultrathin films, molecular beam epitaxy
Research Abstract
Professor Hopster received his Diploma in Physics from the University of Munster in 1974 and his Ph.D. degree from the Technische Hochschule Aachen in 1977. After spending one year as a postdoctoral fellow at the IBM Research Center in San Jose, California, he became a staff member at the Institute for Solid State Physics at the Research Center in Julich, Germany. He joined the Physics Department at UCI in 1984.

The magnetism in ultrathin films (i.e., a few atomic layers thick) and at interfaces has recently become a field of great interest. Advances in preparation techniques, like molecular beam epitaxy (MBE) under ultrahigh vacuum (UHV) conditions (10^-10 Torr), allow the fabrication of high-quality magnetic systems with sharp interfaces on the atomic scale. In our laboratory we study the magnetic properties of these two- dimensional systems by spin polarized electron spectroscopies. In one experiment a spin polarized electron beam is scattered off the sample surface, and the polarization and intensity of the scattered electrons are measured as a function of energy loss. In this way, one is able to determine the spin dependent electronic excitations (electron-hole pairs, Stoner excitations) at the surface. Indirectly, one gains information on the surface magnetic moments. This method of spin polarized electron energy loss spectroscopy (SPEELS) has recently been shown to yield valuable information on novel magnetic systems, like the magnetic coupling of transition metals to Fe(100) surfaces. In another experiment we probe the surface magnetization by measuring the spin polarization of secondary electrons or photoemitted electrons. This allows for the direct determination of the magnetization direction at the surface. For instance, in ultrathin Fe films on Cu and Ag (100) the magnetization was found to switch between perpendicular and in-plane as a function of thickness and temperature. On Gd(0001) the surface spins have been found to be canted. We have recently added structural characterization techniques to our experimental program. One component is a PC-based video system for data acquisition of low-energy electron diffraction (LEED) images. In combination with dynamic LEED calculations, this gives information on the interlayer spacing. In addition, we have constructed a UHV scanning tunneling microscope (STM).
Reversible Transition between Perpendicular and Inplane Magnetization in Ultrathin Films, D.P. Pappas, K.-P. Kaemper, and H. Hopster, Phys. Rev. Lett. 64, 3179 (1990).
Reduction of Magnetic Order in Ultra-thin Fe Films as the Magnetic Orientation Changes, D. P. Pappas, C. R. Brundle, and H. Hopster, Phys. Rev. B45, 8169 (1992).
Magnetic Ordering of Cr Layers on Fe(100), T.G. Walker, A. Pang, H. Hopster, and S.F. Alvarado, Phys. Rev. Lett. 69, 1121 (1992).
Anomalous Behavior in the Spin Polarization of Low-energy Secondary Electrons from Gd(0001), Phys. Rev. B47, 5049 (1993).
Spin Polarized Photoemission Study of Epitaxial Gd(0001) Films on W(110) , H. Tang, D. Weller, T. G. Walker, J. C. Scott, C. Chappert, H. Hopster, D. P. Pappas, A. W. Pang, and D. S. Dessau, J. Appl. Phys. 73, 6770 (1993).
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