IEEE Magnetics Society
Santa Clara Valley Chapter
Meeting Presentation Summary

Tuesday, January 12^th, 2010

Western Digital, 1710 Automation Parkway, San Jose, CA
Directions and Map
Cookies, Conversation & Pizza too at 7:00 P.M.
Presentation at 7:30 P.M.

A New Paradigm for Exchange Bias in Polycrystalline Films
Click above for copy of underlying paper

Kevin O'Grady
The University of York
(2010 IEEE Magnetics Society Distinguished Lecturer)

Abstract

The phenomenon of exchange bias has remained something of a mystery since it was first discovered in core shell particles in 1956 [1]. Over the subsequent years many different models have been attempted to try and explain this effect most of which agree with some experimental data that can be found in the literature. However no single theory has ever been able to put a theoretical line consistently through data for different systems.

In this lecture the reason for our inability to explain exchange bias will be reviewed. Subsequently a new paradigm to explain exchange bias in sputtered polycrystalline films will be presented. This new paradigm is based on an original granular model due to Falcomer and Charap [2]. The basis of the new paradigm is that very careful thermal and magnetic cycling is required to ensure that the order in the antiferromagnetic grains is controlled carefully. Without such careful control reproducible data cannot be obtained.

These measurement procedures which are time consuming and complex, we refer to as the York Protocol and have been developed over the last 9 years. It will be shown that using the York Protocol and an extension of the former granular model, effects such as the film thickness dependence and grain size dependence of exchange bias can be fully explained with an excellent fit between theory and experiment [3]. The York Protocol also allows for the measurement of the anisotropy constant of antiferromagnetic grains [4].

The above model allows for an understanding of the behaviour of the individual AF grains in detail. Since the behaviour of the "bulk" of the antiferromagnetic grains is now known preliminary data describing the behaviour of the interface spins can now be distinguished from the behaviour of the bulk. Possible mechanisms for the behaviour of the interfaces themselves will also be presented.

• [1] W. H. Meiklejohn, and C. P. Bean, Phys. Rev. 102 (1956) 1413
• [2] E. Fulcomer, and S. H. Charap, J. Appl. Phys. 43 (1972) 4190.
• [3] G. Vallejo-Fernandez, L. E. Fernandez-Outon, and K. O'Grady, J. Phys. D: Appl. Phys.41 (2008) 112001.
• [4] G. Vallejo-Fernandez, L. E. Fernandez-Outon, and K. O'Grady, Appl. Phys. Lett. 91 (2007) 212503.

Copy Of Presentation

Biography

Kevin O'Grady

Prof. K O'Grady was educated at The University of Wales in Bangor studying for a degree and a PhD in Physics. Subsequently he was employed as a junior professor in Bangor, Loughborough University of Technology and subsequently in the School of Electronic Engineering at Bangor where he achieved a full professorship. In the year 2000 Prof. O'Grady relocated to the Physics Department at The University of York. Prof. O'Grady's work concentrates on magnetisation reversal in a wide range of materials but particularly those finding application in the information storage industry i.e. magnetic hard disk drives.

In his early career Prof. O'Grady studied fundamental fine particle magnetism utilizing colloidal dispersions or ferrofluids as the medium for study. His current research interests are in the field of materials for hard disks but also in exchange bias materials where an antiferromagnet is grown in contact with a ferromagnet and is used to pin one layer in a GMR stack in disk drive read sensors. It is this subject that will be the basis of the Distinguished Lecture. Prof. O'Grady has published over 250 refereed works on the subject of magnetisation reversal. He is a former President of the IEEE Magnetics Society and an Editor of The Journal of Physics D: Applied Physics. He has been associated with the Intermag Conference organisation being ProCom Co-chair in 1996, 2002 and 2008. He is the General Chair for the 2010 Joint Conference. He is also the founding Director of the recently formed York Materials Research Institute.