1500 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4,JULY 2001 Magnetic Field and Thermal Reversal Properties of Exchange-Bias Recording Films Z. S. Shan, D. Jin, H. B. Ren, J. P. Wang, S. N. Piramanayagam, S. I. Pang, and T. C. Chong Abstract—Effect of exchange-bias coupling between ferro- magnetic (FM) layer and antiferromagnetic (AFM) underlayer on the H-field and thermal reversal properties was studied analytically by minimizing the free energy of the bilayer system. Correlation between the effective-thermal-stability-factor , coercivity and parameters of the bilayer system is presented. Emphasis is on analyzing the energy-barrier influence upon the moment reversal properties. Index Terms—Antiferromagnetism, exchange-bias, switching volume, thermal activation, thermal stability. I. INTRODUCTION I T IS WELL recognized that the thermal stability and low noise are two key points to further increase the areal density in current longitudinal recording media. The ratio of is a valuable parameter for measuring the degree of thermal stability [1]. Many efforts have been devoted in the search for high media, such as Co Sm with erg/cm , fct-CoPt and fct-FePt with erg/cm . However, the anisotropy value of those thin films are much lower than that of bulk materials, and more work needs to be done before these material come into practical use. For the conventional single-layer-media, the increase in , which raises , will degrade the film noise behavior because the magnetic moment per magnetic grain, , increases. In the case of the coupled-layer-media, such as coupled ferromagnetic layer and ferromagnetic layer (FM–FM) [2]–[4] or coupled ferromagnetic layer and anti- ferromagnetic layer FM–AFM [5], [6], it may be possible to improve the effective thermal stability factor without degrading the medium noise behavior significantly. In this paper we report our studies of the effect of exchange-bias coupling between FM magnetic layer and AFM underlayer on the reversal properties of media films. The systematic study is based on minimizing the free energy of the bilayer system, whose energy equation is in SW(Stoner–Wohlfarth) form, and analyzing the effects of exchange-bias coupling and the applied -field on the energy barrier, and consequently the -field and thermal reversal properties of media films were investigated. It is emphasized that the -field switching, i.e., the coercivity , is determined by , which is more precise than (or ). Manuscript received October 13, 2000. The authors are with Data Storage Institute, Singapore 117608 (e-mail: zheng.shan@komag.com; jpwang@dsi.nus.edu.sg). Publisher Item Identifier S 0018-9464(01)05794-6. Fig. 1. (a) Schematic of coupled FM–AFM bilayer film; (b) configuration of magnetization , and applied -field with respect to the energy easy axis. II. MODEL AND SOLUTION A. Energy Equation and Stability of Bilayer System The thin FM layer is assumed to be coupled with an AFM un- derlayer by exchange-bias as shown Fig. 1(a), and the configu- ration of moment and -field is shown in Fig. 1(b). The energy of this coupled bilayer system can be written as: (1) where and , and , and are the magnetization, anisotropy con- stant, and layer-thickness for AFM and FM layer, respectively. is the applied field and is the coupling constant between the FM and AFM layers. and are the angle of , or with respect to the easy-axis, respectively [5]. It is noticed that the two moment sub-networks have opposite magnetization directions and thus the resultant moment of AFM layer vanishes [Fig. 1(a) and (b)], however, the anisotropy of AFM layer and coupling between FM and AFM layer may give significant contributions to the reversal proper- ties of FM layer as will be shown below. Moment-reversal is controlled by the energy-barrier character from the physical point of view. In order to see the coupling effect on the energy-barrier character, a set of normalized en- ergy curves, “ vs ,” is plotted in Fig. 2. It is seen clearly that: the value of at decreases with increasing and conse- quently the energy-barrier-height increases with increasing 0018–9464/01$10.00 © 2001 IEEE