3134 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 Fast Magnetization Switching With Circularly Polarized Fields and Short Pulses Werner Scholz , T. M. Crawford , Gregory J. Parker , T. W. Clinton , T. Ambrose , Shehzaad Kaka , and Sharat Batra Seagate Technology, Pittsburgh, PA 15222 USA USC Nanocenter, University of South Carolina, Columbia, SC 29208 USA GE Global Research, Niskayuna, NY 12309 USA In this paper, we study the magnetization reversal process of single-domain Stoner–Wohlfarth particles subject to circularly polarized fields and short pulses using analytical and numerical models. We investigate the effect of short unipolar, bipolar field, and circularly po- larized field pulses, which are applied perpendicular to the uniaxial magnetocrystalline anisotropy axis, on the magnetization switching dynamics. Index Terms—Fast switching, Landau–Lifshitz–Gilbert (LLG) equation, magnetization reversal. I. INTRODUCTION A N increase in magnetic recording density requires an in- creased anisotropy in granular recording media to main- tain thermal stability. However, the head field from perpendic- ular write heads is limited and might eventually be insufficient to write the media. New technologies like heat assisted mag- netic recording, bit patterned media, ferromagnetic resonance assisted recording, etc., are being investigated to overcome this problem by effectively reducing the switching field. Fast magnetization switching with circularly polarized fields and short pulses offers another possible solution by taking advantage of the dynamic magnetization response. Short field pulses of high frequency linearly or circularly polarized magnetic fields can switch the magnetization of the recording medium with fields, which are smaller than the Stoner–Wohlfarth switching field [1]–[6]. II. MACROSPIN MODEL In the limit of very small damping it can be shown analyt- ically that switching of the magnetization of a single-domain particle (macrospin) is possible with arbitrarily small circularly polarized external magnetic fields independent of the magne- tocrystalline uniaxial anisotropy (field). In the limit of negligible damping, the Landau–Lifshitz–Gilbert (LLG) equation (1) reduces to the torque equation (2) Assuming an in-plane circularly polarized field which is always perpendicular to the projection of the magnetization vector into the -plane and an anisotropy field parallel to the z-axis Digital Object Identifier 10.1109/TMAG.2008.2001600 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. (we neglect the effect of magnetostatic fields or assume an el- lipsoidal spin domain) (3) (4) and rewriting the LLG equation in terms of the azimuthal angle for the magnetization component parallel to the z-axis (5) we find (6) which is independent of the anisotropy field . Since the ex- ternal field is always perpendicular to magnetization there is continuously torque acting on the magnetization, which pulls it away from the anisotropy axis. This leads to switching of the magnetization independent of the magnitude of the anisotropy field. We can calculate the switching time (time until the magne- tization crosses the plane perpendicular to the anisotropy axis) using (6) with and find (7) This means that we can switch the magnetization with an in-plane circularly polarized field of 1 T in 112 ps or with a field of 1000 Oe in 1.12 ns. The precession frequency, which depends on the azimuthal angle , can be calculated similarly from the - or -component (8) 0018-9464/$25.00 © 2008 IEEE Authorized licensed use limited to: University of South Carolina. Downloaded on March 4, 2009 at 12:41 from IEEE Xplore. Restrictions apply.