by Rok Dittrich 
The nudged elastic band method is applied to
calculate minimum energy paths for the thermally induced switching in AFC
media. The height of the energy barrier depends on the strength of the
antiferromagnetic coupling strength between the two layers. For a weak
coupling strength (0.2 mJ/m2) a two step reversal occurs, and the system
visits a metastable state.
The uniaxial anisotropy constant "Ku" = 230 kJ/m3 for both layers. The "Ru" interlayer was 0.8 nm thick, and "Js" was set to 0.375 T. In the bulk material the an exchange constant A=10 pJ/m was used. The interface are for the here shown AFC grain is 96.15 nm2. 

For a weak coupling strength (0.2 mJ/m2) a two step reversal occurs, and the system visits a metastable state. The animation shows (on the left) the sequence of magnetization states when we take a "walk" trough the micromagnetic energy landscape following the minimum energy path from one energy minimum to the other. The corresponding energy of the magnetization state is shown on the right. Along this walk we pass one ore more saddle points, so called "transition states", which determine the height of the energy barrier(s). 

0.2 mJ/m2  0.6 mJ/m2  1.1 mJ/m2  5 mJ/m2 
For a coupling strength stronger that 1.1 mJ/m2 the reversal changes to a mode with a single barrier only and the above shown metastable state disappears.The animations again show sequences of magnetization states along the minimum energy paths. 

The energy barrier increases with increasing interface coupling strength and reaches saturation for J > 1.5 mJ/m2. The reversal mode changes from a two step reversal to a single barrier reversal.  The ratio of the energy barrier "Eb" versus the coercive field "Hc" shows that there is an optimal coupling strength where the best tradeoff between energy barrier and switching field is obtained.  Compared with conventional media, the energy barrier in AFC media can be increased by ~15 % at the same switching (writing) field. 

The energy barrier is about 3% smaller when the AFC grain is located in the data bit transition region as compared to a completely isolated grain. 

Rok Dittrich, "Finite element computation of energy barriers in magnetic systems", Ph.D. thesis, Vienna University of Technology, 2003
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