Synthetic antiferromagnet for hard layer of exchange coupled composite media

Stephanie Hernandez (ECE)
Advisor:  Randall Victora (ECE)

In recent years, it has been shown [1][2] that media grains consisting of coupled magnetically hard and soft regions (exchange coupled composite media [ECC]) exhibit a superior thermal stability to switching field ratio () compared to conventional perpendicular media. However, optimal media designs have employed a low magnetization hard layer that, when coupled with an otherwise achievable anisotropy K, produces a very high anisotropy field 2K/M. Here, we replace the hard layer with a synthetic antiferromagnet consisting of two ferromagnetic hard layers coupled by an antiferromagnetic exchange interaction. For example, for a hard layer magnetization of 900 emu/cc and a soft layer magnetization of 1650 emu/cc we find  (within a three spin model), where each of the two hard layers and the soft layer have the same volume. This reduces the magnitude of the required anisotropy field to a more conventional 30 kOe and keeps the remanent magnetization of the grain at 550 emu/cc. Furthermore, subdividing each layer into multiple elements further reduces the switching field by almost 40%, suggesting that even higher values of the stability ratio are achievable. Interestingly, we find that not only is a synthetic antiferromagnet more easily achievable experimentally, but also has a higher thermal stability ratio than the equivalent hard material. For example, for two ferromagnetically coupled hard regions with a magnetization of 1 emu/cc and the same soft material we find , again within the three spin model. This means that the synthetic antiferromagnet offers a density improvement of approximately 25%. In summary, we find that the addition of a synthetic antiferromagnet to ECC media increases achievable density by roughly factors 3 and 1.2 relative to conventional perpendicular and the previously described ECC media, respectively.

References:

[1] R. H. Victora and Xiao Shen, IEEE Trans. Magn., Vol. 41, Issue 2, Pages 537 – 542, Feb. 2005

[2] D. Suess, T. Schrefl, S. Fähler, M. Kirschner, G. Hrkac, F. Dorfbauer, J. Fidler, Appl. Phys. Letters, 87, 012504 (2005)