Superparamagnetic effect limits the achievable areal recording density. The stability ratio of the order of U_ef/k_BT ~ 60 is necessary to store information at the densities of the order of 100 Gbits/in² or higher. Here T is the absolute temperature and k_B is the Boltzmann’s constant. For the stability ratio with such high value the thermal decay of single-domain particle magnetization is governed by the time constant τ ~ exp(-U_ef/k_BT). Therefore, a pre-exponential factor and its temperature dependence are negligible. Thermal agitation also decreases the coercive force of a single-domain particle. Besides, it was reported that for switching times less than 1 ns or so, the magnetization reversal is controlled mainly by the gyro-magnetic precession. Actually, during fast switching there is not enough time for thermal energy to make an appreciable effect on the magnetization reversal. Therefore, it is sufficient to estimate the dimensionless ratio H_cM_s/U_ef, that can be used to characterize the recording properties of ferromagnetic media, based on usual micromagnetic expressions for the energy barrier and coercive force.

The aim of this study is to show the way to decrease this ratio for a single-domain particle itself. It is shown that this ratio depends on the shape of the energy barrier separating equivalent energy minima. The optimum shape of the barrier is obtained for particles with uniaxial and combined (both uniaxial and cubic) types of magnetic anisotropy. It is found that for the optimum shapes the ratio H_cM_s/U_ef can be lowered down by a proportion of 20 - 40 % with respect to the usual case of Stoner-Wohlfarth particle.

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