MAGNETIC PROPERTIES OF REGULARLY ARRAYED NANODOTS AND THEIR APPLICATION TO ULTRA-HIGH DENSITY

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In a future ultra-high density magnetic memory, the size of each memory bit will be reduced down to 10×10 nm^2, indicating that each bit consists of one nanoparticle. Therefore, the magnetization reversal process and thermal stability of each nanoparticle have to be clarified for development for the future magnetic memory. However, it has been difficult to measure the magnetic properties of such a nanoparticle because of the very poor sensitivity of 10^<-6> emu for conventional magnetometers. To overcome this difficulty, we have developed a very sensitive Hall effect method for a very small nanoparticle. The sensitivity reaches 〜 10^<-14> emu for FePt. By using this technique, we have investigated the magnetic properties and thermal stability of a single crystal L1_0-FePt (001) nanodot by patterning an well characterized epitaxial film. For the particle diameter of D_m < 20 nm, all the magnetic properties, such as the angular dependence of the irreversible switching field H_r, the magn … More itude of coercivity H_c, and their temperature dependences, are completely explained by the coherent rotation model (Stoner-Wohlfarth model) taking thermal relaxation into account. As the particle size D_m exceeds 20 nm, the magnetic behaviors obviously deviate from the coherent rotation model, indicating that the magnetization reversal mode changes from coherent to incoherent rotation modes. For this incoherent region, the activation energy has been evaluated to be about 10^<-12> erg regardless of the dot size, which is much smaller than the product of the anisotropy energy and dot volume but nearly equal to γδ^2 (γ: domain wall energy density, δ: wall width), suggesting that the magnetization reversal is initiated by nucleation of a reversed embryo with the dimension of the exchange length. The critical diameter D_m 〜 20 nm at which the reversal mode changes almost agrees with the critical diameter predicted by the micromagnetic theory. Based on these results, we have successfully designed the detail specifications for a prospective ultra-high density magnetic memory with the area density of 1 terabits/inch^2. Less