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

在未来的超高密度磁存储器中，每个存储位的尺寸将减小到10×10 nm^2，这表明每个位由一个纳米颗粒组成。因此，每个纳米颗粒的磁化反转过程和热稳定性必须澄清为未来的磁存储器的发展。然而，由于传统磁力计的10 μ emu的灵敏度非常差，因此难以测量这种纳米颗粒的磁特性<-6>。为了克服这一困难，我们开发了一种非常灵敏的霍尔效应方法，用于测量非常小的纳米颗粒。对FePt的灵敏度可达100 ~ 100 μ <-14>emu。利用这种技术，我们研究了单晶L1_0-FePt（001）纳米点的磁性和热稳定性，通过图案化一个良好的表征外延膜。当粒径D_m &lt; 20 nm时，不可逆翻转场H_r的角度依赖性、磁感应强度 ...更多信息 用考虑热弛豫的相干转动模型（Stoner-Wohlfarth模型）完全解释了H_c的大小及其随温度的变化。当粒径D_m超过20 nm时，磁性行为明显偏离相干旋转模型，表明磁化反转模式由相干旋转模式转变为非相干旋转模式。对于这个非相干区域，无论点的大小如何，激活能都被估计为大约10^<-12>erg，它远小于各向异性能和点体积的乘积，但几乎等于γδ^2（γ：畴壁能量密度，δ：壁宽），这表明磁化反转是由具有交换长度尺寸的反转胚的成核引发的。反转模转变的临界直径D_m约为20 nm，与微磁理论预测的临界直径基本一致。基于这些结果，我们成功地设计出了一种面密度为1太比特/英寸^[2]的超高密度磁存储器的详细规格。少

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