Fabrication of Curie point written solid state magnetic memories

研究了用于热磁记录的微图案化非晶薄膜、采用非晶合金层的TMR薄膜、交换偏置薄膜和热磁记录工艺。(1)采用射频磁控溅射方法制备了TbFe非晶合金薄膜，并用聚焦离子束(FIB)和常规光刻方法进行了微细加工。在产生焦耳热的电流脉冲后，证实了TbFe元件磁化强度的转换。发现开关所需的电流密度随着元件尺寸的减小而减小，在低至1.6×10~(6)A/cm~(2)的电流密度下，0.5μm平方元件被开关。此外，包括TbFe非晶态合金层在内的隧道结具有足够大的磁阻比。(2)用离子轰击技术制备了NiFe/Mnlr交换偏置薄膜，并用磁力显微镜观察了其磁畴结构。NiFe/Mn1R的易磁化方向在薄膜平面内，对于这种长径比为1：1的平面内元素，很难获得单畴状态。因此，当TbFe等垂直磁化元件具有1：1的长宽比时，可以有效地实现单畴状态。(3)对热磁记录过程进行了模拟，得到了小于100 nm的磁畴记录所需的材料参数。为了在富稀土的TbFeCo介质上记录小于50 nm的磁畴，需要增加矫直力，这可以通过控制TbFeCo薄膜的微结构来实现。

Micropatterned amorphous films for thermomagnetic recording, TMR films using amorphous alloy layers, exchange biased films, and thermomagnetic recording process were investigated.(1) TbFe amorphous alloy films were prepared by rf magnetron sputtering, and microfabricated by focused ion beam (FIB) and conventional photolithography methods. Switching of the magnetization of TbFe elements was confirmed after current pulses which give rise to Joule heat in the elements. The current density necessary for the switching was found to decrease with decreasing the element size, and the 0.5μm squared element was found to be switched by applying the current density as small as 1.6 x 10^6 A/cm^2. In addition, tunnel junctions including TbFe amorphous alloy layer was found to exhibit sufficiently large magneto resistance ratio.(2) NiFe/Mnlr exchange biased films were microfabricated by FIB and their magnetic domain structures were observed by magnetic force microscopy (MFM). The easy direction of the NiFe/Mnlr elements lay in the film plane, and for such in-plane elements having 1 : 1 aspect ratio, it is difficult to obtain sigle domain state. Thus, the perpendicular magnetized elements such as TbFe were found to be effective to accomplish single domain state if the elements have 1 : 1 aspect ratio.(3) Simulation of thermomagnetic recording process was carried out, and the simulation was used to obtain material parameters which are necessary for recording of domains smaller than 100 nm. For the recording of domains smaller than 50 nm on an RE-rich TbFeCo medium, the increase of coercivity, which might be realized by controlling the microstructure of the TbFeCo film, was found to be needed.