GIANT EXCHANGE ANISOTROPY OF FERROMAGNETIC/ANTIFERROMAGNETIC BILAYERS INDUCED BY THE CONTROLE OF THEIR MICROSTRUCTURE AND INTERFACE

研究了具有（110）、（001）和（111）等不同晶向的Mn-Ir/Co-Fe准单晶双层膜的交换各向异性。结果表明，单向各向异性常数J_K与晶体取向密切相关，而AF层的临界厚度d_<AF>（？<cr>）结果表明，d_ = 4 nm的（110）外延双层膜的J_K<AF>值高达0.73 erg/cm^2。由饱和扭矩幅值确定的Mn-Ir层的磁各向异性分别为：（110）-8.5×10^5 erg/cm ^3，（001）-5.0×10^4 erg/cm ^3，（111）-10^3 × 10^4 erg/cm ^3。从Mn-Ir层的交换各向异性与磁各向异性之间的关系可以看出，畴壁模型不足以解释这些实验结果，而单自旋模型可以定性地解释这些实验结果，并假定界面交换耦合能在各个晶体学取向上不同.我们还研究了长时间退火对多晶Mn_<75>Ir_<25>d_<AF>/Co_<70>Fe_<30>4纳米双层膜交换各向异性的影响，从而在很薄的反铁磁层上产生大的单向各向异性常数J_K.在250℃下退火200 h后，在直径为5 nm的PtMn/Co-Fe双层膜中获得了极大的J_K值，J_K = 0.87 erg/cm^2<AF>，比PtMn/Co-Fe双层膜的J_K最大值大2倍。根据作者建立的单自旋系综模型，解释了长时间退火使J_K增大的原因是反铁磁自旋方向分布发生了变化。根据实验获得的J_K过程的激活能，确定Mn-Ir晶粒的磁各向异性能为7×10^3 erg/cm^3。

The exchange anisotropy of pseudo-single crystalline Mn-Ir/Co-Fe bilayers with different crystallographic orientations, such as (110), (001), and (111), was investigated. As results, we found that the unidirectional anisotropy constant, J_K, strongly depends on the crystallographic orientation, while the critical thickness of the AF layer, d_<AF>^<cr>, is similarly 〜 3 nm. As a notable result, the J_K of the (110)-epitaxial bilayer with d_<AF> = 4 nm shows extra large value of 0.73 erg/cm^2. The magnetic anisotropy of the Mn-Ir layer determined from the saturation torque amplitude was 8.5×10^5 erg/cm^3 for (110)-, 5.0×10^4 erg/cm^3 for (001)-, and 10^3 〜 10^4 erg/cm^3 for (111)-bilayer, respectively. From the correlation between the exchange anisotropy and the magnetic anisotropy of the Mn-Ir layer, we conclude that the domain wall model is inadequate to explain these experimental results and the single spin model can do it qualitatively, assuming that the interfacial exchange coupling energy differs in the respective crystallographic orientation.The effect of long-time annealing on the exchange anisotropy of polycrystalline Mn_<75>Ir_<25> d_<AF>/Co_<70>Fe_<30> 4 nm bilayers was also investigated to induce large unidirectional anisotropy constant, J_K, with very thin antiferromagnetic layer. As a notable result, extra large value of J_K = 0.87 erg/cm^2 was obtained in the bilayer with d_<AF> = 5 nm after 200-h-annealing at 250℃, which is larger than the twice of the maximum value of PtMn/Co-Fe system. According to the single spin ensemble model, established by the investigator, the enlargement of J_K by the long-time annealing is explained as a result of the change of the distribution of antiferromagnet spin directions. The magnetic anisotropy energy of Mn-Ir grains was determined as 7×10^3 erg/cm^3 from the activation energy of the enlarging J_K process, experimentally obtained.