서강대학교

초록/요약

Mn3Ga has received renewed attention in recent years because of the variation of magnetic properties depending on the crystal structure[1-7]. One is a triangular antiferromagnet for the hexagonal (D019) phase, which is easily obtained by arc melting[8,9]. The other is a ferrimagnet for the tetragonal (D022) phase, which is achieved by annealing the hexagonal material at high temperatures[8,9,11]. Another is an antiferromagnet for the cubic Heusler (D03) phase, which is synthesized using a nonequilibrium technique, but is not stable in experiments[3,12]. Among them, the tetragonal phase and cubic phase Mn3Ga are one of the Heusler compounds which have high potential for application to spintronics. Tetragonal Mn3Ga has the ferrimagnetic spin ordering with low magnetic moment, strong perpendicular magnetic anisotropy (PMA), high spin polarization, and high Curie temperature satisfying the criteria for spin-transfer-torque (STT) based devices. Since there is scarcely a PMA material with high spin polarization, Mn3Ga is interesting to explore. For this reason, thin film samples are more attractive than the bulk. Clear hysteresis loops with PMA were reported in Mn3Ga thin films grown on SrTiO3 (001) and Si (001)[2,7]. Further studies were carried out in thin films deposited on MgO (001) with buffer layers of Pt or Cr for the concept of MgO-based magnetic tunnel junction (MTJ) structure[4]. Without buffer layers, the PMA properties as well as the morphology were not sufficiently good[1]. In addition, the growth temperature and the film thickness were limited up to 375oC and 70 nm, respectively. These results can be obstacles to the memory device applications. On the other hand, cubic Mn3Ga has compensated antiferromagnetic property unlike tetragonal phase. However, the cubic Heusler (D03) structure Mn3Ga is unstable structure Mn3Ga in bulk. In this study, both cubic phase and tetragonal phase Mn3Ga are obtained in film which is deposited on the MgO(001) substrate by using RF magnetron sputtering method with varying the power of sputtering gun without buffer layer. X-ray diffraction data revealed that films growth as the cubic structure, when below the certain point of RF gun power. Scanning electron microscope images showed that the morphology changes as tuning the gun power. Data of the magnetic properties shows that the tetragonal Mn3Ga has ferromagnetism and the cubic Mn3Ga has antiferromagnetism. As the transport data, the tetragonal Mn3Ga has metallic behavior and cubic Mn3Ga is like a non-metallic behavior. Furthermore, mixed phase that both tetragonal and cubic phase coexist Mn3Ga film also obtained, which is the key to understand how the Mn3Ga crystalline on the large lattice mismatched substrate of MgO.