56th Magnetic Materials Center Seminar
September 28, 2007, 9:00 - 11:00
7th floor seminar room, Sengen
Ab-initio study of Co-ferrite
Z. Gercsi
The extensive magnetic properties make the Cobalt ferrite a promising candidate for spintronics, magnetoelectrics and magnetic recording applications. Recently, several papers reported high coercivity up to 12.5kOe in highly strained Co ferrites in both thin film and mechanical milled powders [1-3]. Although the magnetic properties of these matters
also depend on the microstructure, there is a clear correlation between the sign of magnetic anisotropy and strains [4]. Ab-initio calculations on CoFe_2 O_4 were performed to investigate the effect of tetragonal deformation on the electronic structure. The GGA theorem was used for the calculations including the spin-orbit interactions (SOI) because it can correctly predict the experimental lattice parameter (V/V_exp =1) as compared to the LDA method, which rather underestimates it (V/V_exp ?0.95). Moreover, the various spinel scenarios (normal, inverse, mixed)
were applied. The sign of the calculated anisotropy energies depends on the strain conditions such as compression or tension which turns out to be in good agreement with experimental results. GGA+U theorem was also extensively studied with various U parameters.

Magnetic properties and microstructure of FePt films on MgO underlayer
T. O. Seki
For application of FePt to the practical media, the (001) oriented FePt particles are required on glass substrates. In addition, reducing ordering temperature and the fine particle size of less than 10 nm is necessary. Our goal is to make the media structure with FePt which satisfies all the above requirements. As the first step, we prepared FePt/MgO films on SiO2/Si and glass substrates. Previously, Perumarl et al. reported that structures and magnetic properties of FePt/MgO films depending the power, deposition temperature and MgO thickness. And then, the FePt 3 nm/MgO 10 nm film deposited at 500 C showed the high ordering, resulting in large coercivity of 18 kOe. However the in-plane coercivity was also shown slightly. Therefore, in order to reduce the in-plane coercivity, the deposition condition for MgO underlayer should be more optimized. In this my talk, we show magnetic properties and microstructure of the FePt film on optimized MgO underlayer.

Spin-transfer effect in MgO-based magnetic double tunnel junctions
H. Sukegawa
A current-induced magnetization switching (CIMS) technique in magnetic tunnel junctions has to establish in order to put a large-capacity magnetoresistive random access memory (MRAM) to practical use. The CIMS is a revolutionary technology, because only flowing electric current from a ferromagnet is needed to reverse magnetizations of nano-scale magnets. The flowing spin polarized current provides a spin torque, which causes the magnetization reversal of a free layer. However, a large current density (107 A/cm2) is needed for the reversal and it is still a big problem on practical use. I applied the double tunnel junction (DTJ) structures for high efficient CIMS observation. Typical DTJ structure is buffer/IrMn (14)/CoFe (3)/Ru (0.8)/Co40Fe40B20 (2.5)/MgO (0.88)/Co40Fe40B20 (2.1)/MgO (0.85)/Co40Fe40B20 (1.5)/CoFe (0.5)/Ru (1.2)/CoFe (1.5)/IrMn (14)/cap (in nm) on a thermally oxidized Si wafer, fabricated by using RF magnetron sputtering system. A DTJ [junction area: 330x150 nm2] was patterned by using microfabrication techniques. The CIMS behaviors are characterized by using DC 4-probe method and AC lock-in / pulse current measurement at room temperature. The top and bottom pinned layers are prepared to be aligned antiparallel in order to enhance the spin torques from the pinned layers. I have found that the switching current density of the DTJ can be reduced by nearly half (5x106 A/cm2) compared with an existing single tunnel junction (1x107 A/cm2). This fact indicates CIMS occurs more efficiently in DTJs with antiparallel aligned pinned layers.