In this study, recording/reproduction simulation method was studied to establish design method that would be used to evaluate tolerance levels, especially during recording and reproduction. Simulation methods were developed for both 3-dimensional beam propagation method and 2-dimensional multi-layered method. By using these two methods, the recording/reproduction characteristics are analyzed and a tolerance examination about influence of wavefront aberration to the reproduction signal quality is studied in this paper.
In holographic memories, photopolymer is a hopeful material as a recording medium. We simulated the formation of diffraction gratings by using the diffusion model and analyzed their diffraction characteristics by beam propagation method and we examined a recording characteristics of the pulse train recording by read/write simulation in the photopolymer medium. As a result, the recording sensitivity for pulse recording is deteriorated compared with the single long pulse recording.
In general, the optical configuration of angularly multiplexing holographic memory system is complicated because optics for both signal and reference beam are necessary. As an attractive candidate for a compact holographic memory system, we have developed a Monocular architecture and examined the feasibility of 500GB system using this architecture. In this study, high density recording for 500GB system was achieved by a new test-bed composed of a blue laser system and high NA objective lens. In addition, tolerance analysis for margin allocation was performed to make clear the technical difficulty of 500GB system.
MnBi (15nm) thin films were prepared by the magnetron sputtering of Mn/Bi multilayers followed by vacuum annealing of 350℃, and the variation of their structures and magnetic properties with 30keV Kr^+ ion irradiation was studied. The Mn_<52>Bi_<48> (15nm) film exhibited a magnetization of 180emu/cc, which was 1/3 of the reported value, but it exhibited a large coercivity of 10kOe resulting from large perpendicular anisotropy of this film. The magnetization and coercivity of the Mn_<52>Bi_<48> film were disappeared after a low ion dose of 3.1 x 10^<14>ions/cm^2. However, the existence of NiAs type MnBi phase and uni-directional anisotropy was confirmed by X-ray diffraction and torque magnetometer, respectively, even after the ion irradiation of 1 x 10^<15>ions/cm^2. From atomic force microscope and magnetic force microscope observations, there existed convex surface structures with a height of 〜50nm on the MnBi films, and MnBi phase underneath the convex structures exhibited ferromagnetism even after the ion irradiation of 1 x 10^<15>ions/cm^2. The surviving MnBi phases were considered to be magnetically isolated, and the existence of the isolated MnBi phases explained the variation of the structure and magnetic properties with the ion irradiation. From these results, transformation between ferromagnetic and nonmagnetic MnBi alloys by a low dose ion irradiation of 〜3 x 10^<14>ions/cm^2 was expected.
L1_0-FePt ordered alloy thin films were fabricated by annealing Fe/Pt/X trilayers to control magnetic properties suitable for next generation perpendicular magnetic recording media. Cu and Au were used to get magnetic properties suitable for heat assisted magnetic recording media and to fabricate FePt ordered phase at low annealing temperature, respectively. It was confirmed that the curie temperature of FePtCu films was 280℃ and perpendicular coercivity decreased to 1kOe at around the curie temperature. Though it needed at least 550℃ of annealing temperature to get L1_0-FePt ordered alloy, FePt was successfully attained at lower annealing temperature (500℃) by adding Au into FePt.
We investigated the local magnetism and its temperature dependence for Co-based Heusler alloy films with different degrees of crystalline order and the different kinds of interfaces. Two series of samples were grown by atomically-controlled alternate deposition; Co_2TiSn films with different degrees of crystalline order, and Co_2MnSn/(Cr or Ag) multilayered films with different interfacial atomic species. The magnetic hyperfine field was measured by ^<119>Sn Mossbauer spectroscopy. It was found that the temperature dependence of the local magnetism in the Co_2TiSn films depends on the degree of L2_1 order. It was also observed for Co_2MnSn/(Cr or Ag) multilayers that the influence of interfacial atoms on the local magnetism is large in the case of Cr and small in the case of Ag, but no significant temperature dependence was observed in both systems.
We investigated magnetic tunnel junctions (MTJs) La_<0.7>Sr_<0.3>MnO_3(LSMO)/LaTiO_3/La_2NiMnO_6(LNMO)/Cr/Au which were fabricated on SrTiO_3 substrates to examine spin-filtering effect through the ferromagnetic insulator LNMO by the measurement of tunnel magnetoresistance (TMR). The MTJ showed TMR ratio of about 19% at 50K and about 10% at 110K accompanied by the change of magnetization configuration of LSMO and LNMO from parallel to antiparallel. The spin polarization of the tunnel current through the LNMO barrier was estimated to be about 9% at 50K and about 4% at 110K.
In order to clarify the relationship between lattice distortion and magnetic properties of Heusler alloy, the structural and magnetic properties of Fe_2VSi single layer and [Fe_2CrSi/Fe_2VSi]_5 artificial lattice deposited on Mg_2Al_2O_4 substrate have been investigated. The lattice misfit between Fe_2YSi(Y=Cr, V) and MgAl_2O_4 is as small as 0.7%, and it is known that Fe_2CrSi and Fe_2VSi shows antiferomagnetism and ferromagnetism. The lattice distortion of Fe_2VSi films have been provided by using MgAl_2O_4 substrate and the Neel temperature (T_N) increases with decreasing c/a value. The maximum T_N of Fe2VSi on MgAl2O4 substrate is 57% larger than bulk value. The saturation magnetization artificial lattice using ultra thin (1.4nm) Fe_2VSi layers is a little larger than Fe_2CrSi single layer.