Takuya Ono

Magnetic anisotropy and order structure of L10-FePt(001) single-crystal films grown epitaxially on (001) planes of MgO, SrTiO3, and MgAl2O4 substrates

L10–FePt(001) single-crystal films were grown epitaxially on SrTiO3(001), MgAl2O4(001), and MgO(001) substrates. Their uniaxial magnetic anisotropy Ku and the order structure were examined for the film thickness t range of 2–14 nm. All series of films show large Ku of 4 × 107 erg/cm3 in the thickness range higher than 10 nm, with order parameter S of 0.8 and saturation magnetization Ms of 1120 emu/cm3. Ku decreased gradually as t decreased. The Ku reduction was considerable when t decreased from 4 nm to 2 nm. No marked difference in the thickness dependence of Ku was found in any series of films, although the lattice mismatch between FePt and the substrates was markedly different. Ku reduction showed good agreement with the reduction of S for the films on MgAl2O4 and MgO. The Ku ∼ S2 plot showed an almost linear relation, which is in good agreement with theoretical predictions. Transmission electron microscopy images for a FePt film on MgO substrate revealed that the lattice mismatch between FePt(001) and...

Granular nanostructures and magnetic characteristics of FePt-TiO2/FePt-C stacked granular films

To realize a granular film composed of L10-FePt grains with high uniaxial magnetic anisotropy energy, Ku, and segregants for heat-assisted magnetic recording, the FePt-TiO2/FePt-C stacked film was investigated. The FePt-TiO2/FePt-C stacked film has well-isolated granular structure with average grain size of 6.7 nm because the FePt-TiO2 film follows the FePt-C template film in microstructural growth. However, the Ku value is quite low for total thickness of 9 nm: 5 × 106 erg/cm3. Exploration of the thickness dependence of L10-FePt(001) peaks in XRD spectra and cross-sectional TEM images suggest that degradation of the L10 ordering appears near the middle of the FePt-TiO2 layer. The EDX-STEM mapping reveals that Ti atoms exist within the FePt grains in addition to the grain boundary. This indicates the possibility that TiO2 tends to be incorporated into the FePt grains and that it prevents L10-ordering of the FePt grains along the normal-to-plane direction.

Magnetic characteristics and nanostructures of FePt granular films with GeO2 segregant

To realize a granular film composed of L10-FePt grains with high uniaxial magnetic anisotropy energy, Ku, and segregants for energy-assisted magnetic recording, a FePt-GeO2/FePt-C stacked film was investigated in the engineering process. The FePt-GeO2/FePt-C stacked film fabricated at a substrate temperature of 450 °C realized uniaxial magnetic anisotropy, Kugrain, of about 2.5 × 107 erg/cm3, which is normalized by the volume fraction of FePt grains, and a granular structure with an averaged grain size of 7.7 nm. As the thickness of the FePt-GeO2 upper layer was increased to 9 nm, the Ku values were almost constant. That result differs absolutely from the thickness dependences of the other oxide segregant materials such as SiO2 and TiO2. Such differences on the oxide segregant are attributed to their chemical bond. The strong covalent bond of GeO2 is expected to result in high Ku of the FePt-GeO2/FePt-C stacked films.

Addition of Ru to L10-FePt thin film to lower Curie temperature

The addition of Ru to L10 (fct)-FePt thin films was investigated to lower the Curie temperature (T c) of the films, which is beneficial for heat-assisted magnetic recording. The FePtRu films exhibited L10-ordering and perpendicular anisotropy for Ru contents up to at least 18 at. %, and T c was reduced by ~100 °C/6 at. % Ru independent of the Fe/Pt ratio between 0.8 and 1.1. The T c versus uniaxial magnetic anisotropy diagrams exhibited a linear trend similar to that of FePtCu films but with a larger slope. Systematic analysis clarified that the Ru additive caused a slight decrease of the anisotropic field but greatly decreased the saturation magnetization, which was related to the observed reduction in T c.

Experimental investigation of off-stoichiometry and 3d transition metal (Mn, Ni, Cu)-substitution in single-crystalline FePt thin films

In L10 (fct)-FePt thin films, both tuning Fe and Pt concentrations and substitution with third-metal were studied for magnetic characteristic optimization. We investigated single-crystalline FePt-X (X = Mn, Ni, Cu) thin films grown epitaxially on MgO(001) substrates at a substrate temperature of 350  °C by changing Fe, Pt, and X contents, and explored the effects of off-stoichiometry and 3d-metal-substitution. The magnetic moment per atom (m) of FePt-X films as a function of the effective number of valence electrons (neff) in 3d metal sites follows the Slater-Pauling-type trend, by which m decreases by the neff deviation from neff = 8, independently of the X metal and the Pt concentration. The magnetic anisotropy (Ku) exhibits neff dependence similar to m. This trend was almost independent of the Pt concentration after compensation using the theoretical prediction on the relation between Ku and Fe/Pt concentrations. Such a trend has been proved for stoichiometric FePt-X films, but it was clarified as robu...

Application of magnetic printing method to hard-disk media with double recording layers

The magnetic printing method, which can duplicate soft magnetic patterns containing digital information such as servosignals formed on a master disk onto recording media, enables signals to be written to hard-disk media having high coercivities above 6000 Oe. We propose the application of the magnetic printing method to a hard-disk medium having double recording layers, one layer of which has high coercivity and is to be printed with digital information. This double recording layer medium is a hard-disk medium that has a magnetic read-only-memory (MROM) layer. In this study, we demonstrated a method for printing to this medium, which has MROM, and discussed the magnetic properties and recording performances of this medium.

Magnetic Anisotropy and Crystal Domain Variant in

We fabricated thin L1₀-Fe_xPt_100-x (x = 46-54 at.%) polycrystalline perpendicular films, and examined the x dependences of the first- and second-order uniaxial magnetic anisotropy, K_u1 and K_u2, in relation to variant formation. The values of K_u1 and K_u2 were estimated from magnetic torque curves at an applied field of 70 kOe obtained using anomalous Hall effect. K_u1 for films with thickness d of 10 nm showed a maximum at x = 50 at.%, which was consistent with the behavior of the order parameter. K_u1 decreased markedly as x increased from 50 at.%, accompanied by an increase of K_u2. In-plane X-ray diffraction analysis revealed that the variant formation increased markedly as x increased, especially beyond x = 50 at.%. Moreover, the relative lattice strain evaluated using a Williamson-Hall plot increased remarkably. The significant reduction of K_u1, accompanied by the enhancement of K_u2, was coincident with variant formation. K_u1 for films with d = 2 nm also showed a maximum at x = 50 at.%. However, the K_u1 ~x relation showed a good symmetry against Fe content variations from 50 at.%. No increase of K_u2 with increasing x was observed. It is likely that a large tensile stress in the initial film growth, caused by the lattice misfit between FePt and MgO underlayers, suppressed the variant formation in these very thin films.

{L} 1_{{0}}

In order to study the application potential of spin transfer switching in magnetic tunnel junctions (MTJs), a synthetic parallelly coupled layered structure such as a hard CoFeB/Ru/soft NiFe layers deposited on a MgO layer is investigated. The magnetic coupling between the layers is maintained after post-annealing at 300 °C, while annealing at 350 °C reduces the coupling strength. The observation of spin transfer switching in the junction indicates that parallel-to-antiparallel transition does not occur when the applied current pulse width is in the sub-millisecond range, which is far from the precessional range. This result indicates that spin transfer from NiFe to CoFeB might affect the dynamics of CoFeB magnetization.

-FePt Polycrystalline Films

The orientation ratio (OR) is a key issue to achieve high density in longitudinal recording media. Recent recording media have seed layers which exclusively promote small grain growth of the recording layer. In this paper, the effect of seed layers on OR was investigated by the use of various seed materials deposited twice. OR tends to decrease as the thickness of the seed layer is increased; however, the degree of decrease is quite different with seed material. A seed material with small Young’s modulus is preferable for a large OR. These results suggest that OR depends on the interface stress which is relieved as seed layer thickness is increased. Moreover, the relation between the stress in the seed layer and OR was also considered with a multilayer model.