Akiyoshi Itoh

In situ measurement of stress in Co/Cu, Co/Pd, and Co/Au compositionally modulated multilayer films

Temporal changes of sign and magnitude of the intrinsic stress during ultra‐high vacuum deposition of Co/Cu, Co/Pd, and Co/Au compositionally modulated multilayer films (CMF’s) were investigated by means of a cantilever technique. The intrinsic stress corresponding to monatomic layers could be detected during deposition. The intrinsic stress in Co layers deposited on Cu, Pd, and Au layers is tensile and the stress of Cu, Pd, and Au layers on Co is compressive. The intrinsic stress of a Co layer on Cu in Co/Cu CMF approaches near to the ideal critical shear stress of the perfect crystal, 1×1011 dyn/cm2.

Effects of heat treatment conditions and impurity doping on garnet films for M-O recording prepared by pyrolysis

The heat-treatment conditions of Bi-doped iron garnet films for magnetooptical recording prepared by pyrolysis onto glass substrates were studied. The effects of impurities such as Li, Na, Rb, and Cs were also studied. A low annealing temperature for crystallization and a low drying temperature were found to be effective in improving the morphology of the films. Doping with Na, Rb, and Cs were effective for obtaining films having improved morphology. It made coercivity lower and also made the Faraday hysteresis loop square compared with nondoped films. The rate of crystallization of the films doped with these impurities (except Li) was made much higher than for nondoped films. >

Growth induced magnetic anisotropy in Co-doped garnet and its capability of application to magneto-optical memory materials

Growth-induced magnetic anisotropy (Kug) normal to the film surface in Co-doped iron garnet films grown on GGG substrates by the LPE method has been quantitatively discussed. Kug is increased with increasing Co content and growth rate (Vg). From the measurement of temperature dependence of magnetocrystalline anisotropy K 1 , the parameter αλ is evaluated to be 80 cm-1through a curve fitting procedure using Slonczewskis theory of Co single-ion model for the anisotropy. After the annealing at temperatures above 900°C, Kug is disappeared and K 1 is increased: These changes are successfully interpreted by the theory using the above value αλ. Also shown is the magnetic field effect on the perpendicular anisotropy of poly-crystalline Co-doped BiDyGeIG films prepared by the pyrolysis method on glass substrates for the purpose of application to magneto-optical memory materials. In these films, (Ge(CH 2 ) 2 COOH) 2 O 3 is found to be a suitable solute for adding tetravalent Ge ions. In this method, an increase of Co amount raises the crystallization temperature, while doping of Ge lowers it. A magnetic field perpendicularly applied to the film during the heat treatment for crystallization enhances the anisotropy, whereby Ku is increased to about 170% of those of films prepared with no field.

Optical and magneto-optical characteristics at 1.54µm of Co, Ge doped YNdIG LPE garnet films

Contribution of the Co ion to the growth-induced uniaxial magnetic anisotropy (Kug) has been observed in Co,Ge substituted yttrium-neodymium-iron garnet films grown on GGG substrates by the LPE method. Kug of the films on

Ultrafast Magnetism as Seen by X-rays

Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for magnetic recording industry. Exchange interaction is the strongest force in magnetism, being responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically perturbed on an ultrashort timescales pertinent to the characteristic time of the exchange interaction? Here we demonstrate that femtosecond measurements of X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism. In particular, we show that upon femtosecond optical excitation the ultrafast spin reversal of Gd(FeCo) - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic state. The latter one emerges due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction.

Bi-Substituted DyIG Thin Films Prepared by Thermal Decomposition Method for Magneto-Optic Recording Medium

Highly Bi-substituted DyIG thin films on glass substrates with perpendicular anisotropy, which are candidates for use in magneto-optical recording, have been prepared by a thermal decomposition method. Nitrate solutions were spin-coated on glass substrates, and the resulting films were crystallized by annealing at temperatures between 560 and 660°C. X-ray diffraction studies verified that films were single-phase polycrystalline garnets. Dy substitution resulted in uniaxial anisotropy energies (Ku) of as high as 8 × 104 erg/cm3. Compositional dependences of the saturation magnetostriction constant were measured using the cantilever method. Comparison of the measured and calculated anisotropies revealed that the large Ku values of Dy-substituted films originates in magnetostriction and internal stress. In static recording experiments, written bits about 1.5 ¿m in diameter were obtained. The Faraday rotation coefficient was 3. 7 deg/¿m at a wavelength of 633 nm when the film Bi content was 1.55/formula unit.

Laser-Induced Spin Dynamics in Amorphous NdFeCo

We studied magnetization dynamics in the amorphous light rare earth-transition metal compound NdFeCo. We report a two step demagnetization process. Furthermore, an unusual overshooting behaviour is observed that can be linked to the particular magnetic structure of the sample.

Improving the Efficiency of Ultrafast Optical Control of Magnetism in GdFeCo Continuous Films and Submicron Structures

The goal of this work is to define conditions for the most efficient ultrafast optical control of magnetism. Results show that tuning the composition of the GdFeCo alloys towards the magnetization compensation point as well as reducing the sizes of structures allowing one to reduce the energy of the laser pulse required for ultrafast demagnetization or magnetization reversal.

Laser-Induced Giant Skyrmions and Skyrmion-Compounds in a Thin Magnetic Film of TbFeCo

Here we report about the formation of topologically protected Skyrmions through illumination of a thin metallic film with short laser pulses (about 150 fs). By tuning the laser fluence it is possible to change the topological properties of the created structure.

Influence of the Magnetization Compensation Point on the All-Optical Magnetization Switching

Combining femtosecond transmission measurements with picosecond time-resolved photo-emission electron microscopy, both using x-ray magnetic circular dichroism, new insights into the all-optical magnetization switching mechanism in GdFe based rare-earth transition metal ferrimagnetic alloys is provided, with emphasis on the role played by the magnetization compensation temperature TM of the alloy.