Domain growth in amorphous rare-earth-transition-metal alloy films was theoretically and experimentally studied. The domain wall motion was considered as a thermoactivated process. The domain patterns are shown to be fractal structures. Good agreement between simulated and experimentally observed domain structures was obtained. >

Fractal domain structures in thin amorphous films

Coercivity mechanisms in thin 1000-AA Permalloy films due to inhomogeneous distributions of in-plane uniaxial anisotropy are investigated numerically, utilizing a two-dimensional domain wall model. The distribution of anisotropy could be attributed to the polycrystalline grain structure of thin films, as well as magnetostrictive stress. Typical anisotropy orientation dispersion induced by polycrystalline grains does not appear to account for the wall coercive force observed in soft Permalloy films. Other mechanisms, such as stress-induced anisotropy, may be dominant. >

Inhomogeneities and coercivity of soft Permalloy thin films

A micromagnetic model is established to analyze the read and write processes in a magnetoresistance random access memories (MRAM) cell. The magnetoresistive curves of NiFeCo/interlayer/NiFeCo cells are analyzed and compared with available experiments. The switching fields of the two magnetic layers A and B in a MRAM cell are studied versus different geometrical parameters of a cell. The word current IwA and IwB corresponding, respectively, to the switching of films A and B, and the related error of IwA and IwB under a given sense current Is, are analyzed and compared with experiment. In a cell with a given width W, proper geometrical parameters such as the thickness of the magnetic layers and the aspect ratio (length over width) are found based on the analysis of the IwA−Is and IwB−Is curves.

Micromagnetic studies of read and write process in magnetoresistive random access memory

Wall and vertical Bloch line (VBL) coercivity arising from spatial nonuniformity in the material parameters has been investigated for a typical 5‐μm bubble garnet film by means of a two‐dimensional micromagnetic computation. Two‐dimensional sinusoidal modulations in the magnitude of the uniaxial anisotropy were assumed as a model for a compositional nonuniformity. Nonuniformities with a spatial wavelength comparable to the wall width were found to exert the largest pinning effect. The typical computed value for the wall and VBL coercivity were 0.7 and 2.2 Oe, respectively, where 10% variation and wavelength of 0.47 μm were assumed. The tendency, that the VBL coercivity is larger than that for the domain wall, agrees with the experimental results reported previously. The wall and VBL coercivity caused by a nonuniform exchange constant have also been computed and compared to the analytical solutions due to the step‐like variation of wall and VBL energies, respectively. The larger VBL coercivity compared to that for the wall was observed for a local modulation with the size less than wall width and period more than 1 μm.

Micromagnetic computation for wall and Bloch line coercivity in thin films with perpendicular anisotropy

A micromagnetic model is established to analyze the permeability of Fe–Ta–N and Fe–Al–N thin films in a wide frequency range from 10 to 2200 MHz. The simulated real part of the permeability is about 2500 and 5000 at low frequencies for macroscopic Fe–Al–N and Fe–Ta–N films, respectively. It then decreases by a factor of 2 around 500 and 300 MHz. The corresponding imaginary part of the permeability increases from zero to a maximum value. In mesoscopic Fe–M–N films, huge magnetic poles on the edge of a layer play a significant role in the frequency response. As a result, the real part of the permeability in a mesoscopic film becomes much smaller than that in a macroscopic film. The spin patterns of mesoscopic Fe–M–N films in an external oscillatory field are also shown to understand the frequency response with strong magnetic poles.

Micromagnetic studies of high frequency permeability in Fe–M–N thin films with macroscopic and mesoscopic sizes

The wall coercivity caused by the magnetic nonuniformity has been studied numerically. The Landau‐Lifshits‐Gilbert (LLG) equation is integrated by an explicit scheme of the modified Dufort‐Frankel method. The computation was carried out for a two‐dimensional grid system representing the magnetization direction in a magnetic film plane. Typical magnetic parameters for magneto‐optic recording media were assumed. Spatial variations of uniaxial anisotropy were treated as the magnetic nonuniformity. The validity of our numerical approach was demonstrated with preliminary one‐dimensional computations, compared to analytical solutions. A wall coercive field of 2.5 kOe was observed for the wall coupling with a pinning site (30‐A width and 360‐A spacing along the wall) of 10 times larger anisotropy compared to the ordinary region. A two‐dimensional anisotropy variation (K=106–107 erg/cm3) with a wavelength larger than 60 A also caused a wall coercivity on the order of 1 kOe, compared to those in magneto‐optic recording media. It was also found that a fine pinning site on the order of 100 A caused a notable coercivity for the bubble domain wall surrounding it.

Y. Hirokado

Papers

Micromagnetic computation for wall coercivity caused by magnetic nonuniformity