Morimasa Nagao

Optimum Thickness of MnBi Films for Magnetooptical Memory

Magnetic behavior of thermomagnetically written domains in MnBi films (400 A to 1200 A thick) has been studied. The written domains were circular in shape but became deformed and irregular with increasing film thickness. The circular domains of 1.5 microm in diameter were stable only in the films thinner than 700 A for writing and erasing processes. The stability of written domains can be explained by introducing measured wall coercivity values into a domain theory. The film wall coercivity was varied from 250 Oe to 800 Oe in inverse proportion to the film thickness.

Dynamic observation of thermomagnetic writing on MnBi films

Thermomagnetic writing on MnBi films was studied by dynamic observation on a change in a magnetization state using a two-beam method. As a writing laser beam increased in pulse duration, four writing characteristics were successively observed: I) nonwritable, II) writable without the first order phase transition, III) writable with the transition, and IV) decomposed. Observation of a nonmagnetic state in a writing cycle in II) revealed that the thermomagnetic writing is the extrapolated Curie point writing. The domain expansion phenomena observed in a writing cycle in II) are discussed based on a domain theory.

Optical and Magneto-Optical Properties of MnBi Films

For oriented MnBi films, the diagonal and off-diagonal components of the complex dielectric tensor were determined from optical and magneto-optical data measured in the 425 to 1000 nm wavelength range. The complex dielectric tensor indicates that several electronic transitions contributed from the magnetic electrons are located in the 600 to 1000 nm wavelength range, and may be responsible for the large Faraday rotation of MnBi films over this wide wavelength range. The Kerr rotations for s and p polarized waves were measured and calculated from the dielectric tensor as a function of the incident angle at 632.8 nm. The calculated results can explain the general behaviour of the dependence of the measured Kerr rotations on the incident angle.

Properties and Memory Applications of Cylindrical Domains in MnBi Films

Properties of a cylindrical domain in high wall-coercivity films have been studied theoretically and experimentally. A bubble domain theory in which the wall-coercivity is taken into consideration is shown. From the theory, experimental methods for obtaining wall-coercivity Hwc, curves for the total field acting on a wall versus domain radius, minimum domain radius rm without the applied field and wall energy density σw are derived. These methods are checked by measurement of the change in domain radius with respect to the applied field for thermomagnetically written domains in 1000 A thick MnBi films using a magneto-optical method. rm is measured to be from 0.62 to 0.43 µm corresponding to Hwc of 510 to 660 Oe, and σw is evaluated to be 18±5 erg/cm2. Critical wall-coercivity which depends on film thickness is determined for films used in a thermomagnetic writing-erasing type memory.

Intermediate Compounds in the Thin Films of Mn-Bi System

New birefringent compounds are found in films which are prepared by annealing double layers of successively evaporated Mn and Bi with various Mn to Bi ratios. The compounds are classified into three patterns by observation with a polarization microscope: needle-shaped, leaf-shaped and patched patterns which differ from a magnetic domain pattern in MnBi films. These patterns are observed in the films obtained from double layers with Mn to Bi starting atomic ratios, 0.8–1, 1–1.5 and 1.5–3, respectively. Electron probe microanalyses reveal that an Mn to Bi composition ratio of the leaf-shaped compound is about 0.5, and that of the patched compound is slightly greater than 1.