The Curie temperature Tc variation problem in heat-assisted magnetic recording is discussed. We describe the physical implication of the Tc variation problem, and provide ways of improving the writing property by employing our simplified model calculation. The bit error rates for mean Curie temperatures of 600 and 700 K are examined. The Tc variation increases both write-error (WE) and erasure-after-write (EAW). The main related calculation parameters are the grain column number in one bit and the thermal gradient for the down-track direction. Increasing the grain column number is effective in reducing WE and EAW caused by the Tc variation. Furthermore, increasing the thermal gradient is necessary since EAW is high. Although a higher writing field of 12 to 14 kOe is necessary, a bit error rate less than 10-3 can be achieved for recording densities of 4 or 2 Tbpsi under the conditions used in this study even though the standard deviation of the Curie temperature is 4 %.
L10-FeNi films textured with the a-axis perpendicular to the film plane were successfully fabricated by denitriding FeNiN films. 20-nm-thick FeNiN films with two variants were epitaxially grown on SrTiO3(001), MgAl2O4(001), and MgO(001) substrates by molecular beam epitaxy. Denitriding was performed by annealing at 300 °C for 4 h under an H2 gas atmosphere. The epitaxial relationships were L10-FeNi(100) || substrate(001) and L10-FeNi(100) || substrate(001). The uniaxial magnetic anisotropy energy (Ku) of the L10-FeNi film was estimated to be 4.4 × 106 erg/cm3 at room temperature by magnetic torque measurement. This Ku value corresponds to a degree of long range order of 0.4.
The authors are developing magnetic stimulation coils for use in the treatment of dysphagia. The prototype coil manufactured in our previous research can induce large contraction of the suprahyoid muscles, which should be trained for normal swallowing. In this study, the heat generation in the coil during magnetic stimulation is investigated and suppressed. A numerical analysis showed that the main cause of the generated heat is the eddy current in the coil conductor induced by magnetic flux from the tip of the magnetic core. The analysis also showed that a parallel coil with a cross connection has an ideal current distribution in the coil conductor and thus best suppresses heat generation during magnetic stimulation. The heat-suppressing coil has parallel divided coil conductors and an appropriate connection between the conductors to level the current in each conductor. Moreover, measurements of the temperature rise in prototype coils during magnetic stimulation confirm that heat generation can be greatly suppressed by dividing the conductor and using an appropriate connection between the divided conductors.