In this work we examine grain arrangement and medium structure, temperature profile during writing, adjacent track interference, information stability during 10 years of archiving, information stability in a high Curie temperature (HC) layer during writing in a low Curie temperature (LC) layer, and writing sensitivity on 4 Tbpsi shingled heat-assisted magnetic recording (HAMR) with a bit aspect ratio of 1.5 and 2 Tbpsi/layer (total density of 4 Tbpsi) three-dimensional (3D) HAMR with bit aspect ratios of 1.0 and 2.0. A small bit aspect ratio is preferable in 3D HAMR. The grain aspect ratios for both HAMR and 3D HAMR may be too large when it comes to manufacturing the recording layer. The low Curie temperature of the LC layer may be disadvantageous with respect to the writing property. The readout field is small for both the LC and HC layers in 3D HAMR. The writing sensitivity for HAMR is worse than that for 3D HAMR because the statistical factor is affected by the readout grain number and erasure-after-write is affected by the grain column number. Although both HAMR and 3D HAMR have disadvantages, the poor writing sensitivity in HAMR is a serious problem.
Acicular spinel iron oxide particles with a core-shell structure containing manganese ferrite (MnFe2O4) and magnetite (Fe3O4) were synthesized by uniformly crystallizing MnFe2O4 on the surface of the magnetite particles in an alkaline dispersion. Crystallizing 50 wt% of MnFe2O4 on Fe3O4 led to an increase in the apparent lattice constant from 0.839 nm to 0.841 nm. Transmission electron microscopy (TEM) observations of the particle shape and lattice images showed that MnFe2O4 was uniformly crystallized on the surface of the magnetite particles and their acicular shape was maintained. The coercive force and saturation magnetization were almost constant at 29.5 kA/m and 80-81 Am2/kg, respectively, when the crystallized MnFe2O4 content ranged from 0 to 30 wt%. When the MnFe2O4 content was increased to 50 wt%, the coercive force and saturation magnetization slightly decreased to approximately 28.0 kA/m and 74.1 Am2/kg, respectively, reflecting the lower magnetization of MnFe2O4 compared to that of Fe3O4.
Topological spin textures on artificial pinning landscape may show unique static and dynamic properties. Here, we computationally show that the helicity of frustrated skyrmions on an artificial square-grid obstacle pattern can be switched by a spin current pulse. The obstacle pattern is formed by defect lines with enhanced perpendicular magnetic anisotropy, which could protect the skyrmion from being annihilated at the sample edge. It is found that the skyrmion driven by a moderate current shows a circular motion guided by the boundary of the obstacle pattern, while it shows an almost straight motion toward the sample edge in the absence of the obstacle pattern. By applying a short pulse current to drive the skyrmion in a sample with the obstacle pattern, we find that the helicity of the skyrmion could be switched between Bloch-type configurations favored by the dipole-dipole interaction. Besides, we demonstrate the possibility of switching the helicity of an array of skyrmions on the square-grid obstacle pattern using the same method. Our results could be useful for the helicity control of topological spin textures, and may provide guidelines for building future helicity-based spintronic functions.