L10-type alloys and L21-type Heusler alloys are key materials for future spintronic and magnetic storage devices. L10-FePt with high uniaxial magnetic anisotropy is a promising material for ultrahigh density recording because of its high thermal stability of magnetization at a nanometer scale. Co-based Heusler alloys showing high spin polarization of conduction electrons enable us to enhance the magnetoresistance effect. In addition, utilizing the magnetization dynamics in these ordered alloys provides us with new paths in the development of spintronic and magnetic storage devices. In this review, we introduce the control of magnetization switching field for L10-FePt exchange-coupled with Ni81Fe19 by utilizing the spin waves in the bilayers, which will be useful for information writing. A nanometer-scaled rf oscillator is also introduced, in which the magnetization dynamics is excited by spin angular momentum transfer. We can improve both the rf output power and the oscillation quality simultaneously by using Co2(Fe0.4Mn0.6)Si Heusler alloy.
We calculate the writing field dependence of the bit error rate for Gilbert damping constants of 0.1 and 0.01 in heat-assisted magnetic recording (HAMR) using a new model calculation. The attempt period used in the new model calculation is considered in detail. The writing properties are examined for various thermal gradients, linear velocities, and anisotropy constants. When the damping constant is equal to 0.1, write-error is smaller, and erasure-after-write is larger than that for 0.01 since the attempt period is short. The physical implication of the results is discussed. We also compare the results of the new model calculation and the conventionally used micromagnetic calculation. The overall tendencies of the results are the same. Therefore, the outline of the impact of the damping constant on the bit error rate in HAMR can be understood by the grain magnetization reversal probability and the attempt period used in the new model calculation.
Thin Films, Fine Particles, Multilayers, Superlattices
Metastable α”-Fe16N2 have attracted much interest as a candidate for rare-earth-free hard magnetic materials. To realize high coercivity, it is necessary to utilize not only the magnetocrystalline anisotropy but also the shape anisotropy of α”-Fe16N2 nanoparticles assemblies. An increase in magnetostatic couplings and intergranular exchange couplings among particles typically reduces the coercivity. Therefore, it is very important to evaluate the anisotropy and magnetic interactions among α”-Fe16N2 nanoparticles. We have examined the changes in morphology, structure and magnetic properties through the synthesis of α”-Fe16N2 nanoparticles from various materials such as α-FeOOH, ɤ-Fe2O3, and Fe3O4. The magnetic interactions were also estimated based on experimental results obtained by analysis of the rotational hysteresis loss of randomly oriented nanoparticles. Hc and Hkptc for the α”-Fe16N2 nanoparticle assemblies for different starting materials ranged from 2.2 to 1.1 kOe, and from 11 to 12 kOe respectively. Experimental results of the normalized coercive force and normalized switching field suggests that the existence of large magnetic interactions among α”-Fe16N2 nanoparticles.
The dynamic magnetic loss in ferrites is obtained by subtracting the hysteresis loss, which is independent of the excitation frequency, from the iron loss. In the high frequency excitation region, the dynamic magnetic loss is the dominant component of the iron loss in ferrites. The iron loss in ferrite is temperature-dependent and this dependence has been described in product catalogs, where the hysteresis and dynamic losses are not separated. The catalog data are measured using sinusoidal wave voltage excitation, whereas ferrite cores are commonly used under rectangular wave voltage excitation in DC-DC converters. In this paper, the experimentally obtained temperature characteristics of the hysteresis and dynamic magnetic losses for rectangular wave voltage excitation are shown separately, and it is found that the two are different. This suggests that the physical mechanisms involved are different as well. Thus, it is important to consider the temperature characteristics of the dynamic magnetic loss to produce low-loss ferrites.
The magnetic-field-induced blend films, poly(L-lactide)(PLLA10) (Mn = 1.0 × 105) and poly(DL-lactide) (Mw = 1.0 × 104 (PDLLA1) and 1.0 × 105 (PDLLA10)) were prepared in isothermal process under the magnetic-field 10 T. The effects of amorphous region, i.e., molecular weight of PDLLA effect on crystallization and orientation of PLLA in the films were investigated using wide-angle X-ray diffraction and polarizing microscopy. The crystallinity of each films showed 60% at crystallization growth time tc = 30 hrs irrespective of PDLLA molecular weight. The degree of orientation of PLLA10/PDLLA10 film increased with increasing tc. We concluded that the blending with amorphous high-molecular-weigh PDLLA and crystallization in an applied high magnetic field is effective to achieve the PLLA film that has higher crystallinity and orientation.