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After the development of Nd–Fe–B magnets, rare-earth magnets are now essential components in many fields of technology, because of their ability to provide a strong magnetic flux. There are two, well-established techniques for the manufacture of rare earth magnets: powder metallurgy is used to obtain high-performance, anisotropic, fully dense magnet bodies; and the melt-spinning or HDDR (hydrogenation, disproportionation, desorption and recombination) process is widely used to produce magnet powders for bonded magnets. In the industry of sintered Nd–Fe–B magnets, the total amount of production has increased and their dominant application has been changed to motors. In particular, their use for motors in hybrid cars is one of the most attractive applications. Bonded magnets have also been used for small motors, and the studies of nanocomposite and Sm–Fe–N magnets have become widespread. This paper reviews the current status and future trend in the research of permanent magnets.

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co 2 Cr 1−x Fe x Al(CCFA(x)) and Co 2 FeSi 1−x Al x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co 2 FeSi 0.5 Al 0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2 1 than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe 2 film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe 2 O 4 (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe 2 (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe 2 /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of −124% at 10 K when the configuration of the magnetizations of CFO and CoFe 2 changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The −124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.

We here experimentally demonstrate that gyration modes of coupled vortices can be resonantly excited primarily by the ac current in a pair of ferromagnetic disks with variable separation. The sole gyration mode clearly splits into higher and lower frequency modes via dipolar interaction, where the main mode splitting is due to a chirality sensitive phase difference in gyrations of the coupled vortices, whereas the magnitude of the splitting is determined by their polarity configuration. These experimental results show that the coupled pair of vortices behaves similar to a diatomic molecule with bonding and anti-bonding states, implying a possibility for designing the magnonic band structure in a chain or an array of magnetic vortex oscillators.Magnetic vortex structure [1,2] is one of the fundamental spin structures observed in submicron-sized ferromagnetic elements. It is well characterized by two degrees of freedom, one is 'chirality' (c = ±1), direction of the inplane curling magnetization along the disk circumstance, and the other is 'polarity' (p = ±1), direction of the outof-plane core magnetization. Particularly in the case of the disk, only the core region whose typical size is ∼ 10 nm generates the stray field, the static interaction between vortices in an array is thus negligibly small in the ground state. However in the low frequency excitation state called the gyration (or translational) mode [3,4], the surface magnetic charges appear with the core motion, that brings about the dynamic dipolar interaction between vortices.Coupled pair of magnetic vortices can be considered as a vortex molecule bound via dipolar interaction, which is a mimic of diatomic molecule with the van der Waals bonding [5]. The bonding or anti-bonding state respectively corresponds to in-phase or out-of-phase gyration of coupled vortices whose detailed energy levels are decided by combination of chiralities and polarities. This dynamic coupling is also effective in a two-dimensional array system [6,7], and thus allows us to design the density of states of the eigenfrequencies, the so-called "magnonic band structure", by arranging the core polarizations in a two-dimensional array. At the moment, there are few experimental reports for the coupled vortices via direct exchange interaction [8,9] and also dipolar interaction in physically separated vortices [10][11][12][13]. However the problem is still wide open in terms of experimental determination of the detailed condition for the mode splitting and the magnitude of the dipolar coupling in interacting vortices.Herein we demonstrate the experimental evidence of the resonant excitation of a magnetostatically coupled pair of vortices as a clear mode splitting of spectra.The partial excitation using ac current causes the energy transfer via dipolar interaction between two vortices, which results in a collective excitation of coupled gyrations. The observed mode splitting is reproduced by both micromagnetic simulation and analytical calculation. The different combinations of core pola...