O. Kitakami

Magnetic-field-induced shape recovery by reverse phase transformation

Large magnetic-field-induced strains have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.

Metamagnetic shape memory effect in a Heusler-type Ni43Co7Mn39Sn11 polycrystalline alloy

Shape memory and magnetic properties of a Ni43Co7Mn39Sn11 Heusler polycrystalline alloy were investigated by differential scanning calorimetry, the sample extraction method, and the three-terminal capacitance method. A unique martensitic transformation from the ferromagnetic parent phase to the antiferromagneticlike martensite phase was detected and magnetic-field-induced “reverse” transition was confirmed in a high magnetic field. In addition, a large magnetic-field-induced shape recovery strain of about 1.0% was observed to accompany reverse martensitic transformation, and the metamagnetic shape memory effect, which was firstly reported in a Ni45Co5Mn36.7In13.3 Heusler single crystal, was confirmed in a polycrystalline specimen.

Effect of magnetic field on martensitic transition of Ni46Mn41In13 Heusler alloy

Magnetic and martensitic transition behaviors of a Ni46Mn41In13 Heusler alloy were investigated by differential scanning calorimetry and vibrating sample magnetometry. A unique martensitic transition from the ferromagnetic austenite phase to the antiferromagneticlike martensite phase was detected and magnetic-field-induced “reverse” transition was confirmed in a high magnetic field. In addition, a large positive magnetic entropy change, which reached 13J∕kgK at 9T, was observed to accompany reverse martensitic transition. This alloy shows promise as a metamagnetic shape memory alloy with magnetic-field-induced shape memory effect and as a magnetocaloric material.

Lowering of ordering temperature for fct Fe-Pt in Fe/Pt multilayers

We have explored magnetic properties and structural characteristics of Fe/Pt multilayers after annealing at various temperatures in order to clarify the growth process of the ordered Fe–Pt phase in the Fe/Pt multilayer structures. It is found that rapid diffusion at Fe/Pt interface occurs at temperatures 275–325 °C and the multilayer structure directly transforms to the ordered (fct) phase when Fe and Pt layer thickness is almost equal. The ordering parameter S is evaluated to be 0.50–0.65 after annealing at 300–325 °C, and then is significantly enhanced to ∼1.00 with increasing annealing temperatures. By comparing the thermal processes of these different multilayer structures, it is found that the rapid formation of the fct phase in the multilayers with Fe/Pt≅1 is due to relatively rapid diffusion at the interface.

Effects of B and C on the ordering of L10-CoPt thin films

We have investigated the effects of B and C on the ordering of L10-CoPt films and confirmed the following quite different role between B and C in CoPt. A small amount of B considerably decreases the temperature for ordering and is interstitially incorporated into L10-CoPt along its c axis due to the large negative heat of solutions with Co and Pt. X-ray photoelectron spectroscopy analyses have revealed that B atoms combine with Co and Pt. In contrast, C is not incorporated into the CoPt lattice due to its positive heat of solution, resulting in no decrease in the ordering temperature. The present results reveal that CoPt films containing a small amount of B are promising for high density recording media.

Ordering and orientation of CoPt/SiO2 granular films with additive Ag

We have studied the chemical ordering of granular films (CoPt)1−xAgx/SiO2 with the concentration x=2–6 at. %. It was found that the additive Ag efficiently promotes the ordering process, reducing the ordering temperature by 100 °C compared with that of without Ag. The additive Ag also stimulates nearly perfect (001) orientation of the CoPt-ordered grains in the films with a thickness of less than 50 nm, resulting in a large perpendicular magnetic anisotropy.

Sensitive detection of irreversible switching in a single FePt nanosized dot

Magnetization of an isolated single dot as small as 60 nm in diameter fabricated from a single crystal L10-FePt(001) film has been measured by detection of the anomalous Hall effect in the temperature range from 10 to 300 K. Over the whole temperature range, the dots with diameter ranging from 60 nm to 12 μm exhibit perfect rectangular magnetization loops with coercivity almost constant regardless of the very large difference in diameter. The activation energy has been evaluated to be about 4×10−19 J, equivalent to the domain-wall energy times the square of the domain-wall thickness, suggesting that the magnetization reversals are initiated by nucleation of reversed embryo with the dimension of the exchange length.

Novel magnetostrictive memory device

A stress-operated memory device consisting of an ellipsoidal magnetic particle array and an electrostrictive grid is proposed. In the device, the magnetic state of the particle can be controlled only by the magnetostriction effect. Each particle is located at the intersection of the grid and has an in-plane uniaxial anisotropy. A pair of electric contacts is connected to the end of each wire. In the writing process, the driving voltages are simultaneously applied to two pairs of the selected contacts. This allows to apply a local electric field whose direction and amplitude can be regulated by varying the voltage intensity and polarity. The exerting stress on the magnetic particle results in the linear magnetostriction and hence an additional anisotropy energy in the particle. The in-plane total energy minimum, corresponding to the magnetization direction, follows the local electric field. Consequently the magnetization of the single magnetic particle located at the intersection can therefore be selective...

Magnetization switching behavior with microwave assistance

Microwave assisted switching of magnetization has become one of the key techniques for future advanced recording. We have studied the magnetization switching behaviors excited by circularly and linearly polarized microwaves based on the Landau–Lifshitz–Gilbert equation. The precessional motion of magnetization is excited by the microwave, resulting in significant reduction of the irreversible reversal field. The reversal takes place through very complicated precessional motion, which proceeds via nearly steady precession at an early stage, subsequent unstable motion, and finally abrupt irreversible switching. We discuss the detailed effect of microwave polarization on magnetization reversals.

Study on the in-plane uniaxial anisotropy of high permeability granular films

We have investigated the uniaxial magnetic anisotropy of the high resistivity Fe–Al–O granular films prepared by sputtering. It is found that application of external dc field during film deposition and post-annealing treatments give rise to no serious effects on the magnetic anisotropy. In contrast, the incident angle of sputtered particles to the substrate greatly affects the strength and the direction of the anisotropy. These results suggest that the magnetic anisotropy is induced by a shape anisotropy due to anisotropic morphology in the films. The temperature dependence of the anisotropy constant, however, does not obey the usual shape anisotropy relationship Ku(T)∝Ms(T)2. In the present article, we propose an anisotropic coupling model in order to explain the uniaxial anisotropy found in these granular films, where the magnetic coupling between Fe grains is assumed to be anisotropic in the film plane due to the presence of low Tc intergranular phase which bridges the gap between Fe grains in one dire...