Katsuya Mitsuoka

Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single-crystal films (invited)

Single‐crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X‐ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic mom...

Magnetic and Mössbauer studies of single‐crystal Fe16N2 and Fe‐N martensite films epitaxially grown by molecular beam epitaxy (invited)

Single‐phase, single‐crystal Fe16N2(001) films and Fe‐11 at. %N martensite films of 200–900 A thickness have been epitaxially grown on In0.2Ga0.8As(001) substrates by evaporating Fe in an atmosphere of mixed gas of N2 and NH3, followed by annealing. The saturation magnetizations 4πMs’s for Fe16N2 and Fe‐N martensite films have been measured to be around 29 and 24 kG at room temperature, respectively, and almost constant in the above thickness range by using a vibrating sample magnetometer. 4πMs for Fe‐N martensite films has been increased with ordering of N atoms caused by annealing and finally reached around 29 kG for Fe16N2. Mossbauer spectra have been measured for those films. The spectrum for Fe‐N martensite films was a superposed one with hyperfine fields of 360, 310, and 250 kOe, similar to those previously reported for martensite. While the spectrum became simpler with ordering, finally reaching a single hyperfine field of 330 kOe for Fe16N2. 4πMs of 29 kG for Fe16N2 (3.2 μB/Fe atom) and 4πMs of 24...

Distribution of blocking temperature in bilayered Ni81Fe19/NiO films

Exchange paths were investigated for unidirectional exchange coupled 40 nm Ni81Fe19/50 nm NiO films by performing several field cooling experiments. Our experimental data were very consistent with the assumed existence of a variety of exchange paths. Each exchange path seemed to produce its own local unidirectional anisotropy and different local blocking temperature. The measureable exchange coupling could be described as consisting of the sum of the respective exchange paths, each with its own local blocking temperature. On the other hand, an observed blocking temperature of about 230 °C was determined from the exchange paths having the highest local blocking temperature. The local blocking temperatures were thought to be widely distributed, ranging from room temperature to about 230 °C, and the maximum existence probability was most likely at about 215 °C. This indicated that the exchange paths having the local blocking temperature of 215 °C made the largest contribution to the exchange coupling field a...

Ferromagnetic resonance studies of Fe16N2 films with a giant magnetic moment

The g factor and 4π Ms for epitaxially grown Fe16N2(001)/In0.2Ga0.8As(001) films have been investigated by ferromagnetic resonance along with Fe films for comparison. Angular dependence of the resonance fields in the film plane of Fe16N2 films had four‐fold symmetry, which was attributed to the in‐plane anisotropy. The g factor for Fe16N2 films was about 2.0, which means that the magnetic moment originates mainly from spin. Thus, nothing unusual is seen about the g factor. The g factor for Fe films was about 2.1, which is very similar to the value reported previously. 4πMs values for Fe16N2 and Fe films were 2.8×104 and 2.1×104 G, respectively, which agree well with the previous data obtained by a vibrating sample magnetometer. This confirmed that Fe16N2 has a giant magnetic moment. Torque magnetometer measurements showed that Fe16N2 films have a larger perpendicular anisotropy of 7.8×106 erg/cm3, which can originate from its bct structure.

Design and fabrication of thin‐film heads based on a dry process (invited)

High performance thin‐film heads for disk drive systems have been developed based on a dry process. Heads were computer simulated and optimal design was carried out. Relationships among Ni‐Fe composition, domain structure and wiggle of the read‐write waveform were obtained. Based on these results, optimum Ni‐Fe composition range was determined. A planarization procedure for an inbedding insulator of the conductor coil was developed. Also narrow track patterning and gap depth controlling procedures were developed. Using these procedures, a two‐layered seventeen‐turn thin‐film head for a large capacity disk drive system (23 Mb/in.2) has been developed. The head exhibited excellent read‐write characteristics.

Challenges for Perpendicular Write Heads at High Recording Density

Perpendicular recording technology has become the main stream for 130 Gb/in2 HDD products. In this paper, challenges in perpendicular write head are discussed. Design tradeoffs and concerns of narrow-track single-pole heads, trailing shield heads, floating TS heads, and wrapped-around-shield (WAS) heads are discussed. Experimental data show that WAS heads provide good narrow track and high linear density performance. An areal density of 343 Gb/in2 has been achieved at a very aggressive magnetic spacing condition

Magnetic domains of permalloy films for magnetic recording thin film heads observed by spin-polarized SEM

The domain structures of cores patterned in thin film heads for disk drive systems were investigated by using spin-polarized SEM. This method made it possible to determine the direction of the magnetization at the domains and domain walls in magnetic cores accurately. Domain structures in the track width, as small as 10 μ m could be observed. Domain structure could also be observed at the tapering area of a thin film head. It was found that the triangular domains without the closure domains are located at the edge of the magnetic core in single- and six-layered permalloy films in some cases. Such a domain structure was regarded as causing the wiggles in the read-out waveforms.

Observation of Néel structure walls on the surface of 1.4‐μm‐thick magnetic films using spin‐polarized scanning electron microscopy

Neel‐type surface magnetic wall structure is observed on thick samples such as 1.4‐μm‐thick Permalloy polycrystal film and 1‐μm‐thick Co‐based amorphous films. The structure is observed by using spin‐polarized scanning electron microscopy. These observations are consistent with Hubert’s two‐dimensional domain wall model for thick films [Z. Angew. Phys. 32, 58 (1971)].

Giant Magnetoresistance of Spin Valve Films with NiO Antiferromagnetic Films

The magnetic and magnetoresistive properties of films having spin valve structures were investigated. The basic structure of such films consists of two ferromagnetic layers separated by a Cu layer. One of the ferromagnetic layers is strongly biased by an NiO antiferromagnetic film in direct exchange contact with it. Spin valve films having an NiO/NiFe/Cu/NiFe structure exhibit good sensitivity, and the MR ratio (¿¿/¿) is 4% in an applied field of 10 Oe. Films with a layered NiO/NiFe/Cu/NiFe/Cu/NiFe/NiO structure have an MR ratio of 7% in a field of 10 Oe. On the other hand, NiO/NiFe/Co/Cu/Co/NiFe films exhibit an MR ratio of 6% to 7.5%. The thermal stability of spin valve films was also studied.

Crystal Structures and Magnetic Properties of Fe1 e N2 Films

Fe-N films with thicknesses of 500–1000 A have been epitaxially grown on ln o-2 Ga o-e As(001) substrates using MBE system. Deposition atmosphere was N 2 + 20%NH 3 . and pressure during deposition was around 1 × 10 −4 Torr. Two electron guns were used in order to control nitrogen concentration of 11 at% in Fe-N films. When substrate temperature was 150 °C, an Fe 1 e N 2 single crystal films were found to be formed after annealing at 150 °C. When the substrate temperature was around 300 °C, Fe 1 e N 2 . films were grown epitaxial ly on ln o-2 Ga o-3 As without annealing, while Fe 1 e N 2 could not be grown at substrate temperature higher than 350 °C because of the formation of Fe 2 As caused by the reaction between substrate and Fe–N films. RHEED and XRD patterns showed that Fe 1 e N 2 films was grown epitaxial ly and crystal orientation of Fe 1 e N 2 films are Fe 1 e N 2 (001)//ln o-2 Gao o-e As(001) and Fe 1 e N 2 [100]//ln o-2 Gao o-e As [100]. Saturation magnetic flux density of Fe 1 e N 2 is 2.8˜3.0T.