Minoru Matsumoto

The crystal structure and phase transitions of the magnetic shape memory compound Ni2MnGa

High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni2MnGa have been carried out. They enabled the sequence of transformations which take place when the unstressed, stoichiometric compound is cooled from 400 to 20 K to be established. For the first time the crystallographic structure of each of the phases which occur has been determined. At 400 K the compound has the cubic L21 structure, and orders ferromagnetically at TC ≈ 365 K. On cooling below ~ 260 K a super-structure, characterized by tripling of the repeat in one of the 110cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at TM ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters aortho = 1/√2acubic, bortho = 3/√2acubic, cortho = acubic and space group Pnnm. Below TM the compound has a related orthorhombic super-cell with bortho ≈ 7/√2acubic, which can be described within the same space group. The new modulation appears abruptly at TM and remains stable down to at least 20 K.

Premartensitic transition in Ni2+xMn1-xGa Heusler alloys

The temperature dependencies of the resistivity and magnetization of a series of Ni2+XMn1-XGa (X = 0 - 0.09) alloys were investigated. Along with the anomalies associated with ferromagnetic and martensitic transitions, well-defined anomalies were observed at the temperature of premartensitic transformation. The premartensitic phase existing in a temperature range 200 - 260 K in the stoichiometric Ni2MnGa is suppressed by the martensitic phase with increasing Ni content and vanishes in Ni2.09Mn0.91Ga composition.

On the Forced Magnetostriction of Fe-Ni Invar Alloys at 4.2°K

Transverse and longitudinal forced magnetostrictions of Fe-Ni Invar alloys were measured by means of the paper strain gauge technique at room temperature, 77° and 4.2°K. From the measured values of the forced magneto-strictions, the pressure coefficients of the absolute saturation magnetization were estimated. The absolute values of the pressure coefficients of the absolute saturation magnetization were found to decrease monotonically with an increase of Ni concentration. On the basis of Sidorovs theory the pressure effect on the absolute saturation magnetization is discussed.

Influences of sputtering gas pressure on microtexture and crystallographic characteristics of Ba-ferrite thin films for high density recording media

Ba-ferrite films with small c-axis dispersion angle Δθ 50 are suitable for ultrahigh density recording media. The dependences of partial oxygen gas pressure P O2 and total discharge gas pressure P Total on the characteristics of Ba-ferrite films were clarified. It was found that there were three regions, where films were composed of single layer of spinel type ferrite(P O2 ≤0.002mTorr), mixed layer of spinel and magnetoplumbite type ferrites ( 0.003 \leq P_{O2} \leq 0.2 mTorr) and single layer of magnetoplumbite type ferrite (P O2 >0.3 mTorr). In the range of P 02 between 0.005 and 0.2 mTorr, spinel-like ferrite layer plays very important role as an underlayer to decrease the Δθ 50 of the Ba-ferrite layer. Films deposited at relatively low P O2 and P Total exhibit very smooth surface and Δθ 50 as small as 2.5\sim3.5\deg .

Phase transitions in intermetallic compounds Ni-Mn-Ga with shape memory effect

The effect of a magnetic field on the formation of structural domains at martensitic transition in the intermetallic system Ni/sub 2+x/Mn/sub 1-x/Ga was studied. For the compositions x whose temperatures of structural and magnetic transitions are close this effect is most pronounced. In the field of 0.2 to 1 T the increase in the transition temperature is linear with the coefficient of 0.015 K/T. Estimates of the transition temperature shift based on thermodynamic calculations are in good agreement with the experimental data. It is shown that the switching on the magnetic field at some critical temperature induces partial transformation of austenite into martensite.

Read/write characteristics of a Ba-ferrite sputtered disk

Ba-ferrite thin film rigid disks with various c-axis dispersion angle Δθ 50 and various film thickness have been prepared by rf diode sputtering. Read/write characteristics of Ba-ferrite disks were evaluated by means of ring type Mn-Zn ferrite head. Reproduced wave forms of isolated magnetization transition showed dipulse and dipulse ratio depended on recording current. Dipulse ratio decreased with increase of recording current. PW 50 ( pulse width at half height of reproduced output at low density ) and λ 50 ( recorded wave length at D 50 )depended on Δθ 50 of Ba-ferrite thin film and both PW 50 and λ 50 increased with increase of Δθ 50 . Over-write ratio of -31dB has been obtained at 2F of 40 kfrpi. D 50 of disks with Δθ 50 of 2.5∼2.8° depended on the film thickness. 88, 95 and 100 kfrpi in D 50 for the thickness of 0.26, 0.16 and 0.066 μm, respectively, have been ontained with a head with gap length of 0.4 μm.

Effect of substrate temperature on magnetic properties of strontium ferrite thin films

Strontium hexagonal ferrite (SrM) thin films were prepared by a dc magnetron sputtering system, and the crystal structure and magnetic properties were studied. It was found that crystallization temperature of SrM film was around 500/spl deg/C. Preferential orientation of c-axis was observed at the substrate temperature (Ts) of 550/spl deg/C. The films prepared at Ts of 550/spl deg/C exhibited the maximum coercivity of 3.5 kOe in the perpendicular direction.

Magnetic properties of Heusler type Ni2+xMn1−xGa

Abstract Ferromagnetic Heusler type Ni 2+ x Mn 1− x Ga ( x =0–0.19) alloys were made by the arc melt method. The Curie temperature, T c , and the transformation temperatures, M s , M f , A s and A f , were measured. T c decreased, while M s , M f , A s and A f increased with increase of Ni composition. The specific heat at constant pressure for x =0.16–0.19 was measured. The specific heat at constant volume was estimated. The lattice and structural transformation terms in the specific heat at constant volume and the Debye temperature were estimated.

Modeling and Numerical Simulation on Thermomechanical Behavior of SMA Plates with Two-Way Shape Memory Effect

Phenomenological modeling on the thermomechanical behavior of shape memory alloy (SMA) plates with two-way shape memory effect (TWSME) is presented in this paper. Two important characteristics of SMA materials as sensors or actuators; the thermal responses and the transformation accompanied deformation; are described by the proposed models. The latent heat effect during transformation is approximately expressed by an apparent specific heat as a function of transformation temperatures. The transformation-induced deformation of SMA plates is described by newly defined transformation expansion coefficients, based on the physical understanding of the mechanism of TWSME. The proposed model is verified by the comparison between numerical simulation and experiments on the thermomechanical behavior of SMA plates.

Magnetic properties of ordered alloy AuMn2

Abstract Magnetic measurements have been done for AuMn 2 ordered alloy at temperatures down to 1.9 K and in magnetic fields up to 50 kOe. The magnetic susceptibility of the alloy is very small and metamagnetic behaviors are obsereved at low temperatures. The results indicate AuMn 2 to be an intinerant magnet.