The Forced Magnetostrictions and Magnetic Properties of Ni2MnX (X = In, Sn) Ferromagnetic Heusler Alloys.

Experimental studies into the forced magnetostriction, magnetization, and temperature dependence of permeability in Ni2MnIn and Ni2MnSn ferromagnetic Heusler alloys were performed according to the spin fluctuation theory of itinerant ferromagnetism proposed by Takahashi. We investigated the magnetic field (H) dependence of magnetization (M) at the Curie temperature TC, and at T = 4.2 K, which concerns the ground state of the ferromagnetic state. The M-H result at TC was analyzed by means of the H versus M5 dependence. At 4.2 K, it was investigated by means of an Arrott plot (H/M vs. M2) according to Takahashi's theory. As for Ni2MnIn and Ni2MnSn, the spin fluctuation parameters in k-space (momentum space, TA) and that in energy space (frequency space, T0) obtained at TC and 4.2 K were almost the same. The average values obtained at TC and 4.2 K were TA = 342 K, T0 = 276 K for Ni2MnIn and TA = 447 K, T0 = 279 K for Ni2MnSn, respectively. The forced magnetostriction at TC was also investigated. The forced linear magnetostriction (ΔL/L) and the forced volume magnetostriction (ΔV/V) were proportional to M4, which followed Takahashi's theory. We compared the forced volume magnetostriction ΔV/V and mechanical parameter, bulk modulus K. ΔV/V is inversely proportional to K. We also discuss the spin polarization of Ni2MnIn and other magnetic Heusler alloys. The pC/pS of Ni2MnIn was 0.860. This is comparable with that of Co2MnGa, which is a famous half-metallic alloy.

The Characteristic Properties of Magnetostriction and Magneto-Volume Effects of Ni2MnGa-Type Ferromagnetic Heusler Alloys.

In this article, we review the magnetostriction and magneto-volume effects of Ni2MnGa-type ferromagnetic Heusler alloys at the martensitic, premartensitic, and austenitic phases. The correlations of forced magnetostriction (ΔV/V) and magnetization (M), using the self-consistent renormalization (SCR) spin fluctuation theory of an itinerant electron ferromagnet proposed by Takahashi, are evaluated for the ferromagnetic Heusler alloys. The magneto-volume effect occurs due to the interaction between the magnetism and volume change of the magnetic crystals. The magnetic field-induced strain (referred to as forced magnetostriction) and the magnetization are measured, and the correlation of magnetostriction and magnetization is evaluated. The forced volume magnetostriction ΔV/V at the Curie temperature, TC is proportional to M4, and the plots cross the origin point; that is, (M4, ΔV/V) = (0, 0). This consequence is in good agreement with the spin fluctuation theory of Takahashi. An experimental study is carried out and the results of the measurement agree with the theory. The value of forced magnetostriction is proportional to the valence electron concentration per atom (e/a). Therefore, the forced magnetostriction reflects the electronic states of the ferromagnetic alloys. The magnetostriction near the premartensitic transition temperature (TP) induces lattice softening; however, lattice softening is negligible at TC. The forced magnetostriction at TC occurs due to spin fluctuations of the itinerant electrons. In the martensitic and premartensitic phases, softening of the lattice occurs due to the shallow hollow (potential barrier) of the total energy difference between the L21 cubic and modulated 10M or 14M structures. As a result, magnetostriction is increased by the magnetic field.

Magnetoresistance and Thermal Transformation Arrest in Pd2Mn1.4Sn0.6 Heusler Alloys.

The magnetization, electric resistivity, and magnetoresistance properties of Pd 2 Mn 1 . 4 Sn 0 . 6 Heusler alloys were investigated. The Curie temperature of the parent phase, martensitic transformation temperatures, and magnetic field dependence of the martensitic transformation temperatures were determined. The magnetoresistance was investigated from 10 to 290 K, revealing both intrinsic and extrinsic magnetoresistance properties for this alloy. A maximum of about - 3 . 5 % of intrinsic magnetoresistance under 90 kOe and of about - 30 % of extrinsic magnetoresistance under 180 kOe were obtained. Moreover, the thermal transformation arrest phenomenon was confirmed in the Pd 2 Mn 1 . 4 Sn 0 . 6 alloy, and an abnormal heating-induced martensitic transformation (HIMT) behavior was observed.

Investigation of the Itinerant Electron Ferromagnetism of Ni2+xMnGa1-x and Co2VGa Heusler Alloys.

Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in Ni2+xMnGa1-x (x = 0.00, 0.02, 0.04) and Co2VGa Heusler alloy ferromagnets were performed following the spin fluctuation theory of itinerant ferromagnetism, called as "Takahashi theory". We investigated the magnetic field dependence of magnetization at the Curie temperature TC, which is the critical temperature of the ferromagnetic⁻paramagnetic transition, and also at T = 5 K, which concerns the ground state of the ferromagnetic state. The field dependence of the magnetization was analyzed by means of the H vs. M5 dependence, and the field dependence of the ground state at 5 K was investigated by means of an Arrott plot (H/M vs. M²) according to the Takahashi theory. As for Ni2+xMnGa1-x, the spin fluctuation parameter in k-space (momentum space, TA) and that in energy space (T0) obtained at TC and 5 K were almost the same. On the contrary, as for Co2VGa, the H vs. M5 dependence was not shown at TC. We obtained TA and T0 by means of an Arrott plot at 5 K. We created a generalized Rhodes⁻Wohlfarth plot of peff/pS versus TC/T0 for the other ferromagnets. The plot indicated that the relationship between peff/pS and T0/TC followed Takahashi's theory. We also discussed the spontaneous magnetic moment at the ground state, pS, which was obtained by an Arrott plot at 5 K and the high temperature magnetic moment, pC, at the paramagnetic phase. As for the localized ferromagnet, the pC/pS was 1. As for weak ferromagnets, the pC/pS was larger than 1. In contrast, the pC/pS was smaller than 1 by many Heusler alloys. This is a unique property of Heusler ferromagnets. Half-metallic ferromagnets of Co2VGa and Co2MnGa were in accordance with the generalized Rhodes⁻Wohlfarth plot with a km around 1.4. The magnetic properties of the itinerant electron of these two alloys appeared in the majority bands and was confirmed by Takahashi's theory.

Forced Magnetostrictions and Magnetizations of Ni2+xMnGa1-x at Its Curie Temperature.

Experimental investigations into the field dependence of magnetization and the relationship between magnetization and magnetostriction in Ni2+xMnGa1-x (x = 0.00, 0.02, 0.04) alloy ferromagnets were performed following the self-consistent renormalization (SCR) spin fluctuation theory of itinerant ferromagnetism. In this study, we investigated the magnetization of and magnetostriction on Ni2+xMnGa1-x (x = 0.02, 0.04) to check whether these relations held when the ratio of Ni to Ga and, the valence electron concentration per atom, e/a were varied. When the ratio of Ni to Ga was varied, e/a increased with increasing x. The magnetization results for x = 0.02 (e/a = 7.535) and 0.04 (e/a = 7.570) suggest that the critical index δ of H ∝ Mδ is around 5.0 at the Curie temperature TC, which is the critical temperature of the ferromagnetic⁻paramagnetic transition. This result confirms Takahashi's spin fluctuation theory and the experimental results of Ni2MnGa. The spontaneous magnetization pS slightly decreased with increasing x. For x = 0.00, the spin fluctuation parameter in k-space (momentum space; TA) and that in energy space (T0) were obtained. The relationship between peff/pS and TC/T0 can also be explained by Takahashi's theory, where peff indicates the effective magnetic moments. We created a generalized Rhodes-Wohlfarth plot of peff/pS versus TC/T0 for other ferromagnets. The plot indicates that the relationship between peff/pS and T0/TC follows Takahashi's theory. We also measured the magnetostriction for Ni2+xMnGa1-x (x = 0.02, 0.04). As a result, at TC, the plot of the magnetostriction (ΔL/L) versus M4 shows proportionality and crosses the origin. These magnetization and magnetostriction results were analyzed in terms of Takahashi's SCR spin fluctuation theory. We investigated the magnetostriction at the premartensite phase, which is the precursor state to the martensitic transition. In Ni2MnGa system alloys, the maximum value of magnetostriction is almost proportional to the e/a.

Magnetic Properties of the Ferromagnetic Shape Memory Alloys Ni50+xMn27-xGa23 in Magnetic Fields.

Thermal strain, permeability, and magnetization measurements of the ferromagnetic shape memory alloys Ni50+xMn27-xGa23 (x = 2.0, 2.5, 2.7) were performed. For x = 2.7, in which the martensite transition and the ferromagnetic transition occur at the same temperature, the martensite transition starting temperature TMs shift in magnetic fields around a zero magnetic field was estimated to be dTMs/dB = 1.1 ± 0.2 K/T, thus indicating that magnetic fields influences martensite transition. We discussed the itinerant electron magnetism of x = 2.0 and 2.5. As for x = 2.5, the M4 vs.B/M plot crosses the origin of the coordinate axis at the Curie temperature, and the plot indicates a good linear relation behavior around the Curie temperature. The result is in agreement with the theory by Takahashi, concerning itinerant electron ferromagnets.