Search Results (6)

Ni₄₆Mn₄₁In₁₃ (close to 2-1-1 system) Heusler alloy was studied by magnetization measurement dependence on the temperature in magnetic fields of up to 13.5 T. The magnetocaloric effect measured by the direct method in quasi-adiabatic conditions showed a maximum value of ∆T_ad = −4.2 K at a temperature T = 212 K in a magnetic field of 10 T in the region of martensitic transformation. The structure of the alloy was studied by transmission electron microscopy (TEM) as a function of the temperature and the thickness of the sample foil. In the temperature range from 353 to 215 K, at least two processes were established. The results of the study indicate that the concentration stratification occurs according to the mechanism of spinodal decomposition (conditionally spinodal decomposition) into nanoscale regions. At a temperature of 215 K and lower, martensitic phase with 14 M modulation is observed in the alloy at thicknesses greater than 50 nm. Some austenite is also observed. In foils with thickness of less than 50 nm in a temperature range from 353 to 100 Km only the initial austenite, which has not transformed, was found. Full article

Using the methods of electron microscopy and X-ray analysis in combination with measurements of the electrical resistance and magnetic susceptibility, the authors have obtained data on the peculiar features of pre-martensitic states and martensitic transformations, as well as subsequent decomposition, in the alloys with shape memory effect of Cu–14wt%Al–3wt%Ni and Cu–13.5wt%Al–3.5wt%Ni. For the first time, we established the microstructure, phase composition, mechanical properties, and microhardness of the alloys obtained in the nanocrystalline state as a result of severe plastic deformation under high pressure torsion and subsequent annealing. A crystallographic model of the martensite nucleation and the rearrangements β₁→β₁′ and β₁→γ₁′ are proposed based on the analysis of the observed tweed contrast and diffuse scattering in the austenite and the internal defects in the substructure of the martensite. Full article

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, T_C 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 (T_P) induces lattice softening; however, lattice softening is negligible at T_C. The forced magnetostriction at T_C 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. Full article

The temperature and field dependences of internal friction and Young’s modulus are studied using a high-resolution ultrasonic (90 kHz) technique in stoichiometric ferromagnetic Ni₂MnGa shape memory alloy close to the premartensitic transformation temperature, T_PM, in the demagnetized state and under moderate fields. Several new effects observed like an apparent Young´s modulus softening close to T_PM under moderate fields, instead of the hardening outside this range, as well as existing controversies in the apparent elastic and anelastic properties of Ni₂MnGa close to T_PM are explained by microeddy and macroeddy current relaxations that to date have been disregarded. Full article

Experimental investigations into the field dependence of magnetization and the relationship between magnetization and magnetostriction in Ni_2+xMnGa_1−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 Ni_2+xMnGa_1−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 ∝ M^δ is around 5.0 at the Curie temperature T_C, 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 p_S slightly decreased with increasing x. For x = 0.00, the spin fluctuation parameter in k-space (momentum space; T_A) and that in energy space (T₀) were obtained. The relationship between p_eff/p_S and T_C/T₀ can also be explained by Takahashi’s theory, where p_eff indicates the effective magnetic moments. We created a generalized Rhodes-Wohlfarth plot of p_eff/p_S versus T_C/T₀ for other ferromagnets. The plot indicates that the relationship between p_eff/p_S and T₀/T_C follows Takahashi’s theory. We also measured the magnetostriction for Ni_2+xMnGa_1−x (x = 0.02, 0.04). As a result, at T_C, 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. Full article

Among the functionalities of magnetic Heusler alloys, magnetostriction is attracting considerable attention. The alloy Ni₂MnGa has a premartensite phase, which is a precursor state to the martensitic transition. Some researchers have observed magnetostriction in this alloy in the premartensite phase. We performed magnetostriction studies on the premartensite phase of related Cr-substituted Ni₂Mn_1−xCr_xGa alloys and measured the thermal strain, permeability, magnetisation, and magnetostriction of polycrystals. Our thermal expansion measurements show an anomaly that indicates the occurrence of lattice deformation below the premartensitic transition temperature T_P. Our permeability measurements also showed an anomaly at the premartensitic transition. From our magnetisation results, we obtained the magnetic-anisotropy constant K₁. In the martensite phase, we found that the magnetic-anisotropy constant of the x = 0.00 alloy is larger than that of the x = 0.15 alloy. At 0.24 MA/m, we obtained a magnetostriction of −120 ppm for the x = 0.15 alloy. Magnetostriction in the premartensite phase is larger than that in the austenite and martensite phases at low magnetic-field strength, thus indicating that it is related to lattice softening in the premartensite phase. The e/a is proportional to the magnetostriction and T_P, which indicates that the electron energy, the magnetostriction, and the T_p are correlative each other. Full article