We report the results of experimental investigation of the crystal structure, magnetization and Sn-119 M & ouml;ssbauer spectroscopy of Rh2Mn1+xSn1-x (0 <= x <= 0.6). The samples crystallize in the cubic L2(1)-type structure for 0 <= x <= 0.4 and the cubic B2-type structure for x = 0.5 and 0.6. X-ray diffraction patterns with the samples with 0 <= x <= 0.4 indicate that the excess Mn atoms occupy the Sn sites. The samples with 0 <= x <= 0.4 exhibit the typical ferromagnetic behavior. The magnetic moment at 5 K of 4.27 mu(B)/f.u. for x = 0 (Rh2MnSn) initially increases with increasing x and saturates for x >= 0.2. On the other hand, the Curie temperature for 0 <= x <= 0.4 decreases with increasing x. It is presumed that both samples with x = 0.5 and 0.6 are in the heterogeneous magnetic state. The Sn-119 M & ouml;ssbauer spectroscopy measurements showed that the hyperfine field at 90 K for 0 <= x <= 0.6 increases with increasing x. The concentration dependence of the magnetic moment and the Curie temperature for Rh2Mn1+xSn1-x (0 <= x <= 0.6) are discussed qualitatively based on the Mn-Rh and Mn-Mn interactions in the crystal.

The effect of pressure on the crystal structure of Ni2MnGa was investigated up to 41 GPa using in situ angledispersive x-ray diffraction with synchrotron radiation, an imaging plate detector, and a diamond anvil cell. A pressure-induced transition from a cubic L2(1) structure (space group Fm(3) over bar m) to a 10-layer modulated monoclinic structure (C2/m) with a distortion of beta similar to 92(degrees) occurred at a pressure of 19.1 GPa or lower. The volume change at the martensitic transition is a minute value of about -0.1%. The bulk modulus for the low-pressure phase is 147(2) GPa, consistent with previous studies. In contrast, the bulk modulus for the high-pressure phase is 229(4) GPa, significantly higher than the former bulk modulus. Initial permeability measurements of the Ni2MnGa were also conducted at various pressures up to 1 GPa. The pressure dependencies of the Curie temperature, premartensitic, and martensitic transition temperatures are discussed in detail.

The results of experimental investigations of the crystal structure and magnetization of the magnetic shape memory alloy Ni2Mn1.48Sn0.52 are reported. This alloy undergoes the martensitic transition from the cubic L2(1)-type structure to the orthorhombic 4O-type one with decreasing temperature. The Curie temperature T-C(A) in the austenite phase is 310 K. The magnetic state just below the martensitic transition temperature T-M is the low magnetization state or something like that. With further decrease of temperature, this alloy regains the ferromagnetism. The unit cell volume contracts discontinuously at T-M with decreasing temperature. The percentage of the volume change was estimated to be similar to 0.7%. The nearest neighbor Ni-Mn and Mn-Mn distances in both austenite and martensite phases of Ni2Mn1.48Sn0.52 were estimated using the X-ray diffraction analysis. On the basis of the experimental results, the magnetic properties of Ni2Mn1.48Sn0.52 are discussed.

The results of experimental investigations of the crystal structure, magnetization and Mössbauer spectroscopy of the Heusler alloy Ni2Mn1.48Sn0.52 are reported. The alloy undergoes the martensitic transition from the L21-type cubic structure into 4O-type orthorhombic structure with a thermal hysteresis of ∼23 K. The martensitic transition temperature TM is 309 K. The Curie temperature in the austenitic phase TCA of Ni2Mn1.48Sn0.52 is found to be 316 K. The Mössbauer spectra from the high temperature paramagnetic state at 330 K and the martensitic state at 283 K were both singlets, showing a typical paramagnetic feature in the narrow temperature range just below TM. With further decrease of temperature, the ferromagnetic ordered state also appears in the martensitic phase. The Curie temperature in the martensitic phase TCM is found to be 256 K. Furthermore, the exchange bias effect is observed in the temperature range 1.8 K ≤ T ≤ TB (∼130 K), where TB is an exchange bias blocking temperature. The values of a hyperfine magnetic field at Sn nuclei of Ni2Mn1.48Sn0.52 fall significantly below the molecular magnetic field approximation for S = 1. On the basis of the experimental results, the electronic and magnetic properties of Ni2Mn1.48Sn0.52 are discussed.

The results of experimental investigations of the crystal structure, magnetization and Mo center dot ssbauer spectroscopy of the Heusler alloy Ni2Mn1.48Sn0.52 are reported. The alloy undergoes the martensitic transition from the L21-type cubic structure into 4O-type orthorhombic structure with a thermal hysteresis of -23 K. The martensitic transition temperature TM is 309 K. The Curie temperature in the austenitic phase TCA of Ni2Mn1.48Sn0.52 is found to be 316 K. The Mo center dot ssbauer spectra from the high temperature paramagnetic state at 330 K and the martensitic state at 283 K were both singlets, showing a typical paramagnetic feature in the narrow temperature range just below TM. With further decrease of temperature, the ferromagnetic ordered state also appears in the martensitic phase. The Curie temperature in the martensitic phase TCM is found to be 256 K. Furthermore, the exchange bias effect is observed in the temperature range 1.8 K <= T <= TB (-130 K), where TB is an exchange bias blocking temperature. The values of a hyperfine magnetic field at Sn nuclei of Ni2Mn1.48Sn0.52 fall significantly below the molecular magnetic field approximation for S = 1. On the basis of the experimental results, the electronic and magnetic properties of Ni2Mn1.48Sn0.52 are discussed.

The results of experimental investigations of the crystal structure, permeability, magnetization and Mossbauer spectroscopy of Co2ZrSn are reported. The sample, which was prepared by arc melting, comprised of a Heusler structure Co2ZrSn that coexisted with a small amount of an impurity phase. The Curie temperature T-C of Co2ZrSn was found to be 452.3 K. At 1.8 K the magnetic moment of Co2ZrSn was estimated to be 2.0 mu(B)/f.u., confirming that the magnetic moment of Co2ZrSn obeys the Slater-Pauling rule. The temperature variation of the spontaneous magnetization M-s roughly follows the Brillouin function for spin value S = 1/2. However, in the temperature region of T/T-C > 0.6, M-s is distinctly higher than the Brillouin function. The T-C of Co2ZrSn was almost independent of pressure. The Mossbauer spectroscopy measurements showed that the isomer shift I.S. (1.5 mm/s at 295 K) was approximately temperature independent. At 95 K the value of the hyperfine magnetic field H-hf(Sn) at the Sn nuclei was found to be 101 kOe. The temperature variation of H-hf(Sn) of Co2ZrSn roughly followed the Brillouin function for S = 1/2. On the basis of the experimental results, the electronic and magnetic properties of Co2ZrSn are discussed.

Observation of inverse magnetocaloric effect in magnetic-field-induced austenite phase of Heusler Alloys Ni50-xCoxMn31.5Ga18.5 (x = 9 and 9.7) T. Kihara*, T. Roy, X. Xu, A. Miyake, M. Tsujikawa, H. Mitamura M. Tokunaga, Y. Adachi, T. Eto, and T. Kanomata 1 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan 2 Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi, Japan 3 Department of Materials Science, Tohoku University, Sendai, Miyagi, Japan 4 The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan 5 Graduate School of Science and Engineering, Yamagata University, Yonezawa, Yamagata, Japan 6 Kurume Institute of Technology, Kurume, Fukuoka, Japan 7 Research Institute for Engineering and Technology, Tohoku Gakuin University, Tagajo, Miyagi, Japan

This study experimentally investigates a series of phase transitions of ferromagnetic shape memory alloys, Ni2+xMnGa1-x, in the composition range 0.02 <= x <= 0.27. An experimental phase diagram of these alloys was constructed by the use of electrical resistivity, thermomagnetization, and thermoanalysis measurements, and compared with both the Ni-rich Ni2+xMn1-xGa and Mn-rich Ni2Mn1+yGa1-y alloy systems which have been extensively studied as proto-materials for ferromagnetic Heusler alloys. The comparison makes it clear that there are remarkable composition dependencies for the following characteristic temperatures. All three systems have the common property that Curie temperature decreases and martensitic transition temperature increases with increasing x till the both temperatures coincide each other. As for the magnetostructural coupling temperature, however, the Ni-rich system has a wide temperature range, and the Mn-rich system does not show signs of the magnetostructural coupling, whereas the magnetostructural coupling was observed in Ni2+xMnGa1-x alloys in the range 0.10 <= x <= 0.12 which has similar characteristics between the former two systems. The premartensitic transition temperature was also observed in Ni2+xMnGa1-x alloys; it reached the ambient temperature at x = similar to 0.04, the highest of the three alloy systems. In addition, an intermartensitic transition appeared in Ni2+xMnGa1-x alloys in the composition range 0.06 <= x <= 0.10, which showed a drastic change especially in the electrical resistivity. These characteristic temperatures are also compared in terms of the number of valence electrons among the Ni-Mn-Ga Heusler alloy systems, and the differences are clarified and discussed in detail. (C) 2021 Elsevier B.V. All rights reserved.

Magnetoresistance of the correlated narrow-gap semiconductor FeSi was investigated by the radio frequency self-resonant spiral coil technique in magnetic fields up to 500 T, which is supplied by an electromagnetic flux compression megagauss generator. Semiconductor-to-metal transition accomplishes around 270 T observed as a sharp kink in the magnetoresistance, which implies the closing of the hybridization gap by the Zeeman shift of band edges. In the temperature-magnetic field phase diagram, the semiconductor-metal transition field is found to be almost independent of temperature, which is in contrast to a characteristic magnetic field associated with the hopping magnetoconduction in the in-gap localized states, exhibiting a notable temperature dependence.

The compound Co2NbGa crystallizes in the Heusler-type structure. Magnetization measurements of Co2NbGa have been made under ambient pressure using a SQUID magnetometer. The spontaneous magnetic moment at 5 K and the Curie temperature are 1.81 mu(B)/f.u. and 351 K, respectively. Many magnetic properties of the itinerant electron ferromagnets are explained by the Takahashi's spin fluctuation theory. In the Takahashi's spin fluctuation theory, spectral parameters of spin fluctuations, T-o and T-A, are involved in various magnetic properties derived theoretically. In this work, we estimated T-o and T-A of Co2NbGa by using the results of magnetization measurements at 5 K: T-o = 2.1 x 10(3)K and T-A = 4.7 x 10(3)K. This experiment proved that the squared spontaneous magnetization M-s(T)(2) is proportional to T-2 at low temperature. The T-A value estimated from the M-s (T)(2) vs. T-2 curve is 6.6 x 10(3) K. Furthermore, the field-induced magnetization M-4 around T-c is found to be proportional to H/M in the low magnetic field region.

Magnetic properties under pressure have been investigated on the weak itinerant electron ferromagnet Rh2NiGe. The pressure derivative of the Curie temperature TC has been obtained from the results of the temperature dependence of the initial permeability under pressure up to 10kbar. The value of dTC/dp is found to be −0.60K/kbar. The pressure change of the spontaneous magnetization σs at 4.2K for Rh2NiGe has been determined by the subtraction method under pressure up to 10kbar. The value of dσs/dp is found to be −0.014emu/gkbar. The obtained results are analyzed using the spin fluctuation theory.

Magnetic phase diagrams of the metamagnetic shape memory alloys Ni50-xCoxMn31.5Ga18.5 (x = 9 and 9.7) were produced from high-field magnetization measurements up to 56 T. For both compounds, magnetic field induced martensitic transformations are observed at various temperatures below 300 K. Hysteresis of the field-induced transformation shows unconventional temperature dependence: it decreases with decreasing temperature after showing a peak. Magnetic susceptibility measurement, microscopy, and X-ray diffraction data suggest a model incorporating the magnetic anisotropy and Zeeman energy in two variants, which qualitatively explains the thermal and the magnetic field history dependence of the hysteresis in these alloys.

An investigation on the relationship between magnetostriction and magnetization by means of the self-consistent renormalization (SCR) theory of an itinerant ferromagnet was performed for Ni2MnGa alloy. The magnetization results at the Curie temperature T-C suggest that the critical index delta of H alpha M-delta is 4.7-5.0, and this confirms Takahashi's spin fluctuation theory and the result of Nishihara et al. As a result, at TC, the relation between magnetostriction and M-2 deviates from proportionality and the magnetostriction is proportional to M-4. At T-C, the plot of Delta L/L vs M-4 crossed the point of origin. This result is in accordance with Takahashi's spin fluctuation theory. Published by AIP Publishing.

Effect of pressure on the Curie temperature TC of the Mn-rich Heusler alloys Mn2RuSn and Mn2PdSn has been investigated by measuring the temperature dependence of initial permeability at various pressures up to 10 kbar. It was found that the Curie temperatures of Mn2RuSn and Mn2PdSn decrease with increasing pressure. The pressure derivatives of TC were estimated to be −0.59K/kbar for Mn2RuSn and −0.80K/kbar for Mn2PdSn. On the basis of the experimental results, the relationship between the magnetic transition temperature and the Mn-Mn distance is discussed.

Intermetallic compound Ni 3 A1 with the L1 2 -type structure is a weak itinerant electron ferromagnet (WIFE) with a Curie temperature TC of 45 K. For WIFE, Takahashi proposed a spin fluctuation theory, in which quantum spin fluctuations (zero-point spin fluctuations) were considered [1], [2]. In this study, we performed magnetization measurements of Ni 3 A1 under high pressures up to 1.2 GPa. The obtained data were analyzed based on Takahashi's theory [1], [2]. In this paper, we discuss the pressure dependence of the characteristic parameters of Takahashi's spin fluctuation theory.

Spintronic devices, such as magnetic random access memory (MRAM), are built from materials with high spin polarization. Half-metals which have only one spin band at the Fermi level are necessary to improve the performance of the spintronic devices. Co-based Heusler alloys are predicted relatively high Curie temperature and high spin polarization from the first-principle band calculations. We therefore focus on the spin polarization and magnetization of Co-based Heusler alloys Co 2 TiGa 1-x Sn x .

The effect of pressure on the Curie temperature TC and the martensitic transition temperature TM of the ferromagnetic shape memory alloys Ni50+x(Mn0.5Fe0.5)25Ga25–x has been investigated by measuring the temperature dependence of the initial permeability at various pressures up to ~10kbar. The pressure derivatives of TC and TM, dTC/dp and dTM/dp, of Ni50(Mn0.5Fe0.5)25Ga25 (x=0) are found to be 0.23K/kbar and 0.77K/kbar, respectively. The values of dTC/dp and dTM/dp of Ni50+x(Mn0.5Fe0.5)25Ga25–x alloys decrease with increasing x. Moreover the sign of dTC/dp changes from positive to negative with increasing x. The pressure dependence of TC for Ni50+x(Mn0.5Fe0.5)25Ga25–x alloys is discussed qualitatively based on the dependence of the exchange interactions on interatomic distance.

Ab initio electronic structure calculation of a series of Co2FeAl1−xSix Heusler alloys has been performed, using the Korringa–Kohn–Rostoker-coherent potential approximation method to explain experimental 59Co NMR spectra. Two prominent features are explained semi-quantitatively—a global shift of the 59Co resonance line due to alloying with Al and Si atoms in Co2FeAl1−xSix, and the effect of local disorder in creating distinct satellite lines of 59Co NMR in Co2FeAl. The importance is stressed of the positive contribution to the 59Co hyperfine field from valence electron polarization, which emerges from the half-metallic band structure inherent in Co-based Heusler alloys.