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1.
The phase diagram of ferromagnetic alloys Ni2+x Mn1?x Ga is reconstructed on the basis of temperature dependences of the resistance. It is seen from this diagram that for small x, structural transitions from the cubic to the tetragonal phase are preceded by structural transformations in the cubic phase. In the framework of the phenomenological Landau theory of phase transitions, phase diagrams of the structural and magnetic phase transitions in these alloys are analyzed with regard for the modulation order parameter. It is shown that premartensitic and postmartensitic phase transitions related to the appearance of the modulated structure can occur along with martensitic transformations. The strain and modulation order parameters substantially affect the magnetic phase transitions via the interaction with the magnetic order parameter.  相似文献   

2.
Within the phenomenological model of the interacting parameters of magnetic and structural orders, magnetic and structural transitions in magnetocaloric alloys of the Mn1–x Cr x NiGe system are analyzed. Based on the calculated isobaric temperature dependences of the parameters of magnetic and structural orders, a magnetic susceptibility jump in the first-order structural transition region is predicted and confirmed experimentally; the change in the magnetic ordering type during the approach of magnetic and structural transitions is justified. The change in the phase transition type during the reverse change in the temperature and magnetic field, which is observed in a number of samples of the system under study, is explained. The efficiency of the use of the transitions induced by the magnetic field in magnetocaloric applications is analyzed.  相似文献   

3.
In Ni2+x Mn1?x Ga shape-memory ferromagnetic alloys with coincident magnetic and structural phase transitions, a reversible structural field-induced phase transition was observed at constant temperature and pressure in magnetic fields of about 10 T. Computational results are in qualitative agreement with experiment.  相似文献   

4.
The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni2+x Mn1−x Ga (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T M and magnetic T C (Curie point) phase transitions to converge. In the region where T C =T M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. Zh. éksp. Teor. Fiz. 115, 1740–1755 (May 1999)  相似文献   

5.
Zincblende-to-diamond-lattice structural phase transitions should occur in metastable (III–V)1?xIV2x alloys at a transition composition xc that is controllable by growth conditions. The effect of this transition should be visible both in the electronic as well as the vibrational properties of these alloys. For example, in the prototypical (III–V)1?xIV2x alloy, (GaAs)1?xGe2x, the observed anomalous V-shaped bowing of the direct gap is explained in terms of the phase transition, which occurs at the minimum of the “V.” Predictions are made for the band gaps of new metastable alloys, such as (GaSb)1?xSn2x. Consequences of this transition for (III–V)-IV superlattices are also discussed.  相似文献   

6.
The interaction of magnetic (Tc) an first-order distortion-type structural (TD) transitions in the CoxNi1?xMnGe system was investigated using an ac susceptibility method under hydrostatic pressure up to 1.3 GPa. For all compositions TC increases and TD decreases with pressure. At the first triple point (0.3–0.6 GPa depending on x) these transitions go through a simultaneous magnetic-structural transition, as observed by Anzai and Ozawa in NiMnGe. However, for the composition x = 5, at the second triple point (at 0.8 GPa) the magnetic transition separates from the structural one, with TC greater than TD. This appears to be the first example of physically different, first and second-order phase transitions merging at a particular value of an external parameter, i.e. pressure PTR1 and decoupling again at PTR2 > PT1. The (P, T) phase diagrams and the nature of the triple points are discussed using a simple Landau-type theory  相似文献   

7.
《Surface science》1991,255(3):L579-L584
Interrelation of surface an bulk order-disorder transitions in binary alloys AxB1−x is discussed within the framework of the Bragg-Williams approximation, using the fcc (100) and bcc (110) surfaces as an example. In an fcc alloy with a (100) surface, layers parallel to the surface undergo alternately second-order transitions. On the other hand, the bulk fee alloy reveals a first-order transition from the disordered phase to a mixture of the disordered and the AB3- (or A3B-) type ordered phases except for X = 0.5. The origin of the bulk first-order transition is discussed by taking into account order-disorder transitions in the surface region. For comparison, the bulk second-order transition in a bcc alloy is also discussed.  相似文献   

8.
Low field interband Faraday rotation and a.c. magnetic susceptibility are investigated in Hg1?kMnkTe and Hg1?k?xMnkTe semimagnetic semiconductors (k?0.5). The spin glass phase transition is experimentally manifested by characteristic cusps in χ(T) and kinks in θ(T). The transition temperatures obtained from both types of experiments are well correlated which enable us to determine the phase diagram for Hg1?kMnkTe system. The spin glass phase transition occurs at low temperature in quaternary alloys than in Hg1?kMnkTe alloys with identical Mn composition. This implies that the antiferromagnetic interaction between Mn2+ ions due to the virtual valence to conduction band transitions plays a significant role in the mechanism responsible for the spin glass formation.  相似文献   

9.
The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni43Mn46Sn11−xAlx (x=0-2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.  相似文献   

10.
The influence of the substitution of manganese by boron on the crystal structure and magnetic properties of Ni2Mn1−xBxGa Heusler alloys with 0?x?0.5 has been investigated using X-ray diffraction, thermal expansion, resistivity, and magnetization measurements. The samples with concentrations x<0.25 were found to be of single phase and belonged to the cubic L21 crystal structure at room temperature. Crystal cell parameters of the alloys decreased from 5.830 to 5.825 Å with increasing boron concentration (x) from 0 to 0.25. The alloys were ferromagnetically ordered at 5 K and the saturation magnetization decreased with increasing boron concentration. The ferromagnetic ordering and structural transition temperatures for 0?x?0.3 have been observed and the phase (xT) diagram of the Ni2Mn1−xBxGa system was constructed. The phase (xT) diagram indicates that the ground state of Ni2Mn1−xBxGa alloys belongs to ferromagnetic martensitic, premartensitic, and austenitic phases in x?0.12, 0.12<x?0.18, and 0.18<x?0.3, respectively. The relative influence of cell parameters and electron concentrations on the phase diagram is discussed.  相似文献   

11.
The H-T phase diagram of antiferromagnetic cobalt fluoride in an external magnetic field H perpendicular to the easy magnetization axis A is completed and used to construct a phase diagram in the variables H z and H y . In this diagram, the lines corresponding to second-order phase transitions (between an angular phase and a phase with antiferromagnetic vector IA) begin and end in fields of a spin-flip transition (i.e., in an exchange field). A peculiarity of these lines of phase transitions is that each of them has two tricritical points at which this line of second-order phase transitions transforms into a line of first-order phase transitions. A critical angle between the direction of the external magnetic field and the basal plane within which the first-order phase transition takes place is determined.  相似文献   

12.
The phase diagrams of a two-sublattice Ising metamagnet at finite temperature in a mixed longitudinal field and a transverse magnetic field are investigated by the use of an effective-field theory (EFT) with correlations. In addition to the second-order transition lines, the first-order transition lines are also presented in the phase diagrams, since the Gibbs free energy can be calculated numerically. The results show that there is no fourth-order critical line in the phase diagrams given by using EFT as found by using mean-field theory (MFT). The tricritical lines and their projection in the thx plane obtained by using EFT are also quite different from those by using MFT. Only one type of phase diagram is obtained by using EFT while three kinds of phase diagrams are obtained by using MFT, which indicates that only the first kind of phase diagrams obtained by using MFT is reliable. Furthermore, it is shown that the region of first-order transitions increases as the transverse magnetic field hx decreases.  相似文献   

13.
Temperature dependences of the permittivity and of the dielectric hysteresis loops in ceramic samples of nominally pure CdTiO3 and a Sr1?x CdxTiO3 solid solution were studied. At 76.5±0.5 K, CdTiO3 was established to undergo a ferroelectric phase transition close to the tricritical point. The temperature dependence of spontaneous polarization of CdTiO3 is described within the Landau theory of phase transitions with the critical order parameter exponent ≈0.25. The phase diagram of the Sr1?x CdxTiO3 solid solution was drawn in (T, x) coordinates, and the critical concentration x c =0.002, above which an induced polar state sets in the solid solution, was determined.  相似文献   

14.
The structure of Ni1 ? x Co x Cr2O4 solid solutions has been investigated by synchrotron X-ray powder diffraction in the temperature range of 90–350 K for compositions x = 0, 0.005, 0.01, 0.015, 0.02, 0.1, 0.5, and 0.8, and their structures have been refined using the Rietveld analysis. The T-x phase diagram for solid solutions in the studied range of temperatures and concentrations has been constructed based on the obtained data. The revealed structural phase transitions have been explained from the viewpoint of the crystal field theory and the Jahn-Teller cooperative effect. Distortions of polyhedra, which are the cause of the structural phase transition, have been evaluated. The applicability of the Landau phenomenological theory and the possibility of using the magnitude of spontaneous strains as the order parameter have been discussed.  相似文献   

15.
The T-x magnetic phase diagram of Mn1 ? x Fe x Si solid solutions is probed by magnetic susceptibility, magnetization and resistivity measurements. The boundary limiting phase with short-range magnetic order (analogue of the chiral liquid) is defined experimentally and described analytically within simple model accounting both classical and quantum magnetic fluctuations together with effects of disorder. It is shown that Mn1 ? x Fe x Si system undergoes a sequence of two quantum phase transitions. The first “underlying” quantum critical (QC) point x* ~ 0.11 corresponds to disappearance of the long-range magnetic order. This quantum phase transition is masked by short-range order phase, however, it manifests itself at finite temperatures by crossover between classical and quantum fluctuations, which is predicted and observed in the paramagnetic phase. The second QC point x c ~ 0.24 may have topological nature and corresponds to percolation threshold in the magnetic subsystem of Mn1 ? x Fe x Si. Above x c the short-range ordered phase is suppressed and magnetic subsystem becomes separated into spin clusters resulting in observation of the disorder-driven QC Griffiths-type phase characterized by an anomalously divergent magnetic susceptibility χ ~ 1/T ξ with the exponents ξ ~ 0.5–0.6.  相似文献   

16.
The magnetic structure and magnetic phase transitions of ordered (Fe1-xMnx)Pt alloys were investigated by neutron diffraction in the temperature range 4.2–900 K. A complete magnetic phase diagram is drawn up, where the regions for different magnetic states and the character of transitions between them are shown. The theoretical analysis of the magnetic phase stability is given.  相似文献   

17.
Magnetic and structural transitions of non-stoichiometric Ni50+xMn25−x/2Ga25−x/2 (x=2–5) alloys are systematically investigated. Differential scanning calorimetry and modified thermogravimetry (TG) are used to measure magnetic and structural transitions simultaneously. The structural transition temperatures increase monotonically with increasing Ni substitution for Mn and Ga. Different magnetic transition sequences on heating were observed from ferromagnetic martensite to ferromagnetic autensite, then to paramagnetic autensite, from ferromagnetic martensite to paramagnetic austensite or from ferromagnetic martensite to paramagnetic martensite, respectively. Three kinds of NiMnGa alloys were obtained according to the sequence of the structural and magnetic transition, whose structural transition temperatures are lower, equal to or higher than the magnetic transition temperatures.  相似文献   

18.
Lead zinc niobate-lead titanate[(1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3] (PZN-PT) crystals with x=4.5% and x=12% have been investigated using dielectric and Raman measurements over a range of temperatures. Above room temperature, dielectric measurements show that both compositions exhibit structural phase transitions according to the phase diagram proposed by Kuwata et al. [Ferroelectrics 387 (1981) 579]. Below room temperature, an anomaly at around 180 K for the x=12% sample is observed, suggesting another phase transition. Raman measurements are used to study all phase transitions.  相似文献   

19.
We performed Monte Carlo simulation of phase transitions from isotropic stripe phase with short-range order to long-range stripe phase in a model with competing ferromagnetic exchange and antiferromagnetic dipolar interactions on triangular lattice. We calculated phase diagram for different values of exchange and dipolar interaction constants ratio, η. We also determined the order of the transitions to stripe phases AFh of different stripe widths h: first-order phase transition was found to transitions into AF1 and AF2 phases, while transitions to AF3 and AF4 phases were of the second order. In the phase diagram the tricritical point was determined at the AF2 and AF3 phase boundary. We observed the peak of nematic phase at the transition region to the AF1 phase, but found it metastable at low values of η. We have also found that in AF1 phase spin relaxation corresponds to the Ising model dynamics. In phases AF3 and AF4 the dynamics slows down, and stripe domain growth with time is proportional to logt.  相似文献   

20.
The magnitudes of the temperature hysteresis and diffuse dielectric anomaly corresponding to the transition from the antiferroelectric phase to the ferroelectric phase in PbZr1 ? x TixO3 (0.03 ≤ x ≤ 0.05) ceramics can be reversibly changed by varying the temperatures of heating and cooling in the course of thermocycling. The results obtained indicate that the antiferroelectric-ferroelectric transition in the PbZr1 ? x TixO3 ceramics materials is a smeared first-order phase transition.  相似文献   

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