Phase transitions in ferromagnetic Ni2+x Mn1−x Ga alloys with regard for the modulation order parameter

The phase diagram of ferromagnetic alloys Ni_2+x Mn_1?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.     

The magnetic phase transitions and magnetocaloric effect in MnNi1−xCoxGe alloys

The magnetic phase transitions and the magnetocaloric effects in MnNi_1−xCo_xGe (x=0.38 and 0.40) alloys were investigated. The substitution of Co for Ni in the MnNiGe antiferromagnet results in the metamagnetic transitions from antiferromagnetic to ferromagnetic state, which associates with very small thermal and magnetic hystereses. Positive and negative values of magnetic entropy changes are exhibited around the metamagnetic transition temperature and Curie temperature, respectively. The relatively large refrigerant capacity in low magnetic field along with the good reversibility suggest that MnNi_1−xCo_xGe (x=0.38 and 0.40) alloys are potential candidates for magnetic refrigeration.     

Phase transformations in perovskites TbBaCo2−x FexO5 + γ

The transport, magnetic, and elastic properties of TbBaCo_2?x Fe_xO_{5 + γ} are investigated. It is shown that these compounds exhibit first-order metal-insulator and antiferromagnet-weak ferromagnet transitions in the orthorhombic phase (x < 0.12), while these transitions are not observed in the tetragonal phase (x > 0.12). In the concentration range corresponding to the orthorhombic phase, doping with iron stabilizes the weakly ferromagnetic phase. However, the tetragonal phase is antiferromagnetic. Oxygen vacancies are assumed to be ordered in the orthorhombic case and disordered in the tetragonal phase. An analysis of Young’s modulus, magnetostriction, and effects of pressure and substitution of the O¹⁸ oxygen isotopes for O¹⁶ indicates a weak correlation between magnetic transformations and the crystal lattice.     

La(Fe,Si)13-based magnetic refrigerants obtained by novel processing routes

The structure and magnetic properties of LaFe_13−xSi_x and Co-substituted LaFe_11.8−xCo_xSi_1.2 alloys prepared by melt spinning, as well as of LaFe_11.57Si_1.43H_x hydrides prepared by reactive milling are investigated. The hysteresis in the temperature- and field-induced phase transitions is significantly reduced as compared with conventional bulk alloys, which makes these materials very attractive for magnetic refrigerant applications. The unusual combination of features characteristic of first- and second-order phase transitions in the La(Fe,Si)₁₃-based compounds is discussed on the basis of density-functional electronic structure calculations.     

Quantum bicriticality in Mn1 − x Fe x Si solid solutions: Exchange and percolation effects

The T-x magnetic phase diagram of Mn_{1 ? 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 Mn_{1 ? 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 Mn_{1 ? 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.     

Low-temperature heat capacity of ErxY1−xRh4B4

Resistivity and specific heat data have been used to establish the presence of antiferromagnetic transitions in the Er_xY_1−xRh₄B₄ system for concentrations x=0.1 and 0.3. The relationship of these transitions to the general magnetic and superconducting phase diagram for this pseudoternary system is discussed.     

Acoustic anomalies near phase transitions in manganite

The behavior of the velocities and attenuation of ultrasonic waves propagating in La_1?x Sr_xMnO₃ (x=0.175) manganite at frequencies of 700–800 MHz is studied in the temperature interval from 320 to 180 K, and the effect of magnetic field on the acoustic parameters is investigated. The transformation of acoustic modes in the vicinity of the magnetic phase transition is observed. The changes in the acoustic parameters near the structural and magnetic phase transitions are shown to be related to the strong spin-phonon and electron-phonon interactions.     

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni43Mn46Sn11−xAlx alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni₄₃Mn₄₆Sn_11−xAl_x (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.     

Reversible structural phase transition in Ni-Mn-Ga alloys in a magnetic field

In Ni_2+x Mn_1?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.     

Magnetic phase transitions in PrMn2−xCrxGe2

The structural and magnetic properties of PrMn_2−xCr_xGe₂ (0⩽x⩽1.0) were studied by X-ray diffraction and magnetization measurements. The powder samples crystallize in the ThCr₂Si₂-type structure, and the lattice constants at room temperature show almost no variation as Cr substitutes Mn. The observed phase transitions are summarized in a proposed magnetic x−T phase diagram and compared with previous Moessbauer spectroscopy and neutron diffraction results for x=0.     

Structural and magnetic properties of Ga-substituted Co2−W hexaferrites

Precursor powders of BaCo₂Fe_16-xGa_xO₂₇ with 0.0 ≤ x ≤ 0.8 were prepared using high-energy ball milling, and the effects of chemical composition on the structural and magnetic properties of the powders sintered at 1300 °C were investigated using x-ray diffractometer (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). XRD patterns of all samples indicated crystallization of pure BaCo₂−W (BaCo₂Fe₁₆O₂₇) hexaferrite phase. SEM measurements revealed large step-like formations with hexagonal crystallites. The magnetic data revealed small fluctuations of the saturation magnetization below the value 72.56 emu/g corresponding to the unsubstituted sample. The coercive field H_c of all samples ranged between 70 Oe and 130 Oe, indicating soft magnetic phase. Curie temperature determined from the thermomagnetic curves of the samples decreased from 485 °C at x = 0.0 down to 451 °C at x = 0.6. Also, the thermomagnetic curves revealed the presence of a minority magnetic phase with enhanced superexchange interaction, and the occurrence of complex magnetic phase transitions associated with spin reorientation transitions above room temperature.     

The influence of magnetoelastic interaction on structural phase transitions in cubic ferromagnetics

In the framework of the Landau theory of phase transitions, the influence of the magnetoelastic interaction on structural transitions in cubic ferromagnetics with a positive first magnetic anisotropy constant is analyzed. It is shown that structural transitions are not accompanied by a reorientation of magnetization in this case. The phase diagrams of such ferromagnetics either contain a termination point of the structural transition or a critical point in which the first-order transition is replaced by a second-order one. Magnetoelastic interaction also leads to the appearance of an interval of the ferromagnetic parameters in which a coupled first-order structural-magnetic transition exists. The phase T?x diagram for Heusler Ni_2+x Mn_1?x Ga alloys is calculated, which is in good agreement with the experimental phase diagram of these alloys.     

Phase transitions induced by a strong magnetic field in electron-doped manganites

The magnetic and magnetoelastic properties of single crystals of electron-doped rare-earth manganites La_1?x Sr _x MnO₃ are studied. Phase transitions from the A-type antiferromagnetic phase to the C-type anti-ferromagnetic phase in a strong magnetic field are revealed in La_1?x Sr _x MnO₃ manganites with a strontium content x = 0.65. A similar phase transition is observed in manganites with a strontium content x = 0.8, at which the La_0.2Sr_0.8MnO₃ manganite is assumed to transform from the C-type antiferromagnetic phase to the G-type antiferromagnetic phase.     

Spin-reorientation transitions and domain structure in TbFe<Subscript>11−<Emphasis Type="Italic">x</Emphasis></Subscript> Co<Subscript>x</Subscript>Ti single crystals

The magnetic structure of single-crystal TbFe_11?xCo_xTi compounds has been studied over a broad temperature range and in strong magnetic fields (up to 14 T). Measurements of magnetization and magnetostriction and a study of the domain structure revealed that spin-reorientation transitions (SRTs) in TbFe_11?xCo_xTi single crystals depend substantially on the cobalt concentration. It was established that the SRT temperatures and threshold magnetic fields are governed by the interplay between the magnetic anisotropies of the 3d and terbium sublattices. It is shown that, in these compounds, the low-temperature phase with planar anisotropy is separated in temperature from the high-temperature phase with uniaxial anisotropy by an intermediate metastable phase containing domains of the uniaxial or planar phase.     

Transition from antiferromagnetic to ferromagnetic state of systems LaMnO3+λ and La1−x Srx(Mn1−x/2Nb x/2)O3

The crystal structure and magnetic and elastic properties of the system LaMnO_3+λ are investigated for various concentrations of oxygen. Upon an increase in the oxygen concentration, the orbital-ordered phase is transformed into an orbital-disordered phase via a two-phase crystal-structure state in the interval 0.04<λ<0.06. The transition is accompanied by a jumplike increase in the Curie temperature and spontaneous magnetization. An analysis of the magnetic properties in weak fields and of the temperature dependence of the Young modulus reveals the properties typical of the orbital-ordered antiferromagnetic phase up to λ=0.08. It is proposed that the two-phase state is associated with the martensite type of the orbital order-disorder phase transformation. The system La_1?x Sr_x(Mn_{1 ?x/2}Nb _x/2)O₃ in which all manganese ions are in the trivalent state exhibits a sequence of antiferromagnetic-ferromagnetic (x>0.2) and ferromagnetic-spin glass (x>0.4) transitions. In both systems, the orbital-disordered phases are ferromagnetic, indicating the crucial role of orbital ordering in the formation of magnetic properties.     

Structural features of the ferromagnetic order formation in the Mn1<Subscript>–<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NiGe system

Within the phenomenological model of the interacting parameters of magnetic and structural orders, magnetic and structural transitions in magnetocaloric alloys of the Mn_1–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.     

Magnetic properties,phase transitions and microstructural effects in mixed GdLa-based glasses

Magnetic properties of amorphous Gd_xLa_65-xCo₂₅B₁₀ are investigated from 4.2 to 300 K. AC susceptibility and magnetization measurements to 80 kOe are used to determine a phase diagram including paramagnetic, ferrimagnetic and spin-glass regions. For x ≲ 40 diffraction and susceptibility data indicate the increasing development of chemical short-range order and phase separation. An analysis of the magnetic structure and local-moment magnitudes is presented. Results of a scaling analysis for one paramagnetic-ferrimagnetic transition are discussed and some conclusions are drawn concerning the relationship between phase transitions and chemical short-range-order in ternary or higher-order magnetic glasses.     

X-ray,Mössbauer,and magnetic research of system Y(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript> alloys

The structural transformations and magnetic phase transitions in the quasibinary system Y(Fe_{1 ? x} Al _x )₂ have been investigated by Mössbauer spectroscopy, X-ray diffractometry, and magnetic measurements of polycrystals.     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.