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Structure and its defects, magnetic resonance and magneto-transport properties of La0.6-xEuxSr0.3Mn1.1O3-δ magnetoresistive ceramics were investigated by x-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR and magnetoresistance methods. It was found that isovalent substitution of lanthanum by europium A-cation of a smaller ionic radius increases the structural imperfection and leads to a symmetry change from the rhombohedrally distorted perovskite structure of R3c symmetry to the pseudocubic type. It was shown that the real structure contains anionic and cationic vacancies, the concentrations of which increases with the Eu concentration and the sintering temperature tann. A decrease in the temperature of the metal-semiconductor Tms and ferromagnetic-paramagnetic TC phase transitions as well as an increase in the resistivity ρ and the activation energy Ea with increasing x are due to an increase in vacancy concentration, which weakens the high-frequency electron double exchange Mn3+ ↔ O2- ↔ Mn4+. The crystal structure of the compositions x = 0 and 0.1 contains nanostructured planar clusters, causing anomalous magnetic hysteresis at T = 77 K. Broad asymmetric 55Mn NMR spectra confirm high-frequency electron double exchange Mn3+(3d4) ↔ O2-(2p6) ↔ Mn4+(3d³) and indicate inhomogeneity of the manganese environment due to the surrounding ions and vacancies. The effective local fields of the hyperfine interaction HHF at 55Mn nuclei have been calculated by decomposing asymmetric NMR spectra into three Gaussian components. The constructed experimental phase diagram of the magnetic and conducting states of the La0.6-xEuxSr0.3Mn1.1O3-δ ceramics revealed strong correlation between the composition, structural imperfection, phase state, and magnetotransport properties of rare-earth manganites. [ABSTRACT FROM AUTHOR]

The results of magnetic measurements and x-ray analysis of the Mn0.89Cr0.11NiGe alloy are presented. It is shown that the temperatures of the first-order paramagnetic structural transition from hexagonal to orthorhombic phase and paramagnetic-ferromagnetic magnetic phase transition in the orthorhombic phase can be aligned by quenching the sample. This alignment results in the change of the magnetic phase transformation order: the initially isostructural phase transition of the second order becomes a magnetostructural transition of the first order (hexagonal paramagnet-orthorhombic ferromagnet). Moreover, the character of the low-temperature ferromagnetic-antiferromagnetic transition observed in magnetic fields below 3.5 T does not change substantially. The mechanisms of the giant anisotropic magnetostrictive (up to 10%) and magnetocaloric (up to 28 J/(kg·K) upon changing the magnetic field from 0 to 5 T) effects are discussed. [ABSTRACT FROM AUTHOR]

The results of investigations of the magnetic and magnetocaloric properties of alloys from the system Mn2-xFexP0.5As0.5 (0≼x≼0.5) are presented. The magnetization measurements are performed in the temperature interval 4.2–700 K in magnetic fields up to 8 T. The entropy changes ΔS with the magnetic field changing from 0 to 2, 4, 5, and 8 T are determined from the magnetization isotherms obtained near temperatures of the spontaneous appearance of the ferromagnetic state (TC,TAF–FM1), and the curves ΔS(T0) are constructed. It is found that TC and TAF–FM1 decrease monotonically with increasing manganese concentration and that the ferromagnetic phase is completely suppressed in Mn1.5Fe0.5P0.5As0.5. It is found that the concentration dependences of the maximum entropy jump (and the corresponding cold-storage capacity) and the magnitudes of the ferromagnetic moment of the unit cell with maxima for x=0.9 and 0.8 show extremal behavior. The data obtained are compared with the ferromagnetic moments calculated from first principles by the Korringa-Kohn-Rostoker method using the coherent-potential approximation (KKR-CPA)—the discrepancy for 0.5≼x≼0.7 is attributed to the appearance of an antiferromagnetic component of the magnetic structure. It is concluded that the alloys Mn2-xFexP0.5As0.5 have promise for use in magnetic refrigerators operating at room temperature. [ABSTRACT FROM AUTHOR]

The phase diagram of the system Mn2-xFexAs0.5P0.5 under pressure is investigated experimentally and theoretically. It is found that the spontaneous and magnetic-field-induced low-temperature phase in the region 0.5≼x<0.8 does not suffer significant changes under hydrostatic pressure up to 2 kbar. Based on ab initio calculations of the electronic structure of the alloys Mn1.5Fe0.5As0.5P0.5, it is established that the degree of filling of the 3d electron band changes upon ferromagnetic polarization and compression of the crystal lattice. A model is proposed by which one can take into account the main features of the antiferromagnetic and canted ferromagnetic structures. The parameters of the model are the degree of filling of the d band, the nonmagnetic electronic density of states, and the intra-atomic exchange integral. Their values are estimated directly from the data of the first-principles electronic structure calculations. It is shown in the framework of the model that the stability of the magnetic characteristics of the canted ferromagnetic structure with respect to pressure is due to an increase of the number of electrons in the magnetically active band upon a decrease of the unit cell volume. [ABSTRACT FROM AUTHOR]

The magnetic properties of the system Mn2-xZnxSb are investigated experimentally in static and pulsed magnetic fields. It is found that the spontaneous first-order transitions from the ferro=imagnetic phase Fi to the weakly ferrimagnetic phase If which are observed on decreasing temperature in samples with x=0.1 and 0.2 are accompanied by an almost twofold decrease of the magnetization and magnetostriction with no change of the easy-plane character of the anisotropy. Studies of the effect of a pulsed magnetic field in the temperature region of stability of the weakly ferrimagnetic phase reveal reversible magnetic-field-induced first-order phase transitions If↔Fi. Quenching of the samples from 570 K into liquid nitrogen enhances the stability of the weakly ferrimagnetic phase with respect to the influences of temperature and external magnetic field. This is manifested in an increase of the temperature of the spontaneous transition by 45 degrees and an increase in the critical field for its induction from 80 to 200 kOe. As the quenched sample is heated to room temperature, the initial characteristics of the weakly ferrimagnetic phase are gradually restored. The results are analyzed in a model of itinerant magnetism with the use of first-principles calculations of the electron density of states and total energy of the system. It is shown that the weakly ferrimagnetic phase with the doubled crystal-chemical cell can be described by the Hubbard model. The properties of the quenched samples can be explained by assuming the presence of two stable states of the system, separated by an energy barrier and associated with the possibility of a redistribution of the zinc ions over two crystallographically inequivalent lattice positions. [ABSTRACT FROM AUTHOR]

X-ray studies of TmFeO3 single crystals in the spontaneous spin-reorientation region demonstrate the absence of lattice distortions and no lowering of the symmetry of the crystal from rhombic to monoclinic and thus confirm the purely magnetic nature of the Γ2–Γ24–Γ4 spin reorientation in orthoferrites. [ABSTRACT FROM AUTHOR]

Spontaneous and magnetic-field-induced first-order transitions of the order-order type in Fe0.5Mnl.lAs and Fe0.55Mnl.04As single crystals with the C38 lattice (symmetry, space group P4/nmm) are investigated at hydrostatic pressures tip to 7 kbar in the temperature range from 4.2 to 300 K. The temperature and field dependences of the magnetization at different values of the pressure are measured in static and pulsed magnetic fields. It is established on the basis of the results that the first-order transitions from a low-magnetic to a high-magnetic please induced by magnetic field pulses at low temperatures are irreversible, but with a finite existence time of the high-magnetic phase. A qualitative explanation of the observed effect is proposed on the basis of the theory of magnetostrictive blocking of the formation of new- phase nuclei at first-order phase transitions. [ABSTRACT FROM AUTHOR]

The spontaneous and magnetic-field-induced first-order magnetic phase transitions in single-crystal samples of the alloy system Mn[sub 2-x]Cr[sub x]Sb (symmetry space group P4/nmm) are investigated in samples with x=0.06 and x=0.12. It is found that the spontaneous first-order transitions from the high-temperature to the low-temperature phase are not accompanied by complete vanishing of the magnetization. At the induced transitions the magnetization before and after the transition is an increasing linear function of field. It is concluded on the basis of the results that the ground state in these alloys cannot be one of collinear antiferromagnetism, as had been thought previously. Other possibilities for interpreting the ground state and the mechanisms of order–disorder transitions inherent to magnets with itinerant carriers of magnetism are examined. It is conjectured that the local magnetic moment has a ferromagnetic and a periodic component which are formed by itinerant d electrons and coexist in low- and high-temperature phases. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The crystal structure of the manganite Nd[sub 0.5]Sr[sub 0.5]MnO[sub 3] is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P2[sub 1]/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interrelationships between structural phase transformations and the magnetic characteristics of cobalt fluorosilicate hexahydrate are determined in the temperature interval 400-15 K under hydrostatic pressure up to 220 MPa. It is shown that the values of the magnetization and magnetic susceptibility in the different structural phases realized in P-T space are practically independent of the amount of compression but undergo jumps when the symmetry of the crystal lattice changes. The results obtained are used to determine the values of the g factors along two crystallographic directions for the phases studied. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The phase diagram of the system Mn2-xFexAs0.5P0.5 under pressure is investigated experimentally and theoretically. It is found that the spontaneous and magnetic-field-induced low-temperature phase in the region 0.5≼x<0.8 does not suffer significant changes under hydrostatic pressure up to 2 kbar. Based on ab initio calculations of the electronic structure of the alloys Mn1.5Fe0.5As0.5P0.5, it is established that the degree of filling of the 3d electron band changes upon ferromagnetic polarization and compression of the crystal lattice. A model is proposed by which one can take into account the main features of the antiferromagnetic and canted ferromagnetic structures. The parameters of the model are the degree of filling of the d band, the nonmagnetic electronic density of states, and the intra-atomic exchange integral. Their values are estimated directly from the data of the first-principles electronic structure calculations. It is shown in the framework of the model that the stability of the magnetic characteristics of the canted ferromagnetic structure with respect to pressure is due to an increase of the number of electrons in the magnetically active band upon a decrease of the unit cell volume. [ABSTRACT FROM AUTHOR]