Microscopic magnetic properties of (V1−xTix)2O3 near the phase boundary of the metal–insulator transition

Magnetic susceptibility (χ) and ⁵¹V NMR have been measured in (V_1−xTi_x)₂O₃ near the phase boundary of the metal–insulator transition. It is established that the transition from antiferromagnetic insulating (AFI) to antiferromagnetic metallic phases near x_c≈0.05 is not quantum critical, but is discontinuous with a jump of the transition temperature. In the AFI phase at 4.2 K, we observed the satellite in the zero-field ⁵¹V NMR spectrum around 181 MHz in addition to the ‘host’ resonance around 203 MHz. The satellite is also observable in the paramagnetic metallic phase of the x=0.055 sample. We associated the satellite with the V sites near Ti, which are in the V³⁺-like oxidation state, but has different temperature dependence of the NMR shift from that of the host V site. The host d-spin susceptibility for x=0.055 decreases below ∼60 K, but remains finite in the low-temperature limit.     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000     

伊辛模型的四分支临界面及其临界行为

本文通过等标度同普适类变换理论,使严格的重正化群递推方程得以直接应用。对伊辛模型的计算产生了四个不动点、四支临界面。对应于铁磁及反铁磁相变,其结果与已知的严格解及最好近似解一致。对于超反铁磁相变,本文提出临界曲面方程为—ch(h)=sh(2K_x)sh(2K_y)。 关键词：     

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.     

Electronic and magnetic structure of LaCuO2:5

La₁-_χxSr_χCuO_2:5 is composed of coupled cuprate ladders, and admits hole doping without structural alteration. It is thus a candidate novel superconductor, although to date superconductivity has not been found. Here we study the undoped, parent state to elucidate the interplay between spin liquid and antiferromagnetic ground states in systems with potential for superconductivity     

Magnetic phase transitions and magnetocaloric effect in the Fe-doped MnNiGe alloys

The magnetic phase transition and magnetocaloric effects in Fe-doped MnNiGe alloys are investigated. The substitution of Fe for Ni decreases the structural transition temperature remarkably, resulting in the magnetostructural transition occurring between antiferromagnetic and ferromagnetic states in MnNi_{1 - x}Fe_xGe alloy. Owing to the enhanced ferromagnetic coupling induced by the substitution of Fe, metamagnetic behaviour is also observed in TiNiSi-type phase of MnNi_{1 - x}Fe_xGe alloys at temperature below the structural transition temperature.     

Magnetic properties of the CoxNi1−xMnGe system

Magnetic properties of the Co_xNi_1?xMnGe system have been studied by magnetometric measurements. It was shown that the solid solutions exist for 0 ? χ ? 1, with the crystal structure of NiTiSi type. The samples with the low Co concentrations (0 ? χ ? 0.2) have antiferromagnetic properties. At higher concentrations (0.3 ? χ ? 0.5) the samples exhibit a more complex behaviour, at low temperatures having antiferromagnetic properties. At growing temperatures the change of the magnetic ordering is observed. The samples with large Co concentration (0.6 ? χ ? 1.0) are ferromagnets.     

Mössbauer study of spin-glass Fe x Zn1−x F2 system

⁵⁷Fe Mössbauer effect measurements in the diluted Ising antiferromagnet Fe _x Zn_1?x F₂ withx=0.25 andx=0.57 at temperatures between 4.2 and 55 K, are reported. DC suceptibility measurements show a spin-glass (SG) phase at low temperatures forx≤0.31. Our Mössbauer spectra show a phase transition to a SG state with antiferromagnetic order (AFSG) forx=0.25 and only antiferromagnetic order forx=0.57.     

d = 3 anisotropic and d = 2 tJ models: phase diagrams, thermodynamic properties, and chemical potential shift

The anisotropic d=3 tJ model is studied by renormalization-group theory, yielding the evolution of the system as interplane coupling is varied from the isotropic three-dimensional to quasi-two-dimensional regimes. Finite-temperature phase diagrams, chemical potential shifts, and in-plane and interplane kinetic energies and antiferromagnetic correlations are calculated for the entire range of electron densities. We find that the novel τ phase, seen in earlier studies of the isotropic d=3 tJ model, persists even for strong anisotropy. While the τ phase appears at low temperatures at 30–35% hole doping away from 〈 n_i〉=1, at smaller hole dopings we see a complex lamellar structure of antiferromagnetic and disordered regions, with a suppressed chemical potential shift, a possible marker of incommensurate ordering in the form of microscopic stripes. An investigation of the renormalization-group flows for the isotropic two-dimensional tJ model also shows a clear pre-signature of the τ phase, which in fact appears with finite transition temperatures upon addition of the smallest interplane coupling.     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Lattice dynamics in mixed valent Rare Earth compounds

In the frame of a single coherent picture for the electron lattice interactions in mixed valent Rare Earth compounds, the phase diagrams and phonon dispersions are studied. Our model is introduced on a microscopic level and discussed in detail. The mechanism of the phase transition and the role of the lattice is illuminated and various properties are considered in a model calculation. Anomalies in the phonon dispersions of Sm_1–x Y _x S, which have recently been found, find a satisfying explanation.     

Specific features of magnetoresistance during the antiferromagnet—paramagnet transition in Tm1 − x Yb x B12

The transverse magnetoresistance Δρ/ρ(H, T) of Tm_{1 ? x} Yb _x B₁₂ single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions with x ≤ 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -Δρ/ρ ∝ H ₂, the magnetically ordered phase of these compounds is found to have component Δρ/ρ ∝ H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm_{1 ? x} Yb _x B₁₂ is analyzed in terms of the Yosida model for a local magnetic susceptibility.     

Ferromagnetic transition in ordered double perovskites and related alloys

We study the electronic and magnetic properties of ordered perovskites Sr ₂FeMO₆ (where M is a transition metal), among which some compounds, like Sr₂FeMoO₆ are half-metallic with high Curie temperature while Sr₂FeWO₆ is an antiferromagnetic insulator. Using a double exchange type model with an interaction between localized spins and conduction electrons together with a tight-binding Hamiltonian and the renormalized perturbation expansion method, we study the behavior of T_c as a function of the number of conduction electrons and also as a function of the Fe-M energy difference. We also consider the Sr₂FeMo_xW_1-xO₆ compounds which present a magnetic and metal-insulator transition as a function of doping.     

Effect of Si/Ge ratio on resistivity and thermopower in Gd5SixGe4−x magnetocaloric compounds

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd₅Si_xGe_4−x compounds with x=1.7-2.3. Microstructural studies reveal the presence of Gd₅(Si,Ge)₄-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8 μV/K was obtained at the magneto-structural transition for the x=2 compound.     

Magnetic properties of the Nd0.9MnOx cation-deficient manganites

This paper reports on the results of x-ray diffraction and magnetic studies of manganites in the Nd_0.9MnO_x system with an oxygen content varying in the range 2.84 < x < 2.93. A sample with an oxygen content x = 2.84 undergoes a first-order phase transition at a temperature close to T = 1050 K, whereas an anomaly observed in the elastic properties for a sample with x = 2.93 indicates a phase transition near T ≈ 500 K. It is assumed that these transitions are governed by cooperative ordering of Mn e _g orbitals of the same type as in stoichiometric NdMnO₃. The manganite at an oxygen content x = 2.85 is an antiferromagnet with a Néel temperature T _N = 85 K, whereas the magnetic properties of the manganites at x = 2.90 and 2.93 suggest that an antiferromagnetic component coexists with a ferromagnetic component. The magnetic interaction between the ferromagnetic and antiferromagnetic components manifests itself in the fact that the magnetic moment becomes opposite in direction to the external magnetic field. The properties of the samples are consistent with the hypothesis that part of the neodymium ions (up to 5%) can be substituted for manganese ions.     

Spin glass phase transition in Hg1−kMnkTe semimagnetic semiconductors

Low field interband Faraday rotation and a.c. magnetic susceptibility are investigated in Hg_1?kMn_kTe and Hg_1?k?xMn_kTe 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 Hg_1?kMn_kTe system. The spin glass phase transition occurs at low temperature in quaternary alloys than in Hg_1?kMn_kTe 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.     

An electron paramagnetic resonance study of phase segregation in Nd0.5Sr0.5MnO3

We present results of an electron paramagnetic resonance (EPR) study of Nd_1−xSr_xMnO₃ with x=0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T_c) and the antiferromagnetic, charge ordering transition (T_N=T_co) at 150 K. The results are compared with those on Nd_0.45Sr_0.55MnO₃ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T_N=230 K and on La_0.77Ca_0.23MnO₃ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x=0.5, the EPR signals below T_c consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x=0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La_0.77Ca_0.23MnO₃. The comparison of behaviour of A-type AFM signal observed in both Nd_0.5Sr_0.5MnO₃ and Nd_0.45Sr_0.55MnO₃ with the two FM phases of La_0.77Ca_0.23MnO₃, vis-à-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd_0.5Sr_0.5MnO₃ undergoes phase separation into A-type AFM and FM phases.     

The low temperature magnetic phase diagram of EuxSr1−xTiO3

Specific heat and magnetization measurements demonstrate that the antiferromagnetic (AFM) phase transition at T _N  = 5.7 K of EuTiO₃ is rapidly suppressed with Sr doping in Eu _x Sr_1?x TiO₃. Close to x = 0.25, T _N  = 0 K and AFM order vanishes. Above this critical concentration a finite transition temperature to an AFM phase is observed. The exchange couplings are derived as a function of x and the corresponding low temperature phase diagram is presented.     

Exchange-inversion interaction and acoustical magnon soft mode in Mn2-xCrxSb

A magnon soft mode in Mn_2-xCr_xSb in the vicinity of the Brillouin zone boundary in c¹ direction, for the first order antiferro-ferrimagnetic phase transition, and the magnetization curve of the antiferromagnetic phase were measured. It was found that the soft mode energy squared is a linear function of temperature over a wide temperature range below and above the phase transition point.