Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration x_c ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Superconductivity and electronic specific heat in V-Mo system

The parameters of superconductivity and specific heat are determined from the low temperature specific heat measurements of V_(1−x)Mo_x (0 ⩽ x ⩽ 1) solid solutions. The coefficient of electronic specific heat γ decreases with increasing Mo concentration and shows a minimum around 80 at. %Mo. Debye temperature θ_D varies slightly over the whole composition range. The superconducting transition temperature T_c also decreases with increasing Mo concentration. The variation of T_c is explained by the variation of γ and discussed in terms of the band structure.     

57Fe Mössbauer spectroscopy and magnetization study of YFexAl12−x (4.4⩽x⩽5)

YFe_xAl_12−x in the composition range 4.4⩽x⩽5 was prepared by induction melting followed by annealing in vacuum at 1270 K. Magnetization data below 150 K show complex magnetic behaviour dependent on applied field, composition and temperature. The transition temperature T_c, corresponding to the main maximum of the magnetization vs. temperature curves and below which magnetic interactions are observed for a significant fraction of the Fe atoms in the Mössbauer spectra, decreases from 180 K for x=4 down to 100 K for 4.2⩽x⩽4.7 and rises again up to 160 K for x=5. The analysis of the spectra obtained at 5 K is consistent with full occupation of the 8f sites by Fe atoms and sharing of the 8j sites by Fe and Al as deduced from the Rietveld analysis of X-ray powder diffraction data. The Mössbauer spectra further show a dependence of magnetic hyperfine fields and isomer shifts on the crystallographic site and on the number of the Fe nearest neighbours similar to that observed in UFe_xAl_12−x (4⩽x⩽6) and RFe_xAl_12−x (R=Y, Lu, x=4, 4.2). The magnetic properties of the UFe_xAl_12−x and YFe_xAl_12−x series are compared and the magnetic interactions between the different Fe sublattices are discussed.     

Co-Si-B and Fe-Co-B amorphous alloys: Induced anisotropy and various magnetic properties

The dependence on the metalloid content of some magnetic properties of Co_100−x(Si_0.6B_0.4)_x (22.5 ⩽ x ⩽ 30) and Co₇₅Si_25−xB_x (10 ⩽ x ⩽ 25) amorphous alloys has been studied.Ribbons were subjected to different kinds of heating treatments: field annealing, stress annealing and stress-field annealing (tensile stress and longitudinal magnetic field applied simultaneously). While the anisotropies induced by simple field annealings are of the order of magnitude of 0.1 kJm^-3, the anisotropy induced by stress-field annealing can reach values up to 0.5 kJm^-3. The preferred axis is longitudinal for most of the annealing conditions. The temperature and composition dependence of the magnetostriction have been studied too.Stress, field and stress-field induced anisotropies have also been measured in Co₆₆Fe₉B₂₅ samples (λ_s > 0). In this case the preferred axis is transverse to the ribbon axis.     

Pressure effect on magnetic properties of NdMn2−xFexGe2 (0.1⩽x⩽1.0) series of solid solutions

Temperature and pressure dependence of magnetic properties in the NdMn_2−xFe_xGe₂ series of solid solutions (0.1⩽x⩽1.0) are reported. The (P, T) magnetic phase diagrams are determined on the basis of the AC magnetic susceptibility measured in a weak magnetic field. The measurements were carried out under hydrostatic pressure up to 1.5 GPa in the temperature range 80−430 K. The reported data show that in the studied series of solid solutions, a drastic change in magnetic properties takes place in a narrow dilution parameter range (0.4⩽x⩽0.5). While taking into account the magnetic properties, the studied range of Fe content could be divided into four regions. Only in the case of x=0.3 and 0.4, the external pressure significantly influences the magnetic properties of the samples.     

Octahedral vs tetrahedral coordination of the co(II) ion in layer compounds: CoxZn1−xIn2S4(O⩽x⩽0.46) solid solution

Large crystals of Co_xZn_1−xIn₂S₄ (0.00 ⩽ x ⩽ 0.46) solid solution were grown from the vapour phase by chemical transport modified by the time variation of the temperature profile procedure. Co²⁺ ions were found, by XPS analysis, to occupy tetrahedral as well as octahedral sites of the ZnIn₂S₄-type structure. The magnetic properties of Co_0.46Zn_0.54In₂S₄, crystallizing in space group R 3m, were explained in terms of the short-range magnetic order arising from the presence of isolated clusters of magnetic ions.     

X-ray diffraction and Mössbauer Studies of γ′-(Fe1−xSnx)4N compounds (0.0⩽x⩽0.3)

The structure and magnetic properties of γ′-(Fe_1−xSn_x)₄N samples have been studied using X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy. It has been shown that the single-phase (Fe_1−xSn_x)₄N compounds can be prepared in the composition range of 0.0⩽x⩽0.3, which have the similar structure as γ′-Fe₄N The lattice parameter with the increase of Sn concentration can be well fitted with two linear relationships a₀(x)=3.795+0.019x (with x⩽0.10 ) and a₀(x)=3.797+0.228(x−0.10) (with 0.1⩽x⩽ 0.3). The fitting results of Mössbauer spectra indicate that the hyperfine parameters have the same changing tendency with lattice parameter, and the Sn atoms have a preference to be located at the corner site.     

Near infrared magneto-absorption in the laser material KZn1−xCoxF3

Infrared absorption (1.4–2μm) of KZn_1−xCo_xF₃ (0⩽ x<0.15) at 4.2 K has been studied in a magnetic field (B⩽7 T). Large field-induced splittings were observed for Co²⁺-ion lines. Details of the cobalt concentration dependence of the absorptivity at B = 0 and at field are given along with the dependences on field direction and strength. Assignments are made of the observed Co²⁺ single-ion and pair transitions. The data has enabled the determination of the infrared (IR) transition g values.     

Intrinsic low field susceptibility in pseudobinary DyxY1−&gt;x&gt;Al2andTbxGd1−xAl2 cubic intermetallic compounds

Low field (H_ac ≈ 3.5 x 10^-6T) and low frequency (15 Hz) ac susceptibilities, χ_i, have been measured between 3.3 K and well above the Curie temperatures, for the intermetallic Laves phase compounds Dy_xY_1-xAl₂ and Tb_xGd_1-xAl₂. It is concluded that χ_i is, at low enough temperatures, dominated by the domain wall excursions around the minima of the anisotropy Peierls potential wells. The derived model relation χ_i(T)H_c⁽ⁱ⁾(T) = const., where H_c⁽ⁱ⁾ is the intrin sic pinning coercive field, is confirmed at low enough temperatures. The shape of the observed susceptibility cusps around the Curie temperatures in the series Dy_xY_1-xAl₂ (x 0.35) suggests that these systems are magnetically inhomogenous. The magnetic domain sizes estimated from these measurements seem quite small (between around 0.01 and 0.1 μm).     

Mössbauer study of an amorphous iron III flouride: FeF3, xHF (0 ⩽ x ⩽ 1)

A ferric amorphous flouride FeF₃, xHF with 0 ⩽ x ⩽ 1 synthesized by a soft chemistry reaction was studied by Mössbauer spectroscopy. The hyperfine characteristics are very close to those of deposited variety previously published: they show that the Fe³⁺ ions are surrounded by weakly distorted octahedra of flourine atoms. The study under external magnetic field indicates that the spins are randomly distributed: FeF₃, xHF has a speromagnetic behaviour like deposited amorphous FeF₃. Elsewhere the similarity between the two amorphous forms is confirmed by susceptibility measurements.     

A magnetic study of HoMn6Sn6−xGax single crystals (0.14⩽x⩽1.89)

Single crystals of hexagonal HfFe₆Ge₆-type HoMn₆Sn_6−xGa_x compounds (0.14⩽x⩽1.89) have been obtained by a flux method and studied by magnetisation measurements. All the compounds order ferrimagnetically (308⩽T_c⩽386 K) with moments lying in the (0 0 1) plane and undergo a moment reorientation transition at lower temperatures (156⩽T_SR⩽195 K). At 5 K, the moments are aligned along an intermediate direction (44⩽φ_c⩽50°). These results are discussed and compared with the neutron diffraction results related to the isotypic TmMn₆Sn_6−xGa_x and TbMn₆Sn_6−xGa_x series where a change of the easy direction is observed with increasing gallium contents.     

High-pressure synthesis of superconducting Nb1−xB2 (x=0–0.48) with the maximum Tc=9.2 K

Superconductivity with T_c above 9 K was found in metal-deficient NbB₂ prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB₂. The superconductivity was observed above x=0.04 in Nb_1−xB₂, and the lattice parameters also changed abruptly at x=0.04. As x increased, the transition temperature T_c slightly rose and fell with the maximum value of 9.2 K at x=0.24 for the samples sintered at 5 GPa and 1200 °C. The T_c-value changed in the range from 7 to 9 K, depending on the sintering pressure. A series of Ta_1−xB₂ (0⩽x⩽0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.     

Weakferromagnetism in FexNi1−xS2 (x = 0−0.075)

The effects of nonmagnetic impurities (Fe²⁺ in a low spin state) on the weakferromagnetism in NiS₂ were investigated. The weak-ferromagnetic moment decreases rapidly with Fe content (x) and almost vanishes at about x = 0.05. On the other hand, T_c remains quite constant. Above T_c, two anomalies in the temperature dependence of the susceptibility were observed except for samples with x = 0 and 0.006. The effective magnetic determined from the susceptibility above T_c is about 3 μ_B/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve.     

23Na nuclear magnetic resonance investigation of the lattice distortion in α-NaxV2O5

The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Na_{x 2}O₅ (x = 0.996). Above the spin-Peierls transition (T _c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below T_c, eight distinct quadrupolar²³Na sites are observed according to the distortion wave vector k_c = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift²³K at T_c. The results are discussed in the context of a charge-order-driven spin-Peierls transition.     

Quasi two-dimensional magnetic order of Tb3+ spins in Pb2Sr2Tb1−x Ca x Cu3O8 (x = 0 and 0.5)

Neutron diffraction experiments were performed to study the magnetic ordering of the Tb⁺³ sublattice in the high-T _c superconductor Pb₂Sr₂Tb_1?x Ca _x Cu₃O₈ (x = 0.5) and the undoped parent compound (x = 0). For the parent compound, a quasi two dimensional (2D) phase with a finite antiferromagnetic correlation along the c-direction and a three dimensional phase with ferromagnetic correlation along the c-direction were found. The coexistence of the two phases is likely to be related to structural imperfections such as stacking faults, strains, oxygen disorder or cation vacancies. The superconductor with a superconducting transition temperature of T _c = 71K exhibits a quasi 2D magnetic ordering of the Tb sublattice with a finite ferromagnetic correlation along the c-direction. The correlation lengths along the c-direction for the quasi 2D phases are 32 and 26Å for x = 0 and 0.5, respectively. The magnetic saturation moments are with 9.1 and 8.8 μ _B in excellent agreement with our mean-field crystal-field calculations. 2D short range correlation could be observed up to about 8T _N and is described by a Lorentzian distribution of magnetic intensity perpendicular to the 2D rod in reciprocal space.     

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.     

Magnetic characteristic of the intermetallic system Tb2Ni17-xAlx

The magnetic behaviour of the intermetallic compounds Tb₂Ni_17-xAl_x (0⩽x⩽17) has been investigated in the ferrimagnetic and paramagnetic regions. The thermal variation of the magnetization may be interpreted as a second-order phase transition of order-order type, i.e. from collinear to non-collinear ferrimagnetic structure.     

Photoluminescence of semimagnetic semiconductor Hg1-xMnxTe

Photoluminescence (PL) of Hg_1−xMn_xTe for 0.12 ⩽ x ⩽ 0.13 corresponding E₀ ≃ 0.27 eV was measured at 4.2 K vs magnetic field up to 8.3 T. The spectral dependence of PL shows two maxima connected with band-band and band-acceptor recombination processes. Magnetic fields cause the shift of both maxima to lower energies. The intensity of the PL increases over 50 times for band-acceptor transitions as function of magnetic field, reaches a maximum at 6T than decreases. The results are explained on the base of band structure and acceptor behaviour in magnetic field characteristic for semimagnetic semiconductors.     

Magnetic-Field-Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs2CuCl4-xBrx Mixed System

The triangular antiferromagnetic Cs₂CuCl_4-xBr_x mixed system is studied by neutron single-crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field B_c is found for a Br concentration x = 0.8 at B_c = 8.10(1) T and for x = 1.1 at B_c = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at B_c = 0.99(3) T. For magnetic fields larger than the respective B_c, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature T_N = 0.51(1) K for Cs₂CuCl_3.2Br_0.8, and q = (0, 0.418, 0) below T_N = 1.00(6) K for Cs₂CuCl_0.3Br_3.7. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at B_c = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase.     

Effect of Ti4+ ions on the magnetic and dielectric properties of Mg-ferrite

X-ray diffraction, the real part (ε′), the imaginary part (ε″) of dielectric constant, and the molar magnetic susceptibility (χ_M) for Mg_1+xTi_xFe_2−2xO₄ ferrite (0.1⩽x⩽0.9) were studied. The date of X-ray diffraction showed that the unit cell parameter increases with Ti concentration and ascribed to the predicted variation of the cation distribution, while Mg²⁺ ions are highly diffusible and very sensitive to heat. The effect of dilution by Ti ions is discussed in terms of increasing superparamagnetic and single domain (SP/SD) grains. The measurements of ε′ were performed at different temperatures as a function of frequency, while the magnetic susceptibility was studied at different magnetic field intensities. The variation of the dielectric properties depends mainly on the valence exchange between the different metal ions in the same site or in different sites. All parameters such as ε′, ε″, χ_M showed a decrease in value with increasing Ti and Mg concentration. The dispersion in ε′ with frequency disappeared gradually with increasing Ti concentration.