Novel magnetic behaviour of the system ZnxCa1−xFe2O4

New magnetic system Zn_xCa_1−xFe₂O₄ (x = 0.4 to 0.6) is reported having extremely unusual properties like small ordered moments ≈ 0.13 ∼ 0.23 μ_B, large ordering temperatures T_c ≈ 700–750 K and surprising lack of hyperfine field (HF) at Fe for temperatures greater than T¹(≈ 0.25 T_c) in spite of bulk magnetization in the system. The HF appears below T¹ implying evolution of local moments far below T_c. Sharp susceptibility maxima resembling those of spin-glasses are observed for x = 0.4 and 0.5, which may be attributed to the single domain-superparamagnetic (SD-SP) transition and the overall behaviour shows a cluster spin-glass type of ordering.     

Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

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.     

Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd³⁺ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa₂Cu₃O_6+x (123) and YBa₂Cu₄O₈ (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T _c = 90 K), theT ₁ behavior within 50 <T< 200 K reveals the [1 ? tanh²(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT ₁ is limited by the Korringa-like behaviorT ₁ T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T _c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T _c = 82 K), the Gd³⁺ relaxation rate below 60 K is found to obey a power lawT _n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT ₁ temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd³⁺ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.     

Mössbauer and magnetization studies of the U1−xNpxO2 fluorites

²³⁷Np Mössbauer effect and magnetic susceptibility measurements of the U_1?Np_xO₂ fluorite solid solution have been performed in the composition range 0.15 ? x ? 0.75. For x = 0.15 and 0.25, the Np ions order magnetically at a lower temperature T₀ than the bulk material (T_N) (T₀ ～ 19 K, T_N ～ 27 K for x = 0.15). For x = 0.50, T₀ ～ 10 K (T_N ～ 12 K from recent neutron diffraction measurements). For x = 0.75, T₀ ～ T_N ～ 9 K. The Np (induced) ordered moment is ～ 0.5 μ_B. The ²³⁷Np Mössbauer isomer shift shows that the Np ions are in a IV charge state.     

Electron spin-lattice relaxation in noncommon metals YBa2Cu3Ox and Rb1C60

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa₂Cu₃O_x, high-T_c material doped with 1% Gd, and Rb₁C₆₀, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd³⁺ longitudinal relaxation time T₁, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below T_c = 56 K. Atx = 6.95 (T_c = 90 K), the T₁ behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb₁C₆₀, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm^?1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.     

Complex competition between ferromagnetism and antiferromagnetism in the CMR manganites Pr1−x Ca x MnO3

The systematic investigation of the magnetic susceptibility of the CMR manganites Pr_1?x Ca _x MnO₃ versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, T _N = T _CO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x < temperature (T > 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Nuclear spin relaxation in itinerant electron antiferromagnetic Cr1−xVx system

Nuclear spin relaxation rate T^?1₁ for ⁵¹V in an incommensurate antiferromagnetic Cr_1?xV_x system has been measured in a temperature range between 1.3 and 4.2 K and in a range of magnetic field from 0 to 13.3 kOe by using a field-cycling nuclear magnetic resonance technique. In the (T₁T)^?1 vs x curve a pronounced maximum was observed near the critical concentration (x_c～0.040). Furthermore for alloys with x = 0.038 and 0.040 a deviation from the Korringa relation, T₁T = constant, was observed. The experimental results of (T₁T)^?1 are interpreted in terms of the spin-fluctuation and d-orbital contributions.     

Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

Relationship between the crystal structure and the reentrant superconducting properties of (Sn1-xErx)Er4Rh6Sn18

Single crystals of (Sn_1-xEr_x)Er₄Rh₆Sn₁₈ are superconducting (T_c = 1.3K) for x 0, are reetrant superconducting (T_c = 1.24K and T_M = 0.34K) for x ～.30 and undergo a singel magnetic transition (T_M=0.68K) for x～.75. Since the occupancy of the [Sn(1)_1-xEr(1)_x]Er(2)₄ sublattice is responsible for the variation of the low-temperature properties, one can make predictions as to new reentrant superconductors in the MRh_xSn_y series (M=RE). This appears to be the first system of reentrant superconductors where stoichiometry within a sublattice controls both magnetic ordering and superconductivity.     

Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

Magnetic hyperfine interaction and curie temperatures of amorphous (FexNi1−x)80P14B6

Amorphous magnetic glasses (Fe_xNi_1?x)₈₀P₁₄B₆ (0.1 ? x ? 1) have been studied by Mössbauer spectroscopy. At 4.2 K, well-defined and very similar spectra have been observed for samples with different Fe concentration. This suggests that the Fe hyperfine field distribution is insensitive to the amount of dilutant. The value of the average hyperfine field at 4.2 K increases with Fe concentration. A value of 232 kOe is obtained by extrapolating to zero Fe concentration. The relationship $\text{H}{}_{\mathrm{eff}}\text{(O) = (232 + 33}\text{μ}\text{)}\text{kOe}$, similar to that of crystalline Fe-Ni alloys describes the results. The values of T_c = 60 K, 234 K and 425 K have been determined for samples with x = 0.125, 0.25 and 0.375 respectively. The T_C vs. x relation suggests a critical concentration of x_c ? 0.1, above which ferromagnetism exists. By fitting the measured values of T_C to the results of a coherent potential approximation, exchange interaction temperatures of J_FeFe = 617 K, J_FeNi = 800 K and J_NiNi ? 0 K have been obtained.     

Evidence against reentrant magnetic behaviour in AuFe alloys

The temperature and concentration dependence of the transferred hyperfine field in Au_1−xFe_x with 0.15<x<0.28, was measured by perturbed angular correlation spectroscopy. The temperature dependence of the hyperfine field at the ¹¹¹Cd probe shows a slope change at T/T_c=0.25 for all concentrations below 20 at% Fe, but an almost continuous behaviour for higher concentrations. These results manifest a percolation-like magnetic behaviour but no magnetic double transition.     

Optical investigations of anisotropy of the conducting π-electron system in crystals of the organic superconductor (EDT-TTF)4[Hg3I8]1 − x

The polarized reflectance spectra of microcrystals of the new organic superconductor (EDT-TTF)₄[Hg₃I₈]_{1 ? x} (T _c = 8.1 K for x = 0.027; T _c = 7 K at 0.3 kbar for x = 0.019) have been investigated in the spectral region of 700–6000 cm^?1 (0.087–0.740 eV) at temperatures ranging from 10 to 300 K. A quantitative analysis of the spectra has been performed in the framework of the phenomenological Drude and Drude-Lorentz dispersion models, as well as in the framework of the theoretical “phase phonon” model that takes into account the interaction of free electrons with intramolecular vibrations. The effective mass of charge carriers m*, the width of the initial metallic π-electron band W, and the integral t of the electron transfer between the molecular π-orbitals of neighboring molecules have been determined. In the low-frequency range (700–1600 cm^?1), the vibrational structure associated with intramolecular vibrations of the EDT-TTF molecule has been revealed. It has been shown that the most intense feature (ω = 1330 cm^?1) of this structure is caused by the interaction of “quasi-free” electrons with intramolecular vibrations.     

Magnetization and Mössbauer study of double layer perovskites La1.5Ca1.5Mn2 − x Fe x O7 prepared by sol–gel method

Magnetization and Mössbauer studies have been made for understanding magnetic behavior of three double perovskite systems La_1.5Ca_1.5Mn_2???x Fe _x O₇ corresponding to x = 0.05, 0.10 and 0.50. These have been prepared following sol–gel route. Substitution of Fe does not lead to any major change in the tetragonal cell but increased iron leads to greater distortion in octahedral site. The three samples undergo paramagnetic–ferromagnetic transition. Curie temperature (T _c) for the system with 0.05 Fe is ～150 K which is lower than (190 K) for the system without iron; with 0.50 Fe T _c goes below 50 K. Iron goes as Fe^3?+? and replaces Mn in ab plane. With increasing Fe the valence states of Mn get re-distributed in a way that number of the Jahn–Teller ions Mn^3?+? increases and that of the pairs of Mn^3?+?–O–Mn^4?+? experiencing double exchange decreases.     

Phase transition studies in CoSiF6·6H2O and Co1−xZnxSiF6·6H2O by Mn(II) EPR

Phase transition studies on single crystals of CoSiF₆·6H₂O and Co_1−xZn_xSiF₆·6H₂O (x = 0.082, 0.182, 0.248) using Mn(II) EPR at X-band were carried out in the temperature range 10–300 K. Phase transitions with considerable thermal hysteresis have been detected in CoSiF₆·6H₂O and Co_1−xZn_xSiF₆·OH₂O (x = 0.082, 0.182). In CoSiF₆·6H₂O, the transition temperatures during cooling (T^c_c) and during heating (T^h_c) were found to be considerably less than those obtained previously from magnetic anisotropy experiments, i.e. 230 and 246 K as against 246 and 259 K. These characteristic temperatures for the dilute crystals having x = 0.082 and 0.182 were 205, 218 K and 175, 185 K, respectively. It is thus evident that magnetic dilution with zinc causes a progressive reduction in the transition temperatures as well as in the thermal hysteresis. The phase transition manifests itself in a large discontinuous increase in the Mn(II) zero-field splitting parameter (D), i.e. from −187 ± 1 G to −290 ± 1 G, in these crystals. The hyperfine coefficient (A) also increases discontinuously from 99 ± 1 G to 102 ± 1 G at T_c. In the dilute crystal with x = 0.248 a very slow continuous decrease in D and practically no change in A have been detected with a lowering of the temperature. A small temperature independent positive g-shift (g = 2.020 ± 0.005) has been observed in all the crystals studied. The spin-lattice relaxation time (T₁) of the Co(II) ion in CoSiF₆·6H₂O at different temperatures, both above and below T_c, has been estimated from the observed Mn(II) hyperfine linewidth. A discontinuous change in T₁ at T_c has been observed. Analysis of the temperature dependence of the linewidth has further revealed that a two-level Orbach process describes well the thermal behaviour of the spin-lattice relaxation of the Co(II) ions in these crystals. The location of the first excited level in CoSiF₆·6H₂O determined from such studies at temperatures both above and below T_c has been examined in the light of the ligand field energy scheme of the Co(II) ion in an octahedral field. It is concluded that the phase transition in this crystal is probably accompanied by a change in sign of the axial field parameter (Δ).     

Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T_c in RFeP_1−xAs_xO_0.90F_0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and T_c show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T² to T-linear, and T_c distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances T_c. On the other hand, we could not find any relation between the temperature-dependence of resistivity and T_c in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining T_c besides the antiferromagnetic correlation in this system.     

Re-entrant magnetic behaviour in amorphous Fe-rich Fe100−x Zr x alloys

We report results of⁵⁷Fe Mössbauer measurements on the magnetically concentrated re-entrant amorphous Fe₁₀₀?xZr _x alloys (8≤x≤12) which give convincing evidence for a microscopic origin of the low-temperature magnetization anomaly atT _f<T _c. This is shown by an anomalous increase in the magnetic hyperfine field and in the relative intensities of the (?m=0)-Mössbauer lines belowT _f upon cooling. No evidence was found for an antiferromagnetic phase. The shape of the hyperfine-field distribution atT=4.2 K in zero external field and in a field of 3 T is unchanged, indicating homogeneous behaviour of all Fe moments near saturation. Ferromagnetic order is not long-ranged, but determined by exchange-coupled magnetic clusters. The cluster moment nearT _c is found to decrease strongly with increasing Fe content and extrapolates to zero atx≈96.     

Spectral luminescence properties of Ca1 ? x Sr x F2:Ce3+ (0 < x <1) crystals

The luminescence, reflection, and luminescence excitation spectra of two-component Ca_{1 ? x} Sr _x F₂:Ce³⁺ (0.05 mol %) (x = 0.14, 0.25, 0.4, 0.6, and 0.75) have been studied at room temperature and T = 8 K. It is shown that the luminescence bands (upon 130-eV photon excitation) in the range of 200 to 400 nm are attributed to singlet and triplet self-trapped exciton luminescence and to 5d-4f transitions in Ce³⁺.