Incommensurate antiferromagnetic structures in NdIn3

Among the AuCu₃-type RIn₃ series where R is a rare earth, the NdIn₃ compound presents a complex magnetic phase diagram. In this compound, which orders antiferromagnetically at T_N = 5.9 K, three magnetic phases are separated by two sharp first-order transitions. The phase diagrams, determined for the main crystallographic directions, by magnetization measurements in fields up to 7 T show a strong dependence of the transition temperatures. In order to determine the actual magnetic structures in NdIn₃, a neutron diffraction experiment has been performed on a single crystal under magnetic field. It shows that the magnetic structures are collinear (single q) with the magnetic moments aligned along the fourfold axis. They clearly evidence the existence of two incommensurate phases with q = (1/2, 1/2, τ): for T_N T 5.5 K, τ₁ ≈ 0.037 2π/a and the structure is sine-wave modulated, for 5.5 K > T > 4.7 K, τ₂ = 0.017 2π/a and the structure squares up. Below 4.7 K the q = (1/2, 1/2, 0) commensurate structure is stabilized.     

Anomalous magnetic behavior of the Co0.53Ga0.47 spin glass above the freezing temperature

We present here the detailed analysis of the magnetic behavior of the Co_0.53Ga_0.47 alloy, especially at temperatures above the freezing temperature T_f = 10 K. Low field static magnetization measurements were performed by using the SQUID magnetometer in the temperature range 5–65 K and magnetic fields up to 100 Oe. The temperature dependence of the field cooled susceptibility π_FC(T) at T > T_f has an anomaly, which is displayed in the double change of the curvature near T_s = 24 K. The data of magnetization M_FC in an external field H lie on a universal curve M_FC(H/T) at temperatures T_f < T < T_s. The plots of π^-1_FC(T) and non-linear magnetic susceptibility π^nl_FC(T^-3) are linear lines in the temperature range T_f−T_s. The strong deviation of π^-1_FC(T) and π^nl_FC(T^-3) from straight line, taking place at T T_s, indicates that T_s is an upper temperature limit of the classical superparamagnetic behavior with the constant cluster moment. The results suggest that such phenomena may be fairly universal for spin glasses.     

Incommensurate magnetic structure of CeSi as observed by neutron diffraction

The magnetic ordering of the orthorhombic compound CeSi (Pama space group, FeB type of structure, Z = 4) T_n = 5.6 K was investigated by means of powder neutron diffraction in the temperatures 1.5 and 10 K. Our results show that the magnetic ordering is associated with a three dimensionally amplitude-modulated phase which is incommensurate with the crystal lattice with wave vector q = (q_χ,q_y,q_z). The ordered Ce³⁺ moments are confined to the plane (a,b) at an angle of 16.7° with the b direction. It follows from the present data analysis that the dominant wave vector component is q_z, which approximately corresponds to a transversal modulation, however the incommensurateness involves also the q_x and q_y components. The amplitude of the sine wave is μ⁰_j = 1.667μ_B at 1.5 K which corresponds to quite a reduced ordered moment value compared to the Ce³⁺ free ion value gJ = 2.14μ_B.     

Effect of a magnetic field on MnAs0.88P0.12 below 80 K

MnAs_0.88P_0.12 has been studied by powder neutron diffraction in external magnetic fields up to 15.2 kOe and temperatures down to 4.2 K. MnAs_0.88P_0.12 takes the MnP type atomic arrangement and exists in para-, ferro- and different (essentially) helimagnetic states. The observation of a double 000^± satellite at 4.2 K < T 70 K adds further evidence to the chain of arguments for distinction between the helimagnetic states H'_a (4.2 K < T < T_S,1 ≈ 70 K) and H_a (T_S,2 ≈ 180 K < T < T_N = (243 ± 5) K). External magnetic fields at 4.2 K < T < 70 K evoke a new magnetic state, which is also characterized by a satellite doublet, and is tentatively designated H'_a,fan.     

Layer dependence of the superconducting transition temperature of HgBa2Can−1CunO2n+2+δ

High-pressure methods have been used to synthesize multiphase compositions in the Hg---12{n−1}n homologous series. The phase assemblages were examined by optical, electron diffraction and X-ray diffraction techniques, and their stoichiometries verified by electron microprobe. Transport and magnetic susceptibility measurements were combined with the results of the phase analysis to establish superconducting transition temperatures for both as-prepared and O₂- or Ar-annealed materials. It was found that the transition temperature peaks at T_c = 134 K for N = 3 and then decreases abruptly for n>4, reaching T_c<90 K for n7.     

Re-entrant ferrimagnetism in TbMn6Ge6

The magnetic ordering of the hexagonal compound TbMn₆Ge₆ has been studied by neutron diffraction at various temperatures between 1.8 and 500 K. In almost the whole magnetically ordered regime the magnetic structures are incommensurate with the crystal lattice. The corresponding wave vector q₁ = (0,0,q_z) is strongly temperature dependent and decreases from q_z = 0.1307 r.l.u. at 1.8 K to q_z = 0.091 r.l.u. at 410 K. The low-temperature range (T < 85 K) is characterised by a triple ferrimagnetic spiral structure with net moment along the c-direction. At T_t = 85 K the net moment disappears and the magnetic ordering changes into a flat spiral structure that probably persists up to the magnetic ordering temperature, T_N = 450 K. Before reaching the magnetic ordering temperature, however, an additional ferrimagnetic component perpendicular to the c-direction develops, marking the onset of re-entrant ferrimagnetism. The resulting structure described by two propagation vectors corresponds to a distorted spiral. Various models are discussed. The observed re-entrant ferrimagnetism is in agreement with results of previously reported magnetic measurements.     

Ferromagnetic Zr1−xMnxCo2 laves phase compounds

Structure and magnetic properties of the Zr_1−xMn_xCo_2+δ alloys were studied for 0 x <0.7, δ=0, 0.45. The cubic C15 Laves phase structure shows Mn solubility up to x≈0.4. The other Laves phase with the hexagonal C36 structure found for x0.5 apparently has a small region of Mn solubility in the vicinity of Zr_0.4Mn_0.6Co₂. Though the parent Mn-free compounds are known to be paramagnetic, the Mn-substituted alloys show ferromagnetic behavior with the Curie temperatures up to 625 K and the room-temperature saturation magnetization of about 100 emu/g. The onset of ferromagnetism with the Mn substitution for Zr may be caused by polarization of itinerant 3d electrons, like it was earlier supposed for the off-stoichiometric ZrCo_2+δ. The universal composition dependencies of the intrinsic magnetic properties for different δ can be obtained, if plotted against the amount of zirconium atoms missing in its sublattice. The room-temperature anisotropy with the noticeable anisotropy field of 24 kOe and the 1 1 0 easy magnetization direction laying in a basal plane was found in the hexagonal Zr_0.5Mn_0.5Co₂.     

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

The magnetization of single-crystal HoNi₂B₂C has been measured as a function of applied field (H) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi₂B₂C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c-axis and with a much weaker temperature dependence for H applied parallel to the c-axis, indicating that the Ho⁺³ magnetic moments lie predominately in the tetragonal a−b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T_N=5.0 K. Remarkably, in 20 Oe applied field both superconductivity (T_c=8.0 K) and antiferromagnetism (T_N=5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H−T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H_c2 with a deep minimum at T_N=5 K. The high-field magnetization data for H applied perpendicular to the c-axis also reveal a cascade of three phase transitions for T < 5 K and H < 15 000 Oe, contributing to the rich H versus T phase diagram for HoNi₂B₂C at low temperatures.     

Flux creep and flux flow for thallium ceramics in a pulsed magnetic field

The resistive properties of Tl-ceramics with T_c|=0=114 K were investigated in a pulsed magnetic field (B) up to 30 T and in the temperature range of 4.2 K<T<140 K. It was shown that the character of the field dependence of the resistance differs qualitatively in high-and low-temperature regions. At low (T80K) temperatures the dynamic magnetoresistance arising in the sample is analogous to that observed earlier in LaSrCuO [1] and YBaCuO [2] ceramics. This magnetoresistance is defined by the magnetic field variation rate and leads to the appearance of a minimum at the maximum of the magnetic field pulse, i.e. at . In the region of high temperatures (80 K T<T_c) or magnetic fields (at T60K) the sample resistance rises monotonically with B increase, and dynamic resistance is not observed. In this temperature range the existence of a scaling relation is shown (here B^* and T^* meet the condition k=(B^*, T^*)/ _n(T^*)=const) for the ceramics resistance (B,T), which can be represented as . An estimate for the upper critical field B_c2(0)B_o=1030±40 T is obtained.     

Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems

The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La_1−xCd_xMnO₃ (0x0.5) systems. The ρ(T) curves exhibit a sharp metal insulator transition (T_p1), which is close to paramagnetic to ferromagnetic transition (T_c) obtained from M−T curves for all samples. In addition, ρ(T) curves for Cd doped samples exhibit another broad transition (T_P2) below T_p1. This transition becomes more prominent and the transition temperature (T_p2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ(T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.     

Charge susceptibility and charge correlation function in the d---p model

The properties of the charge fluctuation are investigated in the d---p model with the repulsion U_pd between holes on the nearest-neighbor Cu and O sites and the infinite on-site repulsion U_d at the Cu site. We calculate the charge susceptibility χ^c(q, iω_n) and the charge correlation function S^c(q) = TΣ_ωn χ^c(q, iω_n). It is found that S^c(q) has a peak at the Γ point and a maximum in a ring around the Γ point. The former is due to Tχ^c(q, 0). Its intensity is proportional to temperature T and strongly enhanced by U_pd. The latter is due to TΣ_{ωn ≠ 0} χ^c(q, iω_n) and shows a weak T and U_pd dependence. The intensity of the diffuse X-ray scattering on taking the charge fluctuation into account is also calculated. The result is consistent with the experiments in La_2−δSr_δCuO₄.     

Disruption of antiferromagnetic order in Zn-doped La2CuO4

The magnetic phase diagram of La₂(Cu_1−xZn_x)O₄ has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T_N(x) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T_N. Extrapolation of our results yields T_N → 0 K at x = 0.11 for a single (Cu_1−xZn_x)O₂ plane, and comparison with YBa₂(Cu_1−xZn_x)₃O₆ implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.     

27Al NMR study in UNiAl

The ternary compound UNiAl exhibiting an antiferromagnetic order below T_N = 19.3 K has been studied in the paramagnetic state using the ²⁷Al NMR technique and different magnetically oriented samples. The quadrupole coupling constant e²qQ/h = 1.56 MHz is temperature independent. The dominant, longitudinal component of the Knight shift with respect to the hexagonal c axis, K, is positive and increases upon lowering the temperature down to 50 K. Much smaller in magnitude, the transverse component, K_±, is also positive and only slightly temperature dependent. The plots of the Knight shift vs magnetic susceptibility K(χ) and K_±(χ_±) form the same line, which implies that the transferred hyperfine field of 9.2 kOe/μB for ²⁷Al nuclei should be considered isotropic.     

Temperature dependence of induced magnetic anisotropy in metallic glasses (Fe1 − xCox)78Si10B12

The variations of induced magnetic anisotropy with annealing and measuring temperatures in metallic glasses (Fe_{1 − x}Co_x)₇₈Si₁₀B₁₂ have been measured. It was found that K_u (T) was proportional to Mⁿ_s (T) for T below 200°C, and the index n varied with the cobalt content x and annealing conditions, not being smaller than 3. To the predictions of the existing pair-ordering and single-ion theories, the above results are anomalous. By considering the distributions of exchange integrals and activation energies in metallic glasses, this anomalous behaviour could be explained properly.     

Magnetic moments and Fe hyperfine field interactions in Y(Fe1−xRux)2 ternaries

Measurements of magnetization and ⁵⁷Fe Mössbauer spectra have been made for Y(Fe_1−xRu_x)₂. The C15 type cubic structure is stabilized for xx 0.7. The C15 compounds is ferromagnetic with T_c200 K and its saturation moment decreases monotonically with increasing x, while the ⁵⁷Fe hyperfine field decreases only slightly with x. From these results, it is deduced that the Ru atoms have an induced moment of ≈1μ_B in the range x 0.2. In the C14 type phase, no magnetic ordering develops even at 4.2 K.     

Aspects of “low field” magnetotransport in epitaxial thin films of the ferromagnetic metallic oxide SrRuO3

Epitaxial thin films of the conductive ferromagnetic oxide SrRuO₃ were grown on an (0 0 1) SrTiO₃ (STO) substrate by using DC sputtering technique. The magnetic and magnetoresistive properties of the films were measured by applying the magnetic field both perpendicular (out-of-plane) and parallel (in-plane) to the film plane and ever maintaining the direction of the applied field perpendicular to that of the transport current. The films grown on an (0 0 1) STO substrate showed identical magnetization properties in two orthogonal crystallographic directions of the substrate, [1 0 0]_S and [0 0 1]_S (in-plane and out-of-plane geometry), which suggests the presence of a multi domain structure within the plane of the film. For such samples, no anisotropic field (hard axis) along de [0 0 1]s direction, i.e., perpendicular to the film-plane could be detected. Nevertheless, a distinguishable temperature dependent out-of-plane anisotropic magnetoresistance (MR) along with strong temperature dependent low field hysteretic MR(H) behavior was detected for the studied films. A negative MR ratio MR(T)=[ρ(μ₀H=9 T; T)−ρ( μ₀H=0 T; T)]/ρ( μ₀H=0 T; T) on the order of a few percent, with maximums of 6% and 4% (right at the Curie temperature, T_C 160 K) was calculated for an in-plane and out-of plane measuring geometry, respectively. In addition there is an equally strong MR effect at low temperatures, which might be related to the temperature dependence of the magnetocrystalline anisotropy together with a magnetization rotation. Both the MR(T) behavior and the achieved values (except for T<30 K) are similar to those obtained on SrRuO₃ films grown on 2° miscut (0 0 1) STO substrates with the current parallel to the field and parallel to the direction, which was identified as the easier axis for magnetization.     

Magnetic susceptibility, electrical resistivity and thermal expansion coefficient of NiAs-type V1−xCrxSe

V_1−xCr_xSe(0.05x0.83) shows a temperature dependence of the magnetic susceptibility χ which is similar to that of CrSe. At small x, the magnetic transition temperature T_t(x) and the Weiss constant θ_p(x) decrease with decreasing x, while the effective number of Bohr magnetons per Cr (P_eff) significantly increases.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

Tracking the formation of 2223 BiSrCaCu oxygen compounds by resistivity and susceptibility measurements reaching Tc(0)=115 K

Unsubstituted and lead-antimony substituted 2223 BiSrCaCu Oxygen compounds have been synthesized and compared by resistivity R(T) and susceptibility χ(T) measurements and chemical analysis. The R(T) and χ(T) measurements show that interface effects occur which are reflected in the R(T) characteristics. The chemical analysis of samples with a resistance zero T_c(0)=114 K and a diamagnetic onset at 115 K yields a severe loss of lead from nominally 40 at% down to about 3 at%, whereas the T_c(0)=110 K samples contain up to about 30 at% of lead. It is assumed that for the formation of the 115 K phase lead oxide acts as a catalyst not being incorporated to a considerable amount into the lattice.     

Effective Ru moment in RuSr2Eu2−xCexCu2O10 from high-temperature magnetic susceptibility

We have made high-temperature (250 K<T<800 K) DC susceptibility measurements in the compounds RuSr₂Eu_2−xCe_xCu₂O₁₀ for x=0.6,0.8, and 1.0 in order to determine the Ru effective magnetic moment. After carefully subtracting all contributions to the magnetic susceptibility except that of the Ru ions, we have been able to fit the Ru susceptibility with a law χ_Ru=χ₀+C_Ru/(T−Θ_Ru). We have found that the Ru effective moment falls between the values expected for Ru⁵⁺ in spin states and . We have also found a dependence of μ_eff(Ru) and Θ_Ru with the Ce content x.