Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

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.     

Powder x-ray diffraction and Rietveld analysis of (C_2H_5NH_3)_2CuCl_4

Structural properties of the organic-inorganic hybrid(C_2H_5NH_3)_2CuCl_4 have been investigated by means of x-ray powder diffraction and Rietveld analysis. A structural phase transition from Pbca to Aba2 occurs at T_4= 240 K, which results in a paraelectric–ferroelectric phase transition. The release of the Jahn–Teller distortion with increasing temperature toward T_4 is revealed by the structural analysis.     

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.     

Yb Mössbauer and neutron diffraction investigation of the antiferromagnetic phase in the Kondo lattices YbP and YbN

The Kondo lattices YbP and YbN (nonstoichiometric samples) have been investigated by ¹⁷⁰Yb Mössbauer spectroscopy, in the temperature range 0.085 K to 80 K in zero magnetic field in YbN and with an applied magnetic field at T=0.045 K in YbP and YbN, and by neutron diffraction in YbN. In both compounds, the antiferromagnetic transition at T_N0.7 K is first order and occurs through a progressive growth of paramagnetic domains as the temperature increases. The measured exchange interaction is much larger than k_BT_N in both compounds and it is found to be isotropic in YbP and anisotropic in YbN. The saturated magnetic moment in both compounds is reduced by 50% with respect to the value calculated from the crystal field and exchange interactions. These properties are interpreted in terms of the competition between the Kondo effect and the RKKY exchange interaction; using a model mean field NCA calculation to describe the magnetic phase, the Kondo temperatures T_K are found to be near 5–10 K.     

Field-induced spin-flip and spin-flip transitions in PrGa2

A detailed neutron diffraction study of a single crystal of hexagonal PrGa₂ under an applied field is presented. The field is applied along the easy [100] direction of the hexagonal plane. The compound exhibits several field-induced magnetic structures and, in particular, an uncompensated long period commensurate antiphase structure defined by the propagation vector (4/27, 4/27, 0) at zero field and below T_t = 3.5 K. Concerning the intermediate magnetic structures below T_t, two types of magnetic phase transitions are observed: a spin-flip transition where one moment per magnetic cell turns over, and a spin-slip transition from the low field commensurate propagation vector (4/27, 4/27, 0) to another commensurate vector (1/7, 1/7, 0). Between T_t and T_N = 7.2 K, the structure becomes apmlitude-modulated with a propagation vector (0.148, 0.148, 0.023). Important and original magnetic domain effects are also observed due to the high degeneracy of the easy direction of the hexagonal plane in both T<T_t and T_t<T<T_N regions.     

Nuclear magnetic resonance relaxometry of the normal heart: Relationship between collagen content and relaxation times of the four chambers

The use of nuclear magnetic resonance (NMR) relaxation time measurements for characterization of abnormal cardiac tissue depends upon knowledge of variations of relaxation times of normal myocardium and determinants of these variations. We calculated in vitro NMR T₁ and T₂ relaxation times of canine myocardium from the four cardiac chambers, and determined hydroxyproline concentration (as a measure of collagen) and percent water content of the samples. We found both water content and T₁ relaxation time of the right ventricle to be significantly greater than the left atrium (p < 0.05). T₂ relaxation time of the left ventricle was found to be shorter than each of the other three chambers (p < 0.05). There were significant correlations between the spin-lattice relaxation time and both percent water content (r = 0.58) and hydroxyproline concentration (r = 0.45). A significant correlation was also found between T₂ relaxation time and hydroxyproline concentration (r = 0.49). When T₁ and T₂ were adjusted for water and hydroxyproline content, there was no longer any evidence for significant interchamber differences for either T₁ or T₂. These data suggest that differences in NMR relaxation times exist among the four chambers of the normal canine heart. Furthermore, a major determinant of myocardial spin-lattice relaxation time is tissue water content while both collagen content and percent water content significantly contribute to variability in cardiac chamber T₂ relaxation times.     

Structural changes in RNiO3 perovskites (R=rare earth) across the metal–insulator transition

Rare earth nickel oxide perovskites (RNiO₃, R=rare earth) have, except for LaNiO₃, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature (T_MI) increases as the R-ion becomes smaller. They also present, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (T_N) is close to T_MI, while for heavier R-ions (e.g. Eu, Sm), T_MI and T_N are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Even though Ni³⁺ is a Jahn–Teller ion, no distortion in the NiO₆ octahedra was found for RNiO₃ perovskites with R=Pr, Nd, Sm and Eu. In this work we have measured EXAFS at Ni K edge for samples of PrNiO₃, NdNiO₃ and EuNiO₃. The Fourier transform spectra for the three samples show a clear splitting in the first peak at the insulating phase. This splitting corresponds to two or more different Ni–O distances. This indicates that there is either a distortion in the NiO₆ octahedra or there are two different Ni sites at the insulating phase.     

Selection of pulse sequences producing maximum tissue contrast in magnetic resonance imaging

The importance of spin density [N(H)] and spin-lattice (T₁) and spin-spin (T₂) relaxation in the characterization of tissue by nuclear magnetic resonance (NMR) is clearly recognized. This work considers which optimized pulse sequences provide the best tissue discrimination between a given pair of tissues. The effects of tissue spin density and machine-imposed minimum rephasing echo times (TE_MIN) for achieving maximum signal tissue contrast are discussed. A long TE_MIN sacrifices T₁-dependent contrast in saturation recovery (SR) and inversion recovery (IR) pulse sequences so that spin-echo (SE) becomes the optimum sequence to provide tissue contrast, due to T₂ relaxation. Pulse sequences providing superior performance may be selected based on spin density and T₁ and T₂ ratios for a given pair of tissues. Selection of the preferred pulse sequence and interpulse delay times to produce maximum tissue contrast is strongly dependent on knowledge of tissue spin densities as well as T₁ and T₂ characteristics. As the spin density ratio increases, IR replaces SR as the preferred sequence and SE replaces IR and SR as the pulse sequence providing superior contrast. To select the optimal pulse sequence and interpulse delay times, an accurate knowledge of tissue spin density, T₁ and T₂ must be known for each tissue.     

Magnetic and electronic phase transitions and magnetoresistance effect in the Pr0.5−xLaxSr0.5MnO3 (, 0.15) system

The electronic and magnetic phase transitions of Pr_0.5−xLa_xSr_0.5MnO₃ with x=0.10 and 0.15 were investigated. The M(T) and ρ(T) curves for these samples clearly show transitions from antiferromagnetic insulator to ferromagnetic semiconductor, ferromagnetic metal and finally to paramagnetic semiconductor as the temperature is increased from 5 to 300 K. Especially, two obvious protrudent peaks in the magnetoresistance curves MR(T) for these samples were clearly observed in the relative low magnetic field, 1 T. One peak appears at around the antiferromagnet-ferromagnet transition temperature T_N (150 K) with MR≈−23%, another occurs at around the ferromagnet-paramagnet transition temperature T_C(275 K ) with MR≈−8.2%. In addition, when the magnetic field was increased, the temperature corresponding to the MR peak at T_N shifts to lower temperature while the temperature corresponding to the MR peak at T_C is fixed.     

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.     

Spin ordering in the mixed-ligand antiferromagnet Mn(dca)2(pyrazine)

The flexibility offered by molecular-based systems allows us to introduce or replace specific ligands in a material with the aim of radically altering desired structural and magnetic properties. Specifically, Mn(dca)₂(pyz) {dca = dicyanamide, [N(CN)₂]⁻; PYZ = pyrazine} has a unique interpenetrating ReO₃-like lattice. The Mn²⁺ cations are high-spin ( ) and the material orders antiferromagnetically below T_N = 2.53(2) K. Using neutron powder diffraction we observed a collinear spin structure oriented along the short ac-diagonal of the monoclinic unit cell. Inelastic neutron scattering results show a magnetic excitation at 0.23 meV. The strong dispersion character of this excitation demonstrates that it is related to a low-energy spin wave. Upon warming, the magnon gradually softens and disappears at T_N, while critical scattering becomes evident by a broad quasielastic response above T_N. The energy of the magnon is consistent with the exchange parameter, J, derived from magnetic susceptibility measurements.     

Scaling behavior of the thermopower of the archetypal heavy-fermion metal YbRh2Si2

We reveal and explain the scaling behavior of the thermopower S/T exhibited by the archetypal heavy-fermion (HF) metal YbRh₂Si₂ under the application of magnetic field B at temperature T. We show that the same scaling is demonstrated by different HF compounds such as β-YbAlB₄ and the strongly correlated layered cobalt oxide [BiBa_0.66K_0.36O₂]CoO₂. Using YbRh₂Si₂ as an example, we demonstrate that the scaling behavior of S/T is violated at the antiferromagnetic phase transition, while both the residual resistivity ρ₀ and the density of states, N, experience jumps at the phase transition, causing the thermopower to make two jumps and change its sign. Our elucidation is based on flattening of the single-particle spectrum that profoundly affects ρ₀ and N. To depict the main features of the S/T behavior, we construct a T –B schematic phase diagram of YbRh₂Si₂. Our calculated S/T for the HF compounds are in good agreement with experimental facts and support our observations.     

Magnetostriction and spin reorientation in ferromagnetic Laves phase Pr(Ga_xFe_(1-x))_(1.9) compounds

The magnetostriction, magnetization, and spin reorientation properties in Pr(Ga_xFe_(1-x))_(1.9) alloys have been investigated by high-precision x-ray diffraction(XRD) step scanning, magnetization, and Mo¨ssbauer spectra measurements. Ga substitution reduces the magnetostriction(λ_(||)) with magnetic field H ≥ 8 kOe(1 Oe = 1.33322×10~2 Pa), but it also increases the λ|| value when H ≤ 8 kOe at 5 K. Spin-reorientations(SR) are observed in all the alloys investigated, as determined by the step scanned XRD, Mo¨ssbauer spectra, and the abnormal temperature dependence of magnetization. An increase of the spin reorientation temperature(T_(SR)) due to Ga substitution is found in the phase diagram, which is different from the decrease one in many R(T_x Fe_(1-x))_(1.9)(T = Co, Al, Mn) alloys. The present work provides a method to control the easy magnetization direction(EMD) or T_(SR) for developing an anisotropic compensation system.     

Coexistence of superconductivity and antiferromagentic order in Er2O2Bi with anti-ThCr2Si2 structure

We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er₂O₂Bi with anti-ThCr₂Si₂-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature T_c of 1.23 K and antiferromagnetic transition temperature TN of 3 K are observed in the sample with the best nominal composition. The superconducting upper critical field H_c2(0) and electron-phonon coupling constant λ_e−ph in Er₂O₂Bi are similar to those in the previously reported non-magnetic superconductor Y2O2Bi with the same structure, indicating that the superconductivity in Er₂O₂Bi may have the same origin as in Y₂O₂Bi. The first-principle calculations of Er₂O₂Bi show that the Fermi surface is mainly composed of the Bi 6p orbitals both in the paramagnetic and antiferromagnetic state, implying minor effect of the 4f electrons on the Fermi surface. Besides, upon increasing the oxygen incorporation in Er₂O_xBi, Tc increases from 1 to 1.23 K and T_N decreases slightly from 3 K to 2.96 K, revealing that superconductivity and antiferromagnetic order may compete with each other. The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content, which may account for the variations of T_c and T_N with different oxygen content.     

Anisotropic magnetic susceptibility of single crystal Ca2Y2Cu5O10 with edge-sharing CuO2 chain

We report a successful crystal growth of Ca₂Y₂Cu₅O₁₀ and anisotropic behavior of magnetic susceptibility associated with an antiferromagnetic phase transition. Single crystals were obtained by the floating-zone method using an infrared radiation furnace. Composition and structural characterization shows that the grown crystals are qualified for physical measurements. The magnetic susceptibility along each crystallographic axis shows a typical behavior of antiferromagnetic phase transition at T_N=31 K, and the ordered magnetic moments are found to lie along the b-axis.     

CeAu2In4: A candidate of quasi-one-dimensional antiferromagnetic Kondo lattice

Needle-like single crystals of CeAu₂In₄ have been grown from In flux and characterized as a new candidate of quasi-one-dimensional Kondo lattice compound by crystallographic, magnetic, transport, and specific-heat measurements down to very low temperatures. We observe an antiferromagnetic transition at T_N ≈ 0.9 K, a highly non-mean-field profile of the corresponding peak in specific heat, and a large Sommerfeld coefficient γ =369 mJ·mol^-1·K^-2. The Kondo temperature T_K is estimated to be 1.1 K, being low and comparable to T_N. While Fermi liquid behavior is observed deep into the magnetically ordered phase, the Kadowaki-Woods ratio is much reduced relative to the expected value for Ce compounds with Kramers doublet ground state. Markedly, this feature shares striking similarities to that of the prototypical quasi-one-dimensional compounds YbNi₄P₂ and CeRh₆Ge₄ with tunable ferromagnetic quantum critical point. Given the shortest Ce-Ce distance along the needle direction, CeAu₂In₄ appears to be an interesting model system for exploring antiferromagnetic quantum critical behaviors in a quasi-one-dimensional Kondo lattice with enhanced quantum fluctuations.     

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.