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.     

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.     

Antiferromagnetism and the Kondo behaviour in Ce(Pt1−xPdx)Ga

We present the results of lattice parameters at room temperature, the static magnetic susceptibility and the magnetic resistivity between 1.8 and 300 K, and the low-temperature specific-heat measurements for the series Ce(Pt_1−xPd_x)Ga, (x=0.0, 0.2, 0.5, 0.8 and 1.0). Two maxima in the temperature dependence of the magnetic resistive curve for each sample are observed, one above 100 K, and another at around 4 K, which due to an interplay between crystal-field effect and the Kondo effect. As determined from the peak values of the temperature dependence of the specific heat data C(T), all samples exhibit antiferromagnetic ordering from 1.3 K for CePdGa to 3.4 K for CePtGa. The large reduction of entropy for each sample below T_N is associated with the Kondo effect.     

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.     

The magnetic structure of chalcogenid spinel CoRh2S4

Neutron diffraction study on CoRh₂S₄, having the unexpectedly high Néel temperature T_N, shows the layered antiferromagnetic structure. The magnetic moment is (3.0±0.8)μ_B/Co atom. The T_N is confirmed to be 418 K from the temperature dependence of the intensity of the (200) reflection.     

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.     

Magnetic Raman optical activity in FeF2

Raman optical activity (ROA) of magnons and phonons in antiferromagnetic FeF₂ (T_N=78 K) has been studied as a function of temperature and in applied magnetic field. For exciting light incident along the rutile-structure c-axis, ROA is observed for magnons but not phonons. In zero field, the small anisotropy-induced splitting (0.09 cm⁻¹) of the two acoustic-magnon branches is observed by light scattering for the first time. The splitting in applied magnetic field is found to reduce with increasing temperature in accordance with theory. No ROA was detected for two-magnon excitations.     

Low temperature specific heat of dense kondo ferromagnets and intermediate valence CeNixPtt-x compounds

Specific heat measurements performed in the temperature range 1.5 to 30 K in the compounds CeNi_xPt_{1 - x} with x = 0.5, 0.8 and 0.95 are presented. The analysis of the magnetic entropy enhances the results obtained previously by magnetic measurements: i) the reduction of the cerium moment due to the Kondo effect; ii) the increase of the importance of the Kondo effect in competition with the RKKY interactions when Pt is replaced by Ni. The variation of γ along the series indicates a maximum around x = 0.9 which corresponds to the magnetic instability. It is worth noting that below T_c in the ferromagnetic compounds, C_mag follows a T law, characteristic of magnon dispersion.     

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     

Sublattice magnetization and magnetization process in TbCu2

The TbCu₂ compound exhibits antiferromagnetism and has two nonequivalent Tb sites below 54 K (=T_N). The magnetization for the a-axis increases discontinuously at critical field and saturates at high fields. The magnetization process and magnetic moments have been analyzed in terms of the molecular field theory including the crystal-field interaction.     

Dy3Al5O12磁热性质研究

用量子理论计算了Dy₃Al₅O₁₂的晶场能谱、Zeeman劈裂能级和波函数. 在外磁场H_e为0<H_e<9 T, 温度为3<T<42 K 范围内, 计算了该晶体的磁矩、磁熵变, 计算结果与相关实验数据吻合较好. 该计算结果表明, Dy₃Al₅O₁₂内磁性离子间的交换作用非常微弱, 可以忽略. 从理论上给出了绝热退磁过程中温度变化ΔT与T的关系, 并与Gd₃Ga₅O₁₂晶体进行了比较, 发现不同外磁场下, Dy₃Al₅O₁₂和Gd₃Ga₅O₁₂的低温制冷性能在不同温区有差别. 在进行低温(T<10 K)制冷时, 若外磁场较低, 选择Dy₃Al₅O₁₂作为磁制冷材料较好; 若外磁场较高, 选择Gd₃Ga₅O₁₂作为磁制冷材料较好.     

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.     

Effect of pressure on the Néel temperature of the intermetallic compound YMn2

The effect of pressure on the Néel temperature T_N of the intermetallic compound YMn₂ has been studied up to 3.5 kbar by thermal expansion measurements. We have found that dT_N/dP is large and negative. T_N decreases nonlinearly with increasing pressure and becomes 0 at 3.7 kbar.     

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.     

Helimagnetism in the cubic Laves phase YMn2

Neutron diffraction experiments on YMn₂ using a wave length of λ_N = 2.483 Å show a splitting of the magnetic peaks. The magnetic structure is helimagnetic consistent with an angle modulation of the previously reported antiferromagnetic structure. The NMR spectrum can be explained as arising from a perturbation of the helix by the magnetocrystalline anisotropy. Below T_N the observed frustration of the negative Mn interactions is inherent from the topology of the crystallographic structure. Above T_N, it creates short range ordering whose thermal decrease may explain the increase in the paramagnetic neutron scattering as the temperature is increased.     

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.     

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.     

Diamagnetic impurity effects on the Néel temperature in La2CuO4 and related materials

To investigate why the sensitivity of the Néel temperature T_N of the antiferromagnetic (AF) layered copper perovskites (typically La₂CuO₄) to diamagnetic impurities such as Zn is reportedly much larger than in the AF members of the K₂NiF₄ family, we first treat the effect of a concentration c of impurities on the uncorrelated electronic states in the coherent potential approximation (CPA). Then we consider the Heisenberg hamiltonian as the large correlation limit of the Hubbard hamiltonian for a single band of impurity-modified electronic states. The correlation effects are treated variationally. The model is solved explicity by using a rectangular density of states, and we obtain the c-dependent exchange J, staggered moment S_q, spin wave velocity and transverse susceptibility at zero temperature. We take into consideration several recently proposed formulae for T_N in the clean limit, and include the impurity effects by exploiting the results obtained, in order to test their predictions against the experimental T_N(c) data for La₂Cu_1−cZn_cO₄. Our results suggest that, to explain the difference between the K₂NiF₄ and the La₂ CuO₄ families, one should consider both the sign and the magnitude of the difference I≡ε^B−ε^A between impurity (B) and host (A) ionic potentials. The slowly decreasing trend of T_N(c) in the K₂NiF₄ family is reproduced if I is negative and sizeable, or positive but very small, while the quick decrease typical of the copper perovskites requires a positive and rather large I. For reasonable values of the interaction parameters, among the several models we compare, only the model of Chakravarty, Halperin and Nelson is able to semi-quantitatively reproduce the non-linear behaviour of T_N(c) reported for La₂Cu_1−cZn_cO₄, provided the spin stiffness is assumed to scale with c as appropriate to Fermi liquids.     

Thickness and temperature dependence of subsidiary absorption thresholds in yttrium-iron-garnet thin films

The microwave subsidiary absorption threshold in tangentially magnetized yttrium-iron-garnet (YIG) films was measured as a function of the static magnetic field, film thickness and decrease in the 0.3–3 μm range is observed. The effect is explained quantitatively by using modified Damon-Eshbach dispersion relations which take into account exchange interactions.

The effect of temperature on h_crit, the smallest threshold field amplitude, in the 250<T< 450 K temperature range can be expressed by an empirical relationship h_crit M_s(T)^-1.6 where M_s is the saturation magnetization. This h_crit vs M_s relationship is similar to that obtained for the magnetization dependence of parallel pump thresholds in Li-Ti ferrite compounds.