Deviations from the Curie-Weiss law in the magnetic susceptibility of dilute magnetic alloys

The magnetic susceptibility of dilute magnetic alloys of Mn in Rh and Fe in a Nb-Mo solid solution deviates from the Curie-Weiss law. The following temperature dependence is observed: Δ_χ = C/(T + T_K) + log (T_K/T), with ≡ 0 for T > T_K⁰.     

Reduced s-d model with antiferromagnetically coupled impurity spins

An antiferromagnetic equivalent-neighbour Heisenberg interaction H_i between impurity spins is added to the reduced s-d Hamiltonian H_r previously introduced by simplifying the Kondo s-d exchange Hamiltonian H_K. Asymptotic mean-field theory is developed for H_r + H_i, in the presence and absence of external magnetic field, and applied to (La_1−xCe_x)Al₂ alloys. Specific heat c_i(T) and zero-field susceptibility χ_i(0,T) curves for (La_1−xCe_x)Al₂ are depicted. The coupling constants of H_r + H_i and conduction bandwidth are adjusted so that T_c temperatures for x = 0.2, 0.1 are equal to the experimental values. c_i(T) exhibits a jump at T_c and is decreasing for T < T_c. χ_i(0,T) has a first order pole at T_c which corresponds to the maximum of experimental susceptibility and χ_i(0,0) > 0. These results improve those obtained earlier on the grounds of H_r theory.     

Correlation between the exchange bias and the degree of ordering of antiferromagnetic layer in PtMn/Co–Fe bilayers

Correlation between the unidirectional anisotropy constant, J_K, and the degree of ordering of PtMn layer was investigated for Pt₅₅Mn₄₅/Co₉₀Fe₁₀ bilayer, as a function of the annealing time and the PtMn layer thickness, d_AF. As a result, we found the linear relations between J_K and the degree of ordering, f_FCT·S, in the cases of the bilayers with d_AF=5–50 nm. From the extrapolation of the linear relations to f_FCT·S=1, meaning the perfect ordering of PtMn layer, we obtained the attainable value of J_K and the intrinsic critical thickness of the PtMn layer to be 0.26 erg/cm² and 6 nm, respectively.     

Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H_c2 and parallel H_c2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of H_c2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature T_c and the upper critical fields (H_c2, and H_c2) are also considered. The nonmonotonic T_c behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of T_c as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically.     

The effect of Magic Angle Spinning on proton spin–lattice relaxation times in some organic solids

Proton spectra of solids are usually broadened by strong proton homonuclear dipolar interactions. However, substantial line narrowing may be achieved by Magic Angle Spinning (MAS) in systems of low proton density or in systems in which rapid molecular motions occur. In such conditions, T₁(H) measurements are often used to characterise the dynamics of each resolved proton site. We show that T₁(H) values measured for solid organic compounds with high proton abundance, such as adamantane and glycine, may be strongly dependent on the spinning rate employed, so that care is required when values are compared. The effects of molecular motion and proton density on T₁(H) and its dependence on spinning rate were investigated. We found that an increase in molecular motion leads to an increase of T₁(H) at higher spinning rates. The opposite is found for systems with low proton densities which show relatively lower T₁(H), at higher spinning rates. A possible interpretation is suggested in terms of the reduced spin diffusion efficiency at higher spinning rates.     

Thermoelectric power and irreversibility field of (Hg,Tl)2Ba2Can−1CunOy (n=2–5) superconductors

We have measured the thermoelectric power, S, and the irreversibility field, H_irr, of the superconducting samples of (Hg,Tl)₂Ba₂Ca_n−1Cu_nO_y (n=2–5). S values for the (Hg,Tl)-22(n−1)n above their T_c's drastically increase with increasing n. Judging from the S values at room temperature, the n=2 and 3 samples are located in the overdoped region, the n=4 is in the almost optimally doped and the n=5 is in the underdoped region. H_irr values for the n=4 and 5 samples are higher than those for the as-synthesized and annealed n=3 samples. It is considered that the enhancement in H_irr is due not only to the increase of hole concentration but to the increase in the number of CuO₂ sheets.     

Studies of anisotropy mechanisms in polyphosphate-treated magnetic iron oxide particles

The coercive force of small, acicular ( 2000 Å length, 8:1 length-to-diameter ratio) spinel-type iron oxide particles increases substantially following surface treatment with sodium polyphosphate. Regardless of Fe²⁺/Fe³⁺ cation ratio, H_c always attains a peak value when the polyphosphate/iron oxide weight ratio P/Fe is in the range 0.5–0.6, denoted (P/Fe)_max. The maximum change in H_c is observed when Fe²⁺/Fe³⁺ ≈ 0.10–0.15. When P/Fe0.5–0.6, however, both magnitude and sign of the change in coercive force show strong dependence on the Fe²⁺ content of the oxide, suggesting that the H_c changes are caused by a magnetostrictive mechanism at these high treatment ratios. Calculated anisotropy field distributions of treated specimens show that both the mean anisotropy field δH_kδ_G and predicted H_c reach a peak when particles are treated at (P/Fe)_max, where the distribution becomes very broad. At high treatment ratios both δH_kδ_G and predicted H_c decrease to values below that of the untreated oxide. Mössbauer studies of treated and untreated particles show no significant change in the environment of surface iron ions following treatement at P/FE = 0.5, but indicate a small increase in the Fe³⁺ concentration of the particle core. When P/Fe = 2.5, however, the Fe²⁺ concentration of the core increases markedly.     

Normal state magnetoresistance in stable magnetic fields up to 20 T in single crystalline La1.76Sr0.24CuO4

The in-plane normal state resistance R_n(T, H) in an overdoped La_2−xSr_xCuO₄ crystal (x = 0.24) has been measured in magnetic fields up to 20 T parallel to the c-axis. The R_n(T) curves in constant fields show a quadratic behavior in a wide range of temperature above T_c(H). Some characteristic features in R_n(H) are observed. In the low-field region R_n(H) increases with increasing H, reaches a maximum and then decreases with further increasing H. Possible origins for the observed unusual R_n(T, H) behavior are discussed.     

The magnetic properties of aligned M hexa-ferrite fibres

Aligned and random fibres of strontium hexaferrite (SrM, SrFe₁₂O₁₉) and barium hexaferrite (BaM, BaFe₁₂O₁₉) were manufactured by blow spinning from an aqueous inorganic sol–gel precursor, which was then fired to give the hexagonal ferrite fibre. Their magnetic properties were studied by VSM, investigating the evolution of these properties with firing and measurement temperature, and in particular the effects of fibre alignment. It has been predicted that aligned ferrite fibres will demonstrate an enhanced magnetisation along the axis of alignment with respect to perpendicular to the axis, and this has been demonstrated here for the first time. The optimum firing temperature was 1000 °C, at which point they still had submicron grains. In BaM random fibres M_s=63.8 emu g⁻¹ and H_c=428.1 kA m⁻¹, and in SrM random fibres M_s=63.3 emu g⁻¹ and H_c=452.8 kA m⁻¹, high values for polycrystalline materials. Fibres aligned parallel to the applied field had saturation magnetisation (M_s) values equal to those of the random fibres, whilst fibres aligned perpendicular to the field had M_s values 62% and 75% lower, for BaM and SrM, respectively. There was no change in coercivity (H_c) between random or aligned fibres of any orientation, and fibres aligned 45° and parallel to H appeared identical. Therefore, properties along the axis of alignment were superior when compared to measurements perpendicular to the axis of alignment, giving a directionality to the magnetisation in an otherwise randomly oriented ferrite material.     

Time-dependent magnetization in co-precipitated cobalt ferrite

A study of the magnetic aftereffect in co-precipitated cobalt ferrite is presented. Measurements of the magnetic viscosity S were performed at room temperature along the demagnetization curve for different applied fields H_ap over a wide range of fields (0 kOe<H_ap<−7 kOe). The interrelation function η=(∂M_rev/∂M_irr)_Hi between the DCD reversible M_rev and irreversible M_irr magnetization components was determined as well. The experimental results for S_η(H_i), where H_i is the internal field, showed a broad distribution with a maximum at H_i=2.7 kOe. However, the irreversible susceptibility χ_irr displays a maximum at H_c=0.75 kOe, the coercivity of the material. The experimental behavior of η and the non-proportionality between S_η and χⁱ_irr suggest that the magnetic viscosity in this material is principally supplied by events of nucleation of inverse domains and the depinning of domain walls. When the main mechanism of reversal magnetization changes to rotation of magnetic moments for all the grains, the magnetic viscosity decreases.     

分析指认固体中稀土离子4f-5d跃迁光谱的简单模型

概要介绍和总结最近由段昌奎等人提出,并经发展扩充的用以分析指认固体中稀土离子f-d跃迁光谱的简单模型,以及对Nd³⁺、Sm³⁺、Eu³⁺、Eu²⁺、Tb³⁺、Ho³⁺、Er³⁺的f-d激发光谱及Nd³⁺的d-f辐射谱的成功应用。也论及值得进一步研究的其他离子的f-d谱。     

Anomalous temperature dependence of magnetization in polycrystalline Fe65Ni35 alloy

Magnetization at 0.3 and 140 Hz (0–10 Oe) and magnetic relaxation measurements were carried out in detail in the temperature range between 4.2 and 300 K for a polycrystalline Fe₆₅Ni₃₅ alloy. The typical temperature T_g and the magnetic field H_g which correspond to the anomalous temperature in χ-T curves and inflection field in σ-H curves, respectively, are summarized and a H-T diagram is obtained. A strong magnetic relaxation is observed along the H_g-T line. The temperature dependence of H_g is discussed by the thermal activation of 180° domain wall which is pinned strongly by the antiferromagnetic-like clusters below T_g. It is find that H _g is a linear function of T .     

Surface barrier and magnetic hysteresis of ac permeability in YBaCuO single crystal

The nature of hysteretic behavior of the flux line lattice (FLL) contribution to ac magnetic permeability (μ_v) is analyzed for the case of YBa₂Cu₃O_x single crystal (at applied magnetic field Hc axis). It is shown that hysteresis loops μ_v(H) corresponding to different temperatures (T=70–84 K) are scaled to a universal curve in normalized coordinates. Such a behavior is interpreted in terms of the FLL interaction with the crystal surface. The explicit relationship between μ_v and magnetic induction B is found for the near-surface region of the superconductor. It is shown that the μ_v(H) loops are closely related to the hysteresis of B at cycling of applied magnetic field. The latter hysteresis stems from the Bean–Livingston surface barrier. The estimates demonstrate strong suppression of the surface barrier in YBa₂Cu₃O_x crystal in comparison to that expected for the ideal surface. As a result, the lower branch of the hysteresis loop corresponding to the increasing field is very close to the equilibrium μ_v(H) curve and the surface barrier appreciably affects only the upper branch when magnetic flux leaves the sample. The comparison of theoretical predictions and experimental data provides an opportunity to refine the actual range of stability H_max(B)–H_min(B) for the FLL at fixed B for YBa₂Cu₃O_x crystal in the case of Hc.     

BiFeO3/Ni81Fe19磁性双层膜中的交换偏置及其热稳定性研究

研究了磁场诱导生长的BiFeO₃/Ni₁₈Fe₁₉磁性双层膜中 的交换偏置及其热稳定性. 结果表明: BiFeO₃/Ni₁₈Fe₁₉双层膜中的交换偏置场H_ex未表现出明显的磁练习效应. 在负饱和磁场等待过程中, BiFeO₃/Ni₁₈Fe₁₉双层膜磁滞回线的前支和后支曲 线都随着在负饱和磁场中等待时间t_sat的增加向正场方向偏移. 交换偏置场H_ex的大小随着等待时间t_sat的增加而减小, 矫顽力H_c基本不变. 交换偏置场H_ex的大小随测量温度T_m的升高变化不明显, 表现出良好的热稳定性; 但矫顽力H_c随T_m的升高而显著减小. 良好的热稳定性应该来源于铁电性和反铁磁性间的共同耦合作用. 关键词： 多铁性 磁性薄膜 交换偏置 热稳定性     

Anomalous magnetotransport phenomena in θ-(BEDT-TTF)2I3

Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)₂I₃ crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH²_a + bH²_b + cH²_c)₀^-3/2/H in terms of H = (H_a, H_b, H_c), the zero field resistivity ₀, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b a > c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.     

KAM theory analysis of the dynamics of three coaxial vortex rings

The equations of motion of three coaxial vortex rings in Euclidean 3-space are formulated as a Hamiltonian system. It is shown that the Hamiltonian function for this system can be written as the sum of a completely integrable part H₀ (related to the motion of three point vortices in the plane) and a non-integrable perturbation H₁. Then it is proved that when the vortex strengths all have the same sign and the ratio of the mean distances among the rings is very small in comparison to the mean radius of the rings, H₁/H₀1. Moreover, it is shown that H₁/H₀ is very small for all time for certain initial positions of the rings under the same assumptions. It is proved that the decomposition of the Hamiltonian and the estimates carry over to a reduced form of the system in coordinates moving with the center of vorticity and having one less degree of freedom. Then KAM theory is applied to prove the existence of invariant two-dimensional tori containing quasiperiodic motions. The existence of periodic solutions is also demonstrated. Several examples are solved numerically to show transitions from quasiperiodic and periodic to chaotic regimes in accordance with the theoretical results.     

The condition for the occurence of zero coercive field in amorphous ribbon carrying an ac current

A zero coercive field H_c has been obtained by passing an ac current with appropriate phase, frequency and amplitude through an amorphous Co_70.3Fe_4.7Si₁₅B₁₀ ribbon. The phenomenon is explained in terms of the model for magnetization of amorphous ribbons carrying dc current. The condition for the occurence of H_c=0 has been derived and the evidence provided that the strength of pinning centres responsible for H_c increases at elevated currents. Some applications of the phenomenon are briefly discussed.     

Coherence effects in the Kondo lattice

A simple approach to the coherence effects in the Kondo lattice is presented, using the functional integral method. We show the existence of a coherence temperature T_c smaller than T_K, below which coherence between impurities appears. The density of states is calculated in an approximation which takes into account coherent and incoherent scattering. We also show how the thermodynamic properties reflect the coherence effects.     

Structure of some liquid alcohols, diols and triols: an analysis of dielectric data

This work is a continuation of our studies of the liquid structure of polyhydric alcohols and their solutions. Dielectric parameters (relaxation time, average cluster dipole moment μ¯_c, etc.) have been calculated for 1-hexanol in a wide temperature range in the framework of the cluster model proposed by Dissado and Hill; the values of apparent activation enthalpy ΔH_DHexp^≠ have been evaluated. These data were compared with the parameters previously determined by us for 2,5-hexanediol and 1,2,6-hexanetriol; the ratio μ¯_c/μ¯_v was used for an estimation of the number of molecules in clusters. It was found that at all temperatures in the range 303–393 K, the μ¯_c/μ¯_v values for 1-hexanol are significantly larger than the values of average degree of association p¯ calculated from equilibrium dielectric data; the ΔH_DHexp^≠ values for 1-hexanol are considerably lower than those for 2,5-hexanediol and 1,2,6-hexanetriol.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.