Universal scaling properties of extreme type-II superconductors in magnetic fields

Anisotropic Ginzburg-Landau superconductors of extreme type-II are considered in an approximation where magnetic field fluctuations are neglected. A formulation of the scaling properties is presented for the singular part of the free energy density in the presence of a magnetic field. From the existence of a magnetization, a diamagnetic susceptibility and superconductivity we determine the limiting behavior of the scaling function in the vicinity of the zero field transition temperature, where critical fluctuations dominate. Our predictions for the temperature and field dependence of magnetization, magnetic torque and melting line etc., uncover the universal critical properties and provide an extension of hitherto used mean-field treatments. The results are consistent with experimental data. Received: 24 April 1998 / Accepted: 5 May 1998     

Possible observation of the coexistence of superconductivity and long-range magnetic order in NdRh4B4

The ternary rare earth compound NdRh₄B₄ has been studied by means of critical field, low temperature heat capacity, and static magnetic susceptibility measurements. Features in the upper critical field and heat capacity data at 1.31 K and 0.89 K suggest the occurrence of long-range magnetic order in the superconducting state. The temperature dependence of the static magnetic susceptibility follows a Curie-Weiss law with an effective magnetic moment μ_eff = 3.58 ± 0.05 μ_B and a Curie-Weiss temperature θ_p = ?6.2 ± 1.0 K between 20 K and room temperature. However,, magnetization vs. applied magnetic field isotherms suggest the development of a ferromagnetic component in the Nd³⁺ magnetization at low temperatures.     

The magnetic moment of the proton

In this paper the author calculates the magnetic moment of a bare proton in a non-relativistic approximation on the condition that the proton is a particle which is capable of existing not only in a state with spin 1/2but also in a state with spin 3/2.     

Dynamical theory of thermal spin fluctuations in metallic ferromagnets

A self-consistent dynamical theory of thermal spin fluctuations is developed which describes their spatial correlation. It is based on the functional integral method and utilizes the quadratic representation for the electron free energy in a fluctuating exchange field with renormalized susceptibilities allowing for the interaction of various spin fluctuation modes. Interpolation between the single-site and homogeneous susceptibilities is used, where these susceptibilities are found self-consistently. The average over fluctuations takes account of both long-wavelength and local excitations. A closed system of equations is formed for both unknown quantities: the magnetization and the mean-square exchange field at a site. The basic characteristics of a specific magnet are the density of electron states and the atomic magnetic moment at T=0. A method is proposed for separating the relatively slow thermal-spin fluctuations from the rapid zero-spin fluctuations forming the ground state of the magnets. At T=0 we have a system of equations of mean field theory. The temperature excites thermal spin fluctuations, which are described by taking account of correlation in time and space. The magnetization, susceptibility, magnitude of the spin fluctuations and their distribution over momenta, and the degree of magnetic short-range order in iron are calculated as functions of the temperature in the ferromagnetic and paramagnetic phases, and also at the transition between them, the Curie temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 90–98 (January 1998)     

自旋为1/2的XY模型亚铁磁棱型链的物性和有序-无序竞争

利用不变本征算符法, 计算低温下自旋为1/2的XY模型一维亚铁磁棱型链系统的元激发谱, 讨论在此系统中不同的特殊情形下的元激发能量, 从而给出体系的三个临界磁场强度的解析解H_C1, H_C2, H_peak. 分析不同外磁场下 体系的磁化强度随温度的变化规律, 发现三个临界磁场强度的解析解H_C1, H_C2, H_peak是正确的, 并从三个元激发对磁化强度的贡献进行了说明. 低温下磁化强度随外磁场的变化呈现1/3磁化平台. 体系的磁化率随温度或者外磁场的变化都出现了双峰现象. 这说明双峰源于二聚体分子内电子自旋平行排列的铁磁交换作 用能和二聚体与单基体分子间电子自旋反平行排列的反铁磁交换作用能, 热无序能, 外磁场强度相关的自旋磁矩势能之间的竞争.     

Study of magnetic interactions of the Fe2+ ions and the protons of crystallization water in FeSO4.n H2O hydrates

The paper deals with a study of interactions of protons in crystallization water of paramagnetic hydrates FeSO₄.n H₂O (wheren=1, 4, 5 and 7). The interaction of protons and paramagnetic ions Fe²⁺ and the nuclear dipole-dipole interaction of protons have been studied by means of the parameters expressing the dependence of the second moment of the proton nuclear magnetic resonance (NMR) spectrum on the external magnetic field, by means of the volume magnetic susceptibilities and the relaxation timesT ₁ of the investigated series of hydrates.We are grateful to Prof. Dr. hab. Z.Pajak, director of Department of Radiospectroscopy IF UAM in Poznan, and to his coworkers Dr. J.Kapturczak, Dr. K.Jurga, Dr. S.Glowinkowski and Mgr. F.Brych for facilitating our measurements mentioned in the paper as well as for a valuable discussion.     

Magnetically induced phase transitions in LaCoO<Subscript>3</Subscript> in fields of up to 500 T

The differential magnetization of LaCoO₃ in magnetic fields of up to 500 T has been measured at a temperature of 4.2 K. The magnetization curve reveals several features which suggest a complex pattern of the transition of LaCoO₃ from the low-spin state to the high-spin state. The magnetic moment starts to grow in fields above 50 T to reach a plateau in the 130–240-T region, after which the magnetic moment continues to rise up to saturation in fields ∼500 T.     

Magnetic properties of crystalline boron

Magnetic properties of -rhombohedral boron are investigated measuring the temperature dependence of magnetic susceptibility. It was found that this dependence is similar to that of semiconducting glasses. An explanation of the dependence found is given and the concentration of dangling bonds is checked using thermodynamic consideration.The author thanks Prof. T.Niemyski for supplying the boron samples, Prof. L.Sosnowski and Dr. A.Zarba for valuable discussions and Mr. M.Klígl for technical assistance.     

X-ray photoelectron spectroscopy and magnetism of Mn1−x Al x alloys

X-ray photoelectron spectroscopy (XPS), magnetization and magnetic susceptibility of Mn_1−x Al _x (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.7) alloys are reported. X-ray diffraction measurements showed that all investigated samples have the same crystallographic structure as the parent compound (AuCu₃-structure type). The alloys are disordered for x ≤ 0.5, but become almost crystallographically ordered for higher Al concentration. This change in the crystallographic order is reflected both in the magnetization and Curie temperature values. The exchange interaction is ferromagnetic between the pairs of the near-neighbour Mn-Ni and Ni-Ni magnetic moments and antiferromagnetic for Mn-Mn pairs. The last one is present only in the disordered alloys, which leads to smaller values of the magnetization of these alloys in comparison with the ordered ones. The Mn magnetic moment has the fully ordered value of 3.2 μB in all investigated alloys. The decrease of the Ni magnetic moment as the Al concentration increases may be explained by the hybridization of the Ni 3d and Al 3sp states, which leads to a partial filling of the Ni 3d band. The magnetic susceptibility measurements pointed out the existence of spin fluctuations on Ni sites.      

Magnetic properties of the CuCoAlBO<Subscript>5</Subscript> single crystal

High-quality CuCoAlBO₅ single crystals have been grown, and their crystal structure, magnetic susceptibility, and magnetization have been studied. It has been established that the CuCoAlBO₅ compound is an uncompensated antiferromagnet or ferrimagnet with a small magnetic moment and the magnetic ordering temperature T _N = 28 K. A model has been proposed for the magnetic structure. A strong anisotropy of the magnetic properties has been revealed.     

Magnetic properties of Europium-graphite intercalation compound C6Eu

An ordered state with a complex magnetic structure has been observed below 4CK. The magnetization for H ⊥ c (c; the c-axis) shows a field-induced phase transition from a small magnetic moment state ( ～ 0.6μ_B) at lower fields to a larger moment one ( 2.2 - 2.5μ_B) above 25kG. The transition field depends on temperature and is found to be 5 - 2kG. The magnetization for H//c has only a sublinear field dependence. Above 40K the magnetic susceptibility obeys the Curie-Weiss law with θ = +1.3K and the molar Curie constant C_m = 6.78, which suggests that Eu ion is divalent in C₆Eu.     

Vertical sea level meson-neutrino spectrum derived from the Goddard Space Flight Center measured nucleon spectrum

The vertical sea level meson neutrino spectrum has been derived from the measured primary nucleon spectrum of Goddard Space Flight Centre Group by using the model of Pal and Peters. The calculated result agrees with the work of Osborne et al. at high energies.The author is thankful to Prof.Yash Pal and Prof. J. L.Osborne for sending their works. Thanks are due to C.S.I.R., Govt. of India for financial support.     

The temperature dependence of some thermodynamical quantities of ferromagnetic alloys in the region of magnetic phase transition

This paper deals with the behaviour of ferromagnetic alloys in the region of magnetic phase transition. The temperature dependences of spontaneous magnetization, magnetic susceptibility and specific heat are discused. The theoretical results are compared with experimental data.I wish to thank Professor L.Valenta for his kind guidance and advice given to me during my work on this paper.     

Conductivity and cation distribution in CuxMg1−xFe2O4 system

The electrical conductivity and magnetization of Cu _x Mg_1{x Fe₂O₄ are measured. The electrical conductivity obeys the exponential behaviour characteristic of semiconducting materials. The activation energy in the paramagnetic region is found to be greater than that in the ferrimagnetic region. This is attributed to the change in magnetic ordering. It is observed that the plots of logQ vs 10³/T exhibit a linear relationship throughout the temperature region studied (room temperature to 1000 K). The slope of the plot changes at a particular temperature recognized as Curie temperature. These temperatures agree well with the experimentally obtained Curie temperatures using Laroia technique. Hopping of polarons mechanism is used to explain the conduction behaviour in ferrites. The magnetic moment (n_B) is calculated from the magnetization studies carried out at 78 K. The cation distribution is suggested on the basis of these magnetization data.The authors are thankful to Prof. R. N. Patil for encouragement.     

Antiferromagnetism in the Hubbard model

The energy spectrum of the two-sublattice Hubbard model is obtained in the static-fluctuation approximation. It is shown how the structure of the energy spectrum is modified as the parameters of the Hubbard model are varied. The ground state of the simple Hubbard model of dimension d=2 is the dielectric antiferromagnetic state. The author derives a consistency equation for the magnetization, which has an antiferromagnetic solution. Fiz. Tverd. Tela (St. Petersburg) 39, 1594–1599 (September 1997)     

Magnetic susceptibility and magnetic-field behavior of CuB2O4 copper metaborate

An experimental study is reported regarding the temperature dependence of the magnetic susceptibility of a CuB₂O₄ tetragonal single crystal within the 4.2–200-K range. It has been established that the magnetic susceptibility exhibits anomalies at 21 and 10 K and depends strongly on crystal orientation in the magnetic field. A study has been carried out of the field dependences of the magnetization of CuB₂O₄ at various temperatures and crystal orientations. It is shown that for T>21 K, the crystal is in a paramagnetic state determined by Cu²⁺ copper ions with an effective magnetic moment of 1.77 μ_B. Within the 10–21 K interval, the field dependence of the magnetization is typical of a weak ferromagnet with magnetic moments of the two antiferromagnetically coupled sublattices lying in the tetragonal plane of the crystal. The spontaneous weakly-ferromagnetic moment is 0.56 emu/g at 10 K. The canting angle of the sublattice magnetic moments, determined by the Dzyaloshinski-Moriya interaction, is 0.49°. It is believed that below 10 K, the CuB₂O₄ crystal retains its easy-plane magnetic structure, but with a zero spontaneous magnetic moment.     

Magnetization studied on spin alignments in organic molecule-based ferrimagnetics

A physics picture of spin alignments in molecule-based ferrimagnets is presented from studying the temperature dependence of the effective sublattice magnetic moments and the total reduced magnetization by means of Green’s function theory combined with the Jordan-Wigner transformation. The ferrimagnetic chain includes an S=1 biradical and an S=1/2 monoradical with antiferromagnetic alternating interactions, and the S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculations of the sublattice magnetic moments, the magnetic moment of the S=1 biradical is negative, while that of the S=1/2 monoradical is positive, leading to a ferrimagnetic ground state. With the different kinds of the elementary excitations and the competition between the magnetic interactions and thermal fluctuations, the temperature dependence of the magnetization displays rich thermodynamic properties. Meanwhile, the external magnetic field dependence of the magnetization has a clear plateau at one third of the saturation magnetization, which can be compared with the possible experimental findings.     

Critical behavior studies in Ti-substituted lanthanum bismuth perovskite manganites

Perovskite manganite La_0.4Bi_0.6Mn_1−xTi_xO₃ (x = 0.05 and 0.1) synthesized using conventional solid state route method give rise to critical phenomenon in their magnetic interactions due to the substitution of non magnetic Ti ions. The critical behavior is observed near paramagnetic–ferromagnetic transition and is studied by magnetization measurements. Various techniques like Modified Arrott plot, Kouvel–Fisher method, scaling equation of state analysis and the critical magnetization isotherm were used to analyze the magnetization data on magnetic phase transition. The values of critical exponents β and γ obtained using different techniques are in good agreement. The obtained critical exponents are found to follow scaling equation with the magnetization data scaled into two different curves below and above the transition temperature, T_C. This confirms that the critical exponents and T_C are reasonably accurate. The obtained critical exponents for both the samples deviates from mean-field model and do not completely follow the static long range ferromagnetic ordering. This behavior is consistent with non magnetic nature of Ti substituted at Mn site and can be associated with Griffiths phase like phenomenon.     

Induced moment antiferromagnet in an external magnetic field—I: Molecular field theory

The behavior of a two-level induced moment antiferromagnet in an external magnetic field is investigated in the molecular field approximation. A significant reduction in the critical field and in the sublattice magnetizations is shown. However, the total magnetization rises more rapidly with field and can remain at large value in an external field even at T = 0. The magnetic susceptibility also remains finite at T = 0 in contrast to the case of a permanent moment Ising antiferromagnet. The effects of a ferromagnetic next-nearest neighbor interaction are then examined. It is shown that, in contrast to the usual Ising antiferromagnets, the ferromagnetic coupling has to exceed a certain value depending on the crystal field strength and the antiferromagnetic interaction, to allow for a first order phase transition in a field to occur even at zero temperature.     

Inhomogenous magnetic ground state in CeAgGa

CeAgGa crystallizes in CeCu₂ Imma structure with Ag and Ga atoms randomly distributed at 8h sites. The magnetic and transport properties of the orthorhombic CeAgGa compound have been obtained from the analysis of ac magnetic susceptibility χ _ac , magnetization M vs. magnetic field, specific heat C and electrical resistivity ρ. The results provide evidence for the formation of a spin-glass state with a freezing temperature T _f = 5.1 K. The randomness in the Ce-Ce magnetic exchange interactions seem to arise from a statistical distribution of Ag and Ga atoms on a crystallographic site of the CeAgGa crystal lattice. The results provide also evidence for the formation of ferromagnetic-like order at the temperature T _C ≈ 3.6 K. Band structure calculations for a disordered system give magnetic moment similar to saturation moment obtained from magnetization measurements, however, its calculated value is insensitive on Ga/Ag off-stoichiometry in the 8h position. Complex behavior of CeAgGa Kondo-lattice compound is discussed in terms of interplaying Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, Kondo effect and structural disorder.