Spin-glass state and ferromagnetic order in Cu(II)－Fe(III) cyanides

The static and dynamic magnetic properties of a Prussian blue analogue, molecular magnet Cu^Ⅱ_{3}[Fe^Ⅲ(CN)_6]_2·3NH_3·6H_2O were investigated in detail. The H dependence of the linear AC susceptibility, the irreversibility in the field-cooled (FC)/zero-field-cooled (ZFC) magnetization (M_{FC}/M_{ZFC}) and the relaxation of M_{ZFC} suggest that the magnetic system can be visualized as containing a ferromagnetic cluster of spin below T_C, mixed with small spin-glass clusters formed below temperature T_g less than T_C. The observed magnetic properties are explained with a ferromagnetic-spin-glass phase model. The magnetic ordering of the sample occurs below 19.8K.     

The temperature dependence of the hyperfine interaction of the transition metal osmium in a gadolinium host matrix

Integral perturbed angular correlations of the 931-155keVγγ-cascade of¹⁸⁸Os in Gd have been measured. With this technique the combined magnetic and electric hyperfine interaction of the 155 keV level of¹⁸⁸Os as an impurity in a Gd host has been studied as a function of temperature. The result for the electric field gradient of Os in Gd at 300 K is: $$\left| {V_{zz} \left( {Os:\underline {Gd} } \right)} \right| = \left( {12.8_{ - 1.9}^{ + 3.1} } \right) \cdot 10^{17} {V \mathord{\left/ {\vphantom {V {cm^2 }}} \right. \kern-\nulldelimiterspace} {cm^2 }}.$$ For the magnetic hyperfine field at 4.2 K the value $$H_{hf} \left( {Os:\underline {Gd} } \right) = - 134\left( {26} \right)kG$$ was obtained. Sign and magnitude of the magnetic hyperfine field suggest the existence of a localized moment of about ?0.4 µ _B at the site of Os in Gd. With increasing temperature the magnetic hyperfine field decreases much stronger than the magnetization of the host. Possible explanations for this anomalous temperature dependence are discussed.     

Gleichzeitige Messung von Hyperfeinstruktur,Stark-Effekt und Zeeman-Effekt des39K19F mit einer Molekülstrahlresonanzapparatur

An electric Molecular-Beam-Resonance-Spectrometer has been used to measure simultanously the Zeeman- and Stark-effect splitting of the hyperfine structure of³⁹K¹⁹ F. Electric four pole lenses served as focusing and refocusing fields of the spectrometer. A homogenous magnetic field (Zeeman field) was superimposed to the electric field (Stark field) in the transition region of the apparatus. The observed (Δm _J =±1)-transitions were induced electrically. Completely resolved spectra of KF in theJ=1 rotational state have been measured. The obtained quantities are: The electric dipolmomentμ _e l of the molecul forv=0,1 and 2; the rotational magnetic dipolmomentμ _J forv=0,1; the difference of the magnetic shielding (σ_⊥ ? σ_∥) by the electrons of both nuclei as well as the difference of the molecular susceptibility (ξ_⊥ ? ξ_∥). The numerical values are

$$\begin{array}{*{20}c} {\mu _{e1} = 8,585(4)deb,} \\ {\frac{{(\mu _{e1} )_{\upsilon = 1} }}{{(\mu _{e1} )_{\upsilon = 0} }} = 1,0080,} \\ {{{\mu _J } \mathord{\left/ {\vphantom {{\mu _J } J}} \right. \kern-\nulldelimiterspace} J} = ( - )2352(10) \cdot 10^{ - 6} \mu _B ,} \\ {(\sigma _ \bot - \sigma _\parallel )F = ( - )2,19(9) \cdot 10^{ - 4} ,} \\ {(\sigma _ \bot - \sigma _\parallel )K = ( - )12(9) \cdot 10^{ - 4} ,} \\ {(\xi _ \bot - \xi _\parallel ) = 3 (1) \cdot 10^{ - 30} {{erg} \mathord{\left/ {\vphantom {{erg} {Gau\beta ^2 }}} \right. \kern-\nulldelimiterspace} {Gau\beta ^2 }}} \\ \end{array} $$     

Hyperfine fields on59Co in ReCo2

We report⁵⁷Fe Mössbauer measurements in the diluted antiferromagnetic (AF) Fe_xZn_1?xF₂ withx=0.25 at temperatures between 4.2 and 28 K. DC susceptibility measurements show a spin-glass (SG) phase at low temperatures forx?0.3. Our Mössbauer spectra show a competitive coexistence of an SG phase and AF order. We propose an interpretation in terms of clusters in the AF order and some spins exhibiting SG behavior.     

Infinite differentiability for one-dimensional spin system with long range random interaction

We consider one-dimensional spin systems with Hamiltonian: $$H\left( {\sigma _\Lambda } \right) = - \sum\limits_{t,t' \in \Lambda } {\frac{{\varepsilon _{tt'} }}{{\left| {t - t'} \right|^\alpha }}\sigma _t \sigma _{t'} - h\sum\limits_{t \in \Lambda } {\sigma _t } } $$ , where ? _tt′ are independent random variables and, using decimation and the cluster expansion, we show that, when α>3/2 andE(? _tt′ )=0, for any magnetic fieldh and inverse temperature β, the correlation functions and the free energy areC ^∞ both inh and β. Moreover we discuss an example, obtained by a particular choice of the probability distribution of the ? _tt′ 's, where the quenched magnetization isC ^∞ but fails to be analytic inh for suitableh and β.     

Magnetic properties and magnetocaloric effects in Tb6Co1.67Si3 compound

Magnetic properties and magnetocaloric effects of Tb₆Co_1.67Si₃ have been investigated by magnetization measurement. This compound is of a hexagonal Ce$_{6}$Ni$_{2}$Si$_{3}$-type structure with a saturation magnetization of 187\,emu/g at 5\,K and a reversible second-order magnetic transition at Curie temperature $T_{\rm C} = 186$\,K. A magnetic entropy change $\Delta S = 7$\,J\,$\cdot$\,kg$^{-1}$\,$\cdot$\,K$^{-1}$ is observed for a magnetic field change from 0 to 5\,T. A large value of refrigerant capacity (RC) is found to be 330\,J/kg for fields ranging from 0 to 5\,T. The large RC, the reversible magnetization around $T_{\rm C}$ and the easy fabrication make the Tb₆Co_1.67Si₃ compound a suitable candidate for magnetic refrigerants in a corresponding temperature range.     

Evolution of structural,magnetic and magnetocaloric properties in Sn-doped manganites La0.57Nd0.1Sr0.33Mn1−x Sn x O3 (x = 0.05–0.3)

Structural and magnetic properties of manganites series La_0.57Nd_0.1Sr_0.33Mn_1?x Sn _x O₃ with (0.05 ≤ x ≤ 0.30) have been investigated, and the critical exponents and magnetocaloric effect are studied around the room temperature, to shed light on Sn substitution influence. A solid-state reaction method was used in the preparation. A structural study using Rietveld refinement of XRD patterns indicates rhombohedral structure with R \( \overline{3} \) c space group for (0.05 ≤ x ≤ 0.20) and shows the existence of a secondary phase attributed to the neodymium tin oxide (Nd₂Sn₂O₇) pyrochlore for x = 0.3. The variation of the magnetization (M) vs. temperature (T), under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition at the Curie temperature T _C. In addition, it was discovered that increasing the tin content leads to a reduction in magnetization and a lowering of T _C from 282 K (x = 0.05) to 158 K (x = 0.20) with increasing Sn substitution. The samples exhibit the characteristics of spin/cluster-glass state which is evident from (zero-field-cooled and field-cooled) magnetization vs. temperature curves. Indeed, the thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T ^3/2, in accordance with Bloch’s law. The spin-stiffness constant D obtained from the Bloch constant was determined. A large magnetocaloric effect has been observed in both samples (x = 0.05 and x = 0.10): the maximum entropy change, \( \left| {\varDelta S_{\text{M} }^{\text{peak}} } \right| \) , reaches the highest value of 3.22 J/kg K under a magnetic field change of 5 T with a RCP value of 56 J/kg for x = 0.10 composition. This opens an interesting opportunity to this compound to compete with materials which work as magnetic refrigerants near room temperature. Besides, we show that the samples follow the conventional behavior of a second-order ferromagnetic transition. This was possible by investigating the critical behavior at the transition region by adopting the modified Arrott plot method. The values of the critical exponents (β, γ, δ and n) are determined and they are between those predicted by the three-dimensional Heisenberg model.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

Hyperfine structure,g j factors and lifetimes of excited2 P 1/2 states of Rb

The hyperfine structure of the 6² P _1/2 and 7² P _1/2 state of⁸⁵Rb and⁸⁷Rb and of the 6² P _3/2 state of⁸⁷Rb has been investigated with optical double resonance at intermediate magnetic fields. The magnetic interaction constants,g _j factors and lifetimes are: $$\begin{gathered} 6^2 P_{1/2} state: A\left( {^{85} Rb} \right) = 39.11\left( 3 \right) MHz,A\left( {^{87} Rb} \right) = 132.56 \left( 3 \right)MHz, \hfill \\ g_j = 0.6659\left( 3 \right), \tau = 1.14\left( {13} \right) \cdot 10^{ - 7} \sec , \hfill \\ 7^2 P_{1/2} state: A\left( {^{85} Rb} \right) = 17.68\left( 8 \right)MHz,A\left( {^{87} Rb} \right) = 59.92\left( 9 \right)MHz, \hfill \\ g_j = 0.6655\left( 5 \right), \hfill \\ 6^2 P_{3/2} state: g_j = 1.3337\left( {10} \right), \tau = 1.12\left( 8 \right) \cdot 10^{ - 7} \sec for ^{87} Rb. \hfill \\ \end{gathered} $$ From the hfs coupling constants of then ² P multiplets a 11.5% core polarization contribution to the magnetic hfs of then ² P _3/2 states is obtained, which is found to be independent from the main quantum numbern. The expectation values <r ^?3> _j for thenp valence electrons corrected for core polarization are compared with those derived from the² P fine structure separation. Good agreement is achieved for allnp levels with the choice ofZ _i =Z?3=34 for the effective nuclear charge number. The nuclear quadrupole moments of⁸⁵Rb and⁸⁷Rb are rederived on the basis of this more improved treatment for thep-electron-nucleus interaction yielding $$\begin{gathered} Q_N \left( {^{85} Rb} \right) = + 0.274\left( 2 \right) \cdot 10^{ - 24} cm^2 \hfill \\ Q_N \left( {^{85} Rb} \right) = + 0.132\left( 1 \right) \cdot 10^{ - 24} cm^2 \hfill \\ \end{gathered} $$ where the error does not include the remaining theoretical uncertainty of about 10%.     

Spin waves in paramagnetic Boltzmann gases

As the temperature is lowered we get an interesting temperature region? _d?T?? ²/mr ₀ ² (where? _d is the quantum degeneracy temperature,m the mass of a gas molecule,r ₀ the radius of interparticle interaction) in which the thermal de Broglie wavelength Λ of a particle is considerably greater than its sizer ₀ though Λ turns out to be less than the mean interparticle distanceN ^?1/3?Λ?r ₀. Although the gas molecules obey the classical Boltzmann-Maxwell statistics the system as a whole begins to exhibit a larger number of essentially quantum macroscopic collective features. One of the most interesting and dramatic features is the possibility of propagation of weakly damped spin oscillations in spin-polarized gases (external magnetic field, optical pumping). Such oscillations can propagate both in the low-frequencyθτ?1 regime and the high frequencyθτ?1. The last case is highly non-trivial for a Boltzmann gas with a short range interaction between particles. The weakness of relaxation damping of spin modes implies that the degree of polarization is high enough 1>/|α|?|a|/Λ, whereα=(N ₊?N _?)N,a is the two-particles-wave scattering length. Under these conditions the spectrum of magnons has the form (Bashkin 1981, 1984; Lhuillier and Laloe 1982) 1 $$\omega = \Omega _H + \left( {{{K^2 \nu _{\rm T}^2 } \mathord{\left/ {\vphantom {{K^2 \nu _{\rm T}^2 } {\Omega _{int} }}} \right. \kern-\nulldelimiterspace} {\Omega _{int} }}} \right)\left( {{{1 - i} \mathord{\left/ {\vphantom {{1 - i} {\Omega _{int} }}} \right. \kern-\nulldelimiterspace} {\Omega _{int} }}\tau } \right), \Omega _{int} = {{ - 4\pi ahN\alpha } \mathord{\left/ {\vphantom {{ - 4\pi ahN\alpha } m}} \right. \kern-\nulldelimiterspace} m}, \nu _{\rm T}^2 = {T \mathord{\left/ {\vphantom {T m}} \right. \kern-\nulldelimiterspace} m}$$ where Ω _H is the Larmor precession frequency for spins in the magnetic fieldH. Collisionless Landau damping restricts the region of applicability of (1) with not too large wave vectorsKv _T?|Ω_int|. The existence of collective spin waves has been experimentally confirmed in NMR-experiments with gaseous atomic hydrogen H↑ (Johnsonet al 1984). The presence of undamped spin oscillations means automatically the existence of long range correlations for transverse magnetization. Such correlations decrease with the distance according to the power law 2 $$\delta _{ik} \left( r \right) = 2\left| a \right|\frac{{\left( {\beta N\alpha } \right)^2 }}{\gamma }\delta _{ik} $$ . Hereβ is the molecule magnetic moment. Spin waves being even damped can nevertheless reveal themselves atT?? ²/mr ₀ ² or when |α|?r ₀/Λ. The first experimental discovery or damped spin waves in gaseous³He↑ has been done in Nacheret al 1984. Oscillations of magnetization can also propagate in some condensed media such as liquid³He-⁴He solutions, semimagnetic semiconductors etc.     

The magnetic and superconducting phase diagram of Gd-doped lanthanumsulfides

The system La_3-x( )_xS₄ [where ( ) denotes a vacancy] containing up to 25 at.% Gd is shown to be a model system in which magnetic interactions in metals can be studied. By measuring the temperature dependence of the low field a.c. susceptibility, the superconducting, spin-glass and ferromagnetic transitions can be determined as a function of the Gd concentration. The depression of the superconducting transition temperature mainly follows the theory of Abrikosov and Gor'kov, with a possibility for coexistence of superconductivity and spin-glass magnetic order near a critical concentration of 3 at.% Gd. The transition from spin-glass to ferromagnet is well-defined with the percolation limit for the long range ferromagnetic order at 14 at.% Gd.     

Crystal structure and magnetic state of the LaMn1−x VxO3 perovskites

The crystal structure and the magnetic state of polycrystalline LaMn_1?x V_xO₃ (0.1<x<0.9) compounds have been studied by x-ray and neutron diffraction methods, as well as by magnetization and ac susceptibility measurements. It is shown that substitution of vanadium for manganese ions leaves the orthorhombic crystal structure of the compounds (space group Pnma) unchanged. The magnetic structure is observed to change from a canted antiferromagnetic ordering (wavevector k=[0, 0, 0], with the antiferromagnetic moments aligned with the a axis and the ferromagnetic component of the magnetic moment parallel to the b axis) at vanadium concentrations x<0.4 to a collinear antiferromagnetic ordering (with the magnetic moments parallel to the b axis) at x>0.8; at this transition occurs through an intermediate state exhibiting spin-glass properties.     

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)₂. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.     

Study of the magnetic superconductor,Y9Co7, at high pressure and high magnetic field

We have measured the electrical resistance and magnetic susceptibility of the magnetic superconductor, Y_gCo₇, at pressures up to 20 kbar and in magnetic fields up to 6T. We have found that pressure suppresses the magnetism resulting in a higher superconducting transition temperature and conclude that Y₉Co₇ is an itinerant ferromagnet, not a spin-glass. Pressure also sharpens the superconducting transition and increases the critical magnetic field, signifying that the long range ferromagnetic and superconducting order parameters co-exist but vary spatially. For pressures greater than 6 kbar, the magnetoresistance is always positive, further indicating the suppresion of magnetism by high pressure.     

Magnetic properties of FexMn1−x S sulfides exhibiting the magnetoresistive effect

The magnetic, electrical, and thermal (derived from DTA data) properties of Fe_xMn_1?x S polycrystalline sulfides (0≤x≤0.38) synthesized based on α-MnS (NaCl cubic lattice) and exhibiting colossal magnetoresistance were studied. The studies were conducted at temperatures from 77 to 1000 K and magnetic fields of up to 30 kOe. As the degree of cation substitution in the Fe_xMn_1?x S system was increased, the magnetic order was found to change from antiferromagnetic to ferromagnetic. In the high-temperature domain (550–850 K), the samples undergo two phase transitions with critical temperatures $T_{c_1 }$ and $T_{c_2 }$ , which are accompanied by reversible anomalies in the magnetization and thermal (DTA) properties and by a semiconductor-metal transition.     

Spin-glass behavior of the polyvinyl pyrrolidone-protected Prussian blue analog K1.14Mn[Fe(CN)6]0.88 nanocubes

Prussian Blue analog K_1.14Mn[Fe(CN)₆]_0.88 nanocubes were synthesized by using polyvinyl pyrrolidone (PVP) as a protective matrix. The PVP-protected MnFe PBA nanocubes with face centered cubic structure are well dispersed with a narrow size distribution of around 50 nm. A spin-glass behavior (including hysteresis, a peak in the zero-field-cooled magnetization and frequency-dependent AC magnetic susceptibility) is observed in the nanoparticles. A possible origin of this spin-glass freezing is discussed. Spin disorder due to the structural defects may be the reason that causes the spin-glass freezing in the MnFe PBA nanoparticles.     

Giant magnetocaloric effect in Tb5Ge2-xSi2-xMn2x compounds

The crystal structure,magnetic and magnetocaloric characteristics of the pseduo ternary compounds of Tb5Ge2 xSi2 xMn2x(0 ≤ 2x ≤ 0.1) were investigated by x-ray powder diffraction and magnetization measurements.The x-ray powder diffraction results show that all compounds preserve the monoclinic phase as the majority phase and all the synthesized compounds were observed to be ferromagnetic from magnetization measurements.Magnetic phase transitions were interpreted in terms of Landau theory.Maximum isothermal magnetic entropy change value(20.84 J.kg-1.K-1) was found for Tb5Ge1.95Si1.95Mn0.1 at around 123 K in the magnetic field change of 5 T.