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1.
The magnetic structure of RFe6Ga6 intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, 57Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn12 structure (space group I4/mmm) with the magnetic structure of YFe6Ga6 consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe6Ga6 is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001  相似文献   

2.
Polycrystalline two-layered perovskite La2.5-xK0.5+xMn2O 7 + δ (0 < x < 0.5) samples have been prepared by a modified sol-gel method and their magnetoresistance and magnetocaloric effects have been studied. A large deviation between the metal-insulator (MI) transition temperature (T ρ ) and the magnetic transition temperature (TC) is observed. Large magnetoresistance (MR) effects with Δρ/ρ of 40% at 12 kOe are obtained in wide temperature ranges. The maximum of the magnetic entropy change peaks at its Curie temperature (TC), far above its MI transition temperature (T ρ ). The large magnetic entropy change (1.4 J/kg.K) is obtained in the sample La2.5-xK0.5+xMn2O 7 + δ (x = 0.35) upon 10 kOe applied magnetic field. Received 2 May 2002 / Received in final form 1st October 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: wzhong@ufp.nju.edu.cn  相似文献   

3.
ErCl3 crystallizes in the AlCl3-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3)?, b = 11.7456(5)?, c = 6.3187(3)? and . The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below mK a three-dimensional antiferromagnetic order with a propagation vector of sets in. The magnetic structure of ErCl3 was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er3+ ions are located on two-dimensional honeycomb layers in the ab plane. There are two antiferromagnetically coupled triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the ab plane and amount to 3.3(1) at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent (2) was derived for the magnetic phase transition. Received 1 December 1999 and Received in final form 21 July 2000  相似文献   

4.
The specific heat and resistive upper critical magnetic field of the single crystal La1.85Sr0.15CuO4 are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)  相似文献   

5.
The magnetic properties of ferroborate single crystals with substituted compositions Nd1 − x Dy x Fe3(BO3)4 (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.  相似文献   

6.
The magnetic structures of Mn1-xFexWO4 with x = 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase diagram could be completed in the coexistence range of different magnetic structures up to x = 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector = (±1/4, 1/2, 1/2) was found for x ⩽ 0.22 while the magnetic spins order with = (1/2, 0, 0) for x ≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with = (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x = 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽ x ⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed to a spin arrangement with = (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed Mn2+- and Fe2+-ions in the coexistence range 0.12 ⩽ x ⩽ 0.29 of different magnetic structures related to those of pure MnWO4 and FeWO4. Received 9 October 2002 Published online 14 March 2003  相似文献   

7.
We report on the detection of two narrow peaks of positive magnetoresistivity in the temperature dependence of the magnetoresistivity of a Fe0.95Co0.05Ge2 single crystal in the vicinity of a “smeared” first-order phase transition. The position of these peaks correlates with the position of singularities in the temperature dependence of the temperature derivative of the electrical resistivity and magnetic susceptibility. We show that these singularities in the transport and magnetic properties are, probably due to the presence of two percolation transitions with temperature in the magnetic subsystem of the crystal. Zh. éksp. Teor. Fiz. 112, 690–697 (August 1997)  相似文献   

8.
The magnetic properties of Eu1−x CaxMnO3 have been investigated. As the calcium content increased up to x=0.2, the magnetization and the blocking temperature of the magnetic moments of clusters increased and the magnetic anisotropy decreased. As the calcium content increased further, the magnetization decreased, while the “freezing” temperature of the magnetic moments increased. Anomalies of the magnetic properties were observed in compositions with x=0.4 and 0.5 at T=40 K; these anomalies are attributed to a transition to the antiferromagnetic state in the charge-ordered phase. Fiz. Tverd. Tela (St. Petersburg) 39, 117–120 (January 1997)  相似文献   

9.
The magnetization of R2Sc3Si4 compounds is measured in static magnetic fields up to 14 kOe in the temperature range 77–300 K. It is established that all compounds in the given series are paramagnetic at these temperatures. The paramagnetic Curie points are determined, and the effective magnetic moments are calculated. The measurements are performed on polycrystalline samples. Fiz. Tverd. Tela (St. Petersburg) 41, 1804–1805 (October 1999)  相似文献   

10.
57Fe M?ssbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4. Received: 23 June 1998 / Accepted: 14 July 1998  相似文献   

11.
A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8?, b ≈ 5.8?, c ≈ 11.3?) is structurally characterised by the presence of flattened NiO6 octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R2BaNiO5 ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R3+ cations are close to the free-ion value. The magnetic moment of Ni2+ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R3+, seems to occur below and very close to T N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected. Received 19 July 2001  相似文献   

12.
Abstract

In this work, La0.75Ca0.25FeO3?δ perovskite sample was prepared by the coprecipitation method. The nanoparticle was found to crystallize in the orthorhombic (Pbnm) phase as confirmed by X-ray diffraction (XRD) and transmission electron microscopic (TEM). The oxygen non-stoichiometry (δ) and magnetic states of iron ions (three magnetic sextets and non-magnetic doublet) were investigated by Mössbauer spectroscopy at room temperature (RT). The shape of the magnetic hysteresis loop of the sample reveals the existence of a weak ferromagnetism at RT. The magnetization vs. temperature curves, measured in the 9 to 200 K range, showed that the sample exhibits two magnetic-phase transition temperatures at 29 K (Tg) and 120 K (TCO). The magnetization isotherms, M (H), around these magnetic-phase transition temperatures for the sample are analyzed.  相似文献   

13.
A comprehensive study of the relationship between the electronic specific heat coefficient () and the temperature square coefficient (A) of the electrical resistivity for a single, cubic, heavy fermion alloy system, UPt5-xAux is presented. In this alloy system, whose low temperature properties are consistent with the Fermi-liquid behavior, varies by more than a factor of 10 while the corresponding A coefficient changes by a factor larger than 200. A tracks changes in fairly well, but , postulated to have a universal value for heavy fermions, is not constant and varies from about 10-6 (x = 0, 0.5) to 10-5 cm (mol K/mJ)2 (x > 1.1), thus from a value typical of transition metals to that characteristic of other heavy fermion compounds. We have found a correlation between and magnetic characteristics such as the paramagnetic Curie-Weiss temperature and the low temperature magnetic susceptibility divided by . Received 29 January 1999  相似文献   

14.
Specific heat (CV) measurements in the spin-1/2 Cu2(C2H12N2)2Cl4 system under a magnetic field up to H =8.25 T are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature dependences of both the susceptibility and the low-field specific heat are accurately reproduced by this model, deviations are observed above the critical field HC1 at which the spin gap closes. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuations. Finally, we show that such a Heisenberg ladder, for H > H C1, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation for the observed low temperature ( K-0.8 K) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state. Received: 23 July 1998 / Accepted: 24 August 1998  相似文献   

15.
The evolution of the structural and magnetic properties of the CMR (colossal-magnetoresistance) compound La0.35Pr0.35Ca0.30MnO3 as the temperature changes from 10 to 293 K is investigated by means of neutron diffraction. It is shown that the changes in the transport and magnetic properties are directly related with the rearrangement of the atomic structure. A phase transition to the metallic state occurs together with simultaneous ferromagnetic ordering of the manganese moments and is accompanied by a jump in volume. The static distortions of the oxygen octahedra which are observed to occur prior to the magnetic phase transition and which are practically absent at room temperature and in the FM phase attest to the orbital ordering of oxygen atoms on the bonds, with freezing-in of the Jahn-Teller modes. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 672–677 (10 May 1998)  相似文献   

16.
17.
Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite oxides Pr0.7Sr0.3-x xMnO3 ( 0 ? x ? 0.3) and Pr0.7-x xSr0.3MnO3 ( 0 ? x ? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite structure and R c space group for x ? 0.2 in strontium deficient samples and for x ? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for x = 0 to 90 K for x = 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x = 0 to 315 for x = 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ? x ? 0.1) in the strontium lacunar ones when the temperature decreases. Received 12 April 2000 and Received in final form 8 January 2001  相似文献   

18.
Magnetic phase transition in the CsDyW2O8 magnet has been studied by means of low temperature specific heat C ( T ) measurements. The magnetic ordering temperature of the Dy3+ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy3+ ions. The behavior of the C ( T ) dependencies above and below T N is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state. Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002  相似文献   

19.
The results of investigations of the structure, electrical, and magnetic properties in the system of antiferromagnetic semiconductors FexMn1−x S (0<x⩽0.5) are described. It is established that metal-insulator transitions with respect to concentration and temperature are connected with changes in the magnetic properties of the system. Fiz. Tverd. Tela (St. Petersburg) 40, 276–277 (February 1998)  相似文献   

20.
The magnetic transport properties have been measured for La0.67-xYxCa0.33MnO3 ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003  相似文献   

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