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1.
Magnetization and neutron diffraction studies have been performed on Ce4Sb3 compound (cubic Th3P4-type, space group I4¯3d, no. 220). Magnetization of Ce4Sb3 reveals a ferromagnetic transition at ∼5 K, the temperature below which the zero-field-cooled and field-cooled magnetization bifurcate in low applied fields. However, a saturation magnetization (MS) value of only ∼0.93μB/Ce3+ is observed at 1.8 K, suggesting possible presence of crystal field effects and a paramagnetic/antiferromagnetic Ce3+ moment. Magnetocaloric effect in this compound has been computed using the magnetization vs. field data obtained in the vicinity of the magnetic transition, and a maximum magnetic entropy change, −ΔSM, of ∼8.9 J/kg/K is obtained at 5 K for a field change of 5 T. Inverse magnetocaloric effect occurs at ∼2 K in 5 T indicating the presence of antiferromagnetic component. This has been further confirmed by the neutron diffraction study that evidences commensurate antiferromagnetic ordering at 2 K in zero magnetic field. A magnetic moment of ∼1.24μB/Ce3+ is obtained at 2 K and the magnetic moments are directed along Z-axis.  相似文献   

2.
Magnetic and magnetocaloric properties of the compound Ce5Ge4 have been studied. This compound has orthorhombic Sm5Ge4-type structure (space group Pnma, no. 62) and orders ferromagnetically at ~12 K (TC). The paramagnetic Curie temperature is ~−20 K suggesting the presence of competing ferromagnetic and antiferromagnetic interactions in this compound. The magnetization does not seem to saturate even in fields of 90 kOe at 3 K consistent with the presence of competing interactions. Saturation magnetization value (extrapolated to 1/H→0) of only 0.8μB/Ce3+ is obtained compared to the free ion value of 2.14μB/Ce3+. This moment reduction in the ordered state of Ce5Ge4 could be due to partial antiferromagnetic/paramagnetic ordering of the Ce moments and may also be due to crystalline electric field effects. Magnetic entropy change near TC, calculated from the magnetization vs. field data, is found to be moderate with a maximum value of ~9 J/kg/K at ~11 K for a field change of 90 kOe.  相似文献   

3.
Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic moments along the crystal a axis. From a 151Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure sine-wave just below T N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor. Received 17 January 2001 and Received in final form 20 March 2001  相似文献   

4.
In an attempt to determine the magnetic structures of the heavy rare earth manganites of perovskite type, we have studied first the antiferromagnetic order of manganese in YMnO3. The Néel temperature is about 42 K, the Mn3+ ordering is a helix and derives from an A mode. The propagation vector of the helical structure is along the b axis: k=[0 ky 0] with ky= 0.0786. The Mn3+ ions carry a magnetic moment of only 3.10 ± 0.1 μB at 4.2 K. We present a phase diagram of helical and collinear modes in terms of exchange integrals.  相似文献   

5.
The redox behaviour of a CuO-CeO2/Al2O3 catalyst is studied under propane reduction and re-oxidation. The evolution of the local Cu and Ce structure is studied with in-situ transmission X-ray absorption spectroscopy (XAS) at the Cu K and Ce L3 absorption edges.CuO and CeO2 structures are present in the catalyst as such. No structural effect on the local Cu structure is observed upon heating in He up to 873 K or after pre-oxidation at 423 K.Exposure to propane at reaction temperature (600-763 K) fully reduces the Cu2+ cations towards metallic Cu0. Quick EXAFS spectra taken during reduction show a small amount of intermediate Cu1+ species. Parallel to the CuO reduction, CeO2 is also reduced in the same temperature range. About 25% of the Ce4+ reduces rapidly to Ce3+ in the 610-640 K temperature interval, while beyond 640 K a further slower reduction of Ce4+ to Ce3+ occurs. At 763 K, Ce reduction is still incomplete with 32% of Ce3+.Re-oxidation of Cu and Ce is fast and brings back the original oxides.The propane reduction of the CuO-CeO2/Al2O3 catalyst involves both CuO and CeO2 reduction at similar temperatures, which is ascribed to an interaction between the two compounds.  相似文献   

6.
The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH2O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe3+ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT 2.  相似文献   

7.
TbRh2Ge2 orders antiferromagnetically in the AFI type structure with a magnetic moment of 9.4(3)μB at 4.2 K localized on the Tb3+ ion. TbRu2Ge2 exhibits a square modulated magnetic structure with moments aligned along the c-axis of the crystallographic unit cell. The magnitude of the magnetic moment localized on the Tb3+ ion is 9.1(1)μB at 4.2 K.  相似文献   

8.
The magnetic structure of the rare earth tetraboride TbB4 (crystallographic space group P4/mbm) has been determined by neutron diffraction on a polycrystalline sample. Below the experimentally determined Néel temperature of TN = (43±1) K TbB4 is ordered antiferromagnetically. The data refinement yielded a magnetic moment value of (7.7 ± 0.2) μB/Tb ion at 4.2 K which we interpret as Tb4+. The magnetic structure is antiferromagnetic collinear with the moments perpendicular to the tetragonal axis.  相似文献   

9.
A comparative μSR study of ceramic samples of the EuMn2O5 and Eu0.8Ce0.2Mn2O5 multiferroics is performed in the temperature range from 15 to 300 K. It is found that the Ce doping of the EuMn2O5 sample slightly reduces the temperature of the magnetic phase transition from T N = 45 K for the EuMn2O5 sample to T N = 42.5 K for the Eu0.8Ce0.2Mn2O5 sample. Below the temperature T N for both samples, there are two types of localization of a thermalized muon with different temperature dependences of the precession frequency of the magnetic moment of the muon in an internal magnetic field. The higher frequency in both samples refers to the initial antiferromagnetic matrix. The behavior of this frequency in Eu0.8Ce0.2Mn2O5 follows the Curie–Weiss law with the exponent β = 0.29 ± 0.02, which differs from the value β = 0.39 standard for 3D Heisenberg magnetics and is observed in EuMn2O5, because of the strong frustration of the doped sample. The temperature-independent low frequency is due to the presence of Mn3+–Mn4+ ferromagnetic pairs located along the b axis of the antiferromagnetic matrix and in the regions of phase separation, which contain such ion pairs and e g electrons recharging them. In both samples, polarization losses are the same (about 20%) and are associated with the formation of Mn4+–Mn4+ + Mu complexes near Mn3+–Mn4+ ferromagnetic pairs. In the temperature interval from 25 to 45 K, the separation of the Eu0.8Ce0.2Mn2O5 structure into two fractions where the relaxation rates of polarization of muons differ by an order of magnitude is revealed. This effect is due to a change in the state of regions of phase separation (1D superlattices) at the indicated temperatures. Such effect in EuMn2O5 is significantly weaker.  相似文献   

10.
Magnetometric and neutron diffraction studies of polycrystalline NdCo2GE2, ErCo2Ge2 and PrFe2Ge2 compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ~ 4.2 and 13 K, respectively. The RECo2Ge2 compounds have collinear antiferromagnetic order of +?+? type. For PrFe2Ge2 a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE3+ free ion value. In ErCo2Ge2 the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.  相似文献   

11.
The halide perovskite TiF3, renowned for its intricate interplay between structure, electronic correlations, magnetism, and thermal expansion, is investigated. Despite its simple structure, understanding its low-temperature magnetic behavior has been a challenge. Previous theories propose antiferromagnetic ordering. In contrast, experimental signatures for an ordered magnetic state are absent down to 10 K. The current study has successfully reevaluated the theoretical modeling of TiF3, unveiling the significance of strong electronic correlations as the key driver for its insulating behavior and magnetic frustration. In addition, frequency-dependent optical reflectivity measurements exhibit clear signs of an insulating state. The analysis of the calculated magnetic data gives an antiferromagnetic exchange coupling with a net Weiss temperature of order 25 K as well as a magnetic response consistent with a S = 1/2 local moment per Ti3+. Yet, the system shows no susceptibility peak at this temperature scale and appears free of long-range antiferromagnetic order down to 1 K. Extending ab initio modeling of the material to larger unit cells shows a tendency for relaxing into a noncollinear magnetic ordering, with a shallow energy landscape between several magnetic ground states, promoting the status of this simple, nearly cubic perovskite structured material as a candidate spin liquid.  相似文献   

12.
Neutron powder diffraction has been used to investigate the magnetic order in a series of RMn4Al8 (space group I4/mmm) compounds with R = (Nd, Dy, Ho and Er). For compounds with Mn, no magnetic order was detected down to 1.6 K, whilst for the compound ErCr4Al8, two very weak magnetic reflections were observed at 1.6 K. The observed magnetic peaks cannot be indexed on the tetragonal unit cell, indicating a possible incommensurate magnetic structure. For ErCu4Al8, a type 1 antiferromagnetic structure is observed. The magnitude and temperature dependence of the Er3+ magnetic moment can be adequately modelled by a combined isotropic exchange and tetragonal crystal field Hamiltonian within the Mean Field Approximation.  相似文献   

13.
Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce12Fe57.5As41 and La12Fe57.5As41. The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds. Both compounds exhibited multiple magnetic orders within 2–300 K and metamagnetic transitions at various fields. Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce12Fe57.5As41 and La12Fe57.5As41, respectively, followed by antiferromagnetic type spin reorientations near Curie temperatures. The magnetic properties underwent complex evolution in the magnetic field for both compounds. An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce12Fe57.5As41. The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure. A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce12Fe57.5As41. A temperature-field phase diagram was present for these two rare earth systems. In addition, a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150–300 K, which is rarely found in 3D-based compounds. It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.  相似文献   

14.
Antiferromagnetic Fe0.9S forms a ferrimagnetic superlattice in a narrow temperature range centred at 475 K (γ-transition). We have made a study in single crystals by specific heat, Mössbauer spectroscopy and magnetic susceptibility of the magnetic structure and kinetics and thermodynamics of formation of the ferrimagnetic component.The antiferromagnetic alignment and the spontaneous moment lie in the c-plane. The magnetic anisotropy energy is of the order of 0.4 K per Fe ion. Above 475 K and below TN ≈ 598 K there is a change in ordering of the vacancies and a new antiferromagnetic structure is formed. At lower temperatures our value (100 kJ mol?1) of the activation energy of diffusion of iron vacancies, determined from magnetic susceptibility, is in quite good agreement with the value (88 kJ mol?1) obtained from radiotracer measurements by Condit et al.  相似文献   

15.
Electrical conductivity with and without magnetic field, d.c. magnetization and 151Eu Mössbauer studies were carried out in La0.38Eu0.29Ca0.33MnO3 perovskite manganite system. An insulating ground state is found throughout the temperature range with charge ordered (CO) state emerging at T CO ~ 140 K, where as an external magnetic field of 6 T induces metal-insulator transition at ~120 K. D.C. magnetization measurements show the antiferromagnetic (AFM) transition occurring at T N ≈ 48 K. The temperature dependent 151Eu Mössbauer measurements showed that the substituted Eu replaces La3+ in the 3+ charge state and a small magnetic moment gets induced at the Eu nucleus at low temperatures. The anomalous variation of the f- factor with temperature occurring around T N and T CO corroborates the occurrence of antiferromagnetic (AFM) and charge ordering (CO) transition, respectively.  相似文献   

16.
Cr-doped manganites Sr0.9Ce0.1Mn1−yCryO3 (y=0, 0.05, and 0.10) have been systematically investigated by X-ray, magnetic, transport, and elastic properties measurements. For parent compound Sr0.9Ce0.1MnO3, it undergoes a metal-insulator (M-I) transition at 318 K, which is suggested to originate from a first-order structural transition accompanied by Jahn-Teller (JT) transition. With increasing Cr doping content, the JT transition temperature decreases. The Cr doping suppresses the antiferromagnetic (AFM) state and makes the system spin-glass (SG) behavior at low temperatures. In the vicinity of JT transition temperatures, the softening of Young's modulus originating from the coupling of the orbital (quadrupolar) moment of the eg orbital of Mn3+ ion to the elastic strain has been observed. The anomalous Young's modulus properties imply the electron-phonon coupling due to the JT effect may play an important role in the system.  相似文献   

17.
A complex oxide of the Y2Mn2/3Re4/3O7 composition with pyrochlore-like structure and parameters of hexagonal unit cell a=14.91(1) Å c=17.53(1) Å was synthesized. The magnetic susceptibility and magnetization measurements showed that below 190 K this oxide possesses spontaneous magnetic moment. In the paramegnetic region, the magnetic susceptibility obeys the Curie-Weiss law χ=C/(T?Θ), with C=2.07 cm3 K mol?1 and Θ=?160 K, and the effective magnetic moment corresponding to the cationic combination Mn2+-Re5+. The data obtained allow one to assume that the compound has a noncollinear antiferromagnetic structure.  相似文献   

18.
The Mössbauer hyperfine spectra of the 60 keV resonance of 237Np in powder and single crystal absorbers of NpAs2 were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs2. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μB at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.  相似文献   

19.
Using powder neutron diffraction techniques, we have examined the magnetic order of the pseudoternary compound Ho(Rh0.3Ir0.7)4B4 below the Néel temperature TN=2.7K. The magnetic structure consists of stacked antiferromagnetic basal plane sheets forming a body centered tetragonal unit cell, with a sublattice magnetization corresponding to 9.6±0.6μB per Ho3+ion at 1.5 K. Magnetic intensity versus temperature measurements indicate that the transition is second order and reveal no anomalous effects when the compound becomes superconducting at Tc=1.34K.  相似文献   

20.
Ternary silicides (RE, Th, U)Ru2Si2 have been synthesized from the elements. All the compounds (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were found to be isotypic and to crystallize with the structure type of ThCr2Si2 (ordered derivative of the BaAl4-type). The magnetic behavior of these alloys was studied in the temperature range 1.5 K < T < 1100 K. Magnetic susceptibilities at temperatures T > 300 K closely follow a typical Van Vleck paramagnetism of free RE3+-ions. In the case of CeRu2Si2 susceptibilities are well described for 20 K < T < 1100 K by a Van Vleck paramagnetism of widely spaced multiplets; the observed effective paramagnetic moment μeff = 2.12 BM indicates a high percentage (85%) of Ce3+. SmRu2Si2 yields an effective moment μeff = 0.54 BM, which compares reasonably well with the Hund's rule J = 5/2 ground level for free Sm+ and a low-lying excited level with J = 7/2. For temperatures T > 15 K the magnetic susceptibility as a function of temperature follows the “Van Vleck behavior” for free Sm3+. At low temperatures ferromagnetic ordering was encountered for (Pr, Nd, Ho, Er, Tm)Ru2Si2, whereas antiferromagnetic ordering was observed for (Sm, Gd, Tb, Dy)Ru2Si2. The ordering temperatures are generally below 55 K. No superconductivity was found for temperatures as low as 1.8 K.  相似文献   

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