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1.
The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr0.7Ca0.34?xAxMnO3?δ (A=Sr, Ba), Pr0.7?xLaxCa0.3 MnO3?δ and Pr0.66Ca0.34?x SrxMnO3?δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr0.7Ca0.25Sr0.025MnO3?δ and Pr0.66Ca0.26Sr0.08MnO3?δ that exhibit a variation of resistance of 2.5. 107% and 109% at μ0 H=5 T for T=88 K and 50 K respectively, Pr0.58La0.12Ca0.3 MnO3?δ that exhibits a variation of 6.106% for μ0 H=5 T at T=80 K and Pr0.7Ba0.025Ca0.275MnO3?δ for which a resistance variation of 5.109%, at T=50 K, for μ0 H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature Tmax, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr0.7Ca0.3?xSrxMnO3?δ and Pr0.66Ca0.34?xSrxMnO3?δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation Tmax is decreased of about 50 K by introducing 0.034 hole per Mn mole.  相似文献   

2.
3.
Above Curie temperature, MnBi crystals are aligned in situ along the c-axis in a Bi matrix by a high fabrication magnetic field H f of 10 T. Magnetic testing shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from the alignment. The successful alignment m v result from the fact that the easy magnetization direction is along the c-axis of MnBi and the high fabrication field of 10 T is large enough to rotate the )MnBi crystal to this direction even though the temperature is above the Curie temperature.  相似文献   

4.
Neutron powder diffraction experiments performed on two selected compositions of the yttrium-based solid solution YNixMn1−xO3 clearly reveal a nuclear order between the Ni2+ and Mn4+ ions in the half-substituted compound YNi0.50Mn0.50O3, so that the crystal structure is no longer described in the conventional orthorhombic Pbnm space group, but in the monoclinic P21/n, all over the investigated temperature range (1.5-300 K). However, both X-rays diagrams and neutron patterns of the YNi0.25Mn0.75O3 phase are indexed in the Pbnm orthorhombic-like symmetry, indicating that the Mn and Ni ions are randomly distributed on the octahedral sites.In addition, neutron diffraction points out that the nature of the magnetic ordering is strongly connected to the structural properties. Whereas no long-range 3D-magnetic ordering was detected for the Pbnm YNi0.25Mn0.75O3 phase, the YNi0.50Mn0.50O3 compound exhibits a magnetic transition at The magnetic structure consists of two collinear Mn4+ and Ni2+ ferromagnetic layers (Fx0Fz magnetic configurations) with saturated magnetic moment values of 2.25(2) and 1.57(2) μB for Mn4+ and Ni2+, respectively, at 1.5 K.  相似文献   

5.
Tb2O2SO4 orders antiferromagnetically at 3.9 K in a four-sublattice structure with two of the nearest-neighbour moments antiparallel and two almost perpendicular to that of a central ion. Specific-heat and magnetization measurements were carried out and allowed to establish the phase diagram for the external magnetic field along one of the moments' direction. Starting at the Néel temperature, the boundary of the paramagnetic phase is first shifted to lower temperatures for increasing field and then it stays at constant temperature for further increase of field. The shift is caused by the reduction of the staggered field that is existing in the antiferromagnetic phase. The experimental results are corroberated to a large extent by mean-field calculations.  相似文献   

6.
The orthorhombic holmium oxisulphate orders as a two-sublattice antiferromagnet atT=3.5 K. In external fields along the crystallographica- andc-directions with large and medium-sized magnetic moment, respectively, a ferrimagnetic phase with 1/3 of the saturation magnetization is passed before the paramagnetic phase is reached. Calculations in mean-field theory reveal that for thec-direction the ferrimagnetic phase is not stable atT=0, it only exists for finite temperatures. Magnetization and susceptibility contain large contributions of van Vleck paramagnetism which at any rate have to be taken into consideration. The phase diagram for the two field directions and the magnetic structures of the different phases are established.  相似文献   

7.
A series of Ga doping perovskite cobaltite La2/3Sr1/3 (Co1-y Gay)03 (y = 0, 0.1, 0.2, 0.3 and 0.4) are prepared by the standard solid-state reaction method. Their magnetic properties and Co ions spin state transitions are studied. Upon doping, no appreciable structure changes can be found. However, the corresponding Curie temperature sharply decreases and the magnetization is greatly reduced, indicating that Ga doping destroys the ferromagnetic interaction in the system. In addition, the high temperature magnetization data follow the Curie-Weiss law. At least one kind of Co ions (Co^3+ or Co^4+) favours the mixed spin state, and most Co ions are at the lower spin state (low and intermediate state). With increasing Ga content, more Co ions transit to the higher spin state.  相似文献   

8.
The role of vibrational anisotropy of Mn3+O6 octahedron in the phase separation behavior of La0.67−yPryCa0.33MnO3 (x=0, 0.15, 0.25 and 0.30) has been investigated by means of magnetization M, internal friction Q−1, Young's modulus E along with the X-ray powder diffraction measurements. For the samples with y=0 and 0.15, the Q−1 exhibits three peaks in the ferromagnetic region, which are attributed to the intrinsic inhomogeneity of ferromagnetic phase, i.e. the electronic phase separation with the coexistence of insulating and conducting phases. However, both the samples with y=0.25 and 0.30 undergo a magnetic phase separation with the coexistence of the antiferromagnetic and ferromagnetic phases, and the Q−1 peaks related to the electronic phase separation have not been observed. In addition, the Q−1 exhibits a peak in the paramagnetic region for all samples, which may result from the formation of magnetic clusters. We observed that the evolution from electronic to magnetic phase separation is close related to the rapid increase in the ratio of two kinds of Jahn-Teller distortion modes Q3 and Q2, i.e. Q3/Q2. A schematic phase diagram is given in the text, and it is suggested that the enhancement of vibrational anisotropy of Mn3+O6 octahedron plays a key role in the evolution from electronic to magnetic phase separation.  相似文献   

9.
We have synthesized various half doped lanthanum perovskites (BB′=MnV, MnCr, MnCo, MnNi, FeCr, FeMn, FeCo, FeNi) by solid state reaction method. The crystal structure was orthorhombic in all samples, but the magnetic and transport properties showed a large variation with the composition. A ferromagnetic feature was quite strong in the Mn-based perovskites but weak or non-existent in the Fe-based ones. The resistivity of Ni-doped ones was much smaller than those of the others. For all half doped perovskits, remarkably, the temperature dependent transport was well explained by variable range hopping model with different localization lengths.  相似文献   

10.
The effects of A-site cation size disorder in ABO3 type charge-ordered and antiferromagnetic Pr0.5Ca0.5MnO3 system have been studied by substituting La3+, Sr2+ or Ba2+, while keeping the valency of Mn ions and the tolerance factor (t=0.921) constant in the substituted compounds. We find that the substitutions by these larger cations induce successive sharp step-like metamagnetic transitions at 2.5 K. The critical field for metamagnetism is the lowest for 3% Ba substituted compound, which has the largest A-site cation size disorder and the least distorted MnO6 octahedra, among the compounds reported here. These cation substitutions give rise to ferromagnetic clusters within antiferromagnetic matrix, indicating phase-separation at low temperatures. The growth of the clusters is found to vary with the substitution amount. The local lattice distortion of MnO6 octahedra enhances the charge ordering temperature and reduces the magnetization at high fields (>1 T) in these manganites.  相似文献   

11.
Co3V 2O8 is a spin- 3/2 system on a Kagomé staircase and is known to undergo two magnetic phase transitions between 6 and 11 K. The H-T phase diagram of Co3V 2O8 derived by magnetization measurements on a single crystal is presented. Additionally both ordered magnetic structures were investigated by neutron powder diffraction experiments and solved using Bertaut’s macroscopic theory. For the ferromagnetic phase the magnetic moments of the Co2+ ions were found to be 1.5(3)μB and 2.7(1)μB at 3.5 K along the crystallographic a axis for the (4a) and (8e) sites, respectively. The antiferromagnetic phase exhibits a magnetic cell with a doubled b axis with respect to the nuclear one. The magnetic moments point along the a axis being 1.8(2)μB (4a) and 1.8(1)μB (8e) at 8 K.  相似文献   

12.
The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf1−xTix)Fe2 (0?x?1) intermetallic compounds has been investigated by 57Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf0.4Ti0.6Fe2 compound is completely antiferromagnetic above 200 K.  相似文献   

13.
The results of an inelastic neutron scattering study of the spin wave spectrum for the garnet Fe2Ca3Si3O12(FeSiG) are presented. We compare the exchange parameters for this garnet and for the Ge-species (Fe2Ca3Ge3O12(feGeG)) having the same magnetic structure. We relate the differences found with structural information from powder neutron diffraction. In this way the super exchange paths viap orbitals of intermediate oxygen atoms can be identified. We discuss the effect of a small number (3.2(5)%) of Mn2+ impurities in the 24c sites. These lead to an effective ferromagnetic exchange between the Fe3+ ions and drastically renormalize the average exchange constants. An estimate for the Fe3+–Mn2+ indirect exchange between a and c sites of 6(1) K is obtained. The exchange parameters for the pure FeSiG are found to beJ 1=1.16(4) K,J 1=0.96(4K andJ 2=–1.24(4) K for nearest and next nearest neighbours, respectively. These values apply for a moment of 4.02(4) B per iron atom as obtained from a structure refinement of powder diffraction data. Finally we present results for FeSiG of a high resolution study of the excitations at the zone centre in an attempt to verify our earlier findings of a quantum spin wave gap for FeGeG. In contrast to the earlier measurements, we could follow the acoustical branch to much lower energies using a timeof-flight spectrometer. We found indications for a crossing of the two low lying spin wave branches, the acoustical one extrapolating to the anisotropy gap of 0.005 THz and the antiphase branch extrapolating to the quantum gap of 0.02 THz.  相似文献   

14.
Magnetic second-harmonic generation (SHG) and three-photon difference-frequency generation (DFG) are used to investigate the magnetic phase diagrams of MnO3 3- compounds with five- and sixfold coordination of the Mn3+ (3d4) ion. In the hexagonal manganites, the six basic antiferromagnetic structures following from a one-dimensional representational analysis of the hexagonal unit cell are clearly distinguished on the basis of their different selection rules for SHG. All structures break the geometric frustration of the unit cell by triangular antiferromagnetic ordering of the Mn spins. The stability and the mutual interactions of in-plane and inter-plane exchange and the in-plane anisotropy are investigated. The three quantities act as almost decoupled degrees of freedom, which leads to independent in-plane and inter-plane reorientations of the Mn spin lattice. DFG was observed in the orthorhombic manganites Pr1-xCaxMnO3 and Nd1-xSrxMnO3. The DFG reflects the fourfold anisotropy of the quasicubic crystal, and the independent tensor elements for DFG are determined quantitatively. A metal–insulator transition with a two-order-parameter coupling to antiferromagnetism and charge ordering leads to additional difference-frequency contributions, which allow us to observe the formation of ∼100 μm magnetic domains. The variety of results gained from the two vastly different groups of manganite compounds, with spectral and spatial resolution used as additional degrees of freedom, demonstrates the versatility of nonlinear magnetooptical experiments for the investigation of magnetic structures and symmetries. Received: 16 October 2001 / Published online: 2 May 2002  相似文献   

15.
A structural and thermodynamic study of the newly synthesized single crystal Sr5Rh4O12 is reported. Sr5Rh4O12 consists of a triangular lattice of spin chains running along the c-axis. It is antiferromagnetically ordered below 23 K with the intrachain and interchain coupling being ferromagnetic (FM) and antiferromagnetic (AFM), respectively. There is strong evidence for an Ising character in the interaction and geometrical frustration that causes incomplete long-range AFM order. The isothermal magnetization exhibits two step-like transitions leading to a ferrimagnetic state at 2.4 T and a FM state at 4.8 T, respectively. Sr5Rh4O12 is a unique frustrated spin-chain system ever found in 4d and 5d based materials without a presence of an incomplete 3d-electron shell.  相似文献   

16.
Rather old preparation of the compounds ThCo2Ge2 and ThCo2Si2 and their magnetic study in the temperature range 100–570 K, published by Omejec and Ban [Z. Anorg. Allg. Chem. 380 (1971) 111], indicated that both compounds ordered ferrromagnetically below 100 K. In order to verify the old data, polycrystalline samples of ThCo2Ge2 and ThCo2Si2 have been prepared by arc melting and subsequent annealing, and studied by X-ray diffraction at room temperature (RT), by superconducting quantum interference device (SQUID)-magnetization and AC-susceptibility measurements at 2–320 K, and by dc-magnetization measurements in variable magnetic fields up to 120 kOe at 5, 80, and 283 K. The magnetic measurements confirm the ferromagnetic ordering in both compounds, but with totally different Curie temperatures: ≈120(20) K for ThCo2Ge2 and above 320 K for ThCo2Si2. The paramagnetic values of ThCo2Ge2 and the ordering of both compounds are discussed and compared with the old results of Omejec and Ban.  相似文献   

17.
The nature of the double-exchange (DE) interaction in lanthanum manganites is studied through chemical substitutions, Cs for La, and high-pressure measurements. Static and high-frequency magnetic measurements and high-pressure electrical transport studies were carried out on bulk polycrystalline and radio-frequency sputtered thin films of La0.7-xCsxCa0.3MnO3 for x=0-0.1. The samples are found to be cubic. Curie temperature Tc measurements provide evidence for bond-length-related weakening of DE as x is increased from 0 to 0.03. For higher x, the bond-angle-related changes lead to an increase in the strength of DE. High-pressure mangetoresistance data indicate both bond length and bond-angle-related increase of 10–20 K/GPa in Tc with pressure, with the largest increase measured for x=0.03. The rate of increase in the Curie temperature with pressure decreases with increasing Tc. Anomalies are observed in the magnetic parameters for x=0.03. The Cs-concentration dependence of the low-temperature saturation magnetization shows a minimum close to x=0.03. Ferromagnetic resonance studies at x-band reveal a 5% decrease in the g-value for x=0.03 relative to the end members (x=0 and 0.1). The low-field magnetostriction for x=0.03 indicates a relatively strong electron–phonon spin coupling compared to neighboring compositions. Received: 15 May 2000 / Accepted: 24 July 2000 / Published online: 9 November 2000  相似文献   

18.
The methods of neutron diffraction, X‐ray and magnetic measurements were used to study the structural and magnetic states of disordered samples of manganite La0.825Ba0.175MnO3. A disordered state was attained by irradiation with fast neutrons. It was established that the ferromagnet → spin glass‐like magnetic transformation takes place at a substitution concentration of ~6%. A magnetic state diagram of structurally disordered manganite was constructed. The magnetic state transformations are explained by the effect of localization of e g‐electrons responsible for kinetic ferromagnetic exchange. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
We present the results of a study of electron-doped Sm1−xSrxMnO3 (x>0.5) perovskite manganites by combining high-resolution neutron powder diffraction with measurements of resistivity, magnetization and magnetic susceptibility. Although investigated Sm0.45Sr0.55MnO3 and Sm0.37Sr0.63MnO3 compounds belonging to the same phase diagram area differ significantly in the strontium content, they are homogeneous antiferromagnetic (AF) insulators and do not exhibit CMR. They have different crystallographic symmetries (orthorhombic Pbnm and tetragonal I4/mcm, respectively) in the entire temperature range under study (1.5-288 K), differ in the type of spin ordering at low temperatures (AF-A and AF-C), are characterized by different orbital polarizations (dx2y2 and d3z2r2), and possess two- and one-dimensional magnetic properties, respectively. The lack of magnetoresistance for these compositions is explained by the lack of coexisting magnetic phases involving double exchange ferromagnetism, in contrast to what is observed for the magnetoresistive Sm1−xSrxMnO3 compounds, that is with x?0.52.  相似文献   

20.
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