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1.
Concentration dependent positron annihilation lifetime (PAL) measurements on Cr doped La0.5Pb0.5Mn1−yCryO3 (y=0.075, 0.15, 0.3, 0.35, 0.45) samples showing metal-insulator transitions (MIT) between 178 and 276 K (depending on y) reveal anomalous variation of average and bulk PAL, τav and τB, respectively, around y=0.35. Such anomaly has not, however, been observed from the corresponding magnetic susceptibility and resistivity data. Interestingly, the model parameters (polaron radius, number of ion sites per unit volume) obtained from fitting the high temperature (above MIT, Tp) resistivity data with small polaron hopping model show the signature of the said anomaly around the same concentration. Nonlinear variations of τav and τB support the existence of small polarons (T>Tp) which act as defect centers in such rare-earth manganites. Analysis of X-ray diffraction pattern confirms a change of lattice parameters indicating a structural transformation from rhombohedral (for y≤0.30) to orthorhombic (for y≥0.35) in the present system.  相似文献   

2.
We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2) prepared by sol-gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at Tc=136 K. Both for y=0.1 and 0.2, Tc increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at Tf<Tc which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to Tc and Tf. The value of MR decreases for the Cr substituted samples.  相似文献   

3.
We report a systematic study of the influence of Nb substitution for Fe on the magnetic properties and magneto-impedance (MI) effect in amorphous and annealed Fe76.5−xSi13.5B9Cu1Nbx (x=0, 1, 2, 3, 4, 5, 6, and 7) ribbons. The amorphous ribbons were annealed at different temperatures ranging from 530 to 560 °C in vacuum for different annealing times between 5 and 20 min. We have found that for the as-quenched amorphous ribbons, the substitution of Nb for Fe first increases the saturation magnetization (Ms) and decreases the coercivity (Hc) until x=3, for which the largest Ms∼152 emu/g and the smallest Hc∼1.3 Oe are obtained, then an opposite trend is found for x>3. The largest MI ratio (ΔZ/Z∼38% at f=6 MHz) is achieved in the amorphous ribbon with x=3. A similar trend has been observed for the annealed ribbons. The most desirable magnetic properties (Ms∼156 emu/g and Hc∼1.8 Oe) and the largest MI ratio (ΔZ/Z∼221% at f=6 MHz) are achieved for the x=3 sample annealed at 540 °C for 15 min. A correlation between the microstructure, magnetic properties, and MI effect in the annealed ribbons has been established.  相似文献   

4.
The magnetic properties and the Griffiths singularity were investigated in Mn-site doped manganites of La0.45Sr0.55Mn1−xCoxO3 (x=0, 0.05, 0.10 and 0.15) in this work. The parent sample La0.45Sr0.55MnO3 undergoes a paramagnetic-ferromagnetic transition at TC=290 K and a ferromagnetic-antiferromagnetic transition at TN=191 K. The doping of Co ions enhances the ferromagnetism and suppresses the antiferromagnetism. The enhanced ferromagnetism results from the fact that the Co doping enhances the Mn3+-Mn4+ double-exchange interaction and induces the Co2+-Mn4+ ferromagnetic superexchange interaction. Detailed investigation on the magnetic behavior above TC exhibits that the Griffiths singularity takes place in this series of Mn-site doped compounds. The correlated disorder induced by the Co ionic doping, together with the phase competition from the ferromagnetic and the antiferromagnetic interactions among Mn ions, is responsible for the Griffiths singularity.  相似文献   

5.
In the present work, the temperature dependences of ac transport and magnetoimpedance for sol–gel La0.67Ca0.33MnO3 manganite are investigated at various frequencies. The ac resistance R with absence of field shows a peak around the metal–insulator transition temperature TMI for the dc case. A low magnetic field can shift the metal–insulator transition temperature more easily at high frequency than at low frequency. The valley phenomenon is observed in the temperature dependence of reactance X at 2 and 4 MHz, showing the effects of phase separation or grain boundaries. At 30 MHz, the peak of X appears instead of the valley, since the contribution of insulator capacitance term drops greatly and the inductance of metal dominates the X term with an increase of ac frequency in the sample. At 2 and 4 MHz, La0.67Ca0.33MnO3 sol–gel manganite under H=1 kOe shows negative ac magnetoresistance ΔR/R0, magnetoreactance ΔX/X0 and magnetoimpedance ΔZ/Z0 below 275 K, but positive ac ΔR/R0, ΔX/X0 and ΔZ/Z0 near room temperature much higher than the Curie temperature. The peak temperature of R/R0 moves to low temperature with an increase of the ac frequency from 2 to 4 MHz. At f=30 MHz, the absolute value of negative ac ΔR/R0 increases monotonically with a decrease of temperature for 175 K<T<275 K, whereas the ΔX/X0 is positive below 245 K but negative above 245 K. The ΔZ/Z0 under H=1 kOe is positive below 238 K but negative above 238 K. Such a positive ΔZ/Z0 is due to an increase of permeability induced by field. With an increase of field, the impedance Z experiences a peak, which is connected with the effect of transverse magnetic anisotropy.  相似文献   

6.
We have performed high-resolution photoemission spectroscopy (PES) on FeSr2YCu2O7+δ, of which superconductivity of Tc=49 K was recently reported. We clearly observed opening of a d-wave-like superconducting gap and estimated the maximum gap value (Δmax) to be 10 meV at 15 K. This gap value gives 2Δmax/kBTc∼5, suggesting a strong-coupling nature of superconductivity in FeSr2YCu2O7+δ. Comparative PES study with superconducting and insulating samples shows that the valence band is rigidly shifted as a function of doping without evolution of additional states within the insulating gap.  相似文献   

7.
Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La1−xKxMnO3 polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature (TC) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature (T>TMI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50<T<TMI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below TC is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated.  相似文献   

8.
The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe2O3) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O2− ion into one of the magnetic ions Fe3+) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 103/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature TMo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.  相似文献   

9.
The magnetic and magnetocaloric properties of polycrystalline La0.70(Ca0.30−xSrx)MnO3:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (TIM) and Curie temperature (Tc) are observed at 298 K for x=0.10 composition. Though both TIM and Tc are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La0.70Ca0.20Sr0.10MnO3:Ag0.10 sample. The maximum entropy change (ΔSMmax) at near its Tc (300.5 K) is 7.6 J kg−1 K−1 upon the magnetic field change of 5 T. The La0.70Ca0.20Sr0.10MnO3:Ag0.10 sample having good MR (31%1 T, 49%3 T) and reasonable change in magnetic entropy (7.6 J kg−1.K−1, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.  相似文献   

10.
We report the effects of Al doping on the structure, magnetic properties, and magnetocaloric effect of antiperovskite compounds Ga1−xAlxCMn3 (0≤x≤0.15). Partial substitutions of Al for Ga enhance the Curie temperature (from 250 K for x=0.0 to 312 K for x=0.15) and the saturation magnetization. On increasing the doping level x, the maximum values of the magnetic entropy change (−ΔSM) decreases while the temperature span of ΔSM vs. T plot broadens. Furthermore, the relative cooling power (RCP) is also studied. For 20 kOe, the RCP value tends to saturate at a high doping level (for x=0.12, 119 J/kg at 296 K). However, at 45 kOe, the RCP value increases quickly with increasing x (for x=0.15, 293 J/kg at 312 K). Considering the relatively large RCP and inexpensive raw materials, Ga1−xAlxCMn3 may be alternative candidates for room-temperature magnetic refrigeration.  相似文献   

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