Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Electric and magnetic properties of the system between Cr3S4 and VCr2S4

The electrical resistivity and magnetization of V_xCr_3?xS₄ (x = 0.0 - 1.0) have been measured. Weak ferromagnetic moments are observed at low temperatures for x ≠ 0.0. Curie temperatures are determined from magnetization curves and resistivity anomalies.     

Enhancement of magnetoresistances at room temperature in YSZ doping La0.67Sr0.33MnO3 system

The temperature dependence of the resistance of composite samples (1−x)La_0.67Sr_0.33MnO₃+xYSZ with different YSZ doping level x was investigated at magnetic fields 0-3 T, where YSZ represents yttria-stabilized zirconia. Results show that the YSZ dopant does not only adjust the metal-insulator transition temperature, but also increases the magnetoresistance effect. With increase of YSZ doping level for the range of x<2%, the metal-insulator transition temperature values T_P of the composites decrease, but T_P increases with increase of x further for the range of x>2%. Meanwhile, in the YSZ-doped composites, a broad metal-insulator transition temperature region was found at zero and low magnetic field, which results in an obvious enhanced magnetoresistance in the temperature range 10-350 K. Specially, a larger magnetoresistance value was observed at room temperature at 3 T, which is encouraging with regard to the potential application of magnetoresistance materials.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Effect of the simultaneous substitution of two transition metals on the structural, magnetic and electrical properties of La0.6Sr0.4Mn1−2xCrxFexO3

The structural, magnetic and electrical properties of Cr and Fe simultaneously substituted in the perovskite La_0.6Sr_0.4Mn_1−2xCr_xFe_xO₃ have been studied. The presence of Cr and Fe had no significant effect on the structural properties. Curie temperature and saturation magnetization decrease with increase in Cr and Fe contents. For x=0.20 and 0.25, a steep drop of zero field-cooled (ZFC) magnetization at low temperature signifies the formation of cluster- or spin-glass state. A weak hysteresis at low fields seems to be an indication of phase separation. All the resulting magnetization curves can be explained by a superposition of both ferromagnetic and antiferromagnetic components. All the samples are semiconducting throughout the temperature range studied. Resistivity can be described by the adiabatic small polaron hopping and the variable range hopping model. It was found that the transport mechanism is dominated by the VRH model with an increase of Mott localization energy, which explains the increase of resistivity.     

Study of structural, transport and magneto-resistive properties of La0.7Ca0.3−xCexMnO3 (0≤x≤0.2)

We have synthesized a series of La_0.7(Ca_0.3−xCe_x)MnO₃ (0≤x≤0.2) by standard solid-state reaction method. X-ray diffraction (XRD) measurement was carried out for structural studies and Rietveld refinement was done for structural analysis. The transport properties were studied using four probe technique. The temperature dependence of the resistivity was measured in the temperature range of 20 K to room temperature. It is found that all samples show a systematic variation in metal to insulator transition at transition temperature (T_P) and resistivity (ρ) with the relative concentration of hole and electron doping in the system. The samples showed varying amounts of colossal magnetoresistance depending upon temperature and applied magnetic field. The magnetoresistance values as high as 72% were observed in x=0 sample.     

Influence of magnetic (Fe) and non-magnetic (Ga) ion doping at Mn-site on the transport and magnetic properties of La0.7Ca0.3MnO3

The modifications in electrical and magnetic properties of polycrystalline bulk La_0.7Ca_0.3Mn_1−xT_xO₃ (T=Fe, Ga) samples at relatively higher doping concentration (x=0.08-0.12) are investigated. All the synthesized, single phase samples were subjected to resistivity measurements in the temperature range 50-300 K. No insulator-metal transition (T_P) was observed for Fe doped samples with x=0.12. For all the other samples the transition temperature decreased with increase in doping concentration. The small polaron hoping energy was found to increase, rather slowly, with increase in doping concentration. The effect on magnetic properties is also prominently observed with respect to doping element and doping concentration. Interestingly, with the increase in doping concentration, the Curie temperature (T_C) and T_P separate out significantly indicating decoupling of electric and magnetic properties. Changes in these properties have been analyzed on the basis of magnetic disorder introduced in the system due to the magnetic and nonmagnetic nature of these ions rather than strong lattice effects which is insignificant due to similar ionic radii of Fe⁺³ and Ga⁺³ when compared to that of Mn⁺³.     

Ferromagnetic resonance in the randomly mixed insulating ferromagnet and antiferromagnet Rb2Mn(1-x)CrxCl4

An electron spin resonance experiment has been performed on the Cr-rich concentrations of the random mixture Rb₂Mn(_1-x)Cr_xCl₄ of an insulating ferromagnet (Rb₂CrCl₄) and an insulating antiferromagnet (Rb₂MnCl₄). The resonance fields in x = 0.6, 0.7 and 0.8 samples begin to shift towards low field side at temperatures well above the Curie temperatures, when the external field is applied in the c-plane of the crystal. The shift of the resonance field with temperature of the x = 0.7 sample agreed well with that of the x = 0.8 sample, after scaling the temperature axis, while the shift in the x = 0.6 crystal did not. From this observation, we argue that the x = 0.7 and x = 0.8 samples have a long-range ordered ferromagnetic phase below T_c, and that the low temperature phase of the x = 0.6 sample is not a truely long-range ordered one. A weak resonance line was observed in the x = 0.8 sample below about 20 K. This resonance is discussed in connection with the re-entrant spin-glass behavior of this mixed system found in the ac susceptibility measurement.     

The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Physical properties and anisotropies of the RNiGe3 series (R = Y, Ce-Nd, Sm, Gd-Lu)

We have studied RNiGe₃ (R=Y, Ce-Nd, Sm, Gd-Lu) single crystals by measuring crystal structure and stoichiometry, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, and specific heat. Clear anisotropies as well as antiferromagnetic ordering in the RNiGe₃ series (R=Ce-Nd, Sm, Gd-Tm) have been observed above 1.8 K from the magnetic susceptibility. A metamagnetic transition in this family (except for R=Sm) was detected at 2 K for applied magnetic fields below 70 kOe. The electrical resistivity of this series follows metallic behavior in the high temperature region. Below the antiferromagnetic ordering temperature a significant anisotropy is exhibited in the resistivity and magnetoresistance for different current directions. The anisotropic magnetic, transport, and thermal properties of RNiGe₃ compounds are discussed in terms of Ni site occupancy as well as a combination of the effect of formation of a magnetic superzone gap and the crystalline electric field.     

Evidence for the spin-dependent scattering of conduction electrons on Mn2+ ions in Hg1−xMnxTe and Cd1−xMnxSe mixed crystals

Changes in the resistivity of Hg_1?xMn_xTe and Cd_1?xMn_xSe mixed crystals associated with paramagnetic resonance of the Mn²⁺ ions have been observed at liquid helium temperature in a strong magnetic field. The effect was recorded by monitoring the submillimeter radiation induced photoconductivity in a swept magnetic field. An increase in the resistivity associated with EPR of the Mn²⁺ ions is interpreted in terms of the spin- dependent scattering of electrons on magnetic impurities, the spins of which are selectively depolarised by means of paramagnetic resonance. Some additional effects influencing the experiments are also discussed.     

Magnetic susceptibility and electrical resistivity of CexY1−xAl2

The magnetic susceptibility and the electrical resistivity have been measured for Ce_xY_1−xAl₂ (x = 0, 0.05, 0.15, 0.25) between 4.2°K and 300°K. _x3kT vanishes at low temperature. We find a minimum in resistivity at 60°K (x = 5) and 68°K (x = 0.25).     

Magnetotransport properties of (La0.7Pb0.3MnO3)1−xAgx composites

Magnetic and transport properties of (La_0.7Pb_0.3MnO₃)_1−xAg_x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution with increase of Ag content percentage. Clearly irreversible behavior in the zero-field cooling and field cooling curves at a low field caused by the competition between the magnetization and magnetic domain orientation processes has been observed as x increases. Saturation magnetization decreases as x increases, while ferromagnetic transition temperature remains around 346 K for all composites. The resistivity decreases significantly for (La_0.7Pb_0.3MnO₃)_1−xAg_x composites. It is suggested that introduction of Ag into the niche of grain boundaries forms artificial conducting network and improves the carriers to transport. However, enhancement of magnetoresistance has been observed for the system.     

Anisotropy and FOMP in Tb3 (Fe28−xCox) V1.0 (x=0, 3 and 6) compounds

In this work, the structural and magnetic properties of Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) compounds have been investigated. The structural characterization of compounds by X-ray powder diffraction is an evidence for a monoclinic Nd₃(Fe, Ti)₂₉-type structure (A2/m space group). The refined lattice parameters a and b (but not c) and the unit cell volume V, obtained from the XRD data by the Rietveld method, are found to decrease with increasing Co concentration. The unit cell parameters behavior has been attributed to the smaller Co atoms and a preferential substitution of Fe by Co. The anisotropy field (H_a) as well as critical field (H_cr) was measured using the singular point detection (SPD) technique from 5 to 300 K in a pulsed magnetic field of up to 30 T. At T=5 K, a FOMP of type 2 was observed for all samples and persists at all temperatures up to 300 K. For sample x=0, H_cr=10.6 and 2.0 T at 5 and 300 K, respectively, is equal to that reported earlier. The occurrence of canting angles between the magnetic sublattices during the magnetization process instead of high-order anisotropy contributions (at room temperature are usually negligible) has been considered to explain the survival of the FOMP at room temperature. The anisotropy and critical fields behave differently for samples with x=0, 3 compared with x=6. The observed behavior has been related to the fact that the Co substitution for Fe takes place with a preferential entrance in the inequivalent crystallographic sites of the 3:29 structure. The contribution of the Tb-sublattice in the Tb₃(Fe, V)₂₉ compound with uniaxial anisotropy has been scaled from the anisotropy field measured on a Y₃(Fe, V)₂₉ single crystal with easy plane anisotropy.     

Peculiarities in the electron spectrum of superconducting laves phase ZrxHf1−xV2

Structurally unstable superconducting Laves phases Zr_xHf_1?xV₂ were investigated in the temperature range 4.2 – 300 K for temperature dependence of magnetic susceptibilities χ, resistivity ρ, and X-ray fluorescent emission 2K-spectra of vanadium. Anomalous dependences χ and ρ as compared to common metals was discovered below 150 K. This may be explained in terms of a phase transition of Peierl's type when the lattice instability results from the instability of the electron spectrum due to the dielectric slit that appears on flat parts of the Fermi surface. The presence of regions with localised electron density in the Laves phases on the low-energy side of the vanadium emission spectrum at low temperatures (T=10K) indicates an essential rebuilding in the electron spectrum of valence electrons.     

Magnetocaloric properties of (La0.67−xGdx)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, magnetic property and magnetocaloric effect (MCE) in nanoparticles perovskite manganites of the type (La_0.67−xGd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) synthesized by using an amorphous molecular alloy as precursor have been reported. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature (T_C) of the prepared samples is found to be strongly dependent on Gd content. The Curie temperature of samples is 358.4, 343.2, and 285.9 K for x=0.1, 0.15, and 0.2, respectively. A large magnetocaloric effect close to T_C has been observed with a maximum of magnetoentropy change in all the samples, ∣ΔS_M∣_max of 1.96 and 4.90 J/kg K at 2 and 5 T, respectively, for a substitution rate of 0.15. In addition, the maximum magnetic entropy change observed for samples with different concentration of Gd, exhibits a linear dependence with the applied high magnetic field. These results suggest that (La_0.67−x Gd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) compounds could be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Electronic transport and magnetic studies of La1−xCax−0.08Sr0.04Ba0.04MnO3

Lanthanum based mixed valence manganite system La_1−xCa_x−0.08Sr_0.04Ba_0.04MnO₃ (LCSBMO; x=0.15, 0.24 and 0.33) synthesized through the sol-gel route is systematically investigated in this paper. The electronic transport and magnetic susceptibility properties are analyzed and compared, apart from the study of unit cell structure, microstructure and composition. Second order phase transition is observed in all the samples and significant difference is observed between the insulator to metal transition temperature (T_MI) and paramagnetic (PM) to ferromagnetic (FM) transition temperature (T_C). In contrast to the insulating FM behaviour usually observed in La_1−xCa_xMnO₃ (LCMO) for x=0.15, a clear insulator to metal transition is observed for LCSBMO for the same percentage of lanthanum. The temperature dependent resistivity of polycrystalline pellets, when obeying the well studied law ρ=ρ_o+ρ₂T² for T<T_MI, is observed to differ significantly in the values of ρ_o and ρ₂, with the electrical conductivity increasing with x. The variable range hopping model has been found to fit resistivity data better than the small polaron model for T>T_MI. AC magnetic susceptibility study of the polycrystalline powders of the manganite system shows the highest PM to FM transition of 285 K for x=0.33.