Ferromagnetic spin fluctuations in antiferromagnetic Pr1−xCaxMnO3: An ESR study

Electron spin resonance (ESR) measurements have been performed on polycrystalline samples of Pr_1−xCa_xMnO₃ (x=0.4, 0.5) in the temperature range of 100-300 K. The temperature dependence of ESR intensity, g value and linewidth shows the existence of ferromagnetic spin correlations in the paramagnetic state. With decreasing temperature, the ferromagnetic spin correlations switch to antiferromagnetic spin correlations in the charge ordering state and vanish at the antiferromagnetic ordering temperature T_N.     

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO₃ content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO₃, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO₃. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.     

Hyperfine characterization of Bi1.9Te0.1SrNb1.9Hf0.1O9

The Aurivillius type oxide Bi_1.9Te_0.1SrNb_1.9Hf_0.1O₉ has been studied by Perturbed Angular Correlations spectroscopy using ¹⁸¹Ta probes. The spin precession curves were measured from room temperature up to 873 K. Two sites are occupied by probes and the temperature dependence of both indicates a continuous phase transition at about 625 K. One site is ordered while the other is disordered. This situation is analyzed in terms of simple models already applied to perovskites. The transition temperature of the solid solution Bi_2−xTe_xSrNb_2−xHf_xO₉ (with 0≤x≤0.5) shows a strong dependence on composition.     

The effect of Ru doping on the alkali-doped manganite Pr0.8Na0.2MnO3

Polycrystalline manganites Pr_0.8Na_0.2MnO₃ doped by ruthenium (0.0≤x_Ru≤0.2) were prepared by the sol-gel process. The magnetic field induced metamagnetism was observed to occur with a large resistivity drop at 3 K for x_Ru≤0.02 samples, which can be sorted into the kind of CMR phenomenon. It was found that the 0.01 Ru doping increased T_C and decreased the metamagnetic critical field than that of the undoped sample. The doped manganites show a quick increase in their magnetic moments as x_Ru increases from 0.01 to 0.04, but the larger fraction of Ru doping (0.04≤x_Ru≤0.2) reduces their M and T_C. All the five doped samples have larger magnetic moments than that of the host sample. Ru doping of x_Ru>0.01 results in a rapid disappearance for the observation of long range spin and charge ordering in the samples’ M-T curves, which is characteristic of the undoped sample. It was found that larger low-temperature MR favored decrease in the metamagnetic critical field. Finally, the phase diagram of Ru doping vs. transition temperatures is presented to summarize the experiments.     

X-ray diffraction and magnetic susceptibility of sodium fullerides NaxC60

The X-ray diffraction (XRD), magnetic susceptibility and electron spin resonance (ESR) measurements have been carried out for Na_xC₆₀. The XRD profiles with x<4 can be assigned to a face-centered cubic (fcc) lattice, while those with 4≦x to a hexagonal one. The temperature dependence of magnetic susceptibility χ for Na_xC₆₀ using SQUID was fitted to the Curie law, and estimated temperature-independent component χ₀. The composition x dependence of the χ₀ for Na_xC₆₀ shows two maxima at around x=3 and x=10, and minimum at x=6. The absence of Pauli contribution at x=6 was confirmed using ESR. A trace of superconducting transition at 14 K has been found for some Na_xC₆₀ specimens with 8<x<9.     

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho_2−xY_xTi₂O₇ (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1 −1 0) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6 K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30 mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi₂O₇ and compare our results with pure spin ice Ho₂Ti₂O₇. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.     

High field ESR measurements of spin gap system MCu2(PO4)2

Submillimeter and millimeter wave ESR measurements of spin gap systems SrCu₂(PO₄)₂ and PbCu₂(PO₄)₂, which have four kinds of dimers, have been performed to investigate the magnetic properties of spin gap systems using the pulsed magnetic field up to 35T. The observed ESR spectra of powder sample SrCu₂(PO₄)₂ show sharp and single peak in the temperature range from 4.2 to 80 K. The anisotropy of the g-values turned out to be very small compared to the usual anisotropic powder spectra of copper compounds. The dynamical properties will be discussed from the temperature dependence measurements.     

Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La_0.67Ba_0.33MnO₃ bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (T_IM) and the change in MR around the ferromagnetic transition temperature (T_C). The decrease in MR around T_C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T_IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.     

Spin dynamics in the impurity-doped one-dimensional heisenberg paramagnet (CH3)4NMn(1-x)CuXCl3: Nuclear magnetic relaxation

Proton spin-lattice relaxation rate has been measured at room temperature in the impurity-doped (CH₃)₄NMn_(1-x)Cu_xCl₃ for X = 0.04, 0.1 and 0.17. The result for Hz. snfc;chain-axis as a function of resonance frequency clearly shows that the spectral density of the spin fluctuations in the impure system remains to have the characteristics 1-D diffusive term (ω^{-$\text{1}\text{2}$}), with a slower rate of the spin diffusion in accordance with the theory by Richards. The result for H⊥ chain-axis indicates, however, the existence of a singularity of the fluctuations near ω = 0.     

Magnetic field effect on the critical EPR-dynamics of the cubic ferromagnets CdCr2Se4 and CdCr2S4

We present measurements of the critical behaviour of the EPR linewidths at frequencies 9.21 and 35.5 GHz. In the exchange critical region above T_c (4πx ? 1) the strong field dependence of the linewidths is observed, even when the field variation of susceptibility x is small. This phenomenon is explained by the spin diffusion effect on the linewidth. The spin diffusion coefficients D for CdCr₂Se₄ and CdCr₂S₄ are determined from the linewidths data. The temperature dependence D is found to be consistent with the predictions of the dynamical scaling theory.     

Spin-glass-like ordering in the spinel ZnFe2O4 ferrite

In the present study, spin-glass-like ordering has been observed in the spinel ZnFe₂O₄ ferrite. Field cooled (FC) and zero-field cooled (ZFC) DC magnetizations display divergence at low temperature, which indicates a frozen state with the freezing temperature of T_f=21 K. Frequency dependence of AC susceptibility measurement was performed on the sample. It shows a peak at around T_f, with the peak position shifting as a function of driving frequency, indicating a spin-glass-like transition of the sample. The sample shows a typical spin-glass behavior with a manifestation of non-equilibrium dynamics of the spin glass, such as aging, rejuvenation and memory effects. These experimental findings indicate that Zn-ferrite exhibits a spin-glass-like phase at low temperature and it is not canted antiferromagnetic.     

Phase formation and dielectric phase transition in Ba1−xCaxTi0.6Zr0.4O3 solid solutions

Perovskite types Ba_1−xCa_xTi_0.6Zr_0.4O₃ (with x=0.0-0.5) ceramics have been prepared through solid state reaction route. The room temperature XRD study suggests the compositions with x=0.0 and x=0.1 have single phase cubic symmetry. With further increase in Ca content, solid solution breaks and an orthorhombic CaTiO₃ like phase is developed. The dielectric study on single phase compositions (x=0.0 and 0.1) reveals that the materials are of relaxor type and undergo a diffuse type ferroelectric phase transition. In the Ca containing composition higher transition temperature is observed than the pure BaTi_0.6Zr_0.4O₃ materials. In the paraelectric region (above T_c) lower diffusivity is observed in the Ca containing composition. The strength of relaxation is calculated and found to be more in Ca containing material than that of pure BaTi_0.6Zr_0.4O₃ composition.     

Large enhancement of room temperature magnetoresistance in Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3

The electrical and magnetoresistant properties of La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃/Ag_x (abbreviated by LCBMO/Ag_x) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature (T_c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.     

Effect of sintering atmosphere on the electrical and optical properties of (ZnO)1−x(MnO2)x NTCR ceramics

Nominal composition of (ZnO)_1−x(MnO₂)_x (0.005≤x≤0.2) ceramics have been prepared by the standard solid-state reaction method in three different sintering atmospheres: Ar, air, and reductive atmosphere. The effect of sintering atmosphere on the electron spin resonance (ESR), negative temperature coefficient of resistivity (NTCR), and photoluminescence (PL) properties of (ZnO)_1−x(MnO₂)_x ceramics has been investigated in detail. The results demonstrate that the sintering atmosphere has significant effects on the ESR signals of (ZnO)_1−x(MnO₂)_x; the NTCR of the samples sintered in air is larger than those sintering in Ar and reductive atmosphere; the deep-level PL related to oxygen vacancy increases when sintered in the reductive atmosphere.     

Anomalous spin relaxation and quantum criticality in Mn1 − x Fe x Si solid solutions

We report results of the high frequency (60 GHz) electron spin resonance (ESR) study of the quantum critical metallic system Mn_{1 ? x} Fe _x Si. The ESR is observed for the first time in the concentration range 0 < x < 0.24 at temperatures up to 50 K. The application of the original experimental technique allowed carrying out line shape analysis and finding full set of spectroscopic parameters, including oscillating magnetization, line width and g factor. The strongest effect of iron doping consists in influence on the ESR line width and spin relaxation is marked by both violation of the classical Korringa-type relaxation and scaling behavior. Additionally, the non-Fermi-liquid effects in the temperature dependence of the ESR line width, which may be quantitatively described in the theory of Wölfle and Abrahams, are observed at quantum critical points x* ～ 0.11 and x _c ～ 0.24.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Low-field AC susceptibility in Pr1−xHoxMn2Ge2

We report the structure and magnetic properties of Pr_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Pr leads to a linear decrease in the lattice constants and the unit cell volume. The samples with x=0 and x=0.8 have spin reorientation temperature. The results are collected in a phase diagram.     

Lithium motion in Li0.33 TiS2 and Li0.94 TiS2

We have studied the motion of lithium ions in Li_xTiS₂ (x = 0.33, 0.94) using pulsed NMR techniques. The temperature dependences of the spin lattice relaxation in the rotating frame (T_1?) suggest comparable activation energies for lithium ion diffusion for both samples, 3370 K, but an appreciably longer hopping time for the x = 0.94 sample. Low temperature values of T₂ agree with calculated and measured second moments for both materials.     

X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

The temperature and angular dependence of the X-band electron spin resonance (ESR) and⁵¹V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi_2-xMg_xV₂O₈ (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the⁵¹V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT _N≈ 3.5K) in doped compositions.     

Spin rotations in HoxEr1−xFe2 cubic laves compounds

The temperature dependence of the elastic moduli and the Mössbauer effect in Ho_x,Er_1?x, Fe₂ cubic Laves compounds (x, between 0.3 and 0.9) has been investigated in the temperature region where spin rotation occurs. The composition-dependence minima in the elastic moduli, and the Mössbauer effect measurements, were used to determine the boundaries between the various directions of easy magnetization in these compounds. The experimental spin orientation diagram was found to deviate from the predictions of a one-ion model based on the rare-earth ions alone. From the Mössbauer effect measurement it was deduced that in compounds for which a spin reorientation was observed, the spin rotates continuously with temperature between the major axes of the cubic symmetry. This was attributed to the contribution of higher-order magnetocrystalline anisotropy constants. The ΔE effect, measured in external magnetic fields up to 25 kOe, was found to be constant, and relatively small, in the holmium composition range of x = 0.45–1.00 in the Ho_xEr_1?xFe₂; compounds.