Electron paramagnetic resonance studies of the insulating ferromagnetic manganite Nd_0.8Pb_0.2MnO_3 above the transition temperature

Single crystals of Nd_{1-x}Pb_{x}MnO_{3} with x=0.2 are grown by high temperature solution growth technique using PbO-PbF_{2} flux. Magnetization studies on the samples show a transition to a ferromagnetic state below T_c ~ 125 K and the resistivity measurements show it to be an insulator throughout the temperature range 50 - 300 K. Electron Paramagnetic Resonance studies have been performed for T > T_{C} with a view to comparing the results with those on metallic ferromagnetic manganites. The temperature dependence of various parameters like g-value, linewidth and intensity has been studied in the temperature range 150 - 300 K. It is found that they behave in a manner similar to that exhibited by metallic ferromagnetic manganites.Comment: 8 pages, 4 figure

An electron paramagnetic resonance study of Pr_{0.6}Ca_{0.4}MnO_{3} across the charge ordering transition

We report the first electron paramagnetic resonance studies of single crystals and powders of Pr_{0.6}Ca_{0.4}MnO_{3} in the 300-4.2 K range, covering the charge ordering transition at ~ 240 K and antiferromagnetic transition (T_N) at ~ 170 K. The asymmetry parameter for the Dysonian single crystal spectra shows anomalous increase at T_{co}. Below T_{co} the g-value increases continuously, suggesting a gradual strengthening of orbital ordering. The linewidth undergoes a sudden increase at T_{co} and continues to increase down to T_N. The intensity increases as the temperature is decreased till T_{co} due to the renormalization of magnetic susceptibility arising from the build up of ferromagnetic correlations. The value of the exchange constant, J, is estimated to be 154 K.Comment: Uses Revtex3.

Non-resonant microwave absorption studies of superconducting MgB_2

Non-resonant microwave absorption(NRMA) studies of superconducting MgB_2 at a frequency of 9.43 GHz in the field range -50 Gauss to 5000 Gauss are reported. The NRMA results indicate near absence of intergranular weak links. A linear temperature dependence of the lower critical field H_c1 is observed indicating a non s-wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d-wave symmetry is also not observed.Comment: 8 pages, 2 figure

Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB2_2 samples

The real $R_s(T)$ and imaginary $X_s(T)$ parts of the surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ in polycrystalline MgB$_2$ samples of different density with the critical temperature $T_c\approx 38$ K are measured at the frequency of 9.4 GHz and in the temperature range $5\le T<200$ K. The normal skin-effect condition $R_s(T)=X_s(T)$ at $T\ge T_c$ holds only for the samples of the highest density with roughness sizes not more than 0.1 $\mu$m. For such samples extrapolation $T\to 0$ of the linear at $T<T_c/2$ temperature dependences $\lambda_L(T)=X_s(T)/\omega\mu_0$ and $R_s(T)$ results in values of the London penetration depth $\lambda_L(0)\approx 600$ \AA and residual surface resistance $R_{res}\approx 0.8$ m$\Omega$. In the entire temperature range the dependences $R_s(T)$ and $X_s(T)$ are well described by the modified two-fluid model.Comment: 7 pages, 3 figures. Europhysics Letters, accepted for publicatio

On the analysis of broad Dysonian electron paramagnetic resonance spectra

We analyze the equation used for simulating the lineshapes of broad electron paramagnetic resonance spectra in conducting samples (i.e., broad Dysonian lineshapes) where it becomes necessary to include the effects of both clockwise and counterclockwise rotating components of the microwave magnetic field. Using symmetry arguments, we propose a modification to the equation. We show that the modified equation fits the experimental results better than the equation used in literature

Non-resonant microwave absorption evidence for intrinsic Josephson coupling in Tl2Ba2CaCu2O8 single crystals

We report the results of our non-resonant microwave absorption (NRMA) studies on single crystals of Tl2Ba2CaCu2O8 (Tl 2212) which reflect the occurrence of intrinsic Josephson coupling in these crystals. We have studied the magnetic field induced microwave absorption at various temperatures from 4.2K to T-c (similar to 104K) using a standard CW EPR spectrometer (H-dc parallel to c). We observe the appearance of a characteristic feature in the NRMA signals similar to the ones observed earlier by us in Bi2Sr2CaCu2O8 (Bi 2212) starting a few degrees below T-c, which on cooling passes through a maximum in intensity before disappearing at a further low temperature. This behaviour is attributed to the appearance, strengthening and disappearance of Josephson response consequent to the temperature dependence of the viscosity of the Josephson medium between the CuO2 superconducting sheets

Non-resonant microwave absorption evidence for intrinsic Josephson coupling in Tl2Ba2CaCu2O8Tl_2Ba_2CaCu_2O_8 single crystals

We report the results of our non-resonant microwave absorption (NRMA) studies on single crystals of $Tl_2Ba_2CaCu_2O_8$ (Tl 2212) which reflect the occurrence of intrinsic Josephson coupling in these crystals. We have studied the magnetic field induced microwave absorption at various temperatures from 4.2K to $T_c (-104K)$ using a standard CW EPR spectrometer $(H_{dc}\parallel c)$. We observe the appearance of a characteristic feature in the NRMA signals similar to the ones observed earlier by us in $Bi_2Sr_2CaCu_2O_8 (Bi 2212)$ starting a few degrees below $T_c$, which on cooling passes through a maximum in intensity before disappearing at a further low temperature. This behaviour is attributed to the appearance, strengthening and disappearance of Josephson response consequent to the temperature dependence of the viscosity of the Josephson medium between the $CuO_2$ superconducting sheets

Temperature-dependent electron paramagnetic resonance studies of charge-ordered Nd<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB>

We report electron paramagnetic resonance measurements on single crystalline and powder samples of Nd0.5Ca0.5MnO3 across the charge-ordering transition at Tco=240K down to the antiferromagnetic ordering transition at TN=140K. The changes in the linewidth, g-factor and intensity as functions of temperature are studied to understand the nature of spin-dynamics in the system. We explain the observed large decrease in the linewidth from TN to Tco in terms of motional narrowing caused by the hopping of the Jahn-Teller polarons yielding an activation energy of Ea=0.1 eV. Similar analysis of data on Pr0.6Ca0.4MnO3 published earlier gives Ea=0.2eV. Below Tco, the g-value increases continuously suggesting a gradual strengthening of the orbital ordering. We give a qualitative explanation of the maximum in the asymmetry ratio A/B observed at Tco and its temperature dependence in single crystal spectra which also supports the model of motional narrowing