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We report electron paramagnetic resonance measurements on single crystalline and powder samples of Nd 0.5 Ca 0.5 MnO 3 across the charge-ordering transition at T co ϭ240 K down to the antiferromagnetic ordering transition at T N ϭ140 K. 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 T N to T co in terms of motional narrowing caused by the hopping of the Jahn-Teller polarons yielding an activation energy of E a ϭ0.1 eV. Similar analysis of data on Pr 0.6 Ca 0.4 MnO 3 published earlier gives E a ϭ0.2 eV. Below T co , 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 T co and its temperature dependence in single crystal spectra which also supports the model of motional narrowing.

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.

Rapid-scan electron paramagnetic resonance spectra at 9.8 GHz were obtained on a Bruker E580 spectrometer. Spectra of lithium phthalocyanine (LiPc) needles (T i = 8 p.s, T 2 = 3.4 p.s) and of a 0.2 mM aqueous solution of Nycomed triarylmethyl (trityl-CD3) radical (T~ = 11.5 p.s, T 2 = 8 p.s) were recorded at scan rates between 3.4.10 ~ and 7,5.105 G/s at the center of sinusoidal seans. Signals for LiPc were obtained with a split-ring resonator, a rectangular resonator anda dielectric resonator. At faster scan rates the small bandwidth of the high-Q dielectric resonator filters out highfrequency components of the rapid-scan signals. Field inhomogeneities induced by the rapidly changing magnetic field increase with scan rate and are greater with the dielectric and split-ring resonators than with the rectangular resonator. Data for trityl-CD 3 were recorded with the rectangular resonator. The extended trityl sampLe, about 3 mm Iong, shows larger effects of magnetic field inhomogeneities than the small LiPc crystals. lntroductionIn conventional slow-scan electron paramagnetic resonance (EPR) the magnetic field passes through the spin packet resonance in a time that is long relative to relaxation times (Tt and T2) and the signal is detected with magnetic fŸ moduIation and phase-sensitive detection at the modulation frequency. Typically the microwave magnetic field B 1 is small enough that there is little change in z-axis magnetization during a scan and the line shape is independent of scan rate. On the other extreme, in pulsed spectroscopy the high BI fields cause a large change in the z-axis magnetization, and the signal is recorded with direct detection. In between these two regimes there is a variety of nonlinear responses that ate referred to as passage effects. Bloch first described passage effects for nuclear magnetic resonance (NMR) signals [1]. The initial observation of adiabatic rapid passage EPR by Portis was followed by papers by Weger [2], Hyde [3,4], Mailer [5], among others, that demonstrated the importance of passage effects in EPR spectra. In the notation of Weger [2] the term "rapid" refers to the regime in which Bt/[(dBo/dt)(T~T2) t/E] < 1, where dBo/dt is the magnetic field scan rate. This

We present and compare the results of temperature-dependent electron paramagnetic resonance (EPR) studies on Pr 1−x Ca x MnO 3 for x = 0.64 which is electron-doped with those on the hole doped x = 0.36 composition. The temperature dependence of the various parameters obtained from the powder and single crystal spectra show significant differences between the two manganites. At room temperature the 'g' parameter for the electron doped system is less than the free electron 'g' value 'g e ', whereas for the hole-doped system it is more than 'g e '. Further, the 'g' value and the linewidth obtained from the powder spectra as well as the single crystal spectra show different functional dependences on temperature in the two systems. Quite strikingly, the peak observed at T co in the temperature dependence of the asymmetry parameter, α, of the single crystal spectra in the hole-doped system is absent in the electron-doped system. We understand this contrasting behaviour of the EPR parameters in the two systems in terms of very different nature of microscopic interactions in them.

We present results of an electron paramagnetic resonance (EPR) study of Nd 1Àx Sr x MnO 3 with x ¼ 0:5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T c ) and the antiferromagnetic, charge ordering transition (T N ¼ T co ) at 150 K. The results are compared with those on Nd 0.45 Sr 0.55 MnO 3 which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T N ¼ 230 K and on La 0.77 Ca 0.23 MnO 3 which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x ¼ 0:5; the EPR signals below T c consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x ¼ 0:55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T N : The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La 0.77 Ca 0.23 MnO 3 . The comparison of behaviour of Atype AFM signal observed in both Nd 0.5 Sr 0.5 MnO 3 and Nd 0.45 Sr 0.55 MnO 3 with the two FM phases of La 0.77 Ca 0.23 MnO 3 , vis-" a-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd 0.5 Sr 0.5 MnO 3 undergoes phase separation into A-type AFM and FM phases. r

We show that newly found BF3-doped polyaniline, though highly conducting, remains amorphous. Magnetic studies reveal many unusual properties, while suggesting that the intrinsic conductivity of this system is significantly larger than all other known forms of conducting polyaniline, establishing it as an interesting class of highly conducting amorphous polymer.