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We find that the temperature dependence of the Electron Paramagnetic Resonance (EPR) linewidth observed in Bi$_{0.5}$Sr$_{0.5}$Mn$_{0.9}$Cr$_{0.1}$O$_3$ (BSMCO), as well as in certain other 3-dimensional manganites undergoing antiferromagnetic transitions is satisfactorily described by the Berezinskii-Kosterlitz-Thouless (BKT) model. We understand this unexpected result in terms of an effective 2-dimensional XY easy plane anisotropy induced by the magnetic field applied in the EPR experiment. This conclusion is supported by the field dependence of the BKT correlations observed in the quasi two-dimensional antiferromagnetic compound BaNi$_{2}$V$_{2}$O$_{8}$.

We study the effects of 10% Cr substitution in Mn sites of Bi0.5Sr0.5MnO3 on the antiferromagnetic (AFM) (TN ~ 110 K) transition using structural, magnetic and electron paramagnetic resonance (EPR) techniques. Field cooled (FC) and zero field cooled (ZFC) magnetization measurements done from 400 K down to 4 K show that the compound is in the paramagnetic (PM) phase till 50 K where it undergoes a transition to a short range ferromagnetic phase (FM). Electron paramagnetic resonance measurements performed in the temperature range 300 K till 80 K conform with the magnetization measurements as symmetric signals are observed owing to the paramagnetic phase. Below 80 K, signals become asymmetric. Electron paramagnetic resonance intensity peaks at ~ 110 K, the decreasing intensity below this temperature confirming the presence of antiferromagnetism. We conclude that below 50 K the magnetization and EPR results are consistent with a cluster glass phase of BSMCO, where ferromagnetic clusters coexist with an antiferromagnetic background.

We study and compare magnetic and electron paramagnetic resonance behaviors of bulk and nanoparticles of Nd(1-x)CaxMnO3 in hole doped (x = 0.4;NCMOH) and electron doped (x = 0.6;NCMOE) samples. NCMOH in bulk form shows a complex temperature dependence of magnetization M(T), with a charge ordering (CO) transition at around 250 K, an antiferromagnetic (AFM) transition at around 150 K and a transition to a canted AFM phase/mixed phase at around 80 K. Bulk NCMOE behaves quite differently with just a charge ordering transition at around 280 K, thus providing a striking example of the so called electron-hole asymmetry. While our magnetization data on bulk samples are consistent with the earlier reports, the new results on the nanoparticles bring out drastic effects of size reduction. They show that M(T) behaviors of the two nano samples are essentially similar in addition to the absence of the charge order in them thus providing strong evidence for vanishing of the electron-hole asymmetry in nanomanganites. This conclusion is further corroborated by electron paramagnetic resonance studies which show that the large difference in the g-values and their temperature dependence found for the two bulk samples disappears as they approach a common behavior in the corresponding nano samples., Comment: 12 pages, 7 figures

Mn-doped SrTiO_3.0, when synthesized free of impurities, is a paramagnetic insulator with interesting dielectric properties. Since delocalized charge carriers are known to promote ferromagnetism in a large number of systems via diverse mechanisms, we have looked for the possibility of any intrinsic, spontaneous magnetization by simultaneous doping of Mn ions and electrons into SrTiO_3 via oxygen vacancies, thereby forming SrTi_(1-x)Mn_xO_(3-d), to the extent of making the doped system metallic. We find an absence of any enhancement of the magnetization in the metallic sample when compared with a similarly prepared Mn doped, however, insulating sample. Our results, thus, are not in agreement with a recent observation of a weak ferromagnetism in metallic Mn doped SrTiO_3 system., Comment: 10 pages and 4 figures

Systematic measurements pertinent to the magnetocaloric effect and nature of magnetic transition around the transition temperature are performed in the 10 nm Pr0.5Ca0.5MnO3 nanoparticles (PCMO10) . Maxwell relation is employed to estimate the change in magnetic entropy. At Curie temperature TC, 83.5 K, the change in magnetic entropy discloses a typical variation with a value 0.57 J/kg K, and is found to be magnetic field dependent. From the area under the curve Delta S vs T, the refrigeration capacity is calculated at TC, 83.5 K and it is found to be 7.01 J/kg. Arrott plots infer that due to the competition between the ferromagnetic and anti ferromagnetic interactions, the magnetic phase transition in PCMO10 is broadly spread over both in temperature as well as in magnetic field coordinates. Upon tuning the particle size, size distribution, morphology, and relative fraction of magnetic phases, it may be possible to enhance the magnetocalorific effect further in PCMO10., Comment: Accepted (Journal of Applied Physics) (In press)

We report on isothermal pulsed (20 ms) field magnetization, temperature dependent AC - susceptibility, and the static low magnetic field measurements carried out on 10 nm sized Pr0.5Ca0.5MnO3 nanoparticles (PCMO10). The saturation field for the magnetization of PCMO10 (~ 250 kOe) is found to be reduced in comparison with that of bulk PCMO (~300 kOe). With increasing temperature, the critical magnetic field required to 'melt' the residual charge-ordered phase decays exponentially while the field transition range broadens, which is indicative of a Martensite-like transition. The AC - susceptibility data indicate the presence of a frequency-dependent freezing temperature, satisfying the conventional Vogel-Fulcher and power laws, pointing to the existence of a spin-glass-like disordered magnetic phase. The present results lead to a better understanding of manganite physics and might prove helpful for practical applications.

We report on exchange bias effects in 10 nm particles of Pr0.5Ca0.5MnO3 which appear as a result of competing interactions between the ferromagnetic (FM)/anti-ferromagnetic (AFM) phases. The fascinating new observation is the demonstration of the temperature dependence of oscillatory exchange bias (OEB) and is tunable as a function of cooling field strength below the SG phase, may be attributable to the presence of charge/spin density wave (CDW/SDW) in the AFM core of PCMO10. The pronounced training effect is noticed at 5 K from the variation of the EB field as a function of number of field cycles (n) upon the field cooling (FC) process. For n > 1, power-law behavior describes the experimental data well; however, the breakdown of spin configuration model is noticed at n \geq 1.

We observe a disappearance of electron-hole asymmetry in hole and electron doped manganites of composition Pr(1-x)Ca(x)MnO3 (x=0.36, 0.64), as reflected in the g-parameter values. The bulk property of opposite g-shifts in room temperature seems to have disappeared in nanoparticles. We analyze the results in the light of ferromagnetic fluctuations and melting of charge order in nanoparticles.

Combining experiments with first principles calculations, we show that site-specific doping of Mn into SrTiO3 has a decisive influence on the dielectric properties of these doped systems. We find that phonon contributions to the dielectric constant invariably decrease sharply on doping at any site. However, a sizable, random dipolar contribution only for Mn at the Sr site arises from a strong off-centric displacement of Mn in spite of Mn being in a non-d0 state; this leads to a large dielectric constant at higher temperatures and gives rise to a relaxor ferroelectric behavior at lower temperatures. We also investigate magnetic properties in detail and critically reevaluate the possibility of a true multi-glass state in such systems., Comment: 24 Pages, 2 Tables, 9 Figures

Glass transition and relaxation of the glycerol-water binary mixture system are studied over the glycerol concentration range of 5 - 85 mol% using the highly sensitive technique of spin probe ESR. For the water rich mixture the glass transition, sensed by the spin probe, arises from the vitrified mesoscopic portion of the binary system. The concentration dependence of the glass transition temperature manifests a closely related molecular level cooperativity in the system. A drastic change in the mesoscopic structure of the system at the critical concentration of 40 mol% is confirmed by an estimation of the spin probe effective volume in a temperature range where the tracer reorientation is strongly coupled to the system dynamics., Comment: 6 pages and 5 figures, Typo corrected for equation 5

First systematic spin probe ESR study of water freezing has been conducted using TEMPOL and TEMPO as the probes. The spin probe signature of the water freezing has been described in terms of the collapse of narrow triplet spectrum into a single broad line. This spin probe signature of freezing has been observed at an anomalously low temperature when a milimoler solution of TEMPOL is slowly cooled from room temperature. A systematic observation has revealed a spin probe concentration dependence of these freezing and respective melting points. These results can be explained in terms of localization of spin probe and liquid water, most probably in the interstices of ice grains, in an ice matrix. The lowering of spin probe freezing point, along with the secondary evidences, like spin probe concentration dependence of peak-to-peak width in frozen limit signal, indicates a possible size dependence of these localizations/entrapments with spin probe concentration. A weak concentration dependence of spin probe assisted freezing and melting points, which has been observed for TEMPO in comparison to TEMPOL, indicates different natures of interactions with water of these two probes. This view is also supported by the relaxation behavior of the two probes., Comment: 20 pages, 8 figures, 22 references

Nanoparticles (size 20, 40 and 60 nm) of Pr_0.5Sr_0.5MnO_3 are prepared by sol-gel technique and their magnetic properties are studied using ferromagnetic resonance and magnetization measurements. A comparison with the properties of the bulk material shows that the ferromagnetic transition at 265 K remains unaffected but the anti-ferromagnetic transition at T_N = 150 K disappears in the nanoparticles. Further, the temperature dependence of magnetic anisotropy shows a complex behavior, being higher in the nanoparticles at high temperatures and lower at lower temperatures in comparison with the bulk., Comment: 14 pages including 5 figures;submitted to Applied Physics Letters

We report on the presence of weak links in the intermetallic superconductor MgB2 from the non-resonant microwave absorption (NRMA) studies on the MgB2 thin film. The strong low field signal, which is the characteristics of weak links and the presence of hysteresis, which arises due to flux trapping, are observed. The studies are carried out as a function of temperature, microwave power, magnitude of modulation field and the range of field scan. Presence of weak links is discussed in terms of increase in the oxygen content of grain boundaries in the thin film and because of disorder effects arises due to the aging of the sample. The pinning of flux lines with the changes in sweeping direction leads to hysteresis in the NRMA signal and shows the contribution of flux line motion in the power absorption., Comment: 26 pages, 12 figures

Nanoparticles (dia ~ 5 - 7 nm) of Bi_0.5 X_0.5(X=Ca,Sr)MnO_3 are prepared by polymer assisted sol-gel method and characterized by various physico-chemical techniques. X-ray diffraction gives evidence for single phasic nature of the materials as well as their structures. Mono dispersed to a large extent, isolated nanoparticles are seen in the transmission electron micrographs. High resolution electron microscopy shows the crystalline nature of the nanoparticles. Superconducting quantum interferometer based magnetic measurements from 10 K to 300 K show that these nanomanganites retain the charge ordering (CO) nature unlike Pr and Nd based nanomanganites. The CO in Bi based manganites is thus found to be very robust consistent with the observation that magnetic field of the order of 130 T are necessary to melt the CO in these compounds. These results are supported by electron magnetic resonance measurements., Comment: 13 pages including 4 figures; work presented at the Internatinal Conference on Nanoscience and Technology,ICONSAT2006, New Delhi, March 2006

Nanowires of Pr_0.57 Ca_0.41 Ba_0.02 MnO_3 (PCBM) (diameter ~ 80-90 nm and length ~ 3.5 mm) were synthesized by a low reaction temperature hydrothermal method. Single-phase nature of the sample was confirmed by XRD experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. While the bulk PCBM is known to exhibit charge order (CO) below 230 K along with a ferromagnetic transition at 110 K, SQUID measurements on the nanowires of PCBM show that the charge order is completely absent and a ferromagnetic transition occurs at 115 K. However, the magnetization in the nanowires is observed to be less compared to that in the bulk. This observation of the complete melting of the charge order in the PCBM nanowires is particularly significant in view of the observation of only a weakening of the CO in the nanowires of Pr_0.5 Ca_0.5 MnO_3. Electron paramagnetic resonance experiments were also carried out on the PCBM nanowires using an X-band EPR spectrometer. Characteristic differences were observed in the line width of nanowires when compared with that of the bulk., Comment: Presented at the International Conference on nanoscience and Technology ICONSAT2006, New Delhi, India, March 2006;13 pages including 4 figures

Nd_0.5 Ca_0.5 MnO_3 (NCMO) nanoparticles (average diameter ~ 20 and 40 nm) are synthesized by polymeric precursor sol-gel method and characterized by X- ray diffraction, transmission electron microscopy (TEM), selective area electron diffraction (SAED), superconducting quantum interference device (SQUID) magnetometery and resistivity measurements. Both single crystalline and polycrystalline particles are present in the samples and they are found to retain the orthorhombic structure of the bulk NCMO. However, most strikingly, in the 20 nm particles, the charge ordered (CO) and the antiferromagnetic phases observed in the bulk at 250 K and 160 K respectively are completely absent. Instead, a ferromagnetic (FM) transition is observed at 95 K followed by an insulator to metal transition at 75 K. In the 40 nm particles, though a weak, residual CO phase is observed, transition to FM state also occurs, but at a slightly higher temperature of 110 K. The magnetization is found to increase with the decrease of particle size., Comment: 14 pages, 4 figures

AC losses of granular superconducting MgB2 were measured using non-resonant rf power absorption techniques. The presence of two band-gaps makes the temperature dependent ac loss pattern of MgB2 different from other high Tc superconductors like YBa2Cu3O and BSCCO-2223. Josephson junction decoupling plays a major role in MgB2 polycrystalline samples, both in the presence and absence of magnetic field. We studied the ac loss of MgB2 samples with different crystalline properties, grain sizes and pressures and sintered under different physical and chemical conditions. The effects of frequency and amplitude of ac current on the ac loss are discussed in detail., Comment: 10 pages, 5 figures

The kinetic inductance properties of a thin superconducting sample of BSCCO-2223 compound is studied. A strong dependence of kinetic inductance with the variation of applied magnetic field is found at low temperature in the superconducting state. The kinetic inductance of a thin superconductor increases linearly with increasing magnetic field. This behavior is used for designing a superconducting modulator circuit. This behavior can be useful for device applications and designing of superconducting electronic circuits like superconducting power amplifier, superconducting transmitter and superconducting ac current controller., Comment: 15 pages, 4 figures

Non-resonant radio frequency absorption (NRRA) of superconducting MgB2 pellet is investigated as a function of frequency, temperature, pressure, rf power, magnetic field, sample size and grain size. The NRRA signals of granular MgB2 samples show three separate and distinguish temperature dependent phase reversals, one of them is similar to the phase reversal occurs very often in high Tc superconductors at temperature few degrees below Tc, the other two are anomalous and occur at lower temperatures. Interpretation based on the assumptions in the line shape model indicates that the Josephson junctions are weakly coupled in MgB2 than other high temperature superconductors like YBa2Cu3O7, La1.9Sr0.1CuO4 and Bi2Sr2CaCu2O8. The samples show a significant magnetic field sweep hysteresis, which is a signature of trapped flux. The hysteresis behavior suggests that the vortices in the MgB2 are rather rigid. The effects of passing dc current through the sample on the NRRA signal patterns are discussed. It is seen that the transition temperature Tc measured using the NRRA technique decreases with decreasing sample size, whereas the Tc measured using ac susceptibility and temperature dependent resistivity is independent of the sample size. The results are discussed in terms of resistively shunted junction, flux flow and bond percolation models. Effect of MgO doping is analyzed and discussed. The intergranular coupling energy and the activation energy of MgB2 are calculated and compared with other high temperature superconductors. The effects of sintering on the weak links in MgB2 polycrystalline pellet are discussed., Comment: 29 pages, 16 figures

The 'two-electron' doped rare earth mangnites Ca_1-x Ce_x MnO_3 (x = 0.1,0.2) are probed using resistivity, ac susceptibility and electron paramagnetic resonance (EPR) measurements across their respective charge ordering (CO) temperatures T_CO = 173 K and 250 K. The EPR 'g' factor and intensity as well as the transport and magnetic behaviours of the two compositions are qualitatively similar and are as expected for CO systems. However, the EPR linewidth, reflective of the spin dynamics, for x = 0.1, shows a strongly anomalous temperature dependence, decreasing with decreasing temperature below T_CO in contrast with the sample with x = 0.2 and other CO systems. Keeping in view the evidence for magnetic frustration in the system, we propose that the anomalous temperature dependence of the linewidth is the signature of the occurrence of a disorder driven spin liquid phase, present along with charge ordering., Comment: 19 pages including 4 figures

Nd_0.67Sr_0.33MnO_3 nanoparticles with the grain size of about 30 nm are prepared by sol-gel method.These nanopowders are annealed at four different temperatures viz. 800 oC, 900 oC, 1000 oC and 1100 oC to study the effect of particle size on magnetic, transport and electron magnetic resonance (EMR) spectral parameters. The samples are characterized by XRD, SEM, EDAX and TEM. The ac susceptibility experiments show that as the particle size increases the ferromagnetic to paramagnetic transition temperature (Tc) decreases. The metal-insulator transition temperature also changes with the particle size as revealed by resistivity measurements. EMR spectra of the nanopowders are recorded from room temperature down to 4K using an X-band EPR spectrometer. the spectra could be fitted using two broad-Gaussian lineshapes below Tc and suggested the ferromagnetic nature of the samples. Above Tc a single Lorenzian fits the signals as expetced for paramagnetic samples. The EMR spectral parameters are found to be different from the bulk (polycrystalline)sample data. The spectral parameters show variation with the particle size. The presence of the two signals in the ferromagnetic phase is attributed to core and shell regions in the nanoparticles. We could estimate the shell thickness from the EMR intensity data as 0.7 - 1 nm which agrees with other measurements., Comment: 19 pages including 11 figures

Our experiments show that for two or more pieces of a wire, of different lengths in general, combined in parallel and connected to a dc source, the current ratio evolves towards unity as the combination is cooled to the superconducting transition temperature Tc, and remains pinned at that value below it. This re-distribution of the total current towards equipartition without external fine tuning is a surprise. It can be physically understood in terms of a mechanism that involves the flux-flow resistance associated with the transport current in a wire of type-II superconducting material. It is the fact that the flux-flow resistance increases with current that drives the current division towards equipartition., Comment: Revised version of J.Phys. Condens.Matter; vol. 18(2006) L143-L147 14 pages including 3 figures; provided an explanation in terms of the physical mechanism of flux flow induced resistance that is proportional to the impressed current. We are adding a simple, physically robust derivation of our equipartition without taking resort to the minimum dissipation principle

We have studied rf power dissipation in Bi2Sr2CaCu2O8 single crystals as a function of angle between the applied field and c-axis of the crystal by using a novel technique of measuring direct rf power absorption. We observed two peaks in power absorption as a function of temperature. The amplitude and position of the first peak (peak A) change with orientation and attributed to Josephson junctions decoupling. The second peak (peak B) appears just below Tc has been attributed to the vortex motion and its amplitude depends on the angle of orientation., Comment: 14 Pages, 4 Figures

Even after nearly a century of discovery of superconductivity, there has been no direct experimental proof of the expected zero resistance of superconductors. Indeed, it has been believed that it is impossible to experimentally show that the resistance has fallen exactly to zero. In this work we demonstrate that the dc resistivity of a superconducting material below the transition temperature has to be exactly zero., Comment: 7 pages, 2 figures and one page supplementary information with one figure

Using a novel technique of measuring direct rf power dissipation, the temperature and magnetic field dependence of rf absorption in superconducting single crystals of Bi_2Sr_2CaCu_2O_8 is studied. An unexpected, larger than normal state, dissipation is observed below Tc, which exhibits peaks at magnetic field dependent temperatures. An explanation is provided for this observation in terms of the energy required to decouple the intrinsic Josephson junctions in the crystals., Comment: 13 pages including figures

The design, fabrication and performance of an apparatus for the measurement of direct rf power absorption in colossal magnetoresistive (CMR) and superconducting samples are described. The system consists of a self-resonant LC tank circuit of an oscillator driven by a NOT logic gate. The samples under investigation are placed in the core of the coil forming the inductance L and the absorbed power is determined from the measured change in the current supplied to the oscillator circuit. A customized low temperature insert is used to integrate the experiment with a commercial Oxford Instruments cryostat and temperature controller. The oscillator working in the rf range between 1 MHz to 25 MHz is built around an IC 74LS04. The temperature can be varied from 4.2 to 400 K and the magnetic field from 0 to 1.4 T. The apparatus is capable of measuring direct power absorption in CMR and superconducting samples of volume as small as 1/1000 cm^3 with a signal to noise ratio of 10:1. Further increase in the sensitivity can be obtained by summing the results of repeated measurements obtained at a given temperature. The system performance is evaluated by measuring the absorbed power in La_0.7 Sr_0.3 MnO_3 (LSMO) CMR manganite samples and superconducting Y Ba_2 Cu_3 O_7 (YBCO) samples at different rf frequencies. All operations during the measurements are automated using a computer with a menu-driven software system, user input being required only for the initiation of the measurement sequence., Comment: 32 pages including 14 figures

The origin of AC resistivity in the high Tc superconductors is not addressed adequately in literature. We found out, Josephson Junction (JJ) decoupling is the main origin of the AC resistivity in high Tc superconductors. We have measured the AC resistivity in the superconductors in the low frequency range by measuring the complex AC impedance of superconducting YBa2Cu3O7 (YBCO) polycrystalline samples. Our data shows that under certain conditions when the number density of Josephson Junctions (JJ) present in the sample and the JJ critical current crosses a threshold value, AC resistivity in the superconducting state keeps on increasing with lowering temperature. The underlying mechanism is an interesting interplay of JJ coupling energy, amplitude of the supply AC voltage and the current applied to the superconducting sample. The effect of the applied AC current of different frequencies and the variation of temperature were studied in detail. To find out the exact relation between the JJ coupling energy, JJ number density, applied AC frequency, the amplitude of AC current and the AC resistivity in the superconductors, we have studied samples of different grain sizes, pressurized with different pressure and sintered at different physical situations. These results have important implications for the understanding of the origin of AC resistivity and characterization of superconducting samples. In this paper we also extend the capability of the AC impedance studies in superconductors for the characterization of materials for device applications., Comment: 20 pages, 5 figures

Three long standing problems related to the physics of water viz, the possibility of vitrifying bulk water by rapid quenching, its glass transition, and the supposed impossibility of obtaining supercooled water between 150 and 233 K, the so-called 'no man's land'of its phase diagram, are studied using the highly sensitive technique of spin probe ESR. Our results suggest that water can indeed be vitrified by rapid quenching, it undergoes a glass transition at \~ 135 K, and the relaxation behavior studied using this method between 165 K and 233 K closely follows the predictions of the Adam-Gibbs model., Comment: 13 pages, 3 figures; results on slow cooled water added; four figures compressed in to three

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., Comment: 5 pages, 3 figures

We present and compare the results of temperature-dependent electron paramagnetic resonance (EPR) studies on Pr_1-x Ca_xMnO_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, alpha, 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., Comment: 11 pages, 4 figures

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 figures

We present results of an electron paramagnetic resonance (EPR) study of Nd$_{1-x}$Sr$_x$MnO$_3$ with x = 0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T$_c$) and the antiferromagnetic, charge ordering transition (T$_N$ = 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$_N$ = 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 = 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 = 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 A-type AFM signal observed in both Nd$_{0.5}$Sr$_{0.5}$MnO$_3$and Nd$_{0.45}$Sr$_{0.55}$MnO$_3with the two FM phases of La$_{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.

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 figures

We report Electron Paramagnetic Resonance measurements on single crystalline and powder samples of Nd_0.5Ca_0.5MnO_3 across the charge-ordering transition at T_co=240 K down to the antiferromagnetic ordering transition at T_N = 140 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 = 0.13 eV. Similar analysis of data on Pr_0.6Ca_0.4MnO_3 published earlier gives E_a=0.2 eV. Below T_co, the g-value increases continuously suggesting a gradual strengthening of the orbital ordering. We give a qualitative explanation of the temperature dependence and of the maximum in the asymmetry ratio A/B observed at T_co in single crystal spectra which also supports the themodel 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., Comment: Uses Revtex3.0