M. Pattabiraman

Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd0.7Sr0.3MnO3

The magnetocaloric effect in single crystalline Nd_{0.7}Sr_{0.3}MnO_{3} is investigated by measuring the field-induced adiabatic change in temperature which reveals a single negative peak around 130 K well below the Curie temperature (T_C=203 K). In order to understand this unusual magnetocaloric effect, we invoke the reported {55}^Mn spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on Nd_{0.7}Sr_{0.3}MnO_{3}. We show that this effect is a manifestation of a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom which results in a complex ferromagnetic state. The critical behavior of Nd_{0.7}Sr_{0.3}MnO_{3} near Curie temperature is investigated to study the influence of the coupled degrees of freedom. We find a complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields.

Tricritical point and magnetocaloric effect of Nd1−xSrxMnO3

A tricritical point is observed in the Nd1−xSrxMnO3 (NSMO) (x=0.3, 0.33, and 0.4) manganites at x=0.33 which separates the first-order transition in NSMO-0.3 and second order transition in NSMO-0.4. The ferromagnetic transition of these compounds is further investigated by measuring magnetocaloric effect (MCE) and by applying a theoretical model based on Landau theory of phase transitions. Results indicate that the contributions to the free energy from the presence of correlated clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.

Enhanced magnetocaloric effect in single crystalline Nd0.5Sr0.5MnO3

The magnetocaloric effect in single crystalline Nd0.5Sr0.5MnO3 (NSMO 0.5) is investigated by computing the field dependent entropy change (ΔS) and adiabatic temperature change (ΔTad). At the charge ordering temperature (TCO), the value of ΔSmax is found to be much higher than ΔSmax reported in polycrystalline samples. This “giant” entropy change is attributed to interplay (stronger in single crystals) among spin, charge, lattice, and orbital degrees of freedom resulting in a field induced transition at TCO. In contrast, the change in entropy associated with Curie temperature (TC) is very low. The direct measurements of the field induced temperature change (ΔT) are in agreement with the computed value of ΔS. The presence of short-range correlations with charge/orbital order (COO) above and below TC may be responsible for the suppression of the negative MCE at TC. A critical exponent analysis of the paramagnetic (PM) to ferromagnetic (FM) transition using magnetization data yields mean-field-like values, wh...

Raman scattering study of Nd1−xSrxMnO3 (x = 0.3, 0.5)

We report on polarized Raman scattering of single crystals of Nd1−xSrxMnO3 (x = 0.3, 0.5). Raman spectra of Nd0.7Sr0.3MnO3 show a significant change through the metal–insulator transition. In the ferromagnetic metallic phase phonon modes grow in intensity and number while the electronic continuum becomes more pronounced. We suggest that these effects are due to the strong competition between the localization and the delocalization of carriers which is the origin of the largest colossal magnetoresistance effect ever reported for the manganites. Raman spectra of Nd0.5Sr0.5MnO3, upon cooling through the charge-ordering temperature TCO = 148 K, exhibit several new lines which undergo a substantial hardening. This hardening is interpreted as a freezing of the Jahn–Teller distortions with a gradual decrease of a fraction of the ferromagnetic phase in the CE-type charge/orbital ordered state.

ESR study of spin–lattice correlated clusters in single crystalline Nd0.7Sr0.3MnO3

We report electron spin resonance measurements in single crystalline Nd0.7Sr0.3MnO3 that provide evidence for the existence of spin–lattice correlated clusters above and below TC (= 205 K). The linewidth of the paramagnetic spectrum indicates the presence of strong electron–phonon interaction rather than the spin only interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The observation of ferromagnetic resonance (FMR) spectra only below 185 K (TC = 205 K) and noisy features in the FMR spectra above 140 K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers. The magnetocrystalline anisotropy of the sample splits the FMR line into two below 185 K. An additional splitting observed in the FMR line is attributed to the existence of spin–lattice correlated (insulating) clusters within the ferromagnetic (metallic) phase. The influence of these clusters on the spin dynamics above and below TC is discussed.

Determination of the elastic moduli in LiAlO2

The precise determination of elastic constants of γ‐LiAlO2 has recently become important, as this material is being used as a substrate for intensely studied acousto-optical device structures and no values for the elastic moduli have been published to date. The precision of experimentally determined elastic constants hinges significantly on considering the influence of the acoustic transducer on the sound propagation in the system. Therefore, the influence of the transducer is discussed and an approach for corrections is presented. The compressional elastic constants C11 and C33 and the shear constants C12 and C13 of γ‐LiAlO2 are determined with an acoustic pulse-echo technique and with sampled-continuous-wave spectroscopy.

Electron spin resonance study of Nd0.7Sr0.3MnO3 single crystals

Electron spin resonance (ESR) measurements in Nd0.7Sr0.3MnO3 single crystals provide evidence for the existence of spin and lattice correlated clusters above and below TC=205K. The ESR linewidth deviates from the quasilinear “universal” temperature dependence of the linewidth observed in several manganites. A linear temperature dependence of the linewidth is attributed to the spin-phonon interaction. Thus the linewidth of the paramagnetic resonance line indicates the presence of a strong spin-phonon interaction rather than the spin-spin interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The split in the ESR spectra only below 180K (TC=205K) and more noise in the spectra between 150 and 180K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers.

Sign reversal of Hanle electromagnetically induced absorption with orthogonal circularly polarized optical fields

We study by computation and experiment an electromagnetically induced absorption resonance in the Hanle configuration with a transverse magnetic field on a closed Fg → Fe = Fg+1 transition with co-propagating orthogonal circularly polarized probe and coupling optical fields. At high coupling field intensities, the Hanle resonance changes sign due to a shift in atomic population from Zeeman sublevels associated with a probe field cyclic transition to sublevels associated with a coupling field cyclic transition at zero magnetic field. We also show that a similar sign reversal does not occur for π-polarized and σ-polarized coupling fields.

Development of SQUID-Based System for Nondestructive Evaluation

This article describes the development of a superconducting quantum interference device (SQUID)-based system for nondestructive evaluation. The setup incorporates an in-house developed thin-film-based Nb SQUID with readout flux locked loop electronics and consists of a liquid helium cryostat with adjustable stand-off distance, a precision XY- thetas scanner for studying both flat and cylindrical samples, and a data acquisition system. The system has been used for the detection of artificially engineered subsurface defects in aluminum plates and to track magnetic-to-nonmagnetic phase transformation in stainless steel [grade 316L(N)] weldment specimens subjected to low cycle fatigue deformation.

The Nd–Mn exchange interaction in Nd0.7Sr0.3MnO3

The Nd-Mn exchange interaction in Nd0.7Sr0.3MnO3 is considered by studying its influence on the Mn-55 spin-echo NMR lineshape and spin- spin relaxation time. It is seen that the interaction is of considerable strength well above the Nd spin ordering temperature (approximate to 20 K), with a significant influence on the Mn (electron) spin dynamics.