M. Pattabiraman

Bio: M. Pattabiraman is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Resonance & Zeeman effect. The author has an hindex of 12, co-authored 33 publications receiving 467 citations. Previous affiliations of M. Pattabiraman include Indira Gandhi Centre for Atomic Research & Indian Institutes of Technology.

Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd 0.7 Sr 0.3 MnO 3

Abstract: The magnetocaloric effect in single crystalline ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ is investigated by measuring the field-induced adiabatic change in temperature $(\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}})$ which reveals a single negative peak around $130\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ well below the Curie temperature $({T}_{C}=203\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. In order to understand this unusual magnetocaloric effect, we invoke the reported $^{55}\mathrm{Mn}$ spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$. 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 ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ 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

Abstract: 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.

Hanle electromagnetically induced transparency and absorption resonances with a Laguerre Gaussian beam

Abstract: We describe a computational and experimental study on Hanle electromagnetically induced transparency and absorption resonance line shapes with a Laguerre Gaussian (LG) beam. It is seen that the LG beam profile brings about a significant narrowing in the line shape of the Hanle resonance and ground-state Zeeman coherence in comparison to a Gaussian beam. This narrowing is attributed to the azimuthal mode index of the LG field.

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

Abstract: 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.

Magnetocaloric effect and refrigerant capacity in charge-ordered manganites

Abstract: The influence of first- and second-order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity (RC) of charge-ordered Pr0.5Sr0.5MnO3 has been investigated. The system undergoes a paramagnetic to ferromagnetic transition at TC∼255 K followed by a ferromagnetic charge-disordered to antiferromagnetic charge-ordered transition at TCO∼165 K. While the first-order magnetic transition (FOMT) at TCO induces a larger MCE (6.8 J/kg K) limited to a narrower temperature range resulting in a smaller RC (168 J/kg), the second-order magnetic transition at TC induces a smaller MCE (3.2 J/kg K) but spreads over a broader temperature range resulting in a larger RC (215 J/kg). In addition, large magnetic and thermal hysteretic losses associated with the FOMT below TCO are detrimental to an efficient magnetic RC, whereas these effects are negligible below TC because of the second-order nature of this transition. These results are of practical importance in assessing the usefulness of charge-o...

Structural, Magnetic, Electrical, and Magnetoelectric Properties of Sm- and Ho-Substituted Nickel Ferrites

Abstract: Sm- and Ho-substituted and pure nickel ferrite materials, namely, NiFe1.925Sm0.075O4, NiFe1.925Ho0.075O4, and NiFe2O4, have been synthesized by the solid-state chemical reaction, and their structural, magnetic, dc electrical conductivity, ferromagnetic, ferroelectric, and dielectric properties have been evaluated. Sm- and Ho-substituted nickel ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO3 and HoFeO3 as the additional phase, respectively. X-ray diffraction studies indicate rhombohedral distortion in NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 compounds. The existence of ferroelectricity in NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 has been confirmed from the ferroelectric loops, and the respective transition temperatures are 543 and 677 K. The respective magnetocapacitance values are −1.8% and −1.2%. Magnetoelectric coefficients observed at 500 Oe in the NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 compounds are 1.82 and 1.84 mV cm−1 Oe−1, respectively. A considerable...

Electromagnetically induced absorption in a four-state system

Abstract: Summary form only given. It is well-known that nonlinear interference effects in the resonant atom-light interaction can lead to interesting and important phenomena, such as electromagnetically induced transparency (EIT) of atomic medium, coherent population trapping, lasing without inversion, and others. Common to all these phenomena is the appearance of light-induced coherence between atomic levels, which are not coupled by dipole transitions. Recently Akulshin and co-workers (1998-9) have observed subnatural-width resonances in the absorption on the D/sub 2/ line of rubidium vapor under excitation by two copropagating optical waves with variable frequency offset. It is remarkable that, apart from expected EIT-resonances with negative sign, authors detected positive resonances termed as electromagnetically induced absorption (EIA). More recently, similar effects in a (3+5)-state system have been described as a result of constructive interference of the dipole excitation channels. We propose a simple theoretical model for EIA - a four-state N-atom. Remember that three-state /spl Lambda/- and V-systems give the EIT-resonances only.