arXiv subjects

R. Suryanarayanan

Publications and source records attributed to R. Suryanarayanan.

30 records · Page 2Linked to original sources

arXiv2005

Resistive relaxation in field-induced insulator-metal transition of a (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ bilayer manganite single crystal

We have investigated the resistive relaxation of a (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal, in order to examine the slow dynamics of the field-induced insulator to metal transition of bilayered manganites. The temporal profiles observed in remanent resistance follow a stretched exponential function accompanied by a slow relaxation similar to that obtained in magnetization and magnetostriction data. We demonstrate that the remanent relaxation in magnetotransport has a close relationship with magnetic relaxation that can be understood in the framework of an effective medium approximation by assuming that the first order parameter is proportional to the second order one.

cond-mat.str-el↗

arXiv2004

Ferromagnetism at 300 K in spin-coated anatasea and rutile Ti0.95Fe0.05O2 films

Thin films of Ti1-xFexO2 (x=0 and 0.05) have been prepared on sapphire substrates by spin-on technique starting from metal organic precursors. When heat treated in air at 550 and 700 degrees C respectively, these films present pure anatase and rutile structures as shown both by X-ray diffraction and Raman spectroscopy. Optical absorption indicate a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300 K. However, when the anatase and the rutile films are annealed in a vacuum of 1x10-5 Torr at 500 degrees C and 600 degrees C respectively, the magnetic moment, at 300 K, is strongly enhanced reaching 0.46 $μ$B/Fe for the anatase sample and 0.48 $μ$B/Fe for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K.

cond-mat.mtrl-sci↗

arXiv2004

Ferromagnetism at 300 K in spin-coated films of Co doped anatase and rutile TiO2

Thin films of Ti1-xCoxO2 (x=0 and 0.03) have been prepared on sapphire substrates by spin-on technique starting from metalorganic precursors. When heat treated in air at 550 and 700 C respectively, these films present pure anatase and rutile structures as shown both by X-ray diffraction and Raman spectroscopy. Optical absorption indicate a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300 K. However, when the anatase and the rutile films are annealed in a vacuum of 1x10-5 Torr at 500 oC and 600 oC respectively, the magnetic moment, at 300 K, is strongly enhanced reaching 0.36 B/Co for the anatase sample and 0.68 B/Co for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K.

cond-mat.mtrl-sci↗

arXiv2004

Stretched exponential behavior in remanent lattice striction of a (La,Pr)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ bilayer manganite single crystal

We have investigated the time dependence of remanent magnetostriction in a (La,Pr)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal, in order to examine the slow dynamics of lattice distortion in bilayered manganites. A competition between double exchange and Jahn-Teller type orbital-lattice interactions results in the observed lattice profile following a stretched exponential function. This finding suggests that spatial growth of the local lattice distortions coupled with e$_{g}$-electron orbital strongly correlates with the appearance of the field-induced CMR effect.

cond-mat.str-el↗

arXiv2004

Enhancement of TC and reentrant spin-glass transition in La0.86Ca0.14Mn1-yCryO3 (y = 0, 0.1 and 0.2)

We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La0.86Ca0.14Mn1-yCryO3(y = 0, 0.1 and 0.2) prepared by sol-gel technique. For y = 0, a paramagnetic to ferromagnetic transition was observed at TC = 185 K. For y = 0.1, the value of TC = 200 K, an increase of 15 K and for y = 0.2, the TC = 195 K, an increase of 10 K. The imaginary part of the ac susceptibility of all the three samples shows a secondary transition at Tf < TC. For y = 0, there is no definite law to account for the frequency dependence of Tf and is attributed to a transition arising out of a canted structure. However, in the case of y = 0.1 and 0.2, the frequency dependence indicate the presence of a reentrant spin glass transition at Tf. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR was observed corresponding to TC and Tf. The value of MR decreased for the Cr substituted samples.

cond-mat.mtrl-sci↗

arXiv2001

Work function changes in the double layered manganite La1.2Sr1.8Mn2O7

We have investigated the behaviour of the work function of La1.2Sr1.8Mn2O7 as a function of temperature by means of photoemission. We found a decrease of 55 +/- 10 meV in going from 60 K to just above the Curie temperature (125 K) of the sample. Above T_C the work function appears to be roughly constant. Our results are exactly opposite to the work function changes calculated from the double-exchange model by Furukawa, but are consistent with other measurements. The disagreement with double-exchange can be explained using a general thermodynamic relation valid for second order transitions and including the extra processes involved in the manganites besides double-exchange interaction.

cond-mat.str-el↗

arXiv2000

Magneto-Elastic Coupling in the Layered Manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We have studied the magneto-elastic coupling in the double layered Mn perovskite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ with an ultrasonic velocity technique. The temperature profile of both the in-plane and out-of-plane longitudinal velocities showed a large stiffening anomaly below the insulating paramagnetic to metallic ferromagnetic transition. Magnetic fluctuations effects consistent with the layered structure are evidenced as a frequency dependent velocity softening above the transition. The magneto-elastic coupling has been studied in magnetic field values up to 8 Tesla: the observations are consistent with a substantial magnetic anisotropy and a ferromagnetic order parameter with moments lying in the layers.

cond-mat.str-el↗

arXiv2000

Does EELS haunt your photoemission measurements?

It has been argued in a recent paper by R. Joynt (R. Joynt, Science 284, p 777 (1999)) that in the case of poorly conducting solids the photoemission spectrum close to the Fermi Energy may be strongly influenced by extrinsic loss processes similar to those occurring in High Resolution Electron Energy Loss Spectroscopy (HR-EELS), thereby obscuring information concerning the density of states or one electron Green's function sought for. In this paper we present a number of arguments, both theoretical and experimental, that demonstrate that energy loss processes occurring once the electron is outside the solid, contribute only weakly to the spectrum and can in most cases be either neglected or treated as a weak structureless background.

cond-mat↗

arXiv2000

rf susceptibility of La_{1-x}Sr_{x}MnO_{3} single crystals : magnetic signatures of structural changes

A sensitive tunnel diode oscillator (TDO) operating at 4MHz is used to probe the dynamic response of La_{1-x}Sr_{x}MnO_{3} single crystals for x=0.125,0.175,0.28 and 0.33 doping. Systematics of the measured change in reactance as a function of temperature (30K<T<320K) and DC magnetic field (0<H<6kOe) reveal distinct temperature and field scales associated with the dynamic response of spin. It is notable that these features are far more striking than the corresponding features in static measurements. The results are discussed in the context of structural changes leading to polaron ordering.

cond-mat.str-el↗

arXiv2000

Reentrant spin glass behavior in a layered manganite La1.2Sr1.8Mn2O7 single crystals

We report here a detailed study of AC/DC magnetization and longitudinal/transverse transport properties of La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystals below $T_{c}$ = 121 K. We find that the resistivity upturn below 40 K is related to the reentrant spin glass phase at the same temperature, accompanied by additional anomalous Hall effects. The carrier concentration from the ordinary Hall effects remains constant during the transition and is close to the nominal doping level (0.4 holes/Mn). The spin glass behavior comes from the competition between ferromagnetic double exchange and antiferromagnetic superexchange interactions, which leads to phase separation, i.e. a mixture of ferromagnetic and antiferromagnetic clusters, representing the canted antiferromagnetic state.

cond-mat.str-el↗

arXiv2000

Optical Studies of a Layered Manganite La_{1.2}Sr_{1.8}Mn_2O_7 : Polaron Correlation Effect

Optical conductivity spectra of a cleaved ab-plane of a La_{1.2}Sr_{1.8}Mn_2O_7 single crystal exhibit a small polaron absorption band in the mid-infrared region at overall temperatures. With decreasing temperature (T) to Curie temperature (T_C), the center frequency of the small polaron band moves to a higher frequency, resulting in a gap-like feature, and that it collapses to a lower frequency below T_C. Interestingly, with decreasing T, the stretching phonon mode hardens above T_C and softens below T_C. These concurring changes of lattice and electronic structure indicate that short range polaron correlation exist above T_C but disappear with a magnetic ordering.

cond-mat.str-el↗

arXiv1999

High frequency magneto-impedance of double perovskite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$: secondary transitions at high temperatures

Radio frequency magneto-impedance measurements clearly reveal a pronounced anomaly at 260K besides the main MI transition at 125K in the double perovskite material $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$. This feature is not seen clearly in static resistivity and magnetization measurements. We suggest that this anomaly represents short range magnetic correlations enhanced at radio frequencies, with the easy axis along the c-axis .

cond-mat.str-el↗