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We present angle-resolved photoemission studies of (La1-zPrz)2-2xSr1+2xMn2O7 with x=0.4 and z=0.1,0.2 and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO2-layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ~400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, cause the observed renormalisation of the spectral features., Comment: 4 pages, 3 figures, if not displayed correctly try Adobe Reader

We have demonstrated the effect of hydrostatic pressure on magnetic and transport properties, and thermal transport properties in electron-doped manganites CaMn$_{1-x}$Sb$_{x}$O$_{3}$. The substitution of Sb$^{5+}$ ion for Mn $^{4+}$site of the parent matrix causes one-electron doping with the chemical formula CaMn$^{4+}_{1-2x}$Mn$^{3+}_{x}$Sb$^{5+}_{x}$O$_{3}$ accompanied by a monotonous increase in unit cell volume as a function of $x$. Upon increasing the doping level of Sb, the magnitudes of both electrical resistivity and negative Seebeck coefficient are suppressed at high temperatures, indicating the electron doping. Anomalous diamagnetic behaviors at $x=0.05$ and 0.08 are clearly observed in field cooled dc magnetization. The effect of hydrostatic pressure on dc magnetization is in contrast to the chemical pressure effect due to Sb doping. The dynamical effect of ac magnetic susceptibility measurement points to the formation of the magnetically frustrated clusters such as FM clusters embedded in canted AFM matrix., Comment: 12 pages,11 figures, 3 tables

We have carried out temperature and pressure-dependent Raman and x-ray measurements on single crystals of Tb$_2$Ti$_2$O$_7$. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Gr\"{u}neisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at $\sim$ 9 GPa. We discuss possible implications of our results on the spin-liquid behaviour of Tb$_2$Ti$_2$O$_7$., Comment: 10 figures, 26 pages. Appeared in Physical Review B, vol-79, pp-134112 (2009)

We report a significant decrease in the low-temperature resistance induced by the application of an electric current on the $ab$-plane in the paramagnetic insulating (PMI) state of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. A colossal electroresistance effect attaining -95% is observed at lower temperatures. A colossal magnetoresistive step appears near 5T at low temperatures below 10K, accompanied by an ultrasharp width of the insulator-metal transition. Injection of higher currents to the crystal causes a disappearance of the steplike transition. These findings have a close relationship with the presence of the short-range charge-ordered clusters pinned within the PMI matrix of the crystal studied., Comment: 4 pages 3 figures

We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound $Dy_2Ti_2O_7$ and its non-magnetic analogue $Lu_2Ti_2O_7$. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in $Dy_2Ti_2O_7$, appearing below a temperature of $T_c=$110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O$^{18}$-isotope substituted $Dy_2Ti_2O_7$ confirm the phonon origin of the "new" mode. These findings, absent in $Lu_2Ti_2O_7$, suggest that the room temperature cubic lattice of the pyrochlore $Dy_2Ti_2O_7$ undergoes a "subtle" structural transformation near $T_c$. We find anomalous \textit{red-shift} of some of the phonon modes in both the $Dy_2Ti_2O_7$ and the $Lu_2Ti_2O_7$ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions., Comment: 28 pages, 9 figures (Accepted for publication in Physical Review B)

We present evidence for spin polarized charge carriers in In$_2$O$_3$ films. Both In$_2$O$_3$ and Cr doped In$_2$O$_3$ films exhibit room temperature ferromagnetism after vacuum annealing, with a saturation moment of approximately 0.5 emu/cm$^3$. We used Point Contact Andreev Reflection measurements to directly determine the spin polarization, which was found to be approximately 50$\pm$5% for both compositions. These results are consistent with suggestions that the ferromagnetism observed in certain oxide semiconductors may be carrier mediated., Comment: 5 pages 5 figures

e report the effect of pressure on the steplike magnetostriction of single crystalline bilayered manganite (LaPr)1.2Sr1.8Mn2O8, for our understandings of the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase. The application of pressure suppresses a steplike transformation and causes a broad change in the magnetostriction. The injection of an electric current to the crystal also weakens the steplike variation in both the magnetostriction and magnetoresistance. The stabilization of ferromagnetic interaction or the delocalization of charge carriers is promoted with the applied pressure or applied current, resulting in the suppressed steplike behavior. Our findings suggest that the step phenomenon is closely related to the existence of localized carriers such as the short-range charge-orbital ordered clusters., Comment: 5 pages,4 figures

We report on an investigation of the double-layer manganite series (La(1-z)Pr(z))1.2Sr1.8Mn2O7 (0 <= z <= 1), carried out on single crystals by means of both macroscopic magnetometry and local probes of magnetism (muSR, 55Mn NMR). Muons and NMR demonstrate an antiferromagnetically ordered ground state at non-ferromagnetic compositions (z >= 0.6), while more moderate Pr substitutions (0.2 <= z <= 0.4) induce a spin reorientation transition within the ferromagnetic phase. A large magnetic susceptibility is detected at {Tc,TN} < T < 250K at all compositions. From 55Mn NMR spectroscopy, such a response is unambiguously assigned to the intergrowth of a ferromagnetic pseudocubic phase (La(1-z)Pr(z))(1-x)Sr(x)MnO3, with an overall volume fraction estimated as 0.5-0.7% from magnetometry. Evidence is provided for the coupling of the magnetic moments of these inclusions with the magnetic moments of the surrounding (La(1-z)Pr(z))1.2Sr1.8Mn2O7 phase, as in the case of finely dispersed impurities. We argue that the ubiquitous intergrowth phase may play a role in the marked first-order character of the magnetic transition and the metamagnetic properties above Tc reported for double-layer manganites., Comment: 11 pages, 9 figures. Submitted to Phys. Rev. B

We present here magnetization, specific heat and Raman studies on single-crystalline specimens of the first pyrochlore member $Sm_2Ti_2O_7$ of the rare-earth titanate series. Its analogous compound $Sm_2Zr_2O_7$ in the rare-earth zirconate series is also investigated in the polycrystalline form. The Sm spins in $Sm_2Ti_2O_7$ remain unordered down to at least T = 0.5 K. The absence of magnetic ordering is attributed to very small values of exchange ($\theta_{cw} ~ -0.26 K$) and dipolar interaction ($\mu_{eff} ~ 0.15 \mu_B$) between the $Sm^{3+}$ spins in this pyrochlore. In contrast, the pyrochlore $Sm_2Zr_2O_7$ is characterized by a relatively large value of Sm-Sm spin exchange ($\theta_{cw} ~ - 10 K$); however, long-range ordering of the $Sm^{3+}$ spins is not established at least down to T = 0.67 K, due to frustration of the $Sm^{3+}$ spins on the pyrochlore lattice. The ground state of $Sm^{3+}$ ions in both pyrochlores is a well-isolated Kramer's doublet. The higher-lying crystal field excitations are observed in the low-frequency region of the Raman spectra of the two compounds recorded at T = 10 K. At higher temperatures, the magnetic susceptibility of $Sm_2Ti_2O_7$ shows a broad maximum at T = 140 K while that of $Sm_2Zr_2O_7$ changes monotonically. Whereas $Sm_2Ti_2O_7$ is a promising candidate for investigating spin-fluctuations on a frustrated lattice as indicated by our data, the properties of $Sm_2Zr_2O_7$ seem to conform to a conventional scenario where geometrical frustration of the spin exclude their long-range ordering., Comment: 24 pages, 6 figures, Accepted for publication in Phys. Rev. B

We have successfully grown cm3-size single crystals of the metallic-ferromagnet Sm2Mo2O7 by the floating-zone method using an infrared-red image furnace. The growth difficulties and the remedies found using a 2-mirror image furnace are discussed. Magnetization studies along the three crystalline axes of the compound are presented and discussed based on our recent proposal of an ordered spin-ice ground state for this compound

The recent discovery of Spin-ice is a spectacular example of non-coplanar spin arrangements that can arise in the pyrochlore A2B2O7 structure. We present magnetic and thermodynamic studies on the metallic-ferromagnet pyrochlore Sm2Mo2O7. Our studies, carried out on oriented crystals, suggest that the Sm spins have an ordered spin-ice ground state below about T* = 15 K. The temperature- and field-evolution of the ordered spin-ice state are governed by an antiferromagnetic coupling between the Sm and Mo spins. We propose that as a consequence of a robust feature of this coupling, the tetrahedra aligned with the external field adopt a "1-in, 3-out" spin structure as opposed to "3-in, 1-out" in dipolar spin ices, as the field exceeds a critical value.

We report a steplike lattice transformation of single crystalline (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$bilayered manganite accompanied by both magnetization and magnetoresistive jumps, and examine the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase accompanied by a huge decrease in resistance. Our findings support that the abrupt magnetostriction is closely related to an orbital frustration existing in the inhomogeneous paramagnetic insulating phase rather than a martensitic scenario between competing two phases., Comment: 5 pages,4figures, v4: figures are changed, in press in Phys.Rev.Lett

We observed a large HEMD effect in the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$, a direct consequence of field driven spin-glass insulator to ferromagnetic metal transition. The remnants of the transition can be used to achieve dielectric contrast at room temperature. This discovery suggests that electronic mechanisms such as the metal-insulator transition, charge ordering, and orbital ordering can be exploited to give substantial dielectric contrast in other materials., Comment: 4 pages, 4 figures

The elastic and magnetic properties of a single crystal of 327 bilayer manganese oxide $(La_{1-z} Pr_z)_{1.2} Sr_{1.8} Mn_2 O_7$ with $z$=0.6 have been investigated by ultrasonic and high-field magnetization measurements. A distinct elastic anomaly was observed at low temperatures and in high magnetic fields when crossing the phase boundary. A pronounced elastic softening as a function of magnetic field ($H$) appears across the boundary of the low-temperature magnetic phase below around 40 K, accompanied by a distinct hysteresis. In the high field region, however, these elastic constants exhibit an increase with increasing a magnetic field in the high-field region. The high-field magnetization measurement characterizing the magnetic state clarified a strong connection between elastic strains and magnetic moments. The new metamagnetic transition was found in the high-field region above 40 K, accompanied with no hysteresis. The present results of elastic constants are discussed in terms of a coupling effect between elastic strains associated with sound waves and relevant magnetic moments, and relevant conduction bands. They are explained reasonably in terms of a strong coupling between elastic strains and magnetic susceptibility $chi$$_{m}$=$partial$$M$/$partial$$H$., Comment: 9 pages, 14 figures, regular paper

We have studied the pressure effect on magnetostriction, both in the $ab$-plane and along the c-axis, of a (La,Pr)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ bilayered manganite single crystal over the temperature region where the field-induced ferromagnetic metal (FMM) transition takes place. For comparison, we have also examined the pressure dependence of magnetization curves at the corresponding temperatures. The applied pressure reduces the critical field of the FMM transition and it enhances the remanent magnetostriction. An anomalous pressure effect on the remanent lattice relaxation is observed and is similar to the pressure effect on the remanent magnetization along the c-axis. These findings are understood from the view point that the double-exchange interaction driven FMM state is strengthened by application of pressure., Comment: 7 pages,7 figures

We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. We discuss field- and temperature-induced trends within the framework of the Tomioka-Tokura global electronic phase diagram picture and suggest that controlled disorder near a phase boundary can be used to tune the magnetodielectric response. Remnants of the spin-glass insulator to ferromagnetic metallic transition can also drive 300 K color changes in (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$., Comment: 9 pages, 8 figures

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., Comment: 6 pages,5 figures

We have investigated magnetic, electrical and thermal transport properties (Seebeck effect and thermal conductivity) of LaSr$_{2}$Mn$_{2-y}$Cr$_{y}$O$_{7}$ polycrystalline samples ($y$=0.1, 0.2, 0.4 and 0.6). The Cr$^{3+}$ substitution for Mn$^{3+}$ sites causes a removal of $d_{x^2-y^2}$ orbital of $e_g$-electron resulting in a volume shrinkage of lattice. Magnetic measurements reveal the appearance of a glassy behavior for Cr-doped samples, accompanied by both a collapse of the A-type antiferromagnetic structure and the growth of ferromagnetic clusters. Cr-doping effect on electrical transport strongly enhances an insulating behavior over a wide range of temperature, while it suppresses a local minimum of thermoelectric power at lower temperatures. The phonon thermal conduction gradually rises with increasing Cr content, which is contradictory to a typical impurity effect on thermal conductivity. We attribute this to a suppression of local lattice distortion through the introduction of Jahn-Teller inactive ions of Cr$^{3+}$., Comment: 8 pages, 9figures

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 $\mu$B/Fe for the anatase sample and 0.48 $\mu$B/Fe for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K., Comment: 13 october 2004

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., Comment: 31 july 2004

In this work, we report an atomic scale study on the ferromagnetic insulator manganite LaMnO$_{3.12}$ using $\gamma-\gamma$ PAC spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a Jahn-Teller-distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition (Rhombohedric-Orthorhombic). Two distinct regimes of JT-distortions were found: a high temperature regime where uncorrelated polaron clusters with severe distortions of the Mn$^{3+}$O$_{6}$ octahedra survive up to $T \approx 800 K$ and a low temperature regime where correlated regions have a weaker JT-distorted symmetry., Comment: 4 pages, 4 Figures, submitted to PRL, new version with more data, text reformulated

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., Comment: 3 figures

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., Comment: a slightly modifed version to be published in JPhysCondMatter

The complex lanthanide doping of electron manganites results in enhancement of various phase separation effects in physical properties of these compounds. Selecting Ca0.8Sm0.16Nd0.04MnO3 as a model case we show that the first order structural phase transition from paramagnetic semi-metallic phase into anti-ferromagnetic semi-metallic phase at TS ~ 158 +- 4 K is marked by an abrupt decrease in magnetization, a step like anomaly DL/L = 10-4 in thermal expansion and large latent heat DQ = 610 J/mol. In a certain temperature range below TS, the high field magnetization exhibits hysteretic metamagnetic behavior due to field-induced first order transformation. ac-susceptibility, magnetization and resistivity data suggest rather a non-uniform state in Ca0.8Sm0.16Nd0.04MnO3 at low temperatures. The metal - insulator transition occurs at TMI ~112 +- 3 K, accompanied by a step-like increase in magnetization. These features could be ascribed to "sponging" of electrons from neighboring anti-ferromagnetic matrix by clusters undergoing the ferromagnetic ordering., Comment: submitted to J.Phys. Cond. Matter

Measurements of Seebeck effect in 0 and 1.5 T and Nernst coefficient in 0.3,0.9 and 1.8 T as a function of temperature on a polycrystalline sample Sm0.55Sr0.45MnO3 are presented. The data point out conclusively that an increase inboth the carrier density and the mobility of the charge carriers is responsible for the observed colossal magnetoresistance, Comment: 25 may 2003

The magnetic properties of electron-doped manganite La0.3Ca0.7MnO3 and La0.3Ca0.7Mn0.8Cr0.2O3 polycrystalline samples prepared by sol-gel technique have been investigated between 5 and 300 K in magnetic fields ranging from 0 to 5 T. The transition at 260 K, attributed to charge ordering in La0.3Ca0.7MnO3, is completely suppressed in the Cr-substituted sample while the onset of a magnetic remanence followed by the appearance of a magnetic irreversibility at lower temperatures is observed in both samples. These features indicate that ferromagnetic clusters coexist with either an antiferromagnetic phase for La0.3Ca0.7MnO3 or a spin-cluster glass phase for La0.3Ca0.7Mn0.8Cr0.2O3 at the lowest temperatures. The exponential temperature dependence of the resistivity for the Cr-substituted sample is consistent with the small polaron hopping model for 120 K < T < 300 K, while the data are better described by Mott's hopping mechanism for T < 120 K. Whereas the parent compound La0.3Ca0.7MnO3 is known to show no magnetoresistance, a large negative magnetoresistance is observed in the La0.3Ca0.7Mn0.8Cr0.2O3 sample below 120 K. The appearance of the CMR is attributed to spin dependent hopping between spin clusters and/or between ferromagnetic domains.

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., Comment: 6 pages, 4 figures included in text

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., Comment: 8 figures

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., Comment: 6 pages, figures included. Submitted to PRB

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

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., Comment: 5 pages, 5 figures, submitted to Phys. Rev. B

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., Comment: 4 pages including 5 figures. submitted to Phys. Rev. B

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 ., Comment: four pages, three figures