Current-induced phase control in charged-ordered Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 crystals

Single crystals of Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 show current-induced insulator-metal transitions at low temperatures. In addition, the charge-ordering transition temperature decreases with increasing current. The electroresistive ratio, defined as r0.5/rI where r0.5 is the resistivity at a current of 0.5 mA and rI the resistivity at a given applied current, I, varies markedly with temperature and the value of I. Thermal hysteresis observed in Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 at the insulator-metal transition indicates that the transition is first-order. The current-induced changes are comparable to those induced by magnetic fields, and the insulator-metal transition in Pr0.6Ca0.4MnO3 is accordingly associated with a larger drop in resistivity.Comment: 12 pages, 3 figures, first submitted to submitted to J. Phys. D; applied physics on 18th march 200

Electric-field-induced melting of the randomly pinned charge-ordered states of rare-earth manganates and associated effects

Films of charge-ordered Nd0.5Ca0.5MnO3, Gd0.5Ca0.5MnO3, Y0.5Ca0.5MnO3, and Nd0.5Sr0.5MnO3 show insulator-metal transitions on the passage of small electrical currents. That such an electric-field-induced transition occurs even in Y0.5Ca0.5MnO3 where the charge-ordered state is not affected by magnetic fields is noteworthy. The transition is attributed to the depinning of the randomly pinned charge solid. These materials also exhibit an interesting memory effect probably due to the randomness of the strength as well as the position of the pinning centers

Thin films of cobalt and granular copper-cobalt alloys by nebulized spray pyrolysis

Thin films of cobalt have been prepared by nebulized spray pyrolysis using Co(acac)<SUB>2</SUB> precursor in hydrogen atmosphere. The films show excellent magnetic and metallic properties. Granular Cu-Co alloy films of the composition Cu<SUB>82</SUB>Co<SUB>18</SUB> and Cu<SUB>50</SUB>Co<SUB>50</SUB> have been obtained by the nebulized spray pyrolysis of mixtures of Co(acac)<SUB>2</SUB> and Cu(acac)<SUB>2</SUB> in hydrogen atmosphere. These films show good magnetoresistance at room temperature

Temperature-dependent Brillouin scattering studies of surface acoustic modes in Nd_0.5Sr_0.5MnO₃

Brillouin scattering experiments are carried out to study the surface acoustic waves in Nd0.5Sr0.5MnO3 as a function of temperature in the range of 40–300 K covering the metal-insulator and charge-ordering phase transitions. The surface modes include surface Rayleigh wave, Pseudo-surface Acoustic Wave (PSAW) and high velocity PSAW. The observed softening of the sound velocities for the surface modes below paramagnetic to ferromagnetic transition, Tc is related to the softening of the C44 elastic constant. The subsequent hardening of the sound velocity below the charge ordering transition temperature Tco is attributed to the coupling of the acoustic phonon to the charge ordered state via long range ordering of the strong Jahn–Teller (JT) distortion

Electric-field-induced insulator–metal transitions in thin films of charge-ordered rare-earth manganates

Thin films of charge-ordered Nd<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB>, Y<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB>, and Nd<SUB>0.5</SUB>Sr<SUB>0.5</SUB>MnO<SUB>3</SUB> have been prepared on Si(100) and LaAlO<SUB>3</SUB>(100) substrates by the nebulized spray pyrolysis of organometallic precursors. Small electric fields induce insulator–metal transitions in the films with the resistivity decreasing with increasing current. The current–voltage characteristics are non-ohmic and show some hysteresis. The current-induced negative differential resistance found in these manganate films could have potential technological applications

Collapse of the charge-ordering state at high magnetic fields in the rare-earth manganite Pr0.63Ca0.37MnO3Pr_{0.63}Ca_{0.37}MnO_3

We have investigated the specific heat and resistivity of a single crystal of $Pr_{0.63}Ca_{0.37}MnO_3$ around the charge ordering (CO) transition temperature, $T_{CO}$, in the presence of high magnetic fields $(\leq 12 T)$ which can melt the charge-ordered state. At low magnetic fields $(\leq 10 T)$, the manganite transforms from a chargedisordered paramagnetic insulating (PI) state to a charge-ordered insulating (COI) state as the temperature is lowered. The COI state becomes unstable beyond a threshold magnetic field and melts to a ferromagnetic metallic phase (FMM). This occurs for $T < T_{CO}$. However, above a critical field $\mu_0$ $H_\rho ^*$, the sample shows the onset of a metallic phase for $T > T_{CO}$ and the COI transition occurs from a metallic phase. The onset temperature of the high-field metallic behavior decreases with an increase in the field and above a field $\mu_0 H^*$, the COI transition does not occur and the CO state ceases to occur at all T. The entropy change involved in the CO transition, $\Delta S_{CO} \approx 1.6 J /mol \hspace {2mm}K$at 0 T, decreases with increasing field and eventually vanishes for a field $\mu_0H^*$. The collapse of the CO state above $\mu_0H^*$ is thus associated with a collapse of the entropy that stabilizes the CO state

An electron paramagnetic resonance study of phase segregation in Nd0.5Sr0.5MnO3Nd_{0.5}Sr_{0.5}MnO_3

We present results of an electron paramagnetic resonance (EPR) study of $Nd_{1-x}Sr_xMnO_3$ with x = 0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260K $(T_c)$ 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 $T_N = 230K$ 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 .elds 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 belo $T_N,$ 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 Atype AFM signal observed in both $Nd_{0.5}Sr_{0.5}MnO_3$ and $Nd_{0.45}Sr_{0.55}MnO_3$ with the two FM phases of $a_{0.77}Ca_{0.23}MnO_3$, vis-$\grave{a}$-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

A comparative study of thin films of hole-doped Pr<SUB>0.6</SUB>Ca<SUB>0.4</SUB>MnO<SUB>3</SUB> and electron-doped Pr<SUB>0.4</SUB>Ca<SUB>0.6</SUB>MnO<SUB>3</SUB>

Properties of thin films of hole-doped Pr0.6Ca0.4MnO3 and electron-doped Pr0.4Ca0.6MnO3 are compared. While both are charge-ordered insulators, the hole-doped manganate undergoes an insulator-metal (I-M) transition on the application of magnetic-fields, but the electron-doped manganate does not. Substitution of 3% Cr3+ or Ru4+ in the Mn site has greater effect on the hole-doped manganate. Electrical fields, however, have similar effects on the hole-doped and electron-doped manganates, both exhibiting current-induced I-M transitions. The study not only establishes that the mechanism of the I-M transition brought about by electric and magnetic fields are different, but also suggests that the electronic structures of the hole-doped and electron-doped manganates have basic differences

A study of copper films obtained from the nebulized spray pyrolysis of different precursors

Metallic copper films have been deposited on Si(100) substrates by nebulized spray pyrolysis of Cu(acac)2, Cu(hfac)2 and Cu(dpm)2. The structure and morphology of the films have been examined by X-ray diffraction and scanning electron microscopy. The films are polycrystalline in nature with a preferred (111) orientation. Based on the electrical resistivity data, various transport parameters of the films have been estimated. Growth kinetics and the environmental stability of the films from the precursors have been compared. Taking all the factors into account, Cu(dpm)2 seems to yield the best films of copper metal