Collapse of the charge ordering gap of Nd_{0.5}Sr_{0.5}MnO_{3} in an applied magnetic field

We report results of tunneling studies on the charge ordering compound Nd_{0.5}Sr_{0.5}MnO_{3} in a magnetic field up to 6T and for temperature down to 25K.We show that a gap (2\Delta_{CO} \approx 0.5eV opens up in the density of state (DOS) at the Fermilevel (E_F) on charge ordering (T_{CO}=150K) which collapses in an applied magnetic field when the charge ordered state melts. There is a clear correspondence between the behavior of the resistivity and the gap formation and its collapse in an applied magnetic field. We conclude that a gap in the DOS at E_F is necessary for the stability of the charge ordered state.Comment: 4 pages REVTeX, 5 postscript figures included, submitted to Phys. Rev. Let

Giant magnetoresistance, charge-ordering and related aspects of manganese oxides

Giant magnetoresistance and related properties such as charge-ordering in manganates have been investigated with great vigor in the last two years. The manganates have turned out to be truly exciting systems, exhibiting a wide variety of electronic and magnetic properties

An Infrared Spectroscopic Study of the Insulator-Metal Transition and Charge-Ordering in Rare Earth Manganates, Ln1-xAxMnO3 (Ln=Rare Earth, A=Ca, Sr, Pb)

Infrared absorption spectra of rare earth manganates of the type La(0.7)A(0.3)MnO(3) (A = Pb, Sr, Ca) show that the compositions which are metallic at room temperature do not exhibit phonon bands, while the insulating compositions show sharp bands. La0.7Ca0.3MnO3 shows some changes in the phonon frequencies across the insulator-metal transition, but more importantly, the temperature variation of the relative band intensities parallels that of the electric resistivity. It appears that infrared absorption in these materials occurs when the resistivity is higher than Mott's maximum metallic resistivity. Infrared absorption spectra of Nd0.5Ca0.5MnO3, Y0.5Ca0.5MnO3, and Nd0.5Sr0.5MnO3 have been investigated through their charge-ordering transition temperatures. The band frequencies increase across the charge-ordering transition accompanied by a significant increase in the band intensities. The stretching and bending bands of the MnO6 octahedra show splittings in the charge-ordered manganates consistent with the occurrence of octahedral distortion. Some systematics in band frequencies are found with the average radius of A-site cations in Ln(0.5)A(0.5)MnO(3)

An infrared spectroscopic study of the insulator–metal transition and charge-ordering in rare earth manganates, Ln<SUB>1&#8722;x</SUB>A<SUB>x</SUB>MnO<SUB>3</SUB> (Ln=Rare Earth, A=Ca, Sr, Pb)

Infrared absorption spectra of rare earth manganates of the type Ln<SUB>0.7</SUB>A<SUB>0.3</SUB>MnO<SUB>3</SUB> (A=Pb, Sr, Ca) show that the compositions which are metallic at room temperature do not exhibit phonon bands, while the insulating compositions show sharp bands. La<SUB>0.7</SUB>Ca<SUB>0.3</SUB>MnO<SUB>3</SUB> shows some changes in the phonon frequencies across the insulator–metal transition, but more importantly, the temperature variation of the relative band intensities parallels that of the electric resistivity. It appears that infrared absorption in these materials occurs when the resistivity is higher than Mott's maximum metallic resistivity. Infrared absorption spectra of 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 investigated through their charge-ordering transition temperatures. The band frequencies increase across the charge-ordering transition accompanied by a significant increase in the band intensities. The stretching and bending bands of the MnO<SUB>6</SUB> octahedra show splittings in the charge-ordered manganates consistent with the occurrence of octahedral distortion. Some systematics in band frequencies are found with the average radius of A-site cations in Ln<SUB>0.5</SUB>A<SUB>0.5</SUB>MnO<SUB>3</SUB>

Delamination of Surfactant-Intercalated Brucite-Like Hydroxy Salts of Cobalt and Copper and Solvothermal Decomposition of the Resultant Colloidal Dispersions

Surfactant anion intercalated hydroxy salts of copper and cobalt of the formula M(OH)(2-x)(surf)(x)center dot mH(2)O [M = Cu, Co; surf = dodecyl sulfate. dodecyl benzene sulfonate. and x = 0.5 for Cu and 0.67 for Co] delaminate readily in 1-butanol to give translucent colloidal dispersions that are stable for months. The extent of delamination and the colloidal dispersion observed in these solids is higher than what had been observed for layered double hydroxides. The dispersions yield the corresponding nanoparticulate oxides on solvothermal decomposition. While the copper hydroxy salt forms similar to 300 nm dendrimer-like CuO nanostructures comprising nanorods of similar to 10 nm diameter, the cobalt analogue forms similar to 20 nm superparamagnetic particles of Co3O4

Charge-ordering in manganates

Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln(1-x)A(x)MnO(3) (Ln rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulating properties. This phenomenon is to be contrasted with the ferromagnetic metallic behavior that occurs when double-exchange between the Mn3+ and Mn4+ ions predominates. Two distinct types of charge-ordering can be delineated. In one, a ferromagnetic metallic (FMM) state transforms to the charge-ordered (CO) state on cooling. In the other scenario, the CO state is found in the paramagnetic ground stale and there is no ferromagnetism down to the lowest temperatures. Magnetic fields transform the CO state to the FMM state, when the average radius of the A-site cations is sufficiently large ([r(A)] > 1.17 Angstrom). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also found only when [r(A)] greater than or similar to 1.17 Angstrom. The effect of the size of the A-cations on the Mn-O-Mn angle is not enough to explain the observed variations of the charge-ordering temperature as well as the ferromagnetic Curie temperature T-c. An explanation based on a competition between the Mn and A-cation orbitals for sigma-bonding with the oxygen rho(sigma) orbitals is considered to account for the large changes in T-c and hence the true bandwidth, with [r(A]). Effects of radiation, electric field, and other factors on the CO state are discussed along with charge-ordering in other manganate systems. Complex phase transitions, accompanied by changes in electronic and magnetic properties, occur in manganates with critical values of(rA) Or bandwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-delta

Magnetoresistance in the Double Perovskite Sr2Sr_2CrMoO6CrMoO_6

The double perovskite $Sr_2$$CrMoO_6$ with $Cr^{3+}$$-O-Mo^{5+}$ pairs is ferrimagnetic $(T_N = 450 K)$, but there is considerable site disorder. The disorder appears to be responsible for the low value of magnetization. At 5 or 6 T, the negative magnetoresistance (MR) is <5% at all temperatures, unlike $Sr_2$$FeMoO_6$ which shows MR upto 27%. Tunneling magnetoresistance behavior of $Sr_2$$CrMoO_6$ is comparable to that of $Sr_2$$FeMoO_6$ , but the magnitude is considerably smaller in the former