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Register a new userOn the question of metallicity and ferromagnetism in charge ordered insulating Nd0.5Sr0.5MnO3 nanoparticles
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The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We, revisit this issue and discuss the overwhelming evidence from experiments on nano and bulk manganites as well as the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological understanding on the basis of enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanoparticles of Nd0.5Sr0.5MnO3. We also provide supportive evidence from an ab-initio electron structure calculation and a recent numerical simulation and argue that the mechanism is universal in all nanosize charge ordered manganites.
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We have investigated the magnetic state of Nd0.4Sr0.6MnO3 with the variation of its grain size down to average nanometric diameter (40 nm). The bulk sample is antiferromagnetic (AFM) in nature. However, on reduction of the grain size, emergence of ferromagnetic behavior is experimentally observed. Linear and nonlinear ac magnetic susceptibility measurements reveal that ferromagnetic nature is enhanced as the grain size is reduced. Large coercivity and bifurcation between zero field cooled and field cooled magnetization curves indicate an intrinsic disorder and large anisotropy in the system. Observed behaviors are attributed to surface disorder as well as to possible pressure effect on nano grains.
The double perovskite Y_2NiMnO_6 displays ferromagnetic transition at Tc = 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (M_s) and also, validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P 21/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of frequency dependent dielectric constant and equivalent circuit analysis of impedance data takes into account the bulk contribution to total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T_c). Pyrocurrent measurements register a current flow with onset near Tc and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to re-orientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material
We investigated the electronic states of the quasi-one-dimensional organic conductors $delta_{P}$-(BPDT-TTF)$_2$ICl$_2$ and $delta_{C}$-(BPDT-TTF)$_2$ICl$_2$, both of which are insulating at room temperature owing to strong electron correlations. Through measurements of electrical resistivity, optical conductivity, and magnetic susceptibility, as well as band-structure calculations, we have revealed that the two materials possess completely different ground states, even though they have the same chemical composition and stacking configuration of the donor molecules. We have found that the $delta_P$-type salt with an effective half-filled band behaves as a dimer-Mott (DM) insulator and exhibits a nonmagnetic transition at 25 K, whereas the $delta_C$-type salt with a 3/4-filled band shows a charge ordering (CO) transition just above room temperature and becomes nonmagnetic below 20 K. The optical spectra of the $delta_P$-type salt are composed of two characteristic bands due to intra- and interdimer charge transfers, supporting the DM insulating behavior arising from the strong on-site Coulomb interaction. By contrast, in the $delta_C$-type salt, a single band characterizing the formation of CO arising from the off-site Coulomb interactions is observed. Upon lowering the temperature, the shape of the optical spectra in the $delta_C$-type salt becomes asymmetric and shifts to much lower frequencies, suggesting the emergence of domain-wall excitations with fractional charges expected in a one-dimensional CO chain. The temperature dependence of the magnetic susceptibility of the $delta_P$-type salt is well described by a 2D spin-1/2 Heisenberg AFM model on an anisotropic square lattice in the dimerized picture, while in the $delta_C$-type salt, it can be explained by a 2D spin-1/2 Heisenberg AFM model on an anisotropic honeycomb lattice formed in the CO state.
We describe neutron scattering experiments performed to investigate the magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order at temperatures below ~ 800 K. Magnetic correlations are observed at temperatures below ~ 60 K but static magnetic order only becomes established at 31 K, a temperature at which a kink is observed in the susceptibility. On warming above 31 K we observed a change in the magnetic correlations which we attribute either to a spin canting or to a change in the proportion of inequivalent magnetic domains. The magnetic excitation spectrum is dominated by an intense band extending above a gap of approximately 3 meV up to a maximum energy of 16 meV. A weaker band exists in the energy range 20-30 meV. We show that the excitation spectrum is in excellent quantitative agreement with the predictions of a spin-wave theory generalized to include the full magnetic degrees of freedom of high-spin Co2+ ions in an axially distorted crystal field, coupled by Heisenberg exchange interactions. The magnetic order is found to be stabilized by dominant antiferromagnetic Co2+ -- Co2+ interactions acting in a straight line through Co3+. No evidence is found for magnetic scattering from the Co3+ ions, supporting the view that Co3+ is in the S = 0 state in this material.
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