[en] Temperature dependent conduction electron spin resonance (CESR) was performed in the normal and vortex-state of Mg_1-xB₂. Fine powder of polycrystalline samples with x congruent with 0 and 0.15 were studied at 9.48 GHz (H_r≅3380 Oe). Our results suggest the interesting effect associated with the presence of Mg deficiency in this compound: a phase separated Mg-vacancy rich (insulating) and Mg-vacancy poor (superconducting metal) regions for the x≅0.15 sample. The depinning temperature, T_p, for x congruent with 0.15 sample was obtained and confronted with previous results

[en] The low-temperature magnetization of high-quality samples of Mg_1-xB₂ (x<∼0.15) reveals a reentrant behavior as the deficiency concentration x is increased. This uncommon result is related to the percolative nature of superconductivity in this system, which presents a superconducting phase, poor in Mg-deficiency, coexisting with an insulating phase, vacancy-rich. This contribution reports on the magnetic response of a number of samples of this system. Results are consistent with the idea that Mg-vacancies cluster to form insulating regions. Particularly enhanced in Mg_0.975B₂, the effect on the magnetic response resembles that of a columnar-like structure that percolates throughout the length of the sample, a feature that could hardly be detected by direct observation techniques

[en] The Bi_1-xCa_xMnO₃ system is known to exhibit charge ordering for a much broader range of x than the La_1-xCa_xMnO₃ system. However, the properties of Bi_1-xCa_xMnO₃ over the entire doping range are not well understood. We have performed magnetization and resistivity measurements as well as x-ray absorption and x-ray diffraction measurements on Bi_1-xCa_xMnO₃ to correlate structural, magnetic, and transport properties. The system is insulating and antiferromagnetic for the entire range of x studied (x≥0.4) except near x∼0.9, where we find a canted spin arrangement with approximately one Bohr magneton per Mn site. Detailed magnetization measurements were performed as a function of field and temperature to explore the net moment on the Mn sites as a function of x and reveal the charge ordering and Neel temperatures. X-ray absorption measurements reveal significant structural distortions of the Mn-O bond distributions with increasing Bi content that correlates directly with increasing charge-ordering temperatures. Moreover, the x-ray diffraction data reveal peak splittings consistent with lower-symmetry cells as Bi content increases. These structural-magnetic correlations point to the importance of Mn-O distortions in stabilizing the charge-ordered state in the manganites

[en] We report x-ray scattering studies of short-range structural correlations and diffuse scattering in Nd_0.5Sr_0.5MnO₃. On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states of this compound. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of Nd_0.5Sr_0.5MnO₃

[en] Complementary x-ray emission (XES) and x-ray-absorption (XAS) measurements of La_1-xCa_xMnO₃ as a function of temperature have been performed. For 0.5≤x≤0.8 we find changes in the XAS and XES spectra consistent with a small increase in the average Mn valence accompanying the passage to the low-temperature state. It is suggested that this small Mn-configuration shift could be due to the competition between the energies associated with ferromagnetic (FM) and charge/orbital (CO) correlations. In the region 0.0≤x≤0.4 we observe thermal spectral changes quite different from those found in the x≥0.5 materials. Modest thermal changes in the Mn XAS pre-edge (K-edge), for 0.0≤x≤0.4, were observed to be coupled to the robustness of the FM-metallic ground state. The clearest pre-edge feature variation appears to be related to Mn-e_g-majority-spin state changes. In contrast the thermal XES spectral changes appear similar in character in the antiferromagnetic (AF)-insulating x=0.0 and FM-metallic x=0.3 materials. An enhancement of the effective local moment via a coupling to the internal exchange field, in the magnetically ordered states, is proposed to explain these XES results. The x=1.0 end member (CaMnO₃) was found to exhibit significant temperature dependence of the absorption and emission spectra. These XAS and XES results, for CaMnO₃, are discussed in terms of thermal perturbations on the degree of covalency of this material

[en] We report an inelastic light scattering study of the effects of charge ordering on the spin, charge, and lattice dynamics of Bi_1-xCa _xMnO₃ (x>0.5) . We find that charge ordering results in anomalous phonon behavior, such as the appearance of ''activated'' modes. More significantly, however, the transition to the charge-ordered phase results in the appearance of a quasielastic scattering response with the symmetry of the spin-chirality operator (T_1g ); this scattering response is thus indicative of magnetic or chiral spin fluctuations in the antiferromagnetic charge-ordered phase

[en] Comparison studies of the perovskite compounds system R_1-xCa_xMnO₃ (R=La, Bi) by both Mn K_β-emission spectroscopy and x-ray-absorption near-edge spectroscopy (XANES) are presented. The insensitivity of x-ray-emission measurements to structural distortions coupled with the sensitivity of x-ray-absorption near-edge measurements to changes in both structure and valence enable one to detect the presence of structural distortions, such as local Jahn-Teller (JT) distortions. Theoretical XANES computations for pure cubic perovskite and locally distorted endmembers are used to show the effect of distortions on XANES spectra as well as to comment on the nature of the pre-edge features in the spectra. We show by explicit computations that the near-edge spectra are determined by dipole transitions while pure 1s to 3d electric quadrupole transitions determine a very limited section of the pre-edge region. Simulations of the pre-edge features reveal a direct connection between local distortions and the a1 feature amplitude. The Bi-containing system is found to have significantly higher levels of distortions than the La system. XANES studies of the AMnO₃ (A=La, Pr, and Nd) system reveal a direct relationship between the main line width and the magnitude of the JT distortions

[en] Colossal magnetoresistance manganites are archetypes in which to study the strong coupling between spin, charge and lattice degrees of freedom in materials. We present muon spin-lattice relaxation data in ferromagnetic (FM) ground state materials from the manganite series La_1-xCa_xMnO₃ and La_1-x-yPr_yCa_xMnO₃. These measurements reveal several characteristic relaxation modes arising from the strong spin-charge-lattice interactions. We also present results from neutron-spin-echo spectroscopy, which directly measures the spin-spin correlation function in a time domain comparable to μSR. A qualitative model for the FM transition in the manganites involving microscopic phase separation is suggested by these data

[en] We have made detailed studies of pyrochlore compounds, R₂Mo₂O₇ with R=Nd, Sm, Gd, and Dy. For R=Nd and Sm, we found that apart from the ferromagnetic transition of Mo moments there is another transition of a reentrant spin glass type at lower temperatures, where both the magnetization and heat capacity data show anomalous features. We also observed that the AC susceptibility data are clearly frequency-dependent even in the ferromagnetic phase. In our pressure-dependent magnetization studies, the magnetization data of (Gd_1-xDy_x)₂Mo₂O₇ for x=0, 0.1, 0.2, and 0.4 are found to decrease continuously with pressures and, simultaneously, the ferromagnetic transition temperatures move towards lower temperatures. Eventually the ferromagnetic state of (Gd,Dy)₂Mo₂O₇ becomes unstable at higher pressures, and gives way to a spin glass phase that is found to exist at lower temperatures

[en] We report a low-temperature specific heat study of Pr_0.5(Sr_1-yCa_y)_0.5MnO₃ (0< y<1) as it changes from charge-ordered insulator (y=1) to AFM 2D metal (y=0). A two-phase coexistence for the intermediate compositions is found. The effect of magnetic field on the specific heat of Pr_0.5Sr_0.5MnO₃ is also presented and discussed