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Magnetic properties of bulk SrTi 1− x Co x O 3− δ solid solutions (for x from 0 to 0.05), prepared in air as those partially reduced, were studied by following the influence of the dopant concentration and valence. A strong paramagnetic and/or diamagnetic contribution and an extremely weak ferromagnetic contribution maintained up to room temperature is observed for all the studied compositions, including the undoped samples. While the paramagnetic contribution shows a classical evolution with cobalt concentration and valence, the ferromagnetic part of the magnetization seems to be independent of the doping process. While some of the observations can support the assumption of an intrinsic property of the SrTiO 3 matrix, the hypothesis of a ferromagnetism associated to some contamination with external magnetic impurities cannot be completely discarded and will be retained here.

We have studied the ferromagnetic and antiferromagnetic fluctuations in the charge-ordered Pr0.65Ca0.35MnO3 antiferromagnet by triple-axis neutron spectrometry. Whereas ferromagnetic fluctuations are observed above and below the charge-ordering transition (TCO), the antiferromagnetic fluctuations develop only below TCO .T he dynamical exponent z of both ferromagnetic and antiferromagnetic fluctuations are determined. The ferromagnetic fluctuations are not completely suppressed below TCO and their correlation lengths are short-ranged at all temperatures. The results are discussed with respect to the Zener polaron model recently introduced to describe the charge-ordered state of Pr0.6Ca0.40MnO3.

The double-perovskite oxides Ca2FeMoO6, Sr2FeMoO6 and Ba2FeMoO6 show evidence of ferromagnetic metallic behaviour, with Curie temperatures of 345-426 K and saturation magnetization of 3.5-3.9 µB/formula unit. Ceramics show little anisotropic magnetoresistance, but the low-field isotropic negative magnetoresistance of Sr2FeMoO6 is 7% in 1 T at room temperature and reaches 15% at 77 K. An effect of about 1% is found in 20 mT at room temperature in both ceramics and pressed powders of this compound.

The Pr1-xCaxMnO3 (x≈0.5) manganites display an electronic localization together with a modulated distortion of the structure below a transition temperature TCO. It is commonly attributed to charge and orbital ordering (CO/OO), which are purely electronic processes driven by the cooperative Jahn–Teller effect and the Coulomb interaction. However, an unconstrained structure refinement from a neutron-diffraction experiment below TCO on a Pr0.60Ca0.40MnO3 single crystal, gives us a structure whose distortions invite us to reconsider the microscopic mechanism of the transition. The result strongly suggests that the transition has a magneto-structural origin, associated with the formation of ferromagnetic pairs below TCO. The electronic localization and the magnetic properties of these types of compounds are re-interpreted in this context.

We have studied the magnetism of the half-doped charge ordered manganite YBaMn2O6. A formation of ferromagnetic plaquettes of four Mn atoms in the charge ordered phase below T_{CO} approximately 480 K is inferred from high temperature magnetic susceptibility data and the magnetic structure, as determined by neutron powder diffraction at T=1.5 K. The results indicate that new fourfold Mn paramagnetic units form between T_{N}

We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferromagnetic objects which get stabilized by local double exchange processes. The analysis of short range orbital correlations and the verification of the Grueneisen scaling by hard x-ray, specific heat and thermal expansion data reinforces our conclusions.

We present an inelastic light scattering study of single crystalline ${({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y})}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14$, $y=0$ and $x=1∕8$, $0\ensuremath{\leqslant}y\ensuremath{\leqslant}0.5$). A giant softening up to $20\char21{}30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ of the $\mathrm{Mn}\ensuremath{-}\mathrm{O}$ breathing mode has been observed only for the ferromagnetic insulating (FMI) samples $(0.11\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14)$ upon cooling below the Curie temperature $({T}_{C})$. With increasing Pr-doping the giant softening is strongly suppressed. This is attributed to a coupling of the breathing mode to orbital polarons, which are present in the FMI state. Moreover, the softening that persists to lowest temperature without saturation suggests a continuous, temperature-dependent evolution of orbital polarons below ${T}_{C}$.

The magnetic phase separation in single crystals of the ${\mathrm{P}\mathrm{r}}_{0.67}{\mathrm{C}\mathrm{a}}_{0.33}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ manganites is studied using polarized small angle neutron scattering. The measured spectra give a fractal dimension consistent with a configuration in ferromagnetic filaments of nanometric diameter. We argue here that localized charge carriers hop in a random walk fashion mediating a ferromagnetic "hopping exchange" which coexists with superexchange to create the filamentary phase separation. The arguments for this physical picture are validated by Monte Carlo simulations, where magnetism and transport are treated in a self-consistent manner.

Measuring the magnetization and the specific heat of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals with $0.11l~xl~0.15$ we have investigated the charge order (CO) transition in lightly doped manganites. Though the properties of the ferromagnetic insulating state at low temperatures hardly depend on x, there is a very strong concentration dependence of the specific heat and the magnetization. While the specific-heat anomaly due to ferromagnetic order strongly increases with x, the jumps of both, entropy and magnetization at the CO transition decreases drastically. The analysis of our data shows that the entropy changes at the CO transition are mainly due to spin degrees of freedom, which strongly suggests that the melting of charge order is driven by magnetic energy.

We present magnetization measurements on lightly doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals with $0.06l~xl~0.15$ in external fields up to 14 T at temperatures between 4.2 and 180 K. In contrast to the ferromagnetic behavior which is found for $xg~0.11,$ the samples with $x=0.06$ and $x=0.09$ exhibit a pronounced hysteresis and a constant high-field susceptibility. By analyzing this particular behavior we can rule out a phase separation into ferromagnetic and antiferromagnetic regions. On the other hand, a model of a canted antiferromagnet can account for the main experimental features.

The magnetic properties of (La1 − x Sr x )MnO3 crystals have been studied. The ac susceptibility χ = χ′ + iχ″ has been measured at frequencies ranging from 10 Hz to 1 000 Hz. The x = 0.25 crystal shows a typical ferromagnetic behaviour with a dimensionless SI susceptibility of 6 below T c = 350 K limited by the demagnetizing field. Both x = 0 and x = 0.06 crystals show very sharp peaks in χ′ and χ″ at the Neel points, 139 K and 135 K respectively. This is quite unlike the usual behaviour of an antiferromagnet. Magnetization measurements on an untwinned single crystal of LaMnO3 show a weak moment of 0.18 μ B at 4.2 K which is directed along the c axis. The origin of this canted antiferromagnetic structure is discussed in terms of anisotropy and of Dzyaloshinskii-Moriya interaction. It is concluded that anistropy is insufficient to explain the canted antiferromagnetism.