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We present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy $c$-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10 below the ferromagnetic transition $T_{C}$ as a double step in the magnetization at two critical fields $H_{c1}$ and $H_{c2}$. We map the $H-T$ phase diagram with special focus on the temperature range 50\,K $\le T \le T_{C}$. We find that the critical field $H_{c1}(T)$ connects the field and temperature axes of the phase diagram, whereas the $H_{c2}$ boundary starts at 2.8\,T for the lowest temperatures and ends in a critical endpoint at (1\,T; 80\,K). We conclude from the temperature dependence of the ratio $\frac{Hc1}{Hc2}(T)$ that the double metamagnetic transition is an intrinisc effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

From a systematic study of the electrical resistivity $\rho(T,H)$, magnetic susceptibility $\chi(T,H)$, isothermal magnetization $M(H)$ and the specific heat $C(T,H)$, a temperature-magnetic field ($T$-$H$) phase diagram has been established for GdCo$_{1-x}$Fe$_x$AsO ($x = 0$ and $0.05$) polycrystalline compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic (FM) transition of Co $3d$ electrons ($T_\textup{C}^\textup{Co}$) and antiferromagnetic (AFM) transition of Gd $4f$ electrons ($T_\textup{N}^\textup{Gd}$). For the Fe-doped sample ($x=0.05$), an extra magnetic reorientation transition takes place below $T_\textup{N}^\textup{Gd}$, which is likely associated with Co moments. The two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM) behavior in the magnetic susceptibility. Upon decreasing the temperature ($T < T_\textup{C}^\textup{Co}$), the magnetocrystalline anisotropy breaks up the FM order of Co by aligning the moments with the local easy axes of the various grains, leading to a spin reorientation transition at $T_\textup{R}^\textup{Co}$. By applying a magnetic field, $T_\textup{R}^\textup{Co}$ monotonically decreases to lower temperatures, while the $T_\textup{N}^\textup{Gd}$ is relatively robust against the external field. On the other hand, the applied magnetic field pulls the magnetization of grains from the local easy direction to the field direction via a first-order reorientation transition, with the transition field ($H_\textup{M}$) increasing upon cooling the temperature., Comment: accepted by physical Review B 6 figures and 7 pages

We synthesized a series of GdFe$_{1-x}$Co$_x$AsO polycrystalline samples ($0 \leq x \leq 1$) by using a solid state reaction method and present a systematic study on the physical properties by means of electrical resistivity $��(T)$, magnetic susceptibility $��(T)$ and specific heat $C(T)$. The parent compound GdFeAsO undergoes a spin-density-wave (SDW) transition associated with Fe 3d-electrons around 130 K, followed by an antiferromagnetic (AFM) transition of Gd at $T^\textup{Gd}_\textup{N} \approx$ 4 K. The SDW transition is quickly suppressed by Fe/Co substitution and superconductivity appears in a narrow doping range of $0.05 < x < 0.25$, showing a maximum $T_\textup{sc}$ $\approx$ 20 K around $x = 0.1$. On the other hand, the 4f-electrons of Gd are antiferromagnetically ordered over the entire doping concentration ($0 \leq x \leq 1$), while the Co 3d-electrons exhibit a ferromagnetic (FM) transition above $x \approx 0.8$, with the Curie temperature ($T^\textup{Co}_\textup{C}$) reaching 75 K in GdCoAsO. These two magnetic species (Gd and Co) are coupled antiferromagnetically to give rise to ferrimagnetic behavior in magnetic susceptibility on the Co-rich side. For $0.7 \leq x < 1.0$, the system undergoes a possible magnetic reorientation below $T^\textup{Gd}_\textup{N}$., 8 pages, 8 figures

We study the interplay of 3d and 4f electrons in the iron pnictides CeFe$_{1-x}$Co$_x$AsO and GdFe$_{1-y}$Co$_y$AsO, which correspond to two very different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe 3d electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: $0.05 < x < 0.2$ for CeFe$_{1-x}$Co$_x$AsO and $0.05 < y < 0.25$ for GdFe$_{1-y}$Co$_y$AsO, showing a maximum transition temperature $T_\textup{sc}$ of about 13.5 K for Ce and 19 K for Gd. In both compounds, the $4f$-electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co 3d electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches $T_\textup{C}^\textup{Co} \approx$ 75 K at $x = 1$ and $y = 1$. In the Ce-compounds, the N��el temperature $T_\textup{N}^\textup{Ce}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x \simeq 0.8$ ($T_\textup{N}^\textup{Ce}\approx$ 4 K). Furthermore, evidence of Co-induced polarization on Ce-moments is observed on the Co-rich side. In the Gd-compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For $0.7 \leq y < 1.0$, the system undergoes a possible magnetic reorientation below the N��el temperature of Gd ($T_\textup{N}^\textup{Gd}$). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the 3d-4f electrons give rise to a rich phase diagram in the rare-earth iron pnictides., 13 pages, 15 figures

We measured the magnetic penetration depth $\lambda(T)$ in single crystals of LaOs$_{4}$Sb$_{12}$ ($T_c$=0.74 K) down to 85 mK using a tunnel diode oscillator technique. The observed low-temperature exponential dependence indicates a s-wave gap. Fitting the low temperature data to BCS s-wave expression gives the zero temperature gap value $\Delta (0)= (1.34 \pm 0.07) k_B T_c$ which is significantly smaller than the BCS value of 1.76$k_B T_c$. In addition, the normalized superfluid density $\rho(T)$ shows an unusually long suppression near $T_c$, and are best fit by a two-band s-wave model., Comment: 5 pages, 2 figures

We report the effect of nonmagnetic Ti4+ impurities on the electronic and magnetic properties of Sr3Ru2O7. Small amounts of Ti suppress the characteristic peak in magnetic susceptibility near 16 K and result in a sharp upturn in specific heat. The metamagnetic quantum phase transition and related anomalous features are quickly smeared out by small amounts of Ti. These results provide strong evidence for the existence of competing magnetic fluctuations in the ground state of Sr3Ru2O7. Ti doping suppresses the low temperature antiferromagnetic interactions that arise from Fermi surface nesting, leaving the system in a state dominated by ferromagnetic fluctuations., Comment: 5 pages, 4 figures, 1 table

Anomalous paramagnetic effects in dc magnetization were observed in the mixed state of LuNi2B2C, unlike any reported previously. It appears as a kink-like feature for H > 30 kOe and becomes more prominent with increasing field. A specific heat jump at the corresponding temperature suggests that the anomaly is due to a true bulk transition. A magnetic flux transition from a square to an hexagonal lattice is consistent with the anomaly., 5 pages, 4 figures

We report the coexistence of lamellar charge-ordered (CO) and charge-disordered (CD) domains, and their dynamical behavior, in La0.23Ca0.77MnO3. Using high resolution transmission electron microscopy (TEM), we show that below Tcd~170K a CD-monoclinic phase forms within the established CO-orthorhombic matrix. The CD phase has a sheet-like morphology, perpendicular to the q vector of the CO superlattice (a axis of the Pnma structure). For temperatures between 64K and 130K, both the TEM and resistivity experiments show a dynamic competition between the two phases: at constant T, the CD phase slowly advances over the CO one. This slow dynamics appears to be linked to the magnetic transitions occurring in this compound, suggesting important magnetoelastic effects., Comment: 4 pages, 4 figures

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of PrRu$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density $\rho_{s}$ exhibit an exponential behavior for $T$ $

A study of the dependence of the heat capacity Cp(alpha) on field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, Cp(alpha) exhibits a fourfold variation as the field H < Hc2 is rotated in the [001] plane, with minima along (alpha = 0). A slightly disordered sample, however, develops anomalous secondary minima along for H > 1 T, leading to an 8-fold pattern. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and non-local effects., 5 pages, 4 figures

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of PrOs$_{4}$Sb$_{12}$ down to $\sim$0.1 K using a tunnel-diode based, self-inductive technique at 21 MHz, with the ac field applied along the \textit{a}, \textit{b} and \textit{c} directions. In all three field orientations the penetration depth and superfluid density $\rho_{s}$ tend to follow a power law, $\lambda \sim \mathit{T}^{2}, \rho_{s} \sim \mathit{T}% ^{2}$ at low temperatures. $\rho_{s}$ for various gap functions have been calculated, and data is best fit by the $^{3}$He A-phase-like gap with multidomains, each having two point nodes along a cube axis, and parameter $\Delta_{0}$(0)/\textit{k}$_{B}$\textit{T}$_{c}$ = 2.6. This suggests that PrOs$_{4}$Sb$_{12}$ is a strong-coupling superconductor with two point nodes on the Fermi surface. We also confirm the observation of the double transitions at 1.75 K and 1.85 K seen in other measurements.

The complex magnetic transitions and superconductivity of HoNi2B2C were studied via the dependence of the heat capacity on temperature and in-plane field angle. We provide an extended, comprehensive magnetic phase diagram for B // [100] and B // [110] based on the thermodynamic measurements. Three magnetic transitions and the superconducting transition were clearly observed. The 5.2 K transition (T_{N}) shows a hysteresis with temperature, indicating the first order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the onset of the long-range ordering, displays a dramatic in-plane anisotropy: T_{M} increases with increasing magnetic field for B // [100] while it decreases with increasing field for B // [110]. The anomalous anisotropy in T_{M} indicates that the transition is related to the a-axis spiral structure. The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition, i.e., a small in-plane anisotropy and scaling with Ising model. This last transition is ascribed to the change from a^{*} dominant phase to c^{*} dominant phase., 9 pages, 11 figures

Field-angle dependent heat capacity of the non-magnetic borocarbide superconductor YNi2B2C reveals a clear fourfold oscillation, the first observation of its kind. The observed angular variations were analyzed as a function of magnetic field angle, field intensity and temperature to provide its origin. The quantitative agreement between experiment and theory strongly suggests that we are directly observing nodal quasiparticles generated along by the Doppler effect. The results demonstrate that field-angle heat capacity can be a powerful tool in probing the momentum-space gap structure in unconventional superconductors such as high Tc cuprates, heavy fermion superconductors, etc.

As in such 2D systems as the XY model, topological (or singularity point) defects are thought to play a crucial role in the phase transitions of 3D spin systems. In double exchange (DE) ferromagnets, the conduction electrons are strongly coupled with core spins through Hund's rule, and in the presence of non-trivial spin texture, acquire a Berry phase contribution to the anomalous Hall effect (AHE). We combine Hall and magnetization data on CrO$_2$ with a thermodynamical scaling hypothesis to confirm that the critical behavior of the topological-spin-defect density is consistent with that of the heat capacity. This analysis is the first experimental confirmation of the topological character of critical fluctuations., Comment: 4 Figures

We revisit the transport properties of La-Ca manganites in detail, since they have been confirmed in a number of CMR-exhibiting materials. We also discuss new data considered the hallmark of an unusually high electron-phonon coupling responsible for charge localization and small polaron transport., 23 pages, 7 comb. figures, 98 references. To be published in the proceedings of the Workshop on Physics of Manganites, MSU, July 1998

A microscopic picture of charge transport in manganites is developed, with particular attention being paid to the neighborhood of the ferromagnet-to- paramagnet phase transition. The basic transport mechanism invoked is inelastically-assisted carrier hopping between states localized by magnetic disorder. In the context of the anomalous Hall effect, central roles are played by the Pancharatnam and spin-orbit quantal phases., Comment: 4 pages, 1 figure; Companion Letter cond-mat/9904332

^{139}La NMR measurements of La_{2/3}Ca_{1/3}MnO_3 performed in the paramagnetic state and high magnetic fields are reported. Analysis of the high temperature spectrum measured establishes that the spectrum it is a standard powder pattern broadened by a variation in lattice distortions. At lower temperatures, the spectrum shifts and broadens. Both the shift and broadening exhibit Curie-Weiss behavior, indicating that the shift measures the polarization of the electron spin system, and the broadening reflects a distribution of magnetic susceptibilities. This distribution may result from variations of local susceptibility in the bulk of the sample or from differences in demagnetizing factors among powder grains. The spectrum also indicates that the lattice distortions do not change as the temperature lowers. Spectral diffusion measurements suggest that the temperature dependence of the spectrum shape does not result from freezing out of motion of magnetic polarons. Variation of the nuclear spin-lattice relaxation across the spectrum indicate that magnetic fluctuations, not lattice vibrations, dominate nuclear relaxation. Nuclear spin-lattice relaxation therefore measures electron spin dynamics in this system. The magnetic field dependence of the spin-lattice relaxation indicates that the electron spin-spin correlation function adopts simple single exponential behavior with a slow field-independent correlation time of 10^{-8} seconds near Tc. The spin-spin correlation function changes form at higher temperatures, but remains slow. The spin-lattice relaxation therefore indicates the presence of extremely slow dynamics above Tc., Comment: 10 pages, 11 figures, to be published in Physical Review B

We explore the possibility that polaronic distortions in the paramagnetic phase of La$_{0.67}$Ca$_{0.33}$MnO$_3$ manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting field- and temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the two-component system, and find that a limit with low percolation threshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility., Comment: RevTex, 11 pages, 4 figures. Revised version submitted to PRB, 01/99

The resistivity in the ferromagnetic state of flux-grown La_{2/3}(Pb,Ca)_{1/3}MnO_3 single crystals, measured in magnetic fields up to 7 T, reveals a strong quadratic temperature dependence at and above 50 K. At lower temperatures, this contribution drops precipitously leaving the resistivity essentially temperature independent below 20 K. The Seebeck coefficient also reflects a change of regime at the same temperature. We attribute this behavior to a cut-off of single magnon scattering processes at long wavelengths due to the polarized bands of a double-exchange ferromagnet., 10 pages, TeX, 4 figures. Revised version. Submitted

The Hall resistivity rho_{xy} of a La_{2/3}(Ca,Pb)_{1/3}MnO_3 single crystal has been measured as a function of temperature and field. The overall behavior is similar to that observed previously in thin-films. At 5 K, rho_{xy} is positive and linear in field, indicating that the anomalous contribution $R_S$ is negligible. However, the effective carrier density in a free electron model is n_{eff}=2.4 holes/Mn, even larger than the 0.85-1.9 holes/Mn reported for thin-films and far larger than the 0.33 holes/Mn expected from the doping level. As temperature increases, a strong, negative contribution to rho_{xy} appears, that we ascribe to R_S. Using detailed magnetization data, we separate the ordinary (\propto B) and anomalous (\propto M) contributions. Below T_C, R_S \propto rho_{xx}, indicating that magnetic skew scattering is the dominant mechanism in the metallic ferromagnetic regime. At and above the resistivity-peak temperature, we find that rho_{xy}/rho_{xx}M is a constant, independent of temperature and field. This implies that the anomalous Hall coefficient is proportional to the magnetoresistance. A different explanation based on two fluid model is also presented., Comment: revtex, 11 pages, 4 figures

When heated through the magnetic transition at Tc, La0.7Ca0.3MnO3 changes from a band metal to a polaronic insulator. The Hall constant R_H, through its activated behavior and sign anomaly, provides key evidence for polaronic behavior. We use R_H and the Hall mobility to demonstrate the breakdown of the polaron phase. Above 1.4Tc, the polaron picture holds in detail, while below, the activation energies of both R_H and the mobility deviate strongly from their polaronic values. These changes reflect the presence of metallic, ferromagnetic fluctuations, in the volume of which the Hall effect develops additional contributions tied to quantal phases., 11 pages, 3 figures, final version to appear in Phys. Rev. B Rapid