Search

Spin Seebeck effect has been investigated in Pt/$\varepsilon$-Fe$_2$O$_3$ bilayers. The $\varepsilon$-Fe$_2$O$_3$ thin layer with $40-70$~nm thickness were deposited by a spin-coating method on Y:ZrO$_2$(100) substrates. The prepared layers are highly oriented with the easy magnetic $a$-axis parallel to the film surface. The magnetic hysteresis loops measured at room temperature with magnetic field parallel to the layer exhibit coercive fields up to 11.6~kOe, which is so far the highest value measured for $\varepsilon$-Fe$_2$O$_3$ thin layer samples. The shape of the spin Seebeck hysteresis loops is similar to the shape of magnetization for single phase layers with coercive field around 10~kOe. In some prepared layers a small amount of secondary soft ferrimagnetic phase is revealed by a constricted shape of magnetization loops, in contrast to spin Seebeck loops, where no constriction is observed. A difference in encountered in the case of layers with a small amount ($1-2$~volume\%) of secondary soft ferrimagnetic phase, which is revealed by a constricted shape of magnetization loops, in contrast to spin Seebeck loops, where no constriction is observed., Comment: 7 pages, 5 figures

Spin Seebeck effect (SSE) has been investigated in thin films of two Y-hexagonal ferrites Ba$_2$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn2Y) and Ba$_2$Co$_{2}$Fe$_{12}$O$_{22}$ (Co2Y) deposited by a spin-coating method on SrTiO$_3$(111) substrate. The selected hexagonal ferrites are both ferrimagnetic with similar magnetic moments at room temperature and both exhibit easy magnetization plane normal to $c$-axis. Despite that, SSE signal was only observed for Zn2Y, whereas no significant SSE signal was detected for Co2Y. We tentatively explain this different behavior by a presence of two different magnetic ions in Co2Y, whose random distribution over octahedral sites interferes the long range ordering and enhances the Gilbert damping constant. The temperature dependence of SSE for Zn2Y was measured and analyzed with regard to the heat flux and temperature gradient relevant to the SSE signal., Comment: 7 pages, 9 figures

Two samples of the [Ca$_2$CoO$_{3-t}$]$_{0.62}$(CoO$_2$) misfit cobaltate, often denoted as the Ca$_{3}$Co$_{3.93}$O$_{9}$ phase, were prepared from the same ceramic material by the oxygen and argon annealing, resulting in different carrier concentrations in the conducting CoO$_{2}$ layers, n=0.31 and 0.19 hole/Co, respectively. Electrical and thermal transport properties were studied in dependence of magnetic field up to 140 kOe. The magnetothermopower data reveal an extra spin-entropy contribution to Seebeck coefficient that is not expected for carriers of Fermi liquid character. Its magnitude is unprecedentedly large and makes at zero field up to 50$\%$ of the theoretical limit k$_B$/$e$ ln2$ = 59 \mu VK^{-1}$. This spin-entropy contribution is gradually suppressed with increasing magnetic field, and the saturation is even observed when temperatures are low enough. To understand the results, the thermopower is treated in terms of purely thermodynamic Kelvin formula, and so-called Spin liquid model is evoked, providing a reason for the spin-entropy manifestation in the [Ca$_2$CoO$_{3-t}$]$_{0.62}$(CoO$_2$) misfits., Comment: 26 pages including 11 figures

Infrared reflectivity spectra of cubic SrMnO$_{3}$ ceramics reveal 18 % stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T$_{N}$ = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first principles calculations. Moreover, polar phonons become Raman active below T$_{N}$, although their activation is forbidden by symmetry in $Pm\bar{3}m$ space group. This gives evidence that the cubic $Pm\bar{3}m$ symmetry is locally broken below T$_{N}$ due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small non-stoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers $n_{e}=$3.6 10$^{20}$ cm$^{-3}$ and the sample composition Sr$^{2+}$Mn$_{0.98}^{4+}$Mn$_{0.02}^{3+}$O$_{2.99}^{2-}$. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundaries., Comment: subm. to PRB

The structural and magnetic properties of two mixed-valence cobaltites with formal population of 0.30 Co$^{4+}$ ions per f.u., (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_3$ ($y=0$ and 0.15), have been studied down to very low temperatures by means of the high-resolution neutron diffraction, SQUID magnetometry and heat capacity measurements. The results are interpreted within the scenario of the spin-state crossover from a room-temperature mixture of the intermediate spin Co$^{3+}$ and low spin Co$^{4+}$ (IS/LS) at the to the LS/LS mixture in the sample ground states. In contrast to the yttrium free $y=0$ that retains the metallic-like character and exhibits ferromagnetic ordering below 55 K, the doped system $y=0.15$ undergoes a first-order metal-insulator transition at 132 K, during which not only the crossover to low spin states but also a partial electron transfer from Pr$^{3+}$ 4f to cobalt 3d states take place simultaneously. Taking into account the non-magnetic character of LS Co$^{3+}$, such valence shift electronic transition causes a magnetic dilution, formally to 0.12 LS Co$^{4+}$ or 0.12 $t_{2g}$ hole spins per f.u., which is the reason for an insulating, highly non-uniform magnetic ground state without long-range order. Nevertheless, even in that case there exists a relatively strong molecular field distributed over all the crystal lattice. It is argued that the spontaneous FM order in $y=0$ and the existence of strong FM correlations in $y=0.15$ apparently contradict the single $t_{2g}$ band character of LS/LS phase. The explanation we suggest relies on a model of the defect induced, itinerant hole mediated magnetism, where the defects are identified with the magnetic high-spin Co$^{3+}$ species stabilized near oxygen vacancies.

Crystal and magnetic structures of the $x=0.2$ member of La$_{\rm 0.8-x}$Tb$_{\rm x}$Ca$_{0.2}$CoO$_3$ perovskite series have been determined from the powder neutron diffraction. Enhancement of the diffraction peaks due to ferromagnetic or cluster glass ordering is observed below $T_C=55$ K. The moments evolve at first on Co sites, and ordering of Ising-type Tb$^{3+}$ moments is induced at lower temperatures by a molecular field due to Co ions. The final magnetic configuration is collinear F$_x$ for cobalt subsystem, while it is canted F$_x$C$_y$ for terbium ions. The rare-earth moments align along local Ising axes within \textit{ab}-plane of the orthorhombic $Pbnm$ structure. The behavior in external fields up to $70-90$ kOe has been probed by the magnetization and heat capacity measurements. The dilute terbium ions contribute to significant coercivity and remanence that both steeply increase with decreasing temperature. A remarkable manifestation of the Tb$^{3+}$ Ising character is the observation of a low-temperature region of anomalously large linear term of heat capacity and its field dependence. Similar behaviours are detected also for other terbium dopings $x=0.1$ and 0.3., Comment: 8 pages, 11 figures

Temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at the Pr $L_{3}$- and Tb $L_{3}$-edges was measured for the (Pr$_{1-y}$Tb$_{y})_{0.7}$Ca$_{0.3}$CoO$_{3}$ system, in which a metal-insulator (MI) and spin-state (SS) transition took place simultaneously at a critical temperature $T_{\rm MI}$. A small increase in the valence of the terbium ion was found below $T_{\rm MI}$, besides the enhancement of the praseodymium valence; the trivalent states, which are stable at room temperature, change to a 3+/4+ ionic mixture at low temperatures. In particular for the $y$=0.2 sample, the average valence determined at 8 K amounts to 3.03+ and 3.25+ for the Tb and Pr ion, respectively. In analogous (Pr$_{1-y}$RE$_{y})_{0.7}$Ca$_{0.3}$CoO$_{3}$ samples (RE=Sm and Eu), in which the MI-SS transition also took place, no valence shift of the RE ion was detected in the XANES spectra at the RE ion $L_{3}$-edge. The role of the substituted RE ion for the Pr-site on the MI-SS transition is discussed.

We present an extensive investigation (magnetic, electric and thermal measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and (Pr1-yYy)0.7Ca0.3CoO3 (y=0.0625-0.15) perovskites, in which a peculiar metal-insulator (M-I) transition, accompanied with pronounced structural and magnetic anomalies, occurs at 76 K and 40-132 K, respectively. The inspection of the M-I transition using the XANES data of Pr L3-edge and Co K-edge proofs the presence of Pr4+ ions at low temperatures and indicates simultaneously the intermediate spin to low spin crossover of Co species on lowering the temperature. The study thus definitively confirms the synchronicity of the electron transfer between Pr3+ ions and Co^(3+/4+)O3 subsystem and the transition to the low-spin, less electrically conducting phase. The large extent of the transfer is evidenced by the good quantitative agreement of the determined amount of the Pr4+ species, obtained either from the temperature dependence of the XANES spectra or via integration of the magnetic entropy change over the Pr4+ related Schottky peak in the low-temperature specific heat. These results show that the average valence of Pr3+/Pr4+ ions increases (in concomitance with the decrease of the formal Co valence) below TMI for (Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+ (the doping level of the CoO3 subsystem decreases from 3.30+ to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+ (the decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to 3.46+ (the decrease of doping level from 3.50+ to 3.27+)., Comment: 19 pages, 11 figures

The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed - $T_C=130$ K, 55 K and 25 K and $m = 0.68 \mu_B$, 0.34 $\mu_B$ and 0.23 $\mu_B$ for Nd$_{0.7}$Sr$_{0.3}$CoO$_3$, Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ and Nd$_{0.7}$Ca$_{0.3}$CoO$_3$, respectively. The moment reduction with respect to moment of the conventional ferromagnet La$_{0.7}$Sr$_{0.3}$CoO$_3$ ($T_C=230$ K, $m = 1.71 \mu_B$) in so-called IS/LS state for Co$^{3+}$/Co$^{4+}$, was originally interpreted using phase-separation scenario. Based on the present results, mainly the analysis of Schottky peak originating in Zeeman splitting of the ground state Kramers doublet of Nd$^{3+}$, we find, however, that ferromagnetic phase in Nd$_{0.7}$Ca$_{0.3}$CoO$_3$ and likely also Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ is uniformly distributed over all sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov \textit{et al.} [Phys. Rev. B \textbf{80}, 024423 (2009)]. The ground state of Nd$_{0.7}$Sr$_{0.3}$CoO$_3$ with an intermediate cobalt moment is inhomogeneous due to competing of LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for $4f^3$ (Nd$^{3+}$), $4f^2$ (Pr$^{3+}$) and $4f^1$ (Ce$^{3+}$ or Pr$^{4+}$) are calculated and their magnetic characteristics are presented., Comment: 32 pages, 11 figures

The rhodium doping in the LaCo$_{1-x}$Rh$_{x}$O$_3$ perovskite series ($x=0.02-0.5$) has been studied by X-ray diffraction, electric transport and magnetization measurements, complemented by electronic structure GGA+U calculations in supercell for different concentration regimes. No charge transfer between Co$^{3+}$ and Rh$^{3+}$ is evidenced. The diamagnetic ground state of LaCoO$_3$, based on Co$^{3+}$ in low-spin (LS) state, is disturbed even by a small doping of Rh. The driving force is the elastic energy connected with incorporation of a large Rh$^{3+}$ cation into the matrix of small LS Co$^{3+}$ cations, which is relaxed by formation of large Co$^{3+}$ in high-spin (HS) state in the next-nearest sites to the inserted Rh atom. With increasing temperature, the population of Co$^{3+}$ in HS state increases through thermal excitation, and a saturated phase is obtained close to room temperature, consisting of a nearest-neighbor correlation of small (LS Co$^{3+}$) and large (HS Co$^{3+}$ and LS Rh$^{3+}$) cations in a kind of double perovskite structure. The stabilizing role of elastic and electronic energy contributions is demonstrated in supercell calculations for dilute Rh concentration compared to other dopants with various trivalent ionic radius., Comment: 8 pages, 8 figures

The magnetic, electric and thermal properties of the ($Ln_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_3$ perovskites ($Ln$~=~Pr, Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator transition, which is observed in the praseodymium based samples with $y=0.075$ and 0.15 at $T_{M-I}=64$ and 132~K, respectively. The study suggests that the transition, reported originally in Pr$_{0.5}$Ca$_{0.5}$CoO$_3$, is not due to a mere change of cobalt ions from the intermediate- to the low-spin states, but is associated also with a significant electron transfer between Pr$^{3+}$ and Co$^{3+}$/Co$^{4+}$ sites, so that the praseodymium ions occur below $T_{M-I}$ in a mixed Pr$^{3+}$/Pr$^{4+}$ valence. The presence of Pr$^{4+}$ ions in the insulating phase of the yttrium doped samples (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_3$ is evidenced by Schottky peak originating in Zeeman splitting of the ground state Kramers doublet. The peak is absent in pure Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ in which metallic phase, based solely on non-Kramers Pr$^{3+}$ ions, is retained down to the lowest temperature., Comment: 10 figures

Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively. The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase., Comment: This paper has been withdrawn by the authors. Phys. Rev. B 78, 014432 (2008)

The samples LaCoO3 with dilute substitutions on cobalt sites have been studied using the resistivity, thermopower and magnetic susceptibility measurements over the temperature range up to ~900 K. The Co-site substitution does not affect the magnetic transition at ~100 K and the onset of massive population of hole carriers at ~500 K, characteristic for undoped LaCoO3. On the other hand, the low-temperature transport and magnetism is markedly distinct for samples with extra charge on cobalt ions introduced by the heterovalent dopants (Mg2+, Ti4+) compared to samples with minor non-stoichiometry (LaCoO3, Ga3+ doped sample). Magnetic properties suggest that these extra charges create thermally stable magnetic polarons of total S ~ 2-3. Common features of Co-site doped and La-site doped samples (La1-xSrxCoO3) are discussed, Comment: This paper has been withdrawn by the authors. J. Magn. Magn. Mater. 320, e92 (2008)

The diamagnetic-paramagnetic and insulator-metal transitions in LnCoO3 perovskites (Ln = La, Y, rare earths) are reinterpreted and modeled as a two-level excitation process. In distinction to previous models, the present approach can be characterized as a LS-HS-IS (low-high-intermediate spin) scenario. The first level is the local excitation of HS Co3+ species in the LS ground state. The second excitation is based on the interatomic electron transfer between the LS/HS pairs, leading finally to a stabilization of the metallic phase based on IS Co3+. The model parameters have been quantified for Ln = La, Pr and Nd samples using the powder neutron diffraction on the thermal expansion of Co-O bonds, that is associated with the two successive spin transitions. The same model is applied to interpret the magnetic susceptibility of LaCoO3 and YCoO3., Comment: 52.Conference on Magnetism and Magnetic Materials, November 2007, Tampa

A powder X-ray diffraction study, combined with the magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) Weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions at the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit of magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T = 0 K as 164 K and 2875 K, respectively, fall to zero at T = 230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T = 535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent on the Ln cation and is about 1% in both cases., Comment: This submission has been withdrawn by the authors

A complex structural, magnetic and electric transport investigation shows that the Cr doping on Mn sites in the A-type antiferromagnet Pr0.44Sr0.56MnO3 provokes a non-uniform magnetic state with coexisting FM and AFM regions. Irrespective of the ratio of magnetic phases, the samples exhibit a non-metallic behavior of resistivity and thermopower, pointing to the nanoscopic nature of the phase separation. A particularly large magnetoresistance encountered in a broad range of temperatures for samples with Cr doping of 4 - 6 % supports such idea., Comment: This paper has been withdrawn by the authors. J. Appl. Phys. 93, 8083 (2003)

Measurements of the electrical and thermal conductivities, thermopower and paramagnetic susceptibility have been performed on single crystal samples of Bi1/2Sr1/2MnO3 and complemented with the X-ray powder diffraction data. A pronounced hysteretic behavior, observed below the cubic-to- orthorhombic transition at Tcrit = 535 K, is related to the onset of long-range charge order at TCO = 450 K and its further evolution down to about 330 K. The diffraction data suggest that the charge ordered state is formed by Zener pairs, represented by Mn4+ dimers linked by one extra eg electron, and is possibly stabilized by cooperative Bi,Sr displacements. An extremely low thermal conductivity is observed down to the lowest temperatures, without any recovery at the anti- ferromagnetic ordering temperature TN = 150 K. Such be- havior points to a presence of strong scatterers of phonons. Their possible origin can be linked to "optical-like" oscilla- tions which are associated with fluctuating charges within the Zener pairs., Comment: LATeX file, 4 EPS figures, zipped in 1 file BiSr50.zip

55Mn NMR was used to study the ferromagnetic insulating state in four different manganites. In all cases the coexistence of two types of regions possessing different NMR spectra and nuclear spin dynamics was found. In the ferromagnetic insulating clusters the hopping frequency fhop of the electron holes is slower comparing to the NMR frequency fres and two lines ascribed to 3+ and 4+ valence states of Mn are observed. The relaxation rate increases rapidly with the increasing temperature, so that these spectra can no longer be observed for temperatures higher than approx 60 K. In ferromagnetic metallic clusters fres < fhop and the motionally narrowed spectrum is observed in a broad temperature interval, as the relaxation remains only moderately fast., Comment: 9 pages, 12 figures

Thin films of Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ manganites with colossal magnetoresistance (CMR) properties have been synthesized by the Pulsed Laser Deposition technique on (100)-SrTiO$_{3}$. The lattice parameters of these manganites and correlatively their CMR properties can be controlled by the substrate temperature $T_{S}$. The maximum CMR effect at 75K, calculated as the ratio $\rho (H=0T)/\rho (H=7T)$ is 10$^4$ for a deposition temperature of $T_{S}=680$ degC. Structural studies show that the Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ film is single phase, [010]-oriented and has a pseudocubic symmetry of the perovskite subcell with a=3.77$\AA$ at room temperature. We suggest that correlation between lattice parameters, CMR and substrate temperature $T_{S}$ result mainly from substrate-induced strains which can weaken the charge-ordered state at low temperature., Comment: 9 pages, 4 figures. To be published in Applied Physics Letters