Charge/orbital ordering structure of Pr(1-x)Ca(x)MnO(3)(x = 3/8) examined by low-temperature transmission electron microscopy

The structural phase transition of Pr(1-x)Ca(x)MnO(3)(x = 3/8) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector q(1) = (0,1/2,0) appeared below 230 K, indicating formation of the d(3x(2-r(2))/d(3y(2)-r(2)) type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector q(2) = (1/4,1/4,1/2) appeared, suggesting an additional ordering of excess 1/8 Mn(3+), necessary due to the deviation of x from 1/2, with the occupation of the d(3z(2-r(2)) type of e(g) orbital.     

Structural properties of Pb2MnW(1-x)Re(x)O6 double perovskites

Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.     

Evidence of phase separation in Nd1-xSrxMnO3 （x=0.50, 0.51, 0.52, 0.53, 0.54, 0.55）

The temperature dependence of Mn ion magnetization and the conductivity difference versus temperature provide phase separation evidence in Nd1-xSrxMnO3 (x=0.50, 0.51, 0.52, 0.53, 0.54, 0.55). This is different from other reports and may suggest that a fully charge-ordered state exists in quite a narrow range; the sample preparation procedure affects the charge ordering. The polaron effect also plays an important role in explaining the conductivity.     

Magnetic properties of multiferroics-semiconductors Eu(1-x)Ce(x)Mn2O5

Studies of magnetization, magnetoresistance, and magnetic oscillations in semiconductor-multiferroics Eu(1-x)Ce(x)Mn2O5 (x = 0.2-0.25) (ECMO) at temperatures ranging from 5 to 350 K in magnetic fields up to 6 T are presented. It is shown that phase separation and charge carrier self-organization in the crystals give rise to a layered superstructure perpendicular to the c axis. An effect of magnetic field cycling on the superstructure, magnetization, and magnetoresistance is demonstrated. X-ray diffraction studies of ECMO demonstrating the effect of magnetic field on the superstructure are presented. The de Haas-van Alphen magnetization oscillations in high magnetic fields and the temperature-induced magnetic oscillations in a fixed magnetic field are observed at low temperatures. Below 10 K the quantum corrections to magnetization due to the weak charge carrier localization in 2D superlattice layers occur. It is shown that at all the temperatures the Eu(1-x)Ce(x)Mn2O5 magnetic state is dictated by superparamagnetism of isolated ferromagnetic domains.     

Hidden magnetic configuration in epitaxial La(1-x) Sr(x) MnO3 films

We present an unreported magnetic configuration in epitaxial La(1-x) Sr(x) MnO3 (x ～ 0.3) (LSMO) films grown on strontium titanate (STO). X-ray magnetic circular dichroism indicates that the remanent magnetic state of thick LSMO films is opposite to the direction of the applied magnetic field. Spectroscopic and scattering measurements reveal that the average Mn valence varies from mixed Mn(3+)/Mn(4+) to an enriched Mn3+ region near the STO interface, resulting in a compressive lattice along the a, b axis and a possible electronic reconstruction in the Mn e(g) orbital (d(3)z(2)-r(2). This reconstruction may provide a mechanism for coupling the Mn3+ moments antiferromagnetically along the surface normal direction, and in turn may lead to the observed reversed magnetic configuration.     

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr(1-x)Ca(x)MnO3 (x = 0.0-0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.     

Evolution of magnetic properties in the normal spinel solid solution Mg(1-x)Cu(x)Cr2O4

We examine the evolution of magnetic properties in the normal spinel oxides Mg(1-x)Cu(x)Cr2O4 using magnetization and heat capacity measurements. The end-member compounds of the solid solution series have been studied in some detail because of their very interesting magnetic behavior. MgCr2O4 is a highly frustrated system that undergoes a first-order structural transition at its antiferromagnetic ordering temperature. CuCr2O4 is tetragonal at room temperature as a result of Jahn-Teller active tetrahedral Cu2+ and undergoes a magnetic transition at 135 K. Substitution of magnetic cations for diamagnetic Mg2+ on the tetrahedral A site in the compositional series Mg(1-x)Cu(x)Cr2O4 dramatically affects magnetic behavior. In the composition range 0 ≤ x ≤ ≈0.3, the compounds are antiferromagnetic. A sharp peak observed at 12.5 K in the heat capacity of MgCr2O4 corresponding to a magnetically driven first-order structural transition is suppressed even for small x. Uncompensated magnetism--with open magnetization loops--develops for samples in the x range ≈0.43 ≤ x ≤ 1. Multiple magnetic ordering temperatures and large coercive fields emerge in the intermediate composition range 0.43 ≤ x ≤ 0.47. The Néel temperature increases with increasing x across the series while the value of the Curie-Weiss Θ(CW) decreases. A magnetic temperature-composition phase diagram of the solid solution series is presented.     

Orbital-order-induced metal-insulator transition in La(1-x)Ca(x)MnO3

We present evidence that the insulator-to-metal transition in La(1-x)Ca(x)MnO3 near x approximately 0.2 is driven by the suppression of coherent Jahn-Teller distortions, originating from d-type orbital ordering. The orbital-ordered state is characterized by large long-range Q2 distortions below T(O'- O*). Above T(O'- O*) we find evidence for coexistence between an orbital-ordered and an orbital-disordered state. This behavior is discussed in terms of electronic phase separation in an orbital-ordered insulating and an orbital-disordered metallic state.     

Interplay between Fe 3d and Ce 4f magnetism and Kondo interaction in CeFeAs(1-x)P(x)O probed by 75As and 31P NMR

A detailed (31)P (I = 1/2) and (75)As (I = 3/2) NMR study on polycrystalline CeFeAs(1-x)P(x)O alloys is presented. The magnetism of CeFeAsO changes drastically upon P substitution on the As site. CeFePO is a heavy fermion system without long-range order whereas CeFeAsO exhibits an Fe 3d SDW type of ordering accompanied by a structural transition from tetragonal (TT) to orthorhombic (OT) structure. Furthermore, Ce 4f(1) orders antiferromagnetically (AFM) at low temperature. At the critical concentration where the Fe magnetism is diminished the Ce-Ce interaction changes to a ferromagnetic (FM) type of ordering. Three representative samples of the CeFeAs(1-x)P(x)O (x = 0.05, 0.3 and 0.9) series are systematically investigated. (1) For the x = 0.05 alloy a drastic change of the linewidth at 130 K indicates the AFM-SDW type of ordering of Fe and the structural change from the TT to the OT phase. The linewidth roughly measures the internal field in the ordered state and the transition is most likely first order. The small and nearly constant shift from (31)P and (75)As NMR suggests the presence of competing hyperfine interactions between the nuclear spins and the 4f and 3d ions of Ce and Fe. (2) For the x = 0.3 alloy, the evolution of the Fe-SDW type of order takes place at around 70 K corroborating the results of bulk measurement and μSR. Here we found evidence for phase separation of paramagnetic and magnetic SDW phases. (3) In contrast to the heavy fermion CeFePO for the x = 0.9 alloy a phase transition is found at 2 K. The field-dependent NMR shift gives evidence of FM ordering. Above the ordering the spin-lattice relaxation rate (31)(1/T(1)) shows unconventional, non-Korringa-like behaviour which indicates a complex interplay of Kondo and FM fluctuations.     

Retention of the tetragonal to orthorhombic structural transition in F-substituted SmFeAsO: a new phase diagram for SmFeAs(O(1-x)F(x))

In this Letter we propose a new phase diagram for the SmFeAs(O(1-x)F(x)) system, based on careful analysis of synchrotron powder diffraction data, SQUID, and muon spin rotation measurements. The tetragonal to orthorhombic structural transition is slightly affected by F content and is retained for the superconducting samples, even at optimal doping. These findings relate the AFM transition on a different ground with respect to the structural one and suggests that orbital ordering could be the driving force for symmetry breaking.     

Mn distribution and spin polarization in Ga(1-x)Mn x As

Using the density functional full-potential linearized augmented plane wave approach, the x-ray absorption and magnetic circular dichroism (XMCD) spectra of Ga(1-x)Mn x As are calculated. Significantly, XMCD of Mn is highly sensitive to the change in environment, and thus can be utilized to characterize impurity distribution. The nature of Mn-induced spin polarization on Ga and As sites, vital for the carrier mediated magnetic ordering, is discussed in light of computational and experimental results.     

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.     

Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La(1-x)Ba(x)CoO3

The nature of the magnetic ground state near the insulator-metal transition (IMT) in La(1-x)Ba(x)CoO3 was investigated via neutron scattering. Below the critical concentration, x(c)～0.22, a commensurate antiferromagnetic (AFM) phase appears initially. Upon approaching x(c), the AFM component weakens and a ferromagnetic (FM) ordered phase sets in while in the rhombohedral lattice. At x(c), a spin flip to a new FM structure occurs at the same time as the crystal symmetry transforms to orthorhombic (Pnma). The Pnma phase may be the driving force for the IMT.     

Field-induced thermal metal-to-insulator transition in underdoped La(2-x)Sr(x)CuO(4+delta)

The transport of heat and charge in cuprates was measured in single crystals of La(2-x)Sr(x)CuO(4+delta) (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T-->0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied magnetic field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.     

Origin of the tunnel anisotropic magnetoresistance in Ga(1-x)Mn(x)As/ZnSe/Ga(1-x)Mn(x)As magnetic tunnel junctions of II-VI/III-V heterostructures

We investigated spin-dependent transport in magnetic tunnel junctions made of III-V Ga(1-x)Mn(x)As electrodes and II-VI ZnSe tunnel barriers. The high tunnel magnetoresistance (TMR) ratio up to 100% we observed indicates high spin polarization at the barrier/electrodes interfaces. We found anisotropic tunneling conductance having a magnitude of 10% with respect to the direction of magnetization to linearly depend on the magnetic anisotropy energy of Ga(1-x)Mn(x)As. This proves that the spin-orbit interactions in the valence band of Ga(1-x)M(x)As are responsible for the tunnel anisotropic magnetoresistance (TAMR) effect.     

Displacement-type ferroelectricity with off-center magnetic ions in perovskite Sr(1-x)Ba(x)MnO3

We report a ferroelectric transition driven by the off-centering of magnetic Mn(4+) ions in antiferromagnetic Mott insulators Sr(1-x)Ba(x)MnO(3) with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x ≥ 0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO(3). We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ～70% and increases the soft-phonon energy by ～50%, indicating the largest magnetoelectric effects yet attained.     

Room-temperature ferromagnetism in a II-VI diluted magnetic semiconductor Zn(1-x)Cr(x)Te

The magnetic and magneto-optical properties of a Cr-doped II-VI semiconductor ZnTe were investigated. Magnetic circular dichroism measurements showed a strong interaction between the sp carriers and localized d spins, indicating that Zn(1-x)Cr(x)Te is a diluted magnetic semiconductor. The Curie temperature of the film with x=0.20 was estimated to be 300+/-10 K, which is the highest value ever reported for a diluted magnetic semiconductor in which sp-d interactions were confirmed. In spite of its high Curie temperature, Zn(1-x)Cr(x)Te film shows semiconducting electrical transport properties.     

Microscopic coexistence of superconductivity and magnetism in Ba(1-x)K(x)Fe2As2

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba(1-x)K(x)Fe(2)As(2) exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba(1-x)K(x)Fe(2)As(2) showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.     

Electronic structure of charge- and spin-controlled Sr(1-(x+y))La(x+y)Ti(1-x)Cr(x)O3

We present the electronic structure of Sr(1-(x+y))La(x+y)Ti(1-x)Cr(x)O3 investigated by high-resolution photoemission spectroscopy. In the vicinity of the Fermi level, it was found that the electronic structure was composed of a Cr 3d local state with the t(2g)3 configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.     

Structural characterization of epitaxial Cr(x)Mo(1-x) alloy thin films

To develop a model system containing regularly spaced misfit dislocations for studies of the radiation resistance of nanoscale defects, epitaxial thin films of Cr, Mo, and Cr(x)Mo(1-x) alloys were deposited on MgO(001) by molecular beam epitaxy. Film compositions were chosen to vary the lattice mismatch with MgO. The film structure was investigated by x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and scanning transmission electron microscopy (STEM). Epitaxial films with reasonably high crystalline quality and abrupt interfaces were achieved at a relatively low deposition temperature, as confirmed by STEM. However, it was found by XRD and RBS in the channeling geometry that increasing the Mo content of the CrMo alloy films degraded the crystalline quality, despite the improved lattice match with MgO. XRD rocking curve data indicated that regions of different crystalline order may be present within the films with higher Mo content. This is tentatively ascribed to spinodal decomposition into Cr-rich and Mo-rich regions, as predicted by the Cr(x)Mo(1-x) phase diagram.