Recent progress in new high-Tc superconductors and related multifunctional and magnetic materials

Resume : Iron selenide with the PbO-type structure is a superconductor with a critical temperature of 8 K [1] which increases to 36 K under high pressure [2]. Indications of much higher Tc up to 65 K in one-unit-cell FeSe thin films [3-5] emphasize the potential of FeSe. Also the intercalation of molecular spacer layers was successful and increased the Tc to 43 K in Lix(NH2)y(NH3)1-yFe2Se2 [6]. Very recently LiFeO2Fe2Se2 with anti-PbO-type spacer layers was reported, which show superconductivity at 43 K [7]. During our investigations into soft chemistry approaches to FeSe, we were able to prepare phase pure stoichiometric FeSe by using the hydrothermal method. These samples show only traces of superconductivity at 8 K, which suggests that truly stoichiometric FeSe may not be superconducting. Subsequently we have synthesized the first hydroxide-intercalated iron selenide under hydrothermal conditions. The new compound with composition close to LiOHFe2Se2 according to single crystal x-ray diffraction shows superconductivity up to 43 K [8]. [1] F. C. Hsu, et al., PNAS 105, 14262 (2008). [2] S. Medvedev, et al., Nat. Mater. 8, 630 (2009). [3] Q.-Y. Wang, et al., Chin. Phys. Lett. 29, 037402 (2012). [4] S. He, et al., Nat. Mater 12, 605 (2013). [5] W.-H. Zhang, et al., Chin. Phys. Lett. 31, 017401 (2014). [6] M. Burrard-Lucas, et al., Nat. Mater. 12, 15 (2013). [7] X. F. Lu, et al., Phys. Rev. B 89, 020507 (2014). [8] U. Pachmayr, D. Johrendt, submitted.

Resume : Intercalated iron chalcogenides of the general formula AxFe2-ySe2 (A: Cs, K, Rb, Tl/Rb, Tl/K), were found to be superconducting in recent years and since then are widely investigated. Experimentally determined phase diagrams for AxFe2-ySe2 point out that antiferromagnetic (AFM) ordering appears in those compounds usually at temperatures above 500K, and superconductivity coexists with the AFM ordered phase over a narrow range of compositions. The most peculiar feature of those materials is the coexistence of two phases ? the parent one, which is believed to be insulating ordered Fe vacancy phase with block AFM ordering, known as ?245 phase? of composition A0.8Fe1.6Se2 and the so called minority vacancy free phase: AxFe2Se2. The pronounced coexistence of majority insulating phases and minority superconducting (SC) phases in AxFe2−ySe2 formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. The amount of superconducting phase in the conventionally prepared materials, despite the observed 100% shielding in magnetization measurements, usually is not higher than 10-12%, which makes them difficult for investigation. As the phase separation occurs at pretty high temperatures, the synthesis at low temperature by solvothermal techniques, utilizing liquid ammonia or heterocyclic amines as solvents for alkali metals, can be an alternative route allowing for preparation of intercalated FeSe-based superconductors with considerably enhanced amounts of superconducting phases. Here we report a synthesis and characterization of an iron selenide superconductor with a Tc onset of 45K and the nominal composition Lix(C5H5N)yFe2-zSe2, synthesized via intercalation of layered FeSe with a complexes of lithium and anhydrous pyridine. Magnetization measurements of the as prepared material reveal a superconducting shielding fraction of approximately 30%. The superconducting transition temperature of Lix(C5H5N)yFe2-zSe2 is clearly enhanced by post-annealing of the intercalated material at elevated temperatures. Heat treatment, due to rearrangement of pyridine rings located between the FeSe layers drastically enlarges the c parameter of the unit cell (~44%) and enhances the amount of superconducting shielding fraction to nearly 100%. The influence of the amount of alkali metal and organic moiety on superconducting properties and crystal structure is discussed.

Resume : Directional point-contact Andreev-reflection spectroscopy is a simple but versatile technique to study the number, amplitude and symmetry of the order parameter(s) of superconductors. Its capabilities can be considerably improved if a 3D extension of the Blonder-Tinkham-Klapwijk model, combined with ab-initio and Eliashberg calculations, is used to analyze the results. Here we apply this approach to CaFe2As2 (both Co-doped and under pressure) and to FeSexTe1-x single crystals. In CaFe2As2 at specific values of doping or pressure the Fermi surface is on the verge of a topological 2D – 3D transition, which causes the disappearance of the holelike sheet at the centre of the Brillouin zone. We show that this transition is accompanied by the emergence of a horizontal node line in the hole gap and, rather unexpectedly, corresponds to an enhancement of the critical temperature and of the electron-boson coupling. This indicates that CaFe2As2 can be driven across a transition that makes it bridge the gap between “usual” multiband Fe-based compounds and “anomalous” single-band ones (like FeSe monolayers on SrTiO3 , KFe2As2 or K0.8Fe1.7Se2). In FeSexTe1-x we provide spectroscopic evidence of the presence of two gaps (one isotropic and the other strongly anisotropic) and of intense high-bias structures that can be associated to the electron-boson (spin fluctuation) interaction.

Resume : The magnetic penetration depth \lambda as a function of applied magnetic field and temperature in SrPt_3P(T_c=8.4K) was studied by means of muon-spin rotation (\muSR). The dependence of \lambda^-2 on temperature suggests the existence of a single s-wave energy gap with the zero-temperature value \Delta=1.58(2)meV. At the same time $\lambda$ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multi-gap systems. The multi-gap nature of the superconduicting state is further confirmed by observation of an upward curvature of the upper critical field. This apparent contradiction would be resolved with SrPt_3P being a two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets.

Resume : The investigation of high-Tc superconducting oxides by means of synchrotron radiation presently represents a very active field. Here we report a novel phenomenon with likely practical applications. We have irradiated Bi-2212 superconducting single crystals by means of a 17 keV beam with space resolution of 117  116 nm2 up to a maximum dose of about 3  1012 Gy, while recording both structural and electrical information [1]. Data show a monotonic behavior with a maximum increase in the critical temperature Tc of 1.3 K and a maximum elongation of about 1 Å in the c-axis length, compared to the as-grown crystals. These facts clearly testify a change in the Bi-2212 doping level, which shows close similarity to the appearance of a two-dimensional electron gas in SrTiO3 after exposure to UV synchrotron light [2]. Modeling our experimental conditions by means of the finite element method allowed us to exclude X-ray-induced heating as a possible cause for this doping change. Correspondingly, an important role is suggested for X-ray-induced photoelectrons as a possible source of knock-on for the O atoms in the BiO layers due to their low displacement energy. These results support the possible use in the future of hard X-rays for a novel direct-writing, photoresist-free lithographic process to fabricate superconducting devices with potential nanometric resolution. [1] Alessandro Pagliero et al., Nano Lett., 14, 1583−1589 (2014) [2] W. Meevasana et al., Nature Mater., 10, 114-118 (2011)

Resume : Multiferroic materials are heralded as prime candidates for enhanced magnetoelectric coupling (the induction of electric/magnetic polarization via magnetic/electric fields) due to their large inherent electric/magnetic susceptibilities. We report here a giant linear magnetoelectric coupling in strained pulsed-laser deposited BiMnO3 thin films in which the disorder associated with an islanded morphology gives rise to a convergence of the ferroelectric and ferromagnetic transitions, thus amplifying the magnetoelectric coupling between the ferroelectric and ferromagnetic order parameters. Data showing the proximity of the magnetic and ferroelectric transitions as measured by the overlap of the rapid changes in the temperature-dependent magnetization and the remnant polarization will be presented. We find that the temperature dependence of the cross susceptibility (product of susceptibilities) exhibits a strong peak near the convergent transition temperatures, following closely the separately measured temperature dependence of the magnetoelectric coupling constant. Our interpretation of this correlation derives from a straightforward manipulation of linear relationships between the ferromagnetic and ferroelectric order parameters and the applied magnetic and electric fields derived from a ”gas in a piston” free energy. The results suggest that in the search for new materials with high magnetoelectric coupling, a focus on disordered multiferroics could provide useful new insights.

Resume : We have investigated organic-inorganic hybrids that combine electronic functionality of the perovskite structures and structural flexibility of metal-organic framework compounds. The chemistry of inorganic materials offers a wide range of band gaps or band widths with high carrier density and mobility, magnetic interactions, ferroelectric transitions and thermal stability. On the other hand, organic solids provide nearly unlimited flexibility in structural diversity, good polarizability and they can also be made conductive. The aim of investigating these hybrid materials is to combine properties from both the inorganic and the organic moiety in one material and produce useful combinations or even completely new phenomena. We show that ferroelectricity and (anti-)ferromagnetism coexist in Cu- and Mn-based hybrids Another interesting family is the NiCl3-based organic-inorganic hybrids. Ni-hybrids with general formula NiCl3(YNH3)2, represent 1D magnetic S=1 chains, where magnetism originates from chains of NiCl3 face-sharing octahedra, separated by the organic molecules. We show that the magnetic properties of the inorganic block can be subtly tuned by using different organic moieties. References: A.O. Polyakov, A.H. Arkenbout, J. Baas, G.R. Blake, A.M. Meetsma, A. Caretta, P.H.M. van Loosdrecht, T.T.M. Palstra, Chemistry of Materials 2012, 24, 133–139.

Resume : A couple of years ago, a low temperature ferroelectric phase and an associated magnetoelectric coupling were reported for lightly oxygen-doped La2CuO4+x [1]. Here we report on the observation of ferroelectricity and magnetoelectricity in La2-xSrxCuO4 and La2LixCu1-xO4. We find that Sr doping leads to a distinct ferroelectric behavior along different crystallographic directions and Li doping causes an exceptionally enhanced polarization. In both cases, ferroelectricity can be tuned by Dzyaloshinskii-Moriya interaction resulting to an observed magnetoelectric effect [2, 3]. The above mentioned results establish ferroelectricity as a new generic property of the ground state in underdoped La-214 cuprates. It is proposed that ferroelectricity results from local CuO6 octahedral distortions, associated with the dopant atoms and/or clustering of the doped charge carriers, which break spatial inversion symmetry in local scale [1, 4]. [1] Z. Viskadourakis et al., Phys Rev. B 85, 214502 (2012). [2] S. Mukherjee et al., J. Supercond. Novel Magn. 26, 1649 (2013). [3] S. Mukherjee et al., Phys. Rev B 85, 140405 (2012). [4] Rieus et al., Phys. Rev. B. 73, 144513, (2006)

Resume : The inverse spinel system CoFe2O4 can be considered as a paradigmatic material for the study of magnetic transition metal oxides. The more recent advent of self-assembled perovskite-spinel structures has now opened new possibilities for the engineering of previously inaccessible functionalities, among others by making use of the ferromagnet-antiferromagnet interface coupling of nano-objects with an enhanced surface-to-volume ratio such as the above CoFe2O4 nano-pillars. The well-known “classical” core-shell particle systems may serve as another example for these interface-driven magnetic materials. We have investigated two different nanostructured transition metal oxide sytems: (i) CoFe2O4 nano-pillars in BiFeO3 and (ii) Co/CoO-MgO core/shell nanoparticles in MgO using soft x-ray magnetic circular dichroism (XMCD), SQUID magnetometry, and transport measurements. Single-crystalline CoFe2O4 has also been investigated as a bulk reference material. The results from the XMCD data and their implications on the various bulk properties as well as the underlying microscopic magnetic models will be discussed.

Resume : Cobalt-iron films are extensively investigated because of its significance in manufacturing of tunneling magnetoresistance heads and magnetic-random-access-memory devices. In view of such applications we examine the magnetic properties of thin CoxFe1-x/Ni multilayers, epitaxially grown on Au(111), by ferromagnetic resonance technique. Epitaxial growth has been confirmed by RHEED, and atomic force microscopy (AFM). The out of plane orientation of the easy axis of magnetization for structures with thin CoxFe1-x layer was obtained. The changes of the anisotropy parameters with respect of CoxFe1-x and Ni layers thickness will be presented.

Resume : It is known that magnesium diboride superconducting properties (such as critical current density and irreversibility field) can be modified by adding metals to the material. We have implemented experimental studies synthesis of a superconducting material at high pressures and temperatures with the addition of Zr – up to 10%. It was found that the critical current density in a material under zero magnetic fields was 106 A/cm2 at 10 K and 7×105 A/cm2 at 20 K. The material is characterized by high resistance to strong magnetic fields. The critical current density at 10K 104 remains to 7.5 T. The study of the formation mechanism of the phases showed that the dependence of the superconducting characteristics of the addition of Zr explained by two factors. First, the addition of Zr results creates more number of inclusions MgB12. Second, there are take place the formation of zirconium hydride, which prevents the formation of magnesium hydride. However, this mechanism is limited temperature interval 750-800 °C. Material synthesized at higher temperatures does not differ in their characteristics from pure magnesium diboride. Furthermore, it was found that during the synthesis of the material is no diffusion of Zr.

Resume : Combinatorial solid state chemistry performed by reaction within a system of randomly mixed starting components (Ca, Sr, Ba, La, Y, Pb, Bi, Tl and Cu) showed bulk superconductivity up to 115 K. Application of scanning SQUID microscopy (SSM) revealed local superconducting areas within ceramic samples down to a size of 5 µm and a max Tc of 115 K. Locally formed superconducting matter was extracted by magnetic separation. The analysis single grain (d < 80 µm) performed by X-ray diffraction, elemental analysis and bulk SQUID measurements allowed us to identify Tl2Ca3Ba2Cu4O12, TlCaBaSrCu2O7−δ, BaPb0.5Bi0.25Tl0.25O3-δ, TlCaBaSrCu2O7−δ and Ba2YCu3O7 phases. SSM turned up to be a useful instrument to analyze inhomogeneous reaction products in the solid state chemistry of materials showing magnetic properties.

Resume : For the past decades, a tremendous amount of effort has been devoted to exploring the nature of 3d transition metal oxides where various exotic states and phenomena have emerged such as: high-Tc cuprate superconductivity, colossal magnetoresistivity and metal-insulator transitions. In 5d transition metal oxides the cations’ orbitals are more extended than 3d’s. There is reduced on-site Coulomb interaction strength but much stronger spin-orbit (SO) coupling. Therefore, physical properties of 5d transition metal oxides are more sensitive to lattice distortion, magnetic order, etc. In Na2IrO3 a strong spin-orbit coupling stabilizes the insulating state even though the onsite Coulomb interactions are relatively weak. Another characteristic feature of Na2IrO3 and also Li2IrO3 is their honeycomb structure permitting a realization of a spin liquid ground state proposed by Kitaev. However Na2IrO3 and Li2IrO3 order magnetically at 15 K. It was suggested that a reduction of the chemical pressure along the c-axis can increase JK and therefore induce spin liquid behavior. This can be achieved either by applying uniaxial pressure pressure in the ab plane or substituting Na by smaller Li ions. In this work influence of Li doping on structural and magnetic properties of Na2IrO3 will be discussed.

Resume : The coupling between different magnetic layers in most thin film systems is usually ferromagnetic (FM) (i.e., the layers are aligned parallel to each other). However, other types of couplings such as antiferromagnetic (AFM) (i.e., antiparallel layers) have also been reported. In contrast, the magnetic properties of bi-magnetic core/shell nanoparticles remain relatively unexplored, where experimental work so far has only reported ferromagnetic coupling between the counterparts. Here we present the existence of an interfacial AFM coupling in narrow size distributed ferrimagnetic (FiM) soft/hard and hard/soft core/shell nanoparticles based on Fe3O4 and Mn3O4.1 In contrast to conventional systems, the temperature dependence of the magnetization, M, and the ferromagnetic resonance field, HR, show a downturn at the magnetic ordering temperature of the hard Mn3O4 phase. This decrease in M and HR can be linked to an antiferromagnetic coupling between both phases. Moreover, element selective X-ray magnetic circular dichroism spectra and hysteresis loops confirm that the magnetization of the Mn-containing phase lies opposite to the Fe-containing phase. The presence of positive exchange bias further proves the coupling. Monte Carlo simulations clearly confirm that an AFM interface coupling leads to a magnetization decrease at low temperatures and a positive exchange bias for large cooling fields. Research at ORNL sponsored by DOE-BES. [1] M. Estrader, Nat. Commun. 4, 2960 (2013).

Resume : Electromagnons are spin wave excited by the electric component of the electromagnetic radiation. They become infrared (IR) active due to the dynamic magnetoelectric effect. In multiferroic CaMn7O12, we observed three IR-active excitations below 80 cm-1, which change their frequencies and split in the external magnetic field. Their intensities show remarkable anomalies at the magnetic phase transitions occurring at 90 and 50 K. The frequencies of these excitations correspond to the maxima in the magnon density of states obtained by inelastic neutron scattering. Furthermore, these magnons receive the strength from the polar phonons observed in the IR spectra; therefore they must be electromagnons. Surprisingly, two of them persist in the paramagnetic phase; for this reason, we call them paraelectromagnons. We have observed an electromagnon also in nanograin ceramics of e-Fe2O3. Below 490 K, this material is a ferroelectric ferrimagnet. The IR-active electromagnon activates in the spectra only below 110 K, at the same time as the magnetic structure becomes incommensurately modulated. Inelastic neutron scattering spectra provide an evidence of the magnetic character of the electromagnon. Simultaneously, they show that the electromagnon corresponds to a magnon from the Brillouin-zone boundary. Finally we will show how, by combining IR, THz and inelastic neutron scattering experiments, the electromagnons can be generally discerned from magnons or phonons.

Resume : High-temperature superconductors (HTSs) and other technologically important oxide materials are usually obtained through solid state reaction. This implies long-term (for tens of hours) heating of reactants in powder form at high temperatures (800º-1200ºC) in furnace, which is a highly time and energy consuming process and increases product costs. We report a novel kind of synthesis for oxide materials, which involves the intense irradiation of the powder mixture of the starting oxides by light at elevated temperature. Different cuprate HTSs and magnetic oxides were synthesized using this method. It was demonstrated that light irradiation leads to a dramatic increase of the reaction speed and a lowering of the reaction temperature. The rate of the resulting reaction exceeds the conventional thermal solid state reaction rate in furnace by about two orders of magnitude. The photostimulated solid-state reaction (PSSR) method demonstrated is quite general and opens up the possibility of fast synthesis of a wide range of technologically important bulk and thin-film oxide materials.

Resume : Layered cobaltites RBaCo2O5.5 exhibit a rich spectrum of intriguing properties, e.g., large magnetoresistance, large ionic and electronic conductivity at high temperatures, and several phase transitions of different nature. The objectives of specific heat studies of a series of RBaCo2O5.5 (R = Y, Gd, and Tb) layered cobaltites were investigating the phase transitions appearing in these compounds and explaining an impact of different rare-earth ions on thermal properties of layered cobaltites. The studies were performed over the temperature range 3 − 395 K, in magnetic field ranging from 0 to 9 T. Anomalies accompanying different phase transitions were analyzed. Particular contributions to the specific heat (e.g., phonon, magnon, Schottky) were separated and described theoretically. The strength of the molecular field related to the R−Co exchange interactions in RBaCo2O5.5 was estimated to be ~ 1 T. The work was partly supported by the European Regional Development Fund through the Innovative Economy Grant POIG.01.01.02-00-108/09.

Resume : Structural and physical properties of the layered perovskites RBaCo2O5+δ (R = lanthanide ion or Y, δ = 0-0.9) can be tuned by a charge doping at the R-, Ba- and Co-sites or by oxygen content 5+δ. For NdBaCo2O5.5 a sequence of magnetic and electronic phase transitions is observed with increasing temperature: antiferromagnet-ferromagnet (TN = 230 K), ferromagnet-paramagnet (TC = 260 K), and insulator-metal (TMIT = 350 K). Here we present thermoelectric and magnetic properties of NdBa1 yLayCo2O5+δ with y = 0-0.1 and δ = 0.48–0.55. The local minimum of the Seebeck coefficient (α) seen at T = 80 K for the electron doped NdBa2+0.94La3+0.06Co2O5.5 is suppressed by charge doping with δ equal to 0.48 and 0.55. For NdBa0.9La0.1Co2O5.5 the minimum of α shifts to higher temperatures when δ increases from 0.50 to 0.53, which is an opposite effect to electron doping with La3+. These results are compared to magnetization measurements in order to correlate them with phase transitions at TN and TC. This work was supported by the National Science Centre of Poland under contract no. 1662/B/H03/2011/40.