2014 Fall Meeting
Recent progress in new high-Tc superconductors and related multifunctional and magnetic materials
The ongoing discoveries of new superconductors and other multifunctional materials do not only challenge to explore their microscopic mechanisms but also open new fields of applications. Since both, mechanism and applications, go hand in hand, we pursue to assemble experts of both fields to exchange news and stimulate discussions and teamwork.
The Symposium is meant as an international discussion and presentation forum of novel scientific approaches in the field of complex superconductivity, magnetism, and multiferroic materials (including experimental results, new materials, novel theoretical models). Especially, it is aimed at interchanging material properties from, e.g., superconducting to magnetic, in order to explore common or unlike aspects which can elucidate the microscopic physics behind the relevant phenomenon. Special emphasis will be put on the following subjects:
- High temperature superconductors;
- New materials with strong correlations;
- Novel metallic oxides and anomalous magnetoresistive materials;
- Low dimensional quantum magnets;
- Magnetic multilayers, surfaces, nanostructures, and magnonic crystals;
- Electronic properties under extreme conditions;
- Magneto-electronics and spintronics;
- Data storage and logic devices;
- Magnetic nanoparticles and granular materials;
- Clusters and low dimensional magnetism;
- Magnetocaloric effects and systems;
- Structured materials (ultra-thin films and surface effects, multi-layer films and superlattices, patterned films, nanoparticles and self-assembling);
- Exchange bias, especially in nanostructures;
- Magnetic applications in medicine, biomagnetic applications;
- Multiferroic compounds and their prospective applications.
Hot topics to be covered by the symposium
- Iron based superconductors
- Cuprate superconductors
- Magnesium diboride
- Magnetic perovskites
- Models of electron-lattice interactions
- Hubbard type approaches
- Antiferromagnetic fluctuation induced phenomena
- Polaron and bipolaron physics
List of invited speakers
- M. Angst, Forschungszentrum Julich, Germany
- B. Barbara, Grenoble, France
- A. Bianconi, Rome, Italy
- J. Bonca, Ljubljana, Slovenia
- C. W. Chu, Houston, USA
- M. Doria, Rio de Janeiro, Brasil
- T. Egami, Univ. Tennessee, USA
- M. Ghafari, KIT / INT, Germany
- R. S. Gonnelli, Politecn. Torino, Italy
- A. F. Hebard, Gainsville, USA
- J. E. Hirsch, Univ. California, San Diego, USA
- D. Johrendt, Univ. Munich, Germany
- S. Kamba, Prague, Czech Republic
- H. Kamimura, Tokyo, Japan
- R. Khasanov, PSI, Switzerland
- R. K. Kremer, Stuttgart, Germany
- E. Liarokapis, Athens, Greece
- J. Mannhart, Stuttgart, Germany
- R. Micnas, Poznan, Poland
- J. Nogues, ICREA, Barcelona, Spain
- T. T. M. Palstra, Groningen, The Netherlands
- E. Pomjakushina, PSI, Switzerland
- B. Raveau, Univ. Caen, France
- A. Shengelaya, Tbilisi Univ., Georgia
- L. Sun, Inst. Phys., Chinese Acad. Sci., China
List of scientific committee members
- A. Bussmann-Holder, Max Planck Inst., Germany
- K. Conder, PSI, Switzerland
- H. Keller, Univ. Zurich, Switzerland
- R. Puzniak, Inst. Phys., Polish Acad. Sci., Poland
- H. Szymczak, Inst. Phys., Polish Acad. Sci., Poland
- A. Wisniewski, Inst. Phys., Polish Acad. Sci., Poland
The symposium will be co-organized by the EU 7th Framework Programme under the project REGPOT-CT-2013-316014 (EAgLE)
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Authors : Karol Izydor Wysokinski,
Affiliations : Institute of Physics, M. Curie-Sklodowska University,ul. Radziszewskiego 10, 20-031 Lublin
Resume : We shall present an analysis of the Hall conductivity σxy(ω, T) in time reversal symmetry breaking states of exotic superconductors. The intrinsic Kerr signal appears in a general multiband system. This is a novel mechanism which may explain the Kerr effect observed in strontium ruthenate and possibly other multiband superconductors. The proposed mechanism does not rely on impurity scattering or a finite width of the incident photon beam (in collaboration with James Annett and Martin Gradhand, University of Bristol) K.I. Wysokinski, J.F. Annett, B.L. Gyorffy, Phys. Rev. Lett. 108 077004 (2012); Martin Gradhand, Karol I. Wysokinski, James F. Annett, and Balazs L. Györffy Phys. Rev. B 88, 094504 (2013).
Authors : J. Bonca, J. Kogoj, D. Golez, M. Mierzejewski, P. Prelovsek, Z. Lenarcic
Affiliations : J. Stefan Institute, Faculty of Mathematics and Physics, University of Ljubljana, Slovenia, Institute of Physics, University of Silesia, 40-007 Katowice, Poland
Resume : I will present the relaxation dynamics of an excited charge carrier in the spin background described by the two dimensional t-J model. I will also discuss a simple analytical argument for the unusual scaling of the relaxation time with the exchange interaction. ?In the second part I will discuss the formation of a spin-lattice polaron in one spatial dimension ?after a quantum quench that simulates absorption of the pump pulse in the time resolved experiments. A two-stage relaxation is found where spin and lattice degrees of freedom represent an integral part of the relaxation mechanism. In the first stage the kinetic energy of the spin-lattice polaron relaxes towards its ground state value while relaxation processes via spin and phonon degrees of freedom remain roughly independent. In the second, typically much longer stage, a subsequent energy transfer between lattice and spin degrees of freedom via the charge carrier emerges. The excess local spin energy radiates away via magnon excitations.
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Authors : Ursula Pachmayr and Dirk Johrendt
Affiliations : Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (D), 81377 München, Germany
Resume : Iron selenide with the PbO-type structure is a superconductor with a critical temperature of 8 K  which increases to 36 K under high pressure . 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 . Very recently LiFeO2Fe2Se2 with anti-PbO-type spacer layers was reported, which show superconductivity at 43 K . 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 .  F. C. Hsu, et al., PNAS 105, 14262 (2008).  S. Medvedev, et al., Nat. Mater. 8, 630 (2009).  Q.-Y. Wang, et al., Chin. Phys. Lett. 29, 037402 (2012).  S. He, et al., Nat. Mater 12, 605 (2013).  W.-H. Zhang, et al., Chin. Phys. Lett. 31, 017401 (2014).  M. Burrard-Lucas, et al., Nat. Mater. 12, 15 (2013).  X. F. Lu, et al., Phys. Rev. B 89, 020507 (2014).  U. Pachmayr, D. Johrendt, submitted.
Authors : Anna Krzton-Maziopa 1, Ekaterina Pomjakushina 2, Kazimierz Conder 2, Vladimir Pomjakushin 3
Affiliations : 1)Faculty of Chemistry, Warsaw University of Technology, Noakowskiego St.3, 00-664 Warsaw (Poland) 2)Laboratory for Developments and Methods, Paul Scherrer Institut, 5232 Villigen (Switzerland). 3)Laboratory for Neutron Scattering, Paul Scherrer Institut, 5232 Villigen (Switzerland)
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.
Session 5 : chair - J. Karpinski
Authors : R.S. Gonnelli (a), D. Daghero (a), S. Galasso (a), G.A. Ummarino (a), R.K. Kremer (b), G. Profeta (c), K. Kuroki (d)
Affiliations : (a) Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Torino, Italy; (b) Max Planck Institute for Solid State Research, Stuttgart, Germany; (c) Dipartimento di Fisica, Università dellAquila, LAquila, Italy; (d) Department of Physics, Osaka University, Osaka, Japan.
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.
Authors : R. Khasanov, A. Amato, P.K. Biswas, H. Luetkens, N.D. Zhigadlo, B. Batlogg
Affiliations : aboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland; Laboratory for Solid State Physics, ETH Zurich, 8093 Zurich, Switzerland
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.
Authors : Marco Truccato(1), Alessandro Pagliero(1), Lorenzo Mino(2), Elisa Borfecchia(2), Angelo Agostino(2), Lise Pascale(2), Emanuele Enrico(3), Natascia De Leo(3), Carlo Lamberti (2),Gema Martínez-Criado (4)
Affiliations : (1) Dept. of Physics and NIS, University of Torino, Torino, Italy; (2) Dept. of Chemistry and NIS, University of Torino, Torino, Italy; (3) INRIM, National Institute of Metrological Research, Torino, Italy; (4) Experiments Division, ESRF, Grenoble Cedex, France
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 . 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 . 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.  Alessandro Pagliero et al., Nano Lett., 14, 1583−1589 (2014)  W. Meevasana et al., Nature Mater., 10, 114-118 (2011)
Session 6 : chair - B. Raveau
Authors : Arthur F. Hebard (1), Patrick R. Mickel (2), Hyoungjeen Jeen (3), Amlan Biswas (1), Pradeep Kumar (1)
Affiliations : (1)University of Florida, USA; (2) Sandia National Laboratories, USA;(3) Pusan National University, S. Korea
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.
Authors : Thomas T.M. Palstra, A.O. Polyakov, Graeme R. Blake
Affiliations : Zernike Institute for Advanced Materials University of Groningen, The Netherlands
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, 133139.
Authors : Z. Viskadourakis(1,2), S. Mukherjee(3), S. S. Sunku(4), B. M. Andersen(3) , T. Ito(5), T. Sasagawa(6) and C. Panagopoulos(4,1,7,2)
Affiliations : 1. Crete Center for Quantum Complexity and Nanotechnology, University of Crete, Heraklion 71003, Greece 2. IESL-FORTH, Vassilika Vouton, Heraklion 71110, Greece 3. Niels Bohr Institute, University of Copenhagen, Copenhagen DK-2100, Denmark 4. School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Nanyang Technological University, 637371 Singapore 5. National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan 6. Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan 7. Department of Physics, University of Crete, Heraklion 71003, Greece
Resume : A couple of years ago, a low temperature ferroelectric phase and an associated magnetoelectric coupling were reported for lightly oxygen-doped La2CuO4+x . 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].  Z. Viskadourakis et al., Phys Rev. B 85, 214502 (2012).  S. Mukherjee et al., J. Supercond. Novel Magn. 26, 1649 (2013).  S. Mukherjee et al., Phys. Rev B 85, 140405 (2012).  Rieus et al., Phys. Rev. B. 73, 144513, (2006)
Authors : E. Pellegrin, C. Ge[2,3], Z. Hu, S. Agrestini4, S. M. Valvidares, J. Herrero Martin, P. Gargiani, A. Barla, X. Wan, W.-I Liang, Y.-H. Zou, W. Zou, Y. Dou, and F.-C. Chou.
Affiliations :  CELLS-ALBA Synchrotron Radiation Facility, Carretera BP 1413, km 3.3, E-08290 Cerdanyola del Vallès (Barcelona), Spain;  National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China;  Department of Physics, Jiangsu Institute of Education, Nanjing 210013, China;  Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, D-01187 Dresden, Germany;  Istituto di Struttura della Materia, ISM CNR, S.S. 14 km 163.5, Area Science Park, Basovizza (Ts), Trieste I-34149, Italy;  Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan;  Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan;
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.
Authors : K. Lasek, L. Gladczuk, M. Aleszkiewicz, P. Aleshkevych, P. Przyslupski
Affiliations : Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, PL 02-668 Warsaw, Poland.
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.
Authors : A.Kozyrev, T.Basyuk, T.Prikhna, E.Moshil, N.Sergienko
Affiliations : Institute for Superhard Materials of the National Academy of Sciences of Ukraine
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.
Authors : Mitra Iranmanesh1, Manuela Stir1, John R. Kirtley2, Jürg Hulliger1
Affiliations : 1 Department of Chemistry & Biochemistry, University of Berne, Freiestrasse 3, CH-3012 Berne, Switzerland 2 Center for Probing the Nanoscale, Stanford University, Palo Alto, California, U.S.A.
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.
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Authors : Katharina Rolfs1, Sándor Tóth2, Ekatarina Pomiakushina1, Kazimierz Conder1
Affiliations : 1 Paul Scherrer Institute, Laboratory for Development and Methods, Villigen PSI West, 5303 Switzerland 2Paul Scherrer Institute, Laboratory for Neutron Scattering, Villigen PSI West, 5303 Switzerland
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 3ds. 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.
Authors : M. Estrader1), A. López-Ortega2), S. Estradé1), I.V. Golosovsky3), G. Salazar-Alvarez4), M. Vasilakaki5), K.N. Trohidou5), M.A. Roldan6,7), M. Varela6,7), D.C. Stanley8), M. Sinko8), M.J. Pechan8), D. J. Keavney9), M. Laver10), K.L. Krycka11), J.A. Borchers11), F. Peiró1), S. Suriñach12), M.D. Baró12), J. Nogués13)*
Affiliations : 1)Univ. de Barcelona, Barcelona, Spain, 2)Univ. degli Studi di Firenze, Firenze, Italy, 3)St. Petersburg Nuclear Physics Institute, Gatchina, Russia, 4)Stockholm Univ., Stockholm, Sweden, 5)NCSR Demokritos, Aghia Paraskevi, Greece, 6)Oak Ridge National Lab., Oak Ridge, TN, USA, 7)Univ. Complutense, Madrid, Spain, 8) Miami University, Oxford, OH, USA, 9)Argonne National Laboratory, Argonne, IL, USA, 10)Univ. of Birmingham, Edgbaston, UK, 11)National Institute of Standards and Technology, Gaithersburg, MD, USA, 12)Univ. Autònoma de Barcelona, Bellaterra, Spain, 13)ICREA and ICN2-Institut Catala de Nanociencia i Nanotecnologia, Bellaterra, Spain.
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.  M. Estrader, Nat. Commun. 4, 2960 (2013).
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Authors : S. Kamba, F. Kadlec, V. Goian, C. Kadlec, P. Vaněk, M. Kempa
Affiliations : Institute of Physics, Czech Academy of Sciences, Prague, Czech Republic
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.
Session 11 : chair - K. Conder
Authors : A. Shengelaya (1), D. Daraselia (1), D. Japaridze (1), Z. Jibuti (1), K. A. Müller (2)
Affiliations : 1. Department of Physics, Tbilisi State University, GE-0128, Tbilisi, Georgia; 2. Physik-Institut der Universität Zürich, CH-8057, Zürich, Switzerland
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.
Authors : J. Wieckowski*1, M. U. Gutowska1, A. Szewczyk1, K. Conder2, E. Pomjakushina2, V. P. Gnezdilov3, and S. L. Gnatchenko3
Affiliations : 1 Institute of Physics, Polish Academy of Sciences, Warsaw, Poland 2 Paul Scherrer Institute, Villigen, Switzerland 3 B. Verkin Institute for Low Temperature Physics and Engineering, Kharkiv, Ukraine * firstname.lastname@example.org
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.
Authors : J. Pietosa1, K. Piotrowski1, S. Kolesnik2, B. Dabrowski2
Affiliations : 1 Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland 2 Department of Physics, Northern Illinois University, De Kalb, Illinois 60115, USA
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 = 0.480.55. 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.
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