15 research outputs found
Orbitally induced hierarchy of exchange interactions in zigzag antiferromagnetic state of honeycomb silver delafossite Ag3Co2SbO6
We report the revised crystal structure, static and dynamic magnetic
properties of quasi-two dimensional honeycomb-lattice silver delafossite
Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent
with the onset of antiferromagnetic long range order at low temperatures with
N\'eel temperature TN ~ 21.2 K. In addition, the magnetization curves revealed
a field-induced (spin-flop type) transition below TN in moderate magnetic
fields. The GGA+U calculations show the importance of the orbital degrees of
freedom, which maintain a hierarchy of exchange interaction in the system. The
strongest antiferromagnetic exchange coupling was found in the shortest Co-Co
pairs and is due to direct and superexchange interactions between the
half-filled xz+yz orbitals pointing directly to each other. The other four out
of six nearest neighbor exchanges within the cobalt hexagon are suppressed,
since for these bonds active half-filled orbitals turned out to be parallel and
do not overlap. The electron spin resonance (ESR) spectra reveal a Gaussian
shape line attributed to Co2+ ion in octahedral coordination with average
effective g-factor g=2.3+/-0.1 at room temperature and shows strong divergence
of ESR parameters below 120 K, which imply an extended region of short-range
correlations. Based on the results of magnetic and thermodynamic studies in
applied fields, we propose the magnetic phase diagram for the new
honeycomb-lattice delafossite
Synthesis and characterization of MnCrO4, a new mixed-valence antiferromagnet
A new orthorhombic phase, MnCrO4, isostructural with MCrO 4 (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH4)2Cr2O7 + 2 Mn(NO 3)2, followed by calcination at 400 C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO4 phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn2+Cr 6+O4. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at TN ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above T N. MnCrO4 is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 C to form first Cr 2O3 plus Mn3O4 and then Mn 1.5Cr1.5O4 spinel. © 2013 American Chemical Society
Preparation and characterization of metastable trigonal layered MSb2O6 phases (M = Co, Ni, Cu, Zn, and Mg) and considerations on FeSb2O6
MSb2O6 compounds (M = Mg, Co, Ni, Cu, Zn) are known in the tetragonal trirutile forms, slightly distorted monoclinically with M = Cu due to the Jahn-Teller effect. In this study, using a low-temperature exchange reaction between ilmenite-type NaSbO3 and molten MSO4-KCl (or MgCl2-KCl) mixtures, these five compositions were prepared for the first time as trigonal layered rosiaite (PbSb2O6)-type phases. Upon heating, they irreversibly transform to the known phases via amorphous intermediates, in contrast to previously studied isostructural MnSb2O6, where the stable phase is structurally related to the metastable phase. The same method was found to be applicable for preparing stable rosiaite-type CdSb2O6. The formula volumes of the new phases show an excellent correlation with the ionic radii (except for M = Cu, for which a Jahn-Teller distortion is suspected) and are 2-3% larger than those for the known forms although all coordination numbers are the same. The crystal structure of CoSb2O6 was refined via the Rietveld method: P31m, a = 5.1318(3) Å, and c = 4.5520(3) Å. Compounds with M = Co and Ni antiferromagnetically order at 11 and 15 K, respectively, whereas the copper compound does not show long-range magnetic order down to 1.5 K. A comparison between the magnetic behavior of the metastable and stable polymorphs was carried out. FeSb2O6 could not be prepared because of the 2Fe2+ + Sb5+ = 2Fe3+ + Sb3+ redox reaction. This electron transfer produces an additional 5s2 shell for Sb and results in a volume increase. A comparison of the formula volume for the stable mixture FeSbO4 + 0.5Sb2O4 with that extrapolated for FeSb2O6 predicted that the trirutile-type FeSb2O6 can be stabilized at high pressures. © 2017 The Royal Society of Chemistry.The work was supported by the Russian Foundation for Basic Research under the grant 14-03-01122. A. N. V. acknowledges the support in part from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST (no. K2-2016-066) and by Act 211 of the Government of Russian Federation, contract no. 02.A03.21.0006
Static and dynamic magnetic response of fragmented haldane-like spin chains in layered Li3Cu2SbO6
The structure and the magnetic properties of layered Li3Cu2SbO6 are investigated by powder X-ray diffraction, static susceptibility, and electron spin resonance studies up to 330 GHz. The XRD data experimentally verify the space group C2=m with halved unit cell volume in contrast to previously reported C2=c. In addition, the data show significant Li=Cuintersite exchange. Static magnetic susceptibility and ESR measurements show two magnetic contributions, i.e., quasifree spins at low-temperature and a spin-gapped magnetic subsystem, with about half of the spins being associated to each subsystem. The data suggest ferromagnetic-antiferromagnetic alternating chains with JFM = -285 K and JAFM = 160 K with a significant amount of Li-defects in the chains. The results are discussed in the scenario of fragmented 1D S = 1 AFM chains with a rather high defect concentration of about 17% and associated S =1=2 edge states of the resulting finite Haldane chains. © 2016 The Physical Society of Japan
Study of the process in the energy range = 1.07 -- 2 GeV
The cross section is measured in the center-of-mass
energy range from 1.07 to 2.00 GeV in the decay channel ,
. The data set with an integrated luminosity of 242
pb accumulated in the experiment with the SND detector at the VEPP-2000
collider is analyzed.Comment: 12 pages, 3 figures
Upgrade of the diagnostic neutral beam injector for the TCV tokamak
A diagnostic neutral beam injector (DNBI) [CRPP report LRP 710/01, CRPP-EPFL, 200 1; EPS Conf. Comr. Fusion Plasma Phys., 25A (2001) 365] has been installed on tokamak A configuration variable (TCV) [Plasma Phys. Control Fusion, 36 (1994) 13277; Plasma Phys. Control Fusion, 43 (200 1) A 16 1; Plasma Phys. Control Fusion, to be published] for the purpose of providing local measurements of plasma ion temperature, velocity and impurity density by Charge exchange recombination spectroscopy (CXRS) [EPS Conf. Contr. Fusion Plasma Phys., 25A (2001) 365]. The system recently underwent a technical upgrade, which allowed to increase the full neutral beam current density by a factor of two (from 0.5 to 1 A at 52 keV injection energy) and to extend the operational range of the diagnostic. This was achieved by means of a new, larger ion source, with an increased extraction area and corresponding enhancements of the power supplies. (C) 2003 Elsevier B.V. All rights reserved
Synthesis and Characterization of MnCrO<sub>4</sub>, a New Mixed-Valence Antiferromagnet
A new orthorhombic phase, MnCrO<sub>4</sub>, isostructural with MCrO<sub>4</sub> (M = Mg, Co, Ni, Cu,
Cd) was prepared by evaporation of an aqueous solution, (NH<sub>4</sub>)<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2 Mn(NO<sub>3</sub>)<sub>2</sub>, followed by calcination at 400 °C. It is characterized
by redox titration, Rietveld analysis of the X-ray diffraction pattern,
Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific
heat and resistivity measurements. In contrast to the high-pressure
MnCrO<sub>4</sub> phase where both cations are octahedral, the new
phase contains Cr in a tetrahedral environment suggesting the charge
balance Mn<sup>2+</sup>Cr<sup>6+</sup>O<sub>4</sub>. However, the
positions of both X-ray absorption K edges, the bond lengths and the
ESR data suggest the occurrence of some mixed-valence character in
which the mean oxidation state of Mn is higher than 2 and that of
Cr is lower than 6. Both the magnetic susceptibility and the specific
heat data indicate an onset of a three-dimensional antiferromagnetic
order at <i>T</i><sub>N</sub> ≈ 42 K, which was confirmed
also by calculating the spin exchange interactions on the basis of
first principles density functional calculations. Dynamic magnetic
studies (ESR) corroborate this scenario and indicate appreciable short-range
correlations at temperatures far above <i>T</i><sub>N</sub>. MnCrO<sub>4</sub> is a semiconductor with activation energy of
0.27 eV; it loses oxygen on heating above 400 °C to form first
Cr<sub>2</sub>O<sub>3</sub> plus Mn<sub>3</sub>O<sub>4</sub> and then
Mn<sub>1.5</sub>Cr<sub>1.5</sub>O<sub>4</sub> spinel
Synthesis and Characterization of MnCrO<sub>4</sub>, a New Mixed-Valence Antiferromagnet
A new orthorhombic phase, MnCrO<sub>4</sub>, isostructural with MCrO<sub>4</sub> (M = Mg, Co, Ni, Cu,
Cd) was prepared by evaporation of an aqueous solution, (NH<sub>4</sub>)<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2 Mn(NO<sub>3</sub>)<sub>2</sub>, followed by calcination at 400 °C. It is characterized
by redox titration, Rietveld analysis of the X-ray diffraction pattern,
Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific
heat and resistivity measurements. In contrast to the high-pressure
MnCrO<sub>4</sub> phase where both cations are octahedral, the new
phase contains Cr in a tetrahedral environment suggesting the charge
balance Mn<sup>2+</sup>Cr<sup>6+</sup>O<sub>4</sub>. However, the
positions of both X-ray absorption K edges, the bond lengths and the
ESR data suggest the occurrence of some mixed-valence character in
which the mean oxidation state of Mn is higher than 2 and that of
Cr is lower than 6. Both the magnetic susceptibility and the specific
heat data indicate an onset of a three-dimensional antiferromagnetic
order at <i>T</i><sub>N</sub> ≈ 42 K, which was confirmed
also by calculating the spin exchange interactions on the basis of
first principles density functional calculations. Dynamic magnetic
studies (ESR) corroborate this scenario and indicate appreciable short-range
correlations at temperatures far above <i>T</i><sub>N</sub>. MnCrO<sub>4</sub> is a semiconductor with activation energy of
0.27 eV; it loses oxygen on heating above 400 °C to form first
Cr<sub>2</sub>O<sub>3</sub> plus Mn<sub>3</sub>O<sub>4</sub> and then
Mn<sub>1.5</sub>Cr<sub>1.5</sub>O<sub>4</sub> spinel
New Phase of MnSb<sub>2</sub>O<sub>6</sub> Prepared by Ion Exchange: Structural, Magnetic, and Thermodynamic Properties
A new layered trigonal (<i>P</i>3̅1<i>m</i>) form of MnSb<sub>2</sub>O<sub>6</sub>, isostructural with MSb<sub>2</sub>O<sub>6</sub> (M = Cd, Ca, Sr,
Pb, and Ba) and MAs<sub>2</sub>O<sub>6</sub> (M = Mn, Co, Ni, and
Pd), was prepared by ion-exchange reaction between ilmenite-type NaSbO<sub>3</sub> and MnSO<sub>4</sub>–KCl–KBr melt at 470 °C.
It is characterized by Rietveld analysis of the X-ray diffraction
pattern, electron microprobe analysis, magnetic susceptibility, specific
heat, and ESR measurements as well as by density functional theory
calculations. MnSb<sub>2</sub>O<sub>6</sub> is very similar to MnAs<sub>2</sub>O<sub>6</sub> in the temperature dependence of their magnetic
susceptibility and spin exchange interactions. The magnetic susceptibility
and specific heat data show that MnSb<sub>2</sub>O<sub>6</sub> undergoes
a long-range antiferromagnetic order with Néel temperature <i>T</i><sub>N</sub> = 8.5(5) K. In addition, a weak ferromagnetic
component appears below <i>T</i><sub>1</sub> = 41.5(5) K.
DFT+U implies that the main spin exchange interactions are antiferromagnetic,
thereby forming spin-frustrated triangles. The long-range ordered
magnetic structure of MnSb<sub>2</sub>O<sub>6</sub> is predicted to
be incommensurate as found for MnAs<sub>2</sub>O<sub>6</sub>. On heating,
the new phase transforms to the stable <i>P</i>321 form
via its intermediate disordered variant