Browsing by Author "Buechner, B."

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A new layered triangular antiferromagnet Li4FeSbO6: spin order, field-induced transitions and anomalous critical behavior
(2013)
Zvereva, E. A.
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Savelieva, O. A.
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Titov, Ya D.
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Evstigneeva, M. A.
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Nalbandyan, V. B.
Structure, electrochemical, magnetic and resonance properties of new layered antimonate Li4FeSbO6 were comprehensively studied using powder X-ray diffraction, cyclic voltammetry, magnetic susceptibility, heat capacity, electron spin resonance and Mossbauer spectroscopy. In the crystal structure the iron ions form the triangular network within (LiFeSbO6)(3-) layers alternating with nonmagnetic lithium layers. The electrochemical activity studied implies an Fe3+/Fe4+ redox couple at 4.3 V (ox.) and 3.9 V (red.) thereby revealing that Li can be reversibly extracted. The long-range antiferromagnetic order was found to occur at the Neel temperature, T-N approximate to 3.6 K, confirmed both by the magnetic susceptibility data and specific heat ones. The effective magnetic moment is estimated to be 5.93 mu(B)/f.u. and satisfactorily agrees with theoretical estimations assuming high-spin configuration of Fe3+ (S = 5/2). In the magnetically ordered state, though, the magnetization demonstrates rather peculiar behavior. An additional anomaly on the M(T) curves appears at T-2 < T-N in moderate magnetic field. The positions of transitions at T-N and T-2 separate increasingly with increasing external field. Multiple measurements consistently demonstrated field-sensitive moving of magnetic phase boundaries constituting a unique phase diagram for the compound under study. The complex low-dimensional (2D) nature of magnetic coupling was confirmed by the dynamic magnetic properties study. Electron spin resonance from Fe3+ ions in paramagnetic phase is characterized by a temperature independent effective g-factor of 1.99 +/- 0.01. However, the distortion and broadening of the ESR line were found to take place upon approaching the magnetically ordered state from above. The divergence of the temperature-dependent linewidth is analyzed in terms of both critical behavior close to long-range magnetic order and the Berezinskii-Kosterlitz-Thouless (BKT)-type transition. Heat capacity measurements even at zero field manifested an appearance of the additional anomaly at temperatures below the Neel temperature. The temperature dependence of ESR intensity, linewidth and shift of the resonant field imply an extended region of short-range order correlations in the compound studied. The rich variety of the anomalies in magnetic and resonance properties makes this new antimonate a very interesting system to investigate the multiple phase transitions and competing exchange interaction due to the critical role of the layered structure organization accompanied by the frustration effects in triangular antiferromagnets.
Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance
(2010-10-01)
Grafe, H. -J.
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Curro, N. J.
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Young, B. L.
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Vyalikh, A.
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Vavilova, J.
We report detailed O-17, La-139, and Cu-63,Cu-65 Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La1.875Ba0.125CuO4 single crystal and in oriented powder samples of La1.8-x Eu0.2Sr (x) CuO4. We observe a partial wipeout of the O-17 NMR intensity and a simultaneous drop of the O-17 electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the Cu-63,Cu-65 intensity is completely wiped out at the same temperature. The drop of the O-17 quadrupole frequency is compatible with a charge stripe order. The O-17 spin lattice relaxation rate shows a peak similar to that of the La-139, which is of magnetic origin. This peak is doping dependent and is maximal at x a parts per thousand 1/8.
Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance
(2010-10-01)
Grafe, H. -J.
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Curro, N. J.
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Young, B. L.
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Vyalikh, A.
;
Vavilova, J.
We report detailed (17)O, (139)La, and (63,65)Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La(1.875)Ba(0.125)CuO(4) single crystal and in oriented powder samples of La(1.8-x) Eu(0.2)Sr (x) CuO(4). We observe a partial wipeout of the (17)O NMR intensity and a simultaneous drop of the (17)O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the (63,65)Cu intensity is completely wiped out at the same temperature. The drop of the (17)O quadrupole frequency is compatible with a charge stripe order. The (17)O spin lattice relaxation rate shows a peak similar to that of the (139)La, which is of magnetic origin. This peak is doping dependent and is maximal at x a parts per thousand 1/8.
Charge-transfer energy in iridates: A hard x-ray photoelectron spectroscopy study
(2020-07-13)
Takegami, D.
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Kasinathan, D.
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Wolff, K. K.
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Altendorf, S. G.
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Chang, C. F.
We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir4+ or Ir5+ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight-binding calculations including only the Ir 5d, O 2p, and O 2s orbitals with parameters based on the downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the A(2)BIrO(6) iridates have essentially zero O 2p to Ir 5d charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.
Weak ferrimagnetism and multiple magnetization reversal in alpha-Cr(3)(PO(4))(2)
(2012-01-18)
Vasiliev, A. N.
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Volkova, O. S.
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Hammer, E.
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Glaum, R.
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Broto, J. -M.
The chromium(II) orthophosphate alpha-Cr(3)(PO(4))(2) is a weak ferrimagnet with the Curie temperature T(C) = 29 K confirmed by a lambda-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature Theta = 96 K, indicating an intermediate frustration ratio vertical bar Theta vertical bar/T(C) similar to 3. In its magnetically ordered states alpha-Cr(3)(PO(4))(2) exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e., either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 mu(B)/Cr(2+), as compared to the spin-only moment 4.9 mu(B)/Cr(2+), cannot be ascribed to the occurrence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d(4) Cr(2+) ions. X-ray absorption spectra at the K edge indicate divalent chromium and unravel the high-spin state of these ions at the L(2,3) edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr(2+) in its six crystallographically inequivalent positions.
Weak ferrimagnetism and multiple magnetization reversal in alpha-Cr-3(PO4)(2)
(2012-01-18)
Vasiliev, A. N.
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Volkova, O. S.
;
Hammer, E.
;
Glaum, R.
;
Broto, J. -M.
The chromium(II) orthophosphate alpha-Cr-3(PO4)(2) is a weak ferrimagnet with the Curie temperature T-C = 29 K confirmed by a lambda-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature Theta = 96 K, indicating an intermediate frustration ratio vertical bar Theta vertical bar/T-C similar to 3. In its magnetically ordered states alpha-Cr-3(PO4)(2) exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e., either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 mu(B)/Cr2+, as compared to the spin-only moment 4.9 mu(B)/Cr2+, cannot be ascribed to the occurrence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d(4) Cr2+ ions. X-ray absorption spectra at the K edge indicate divalent chromium and unravel the high-spin state of these ions at the L-2,L-3 edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr2+ in its six crystallographically inequivalent positions.