53 research outputs found
Ising magnetism and ferroelectricity in CaCoMnO
The origin of both the Ising chain magnetism and ferroelectricity in
CaCoMnO is studied by electronic structure calculations
and x-ray absorption spectroscopy. We find that CaCoMnO has the
alternate trigonal prismatic Co and octahedral Mn sites in the
spin chain. Both the Co and Mn are in the high spin state. In
addition, the Co has a huge orbital moment of 1.7 which is
responsible for the significant Ising magnetism. The centrosymmetric crystal
structure known so far is calculated to be unstable with respect to exchange
striction in the experimentally observed
antiferromagnetic structure for the Ising chain. The calculated inequivalence
of the Co-Mn distances accounts for the ferroelectricity.Comment: 4 pages, 3 figures, PRL in press (changes made upon referees
comments
The spin state transition in LaCoO; revising a revision
Using soft x-ray absorption spectroscopy and magnetic circular dichroism at
the Co- edge we reveal that the spin state transition in LaCoO
can be well described by a low-spin ground state and a triply-degenerate
high-spin first excited state. From the temperature dependence of the spectral
lineshapes we find that LaCoO at finite temperatures is an inhomogeneous
mixed-spin-state system. Crucial is that the magnetic circular dichroism signal
in the paramagnetic state carries a large orbital momentum. This directly shows
that the currently accepted low-/intermediate-spin picture is at variance.
Parameters derived from these spectroscopies fully explain existing magnetic
susceptibility, electron spin resonance and inelastic neutron data
Orbital order in La0.5Sr1.5MnO4: beyond a common local Jahn-Teller picture
The standard way to find the orbital occupation of Jahn-Teller (JT) ions is
to use structural data, with the assumption of a one-to-one correspondence
between the orbital occupation and the associated JT distortion, e.g. in O6
octahedron. We show, however, that this approach in principle does not work for
layered systems. Specifically, using the layered manganite La0.5Sr1.5MnO4 as an
example, we found from our x-ray absorption measurements and theoretical
calculations, that the type of orbital ordering strongly contradicts the
standard local distortion approach for the Mn3+O6 octahedra, and that the
generally ignored long-range crystal field effect and anisotropic hopping
integrals are actually crucial to determine the orbital occupation. Our
findings may open a pathway to control of the orbital state in multilayer
systems and thus of their physical properties.Comment: 4+ pages, 4 figure
Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4
Using Co-L_(2,3) and O-K x-ray absorption spectroscopy, we reveal that the
charge ordering in La_(1.5)Sr_(0.5)CoO4 involves high spin (S=3/2) Co^2+ and
low spin (S=0) Co^3+ ions. This provides evidence for the spin blockade
phenomenon as a source for the extremely insulating nature of the
La_(2-x)Sr_(x)CoO4 series. The associated e_g^2 and e_g^0 orbital occupation
accounts for the large contrast in the Co-O bond lengths, and in turn, the high
charge ordering temperature. Yet, the low magnetic ordering temperature is
naturally explained by the presence of the non-magnetic (S=0) Co^3+ ions. From
the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow
band material.Comment: 5 pages, 3 figure
Local electronic structure and magnetic properties of LaMn0.5Co0.5O3 studied by x-ray absorption and magnetic circular dichroism spectroscopy
We have studied the local electronic structure of LaMn0.5Co0.5O3 using
soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found
a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie
temperature. We discovered that samples with lower Curie temperatures contain
low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism
we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned.
We revealed also that the Co^{2+} ions have a large orbital moment:
m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the
presence of a large magnetocrystalline anisotropy in the material and predicts
a non-trivial temperature dependence for the magnetic susceptibility.Comment: 8 pages, 7 figure
Valence, spin, and orbital state of the Co ions in the one-dimensional Ca3Co2O6: an x-ray absorption and magnetic circular dichroism study
We have investigated the valence, spin, and orbital state of the Co ions in
the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray
magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the
octahedral Co_oct and trigonal Co_trig sites are found to be in a 3+ state.
From the analysis of the dichroism we established a low-spin state for the
Co_oct and a high-spin state with an anomalously large orbital moment of 1.7
muB at the Co3+ trig ions. This large orbital moment along the c-axis chain and
the unusually large magnetocrystalline anisotropy can be traced back to the
double occupancy of the d2 orbital in trigonal crystal field.Comment: 5 pages, 4 figure
Orbitally driven spin-singlet dimerization in =1 LaRuO
Using x-ray absorption spectroscopy at the Ru- edge we reveal that
the Ru ions remain in the =1 spin state across the rare 4d-orbital
ordering transition and spin-gap formation. We find using local spin density
approximation + Hubbard U (LSDA+U) band structure calculations that the crystal
fields in the low temperature phase are not strong enough to stabilize the
=0 state. Instead, we identify a distinct orbital ordering with a
significant anisotropy of the antiferromagnetic exchange couplings. We conclude
that LaRuO appears to be a novel material in which the
orbital physics drives the formation of spin-singlet dimers in a quasi
2-dimensional =1 system.Comment: 5 pages, 4 figures, and 1 tabl
Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7
Sr3(Ru1-xMnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn,
is studied by x-ray dichroism and spin-resolved density functional theory. We
find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+
acceptors; the extra eg electron occupies the in-plane 3dx2-y2 orbital instead
of the expected out-of-plane 3d3z2-r2. We propose that the 3d-4d interplay, via
the ligand oxygen orbitals, is responsible for this crystal-field level
inversion and the material's transition to an antiferromagnetic, possibly
orbitally-ordered, low-temperature state.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/MnSr3Ru2O7_XAS.pd
X-ray absorption and x-ray magnetic dichroism study on Ca3CoRhO6 and Ca3FeRhO6
Using x-ray absorption spectroscopy at the Rh-L_2,3, Co-L_2,3, and Fe-L_2,3
edges, we find a valence state of Co^2+/Rh^4+ in Ca3CoRhO6 and of Fe^3+/Rh^3+
in Ca3FeRhO6. X-ray magnetic circular dichroism spectroscopy at the Co-L_2,3
edge of Ca3CoRhO6 reveals a giant orbital moment of about 1.7mu_B, which can be
attributed to the occupation of the minority-spin d_0d_2 orbital state of the
high-spin Co^2+ (3d^7) ions in trigonal prismatic coordination. This active
role of the spin-orbit coupling explains the strong magnetocrystalline
anisotropy and Ising-like magnetism of Ca3CoRhO6.Comment: 7 pages, 6 figure
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