32 research outputs found
Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states
A gradual spin-state transition occurs in LaCoO3 around T~80-120 K, whose
detailed nature remains controversial. We studied this transition by means of
inelastic neutron scattering (INS), and found that with increasing temperature
an excitation at ~0.6 meV appears, whose intensity increases with temperature,
following the bulk magnetization. Within a model including crystal field
interaction and spin-orbit coupling we interpret this excitation as originating
from a transition between thermally excited states located about 120 K above
the ground state. We further discuss the nature of the magnetic excited state
in terms of intermediate-spin (IS, S=1) vs. high-spin (HS, S=2) states. Since
the g-factor obtained from the field dependence of the INS is g~3, the second
interpretation looks more plausible.Comment: 10 pages, 4 figure
Anomalous Magnetic Excitations of Cooperative Tetrahedral Spin Clusters
An inelastic neutron scattering study of Cu2Te2O5X2 (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above T-N, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2O5Cl2 and the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach
Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2
Neutron scattering studies on powder and single crystals have provided new
evidences for unconventional magnetism in Cu2Te2O5Cl2. The compound is built
from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice.
Magnetic ordering, emerging at TN=18.2 K, leads to a very complex multi-domain,
most likely degenerate, ground state, which is characterized by an
incommensurate (ICM) wave vector k ~ [0.15, 0.42,1/2]. The Cu2+ ions carry a
magnetic moment of 0.67(1) mB/ Cu2+ at 1.5 K and form a four helices spin
arrangement with two canted pairs within the tetrahedra. A domain
redistribution is observed when a magnetic field is applied in the tetragonal
plane (Hc≈0.5 T), but not for H||c up to 4 T. The excitation spectrum is
characterized by two well-defined modes, one completely dispersionless at 6.0
meV, the other strongly dispersing to a gap of 2 meV. The reason for such
complex ground state and spin excitations may be geometrical frustration of the
Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having
similar strengths and strong Dzyaloshinski-Moriya anisotropy. Candidates for
the dominant intra- and inter-tetrahedral interactions are proposed