49 research outputs found
Statics and dynamics of an incommensurate spin order in a geometrically frustrated antiferromagnet CdCrO
Using elastic and inelastic neutron scattering we show that a cubic spinel,
CdCrO, undergoes an elongation along the c-axis () at its
spin-Peierls-like phase transition at = 7.8 K. The N\'{e}el phase () has an incommesurate spin structure with a characteristic wave vector
\textbf{Q} = (0,,1) with 0.09 and with spins lying on
the -plane. This is in stark contrast to another well-known Cr-based
spinel, ZnCrO, that undergoes a c-axis contraction and a commensurate
spin order. The magnetic excitations of the incommensurate N\'{e}el state has a
weak anisotropy gap of 0.6 meV and it consists of at least three bands
extending up to 5 meV.Comment: PRL in pres
Spin-lattice instability to a fractional magnetization state in the spinel HgCr2O4
Magnetic systems are fertile ground for the emergence of exotic states when
the magnetic interactions cannot be satisfied simultaneously due to the
topology of the lattice - a situation known as geometrical frustration.
Spinels, AB2O4, can realize the most highly frustrated network of
corner-sharing tetrahedra. Several novel states have been discovered in
spinels, such as composite spin clusters and novel charge-ordered states. Here
we use neutron and synchrotron X-ray scattering to characterize the fractional
magnetization state of HgCr2O4 under an external magnetic field, H. When the
field is applied in its Neel ground state, a phase transition occurs at H ~ 10
Tesla at which each tetrahedron changes from a canted Neel state to a
fractional spin state with the total spin, Stet, of S/2 and the lattice
undergoes orthorhombic to cubic symmetry change. Our results provide the
microscopic one-to-one correspondence between the spin state and the lattice
distortion
Local spin resonance and spin-Peierls-like phase transition in a geometrically frustrated antiferromagnet
Using inelastic magnetic neutron scattering we have discovered a localized
spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated
cubic antiferromagnet . The resonance develops abruptly from
quantum critical fluctuations upon cooling through a first order transition to
a co-planar antiferromagnet at K. We argue that this transition
is a three dimensional analogue of the spin-Peierls transition.Comment: 4 figures, revised and accepted in Phys. Rev. Let
Origin and pressure dependence of ferromagnetism in A2Mn2O7 pyrochlores (A=Y, In, Lu, and Tl)
Non-conventional mechanisms have been recently invoked in order to explain
the ferromagnetic ground state of A2Mn2O7 pyrochlores (A=Y, In, Lu and Tl) and
the puzzling decrease of their Curie temperatures with applied pressure. Here
we show, using a perturbation expansion in the Mn-O hopping term, that both
features can be understood within the superexhange model, provided that the
intra-atomic oxygen interactions are properly taken into account. An additional
coupling between the Mn ions mediated by the In(5s)/Tl(6s) bands yields the
higher Tc's of these two compounds, this mechamism enhancing their
ferromagnetism for higher pressures.Comment: 7 pages and 2 figures submitted to Phys. Rev. B, missing text adde
Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr_(9p)Ga_(12-9p)O_(19)
We present an extensive gallium NMR study of the geometrically frustrated
kagome bi-layer compound SrCr_(9p)Ga_(12-9p)O_(19) (Cr^3+, S=3/2) over a broad
Cr-concentration range (.72<p<.95). This allows us to probe locally the kagome
bi-layer susceptibility and separate the intrinsic properties due to the
geometric frustration from those related to the site dilution. Our major
findings are: 1) The intrinsic kagome bi-layer susceptibility exhibits a
maximum in temperature at 40-50 K and is robust to a dilution as high as ~20%.
The maximum reveals the development of short range antiferromagnetic
correlations; 2) At low-T, a highly dynamical state induces a strong wipe-out
of the NMR intensity, regardless of dilution; 3) The low-T upturn observed in
the macroscopic susceptibility is associated to paramagnetic defects which stem
from the dilution of the kagome bi-layer. The low-T analysis of the NMR
lineshape suggests that the defect can be associated with a staggered
spin-response to the vacancies on the kagome bi-layer. This, altogether with
the maximum in the kagome bi-layer susceptibility, is very similar to what is
observed in most low-dimensional antiferromagnetic correlated systems; 4) The
spin glass-like freezing observed at T_g=2-4 K is not driven by the
dilution-induced defects.Comment: 19 pages, 19 figures, revised version resubmitted to PRB Minor
modifications: Fig.11 and discussion in Sec.V on the NMR shif