11 research outputs found
Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction
High real-space-resolution atomic pair distribution functions of
La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using
high-energy x-ray powder diffraction to study the size and shape of the MnO_6
octahedron as a function of temperature and doping. In the paramagnetic
insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95
and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long
bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and
T=20K, we find a single Mn-O bond-length; however, as the metal-insulator
transition is approached either by increasing T or decreasing x, intensity
progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the
appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong
evidence that charge localized and delocalized phases coexist close to the
metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.
Structural response to O*-O' and magnetic transitions in orthorhombic perovskites
We present a temperature dependent single crystal x-ray diffraction study of
twinned orthorhombic perovskites La1-xCaxMnO3, for x=0.16 and x=0.25. These
data show the evolution of the crystal structure from the ferromagnetic
insulating state to the ferromagnetic metallic state. The data are modelled in
space group Pnma with twin relations based on a distribution of the b axis over
three perpendicular cubic axes. The twin model allows full structure
determination in the presence of up to six twin fractions using the single
crystal x-ray diffraction data.Comment: 13 pages, including 13 figures and 2 table
Exchange coupling in CaMnO and LaMnO: configuration interaction and the coupling mechanism
The equilibrium structure and exchange constants of CaMnO and LaMnO
have been investigated using total energy unrestricted Hartree-Fock (UHF) and
localised orbital configuration interaction (CI) calculations on the bulk
compounds and MnO and MnO clusters. The
predicted structure and exchange constants for CaMnO are in reasonable
agreement with estimates based on its N\'eel temperature. A series of
calculations on LaMnO in the cubic perovskite structure shows that a
Hamiltonian with independent orbital ordering and exchange terms accounts for
the total energies of cubic LaMnO with various spin and orbital orderings.
Computed exchange constants depend on orbital ordering. UHF calculations tend
to underestimate exchange constants in LaMnO, but have the correct sign
when compared with values obtained by neutron scattering; exchange constants
obtained from CI calculations are in good agreement with neutron scattering
data provided the Madelung potential of the cluster is appropriate. Cluster CI
calculations reveal a strong dependence of exchange constants on Mn d e
orbital populations in both compounds. CI wave functions are analysed in order
to determine which exchange processes are important in exchange coupling in
CaMnO and LaMnO.Comment: 25 pages and 9 postscript figure
Specific heat and magnetic order in LaMnO_{3+\delta}
Magnetic and specific-heat measurements are performed in three different
samples of LaMnO_{3+\delta}, with \delta=0.11, 0.15 and 0.26, presenting
important disorder effects, such as carrier localization, due to high amounts
of La and Mn vacancies. For the samples with \delta =0.11 and 0.15, magnetic
measurements show signatures of a two-step transition: as the temperature is
lowered, the system enters a ferromagnetic phase followed by a disorder-induced
cluster-glass state. Spin-wave-like contributions and an unexpected large
linear term are observed in the specific heat as a function of temperature. In
the sample with the highest vacancy content, \delta=0.26, the disorder is
sufficient to suppress even short-range ferromagnetic order and yield a
spin-glass-like state.Comment: RevTeX 2-col, 8 pages, 5 ps figures included, submitted to PR