734 research outputs found
Confined spin waves reveal an assembly of nanosize domains in ferromagnetic La(1-x)CaxMnO3 (x=0.17,0.2)
We report a study of spin-waves in ferromagnetic LaCaMnO,
at concentrations x=0.17 and x=0.2 very close to the metallic transition
(x=0.225). Below T, in the quasi-metallic state (T=150K), nearly
q-independent energy levels are observed. They are characteristic of standing
spin waves confined into finite-size ferromagnetic domains, defined in {\bf a,
b) plane for x=0.17 and in all q-directions for x=0.2. They allow an estimation
of the domain size, a few lattice spacings, and of the magnetic coupling
constants inside the domains. These constants, anisotropic, are typical of an
orbital-ordered state, allowing to characterize the domains as "hole-poor". The
precursor state of the CMR metallic phase appears, therefore, as an assembly of
small orbital-ordered domains.Comment: 4 pages, 5 figure
Spin glass freezing and superconductivity in YBa2(Cu(1-x)Fe(x))3O7 alloys
The dynamics were studied of the iron spins in superconducting YBa2(Cu(0.94)Fe(0.06))3O7 by neutron time of flight measurements. Two samples were studied with slightly different characteristics, as shown by resistivity and neutron diffraction measurements. The same dynamical anomalies are observed by neutrons in both samples. Differences appear qualitative but not quantitative. In the whole temperature range, the q-dependence of the magnetic intensity mainly reflects the magnetic form factor of iron which shows that the iron spins are almost uncorrelated. The elastic and quasielastic intensities strongly vary with temperature. A spin glass like freezing is revealed at low temperature by a sharp decrease of the quasielastic intensity, an increase of the 'elastic' or resolution limited intensity and a minimum in the quasielastic width. The freezing temperature (T sub f - 18 K) corresponds to that already determined by a magnetic splitting in Mossbauer experiments. Above T sub f, the relaxation of the iron spins in the paramagnetic state is modified by the occurrence of superconductivity. An increase was observed of the quasielastic intensity and of the quasielastic width at the superconducting transition
Inhomogeneous Magnetism in La-doped CaMnO3. (II) Mesoscopic Phase Separation due to Lattice-coupled FM Interactions
A detailed investigation of mesoscopic magnetic and crystallographic phase
separation in Ca(1-x)La(x)MnO3, 0.00<=x<=0.20, is reported. Neutron powder
diffraction and DC-magnetization techniques have been used to isolate the
different roles played by electrons doped into the eg level as a function of
their concentration x. The presence of multiple low-temperature magnetic and
crystallographic phases within individual polycrystalline samples is argued to
be an intrinsic feature of the system that follows from the shifting balance
between competing FM and AFM interactions as a function of temperature. FM
double-exchange interactions associated with doped eg electrons are favored
over competing AFM interactions at higher temperatures, and couple more
strongly with the lattice via orbital polarization. These FM interactions
thereby play a privileged role, even at low eg electron concentrations, by
virtue of structural modifications induced above the AFM transition
temperatures.Comment: 8 pages, 7 figure
Dynamics of the iron spins in superconducting YBa2(Cu(1-x)Fe(x))O7
The dynamics of the iron spins in YBa2(Cu(1-x)Fe(x))3O7 alloys (0 = to or less than 0.12) was studied by the means of inelastic neutron scattering. Measurements were performed using the time of flight technique with an excellent resolution of 50 micro eV, in a temperature range of 1.8 to 300 K. The doped samples show an elastic and a quasielastic intensity strongly varying with temperature. A spin glass like freezing is revealed at low temperature by a sudden decrease of the quasielastic intensity, an increase of the elastic or resolution limited intensity and a minimum in the quasielastic width. The freezing temperature corresponds to the one already determinated by a magnetic splitting in Mossbauer experiments. Above freezing, the occurrence of superconductivity slightly modifies the characteristics of the spin relaxation in the paramagnetic state, as shown by measurements in two x = 0.06 samples. In the whole temperature range of measurement, the dependence of the quasielastic intensity with the scattering vector q, mainly reflects the variation of the Iron form factor, which shows that the spins are almost uncorrelated
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