36 research outputs found
Spin wave resonances in La_{0.7}Sr_{0.3}MnO_{3} films: measurement of spin wave stiffness and anisotropy field
We studied magnetic field dependent microwave absorption in epitaxial
LaSrMnO films using an X-band Bruker ESR spectrometer. By
analyzing angular and temperature dependence of the ferromagnetic and spin-wave
resonances we determine spin-wave stiffness and anisotropy field. The spin-wave
stiffness as found from the spectrum of the standing spin-wave resonances in
thin films is in fair agreement with the results of inelastic neutron
scattering studies on a single crystal of the same composition [Vasiliu-Doloc
et al., J. Appl. Phys. \textbf{83}, 7343 (1998)].Comment: 15 pages, 7 figures (now figure captions are included
Cubic anisotropy in high homogeneity thin (Ga,Mn)As layers
Historically, comprehensive studies of dilute ferromagnetic semiconductors,
e.g., -type (Cd,Mn)Te and (Ga,Mn)As, paved the way for a quantitative
theoretical description of effects associated with spin-orbit interactions in
solids, such as crystalline magnetic anisotropy. In particular, the theory was
successful in explaining {\em uniaxial} magnetic anisotropies associated with
biaxial strain and non-random formation of magnetic dimers in epitaxial
(Ga,Mn)As layers. However, the situation appears much less settled in the case
of the {\em cubic} term: the theory predicts switchings of the easy axis
between in-plane and directions as a
function of the hole concentration, whereas only the
orientation has been found experimentally. Here, we report on the observation
of such switchings by magnetization and ferromagnetic resonance studies on a
series of high-crystalline quality (Ga,Mn)As films. We describe our findings by
the mean-field - Zener model augmented with three new ingredients. The
first one is a scattering broadening of the hole density of states, which
reduces significantly the amplitude of the alternating carrier-induced
contribution. This opens the way for the two other ingredients, namely the
so-far disregarded single-ion magnetic anisotropy and disorder-driven
non-uniformities of the carrier density, both favoring the
direction of the apparent easy axis. However, according to our results, when
the disorder gets reduced a switching to the orientation
is possible in a certain temperature and hole concentration range.Comment: 12 pages, 9 figure
Microstructural magnetic phases in superconducting FeTe0.65Se0.35
In this paper, we address a number of outstanding issues concerning the
nature and the role of magnetic inhomogenities in the iron chalcogenide system
FeTe1-xSex and their correlation with superconductivity in this system. We
report morphology of superconducting single crystals of FeTe0.65Se0.35 studied
with transmission electron microscopy, high angle annular dark field scanning
transmission electron microscopy and their magnetic and superconducting
properties characterized with magnetization, specific heat and magnetic
resonance spectroscopy. Our data demonstrate a presence of nanometre scale
hexagonal regions coexisting with tetragonal host lattice, a chemical disorder
demonstrating non homogeneous distribution of host atoms in the crystal
lattice, as well as hundreds-of-nanometres-long iron-deficient bands. From
magnetic data and ferromagnetic resonance temperature dependence, we attribute
magnetic phases in Fe-Te-Se to Fe3O4 inclusions and to hexagonal symmetry
nanometre scale regions with structure of Fe7Se8 type. Our results suggest that
nonhomogeneous distribution of host atoms might be an intrinsic feature of
superconducting Fe-Te-Se chalcogenides and we find a surprising correlation
indicating that faster grown crystal of inferior crystallographic properties is
a better superconductor.Comment: 16 pages, 8 figures, 2 table
Diluted magnetic layered semiconductor InSe:Mn with high Curie temperature
We present a detailed study of layered semiconductor InSe doped with Mn. Xray
and neutron diffraction analyses of (In,Mn)Se single crystals show the presence of a
main phase as In₁−xMnxSe solid solution, the second antiferromagnetic MnSe phase,
and traces of In₄ Se₃ Magnetic measurements reveal ferromagnetic behavior of (In,Mn)Se with the Curie temperature about 800 K. The ferromagnetic cluster model and exchange interaction via 2D electron gas, as the reasons of spontaneous magnetization, are discussed. The dramatic transformation of (In,Mn)Se electron spin resonance (ESR) spectra as a function of temperature is revealed. At the magnetic field perpendicular to crystallographic c axis, a low-field line within the temperature range 70 down to 4.7 K is observed. It shifts to smaller magnetic fields with temperature decrease. Neutron diffraction studies reveal the strong rise for one of the reflection peaks with temperature decrease in the same temperature region where ESR spectra transformation occurs. This peak corresponds to double MnSe interplanar distance in the [111] direction what is a period of its magnetic lattice. Magnetic structure of (In,Mn)Se single crystal is discussed
EPR in Kagome Staircase Compound Mg 2.997
The paramagnetic resonance of Co ions in MgCoVO single crystals are reported. The EPR spectrum shows two groups of resonance lines associated with two crystallographically nonequivalent Co ions positions that are known in the kagome staircase system as "cross-tie" and "spine" sites. No preferential occupation of Co ions in the kagome lattice was observed. The ground state was described by spin-Hamiltonian with an effective electronic spin S=1/2 and nuclear spin I=7/2. The local symmetry of oxygen octahedron surrounding Co ions, main values of the g-factors, and hyperfine structure were determined for both cobalt positions
Magnon Excitations in Manganites
A review of experimental studies of magnon excitations in manganites is presented. Two kinds of techniques: the inelastic neutron scattering (in bulk materials) and the microwave resonance (in thin films) are considered. Experimental studies of spin dynamics by inelastic neutron scattering in metallic ferromagnetic manganites have shown that at low temperature for small wave vectors k → 0 the dispersion relation has a quadratic shape similar to that observed in Heisenberg ferromagnets. However, the above technique although very informative can be used only for sufficiently large samples of bulk materials. A complementary microwave resonance technique allows studying not only bulk properties, but also surface properties. There are two main theoretical approaches used to interpret the spin wave resonance spectra: the volume inhomogeneity and the surface inhomogeneity models. The last one introduced by Puszkarski has allowed for interpretation of the observed surface magnon excitations in thin films