186 research outputs found
B NMR detection of the magnetic field distribution in the mixed superconducting state of MgB
The temperature dependence of the magnetic field distribution in the mixed
superconducting phase of randomly oriented MgB powder was probed by B NMR spectroscopy. Below the temperature of the second critical () field, K, our spectra reveal two NMR signal
components, one mapping the magnetic field distribution in the mixed
superconducting state and the other one arising from the normal state. The
complementary use of bulk magnetization and NMR measurements reveals that
MgB is an anisotropic superconductor with a Tesla
anisotropy parameter
B and Al NMR spin-lattice relaxation and Knight shift study of MgAlB. Evidence for anisotropic Fermi surface
We report a detailed study of B and Al NMR spin-lattice
relaxation rates (), as well as of Al Knight shift (K) of
MgAlB, . The obtained () and K vs. x
plots are in excellent agreement with ab initio calculations. This asserts
experimentally the prediction that the Fermi surface is highly anisotropic,
consisting mainly of hole-type 2-D cylindrical sheets from bonding
boron orbitals. It is also shown that the density of states at the Fermi level
decreases sharply on Al doping and the 2-D sheets collapse at ,
where the superconductive phase disappears
1H NMR investigation of the magnetic spin configuration in the molecule-based ferrimagnet [MnTFPP][TCNE]
Journal Article1H NMR line-shape measurements have been performed in the linear chain molecule-based ferrimagnet [MnTFPP][TCNE].xPhMe (TFPP=tetrakis(4-fluorophenyl)porphinato; TCNE=tetracyanoethylene) as a function of temperature. Hyperfine shifts of opposite sign were observed indicating the presence of two oppositely hyperfine couplings, predominantly between the Mn ions and the 1H nuclei of the planar ring of the [MnTFPP]+ unit. The observation of the hyperfine shifts strongly suggests the existence of two oppositely directed [MnTFPP]+ sites. Since there is no direct exchange coupling between adjacent [MnTFPP] spins, then it can be argued that an ordered spin configuration with antiparallel components may arise from antisymmetric spin-spin exchange interactions. The field independence of the hyperfine shifts at low temperatures and for applied magnetic fields in the range 2-5 T, indicates the presence of ferromagnetic interactions in the ground state of the ferrimagnetic chain
139La NMR evidence for phase solitons in the ground state of overdoped manganites
Hole doped transition metal oxides are famous due to their extraordinary
charge transport properties, such as high temperature superconductivity
(cuprates) and colossal magnetoresistance (manganites). Astonishing, the mother
system of these compounds is a Mott insulator, whereas important role in the
establishment of the metallic or superconducting state is played by the way
that holes are self-organized with doping. Experiments have shown that by
adding holes the insulating phase breaks into antiferromagnetic (AFM) regions,
which are separated by hole rich clumps (stripes) with a rapid change of the
phase of the background spins and orbitals. However, recent experiments in
overdoped manganites of the La(1-x)Ca(x)MnO(3) (LCMO) family have shown that
instead of charge stripes, charge in these systems is organized in a uniform
charge density wave (CDW). Besides, recent theoretical works predicted that the
ground state is inhomogeneously modulated by orbital and charge solitons, i.e.
narrow regions carrying charge (+/-)e/2, where the orbital arrangement varies
very rapidly. So far, this has been only a theoretical prediction. Here, by
using 139La Nuclear Magnetic Resonance (NMR) we provide direct evidence that
the ground state of overdoped LCMO is indeed solitonic. By lowering temperature
the narrow NMR spectra observed in the AFM phase are shown to wipe out, while
for T<30K a very broad spectrum reappears, characteristic of an incommensurate
(IC) charge and spin modulation. Remarkably, by further decreasing temperature,
a relatively narrow feature emerges from the broad IC NMR signal, manifesting
the formation of a solitonic modulation as T->0.Comment: 5 pages, 4 figure
Orbital domain state and finite size scaling in ferromagnetic insulating manganites
55Mn and 139La NMR measurements on a high quality single crystal of
ferromagnetic (FM) La0.80Ca20MnO3 demonstrate the formation of localized
Mn(3+,4+) states below 70 K, accompanied with strong anomalous increase of
certain FM neutron Bragg peaks. (55,139)(1/T1) spin-lattice relaxation rates
diverge on approaching this temperature from below, signalling a genuine phase
transition at T(tr) approx. 70 K. The increased local magnetic anisotropy of
the low temperature phase, the cooling-rate dependence of the Bragg peaks, and
the observed finite size scaling of T(tr) with Ca (hole) doping, are suggestive
of freezing into an orbital domain state, precursor to a phase transition into
an inhomogeneous orbitally ordered state embodying hole-rich walls.Comment: 4 pages, 4 figure
Spin-polarized oxygen hole states in cation deficient La(1-x)CaxMnO(3+delta)
When holes are doped into a Mott-Hubbard type insulator, like lightly doped
manganites of the La(1-x)CaxMnO3 family, the cooperative Jahn-Teller
distortions and the appearance of orbital ordering require an arrangement of
Mn(3+)/Mn(4+) for the establishment of the insulating canted antiferromagnetic
(for x<=0.1), or of the insulating ferromagnetic (for 0.1<x<= 0.2) ground
state. In the present work we provide NMR evidence about a novel and at the
same time puzzling effect in La(1-x)CaxMnO(3+delta) systems with cation
deficience. We show that in the low Ca-doping regime, these systems exhibit a
very strong hyperfine field at certain La nuclear sites, which is not present
in the stoichiometric compounds. Comparison of our NMR results with recent
x-ray absorption data at the Mn K edge, suggests the formation of a
spin-polarized hole arrangement on the 2p oxygen orbitals as the origin of this
effect.Comment: 10 pages, 4 Figures, submitted to PR
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