232 research outputs found
Charge, Orbital and Magnetic Order in Nd0.5Ca0.5MnO3
In the manganite Nd0.5Ca0.5MnO3, charge ordering occurs at much higher
temperature than the antiferromagnetic order (TCO=250K,TN=160K).
The magnetic behavior of the phase TN<T<TCO is puzzling: its magnetization
and susceptibility are typical of an antiferromagnet while no magnetic order is
detected by neutron diffraction.We have undertaken an extensive study of the
cristallographic, electric and magnetic properties of Nd0.5Ca0.5MnO3 and
established its phase diagram as a function of temperature and magnetic field.
The charge disordered, paramagnetic phase above TCO present ferromagnetic
correlations. An antiferromagnetic CE phase prevails below TN, with complete
charge and orbital ordering. In the intermediate temperature range, charge
ordering occurs while orbital ordering sets in progressively, with no magnetic
order. Strong magnetic fields destroy the charge ordered phases in a fisrt
order transition towards a ferromagnetic state.Comment: 17 pages, 11 Figures to appear in Phys. Rev.
Stress deformations and structural quenching in Sm0.5Ca0.5MnO3 thin films allow a huge decrease of the charge order melting magnetic field
Thin films of Sm0.5Ca0.5MnO3 manganites with charge ordering (CO) properties
and colossal magnetoresistance were synthesized by pulsed laser deposition
technique on (100)-SrTiO3 and (100)-LaAlO3 substrates. We first compare the
structural modifications as function of the substrate and film thickness.
Secondly, measuring transport properties in magnetic fields up to 24T, we
establish the temperature-field phase diagram describing the stability of the
CO state and compare it to bulk material. We show that some structural
modification induced by the substrate occurs and that the CO melting magnetic
field is greatly reduced. Moreover, with the temperature decrease, no
modification of the lattice parameters is observed. We then propose an
explanation based on the quenching of the unit cell of the film that adopts the
in-plane lattice parameters of the substrate and thus, prevents the complete
growth of the CO state at low temperature.Comment: to be published in Journal of Applied Physic
Fluctuations of g-factors in metal nanoparticles: Effects of electron-electron interaction and spin-orbit scattering
We investigate the combined effect of spin-orbit scattering and
electron-electron interactions on the probability distribution of -factors
of metal nanoparticles. Using random matrix theory, we find that even a
relatively small interaction strength %(ratio of exchange constant and mean
level %spacing \spacing ) significantly increases -factor
fluctuations for not-too-strong spin-orbit scattering (ratio of spin-orbit rate
and single-electron level spacing 1/\tau_{\rm so} \spacing \lesssim 1), and
leads to the possibility to observe -factors larger than two.Comment: RevTex, 2 figures inserte
Spin excitations in the antiferromagnet NaNiO2
In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S =
1=2 spins. The magnetic order observed below 20K has been described as an A
type antiferromagnet with ferro- magnetic layers weakly coupled
antiferromagnetically. We studied the magnetic excitations with the electron
spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The
bulk of the results are interpreted in terms of a phenomenological model
involving bi-axial anisotropy for the spins: a strong easy-plane term, and a
weaker anisotropy within the plane. The direction of the easy plane is
constrained by the collective Jahn-Teller distortion occurring in this material
at 480 K
Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4
In both spinel compounds GeCoO and GeNiO which order
antiferromagnetically (at and , ) with different Curie Weiss temperatures (=80.5 K and -15 K),
the usual magnetic frustration criterion is not fulfilled.
Using neutron powder diffraction and magnetization measurements up to 55 T,
both compounds are found with a close magnetic ground state at low temperature
and a similar magnetic behavior (but with a different energy scale), even
though spin anisotropy and first neighbor exchange interactions are quite
different. This magnetic behavior can be understood when considering the main
four magnetic exchange interactions. Frustration mechanisms are then
enlightened.Comment: submitted to Phys.Rev.B (2006
Influence of Nd on the magnetic properties of Nd1-xCaxMnO3
The role played by the Nd ions in the magnetic properties of Nd0.5Ca0.5MnO3
and Nd0.7Ca0.3MnO3 is studied using static magnetization, neutron diffraction
and high frequency (9.4-475GHz) Electron Spin Resonance. We show that the Nd
ions are weakly coupled to the Mn ions via ferromagnetic exchange and are
responsible for the peculiar ferromagnetic resonance observed in the FM phase
of both compounds (ground state below 120K for x=0.3, high field state for
x=0.5). We then use ESR to look for magnetic phase separation in the low field,
CO phase of Nd0.5Ca0.5MnO3. We show that there is no trace of the FM phase
imbedded in the CO phase, contrary to what is observed in La0.5Ca0.5MnO3 or
Pr0.5Sr0.5MnO3.Comment: to be published in phys.Rev.B as a Rapid Com
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