51 research outputs found
^{7}Li NMR Study of Heavy Fermion LiV2O4 Containing Magnetic Defects
We present a systematic study of the variations of the ^{7}Li NMR properties
versus magnetic defect concentration up to 0.83 mol% within the spinel
structure of polycrystalline powder samples and a collection of small single
crystals of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report
static magnetization measurements and ac magnetic susceptibility measurements
at 14 MHz on the samples at low temperatures. Both the NMR spectrum and nuclear
spin-lattice relaxation rate are inhomogeneous in the presence of the magnetic
defects. The NMR data for the powders are well explained by assuming that (i)
there is a random distribution of magnetic point defects, (ii) the same heavy
Fermi liquid is present in the samples containing the magnetic defects as in
magnetically pure LiV2O4, and (iii) the influences of the magnetic defects and
of the Fermi liquid on the magnetization and NMR properties are separable. In
the single crystals, somewhat different behaviors are observed. Remarkably, the
magnetic defects in the powder samples show evidence of spin freezing below T ~
1.0 K, whereas in the single crystals with similar magnetic defect
concentration no spin freezing was found down to 0.5 K. Thus different types of
magnetic defects and/or interactions between them appear to arise in the
powders versus the crystals, possibly due to the substantially different
synthesis conditions of the powders and crystals.Comment: 18 pages typeset in 2 columns, 16 figures; submitted to PR
Contact-less measurements of Shubnikov-de Haas oscillations in the magnetically ordered state of CeAgSb and SmAgSb single crystals
Shubnikov - de Haas oscillations were measured in single crystals of highly
metallic antiferromagnetic SmAgSb and ferromagnetic CeAgSb using a
tunnel diode resonator. Resistivity oscillations as a function of applied
magnetic field were observed via measurements of skin depth variation. The
effective resolution of p allows a detailed study
of the SdH spectra as a function of temperature. The effects of the Sm long -
range magnetic ordering as well as its electronic structure (-electrons) on
the Fermi surface topology is discussed
Two-gap superconductivity in single crystal LuFeSi from penetration depth measurements
Single crystal of LuFeSi was studied with the tunnel-diode
resonator technique in Meissner and mixed states. Temperature dependence of the
superfluid density provides strong evidence for the two-gap superconductivity
with almost equal contributions from each gap of magnitudes
and . In the vortex state, pinning
strength shows unusually strong temperature dependence and is non-monotonic
with the magnetic field (peak effect). The irreversibility line is sharply
defined and is quite distant from the , which hints on to enhanced
vortex fluctuations in this two-gap system. Altogether our findings provide
strong electromagnetic - measurements support to the two-gap superconductivity
in LuFeSi previously suggested from specific heat measurements
Local-moment ferromagnetism and unusual magnetic domains in FeTaS crystals
Single crystals of FeTaS have been studied by using
magneto-optical (MO) imaging and radio-frequency (rf) magnetic susceptibility,
. Real time MO images reveal unusual, slow dynamics of dendritic domain
formation, the details of which are strongly dependent upon magnetic and
thermal history. Measurements of show well-defined, local moment
ferromagnetic transition at K as well as thermal hysteresis for
50 K60 K. This temperature range corresponds to the domain formation
temperature as determined by MO. Together these observations provide strong
evidence for local moment ferromagnetism in FeTaS crystals with
large, temperature dependent magnetic anisotropy
Field-dependent AC susceptibility of itinerant ferromagnets
Whereas dc measurements of magnetic susceptibility, , fail to
distinguish between local and weak itinerant ferromagnets, radio-frequency (rf)
measurements of in the ferromagnetic state show dramatic differences
between the two. We present sensitive tunnel-diode resonator measurements of
in the weak itinerant ferromagnet ZrZn at a frequency of 23 MHz.
Below Curie temperature, K, the susceptibility is seen to
increase and pass through a broad maximum at approximately 15 K in zero applied
dc magnetic field. Application of a magnetic field reduces the amplitude of the
maximum and shifts it to lower temperatures. The existence and evolution this
maximum with applied field is not predicted by either the Stoner or
self-consistent renormalized (SCR) spin fluctuations theories. For temperatures
below both theories derive a zero-field limit expression for . We
propose a semi-phenomenological model that considers the effect of the internal
field from the polarized fraction of the conduction band on the remaining,
unpolarized conduction band electrons. The developed model accurate describes
the experimental data
Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals
The London penetration depth, , has been measured in several
single crystals of Ba(FeCo)As. Thermodynamic,
electromagnetic, and structural characterization measurements confirm that
these crystals are of excellent quality. The observed low temperature variation
of follows a power-law, with , indicating the existence of normal quasiparticles down to at least
. This is in contrast to recent penetration depth measurements on
single crystals of NdFeAsOF and SmFeAsOF, which
indicate an anisotropic but nodeless gap. We propose that a more
three-dimensional character in the electronic structure of
Ba(FeCo)As may lead to an anisotropic wave gap
with point nodes that would explain the observed .Comment: 4 page
Non-exponential London penetration depth in RFeAsOF (R=La,Nd) single crystals
The superconducting penetration depth, , has been measured in
RFeAsOF (R=La,Nd) single crystals (R-1111). In Nd-1111, we find
an upturn in upon cooling and attribute it to the paramagnetism of
the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O.
After the correction for paramagnetism, the London penetration depth variation
is found to follow a power-law behavior, at
low temperatures. The same variation of was found in
non-magnetic La-1111 crystals. Analysis of the superfluid density and of
penetration depth anisotropy over the full temperature range is consistent with
two-gap superconductivity. Based on this and on our previous work, we conclude
that both the RFeAsO (1111) and BaFeAs (122) families of pnictide
superconductors exhibit unconventional two-gap superconductivity.Comment: v2 is the same as v1 - error in the arxiv submissio
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