55 research outputs found
Magnetization of rotating ferrofluids: the effect of polydispersity
The influence of polydispersity on the magnetization is analyzed in a
nonequilibrium situation where a cylindrical ferrofluid column is enforced to
rotate with constant frequency like a rigid body in a homogeneous magnetic
field that is applied perpendicular to the cylinder axis. Then, the
magnetization and the internal magnetic field are not longer parallel to each
other and their directions differ from that of the applied magnetic field.
Experimental results on the transverse magnetization component perpendicular to
the applied field are compared and analyzed as functions of rotation frequency
and field strength with different polydisperse Debye models that take into
account the polydispersity in different ways and to a varying degree.Comment: 11 pages, 7 figures, to be published in Journal of Physics
Propagation of defects in doped magnetic materials of different dimensionality
Defects intentionally introduced into magnetic materials often have a
profound effect on the physical properties. Specifically tailored neutron
spectroscopic experiments can provide detailed information on both the local
exchange interactions and the local distances between the magnetic atoms around
the defects. This is demonstrated for manganese dimer excitations observed for
the magnetically diluted three- and two-dimensional compounds KMn(x)Zn(1-x)F(3)
and K(2)Mn(x)Zn(1-x)F(4), respectively. The resulting local exchange
interactions deviate up to 10% from the average, and the local Mn-Mn distances
are found to vary stepwise with increasing internal pressure due to the Mn/Zn
substitution. Our analysis qualitatively supports the theoretically predicted
decay of atomic displacements according to 1/r**2, 1/r, and constant (for
three-, two-, and one-dimensional compounds, respectively) where r denotes the
distance of the displaced atoms from the defect.Comment: 21 pages, 4 figures, 3 table
Faraday Instability in a Surface-Frozen Liquid
Faraday surface instability measurements of the critical acceleration, a_c,
and wavenumber, k_c, for standing surface waves on a tetracosanol (C_24H_50)
melt exhibit abrupt changes at T_s=54degC above the bulk freezing temperature.
The measured variations of a_c and k_c vs. temperature and driving frequency
are accounted for quantitatively by a hydrodynamic model, revealing a change
from a free-slip surface flow, generic for a free liquid surface (T>T_s), to a
surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall
(T < T_s). The change at T_s is traced to the onset of surface freezing, where
the steep velocity gradient in the surface-pinned flow significantly increases
the viscous dissipation near the surface.Comment: 4 pages, 3 figures. Physical Review Letters (in press
Response of a ferrofluid to traveling-stripe forcing
We observe the dynamics of waves propagating on the surface of a ferrofluid
under the influence of a spatially and temporarily modulated field. In
particular, we excite plane waves by a travelling lamellar modulation of the
magnetization. By this external driving both the wavelength and the propagation
velocity of the waves can be controlled. The amplitude of the excited waves
exhibits a resonance phenomenon similar to that of a forced harmonic
oscillator. Its analysis reveals the dispersion relation of the free surface
waves, from which the critical magnetic field for the onset of the Rosensweig
instability can be extrapolated
From powder to solution: hydration dependence of human hemoglobin dynamics correlated to body temperature
A transition in hemoglobin (Hb), involving partial unfolding and aggregation, has been shown previously by various biophysical methods. The correlation between the transition temperature and body temperature for Hb from different species, suggested that it might be significant for biological function. In order to focus on such biologically relevant human Hb dynamics, we studied the protein internal picosecond motions as a response to hydration, by elastic and quasielastic neutron scattering. Rates of fast diffusive motions were found to be significantly enhanced with increasing hydration from fully hydrated powder to concentrated Hb solution. In concentrated protein solution, the data revealed that amino acid side-chains can explore larger volumes above body temperature than expected from normal temperature dependence. The body temperature transition in protein dynamics was absent in fully hydrated powder, indicating that picosecond protein dynamics responsible for the transition is activated only at a sufficient level of hydration. A collateral result from the study is that fully hydrated protein powder samples do not accurately describe all aspects of protein picosecond dynamics that might be necessary for biological function
Magnetic order and exchange couplings in the frustrated diamond lattice antiferromagnet MnScSe
We report the magnetic properties of -site spinel compound MnScSe.
The macroscopic magnetic measurements uncovers successive magnetic transitions
at = 2.04 K, followed by two further transitions at
=1.8 K and =1.6 K. Neutron powder diffraction reveals
that both, and , orders are
associated with the propagation vector =(3/4 3/4 0), while the magnetic
structures are collinear amplitude modulated and helical, respectively. Using
neutron powder spectroscopy we demonstrated the effect of substitution of S by
Se on the magnetic exchange. The energy range of the spin-wave excitations is
supressed due to the chemical pressure of the - ion in MnSc (=S,
Se) spinels.Comment: Accepted in Phys. Rev.
Direct observation of local Mn-Mn distances in the paramagnetic compound CsMnxMg1-xBr3
We introduce a novel method for local structure determination with a spatial
resolution of the order of 0.01 Angstroem. It can be applied to materials
containing clusters of exchange-coupled magnetic atoms. We use neutron
spectroscopy to probe the energies of the cluster excitations which are
determined by the interatomic coupling strength J. Since for most materials J
is related to the interatomic distance R through a linear relation
dJ/dR={\alpha} (for dR/R<<1), we can directly derive the local distance R from
the observed excitation energies. This is exemplified for the mixed
one-dimensional paramagnetic compound CsMnxMg1 xBr3 (x=0.05, 0.10) containing
manganese dimers oriented along the hexagonal c-axis. Surprisingly, the
resulting Mn-Mn distances R do not vary continuously with increasing internal
pressure, but lock in at some discrete values.Comment: 16 pages, 2 tables, 3 figure
The normal field instability under side-wall effects: comparison of experiments and computations
We consider a single spike of ferrofluid, arising in a small cylindrical
container, when a vertically oriented magnetic field is applied. The height of
the spike as well as the surface topography is measured experimentally by two
different technologies and calculated numerically using the finite element
method. As a consequence of the finite size of the container, the numerics
uncovers an imperfect bifurcation to a single spike solution, which is forward.
This is in contrast to the standard transcritical bifurcation to hexagons,
common for rotational symmetric systems with broken up-down symmetry. The
numerical findings are corroborated in the experiments. The small hysteresis
observed is explained in terms of a hysteretic wetting of the side wall.Comment: accepted to New Journal of Physic
Equilibrium Structure, Hydrogen Bonding, and Proton Conductivity in Half-Neutralized Diamine Ionic Liquids
Recent experiments on proton conducting ionic liquids point to half-neutralized diamine-triflate salts as promising candidates for applications in power generation and energy conversion electrochemical devices. Structural and dynamical properties of the simplest among these compounds are investigated by a combination of density functional theory (DFT) and molecular dynamics (MD) simulations based on an empirical force field. Three different cations have been considered, consisting of a pair of amine-ammonium terminations joined by a short aliphatic segment -(CH2)(n)- with n = 2, 3, and 4. First, the ground state structure, vibrational eigenstates, and hydrogen-bonding properties of single ions, neutral ion pairs, small neutral aggregates of up to eight ions, and molecularly thin hydrogen bonded wires have been investigated by DFT computations. Second, structural and dynamical properties of homogeneous liquid and amorphous phases are investigated by MD simulations over the temperature range of 200 <= T <= 440 K. Structure factors, radial distribution functions, diffusion coefficient, and electrical conductivity are computed and discussed, highlighting the inherent structural heterogeneity of these compounds. The core investigation, however, is the characterization of connected paths consisting of cation chains that could support proton transport via a Grotthuss-type mechanism. Since simulations are carried out using a force field of fixed bonding topology, this analysis is based on the equilibrium structure only, using geometrical criteria to identify potential paths for proton conduction. Paths of connected cations can reach a length of 80 cations and 30 angstrom, provided that bridging oxygen atoms from triflate anions are taken into account. The effects of water contamination at 1% weight concentration on the structure, dynamics, and paths for proton transport are discussed
Measuring the anomalous dispersion branch of surface waves on ferrofluids
In this paper we report on the first experimental investigation of the anomalous dispersion branch of surface waves on ferrofluids in a magnetic field. The existence of such an anomalous dispersion branch has been predicted by several authors, but there has been no experimental evidence for its existence. In our experiments we used the well-known Faraday instability of parametrically excited surface waves on a ferrofluid in the presence of a magnetic field oriented perpendicular to the fluid's surface
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