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

Hexagons become second if symmetry is broken

Pattern formation on the free surface of a magnetic fluid subjected to a magnetic field is investigated experimentally. By tilting the magnetic field the symmetry can be broken in a controllable manner. When increasing the amplitude of the tilted field, the flat surface gives way to liquid ridges. A further increase results in a hysteretic transition to a pattern of stretched hexagons. The instabilities are detected by means of a linear array of magnetic hall sensors and compared with theoretical predictions.Comment: accepted for publication by Physical Review E/Rapid Communicatio

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

Hydrogen Dynamics in Lightweight Tetrahydroborates

The high hydrogen content in complex hydrides such as M[AlH4]x and M[BH4]x (M = Li, Na,K, Mg, Ca) stimulated many research activities to utilize them as hydrogen storage materials. An understanding of the dynamical properties on themolecular level is important to understand and to improve the sorption kinetics. Hydrogen dynamics in complex hydrides comprise long range translational diffusion as well as localized motions like vibrations, librations or rotations. All the different motions are characterized by their specific length- and timescales. Within this review we give an introduction to the physical properties of lightweight complex hydrides and illustrate the huge variety of dynamical phenomena on selected example

Magnetic order and exchange couplings in the frustrated diamond lattice antiferromagnet MnSc2_2Se4_4

We report the magnetic properties of $A$-site spinel compound MnSc$_2$Se$_4$. The macroscopic magnetic measurements uncovers successive magnetic transitions at $T_{\rm{N1}}$= 2.04 K, followed by two further transitions at $T_{\rm{N2}}$=1.8 K and $T_{\rm{N3}}$=1.6 K. Neutron powder diffraction reveals that both, $T_{\rm{N2}} < T < T_{\rm{N1}}$ and $T <T_{\rm{N3}}$, orders are associated with the propagation vector $k$=(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 $X$- ion in MnSc$_2X_4$ ($X$=S, Se) spinels.Comment: Accepted in Phys. Rev.

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

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