3,737 research outputs found
Skyrmion Lattice in a Chiral Magnet
Skyrmions represent topologically stable field configurations with
particle-like properties. We used neutron scattering to observe the spontaneous
formation of a two-dimensional lattice of skyrmion lines, a type of magnetic
vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice
stabilizes at the border between paramagnetism and long-range helimagnetic
order perpendicular to a small applied magnetic field regardless of the
direction of the magnetic field relative to the atomic lattice. Our study
experimentally establishes magnetic materials lacking inversion symmetry as an
arena for new forms of crystalline order composed of topologically stable spin
states
Solid-solid phase transition in hard ellipsoids
We present a computer simulation study of the crystalline phases of hard
ellipsoids of revolution. A previous study [Phys. Rev. E, \textbf{75}, 020402
(2007)] showed that for aspect ratios the previously suggested
stretched-fcc phase [Mol. Phys., \textbf{55}, 1171 (1985)] is unstable with
respect to a simple monoclinic phase with two ellipsoids of different
orientations per unit cell (SM2). In order to study the stability of these
crystalline phases at different aspect ratios and as a function of density we
have calculated their free energies by thermodynamic integration. The
integration path was sampled by an expanded ensemble method in which the
weights were adjusted by the Wang-Landau algorithm.
We show that for aspect ratios the SM2 structure is more stable
than the stretched-fcc structure for all densities above solid-nematic
coexistence. Between and our calculations reveal a
solid-solid phase transition
Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn
We report a study of the ferromagnetism of ZrZn, the most promising
material to exhibit ferromagnetic quantum criticality, at low temperatures
as function of pressure . We find that the ordered ferromagnetic moment
disappears discontinuously at =16.5 kbar. Thus a tricritical point
separates a line of first order ferromagnetic transitions from second order
(continuous) transitions at higher temperature. We also identify two lines of
transitions of the magnetisation isotherms up to 12 T in the plane where
the derivative of the magnetization changes rapidly. These quantum phase
transitions (QPT) establish a high sensitivity to local minima in the free
energy in ZrZn, thus strongly suggesting that QPT in itinerant
ferromagnets are always first order
History dependence of the magnetic properties of single-crystal FeCoSi
We report the magnetization, ac susceptibility, and specific heat of
optically float-zoned single crystals of FeCoSi, . We determine the magnetic phase diagrams for all major
crystallographic directions and cooling histories. After zero-field cooling,
the phase diagrams resemble that of the archetypal stoichiometric cubic chiral
magnet MnSi. Besides the helical and conical state, we observe a pocket of
skyrmion lattice phase just below the helimagnetic ordering temperature. At the
phase boundaries between these states evidence for slow dynamics is observed.
When the sample is cooled in small magnetic fields, the phase pocket of
skyrmion lattice may persist metastably down to lowest temperatures. Taken
together with the large variation of the transition temperatures, transition
fields, and the helix wavelength as a function of composition, this hysteresis
identifies FeCoSi as an ideal material for future experiments
exploring, for instance, the topological unwinding of the skyrmion lattice.Comment: 14 pages, 11 figure
Chirality induced anomalous-Hall effect in helical spin crystals
Under pressure, the itinerant helimagnet MnSi displays unusual magnetic
properties. We have previously discussed a BCC helical spin crystal as a
promising starting point for describing the high pressure phenomenology. This
state has topologically nontrivial configurations of the magnetization field.
Here we note the consequences for magneto-transport that arise generally from
such spin textures. In particular a skyrmion density induced `topological' Hall
effect, with unusual field dependence, is described.Comment: 4 pages, 3 figures, to appear in the proceedings of SCES 07 (the
international conference on strongly correlated electron systems 2007 in
Houston, USA
Novel crystal phase in suspensions of hard ellipsoids
We present a computer simulation study on the crystalline phases of hard
ellipsoids of revolution. For aspect ratios greater than or equal to 3 the
previously suggested stretched-fcc phase [D. Frenkel and B. M. Mulder, Mol.
Phys. 55, 1171 (1985)] is replaced by a novel crystalline phase. Its unit cell
contains two ellipsoids with unequal orientations. The lattice is simple
monoclinic. The angle of inclination of the lattice, beta, is a very soft
degree of freedom, while the two right angles are stiff. For one particular
value of beta, the close-packed version of this crystal is a specimen of the
family of superdense packings recently reported [Donev et al., Phys. Rev. Lett.
92, 255506 (2004)]. These results are relevant for studies of nucleation and
glassy dynamics of colloidal suspensions of ellipsoids.Comment: 4 pages, 4 figure
Theoretical proposal predicting anomalous magnetoresistance and quadratic Hall effect in the partially ordered state of MnSi
In [B. Binz, A. Vishwanath and V. Aji, Phys. Rev. Lett. 96, 207202 (2006)], a
magnetic structure that breaks time reversal symmetry in the absence of net
magnetization was proposed as an explanation for the high pressure "partially
ordered" state of MnSi. Here we make explicit the anomalous magneto-transport
properties of such a state: a magnetoresistivity which is linear and a Hall
conductance which is quadratic in the applied magnetic field. Field cooling
procedures for obtaining single domain samples are discussed. The anomalous
effects are elaborated in the case of three geometries chosen to produce
experimentally unambiguous signals of this unusual magnetic state; e.g., it is
predicted that a field in z-direction induces an anisotropy in the x-y plane.
Another geometry leads to a Hall voltage parallel to the magnetic field.Comment: 2 pages, 2 figures, International Conference on Magnetism 2006 in
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