953 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
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
Imaging and manipulation of skyrmion lattice domains in Cu2OSeO3
Nanoscale chiral skyrmions in noncentrosymmetric helimagnets are promising
binary state variables in high-density, low-energy nonvolatile memory.
Skyrmions are ubiquitous as an ordered, single-domain lattice phase, which
makes it difficult to write information unless they are spatially broken up
into smaller units, each representing a bit. Thus, the formation and
manipulation of skyrmion lattice domains is a prerequisite for memory
applications. Here, using an imaging technique based on resonant magnetic x-ray
diffraction, we demonstrate the mapping and manipulation of skyrmion lattice
domains in Cu2OSeO3. The material is particularly interesting for applications
owing to its insulating nature, allowing for electric field-driven domain
manipulation.Comment: 4 pages, 3 figure
Magnon Exchange Mechanism of Ferromagnetic Superconductivity
The magnon exchange mechanism of ferromagnetic superconductivity
(FM-superconductivity) was developed to explain in a natural way the fact that
the superconductivity in , and is confined to the
ferromagnetic phase.The order parameter is a spin anti-parallel component of a
spin-1 triplet with zero spin projection. The transverse spin fluctuations are
pair forming and the longitudinal ones are pair breaking. In the present paper,
a superconducting solution, based on the magnon exchange mechanism, is obtained
which closely matches the experiments with and . The onset of
superconductivity leads to the appearance of complicated Fermi surfaces in the
spin up and spin down momentum distribution functions. Each of them consist of
two pieces, but they are simple-connected and can be made very small by varying
the microscopic parameters. As a result, it is obtained that the specific heat
depends on the temperature linearly, at low temperature, and the coefficient
is smaller in the superconducting phase than in the
ferromagnetic one. The absence of a quantum transition from ferromagnetism to
ferromagnetic superconductivity in a weak ferromagnets and is
explained accounting for the contribution of magnon self-interaction to the
spin fluctuations' parameters. It is shown that in the presence of an external
magnetic field the system undergoes a first order quantum phase transition.Comment: 9 pages, 7 figures, accepted for publication in Phys.Rev.
Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator CuOSeO
Linear dichroism -- the polarization dependent absorption of electromagnetic
waves -- is routinely exploited in applications as diverse as structure
determination of DNA or polarization filters in optical technologies. Here
filamentary absorbers with a large length-to-width ratio are a prerequisite.
For magnetization dynamics in the few GHz frequency regime strictly linear
dichroism was not observed for more than eight decades. Here, we show that the
bulk chiral magnet CuOSeO exhibits linearly polarized magnetization
dynamics at an unexpectedly small frequency of about 2 GHz. Unlike optical
filters that are assembled from filamentary absorbers, the magnet provides
linear polarization as a bulk material for an extremely wide range of
length-to-width ratios. In addition, the polarization plane of a given mode can
be switched by 90 via a tiny variation in width. Our findings shed a
new light on magnetization dynamics in that ferrimagnetic ordering combined
with anisotropic exchange interaction offers strictly linear polarization and
cross-polarized modes for a broad spectrum of sample shapes. The discovery
allows for novel design rules and optimization of microwave-to-magnon
transduction in emerging microwave technologies.Comment: 20 pages, 4 figure
Crystalline phases in chiral ferromagnets: Destabilization of helical order
In chiral ferromagnets, weak spin-orbit interactions twist the ferromagnetic
order into spirals, leading to helical order. We investigate an extended
Ginzburg-Landau theory of such systems where the helical order is destabilized
in favor of crystalline phases. These crystalline phases are based on periodic
arrangements of double-twist cylinders and are strongly reminiscent of blue
phases in liquid crystals. We discuss the relevance of such blue phases for the
phase diagram of the chiral ferromagnet MnSi.Comment: 6 pages, 5 figures (published version
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Configurable multiplier modules for an adaptive computing system
The importance of reconfigurable hardware is increasing steadily. For example, the primary approach of using adaptive systems based on programmable gate arrays and configurable routing resources has gone mainstream and high-performance programmable logic devices are rivaling traditional application-specific hardwired integrated circuits. Also, the idea of moving from the 2-D domain into a 3-D design which stacks several active layers above each other is gaining momentum in research and industry, to cope with the demand for smaller devices with a higher scale of integration. However, optimized arithmetic blocks in course-grain reconfigurable arrays as well as field-programmable architectures still play an important role. In countless digital systems and signal processing applications, the multiplication is one of the critical challenges, where in many cases a trade-off between area usage and data throughput has to be made. But the a priori choice of word-length and number representation can also be replaced by a dynamic choice at run-time, in order to improve flexibility, area efficiency and the level of parallelism in computation. In this contribution, we look at an adaptive computing system called 3-D-SoftChip to point out what parameters are crucial to implement flexible multiplier blocks into optimized elements for accelerated processing. The 3-D-SoftChip architecture uses a novel approach to 3-dimensional integration based on flip-chip bonding with indium bumps. The modular construction, the introduction of interfaces to realize the exchange of intermediate data, and the reconfigurable sign handling approach will be explained, as well as a beneficial way to handle and distribute the numerous required control signals
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