1,088 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
Magnetic phase diagram of MnSi inferred from magnetization and ac susceptibility
We report simultaneous measurements of the magnetization and the ac
susceptibility across the magnetic phase diagram of single-crystal MnSi. In our
study we explore the importance of the excitation frequency, excitation
amplitude, sample shape, and crystallographic orientation. The susceptibility,
dM/dH, calculated from the magnetization, is dominated by pronounced maxima at
the transition from the helical to the conical and the conical to the skyrmion
lattice phase. The maxima in dM/dH are not tracked by the ac susceptibility,
which in addition varies sensitively with the excitation amplitude and
frequency at the transition from the conical to the skyrmion lattice phase. The
same differences between dM/dH and the ac susceptibility exist for Mn1-xFexSi
(x=0.04) and Fe1-xCoxSi (x=0.20). Taken together our study establishes
consistently for all major crystallographic directions the existence of a
single pocket of the skyrmion lattice phase in MnSi, suggestive of a universal
characteristic of all B20 transition metal compounds with helimagnetic order.Comment: 19 pages, 20 figure
Critical spin-flip scattering at the helimagnetic transition of MnSi
We report spherical neutron polarimetry (SNP) and discuss the spin-flip
scattering cross sections as well as the chiral fraction close to the
helimagnetic transition in MnSi. For our study, we have developed a
miniaturised SNP device that allows fast data collection when used in small
angle scattering geometry with an area detector. Critical spin-flip scattering
is found to be governed by chiral paramagnons that soften on a sphere in
momentum space. Carefully accounting for the incoherent spin-flip background,
we find that the resulting chiral fraction decreases gradually above the
helimagnetic transition reflecting a strongly renormalised chiral correlation
length with a temperature dependence in excellent quantitative agreement with
the Brazovskii theory for a fluctuation-induced first order transition.Comment: 5 pages, 3 figure
Peculiar behavior of the electrical resistivity of MnSi at the ferromagnetic phase transition
The electrical resistivity of a single crystal of MnSi was measured across
its ferromagnetic phase transition line at ambient and high pressures. Sharp
peaks of the temperature coefficient of resistivity characterize the transition
line. Analysis of these data shows that at pressures to ~0.35 GPa these peaks
have fine structure, revealing a shoulder at ~ 0.5 K above the peak. It is
symptomatic that this structure disappears at pressures higher than ~0.35 GPa,
which was identified earlier as a tricritical poin
Magnetization under High Pressure in MnSi
The magnetization M(H) has been measured in the weakly helimagnetic itinerant
compound MnSi under high pressure up to 10.2 kbar and high magnetic field up to
9 Tesla. We interpret the simultaneous decrease under pressure of the saturated
magnetization, , and the Curie temperature, in the frame of the
self-consistent renormalization theory (SCR) of spin fluctuations. From the
analysis of the so-called Arrot-plot ( versus ) and
the respective volume dependence of and , we estimate the evolution
of the characteristic spin fluctuation temperatures, and when the
system approaches its critical pressure, =15 kbar, corresponding to the
disappearance of the long range magnetic order at T=0.Comment: 12 pages, 5 figures. Submitted to Phys. Rev.
Hardwarearchitektur für einen universellen LDPC Decoder
Im vorliegenden Beitrag wird eine universelle Decoderarchitektur für einen Low-Density Parity-Check (LDPC) Code Decoder vorgestellt. Anders als bei den in der Literatur häufig beschriebenen Architekturen für strukturierte Codes ist die hier vorgestellte Architektur frei programmierbar, so dass jeder beliebige LDPC Code durch eine Änderung der Initialisierung des Speichers für die Prüfmatrix mit derselben Hardware decodiert werden kann. Die größte Herausforderung beim Entwurf von teilparallelen LDPC Decoder Architekturen liegt im konfliktfreien Datenaustausch zwischen mehreren parallelen Speichern und Berechnungseinheiten, wozu ein Mapping und Scheduling Algorithmus benötigt wird. Der hier vorgestellte Algorithmus stützt sich auf Graphentheorie und findet für jeden beliebigen LDPC Code eine für die Architektur optimale Lösung. Damit sind keine Wartezyklen notwendig und die Parallelität der Architektur wird zu jedem Zeitpunkt voll ausgenutzt
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