838 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
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
A hidden constant in the anomalous Hall effect of a high-purity magnet MnSi
Measurements of the Hall conductivity in MnSi can provide incisive tests of
theories of the anomalous Hall (AH) effect, because both the mean-free-path and
magnetoresistance (MR) are unusually large for a ferromagnet. The large MR
provides an accurate way to separate the AH conductivity from
the ordinary Hall conductivity . Below the Curie temperature
, is linearly proportional to (magnetization) with a
proportionality constant that is independent of both and . In
particular, remains a constant while changes by a factor
of 100 between 5 K and . We discuss implications of the hidden constancy
in .Comment: 5 pages, 4 figures. Minor change
Parasitic small-moment-antiferromagnetism and non-linear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction
We report simultaneous measurements of the distribution of lattice constants
and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and
conventional neutron diffraction, as a function of temperature and pressure up
to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is
purely parasitic and quantitatively originates from the distribution of lattice
constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high
pressure are separated by a line of first-order phase transitions, which ends
in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must
have different symmetry, as expected of the HO being, e.g., incommensurate
orbital currents, helicity order, or multipolar order.Comment: 4 pages, 4 figure
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
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