457 research outputs found
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
Magnon Bose condensation in symmetry breaking magnetic field
Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a
result of unusual form of the Zeeman energy, which has terms linear in the
spin-wave operators and terms mixing excitations differ in the Wave-vector of
the magnetic structure. The following examples are considered: simple
easy-plane tetragonal antiferromagnets (AF), frustrated AF family
where etc. and cubic magnets with the Dzyaloshinskii-Moriya
interaction ( etc.). In all cases the BC becomes important when the
magnetic field becomes comparable with the spin-wave gap. The theory is
illustrated by existing experimental results.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of
International Conference "Highly Frustrated Magnets", Osaka (Japan), August
2006). 8 pages, 5 figure
Ultrasonic studies of the magnetic phase transition in MnSi
Measurements of the sound velocities in a single crystal of MnSi were
performed in the temperature range 4-150 K. Elastic constants, controlling
propagation of longitudinal waves reveal significant softening at a temperature
of about 29.6 K and small discontinuities at 28.8 K, which corresponds to
the magnetic phase transition in MnSi. In contrast the shear elastic moduli do
not show any softening at all, reacting only to the small volume deformation
caused by the magneto-volume effect. The current ultrasonic study exposes an
important fact that the magnetic phase transition in MnSi, occurring at 28.8 K,
is just a minor feature of the global transformation marked by the rounded
maxima or minima of heat capacity, thermal expansion coefficient, sound
velocities and absorption, and the temperature derivative of resistivity.Comment: 4 pages, 4 figure
Ordered Phases of Itinerant Dzyaloshinsky-Moriya Magnets and Their Electronic Properties
A field theory appropriate for magnets that display helical order due to the
Dzyaloshinsky-Moriya mechanism, a class that includes MnSi and FeGe, is used to
derive the phase diagram in a mean-field approximation. The helical phase, the
conical phase in an external magnetic field, and recent proposals for the
structure of the A-phase and the non-Fermi-liquid region in the paramagnetic
phase are discussed. It is shown that the orientation of the helical pitch
vector along an external magnetic field within the conical phase occurs via two
distinct phase transitions. The Goldstone modes that result from the long-range
order in the various phases are determined, and their consequences for
electronic properties, in particular the specific heat, the single-particle
relaxation time, and the electrical and thermal conductivities, are derived.
Various aspects of the ferromagnetic limit, and qualitative differences between
the transport properties of helimagnets and ferromagnets, are also discussed.Comment: 22pp, 8 eps fig
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