401 research outputs found
Microwave power coupling in a surface wave excited plasma
In recent decades, different types of plasma sources have been used for
various types of plasma processing, such as, etching and thin film deposition.
The critical parameter for effective plasma processing is high plasma density.
One type of high density plasma source is Microwave sheath-Voltage combination
Plasma (MVP). In the present investigation, a better design of MVP source is
reported, in which over-dense plasma is generated for low input microwave
powers. The results indicate that the length of plasma column increases
significantly with increase in input microwave power.Comment: 10 pages, 5 figure
Nucleation, instability, and discontinuous phase transitions in monoaxial helimagnets with oblique fields
The phase diagram of the monoaxial chiral helimagnet as a function of
temperature (T ) and magnetic field with components perpendicular (H x ) and
parallel (H z ) to the chiral axis is theoretically studied via the variational
mean field approach in the continuum limit. A phase transition surface in the
three dimensional thermodynamic space separates a chiral spatially modulated
phase from a homogeneous forced ferromagnetic phase. The phase boundary is
divided into three parts: two surfaces of second order transitions of
instability and nucleation type, in De Gennes terminology, are separated by a
surface of first order transitions. Two lines of tricritical points separate
the first order surface from the second order surfaces. The divergence of the
period of the modulated state on the nucleation transition surface has the
logarithmic behavior typical of a chiral soliton lattice. The specific heat
diverges on the nucleation surface as a power law with logarithmic corrections,
while it shows a finite discontinuity on the other two surfaces. The soliton
density curves are described by a universal function of H x if the values of T
and H z determine a transition point lying on the nucleation surface;
otherwise, they are not universal.Comment: Phase diagram refined, with a new tricritical point located; 9 pages,
8 figures; version shortened, published in Phys. Rev.
Understanding the H-T phase diagram of the mono-axial helimagnet
Some unexpected features of the phase diagram of the monoaxial helimagnet in
presence of an applied magnetic field perpendicular to the chiral axis are
theoretically predicted. A rather general hamiltonian with long range
Heisenberg exchange and Dzyaloshinskii--Moriya interactions is considered. The
continuum limit simplifies the free energy, which contains only a few
parameters which in principle are determined by the many parameters of the
hamiltonian, although in practice they may be tuned to fit the experiments. The
phase diagram contains a Chiral Soliton Lattice phase and a forced
ferromagnetic phase separated by a line of phase transitions, which are of
second order at low T and of first order in the vicinity of the zero-field
ordering temperature, and are separated by a tricritical point. A highly non
linear Chiral Soliton Lattice, in which many harmonics contribute appreciably
to the spatial modulation of the local magnetic moment, develops only below the
tricritical temperature, and in this case the scaling shows a logarithmic
behaviour similar to that at T=0, which is a universal feature of the Chiral
Soliton Lattice. Below the tricritical temperature, the normalized soliton
density curves are found to be independent of T, in agreement with the
experimental results of magnetorresistance curves, while above the tricritical
temperature they show a noticeable temperature dependence. The implications in
the interpretation of experimental results of CrNb3S6 are discussed.Comment: 11 pages, 17 figures. Enlarged version, with more details and
results. To be publisehd in Phys. Rev.
Observation of elastic anomalies driven by coexisting dynamical spin Jahn-Teller effect and dynamical molecular spin state in paramagnetic phase of the frustrated MgCrO$
Ultrasound velocity measurements of magnesium chromite spinel MgCrO
reveal elastic anomalies in the paramagnetic phase that are characterized as
due to geometrical frustration. The temperature dependence of the tetragonal
shear modulus exhibits huge Curie-type softening, which
should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic
phase. The trigonal shear modulus exhibits nonmonotonic temperature
dependence with a characteristic minimum at 50 K, indicating a coupling
of the lattice to dynamical molecular spin state. These results strongly
suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical
molecular spin state in the paramagnetic phase, which is compatible with the
coexistence of magnetostructural order and dynamical molecular spin state in
the antiferromagnetic phase.Comment: 6 pages, 3 figure
Crystal and magnetic structures of Cr1/3NbSe2 from neutron diffraction
Under a Creative Commons Attribution (CC BY) license.-- et al.Neutron diffraction measurements of the Cr intercalated niobium diselenide CrNbSe together with magnetization measurements have revealed that this compound exhibits ferromagnetic ordering below T = 96 K unlike a chiral helimagnetic order observed in the sulfide compound CrNbS. As derived from neutron diffraction data, the Cr magnetic moments μ = 2.83 ± 0.03 μ in CrNbSe are aligned within basal plane. The discrepancy in the magnetic states of CrNbS and CrNbSe is ascribed to the difference in the preferential site occupation of Cr ions in crystal lattices. In CrNbSe, the Cr ions are predominantly distributed over 2b Wyckoff site, which determines a centrosymmetric character of the crystal structure unlike CrNbS, where the Cr ions are mainly located in 2c position and the crystal structure is non-centrosymmetric.This work is partly based on experiments performed at the Japan Proton Accelerator Research Complex J-PARC. This work was performed within the state assignment of
the FASO of Russia (No. 01201463334) and supported by Act 211 Government of the Russian Federation (Contract No. 02.A03.21.0006), by the Russian Foundation for Basic Research (Project Nos. 13-02-00364 and 13-02-92104), by the program of UB of RAS (Project No. 15-17-2-22), and by Grants-in-Aid for Scientific Research (Nos. 25220803, 242440590, and 25246006) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work was also supported by the Center for Chiral Science in Hiroshima University (the MEXT program for promoting the enhancement of research universities, Japan) and JSPS Core-to-Core Program, A. Advanced Research Networks. J.C. and Y.K. acknowledge the Grant No.
MAT2011-27233-C02-02.Peer Reviewe
Incommensurate--commensurate transitions in the mono-axial chiral helimagnet driven by the magnetic field
The zero temperature phase diagram of the mono-axial chiral helimagnet in the
magnetic field plane formed by the components parallel and perpendicular to the
helical axis is thoroughly analyzed. The nature of the transition to the
commensurate state depends on the angle between the field and the helical axis.
For field directions close to the directions parallel or perpendicular to the
helical axis the transition is continuous, while for intermediate angles the
transition is discontinuous and the incommensurate and commensurate states
coexist on the transition line. The continuous and discontinuous transition
lines are separated by two tricritical points with specific singular behaviour.
The location of the continuous and discontinuous lines and of the tricritical
points depend strongly on the easy-plane anisotropy, the effect of which is
analyzed. For large anisotropy the conical approximation locates the transition
line very accurately, although it does not predict the continuous transitions
nor the tricitical behaviour. It is shown that for large anisotropy, as in
CrNb3S6, the form of the transition line is universal, that is, independent of
the sample, and obeys a simple equation. The position of the tricritical
points, which is not universal, is theoretically estimated for a sample of
CrNb3S6Comment: 10 pages, 9 figure
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