38 research outputs found
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.
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
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
Incommensurate-commensurate transitions in the monoaxial chiral helimagnet driven by the magnetic field
Under the terms of the Creative Commons Attribution license.-- et al.The zero-temperature phase diagram of the monoaxial 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 behavior. 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 high anisotropy the conical approximation locates the transition line very accurately, although it does not predict the continuous transitions and the tricritical behavior. It is shown that for high 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 CrNb3S6.J.C. and V.L. acknowledge Grant No. MAT2015-68200-C2-2-P from the Spanish Ministry of Economy and Competitiveness. The work of A. S. Ovchinnikov was supported by the Government of the Russian Federation Program (02.A03.21.0006) and by the Ministry of Education and Science of the Russian Federation (Project Nos. 1437 and 2725).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
Chiral Soliton Lattice Formation in Monoaxial Helimagnet Yb(NiCu)Al
Helical magnetic structures and its responses to external magnetic fields in
Yb(NiCu)Al, with a chiral crystal structure of the space
group , have been investigated by resonant X-ray diffraction. It is shown
that the crystal chirality is reflected to the helicity of the magnetic
structure by a one to one relationship, indicating that there exists an
antisymmetric exchange interaction mediated via the conduction electrons. When
a magnetic field is applied perpendicular to the helical axis ( axis), the
second harmonic peak of develops with increasing the field. The
third harmonic peak of has also been observed for the =0.06
sample. This result provides a strong evidence for the formation of a chiral
magnetic soliton lattice state, a periodic array of the chiral twist of spins,
which has been suggested by the characteristic magnetization curve. The helical
ordering of magnetic octupole moments, accompanying with the magnetic dipole
order, has also been detected.Comment: 13 pages, 18 figures, accepted for publication in J. Phys. Soc. Jp
Order and disorder in the magnetisation of the chiral crystal CrNb3S6
Competing magnetic anisotropies in chiral crystals with Dzyaloshinskii-Moriya exchange interactions can give rise to nontrivial chiral topological magnetization configurations with new and interesting properties. One such configuration is the magnetic soliton, where the moment continuously rotates about an axis. This magnetic system can be considered to be one dimensional and, because of this, it supports a macroscale coherent magnetization, giving rise to a tunable chiral soliton lattice (CSL) that is of potential use in a number of applications in nanomagnetism and spintronics. In this paper, we characterize the transitions between the forced-ferromagnetic (F-FM) phase and the CSL one in
CrNb3S6 using differential phase contrast imaging in a scanning transmission electron microscope, conventional Fresnel imaging, ferromagnetic resonance spectroscopy, and mean-field modeling. We find that the formation and movement of dislocations mediate the formation of CSL and F-FM regions and that these strongly influence the highly hysteretic static and dynamic properties of the system. Sample size and morphology can be used to tailor the properties of the system and, with the application of magnetic field, to locate and stabilize normally unstable dislocations and modify their dimensions and magnetic configurations in ways beyond that predicted to occur in uniform films
Spin polarization gate device based on the chirality-induced spin selectivity and robust nonlocal spin polarization
Nonlocal spin polarization phenomena are thoroughly investigated in the
devices made of chiral metallic single crystals of CrNbS and NbSi
as well as of polycrystalline NbSi. We demonstrate that simultaneous
injection of charge currents in the opposite ends of the device with the
nonlocal setup induces the switching behavior of spin polarization in a
controllable manner. Such a nonlocal spin polarization appears regardless of
the difference in the materials and device dimensions, implying that the
current injection in the nonlocal configuration splits spin-dependent chemical
potentials throughout the chiral crystal even though the current is injected
into only a part of the crystal. We show that the proposed model of the spin
dependent chemical potentials explains the experimental data successfully. The
nonlocal double-injection device may offer significant potential to control the
spin polarization to large areas because of the nature of long-range nonlocal
spin polarization in chiral materials.Comment: 8 pages, 8 figure
Chirality-selected crystal growth and spin polarization over centimeters of transition metal disilicide crystals
We performed a chirality-controlled crystal growth of transition metal
disilicide NbSi and TaSi by using a laser-diode-heated floating
zone (LDFZ) method. The crystal chirality was evaluated in the crystals of
centimeters in length by performing single crystal X-ray diffraction as well as
probing a spin polarization originating from chirality-induced spin selectivity
(CISS) effect. The crystals of right-handed NbSi and of left-handed
TaSi were obtained in the conventional LDFZ crystal growth, while the
left-handed NbSi and right-handed TaSi crystals were grown by the
LDFZ method with the composition-gradient feed rods. The spin polarization via
the CISS was observed over centimeters in the NbSi single crystals and
the sign of the CISS signals was dependent on the chirality of crystals. The
correlation between the crystal chirality and CISS signals indicates that the
CISS measurements work as a non-destructive method for chirality determination
even in centimeter-long specimens.Comment: 9 pages, 6 figure