16 research outputs found
Ab initio study on the magneto-structural properties of MnAs
The magnetic and structural properties of MnAs are studied with ab initio
methods, and by mapping total energies onto a Heisenberg model. The stability
of the different phases is found to depend mainly on the volume and on the
amount of magnetic order, confirming previous experimental findings and
phenomenological models. It is generally found that for large lattice constants
the ferromagnetic state is favored, whereas for small lattice constants
different antiferromagnetic states can be stabilized. In the ferromagnetic
state the structure with minimal energy is always hexagonal, whereas it becomes
orthorhombically distorted if there is an antiferromagnetic component in the
hexagonal plane. For the paramagnetic state the stable cell is found to be
orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above
which it remains hexagonal. This leads to the second order structural phase
transition between paramagnetic states at about 400 K, where the lattice
parameter increases above this critical value with rising temperature due to
the thermal expansion. For the paramagnetic state an analytic approximation for
the magnitude of the orthorhombic distortion as a function of the lattice
constant is given. Within the mean field approximation the dependence of the
Curie temperature on the volume and on the orthorhombic distortion is
calculated. For orthorhombically distorted cells the Curie temperature is much
smaller than for hexagonal cells. This is mainly due to the fact that some of
the exchange coupling constants in the hexagonal plane become negative for
distorted cells. With these results a description of the susceptibility as
function of temperature is given
Magnetoelastic effects in low-dimensional magnetic systems
We consider a new realization of magnetoelastic interactions in low-dimensional magnetic systems. We show that low-dimensional spin systems are unstable with respect to the spontaneous appearance of alternating distortions of the positions of the three-dimensional nonmagnetic atoms (ligands), that surround the magnetic ions. Those distortions are supplemented by the spontaneous onset of alternating effective g factors of the magnetic ions in the phase with short-range interactions. We discuss the possibility of observing the effect in an uniform external magnetic field, which in the situation considered produces both magnetization and staggered magnetization of the magnetic subsystem. The connection of the proposed theory with recent experiments on effectively low-dimensional magnetic systems (organic spin chains, heavy-fermion compounds, rare-earth molybdates) is discussed
Electrically active magnetic excitations in antiferromagnets (Review Article)
The magnetic resonance operation by electric field is highly nontrivial but the most demanding function in
the future spin-electronics. Recently observed in a variety of multiferroics materials named the collective electrically
active magnetic excitations, frequently referred to as “electromagnons”, reveal a possible way to implement
such a function. Experimental advances in terahertz spectroscopy of electromagnons in multiferroics as well as
related theoretical models are reviewed. The earlier theoretical works, where the existence of electric-dipole active
magnetic excitations in antiferro- and ferrimagnets with collinear spin structure has been predicted, are also
discussed. Multi-sublattice magnets with electrically active magnetic excitations at room temperature give a direct
possibility to transform one type of excitation into another in a terahertz time-domain. This is of crucial importance
for the magnon-based spintronics as only the short-wavelength exchange magnons allow the signal
processing on the nanoscale distance