When magnetic materials are subject to a temperature gradient, domain walls are known to move towards the hotter end of the system. This paper shows that in certain types of ferrimagnets, domain walls can move in either directions, towards the hotter or the cooler end, depending on temperature. This effect rests on a competition between maximization of entropy and an adiabatic transfer of magnonic angular momentum.

Unveiling domain wall dynamics of ferrimagnets in thermal magnon currents : competition of angular momentum transfer and entropic torque

Spin waves in a system of pseudo-spins forming a simple cubic ferromagnetic lattice are analysed by considering the most general bilinear interaction between nearest neighbors allowed by a four-fold order symmetry of the bond ℋij = J∥SizSjz+J⟂(SixSjx+SiySjy) for a pair (i,j) oriented along the z axis. For that extended Heisenberg system, the excitation energy a t the center of the Brillouin zone is found to be zero with the usual theory, a result which violates the Goldstone theorem predicting a gap a t this point. This gap is calculated through two different but consistent methods and its variation with the anisotropy of the exchange parameters is determined numerically. It is seen that even for a small gap, strongly anisotropic exchange is possible.



On consiekre un systeme de pseudo-spins situes aux noeuds d'un reseau cubique simple, ordonnesferromagnetiquement, soumis a I'interaction bilineaire la plus generale entre premiers voisins autorisee par la symbtrie, ℋij = J∥SizSjz+J⟂(SixSjx+SiySjy), pour une paire (i,j) orientheselon O z. Pour ce systeme, qui est une extension du modele de Heisenberg, on trouve a l'aide de la theorie usuelle des ondes de spin une energie d'excitation nulle au centre de la zone de Brillouin; ce resultat est en contradiction avec le theoreme de Goldstone qui prevoit un gap en ce point. Ce gap est calcule par deux methodes differentes et coherentes et sa variation en fonction de l'anisotropie des parametres d'echange est determinee numeriquement. On montre qu'un faible gap peut s'accompagner d'une forte anisotropie de l'echange.

Calculation of the Spin Wave Energy Gap atk =o for a Simple Cubic Ferromagnet with Anisotropic Exchange Interactions

On the energy gap in the anisotropic Heisenberg ferromagnet

The alkali-trihalogeno-germanates AGeX$_3$ with A a large single positive ion such as Rb, Cs, or organic radicals such as methyl ammonium (MA), and X a halogen (I, Br, Cl, F) along with the corresponding stannates (ASnX$_3$) and plumbates (APbX$_3$) exhibit a large variety of crystal structures, some of which are of the perovskite type. These materials, better known as "halide perovskites'' have recently gained worldwide attention as promising photovoltaic and more broadly opto-electronic materials. But their stability problems relative to the non-perovskite phases is a major issue. Here we show that the phase relations in these materials can be understood in terms of the relative orientation of the GeX$_3$ tetrahedral units, which is ferroelectric in the perovskite phase but antiferroelectric in the competing phases. This suggests that an applied electric field could be used to stabilize the desired phases and trigger a phase transition between two phases of the material with widely different optical and electronic properties.

Understanding the Crystallographic Phase Relations in Alkali‐Trihalogeno‐Germanates in Terms of Ferroelectric or Antiferroelectric Arrangements of the Tetrahedral GeX3 Units

Magnetoelastic interactions in the Belorizky model

2014The configurations of lowest energy of a system of pseudo-spins S = 1/2 forming a simple cubic lattice are analysed by considering the most general interaction, between nearest neighbours, allowed by a fourfold symmetry of the bond : Hij = J~ Siz Sjz + J (Six Sjx + Siy Sjy) for a pair (i, j) oriented along the z axis. In zero external field, the most stable configuration is either ferromagnetic when J~ and J are both negative, or antiferromagnetic in the other cases; in the latter situation various antiferromagnetic configurations are obtained depending on the signs of J~ and J. For all these configurations, derived in a classical way, the spin wave spectrum is calculated in order to check the magnetic stability of the system as well as to evaluate the quantum ground state energy and the mean spin deviation in this ground state. It is also shown that when J~ = 2014 J 0, the antiferromagnetic ground configuration is an eigenstate of the total Hamiltonian. Thus a new three dimensional magnetic system, whose ground state is known exactly, is obtained. LE JOURNAL DE PHYSIQUE TOME 39, JUILLET 1978, Classification Physics Abstracts 75.1OJ 75.30D .

https://hal.archives-ouvertes.fr/docs/00/20/88/13/PDF/ajp-jphys_1978_39_7_776_0.pdf

R. Casalegno

Papers

Ground state configurations of a simple cubic array of pseudo-spins s = 1/2 with anisotropic exchange between nearest neighbours