Optical estimation of the zone-centre magnon gap in Rb₂CrCl₄: a two-dimensional easy-plane ionic ferromagnet

Spin dynamics and absorption-band profiles for the planar two-dimensional easy-plane ionic ferromagnet Rb 2 CrCl 4

“…The intensity is therefore determined by the Bose population of the magnons. Closer inspection of the 631 nm absorption region between 0.8 K and 4.2 K revealed a deviation from the T2 intensity law expected for a square planar Heisenberg ferromag with small anisotropy, because in this temperature range the thermal energy becomes comparable with the anisotropy gap in the magnon dispersion at the Brillouin zone centre (Day et al 1979). Furthermore, when the temperature is low enough to remove most of the intensity from the magnon-annihilation sideband ('h o t' band), a weak magnon-creation sideband ('cold' band) becomes apparent.…”

“…Rb2CrCl4 is a rare example of an ionic insulator which orders as a ferromagnet (Tc = 52.4 K) (see Day 1979 for a review). It is also unusual in having well-defined absorption bands in the visible, whose intensity is strongly reduced with the onset of magnetic order.…”

“…This structure is related to that of K 2NiF4 {DU space group) with a doubling of the unit cell in orthorhombic symmetry. This occurs because the high spin 3d4 configuration of Cr2+ has a ground state 5Eg in an octahedral field, two neighbouring units having their elongated, approximately tetragonal, axis along the directions [100] and [010] of K 2NiF4 (Day et al 1979 ;Munninghoff et al 1980 ;Janke et al 1983). The ground state suffers a zero-field splitting because of second-order mixing with excited states due to spin-orbit coupling.…”

“…The result of the competition between the ferromagnetic exchange and the anisotropy terms is a structure with spins on neighbouring ions canted at an angle + 6 from the easy [110] direction (Janke et al 1983) and the occurrence of a small gap in the acoustic spin-wave branch. Applying a magnetic field along one of the relatively hard [100] or hard [001] axes, this gap should first decrease to zero at a critical field B& or Bc and then rise again (Elliott et al 1980; Harrop The unusual temperature dependence of the visible absorption bands in Rb2CrCl4 has been the subject of several experimental studies (Day 1979 ;Janke et al 1982). Attention has concentrated on two regions, near 631 an 530 nm.…”

Truncated forms of the second-rank orthorhombic Hamiltonians used in magnetism and electron magnetic resonance (EMR) studies are invalid—Why it went unnoticed for so long?

“…(12) and (13) have been used for Rb 2 CrCl 4 and related systems in, e.g. [64][65][66][67][68][69][70][71]. Below only a few selected examples are explicitly provided.…”