Variation du champ seuil avec une tension uniaxiale dans FeCl2 A 4,2°K

In FeCl₂, at low temperature, the Fe²⁺ ions present an antiferromagnetic order characterised by an easy magnetization axis perpendicular to the ion-containing planes. There exists a value h_s of a magnetic field parallel to the axis, which transforms the system from an antiferromagnetic order into a saturated paramagnetic order. There we present the variations of h_s at 4·2°K as well as the deformations of the sample, when we impose upon it a strain parallel to the axis without allowing its lateral dilatation. These results, when added to those already obtained with χ_i in similar experiments enable us to determine the variations of the crystalline anisotropy and anti-ferromagnetic exchange (between Fe²⁺ ions belonging to two consecutive planes) in terms of the deformations. Our results concerning h_s are different from the ones obtained when exerting a hydrostatic strain. We compare the two cases by using the relations between elastic constants of FeCl₂ obtained at 300°K and recorded in the Appendix.