Weak ferromagnets with integer and half-integer spin quantum numbers

Experimental investigations are presented which demonstrate that the weak ferromagnets are special materials of a larger group of magnets which exhibit two magnetic ordering structures. In weak ferromagnets the main order is antiferromagnetic with a small ferromagnetic component existing in addition. On the other hand, materials in which other combinations of the two basic ordering structures coexist are also known. Common to all these non-collinear spin structures is that the moment components of the two magnetic structures are orthogonal. In the weak ferromagnets this orthogonality manifests already in the paramagnetic phase as a crossover from an isotropic to an anisotropic susceptibility with respect to the applied magnetic field: at high temperatures the antiferromagnetic Curie-Weiss law of the total moment is observed while for T→Tc crossover to a second Curie-Weiss law due to the weak ferromagnetic component occurs. For T→Tc the ferromagnetic component is, of course, parallel to the field and the antiferromagnetic component is perpendicular to the field. Arguments will be given that in isotropic magnets with a half-integer spin the two magnetic ordering structures can be identified as different order parameters.