OBSERVATION OF A NOVEL FERROMAGNETIC PHASE OF HO3+ IN HO LU SUPERLATTICES/

The magnetic structures of three Ho/Lu superlattices have been investi gated using neutron diffraction. The low-temperature magnetic structur e adopted by the Ho3+ moments is found to depend on the number n(Ho) o f atomic planes in a single block of Ho. For a sample with n(Ho) congr uent-to 40 the moments form a basal-plane spiral down to the lowest te mperatures investigated, whereas for two samples with n(Ho) less-than- or-equal-to 20 a phase transition occurs from a basal-plane spiral to a structure in which the Ho3+ moments within a single block align ferr omagnetically.