a thin film having a large high kerr rotation angle is formed of a compound having a composition represented by the following formula : j . sub . x l . sub . y q . sub . wherein j : at least one of f , cl , br and i ; l : at least one of b , c , al , si , p , as , sb , bi , se , te , ti , v , cr , mn , ni , ga , ge , zr , nb and mo ; q : at least one of fe and co ; x : a value of 3 - 80 ; and y : a value satisfying the following equation : 3 ≦ x + y ≦ 80 the film is produced by reacting a halogen or halogen - containing gas , which has occurred as a result of decomposition of a halogen compound , with a metal plasma or halogen - containing metal plasma and allowing the resultant component to deposit as a thin film on a substrate .