Polycrystalline ferromagnetic Gd and GdAu films are built up of clusters, the atomic spins being exchange coupled. At low temperatures the clusters are coupled almost exclusively by magnetostatic forces. In this superferromagnetic state the usual large magnetic domains show up. At a critical temperature well below the Curie point the magnetic long‐range order is destroyed and the usual magnetic domains vanish. A short‐range order comprising several aligned neighbouring clusters, however, persists up to the Curie point. These short‐range aligned clusters form microdomains with an average diameter of 200 Å, the cluster diameter being in the order of the crystal size.
Superferromagnetism is investigated using Gd films as an example. Polycrystalline films were found t o split into magnetic clusters. Below the bulk Curie point T O they are ferromagnetic. Due to weakened interactions between them magnetic long-range order is established only below a critical temperature T c r < T,. Applying molecular field and spin wave theory to interacting clusters the experimental film magnetization is fairly reproduced. Cluster diameter, critical temperature, and film magnetization as function of temperature coincide for films with widely different crystal diameters, the cluster diameter (= 9 A) being smaller than the latter (15 to 80 A). In the case of Gd the sub-division of the ferromagnet into clusters is due to decoupling oxides. Es wurde Superferromagnetismus am Beispiel von Gd-Schichten untersucht. Die polykristallinen Schichten bestanden aus m%gnetischen Clustern. Unterhalb des Curiepunkts des kompakten Materials sind diese ferromagnetisch. Wegen geschwiichter Wechselwirkung stellt sich eine magnetische Fernordnung nur unterhalb einer kritischen Temperatur Tcr < Tc ein. Die experimentelle Schichtmagnetisierung wird niiherungsweise durch die Molekularfeld-und Spinwellentheorie, angewandt auf wechselwirkende Cluster, wiedergegeben. ClustergroBe, kritische Temperatur und Schichtmagnetisierung als Funktion der Temperatur stimmen fur Schichten mit stark unterschiedlichen Kristalldurchmessern uberein. Die Cluster (Durchmesser = 9 A) sind kleiner als die Kristallite (15 bis 80 if Durchmesser). I m Fall des Gd ist die Unterteilung des Ferromagneten in Cluster durch entkoppelnde Oxide bedingt.
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