Remanent magnetization of the dilute antiferromagnets Mn1-xZnxF2 at very low magnetic fields

Abstract

The magnetization of Mn1-xZnxF2 single crystals with x = 0.25 and 0.51 was studied in low magnetic fields directed along the easy axis (c axis). Data were taken for axial fields Haxial between approximately 10-3 Oe and approximately 10 Oe. Below the Neel temperature TN, the magnetization M for given H,axial and temperature T depended on the history of the sample. Data obtained while cooling in a constant Haxial (FC procedure) show a dramatic rise of M below TN. This rise corresponds to the development of a thermoremanent magnetization M,. The sign and magnitude of M, depend on the axial field present while cooling; Mr is proportional to Haxial at low axial fields, but approaches saturation at Haxial approximately 1 Oe. The measured values of Mr(t)/Mr(0), where t = T/TN is the reduced temperature, are independent of Haxial and are the same for both samples. They also agree with values of Mr(t)/Mr(0) measured in K2Fe1-xInxCl5.H20, but not with those measured in K2Fe(Cl1-xBrx)5.H20. Fits of the temperature dependence of M, near TN yield an effective critical exponent B,. Extrapolated values of B, very close to TN are between 0.35 and 0.40. Experiments in which Haxial is changed below TN show that the remanent magnetization is controlled only by the field Haxial(TN) present while cooling through TN. The observed properties of Mr, particularly the saturation in axial fields of approximately 1 Oe, cannot be explained in terms of domains caused by random fields. Another explanation that fails is based on the statistical difference between the number of up and down spins in each of the antiferromagnetic domains, which exist even in the absence of random fields. It is possible that Mr arises from the walls between such domains. but since this explanation is yet to be tested other possible explanations cannot be ruled out.