Internal fields in magnetically ordered dysprosium, holmium and erbium

Muon spin rotation data on single-crystalline samples of the heavy rare ear th metals Dy, Ho and Er have been obtained as function of temperature in bo th the antiferromagnetic (afm) and the ferromagnetic (fm) state. In the afm state the temperature dependence of the spontaneous muon spin precession f requency consistently exhibits Brillouin-like behavior. In the fm state we observe, in all three metals, a decrease of frequency on cooling, while one expects a nearly temperature-independent saturation (T --> 0) behavior. Al though the origin of this feature is not clear, it definitely cannot be con nected to a spin reorientation. It is suggested that spontaneous bulk magne tization caused by the strong magnetic anisotropy might be responsible. In Dy and Ho only minor irregularities are seen at T,. In contrast, Er shows a huge drop of the spontaneous frequency at the fm transition temperature, w hich can be directly traced to the behavior of the dipolar field component at the muon site. Saturation values for the dipolar and the contact field a t the muon site for the three metals are given. (C) 2000 Elsevier Science B .V. All rights reserved.