We present an in situ butterfly study of the in plane magnetization reversal of epitaxial hcp Gd(0001) thin films grown on bcc W(110) at T(s) = 450-degrees-C in ultrahigh vacuum (UHV). Two UHV-compatible techniques, viz., transverse biased initial susceptibility (TBIS) and magneto-optic Kerr effect (MOKE) are used to in situ record the so-called butterfly loops well below the bulk Curie temperature (T(cb) is-approximately-equal-to 293 K). The derivative character of these loops allows one to determine the in-plane coercive field (H(c)). Hcp Gd(0001) thin films, 40 and 80 nm thick, are found to be magnetically soft with in-plane H(c) < 30 Oe for T = 109, 132 and 230 K. A low value of H(c) (is-approximately-equal-to 5.8 Oe) is observed at T = 230 K, this temperature being close to the onset of spin-reorientation in bulk gadolinium (T(SR)b is-approximately-equal-to 245 K). Also at T = 230 K, a value of in-plane anisotropy field H(k) is-approximately-equal-to 27 Oe is deduced. We distinguish two cases, both of them being measured in the low-applied-field regime where para-processes are negligible: (i) the Gd(0001) thin films when biased perpendicularly to the basal plane exhibit an anhysteretic behavior and (ii) as soon as the films are biased in the basal plane, the bias-field dependence of both TBIS and MOKE signals becomes hysteretic and the magnetization does reverse. The anhysteretic behavior might be related to the recently observed effect of surface spin canting. In this article, we focus mainly on the case of Gd(0001) thin films biased in the film plane. From the so-called Frolich-Kennelly plots, it is concluded that thinner Gd(0001) films show smaller hysteresis losses accompanying the in-plane magnetization reversal. A comprehensive historical review of the butterfly methods is also given in this paper.
