J A. Bland, S J. Steinmuller, T Trypiniotis, William F. Egelhoff Jr.
In their article High Temperature Ferromagnetism in GaAs-Based Heterostructures with Mn ? Doping , the authors A. M. Nazmul et al. report finding remarkably high Curie temperatures in Mn delta doped GaAs structures. They base their assessment on the observation of clear ferromagnetic hysteresis in the measured Hall resistance loops that persists up to 250 K. However, a close examination of the measurements presented in the paper (Fig. 3 (a)-(e) and Fig. 4(b)-(e)) reveals that the Hall resistance data shown for negative applied magnetic fields is identical to that shown for positive fields (with a change of sign), including the details of the background noise. The probability of this happening for a single loop would certainly be very small, but it is practically zero for nine different independent measurements. We would therefore like to ask the authors to clarify how exactly the published data was obtained, since for the claim of ferromagnetic hysteresis it is absolutely essential to perform a full field sweep (i.e., from negative to positive saturation and then back to negative).