PHILOSOPHICAL MAGAZINE B, 1999, VOL. 79, NO. 3, 449±461 On the mechanism of the paramagnetic-to-ferromagnetic transition in Fe±Al Y. Y ang, I. Baker Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755, USA and P. Martin Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA [ Received 23 June 1998 and accepted 29 August 1998] A bstract Single crystals of Fe±40 at. % Al were cold rolled to a variety of strains up to 48 % , when cracking occurred. The rolled crystals were heated at 10 K min 1 to 973 K in a dierential scanning calorimeter, whereupon three exothermic peaks were observed. The cold rolling also induced a transition from paramagnetism to ferromagnetism. At room temperature, the ferromagnetism disappeared upon annealing above the lowest-temperature exothermic peak, but at temperatures below 225 K the annealed specimen still showed a larger magnetic susceptibility than the virgin single crystal. Analysis of the possible contributions to the ferromagnetic behaviour suggests that antiphase boundaries (APBs) , principally in APB tubes, are the source. § 1. Introduction For well annealed Fe±Al alloys with a well ordered B2 structure and over 35 at. % Al a transition from paramagnetism to ferromagnetism can be induced by plastic deformation. A number of explanations have been proposed for this beha- viour (Taylor and Jones 1958, Kouvel 1969, Beck 1971, Besnus et al . 1975, Human and Fisher 1967). Besnus et al. (1975 ) studied the magnetic behaviour of fully disordered Fe±Al alloy powders, containing from 20 to 51 at. % Al, produced by ball milling. By cal- culating the number of iron nearest neighbours (NNs) around an iron atom, they explained the saturation magnetization using the local environmental theory in which the magnetic moment of an iron atom depends on the number of iron atom NNs. The magnetic moment of an iron atom is about 2.2 ¹ B , where ¹ B is the Bohr magneton, when it has ®ve or more iron NNs. The moment is 1.8 ¹ B when it has four iron NNs, and zero when it has fewer than three iron NNs. The local environment theory has also been used in an attempt to explain the dependence on aluminium concentration of the ferromagnetic behaviour of strained fully ordered Fe±Al alloys (Kouvel 1969, Human and Fisher 1967). Human and Fisher (1967) proposed that antiphase boundarie s (APBs) produced during deforma- tion are the source of ferromagnetic behaviour, since the number of Fe±Fe NNs is greater there. 0141±8637/99 $12. 00 Ñ 1999 Taylor & Francis Ltd.