Strain-Induced Ferromagnetism in Fe-40Al Single Crystals
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Strain-Induced Ferromagnetism in Fe-40Al Single Crystals D. Wu and I. Baker Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, U.S.A. ABSTRACT Recent research [1-3] suggests that strain-induced ferromagnetism in lightly-strained FeAl arises chiefly from anti-phase boundary tubes. Magnetic and calorimetric measurements have been performed on two different orientations of B2-structured Fe-40Al single crystals that had been cold rolled to a variety of strains. The saturation magnetization and magnetic susceptibility are related to the enthalpy associated with the annealing out of antiphase boundary tubes and, hence, to the degree of deformation and crystal orientation. INTRODUCTION Many intermetallics which are paramagnetic when well-annealed become ferromagnetic upon plastic straining, e.g. Fe3Al [4-8], FeAl [1-3, 9-16], CoAl [17], CoGa [18], Ni3Sn2 [19], Fe3Ge2 [19], Pt3Fe [20-23]. Deformation induces disorders so that atoms of the ferromagnetic element are no longer isolated from each other but interact. After severe deformation, e.g. by ball-milling or prolonged crushing [5, 9, 10], the whole lattice is disordered, and the resulting ferromagnetism is well explained by the local environment model [24] in which the magnetic moment of an element depends on the number of like nearest neighbors. After less severe deformation, e.g. tensile testing or rolling, strain-induced ferromagnetism can still be observed [4, 6, 7, 11-16]. In this case, disorder is present not throughout the whole lattice, but largely in antiphase boundaries (APBs). Huffman and Fisher [4] first suggested that these are the source of strain-induced ferromagnetism. Hence, Takahashi et al. [11-16] modeled the magnetic behavior of iron-rich FeAl using the local environment model applied to the APBs between gliding dislocations. However, Takahashi’s approach cannot correctly predict the saturation magnetization of lightly-strained FeAl unless physically unrealistic assumptions are made concerning the width and influence of the APBs. Further, a 500 K anneal of cold-rolled Fe-40Al single crystals which are ferromagnetic at room temperature returns the crystals to their paramagnetic state, but changes neither the density nor configuration of dislocations [2]. Hence, APBs between gliding dislocations cannot account for strain-induced ferromagnetism. Yang and Baker [1] first proposed that strain-induced ferromagnetism in lightly-strained intermetallics arises not from APBs between gliding dislocations but from APB tubes [25]. APB tubes have been observed in a number of lightly-strained intermetallics using transmission electron microscopy (TEM) [3, 26-38]. Further, Yamashita et al. [3] showed that polycrystals of B2-structured Fe-35Al strained under compression at 77 and 300 K contained APB tubes and exhibited spontaneous magnetization, whereas the same material strained at 650 K exhibited neither. They concluded that APBs coupling slip dislocations could not account for the straininduced ferromagnetism and showed that a calculation of t
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