Defect Properties and Magnetism in B2-type CoFe Alloys

Abstract

Point defect properties have been studied for B2-type Co1-cFec alloys (0.4 ≤ c ≤ 0.6 ) by X-ray diffraction and density measurements. The lattice constant increases with increase in composition c and indicates a distinct change in the slope beyond c = 0.5. Further, it shows a decrease with increase in quenching temperature. The obtained vacancy concentration is very low for any composition and temperature concerned. Thus, it can be concluded that defect type in B2 CoFe is an antisite atom. The change in the lattice constant with quenching temperature can be related to the long-range order (LRO): it is well approximated by a linear form with respect to square of the LRO parameter. CoFe alloys in both B2 and higher temperature A2 phase regions show ferromagnetism. Observed results are discussed taking a correlation between the defect structure and the magnetic behavior into account.

Magnetism and point defect in B2-type CoFe alloys

“…As pointed out previously [14,15], such magnetic behavior relates to the change in the number of nearest neighboring Co-Fe atom pairs. Denoting the number as N CoFe , this is given by…”

“…Fig. 7 shows the relations between previously observed a [14] and estimated hmiKhmi A2 for cZ0.48, 0.50 and 0.52 in (a) and cZ0.40 and 0.60 in (b). Eq.…”

“…As mentioned in Section 1, major point defects in B2-type Co 1Kc Fe c alloys are antisite atoms [14], meaning that the atoms occupy the wrong sites, i.e. the antisite Co atom (Co atom on the Fe-site) and the antisite Fe atom.…”

“…4). In the previous work [14], we examined the observed quenching temperature dependence of the lattice constant, a, through the calculated LRO parameter and explored the relation between a and S under the assumption that the alloy quenched from T Q is in the same LRO state as that at TZT Q . As a result, linear relation between the lattice constant and the square of the LRO parameter, S 2 , was found, which is shown in Fig.…”

“…B2-A2 order-disorder and FM-PM magnetic transitions, from the viewpoint of point defects. Recently, we studied the point defect properties of CoFe alloys in the B2 and A2-phase regions from the density and lattice constant measurements as functions of composition and quenching temperature [14]. Resultant i.e.…”