Effects of cobalt substitutions in spring magnets

Abstract

Spring magnets of the type α-Fe/Nd2Fe14B consist of a soft phase (α-Fe) and a hard phase (Nd2Fe14B), which are exchange coupled [1, 2]. These nanocomposite magnetic materials exhibit high magnetic remanence, high coercivity, high energy product, and low cost. In these nanostructures, the enhancement of magnetic properties is related to the exchange coupling between hard and soft nanograins [3–11]. In this paper we propose to study the effect of Co substitutions on the structural and magnetic properties of spring magnets. We employ X-ray diffraction (XRD) and Mossbauer spectroscopy to characterize the main features of Co additions and in particular, to answer the question of whether Co enters the soft phase or the hard phase. Three compositions of spring magnets, Nd11 Fe80.6Co2.6B5.8, Nd10.5Fe78.4Co5.3B5.8, and Nd10Fe76.5 Co7.9B5.6, were prepared by arc melting and annealed for 3 min at 690, 720, 755, and 775 ◦C. XRD measurements were performed using a Rigaku D-2013 diffractometer with Cu Kα1 radiation at λ = 1.5404 A. Transmission Mossbauer measurements were made with a constant acceleration spectrometer. The 50 mCi gamma ray source was 57Co in Rh matrix, maintained at room temperature. All spectra were analyzed with the NORMOS-DIST program, which uses the histogram method to obtain the hyperfine magnetic field distributions. Fig. 1 shows the XRD pattern of the Nd11Fe80.6 Co2.6B5.8 composition, annealed at 690 and 755 ◦C. We could identify the soft phase α-Fe(Co), which is crystalline and the hard phase Nd2Fe14B, which is predominantly amorphous. Similar features were exhibited by the samples Nd10.5Fe78.4Co5.3B5.8 and Nd10Fe76.5Co7.9B5.6, annealed at 690 and 775 ◦C. Fig. 2a and b present the room temperature transmission Mossbauer spectra of the Nd11Fe80.6Co2.6B5.8 system, annealed at 690 and 755 ◦C. In (A) and (B) we give the hyperfine magnetic field distributions extracted from these spectra by nonlinear least-squares fitting. Similar spectra were obtained for the compositions Nd10.5Fe78.4Co5.3B5.8 and Nd10Fe76.5Co7.9B5.6, annealed at 690 and 775 ◦C. In view of the XRD results, all Mossbauer spectra were analyzed with a six-line pattern corresponding to the α-Fe(Co) crystalline phase,