Journal of Magnetism and Magnetic Materials 54-57 (1986) 253-254 253 zyxwvutsr FMR STUDY OF CRYSTALLIZATION IN THE METGLAS 2826 AMORPHOUS ALLOY R.S. de BIAS1 and A.A.R. FERNANDES Cenrro de Pesquisa de Materiais, Insfituto Militar de Engenhrrria, 22290 RIO de Jmero, RJ, Brazrl The ferromagnetic resonance technique was used to study the annealing behavior of the metallic glass Fe,,Ni,,,P,,$ (Metglas 2826). The peak-to-peak FMR linewidth, AH,, was measured at room temperature and 9.5 GHz for several times of isothermal annealing in the temperature range 350-375°C. After an initial decrease, attributed to structural relaxation. the linewidth increases with annealing time for all temperatures investigated. The activation energy for crystallization. E,. is estimated from the times to AH,, = 20 to 80 mT as (412 k 23) kJ/mol. The results suggest that. for sufficiently long annealing times. there is a well-defined relation between AH and the volume fraction of the transformed phase. If this relation is assumed to be linear. the transformed fraction. as derived from linewidth data, satisfies the Johnson-Mehl-Avrami equation with the exponent n -equal to 1.67i0.13. 1. Introduction The thermal stability of metallic glasses is a subject of considerable interest, since the properties of these engineering materials may be significantly changed by the onset of crystallization. Ferromagnetic (FMR) spec- troscopy seems to be a convenient method to study the crystallization of metallic glasses, because it is a fast, sensitive and non-destructive technique. In the present work, the FMR method was used to investigate the annealing behavior of the commercial alloy Metglas 2826, manufactured by Allied Chemical Corporation. 2. Experimental procedure The alloy, of nominal composition Fe,aNi,,P,,$, was supplied in the form of ribbons 2 mm wide and 50 pm thick. Isothermal heat treatments were carried out in air, on small pieces of the ribbon (2-4 mm long), in a tube furnace with a temperature accuracy of f 1°C. First-derivative FMR spectra were recorded at room temperature using an X-band Varian E-12 spectrometer. All measurements were taken with the static field paral- lel to the sample surface and along the long axis of the ribbon. 3. Experimental results and analysis The FMR peak-to-peak linewidth, AHpp, is shown in fig. 1 as a function of annealing time, for four annealing temperatures between 350 and 375°C. Except for very small annealing times, the linewidth is de- scribed well by equations of the form AH,, = A + B[l - exp( -kc”)] with A = 16 mT, B = 82 mT. This sug- gests that, as in the case of the metallic glass Metglas 2826A [l], the linewidth increases linearly with the transformed fraction of the sample (in Metglas 2826, two crystallization phases are present [2]; since the FMR spectra were obtained at a temperature T< T,,, Tc., where T,, = 400 K and Tc. = 730 K are the Curie temperatures [3] of the two ferromagnetic phases, they 01 1 I I 0 20 40 60 60 ANNEALING TIME. t (HOURS) Fig. 1. FMR linewidth as a function of annealing time, for four different annealing temperatures. should reflect the combined effects of the two phases). The initial decrease, which is of the order of 3 mT, is attributed to structural relaxation in the amorphous phase [1,4,5]. The linewidth in untreated samples, 19 mT, is about the same as that reported by Baianu et al. [4], but 9 mT larger than that reported by Bhagat et al. [6]. The reason for this discrepancy is unclear; it could ANNEALING TIME, t ( HOURS) 0.5 1 5 10 50 2, I , 0 4n t* 4 Fig. 2. Avrami plots for four different temperatures. zyxwvutsrqpon 1 0304-8853/86/$03.50 0 Elsevier Science Publishers B.V.