Crystallization of amorphous alloys induced by the rf magnetic field Michael Kopcewicz Institute of Electronic Materials Technology, Warszawa, Wólczyńska Street 133, Poland

Crystallization is of crucial importance for amorphous alloys. Excellent soft magnetic properties of amorphous alloys dramatically deteriorate upon crystallization. The common origin of crystallization is due to thermal effects [1,2]. However, the crystallization may result also from nonthermal effects, e.g., those associated with mechanical deformations. An unusual effect of crystallization of amorphous phase (containing Co) was noticed in the rf-Mössbauer study of Fe 81-x-y Ni x Co y Zr 7 B 12 alloys [1]. The rf field induced crystallization is discussed for the x=30, y=10 (Fe 41 Ni 30 Co 10 Zr 7 B 12 ) and x=20, y=20 (Fe 41 Ni 20 Co 20 Zr 7 B 12 ) alloys and is compared with that of the x=40, y=0 and x=50, y=0 Co-free alloys. Introduction

The rf-Mössbauer technique (see e.g., [3-5]): - the ferromagnetic sample is exposed to the radio-frequency (rf) magnetic field that may induce the rf-collapse and rf-sideband effects. This technique allows us to follow the crystallization of certain amorphous alloys induced by the rf field. In particular the rf-sidebands effect is relevant to the present study. The rf-sidebands effect: - frequency modulation of Mössbauer  -radiation due to rf-induced vibrations of Mössbauer atoms via magneto-acoustic coupling - magnetostriction; Sideband positions are given by  n  r. Intensities: frequency modulation (FM) model; Rf-sidebands can be observed in ferromagnetic magnetostrictive materials (below the Curie point).

Experimental procedure Samples: Amorphous alloys: Fe 41 Ni 30 Co 10 Zr 7 B 12 (Co 10 alloy), Fe 41 Ni 20 Co 20 Zr 7 B 12 (Co 20 alloy) and Fe 41 Ni 40 Zr 7 B 12 and Fe 31 Ni 50 Zr 7 B 12 (Co-free alloys) are prepared by the melt quenching technique. The ribbons were 3-5 mm wide and about 25  m thick. The Mössbauer spectra : recorded at room temperature in the absence of the external radio-frequency (rf) field before and after the exposure to the rf field. The 57 Co-in- Rh source of 25 mCi activity was used. The rf-Mössbauer measurements were performed during exposure of the samples to a rf magnetic field with a frequency of 61 MHz and intensity of 20 Oe.

The Mössbauer investigations were accompanied by the measurements of magnetostriction constants of the alloys studied. The saturation magnetostriction constant s was measured at room temperature using a strain modulated ferromagnetic resonance method (SMFMR) [6, 7].

Results All Co-containing amorphous alloys studied revealed the onset of crystallization of an amorphous phase when exposed to the rf field of intensity up to 20 Oe at 61 MHz. The rf field induced crystallization effect depends strongly on the sample composition, in particular on the Co-content. One can easily observe this crystallization effect by comparing the Mössbauer spectra recorded before and after the rf field exposure. As examples, the spectra recorded for the Fe 41 Ni 30 Co 10 Zr 7 B 12, Fe 41 Ni 20 Co 20 Zr 7 B 12 alloys (Co-containing alloys designated as Co 10 and Co 20, respectively), and the Co-free Fe 41 Ni 40 Zr 7 B 12 alloy are shown in the next frame.

Velocity [mm/s] Transmission Velocity [mm/s]

The spectra of all amorphous alloys recorded before the rf field exposure reveal the shapes typical of ferromagnetic amorphous alloys (broadened sextets), (Figs 1a, 1d, 1g). The spectra recorded for the Co-containing samples after the rf field exposure clearly contain two spectral components (Figs. 1c, 1f): (i) a well-resolved sextet with narrow lines (the hyperfine field of about 36 T) characteristic of the crystalline bcc-FeCo phase, formed due to the rf induced crystallization of amorphous alloys, (ii) the broadened sextet characteristic of the retained amorphous phase. The spectra recorded for the Co-free amorphous alloys before and after rf field exposure are almost identical and consist of a broadened sextet characteristic of the amorphous alloy. They do not show any evidence of the crystallization effect (Figs. 1g and 1i). The rf-Mössbauer spectra recorded during the rf field exposure (Figs. 1b, 1e and 1h) consist of the rf-collapsed central component accompanied by intense rf- sidebands. The relative intensity of the rf-sideband lines provides direct information on the magnetostriction of the alloy studied.

Intensities of the rf-sidebands are particularly large for the Co-containing alloys (Fig.1b, 1e). The rf-sidebands vanish for zero magnetostriction alloys. - Large intensities of the rf-sidebands observed for Co-containing alloys (Figs. 1b, 1e) strongly suggest that the magnetostriction of these alloys is large. - The rf-sidebands observed for Co-free sample (Fig. 1h) are significantly smaller, suggesting a significantly smaller magnetostriction.

In order to suggest the origin of the rf field induced crystallization of the Co-containing alloys it is necessary to estimate the temperature of the sample during the exposure to the rf field applied. - It is assumed that the entire center shift of the spectrum recorded during the rf field exposure is caused by the rf-heating effect and can be described by the second order Doppler (SOD) shift. - The Debye temperature of the sample material must be known. - It is assumed that the typical Debye temperatures of Fe-based amorphous alloys are close to the room temperature [8]. The SOD factor can be determined for the relevant alloys from the linear dependence of the center shifts of the spectra vs. sample temperatures. The temperature of the samples during the rf field exposure was estimated by comparing the changes of center shifts of the rf-Mössbauer spectra (Figs. 1b, 1e, 1h) with those recorded for the same samples at room temperature in the absence of the rf field (Figs. 1a, 1d, 1g) and by dividing these differences by the SOD factor.

It was found that the temperature of the Co-free amorphous sample during the exposure to the rf field of the intensity of 20 Oe was about 200 o C and 230 o C for Fe 31 Ni 50 Zr 7 B 12 and Fe 41 Ni 40 Zr 7 B 12 samples, respectively. The temperatures of the amorphous Co-containing samples were higher, about 260 o C and 290 o C for Fe 41 Ni 30 Co 10 Zr 7 B 12 and Fe 41 Ni 20 Co 20 Zr 7 B 12 alloys, respectively. Thus, these temperatures of the samples were much lower than the temperatures of the first step of crystallization of the corresponding amorphous alloys (about 470 o C and about o C for the Co-free alloy and Co-containing alloys, respectively). The origin of the crystallization effect induced by the rf field as resulting from heating the sample can be excluded as a major mechanism that causes the crystallization. It is inferred that the rf-crystallization effect is of nonthermal origin.

While the common origin of conventional crystallization of amorphous alloys is usually thermal, crystallization may also originate from the nonthermal effects related to mechanical deformations. The crystallization of Co-containing amorphous phase, observed earlier [9], was related to mechanical deformations induced during the high-energy ball milling. Also in that case the concept of a high effective local heating during the milling process was ruled-out.

It is concluded that the rf field induced crystallization of amorphous alloys observed here is of magnetostriction origin. Since, the crystallization effect caused by the rf field is particularly pronounced in the amorphous alloys that reveal significantly large magnetostriction, the crystallization effect was attributed to mechanical deformations induced in the sample via magnetostriction (rf-sidebands effect). The rf field forced, via magnetostriction, enhanced vibrations of atoms as a result of which the amorphous structure was destabilized and partly crystallized. These vibrations cause the frequency modulation of the Mössbauer gamma radiation and the rf-sideband lines are formed in the rf-Mössbauer spectrum. When magnetostriction of the alloy is large (large rf-sidebands effect) then the rf field induced crystallization effect is strong. When the rf-sidebands effect is decreased because of small magnetostriction then the rf-induced crystallization does not occur.

The Mössbauer results are well supported by direct measurements of the saturation magnetostriction constants ( s ) [10] performed by using a strain modulated ferromagnetic resonance (SMFMR). The smallest value of s was obtained for Co-free Fe 31 Ni 50 Zr 7 B 12 alloy ( s  10 x ). The magnetostriction constant of Fe 41 Ni 40 Zr 7 B 12 alloy: s  11 x The magnetostriction constants, determined for Co-containing alloys, were significantly larger: s  15 x for Fe 41 Ni 30 Co 10 Zr 7 B 12 s  23 x for Fe 41 Ni 20 Co 20 Zr 7 B 12. The magnetostriction constant data obtained fully agree with the Mössbauer results.

Conclusion The rf field induced crystallization effect, observed in high magnetostriction Co-containing amorphous alloys, was attributed to mechanical deformations induced in the sample via magnetostriction (rf-sidebands effect). It did not occur in the Co-free amorphous alloys with smaller magnetostriction. The presence of Co is important for this effect, because Co creates significantly larger magnetostriction constants of the Co-containing amorphous alloys.

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These results were presented at the Magnetism and Magnetic Materials Conference (MMM-2007), Tampa, FL, (USA) in November 2007 and and were published in the Journal of Applied Physics 103, 07E717 (2008).