Multipulse excimer laser irradiation of FeB amorphous alloys

Solid State Communications, Vol. 85, No. 8, pp. 717-721, 1993. Printed in Great Britain.

0038-1098/93 $6.00 + .00 Pergamon Press Ltd

MULTIPULSE EXCIMER LASER IRRADIATION OF Fe-B AMORPHOUS ALLOYS D. Sorescu," M. Sorescu, l'° I.N. Mihailescu, D. Barb and A. Hening Institute of Atomic Physics, 76900 Bucharest-Magurele, Romania

(Received 11 February 1992; in revisedform 15 August 1992; acceptedfor publication 27 October 1992 by G. Fasol) Substituted Fe-B amorphous ferromagnetic ribbons have been exposed to multipulse excimer laser irradiation (A=248nm, 7-= 20ns). The effects of laser treatment have been studied by Mrssbauer spectroscopy, X-ray diffraction and scanning electron microscopy. Depending on the number of applied laser pulses and sample composition, relaxation of internal stresses and/or controlled magnetic anisotropy could be obtained without onset of bulk crystallization.

1. INTRODUCTION IN O R D E R to obtain metallic glasses with improved properties for soft magnetic applications, several annealing techniques have been currently employed: isothermal [1] and isochronal [2] annealing, radiofrequency annealing [3] and magnetic annealing [4]. As well, the possibility of using continuous laser annealing to improve magnetic properties of amorphous ribbons, due to structural relaxation without the onset of bulk crystallization has been evidenced [5]. The present paper reports results of M6ssbauer spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) on samples of substituted Fe-B amorphous ribbons exposed to multipulse excimer laser treatment.

quartz lens to a spot size of few mm 2, followed by laser beam scanning of the whole sample surface, placed on a x - y micrometer stage. Room temperature Mfissbauer spectra of samples irradiated with 5, 25, 50 and 75 laser pulses per spot, at a repetition rate of 10Hz, were collected using a constant acceleration spectrometer, with the gamma ray perpendicular to the ribbon plane, in the conventional transmission geometry. The Mrssbauer measurements were carried out using a 57Co source in Cr matrix. Least squares fitting was performed in the assumption of Gaussian line shapes [6] using the program SPECFIT [7]. XRD and SEM analysis were performed on the irradiated surfaces in order to verify the presence of crystallization. 3. RESULTS AND DISCUSSION

2. EXPERIMENTAL The results presented here concern the amorphous ferromagnetic alloys FeslBt3.sSi3.~C 2 (METGLAS 2605 SC) and Fe40Ni~PI4B6 (METGLAS 2826). Square samples of 2cm wide were cut from the ribbon roll (20 #m thick) and exposed on both sides to the ~ = 248nm radiation generated by a KrF* excimer laser (model M 1701), with the pulse width rPWnM = 20 ns, capable of giving a maximum energy per pulse of 50mJ. Energy densities of about 10mJ mm -2 could be obtained within an acceptable degree of homogeneity by beam focusing with a 1 Polytechnical Institute Bucharest, Physics Department, 77206 Bucharest, Romania. * On leave for the Oklahoma State University, Stillwater, OK 74078-0444, USA.

Figures 1(a) and 2(a) show the room temperature Mrssbauer spectra of the FeslBl3.sSi3.sC2 and FeaoNi40Pl4B6 samples in the amorphous as-quenched state, respectively. For the 14.4keV gamma rays of 57Fe, the relative intensity of the second (fifth) to the first (sixth) lines is given, in thin absorber approximation, by [8]: R2z = 4sin2/3/3(1-]-CO82~), where /~ stands for the angle between the gamma ray and the direction of magnetic hyperfine field; the ratio R21 varies from 0 to 4/3 when 13 changes from 0 to 90 ° and for a completely random distribution of magnetic moment directions takes the value 0.67. On these grounds, the determined values of the areal intensity ratio (REl = 1.10 and 1.20, respectively) show that in the as-quenched state, the magnetic moments are preferentially oriented in the plane of the ribbons.

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Fig. 1. Room temperature M6ssbauer spectra of Fe81Bt3.sSi3.sC2 samples: (a) in the as-quenched state; (b) after 5 laser pulses per spot; (c) after 50 laser pulses per spot; (d) after 75 laser pulses per spot. Significant changes in the shape of M6ssbauer spectrum are visible in Fig. 1(b) for the FesiBt3.sSi3.sC2 sample irradiated with 5 laser pulses per spot. The laser induced effects have resulted in a pronounced decrease of the intensity ratio of the second to the first line, the value R2t = 0.11 showing that a rotation of the average magnetic moment

direction from the in-plane (~ = 90 °) to the out-ofplane orientation has taken place. As can be seen in Table 1 for the FesIBI3.sSi3.sC2 sample irradiated with 25 laser pulses per spot and in Figs. l(c) and l(d) for the F%B~3.sSi3.sC2 samples irradiated with 50 and 75 laser pulses respectively, the relative intensity of lines R2t increases again

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Fig. 2. Room temperature M6ssbauer spectra of samples: (a) in the as-quenched state; Fe40Ni40Pl+B6 (b) after 5 laser pulses per spot; (c) after 50 laser pulses per spot; (d) after 75 laser pulses per spot.

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MULTIPULSE EXCIMER LASER IRRADIATION OF Fe-B ALLOYS

Table 1. Relative intensity of lines R21 and hyperfine magnetic field of the substituted Fe-B alloys, exposed to multipulse excimer laser irradiation

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Fe40Nia0P14B6 (As ---- 11 x 10 -6)

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Fig. 3. Relative intensity of the second to the first line of the M6ssbauer spectra, as function of the number of applied laser pulses on the substituted Fe-B alloys: * for FeslBl3.sSia.sC2 and ~ for Fe4oNi40PlaB6. reached even after 5 laser pulses per spot and remained practically unchanged after 75 laser pulses, indicating the relaxation of the internal stresses present in the as-quenched ribbon [9]. For direct comparison, the intensity ratio R21 is plotted in Fig. 3 as a function of the number of applied laser pulses for both studied amorphous systems. These results show that the multipulse excimer laser irradiation proves to be a method of inducing controlled magnetic anisotropy and/or relaxing the internal stresses in Fe-B alloys, depending on the type of substitutions and on the number of laser pulses. The fact that the anisotropy energy induced by the annealing is greater in the higher magnetostriction samples (FesiBla.sSia.sCz) than in the lower As ones (Fe40Ni40PlaB6) is in complete agreement with previously reported results [10]. As well, the occurrence of two opposite effects was also observed in continuous laser annealing [9]: relaxation of the quenching stresses or production of stresses (as

compared to Fig. l(b), taking the corresponding values 0.21, 0.34 and 0.38, which are still far from the value indicating the random distribution of magnetic moment directions. Consequently, the magnetic anisotropy induced by the laser treatment is found to depend on the number of applied laser pulses. Selected M6ssbauer spectra taken at room temperature for the Fe40Nia0P]aB6 samples, after irradiation with the same number of laser pulses per spot as the FeslB13.sSi3.sC2 samples are shown in Fig. 2(b)-(d). The values of the relative intensity R21 (Table 1) indicate a reorientation of the magnetic moment axis, as consequence of the laser treatment performed. However, in contradiction to the behaviour of the Fe81BI3.sSi3.sC2 samples, the random distribution of magnetic moment directions has been

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MULTIPULSE EXCIMER LASER IRRADIATION OF Fe-B ALLOYS

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Fig. 5. SEM examination of Fes~B13.sSi3.sC2 sample after irradiation with 75 laser pulses per spot. The horizontal line at bottom of the photo equals 1 #m. a consequence of the rapid heating and cooling). In addition, it has been shown that the formation of stresses can determine an out-of-plane orientation of the magnetic moment directions [11]. Importantly, however, the pulsed excimer laser treatment performed in the present study did not cause the onset of bulk crystallization. As can be seen in Table 1, the laser irradiation had no detectable effects on the values of the hyperfine magnetic fields in both amorphous systems. Moreover, the XRD measurements performed on Fes~BI3.sSi3.sC2 and Fe40Ni40Pl4B6 samples after 75 laser pulses [Fig. 4(a)-(b)] show a broad intensity maximum at diffraction angles between 40 ° and 50 °, typical for the amorphous state and confirming thus the absence of crystalline phases [12]. In addition, SEM examinations of the FeslBz3.sSi3.sC2 sample after irradiation with 75 pulses (Fig. 5) revealed the existence of relatively large areas, consisting of melt zones which later solidified rapidly, without preserving the dendritic aspect characteristic to a material crystallized by slow cooling.

4. CONCLUSIONS The present results allow us to conclude that multipulse excimer laser treatment of amorphous ferromagnetic ribbons can be used in order to induce controlled changes in the magnetic anisotropy, without causing significant bulk crystallization. The effects of laser irradiation depend on the number of applied laser pulses and on the composition of the amorphous system. Multipulse excimer laser irradiation may represent the alternative method in order to produce structural relaxation and improvement of soft magnetic properties.

A c k n o w l e d g e m e n t - The authors are indebted to Mr. G. Nita for performing the SEM examinations and for helpful discussions.

REFERENCES 1.

D.G. Morris, A c t a M e t . 29, 1213 (1981).

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MULTIPULSE EXCIMER LASER IRRADIATION OF Fe-B ALLOYS

H.N. Ok & A.H. Morrish, J. Phys. FI1, 495 (1981). M. Kopcewicz, Appl. Phys. 23, 1 (1980). A. Gangulle, J.A. Aboaf & R.J. Kobliska, J. Appl. Phys. 52, 2880 (1981). P. Matteazzi, L. Lanotte & V. Tagliaferri, Hyp. Int. 45, 315 (1989). P.J. Schurer & A.H. Morrish, Phys. Rev. B26, 1305 (1982); Solid State Commun. 28, 819 (1978). F.D. Barb, O. Netoiu, M. Sorescu & M. Weiss, Computer Phys. Commun. 69, 182 (1992).

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