Journal of Magnetism and Magnetic Materials 54-57 (1986) 245-246 245

NMR STUDY OF THE ATOMIC STRUCTURE FOR HEAT TREATED METGLAS 2605 CO

J.C. FORD *, J.I. BUDNICK *, W.A. HINES *, M. CHOI *, G.H. HAYES *.

G.E. LONGWORTH +, D.M. PEASE * and D.P. YANG *

* Dept. of Physics, Unio. of Connecticut, Storrs, CT 06268, USA i Nuclear PhJslcs Div., AERE, HamelI, Oxfordshire. UK

Spin-echo NMR measurements of the “B, 57Fe and 59Co internal hyperfine field distributions in both as-fabricated and heat treated Metglas 2605 CO are presented. These spectra provide a clear picture of the evolution of various phases with annealing.

1. Introduction

Metglas 2605 CO (F%,Co1sBi4Si,) exhibits a very high magnetomechanical coupling factor (k,, = 0.71, a measure of the efficiency of conversion of magnetic energy into elastic energy) after undergoing specific magnetic annealing treatments [l]. In order to de- termine the near neighbor atomic environments, how they are affected by the thermal, magnetic and mechani- cal history, as well as investigate the phases that result from crystallization, NMR [2], X-ray diffraction [3], EXAFS and XANES, Mossbauer [4] and magnetization studies have been carried out on this system. The special contribution of the NMR work will be highlighted in

this presentation.

2. Experimental apparatus and procedure

Ribbons of Metglas 2605 CO. prepared by the “planar flow casting” process, are commercially availa- ble from Allied Corp. The NMR work has been carried out on three ribbon samples which have undergone the following annealing treatments: (1) no annealing treat- ment after the initial fabrication; (2) annealing for 10 min at 369°C in a magnetic field of 6.1 kOe which lies in the plane of the ribbon and transverse to its length; (3) annealing for 15 min at 425°C in a magnetic field of 6.1 kOe which lies in the plane of the ribbon and transverse to its length. The sample prepared from the

original material [(I) above] was designated “as- quenched”. The sample prepared by process (2) was designated “annealed” and represents a relaxed glassy state in which a very small amount of crystalline phase has emerged. The annealed sample yields the large value of k,,. The sample prepared by process (3) was desig- nated “crystalline” and represents a partially crystal- lized two-phase situation in which a significant amount of this initial crystalline phase has precipitated, leaving behind a residual amorphous phase with altered com- position. The crystalline sample has a drastically re- duced value of k,,.

Spin-echo NMR measurements of the internal hyper- fine field (HF) distributions for “B, 57Fe and 59Co were made on the three samples at 4.2 K over the frequency

range 25-310 MHz. The variable frequency pulsed NMR

apparatus, sample preparation and data-taking proce- dure have been described in detail elsewhere [2].

3. Results and discussion

Preliminary spin-echo NMR measurements of the “B internal HF distributions observed in heat treated Metglas 2605 CO samples have been reported previ- ously [2]. By systematically varying the pulse excitation conditions in this work, we were able to observe, for the first time, additional spectra associated with the “Fe HF distributions which are characteristic of the amorphous and bee cY-Fe(Co) phases present in the as-quenched, annealed and crystalline Metglas 2605 CO samples. Fig. 1 shows the spin-echo NMR spectrum for the as-quenched sample obtained over the low frequency range 25-55 MHz. The solid circles represent the “B present in the amorphous phase. which was observed here and in the earlier work [2]. The open circles repre- sent the “Fe present in the amorphous phase. The above assignments are based on intensity and pulse excitation considerations, as well as a knowledge of the HF behavior in Fe-B alloys. Furthermore, the mea- sured 57Fe amorphous phase HF (42.0 MHz or 304 kOe) is consistent with that obtained from Mossbauer

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Fig. 1. NMR spin-echo spectrum for as-quenched Metglas 2605 CO.

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Fig. 2. NMR spin-echo spectrum for annealed Metglas 2605 CO.

measurements [4]. Unlike the annealed and crystalline samples discussed below. there was no trace of a 57Fe spectrum associated with the bee cu-Fe(Co) phase in the as-quenched sample. The amount of surface crystallinity (preferrentially ordered bee a-Fe(Co). see ref. [3]) that occurs on the “wheel” side of the ribbon during the fabrication process is apparently too small to be de-

tected by the 57Fe NMR. However. as discussed below. a “‘Co NMR contribution has been attributed to the surface crystallinity occurring in as-quenched Metglas 2605 CO.

Fig. 2 shows the low frequency spin-echo NMR spectrum for the annealed sample. The solid circles represent the “B present in the amorphous phase, while the open circles represent the 57Fe present in the amorphous phase and the crosses represent the 57Fe in the bee n-Fe(Co) phase. Again, the various spectra can

be separated by a variation in the pulse excitation conditions. Fig. 3 shows the low frequency spectrum for the crystalline sample. We note that the “B spectra are identical for the as-quenched and annealed samples. The “corresponding” spectrum for the crystalline sam- ple (solid circles) demonstrates significant asymetry in that intensity is shifted from the high frequency side to the low frequency side. It has been suggested that annealing Metglas 2605 CO at = 425°C (i.e.. the crys- talline sample) causes bee n-Fe(Co) to precipitate until the residual amorphous phase composition reaches

(Fe.Co),,B,,. thereby reducing the transition metal concentration from 85 to 75% [4]. By a comparison with the Fe ,,,,, ~, B, amorphous alloys [5]. we estimate that, correspondingly, the “B HF is reduced from 35.0 MHz (25.5 kOe) to 32.4 MHz (23.6 kOe), while the 57Fe HF is reduced from 42.0 MHz (304 kOe) to 35.5 MHz (257

kOe). Hence. the spectrum for the crystalline sample. which was designated previously as ” B amorphous phase [2]. is probably a convolution of both “B and 57Fe in the residual (Fe.Co) 75 B,, amorphous phase. Measure- ments of the ‘“B NMR are currently in progress to resolve this point. Furthermore, a narrow “Fe spectrum with a peak at 51.3 MHz (372 kOe), which is character- istic of the precipitated bee cY-Fe(Co) phase, is just barely observable in the annealed sample and clearly resolved in the crystalline sample. Since the “Fe hyper- fine field in Fe,,,,,_, Co, is very sensitive to the Co concentration. we have exploited this fact to estimate that the precipitated bee a-Fe contains about 20 at9 Co. These results are in agreement with Mossbauer (41 and EXAFS measurements.

Finally, the entire “‘Co spectrum has been observed to be very broad (120&310 MHz) with a sharp peak-like structure occurring at approximately 245 MHz. This supersedes the incomplete spectrum reported in the earlier work [2]. Such high frequencies for ‘“Co are indicative of Co atoms which have Fe as first nearest neighbors. Specifically, the broad ‘“Co line observed in the as-quenched sample is associated with the amorphous phase and. perhaps, the sharp peak arises from Co in the bee cu-Fe(Co) phase which exists on the surface. Additional work is in progress to confirm the latter identification and obtain similar “Co spectra for

the annealed and crystalline samples.

[I] C.U. Modzelewski, H.T. Savage. L.T. Kabacclff and 4.b. Clark. IEEE Trans. Magn. MAC-17 (1981) 2837.

(21 J.C‘. Ford. W.A. Hines. J.I. Budnick. A. PaoluLi. D.M. Peasz, L.T. Kabacoff and C.U. Modzelewski. J. Appl. Phya. 53 (1982) 2288.

[3] M. Choi, D.M. Pease. W.A. Hines, J.I. Budnick, G.H. Hayes and L.T. Kahacoff. J. Appl. Phys. 54 (1983) 41Y3.

[4] G. Longworth and J. Budnick. AERE Tech. Rep. R-11526 (1984).

[5] J.C. Ford. J.I. Budnick. W.A. Hines and R. Hasegasn. J. Appl. Phys. 55 (1984) 2286.