Calorimetric study of amorphous Fe80.3Ho1.7B18

Materials Letters 18 (1993) 35-38 North-Holland

Calorimetric study of amorphous Fe80.3H01.7B18 G. Ravach a, J.M. Saiter b, A. Fnidiki a and J. Teillet a ’ U.R.A. CNRS 808, FacultP des Sciences de Rouen, P.O. Box 118, 76134 Mont b LECAP, Fact&

Saint Aignan Cedex. France des Sciences de Rouen, P.O. Box 118, 76134 Mont Saint Aignan Cedex, France

Received IO July 1993; accepted 2 August 1993

Amorphous ribbons of Fes,,,Ho,,,B,s have been studied by differential scanning calorimetry and by magnetization measurements. The Curie temperature, determined from both methods, is rc= 628 K and the primary crystallization appears up to T,= 800 K. Comparison with results where the rare earth is replaced by Er shows that Ho leads to better thermal and magnetic stabilities than Er. The crystallization kinetics, analyzed from the Kissinger method, provides an apparent activation energy of 6.4 eV, a value attesting to the good stability of the amorphous state of this system.

1. Introduction

Transition metal-rare earth or transition metalmetalloid based alloys are of great interest judging by the number of works performed during the last decades [ 11. By rapid solidification from the melt, a large range of metastable microstructures can be reached, providing a large variety of physical properties, particularly in the magnetic topics. On the other hand, when the alloys are amorphous, heat treatments also lead to structural modifications which are of great concern in every application [ 2,3 1. As shown by Buschow [ 41, ternary compounds exhibit great advantage because they can give rise to physical properties not present in binary compounds. In many cases, the ternary system is composed of transition metal and rare earths, giving magnetic properties, and as the third element a metalloid, which allows for a better ability to the amorphization of the melt. In this context, amorphous Fe80.3Hol.,B18 alloys were studied by scanning calorimetry. The values of the ferrimagnetic to paramagnetic transition temperature were measured for different heating rates. The crystallization temperatures and the crystallization kinetics analyzed from the Kissinger method were also determined. Finally, our results are com-

pared with others obtained for systems where Ho is replaced by Er.

2. Experimental Amorphous ribbons were prepared by the usual melt-spinning technique. The ingots were first prepared from high-purity components (better than 99.9%) by induction melting in Ar atmosphere using a water-cooled copper crucible. The quenched materials were obtained by planar flow casting in an Arfilled chamber (to avoid Ho oxidation) with a linear velocity of about 30 ms- ’ [ 5 1. The amorphous state was checked by X-ray diffraction and the composition determined by electron-probe microanalysis. Calorimetric measurements were performed with a differential scanning calorimeter ( Perkin-Elmer DSC system 7). The obtained enthalpic curves were normalized to 1 mg of material. Calibration was achieved with indium as reference material. Magnetic measurements were performed using a Faraday balance.

3. Results The DSC scans obtained at different heating rates (r= 5, 10,20, 30,40 K min-’ ) for the Fe80.3H01.7B,8 amorphous alloys are reported in fig. 1. In the same

0167-577x/93/% 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

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200 ,

d ..

.

’ l._Tc . I

575

595

615

i

OC

635

250

TEMPEMTL’RE (K)

350

550

650

temperature

C

750

850

(K)

Fig. 2. Temperature dependence of the magnetization in amorphous Feso.3Hoi.rBia. Table 1 Variation of the crystallization temperature 7’, with the heating rate r for the Fes0.3Hoi.,B,s amorphous system r (K/min)

b

5

10 20 30 40 .

T, (K) 800 806 813 816 818

co

3

Y

41 530

5S0

630

680

TEMPERATURE

730

7so

a

szo

(K)

Fig. 1. Enthalpic curves obtained for amorphous Feso.,Hoi.,Bis at different heating rates: (a) 5 K min-‘, (b) 10 K min-‘, (c) 20 K min-i and (d) 40 K min-i. (e) Expanded view for heating rate 30 K min-‘; (f ) expanded view for heating rate 40 K min-i.

figure, we also report the expanded views of the lowest part of the range of analyzed temperatures (fig. Id). At first, we observe a reaction which seems to be endothermic for temperatures from 603 to 633 K. From these measurements, no evidence of a thermal activation can be clearly observed. The variations of the magnetization (a) with temperature, obtained from measurements on a Faraday balance, show a decrease of a from 160 to 0 emu g- ’ in the range 250 KG T< 650 K (see fig. 2). The inflexion point at 36

T= 628 K, called the Curie temperature (T,), corresponds to the ferrimagnetic to paramagnetic transition. The endothermic reaction previously observed can then be identified with this magnetic transition. Otherwise, for temperatures higher than 780 K, a sharp exothermic peak of crystallization is observed. On increasing the scanning rate, this peak is shifted towards higher temperatures. The crystallization temperature (T,) given in table 1 corresponds to the extremum of the peak.

4. Discussion In table 2 we report the values of T, and T, obtained by different authors for similar amorphous ternary systems. The comparison of our results with those obtained for the Fe,OO_X_,,ErXB,, amorphous system (0.5~~~3.8, 16~y~20) [6] and for the

Table 2 The Curie, Tc, and crystallization, r,,,, temperatures obtained for Fe-Er-B, Fe-Ho-B and Fe-B amorphous systems System

Tc (R)

623 585 Fe79.5Ero.5B20 645 675 ” Fe~d%.dh~ 685 FeaaBzo 630 Fe&s Feso.3Hol.7B1s Feso.lErl.7Bls.2

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T, W)

Ref.

800 765 683

this work

['31 161 [71

715 730

[81 t91

*)Extrapolated from Tc versus x.

FeSz_,HoXB,, amorphous system (XG 16) [7] shows that the substitution of the metalloid by the rare earth element leads to an increase in the crystallization temperature as well as a decrease in the Curie temperature, and that the same behaviour is obtained by the substitution of the transition metal element by a rare earth. The addition of these rare earth elements in FesoBzo or FeBzB18then leads to a better thermal stability and a weaker magnetic stability. This agrees with previous results for closely related systems [ 1O151. The decrease in T, can be understood by taking into account: - The size effect of rare earth atoms leading to an increase in distances between iron atoms, thus weakening the exchange in the 3d sublattice. - The antiparallel coupling between heavy rare earth and transition metal moments. - The hybridization between 4s-5d orbitals of iron and 5s-4f orbitals of holmium. For the binary Fe-B amorphous systems, the exothermic peak corresponds to the primary crystallization of a-Fe [ 16,171. When a rare earth is added to these binary alloys, Das et al. [ 18 ] noticed that the same phenomenon appears at higher temperatures. Thus, the modifications in T,,, values that we observe, when the amount or the nature of the rare earth is varied, can be understood, in a first approach, as the ability of the rare earth elements to shift the precipitation of a-Fe towards higher temperatures. The size effect again, as well as the high chemical affinity of rare earth atoms for boron, can explain the better thermal stability as it hinders the boron diffusion which is known to control the crystallization process in Fe-B metallic glasses. On the

other hand, as Ho gives higher values of Tc and T,,, than those observed for Er, the nature of the rare earth also plays a role, the magnetic and thermal stabilities being better with Ho. This may be due to a larger metallic radius of Ho than of Er [ 19 1, which could induce a stronger atomic size effect. The kinetics of a first-order crystallization process is usually described, in metallic glasses, by the following relationship: T) (1 -z)

dz/dt=K(

,

(1)

where dz/dt is the rate of reaction, t the time, T the temperature, z the crystallized fraction and K is assumed to follow the Arrhenius law: K( T) = K. exp ( - EJkT)

,

(2)

with E, the apparent activation energy and K. a preexponential factor. Assuming that the rate of reaction is maximum at the extremum of the peak ( d2z/ dt2=0), K. exp( -E,/kT,)

=E,r/kT&

(3)

,

where r is the heating rate, and allows one to determine the apparent activation energy from the Kissinger plot (ln(r/TL) versus l/T,,,). Using data from table 1, the Kissinger plot (fig. 3) provides a value of 6.4 eV per atom for the apparent activation energy. Such a high value agrees with those generally observed for this class of materials [ 19 1, and with the result obtained by Clavaguera-Mora et al. [ 2 1 ] for the Fe7,Nd3B2,, amorphous alloy (i.e., 5.9 1 eV per atom), attesting to the good thermal sta-

1 OOOlTm

-13.5

1.21 *mu

1.22

1.23

1.24

1.25

-14 1 Nj

-14.5

2 z

-15 -15.5

!

1

-16 ’ Fig. 3. Kissinger plot of the crystallization process observed in Feso.3HOdls.

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bility of the amorphous state of these systems.

Acknowledgement

We are grateful to R. Krishnan and H. Lassri from the Laboratoire de MagnCtisme et MatCriaux Magnttiques of Meudon (France) for providing us with the studied samples.

References [ 1] K.H.J. Buschow, Handbook on the physics and chemistry of rare earths, eds. K.A. Gschneidner Jr. and L. Eyring (Elsevier, Amsterdam, 1984) p. 265. [2] K.H.J. Buschow, Mater. Sci. Rept. 1 (1986) 1. [ 31 S. Hatta and T. Mizoguchi, Japan. J. Appl. Phys. 27 ( 1988) 2078. [4] K.H.J. Buschow, J. Alloys Comp. 193 (1993) 223. [ 5] H. Lassri, Thesis, Rouen, France ( 1990). [ 61 J. Teillet, H. Lassri, R. Krishnan and A. Laggoun, Hyperfiie Interactions 55 (1990) 1083. [ 7 ] G. Ravach, A. Fnidiki, J. Teillet, R. Krishnan and H. Lassri, Hypertine Interactions, to be published.

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[8] C.L. Chien, Phys. Rev. B 18 (1978) 1003. [ 9] K. Fukamichi, M. Kikuchi, S. Arakawa and T. Masumoto, Solid State Commun. 23 (1977) 955. [ lo] B.X. Gu, H.R. Zhai and B.G. Shen, Phys. Rev. B 42 (1990) 10648. [ 111 B.X. Gu, B.G. Shen, S. Methfessel and H.R. Zhai, Solid State Commun. 70 ( 1989) 933. [ 121 L.X. Liao and Z. Altounian, J. Appl. Phys. 66 (1989) 768. [ 131 B. Idzikowski and A. Wrzeciono, J. Phys. (Paris) 49 C 8 (1988) 1287. [ 141 T. Miyazaki, H. Takada and M. Takahashi, Phys. Stat. Sol. 99a (1987) 611. [ 151 L. Potocky, J. Kovac, L. Novak, E. Kisdi-Koszo and A. Lovas, Rapidly Quenched Met. 12 (1985) 1153. [ 161 F.H. Sanchez, Y.D. Zhang, J.L. Budnick and R. Hasegawa, J. Appl. Phys. 66 (1989) 1671. [ 171 Z. Altoumian, J.O. Strom-Olsen and M. Olivier, J. Mater. Res. 2 (1987) 54. [ 18 ] B.N. Das, P. D’Antonio and N.C. Koon, Metall. Trans. A 21 (1990) 2805. [ 191 P. Hansen, Handbook of magnetic materials, Vol. 6, ed. K.H.J. Buschow (Elsevier, Amsterdam, 1991) p. 335. [ 201 P. Hansen, in: Handbook of magnetic materials, Vol. 6, ed. K.H.J. Buschow (Elsevier, Amsterdam, 199 1) p. 307. I21 ] M.T. Clavaguera-Mora, M.D. Baro, S. Surinach and N. Clavaguera, J. Mater. Res. 5 ( 1990) 1201.