Magnetic softening and nanocrystallization in amorphous Co-rich alloys

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of

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ELSEVlER

Journal of Magnetism and Magnetic Materials 172 (1997) 147-152

Magnetic softening and nanocrystallization in amorphous Co-rich alloys G. Buttino, A. Cecchetti, M. Poppi* Department

of Physics,Istituto Nazionale Fisica Della Materia, Via Paradiso 12, Universih, of Ferrara, I-44100 Ferrara. Italy Received 21 February 1997

Abstract In this work we have analyzed the changes of the magnetic properties in the Co-based Metglas 2714A (made by Allied Chem. Corp., USA) caused by isothermal heat treatments in the range of temperature from room temperature to conventional crystallization temperature T,, = 550°C. The nominal composition of the amorphous alloy is Co66Fe4NilB14Si15. The analysis is made on toroidal samples prepared by winding lengths of amorphous ribbon of about 20 cm. The magnetic properties undergo variations depending on the treatment temperature, except for the saturation magnetization which remains unchanged. For heat treatments of about half an hour around 500°C superior soft magnetic properties are obtained. Particularly, the initial permeability reaches values up to ten times the value of permeability in the as-received sample. Analysis by the transmission electron microscopy of the sample annealed around 5OO’C reveals the formation of a nanocrystalline phase, with average grain size of 2 nm, embedded in a residual amorphous matrix. The occurrence of permeability increases in concomitance with the formation of the nanocrystalline phase is ascribed to a drastic reduction in the local magnetocrystalline anisotropy randomly averaged out by the exchange interactions, similar to the case of the annealed Fe-based alloys containing Cu. PACS:

75.52.Kj; 75.60.Nt

Keywords: Co-rich amorphous

alloys; Heat treatments; Nanocrystallization;

magnetic properties. anisotropy direction

1. Introduction It is commonly

assumed

that

annealing

treat-

ments of amorphous materials below their crystallization temperature, relaxes the amorphous structure giving rise to a moderate softening of the

*Correspondingauthor. Tel.: + 39 532 781811; fax: + 39 532 781810.

Soft magnetic materials

Changes in the pre-existing have been observed in some

cases, due to the rearrangement of the atom pairs also in absence of external magnetic field and applied stress [l]. Recently, Quintana et al. [2] observed that suitable thermal treatments below the conventional crystallization temperature give rise, in the Co-based amorphous alloy 6025V of composition Co66Fe4MoZSi16B12, to the formation of nanocrystallites accompanied with a large increase

0304-8853/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO304-8853(97)00126-l

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G. Buttino et al. J Journal of Magnetism and Magnetic Materials I72 (1997) 147-152

of the initial permeability and a reduction of the Bloch wall relaxation frequency. They interpreted this result in terms of free-wall surface area among pinning centers and dependence of the wall bulging on anisotropy. On the other hand, according to Herzer [3,4] the formation of the nanocrystalline phase in the Fe-based alloy containing Cu, annealed above the crystallization temperature, gives rise to a softening of the magnetic properties which is ascribed to the reduction of the effective anisotropy because the random oriented anisotropies are averaged out by the exchange interaction. The purpose of this work is to analyze, through a set of thermal treatments below the conventional crystallization temperature, the possible formation of nanocrystalline phases and the relative softening of the magnetic properties in the amorphous alloy named Metglas 2714A (made by Allied Chem. Corp., USA). The nominal composition of this alloy is Co66Fe4NiIB,,SiIS, the saturation magnetization I, = 0.55 T, the crystallization temperature T,, = 550°C and the saturation magnetostriction is less than 10m6.

2. Experimental and results Toroidal specimens, with mean diameter of 48 mm, were prepared by winding lengths of about 20 cm of ribbon 25 mm wide and 20 urn thick. The specimens were inserted in a preheated furnace, whose thermal stability was about 2°C and then removed and cooled at room temperature after an isothermal treatment having a time length Atisotin the range 2@40 min. Measurements of the initial complex permeability are carried out by the automatized lock-in amplifier technique which gives the components of the initial permeability in phase and out of phase with the driving sinewave magnetic field parallel to the longitudinal axis of the ribbon. Coils were suitably wound around the rings to obtain uniform driving fields and to monitor the induced signals. The measurements of permeability were performed at room temperature with an AC field of amplitude ho = 0.05 A/m at a frequency f= 1 kHz. Under these conditions only the real component of the complex permeability is detectable since no irreversible magnetization processes

4x104 3

0

0

100

200

300 Th,

400

500

600

WI

Fig. 1. Initial permeability pi against heat treatment temperature Th,.

are generated and the effects of the eddy currents are negligible. Fig. 1 shows the relative initial permeability hi as a function of the thermal treatment temperature Tht. The results refer to Atisot= 30 min since this time turns out to be the most suitable for obtaining the best soft magnetic properties which occur when heat treatment temperature was around 500520°C. The initial permeability increases for thermal treatments in the range of temperature 150-350°C. This increase is due to internal stress relief. The successive decrease towards the minimum at Tht = 430°C is ascribed to both a possible local anisotropy induced by atomic rearrangement along directions deviated from the longitudinal direction [l] and an initial formation of crystallites which act as pinning centers for the domain Bloch walls [S]. The increase of pi for Tht > 430°C is associated with the formation of a nanocrystalline phase as it results from TEM micrographs, see Fig. 2, where grains with size of 2 nm appear embedded in a residual amorphous matrix. For Tht > 550°C the system tends to a complete devitrification with increasingly coarser microstructure and then the permeability decreases. We have also analyzed the evolution of the hysteresis loops with the treatment temperature. The measurements of hysteresis are performed by a quasi-static (at a frequency f = 0.1 Hz) hysteresis-graph [6]. Fig. 3 shows the hysteresis loop of

149

G. Buttino et al. /Journal of Magnetism and Magnetic Materials 172 (1997) 147-152

s

a,

i,.4-

-*

.

_“_

z

’ ;.

.

,’

Fig. 2. TEM micrograph of the sample subjected mal treatment of 30 min at Th, = 520°C.

to an isother-

the as-received sample and the hysteresis loops of other three samples heated at different temperatures, for Atisol = 30 min. Appreciable shape variations appear in the region from saturation to remanence: the remanence is somewhat reduced while the saturation magnetization does not change. This result indicates that a fraction of easy magnetization directions are deviated from the longitudinal direction.

Fig. 4 shows the behavior of the coercivity, h,, and that of the squareness ratio, ZJI,, against the treatment temperature. The initial increase of h, is ascribed to rearrangements of atom pairs in the free volumes of the amorphous matrix giving rise to local induced anisotropies. The successive lowering of h, until the minimum around 500°C is due to the softening of the magnetic properties imputable to the formation of the nanostructures which are responsible for the large increase of permeability. The initial decrease of Z,/IS is ascribed to a deviation of local easy magnetization directions from the longitudinal direction as mentioned above. Then Z,/I, recovers its initial value around Tht = 40&43o”C and this latter effect is imputable to the formation of the crystallites which, acting as pinning centers for the domain walls, prevent the demagnetization. Finally, the successive decrease of the squareness ratio for higher Tht, i.e. when the system tends to a more complete crystallization, suggests that the fraction of uniaxial anisotropy crystals is prevailing since the squareness ratio of

-40

H[A/ml Fig. 3. Hysteresis loops of the as-received temperature Th,.

sample and of other three samples subjected

-20

0

20

40

HIP/ml to isothermal

treatments

for 30 min at different

150

G. Buttino et al. /Journal

of Magnetism and Magnetic Materials

I72 (1997) 147-152

METGLAS 27 14A 0.5 -

Atisol =

30

min

0.0

Fig. 4. Coercivity h, and squareness ratio I,/!. as a function of the heat treatment temperature T,,,.

a system of randomly oriented uniaxial anisotropies is 0.5, while if the crystals are of cubic type ZJZ, should be 0.8 [7].

3. Discussion As mentioned in the previous section the evolution of the permeability with increasing thermal treatment temperature until Tht z 450°C (Fig. 1) is ascribed to balance effects among the stress relief, the variations of local induced anisotropies and the initial formation of crystallites which may act as pinning centers for the domain Bloch walls [S]. When the nanocrystalline fraction drastically increases, pi drastically increases too (Tht = 5OCL52O”C).This behavior suggests us to apply the random anisotropy model as described by other authors for the Fe-Cu-Nb-Si-B alloys [3-S]. The nanocrystalline phase is prevailingly constituted of pure crystals of Co and some Co compounds (Co3B, Co2B, ...) [9]. The random distribution of the nanocrystalline orientation and the small size of the particles make the system quite similar to an isotropic amorphous state where any macroscopic anisotropy is eliminated.

The lowering of the average anisotropy constant may also be well evidenced by calculating the anisotropy field distribution following the procedure proposed by Barandiaran et al. [lo]. According to them this function is given by P(H) = - H(d2m/dH2),

(1)

where m = I/I,is the reduced magnetization and the second derivative is evaluated in the range from saturation to remanence of the hysteresis loop. Fig. 5 shows P(H) for some of the analyzed samples. The magnetic softening due to the thermal treatment shifts the peak of P(H), which corresponds to the most probable anisotropy field H,, of the system, towards the low fields, For instance, for the sample heat treated at Tht = 520°C which shows the highest permeability, H,, = 5 A/m. According to Garcia-Arribas et al. [ 111, the function P(H) represents, in the present case where the anisotropy axis of the grains are randomly oriented, the ‘apparent’ anisotropy field distribution since the basic concept which supports this procedure is that the anisotropy field H, of a grain is the field which saturates the grain perpendicularly to its uniaxial anisotropy direction [12]. Therefore, only the local anisotropy directions at an angle of n/2 with the applied field, furnish the

G. Buttino et al. /Journal

of Magnetism

METGLAS 27 14A

and Magnetic Materials

It

172 (1997) 147-152

spectively [13], while the exchange stiffness constant is A Z 10-i ’ J/m for both types of crystals. Lower values of A, down to 0.4 x lo-” J/m, must be used for some Co compounds [14]. The random anisotropy model may be applied when the grain size is smaller than the exchange correlation length defined by

L,,= (A/Ky,

Fig. 5. Anisotropy field distribution P(H) for samples subjected to heat treatments at different temperatures.

intrinsic anisotropy fields. All the other orientations show lower anisotropy fields. Thus, the effective anisotropy field drops near the intercept of the function P(H) with the field axis of Fig. 5. This fact is confirmed by deducing the anisotropy field H, by the well-known equation

(k)rK4D6K3.

Ha x 1,/2.

For the sample heated

(2) at 520°C we obtain

H,z 12 A/m which approaches the field value where the P(H)curve crosses the H axis (see Fig. 5) and by using this value in Eq. (2) we obtain an average anisotropy constant (k) FZ 3.30 J/m3. On the other hand, by considering that the anisotropy constant for spin rotation is proportional to the reciprocal of the initial permeability, we may evaluate the average anisotropy constant of our assembly of random crystallites by the relationship (3) In correspondence with the best value of pi detected in the sample treated at a temperature around 520°C i.e., for pi = 3.7 x lo4 (Fig. l), being I, = 0.55 T, we obtain (k) = 3.25 J/m3 which is a very low value in comparison with the anisotropy constants of cobalt and agrees with the value obtained by Eq. (2). For Co HCP and BCC crystals K = 4.5 x lo5 J/m3 and 5 x lo4 J/m3, re-

(5)

But, in order to account also for the influence of the intergranular amorphous matrix according to Hernando et al. [S], Eq. (4) becomes L = Lexr2(%)- l

(6)

and the Eq. (5) becomes (k)

=

(4)

where A is the exchange stiffness constant and K the anisotropy constant. As mentioned above [3,4,8] and, according to this model, the average anisotropy constant should be given by

H[A/ml

(k)

151

=

v,,(~/~)~K~D’$A)

3 9

(7)

where v,, is the nanocrystalline volumetric concentration and y is a parameter (0 d y < 1) related to the correlation length of the amorphous matrix. An evaluation of the nanocrystallite concentration may be obtained by the TEM micrographs which give v,, z 0.6. By using this value we evaluate, following the procedure indicated in [S], the parameter y, which is equal to about 0.99. By substituting the experimental value of the average anisotropy (k) x 3.25 J/m3 in Eq. (7) we obtained, for A = lo-“J/m and A =0.4x lo-” J/m, the lowest and the highest values of the anisotropy constant of the crystallites: they are K = 0.59 x lo5 J/m3 and K = 1.18 x lo5 J/m3. For those values the correlation length results are 21 and 10 nm, respectively and thus it is higher than the grain size. Besides, the above anisotropy constants being higher than the anisotropy constant of the cubic Co, we may conclude that in the nanocrystalline phase the presence of the uniaxial HCP nanocrystals is prevailing.

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G. Buttino et al. /Journal

of Magnetism and Magnetic Materials 172 (1997) 147-152

This result agrees with the previous conclusions deduced from the value of the squareness ratio which is close to 0.5 for the sample treated at 520°C and indicates a predominance of randomly oriented uniaxial anisotropies. In conclusion, improvements in the soft magnetic properties of the Co-based amorphous alloy 2714A of composition Co66Fe4Ni1B14Si15 may be obtained by isothermal treatments around 5OCL 520°C i.e. below the conventional crystallization temperature, for times around 2WO min. Since the thermal treatments give rise to the formation of a nanocrystalline phase with grain size around 2 nm, embedded in a residual amorphous matrix, the magnetic softening may be ascribed to the dominant effect of the exchange interaction on the magnetocrystalline anisotropy whose effective average value is of the order of few J/m3.

Acknowledgements The authors are indebted to Dr. P. Palmieri of the Centro di Microscopia Elettronica of the University of Ferrara for TEM analysis.

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