Magnetic annealing in electrodeposited Co-P amorphous alloys

Journal of Magnetism and Magnetic Materials 37 (1983) 155-160 North-Holland Publishing Company

155

MAGNETIC A N N E A L I N G IN E L E C T R O D E P O S I T E D Co-P AMORPHOUS ALLOYS

J.M. RIVEIRO and J.M. de FRUTOS Lab. Magnetismo, Fisicas, Univ. Complutense, Madrid, Spain Received 3 December 1982; in revised form 14 January 1983

The different diffusion processes that occur in electrodeposited Co-P amorphous alloys when they are subjected to magnetic annealing at different temperatures are studied by Coercive field measurements. Different processes are identified with different activation energies: 0.19 eV (stress relaxation): 0.75 eV (directional order); 2.9 eV (crystallization process). The low value of the activation energy for the first mechanism is identified with the diffusion of H, and that of the second with the diffusion of P.

1. Introduction

Metallic glasses have a state that is outside the thermodynamic equilibrium, and because of this they evolve towards a stable state, which is in this case crystalline. Apart from this we know that another complimentary evolution takes place: that of the "real amorphous" state which evolves towards what can be called the "ideal amorphous" state, one whose free volume fraction is minimum for a certain temperature. This second evolution needs a low activation energy because it is preferentially connected to atoms in rather unstable positions around the free volume. This rearrangement of atoms, which is accompanied by a reduction in the free volume, is an irreversible process and affects both metallic and metalloid atoms. Before crystallization, other reversible phenomena are observed within the amorphous state. One of these is the anisotropy, induced by magnetic annealing. The activation energy necessary to activate this mechanism is also low (~ 1 eV, e.g. refs. [1,2]), and is therefore associated with movements of metalloid atoms. These, when activated thermically, place themselves in preferential positions and directional order is established.' The influence of local magnetization through the "pseudodipolar interaction" between magnetic moment makes some position become more privileged than others.

It is difficult to differentiate between these two phenomena because usually they tend to have similar activation energies. The phenomenon associated with the reduction in the free volume depends, as is to be expected, very much on the way in which the sample was obtained. The directional order is expected to be similar for the amorphous state with a similar composition. Both atomistic rearrangements that occur during relaxation phenomenon are associated by Egami [3] with two different forms of relaxation: a) Changes in topological short-range order (TSRO) that involve largely the elimination and redistribution of free volume; b) Chemical short-range order (CSRO), as in nearest neighbour atom pairs. The binary "metal-metalloid" amorphous alloys seem to be the simplest in which to study these phenomena. In the actual study we have chosen electrodeposited Co-P amorphous alloys. The parameter used to study the structural changes that occur during relaxation was the coercive field (He). The Co-P amorphous alloys conventionally electrodeposited display perpendicular magnetic anisotropy. Using the Bitter technique we can observe a stripe domain structure with closure domains (semiclosed flux) (see ref. [4]); Its hysteresis loop corresponds to a cycle of coherent rotation in the hard direction, superimposed by a small hysteresis for low fields. The small hysteresis,

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156

J.M. Riveiro, J.M. de Frutos / Magnetic annealing in C o - P alloys

according to ref. [5], corresponds to the magnetization process that occurs in the closure domains through movements of 180 ° walls and are those that contribute to H c. Samples obtained by electrolysis also contain H. T h e amorphous alloys electrodeposited, by the conditions in Which they were obtained (low pH), contain a large amount of H which produces sample embrittlement. This has been proved experimentally by comparing amorphous alloys obtained by quenching and with those obtained by electrolysis [6]. H embrittlement in metallic glasses is being studied by numerous researchers e.g. refs. [7-9]. The studies already completed indicate that the amount of absorbed H is higher than in crystalline metals; this induces large elastic strains in the sample. When the sample is degassified using heat treatment, it is to be expected that a great relaxation of internal stress will occur, which has been detected by Riveiro in electrodeposited C o - P amorphous alloys [10], measuring the changes observed in the magnetic anisotropy. The mechanism of H embrittlement is not clear. Probably H is stored largely in the distorted regions close to the free volume. If this is so, it is to be expected that H will participate in the redistribution of the free volume.

2. Experimental method Two foils of C o - P amorphous alloys were electrodeposited on Cu plates which were later chemically dissolved. We cut the samples to be used (13 × 13 m m 2) out of the middle of the foils. The P content was = 14 at%, as determined by conventional wet chemical techniques. The thickness of one sample (named A) was 37 ~tm, and the other (named B) 40 ~tm. Using sample A, isochronal annealing was carried out at = 20°C steps from 85 to 360°C. The time for each heating was 70 min. Which is thought to be sufficient to reach a situation close to equilibrium. H~ was measured with a Frrster coercimeter in an earth compensate d solenoid. Measurements carried out at room temperature in as-obtained samples, similar to those used in the present work, revealed that He decreases with the aging time. However, this varia-

tion is very slow and can only be observed in long periods of time of about one day. In order to avoid this effect in the experiment with sample A, the first measurement was done three hours after the sample was obtained. The second measurement was done during the same day. The time elapsed between the second and the third was about 12 h. N o changes in the corresponding value of H~ was observed after this time. Then another experiment was carried out using sample B. It was subjected to two successive annealings for 70 min at 130°C, applying a magnetic field in the second treatment perpendicularly to the direction in which it was applied in the first (both directions being in the plane of the sample). After these treatments, the process was repeated at a T of 185°C. H c was measured after each treatment in five directions of the plane from the direction in which the magnetic field was applied during the heat treatment. The magnetic field was 200 Oe.

3. Results and discussion The variation of H c in the sample a, after the magnetic annealings (He corresponding to the direction in which the magnetic field was applied during annealing), is shown in figs. 1 and 2. We separated the results by their different orders. Fig. 1 corresponds to the interval (85-240°C) in which an appreciably great decrease in H c can be observed, reaching a minimum value of • 30 mOe at 220°C. "Fig. 2 shows the posterior increase of He. This growth means that the crystallization process occurs. The H~ proves to be a much more sensitive parameter to crystalline order than X-ray scattering. The rise in Hc is not continuous, which could be related to the appearance of different crystalline phases at a different T. This would agree with Simpson et al. [11], who observed an initial a-Co phase at --270°C. At a slightly higher T a new crystalline Co2P phase can be observed by X-ray diffraction. We could deduce that the first crystalline phase would occur in an arrangement of Co atoms that are hardly or not all affected by the presence of P atoms and that the second phase would occur in the remaining ones. These different

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J.M. Riveiro, J.M. de Frutos / Magnetic annealing in C o - P alloys

Hc(Oe)

0.6

0.4

0.2

I

100

I

120

I

f

I

140

160

180

I

I

200

I

220

24.0

T('C)

Fig. 1. Temperature dependence of the coercive field between 80 and 240°C (sample A).

arrangements have been detected experimentally [12,13]. We shall see later on how our results could throw some light on this. As the H c variations are due to different diffusion p h e n o m e n a that are activated thermically, we have placed the corresponding values of figs. 1 and 2 in "Arrhenius coordinates" in figs. 3 and 4. This was done in an attempt to distinguish the different processes, and try to evaluate the activation energies of each one. In fig. 3 two straight lines can be distinguished, corresponding to the intervals --- 8 5 - 1 6 0 ° C and -~ 160-200°C, Both intervals can be adjusted to the function: He ( T ) = constant × e x p ( Q , / k T ) .

Hc(Oe) 20

15

10

o

(1)

By adjusting the straight lines in the following values for the activation energies (Qi) can be obtained: Ql = 0.19 eV ( 8 5 - 1 6 0 ° C ) and O2 -- 0.75 eV (160-200°C).

~20

Z60

300

340

T('C)

Fig. 2. Temperature dependence of the coercive field between 220 and 360°C (sample A).

J.M. Riveiro, J.M. de Frutos / Magnetic" annealing in C o - P alloys

158

o0[

Ln Hc (Hc~Oe)

I i0 t

o

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-3.0

_ _ _

io

o

2.0

r

2,2

2

t

2,4

2.6

T-l(K)

2.8

.I0-3

Fig, 3. Arrhenius plot of the coersive field variation between 80 and 240°C.

Where: H,° is due to magnetoelastic coupling H~ r is due to directional ordering of atom pairs: and n s u r f is due to surface we shall discount th~ influence of HS~urr.

The coercive field in amorphous alloys may be written as [14,15]: nc=

(2)

n f f -l- n s r d- I4 surf

Ln Hc ( Hc÷Oe )

o o

I

0

-i

-2

-3

I

1,6

_ _

J

I

J

L

1.7

1.8

i .9

2.0

T'I(K)

Fig. 4. Arrhenius plot of the coersive field variation between 220 and 360°C.

.I0-3

J.M. Riveiro, J.M. de Frutos / Magnetic annealing in C o - P alloys

Riveiro detected in ref. [10] the possibility of inducing directional order anisotropy in C o - P amorphous alloys for T > 140°C. Therefore, it seems logical to attribute the behavior of He between 160-200°C to this phenomenon (influence of H~ r through changes in the CSRO). The value obtained for Q2 proves to be slightly less than other usual values found in the literature which are, as already indicated in the introduction, of the order - 1 eV. Stress is relaxed during heating while the free volume is redistributed (changes in the TSRO). The value of Q2 is significantly lower than values habitually observed in this kind of mechanism by different authors, who usually found values between = 0.5-0.7 eV [16-18], but we must point out that the samples used were obtained by "quenching". The value for Q1, however, agrees

Hc(Oe) 0.5

as obtained I~

o

o

~ ~

0.4

TI(1300C)

I

0.3

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/

L.

.

.

.

--

/

/

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T2(130"C)

--

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~

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T3(185"C)

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0.1 .-

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T4(185.C)

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I 90*

(e)

Fig. 5. Hc variation of sample B after various magnetic annealings. For different positions of the direction in which,magnetization occurs with respect to the direction of H during magnetic annealing.

159

well with the activation energy obtained for H diffusion in the amorphous Fe70Crl0Pl3C 7 alloy [7]. From this we can deduce that the possible cause that helps the redistribution of TSRO is, in our case, the diffusion of H stored in its neighbourhood. In order to study whether the observed phenomena are reversible or not, we carried out the experiment with sample B, as already described. The results can be seen in fig. 5. H c in the untreated sample has almost isotropic magnetic behaviour. The slight difference observed in Hc as function of 0 (angle formed by the direction in which the sample was magnetized with the direction in which magnetic field was applied during the 1st heat treatment) must be a consequence of H~ urf. The two successive magnetic annealings at 130°C show the irreversibility of the process that occurs for T < 160°C. After the first heating, markedly anisotropic behaviour was observed in H~ which was much reduced after the second annealing, but the latter does not totally eliminate the anisotropic effect during the first annealing. In the 3rd and 4th magnetic annealing at 185°C, a reversible anisotropic effect was detected. The slight displacement of maximum and minimum H c values that was observed can probably also be attributed to effects of the surface ( H i). The discussion of the obtained results suggests to us that the redistribution of TSRO in electrodeposited amorphous samples is governed principally by elimination of H stored in the neighbourhood of the free volume. Directional order is the second mechanism that influences the behaviour of He. This order is probably due to "monoatomic directional ordering" of single metalloid atoms by interstitial diffusion within holes present in the amorphous structure, as is suggested by Allia and Vinai [19]. Fig. 4 corresponds to the processes that occur during crystallization. In the Arrhenius diagram of the evolution of H~, two well-differentiated straight lines can be observed with the following activation energies: 03 = 2.9 eV and Q3 = 0.74 eV. The Q3 value corresponds to its own crystallization process and agrees quite well with other values found in literature [20-22] which are of the order Q3 -- 3 eV. This process occurs for T < 270°C, which in

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J.M. Riveiro, J.M. de Frutos / Magnetic annealing in C o - P allq~'s

agreement with Simpson and Brambley [l l] and our own analysis, corresponds to the appearance of a predominant a-Co phase and only indications of crystallized Co 2 P. The Q4 value has, curiously, the same value as Q2 (associated with P diffusion); this value for Q4 appears to be very low to associate it with crystallization of a second crystalline phase where X-rays can be clearly detected for T > 270°C. This value for Q2 suggests a possible formation of the second phase: The initial crystalline CO2P nuclei, formed during the crystallization process, would grow to the expense of the crystalline Co which surrounds them. This growth would be favoured by the diffusion of P impurities which contains the crystalline Co. Finally, in order to test the reproducibility of results shown in figs. 1 and 2, we have repeated the experiment with another sample identical to sample A. As there is always a slight difference in the values of H~, due to "uncontrolled" changes in the obtaining conditions, the results were normalized to the initial value of H~ corresponding to sample A'. The new points fitted well to the ones of figs. 1 and 2, the differences less than 10%.

4. Conclusions From the different behaviour of H c in electrodeposited amorphous C o - P alloys when subjected to magnetic annealing, we have obtained the activation energies for the diffusion phenomena that occur during heat treatments. From the value of the activation energy for the relaxation of stress, it can be suggested that this process is associated with the diffusion of H stored in the proximities of the free volume. A second mechanism with an activation energy of 0.75 eV is identified with the diffusion of P in the amorphous matrix. A third mechanism with an activation energy of 2.9 eV is identified with crystallization, but when it seems that crystallization is still not complete ( T ~ 300°C), another diffusion mechanism is detected which has an activation

energy of 0.74 eV, which is surprisingly the same as the value for the diffusion of P in the amorphous matrix. It has been suggested the mechanism could be due to the growth of the initial Co 2P crystalline nuclei, a growth favoured by the diffusion of P, which in the guise of impurity is contained in the Co crystalline ambient. If this hypothesis, based on the results obtained here, proves correct, it implies that the diffusion of P would not vary appreciably between the amorphous and the crystalline state.

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