Thermomagnetic study of the amorphous cobalt-erbium borides Co80ErxB20−x (0 ⩽ x ⩽ 4)

Thermomagnetic study of the amorphous cobalt-erbium borides Co ₈₀ Er x B _20x (0 6 x 6 4)

A. Dahmani ^a,b , O. Sassi ^b , M. Taibi ^b,* , J. Aride ^b , E. Loudghiri ^a , A. Hassini ^c , H. Lassri ^c , A. Belayachi ^a

a

Laboratoire de Physique des Mate´riaux, Faculte´ des Sciences, Universite´ Mohammed V, B.P. 1014 Rabat, Morocco

b

Laboratoire de Physico-chimie des Mate´riaux, Associe´ a` l’AUF (LAF 502) ENS, B.P. 5118 Takaddoum, Rabat, Morocco

c

Laboratoire de Physique des Mate´riaux et Micro-e´lectronique, Faculte´ des Sciences, Ain Chock, B.P. 5366 Casablanca, Morocco Received 8 February 2007; received in revised form 5 October 2007

Abstract

Calorimetric, magnetic and X-ray diffraction measurements have been used to study the magnetic susceptibility and thermal stability of Co

Er

B

with (0 6 x 6 4) amorphous ribbons. The compounds are found to crystallize in Co

B and b-Co, after precipitation of the tetragonal Co

B phase. The addition of erbium shifts up the crystallization temperature leading to the increase in the stability of the amorphous state. Magnetic susceptibility measurements show that the addition of erbium increases the Curie temperature and induces noncollinear magnetic behavior. This latter fact is explained on the basis of random local magnetic anisotropy related to the rare earth atoms in amorphous materials.

2007 Elsevier B.V. All rights reserved.

PACS: 75.50.Kj

Keywords: Amorphous metals, metallic glasses; Glass transition

1. Introduction

Amorphous materials in Co–B system are the subject of intensive studies but many ambiguities still remain when trying to explain some of their properties in terms of micro- scopic parameters. It was reported that the crystallization of Co

B

starts with the precipitation of Co

B in the amorphous matrix when the sample is annealed at T

= 600–700 K [1]. This first step is followed by a decom- position of the Co

B to give the more stable phases Co (f.c.c.) and tetragonal Co

B at T

= 800–900 K.

The thermo-magnetic study of the Co

B

materials (18 6 x 6 25), showed that the microstructure of the crys- tallized samples consists of Co (f.c.c.), Co

B and Co

B [2]. It was found that for x = 18 (Co

B

) annealed at

573 K, for 1 h leads primary a precipitation of Co (h.c.p.) while the rest part stays amorphous. After additional annealing for 17 h at the same temperature the amorphous part crystallizes in Co

B (orthorhombic) form [3].

Using X-ray diffraction and DSC techniques, many authors reported that Co

B

crystallizes through poly- morphic transformation to produce Co

B with the presence of b-Co (f.c.c.) [4–8].

On the other hand, by transmission electronic micros- copy, it was reported the strong dependence of crystalliza- tion mechanism and products on the annealing conditions (time and temperature) [9]. Entire crystallized samples were obtained for annealing temperature higher than 685 K. The microstructure consists of Co

B, Co (f.c.c.) and Co

B. The decomposition of Co

B into Co

B and Co can be observed up to 873 K.

Using electrical measurements, it was found that the crystallization of Co

B

takes place at 600 K. With con-

0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.10.011

*

Corresponding author. Tel.: +212 37 75 22 61; fax: +212 37 75 00 47.

E-mail address: taibiens@yahoo.fr (M. Taibi).

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Journal of Non-Crystalline Solids 354 (2008) 1817–1821

tinued heating other peaks are detected and are identified as belonging to other changes in the crystallized materials structure [10].

During crystallization process, promising properties of the metallic glasses have been found deteriorated which reduces the use of amorphous alloys [11]. Thus enhancing the thermal stability of these materials is very suitable for technological applications as well as fundamental interest.

The possible way to achieve this goal is small addition of a third element to the binary system which could shift up the crystallization temperature [12].

In a previous work Lassri et al. [13] have presented the thermal variation of Co

Er

B

amorphous ribbons magnetization in terms of the molecular field theory. The exchange interactions J

and J

have been determined.

The aim of this work is to present a comparative exper- imental study, using DSC and magnetic susceptibility, X- ray diffraction to explain crystallization of the system Co

Er

B

with (0 6 x 6 4). We discuss the effect of variable erbium content on the thermal stability and mag- netic properties of the amorphous ribbons.

2. Experimental

Amorphous Co

Er

B

(x = 0, 1, 2, 3 and 4) were prepared by the melt spinning technique under pure argon atmosphere in ribbons form of about 2 mm wide and 30–

40 lm thickness. Both the amorphous and the crystallized states were checked out by X-ray diffraction performed with a Siemens D5000 diffractometer. The calorimetric investigation was performed using a differential scanning calorimeter (Setaram DSC 121) between 300 and 1000 K under purified Ar flux at constant heating rate. Direct cur- rent magnetic susceptibility measurements v

were carried out on a DSM4 magnetometer in the temperature range 300–950 K.

3. Results

3.1. Calorimetric study

Fig. 1 shows the obtained DSC thermograms for Co

B

at a constant heating rate of 10 K min

. Thermo- grams (1) and (2) correspond to the first and second heat- ing, respectively, while curve (3) is established from the two cycles (1 and 2) with corrected base lines.

On curve (1) an irreversible exothermic peak is observed at about 670 K, attributed to the crystallization of the amorphous state. The determined crystallization tempera- ture value is in agreement with that obtained by other authors [7,14]. However, in contrast to Hernando et al.

[7] who reported two DSC peaks, the crystallization pro- cess in our case is manifested by one peak suggesting that it takes place in a single step. Furthermore, during the sec- ond heating process (curve 2), taken for a sample left at room temperature for one day after the first heating run,

an endothermic peak occurs at about 385 K followed by a break of the DSC base line at 430 K. The later phenom- enon is close with the ferromagnetic–paramagnetic transi- tion of Co

B [2].

Fig. 2 shows DSC thermograms of the all studied sam- ples Co

Er

B

(x = 0, 1, 2, 3 and 4) obtained at heating rate of 10 K min

. We can note some exceptions to the Co

B

thermograms: for the erbium substituted com-

Fig. 1. DSC thermograms obtained for Co

B

, (1) first heating, (2) second heating and (3) thermograms with corrected base lines at 10 K min

.

Fig. 2. DSC thermograms obtained for amorphous Co

Er

B

(x = 0,

1, 2, 3 and 4) samples at heating/cooling rate of 10 K min

.

pounds other thermal phenomenon is observed as a break of the base line of the DSC curve at a temperature lower than the crystallization one. This fact is attributed to the transition from ferromagnetic to paramagnetic amorphous states [15]. In the case of Co

B

(x = 0) the Curie transi- tion relative to the amorphous state is not observed. This suggests that it is probably truncated by the crystallization.

On the other hand for x = 1 and x = 2 the crystallization is manifested by two overlapping exothermic irreversible peaks, due to the presence of inhomogeneities in these com- pounds as has been observed by nuclear magnetic reso- nance (NMR) data [13]. The peak corresponding to the formation of Co

B occurs always at the same temperature independently on the erbium content, which is a proof that the endothermic peak recorded in the second heating corre- sponds to the magnetic transition of the crystalline phase Co

B. The crystallization temperatures (T

) are plotted versus Er content in Fig. 3. We note that T

increases with increasing Er content until x = 3 where a saturation seems to be reached.

3.2. X-ray diffraction analysis

The X-ray diffraction analysis is performed on materials before and after the two DSC scans. Before heat treatments the X-ray patterns are typical of the amorphous state.

After passing the two runs of DSC, the X-ray diffraction patterns show the presence of Co

B lines (CuAl

tetragonal structure type) and f.c.c. b-Co crystalline phases. The X- ray diffraction patterns taken for Co

B

and Co

Er

B

after heat treatment are reported in Fig. 4.

3.3. Dc magnetic susceptibility

In order to determine the correlation between the struc- tural and magnetic properties occurring during crystalliza-

tion process, a dc magnetic susceptibility measurements are performed in the temperature range 300–850 K. An applied magnetic field of 0.05 T was sufficient to saturate the sam- ples signal. The magnetic susceptibility versus temperature curves obtained for the Co

B

are reported in Fig. 5.

During the heating process, we have observed two rapid decreases of the susceptibility, at about 660 and 750 K, respectively. In order to identify this behavior we have recorded magnetic measurements in cooling. The obtained curve shows two phenomena:

• The first at about 748 K close to the ferromagnetic–

paramagnetic transition of the crystalline phase Co

B.

• The second at 430 K, attributed to the Curie transition of Co

B.

650 700 750 800 850 900

0 1 2 3 4

x(Er) T

(K )

Fig. 3. Variation of the crystallization temperatures obtained by DSC measurements versus Er content (errors were given automatically by the software of thermogram analysis).

Fig. 4. X-ray diffraction patterns of Co

Er

B

after the two DSC scans (x = 0 and x = 3): ( ) Co

B, ( h ) b-Co.

300 400 500 600 700 800

T (K) 3

4 5 6 7 8 9

(emu/mole)

x=0

χ

Fig. 5. Magnetic susceptibility versus temperature for Co

B

during

heating (!) and cooling ( ) processes.

We conclude that in the course of the heating process the first transition at T = 660 K, is identified as the crystal- lization temperature of the amorphous sample Co

B

while the second event is attributed to the ferromagnetic–

paramagnetic transition of the crystalline phase Co

B.

The determined crystallization temperature is in agreement with that obtained from DSC measurements.

In order to investigate the erbium effect on the thermo- magnetic properties in the system Co

Er

B

, we have performed the magnetic susceptibility measurements for all compositions in temperature interval 300–850 K. The obtained results for x = 1, 2, 3 and 4 are plotted in Fig. 6.

For Co

Er

B

(x = 1), the first anomaly observed at 706 K is identified as the Curie transition of the amorphous state. The second at about 785 K is attributed to the crys- tallization of the material. During the cooling process the magnetic transition relative to Co

B is observed at approx- imately 754 K while the Curie transition occurring at about 430 K is weak in comparison with Co

B

case.

For Co

Er

B

(x = 2), at the high temperature limit of our measurements (T

= 850 K), the crystallization tem- perature is not reached. Thus during cooling process we have noted the transitions relative to Co

B and Co

B crys- talline phases.

For Co

Er

B

(x = 3), and Co

Er

B

(x = 4), the samples are not entirely crystallized at T

= 850 K. Con-

sequently the curves recorded during cooling do not follow those obtained during heating process.

For all samples the Curie transition of the amorphous state is observed during heating. The obtained Curie tem- peratures are plotted versus erbium content in Fig. 7. As for DSC results T

increases slightly when increasing the Er concentration.

4. Discussion

The non-isothermal crystallization of Co

B

proceeds by complex mechanisms. During the first heat the amor- phous material crystallizes to produce Co

B (probably the f.c.c. Co) crystalline phases, followed by the decompo- sition of Co

B to give the more stable phases Co

B and b- Co. In fact, other researchers have observed this decompo- sition with a doubt about its kinetic. It was suggested that the transformation might be a slow process without giving sufficient arguments [9].

Magnetic susceptibility measurements showed that the phase transition related to Co

B is always present even after the decomposition process. This must indicates that Co

B does not decompose thoroughly into Co

B and Co.

However, X-ray diffraction patterns taken after the two runs do not show any line of Co

B. This means that the residual fraction of Co

B is not detected by X-ray diffrac- tion but it is detected by magnetic measurements. In fact Zern [9] observed that with increasing annealing tempera- ture the volume fraction and grain size of Co

B decrease in the benefit of Co

B and Co ones. However, long time (several days) annealing treatments at high temperatures are necessary to remove the Co

B.

300 400 500 600 700 800 900

T(K) 4

6 8

x=1

6

7 8

x=2

5

6

7

3.5 4.0 4.5 5.0

x=4

χ (emu/mole)

Fig. 6. Magnetic susceptibility versus temperature for Co

Er

B

alloys (x = 1, 2, 3 and 4) during heating (!) and cooling ( ) processes.

650 700 750

0 1 2 3 4 5

x(Er) T

(K )

Fig. 7. Variation of Curie temperatures obtained by DSC ( m ) and

susceptibility (4) measurements of Co

Er

B

compounds (x = 1, 2, 3

and 4) versus x (errors were given automatically by the software

(pendown) of curve analysis).

When an amount of boron is substituted by erbium, the crystallization temperature was shifted up significantly.

This result means that thermal stability of the amorphous state is enhanced. We affirm that the addition of small amount of Er does not change the crystallization mode.

The similarity of the DSC thermograms for all composi- tions with, in particular, the occurrence of the endothermic peak of Co

B formation supports this hypothesis. Indeed the crystallization occurs by nucleation and crystal growth, which are predominated by diffusion mechanism. Kineti- cally the large atomic size of Er (the atomic radius are 1.75 A ˚ and 0.92 A˚ for Er and B, respectively) makes large-scale atomic diffusion extremely difficult reducing the nucleation and growth rates. In fact, it has been estab- lished that the composition and microstructure of amor- phous materials are much different from those of the corresponding crystallized phases. This suggests that the crystallization needs substantial redistribution of the com- ponent elements [16]. The addition of Er makes the redistri- bution of atoms on a large scale more difficult. This restrains the nucleation and growth of the crystalline phases. This is perhaps the major factor why a small amount of Er enhances thermal stability of the amorphous borides [17,18].

We can believe that not only the nucleation rate has been delayed by Er addition but also that it can be com- pletely suppressed as has been observed in Fe–B–Si metal- lic glasses [19]. Consequently, the crystal growth occurs only on the quenched-in nuclei which increase the devitrifi- cation temperature [20].

Both DSC and magnetic measurement techniques revealed an increase of the Curie temperature of the amor- phous state with increasing Er concentration. The behavior is strange in regard to the antiferromagnetic coupling between Co and Er. Indeed, using magnetic and NMR measurements, Lassri et al. [13] have reported that Co moment increases with increasing Er concentration.

Related to the hybridization of 5d–3d orbitals, the Co moment would decrease. Nevertheless, this decrease is off- set by the diminution of B since the addition of boron leads also to a decrease in l

due to the charge transfer between 3d orbital of Co and p orbital of B. The above explanation is however justified only for small rare-earth content in the order of 5% [21], which is the case of the studied samples.

On the other hand, it has been established that a great amount of magnetic anisotropy is associated with rare earth atoms in metallic glasses [22]. This anisotropy was identified to be completely random and of uni-axial type.

In the studied samples it tends to align Er moments along random directions, leading a reduction of its value [13].

Competition between anisotropy and exchange results in (more or less random), conducts to a noncollinear mag- netic structures [23]. In fact, some metallic glasses of similar formula are reported to exhibit sperimagnetic behavior [21]. We believe that this noncollinear structure related to the admixture of Er is manifested in the studied samples by broadening of the magnetic transition.

5. Conclusion

Calorimetric and thermomagnetic techniques were used to study the non-isothermal crystallization occurring in the Co

Er

B

amorphous materials with (0 6 x 6 4) and to check out the correlation between magnetic and structural rearrangements accompanying the crystallization process.

The crystallization of the binary boride Co

B

proceeds in one step to produce Co

B and Co (f.c.c.). The devitrifi- cation is probably of eutectic type. The decomposition of Co

B phase gives Co

B and Co.

The addition of Er stabilizes the amorphous phase shift- ing up the crystallization temperature by about 220 K. In addition of small amount of Er induces slight increase in the Curie temperature. Such strange result is explained by the diminution of B content and thus a diminution of charge transfer. The magnetic phase transition of the amor- phous phase became weak when increasing Er content.

This fact is attributed to the local random anisotropy effect.

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