Crystallization and magnetic properties of (Co0.75Cr0.25)80Si5B15 metallic glass

Journal of Alloys and Compounds 368 (2004) 283–286

Crystallization and magnetic properties of (Co0.75Cr0.25)80 Si5B15 metallic glass T.Y. Byun, Y. Oh, C.S. Yoon∗ , C.K. Kim Department of Materials Science and Engineering, Hanyang University, 17 Haengdaqng-dong Seongdong-ku, Seoul 133-791, South Korea Received 12 January 2003; received in revised form 14 April 2003; accepted 20 June 2003

Abstract (Co0.75 Cr0.25 )80 Si5 B15 amorphous alloy was thermally annealed to study the crystallization and magnetic properties. The amorphous alloy crystallized in two stages. The primary crystallization of hexagonal closed packed (Co, Cr) occurred in the initial stage and the rejection of non-magnetic Cr from the crystallized (Co, Cr) crystals was investigated by the temperature dependence of magnetization using a vibrating sample magnetometer. During the second stage, the precipitation of ␴-CoCr occurred instead of the eutectic crystallization of borides that is typically observed in most metal-boride glasses. The apparent activation energies for two crystallization stages were estimated to be 318 (±12) and 1690 (±23) kJ/mol, respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Crystallization; Magnetic properties; Metallic glass

1. Introduction The Co-rich metallic glass has attracted great interest for a variety of applications including electronics, magnetic recording, and magnetic sensors due to its near-zero magnetoresistive behavior [1–3]. It is known that the alloying by a small percent of transition metals such as Fe, Mn, V, and Cr with cobalt-based amorphous alloys can effectively reduce the magnetostriction of the materials to zero [4]. The crystallization behavior of metallic glasses is interesting because it is connected with the changes involved in physical and chemical properties which determine most applications [4,5]. During the heat-treatment well below its bulk crystallization temperature, Co75.26−x Fe4.74 (BSi)20+x amorphous alloy underwent surface-oxide formation. Depending on the composition, expecially B and Si concentrations, the alloy exhibited a complex surface oxidation behavior [6]. The bulk crystallization of Co68 Fe4 Cr4 Si13 B11 amorphous alloy underwent polymorphic transformation of cobalt and subsequent eutectic crystallization of orthorhombic Co3 B when annealed under vacuum [7].

∗

Corresponding author. Tel.: +82-2-22900409; fax: +82-2-22901838. E-mail address: [email protected] (C.S. Yoon).

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In this work, we investigated the effect of alloying by a large amount of Cr on the crystallization of Co-based metallic glass, which showed quite different behaviors compared with the alloy that has a small content of Cr. The bulk crystallization behavior of the (Co0.75 Cr0.25 )80 Si5 B15 amorphous alloy was measured using differential scanning calorimeter (DSC), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Magnetic properties were also measured using vibrating sample magnetometer (VSM) to associate with the variation of crystallization processes.

2. Experimental details (Co75 Cr25 )0.8 Si5 B15 amorphous alloy was produced using melt spin method. Typical ribbons produced are 20 ␮m thick and 2 mm wide. Co75 Cr25 alloy was also prepared by induction heating in Ar atmosphere. Temperature dependence of the saturation magnetization at 5 kOe was measured with VSM (Lakeshore Model 7300) at the heating rate of 6 K/min. The crystallization behavior of the as-cast ribbon was determined by DTA (TA Instrument, SDT2960) with various heating rates between 3 and 20 K/min. Samples were crystallized in vacuum (10−3 Pa) at temperatures between 773 and 973 K for 30 min. Crystalline phases formed during

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devitrification were identified by X-ray diffraction (XRD, Rigaku Rint-2000) using Cu K␣ radiation. Microstructures were observed using transmission electron microscopy (TEM, Jeol 2000). For the preparation of TEM specimens, double-sided ion milling was performed with a liquid nitrogen cooled cold stage to avoid possible beam heating.

3. Results and discussion 3.1. Crystallization The crystallization behavior of the amorphous alloy was investigated by DTA continuous heating curve with various heating rates and is shown in Fig. 1. The crystallization of the amorphous (Co75 Cr25 )0.8 Si5 B15 alloy occurred in two stages. The onset temperature of each crystallization process in the DTA curve with the heating rate of 10 K/min is 760 K (Tp1 ) and 848 K (Tp2 ), respectively. The apparent activation energies for the crystallization stages can be determined by the Kissinger’s method [8] and was estimated to be 318 (±12) kJ/mol for the first peak and 1690 (±23) kJ/mol for the second. The activation energies for the crystallization of metallic glasses are typically in the order of a few hundred kilojoule/mole, e.g. the activation energies for Co26 Fe54 B14 Si6 glass were 208 and 409 kJ/mol [9] and the activation energies for Co68 Fe5 Cr4 Si13 B11 glass were 380 and 260 kJ/mol [7] for the first and the second crystallization, respectively. The activation energy for the second crystallization peak of the amorphous (Co75 Cr25 )0.8 Si5 B15 alloy is extraordinarily large. To analyze the crystalline phases that were formed during each crystallization stage, specimens that underwent each crystallization stage were prepared. Fig. 2 shows the XRD

Fig. 1. DTA continuous heating curves measured with the heating rates from 6 to 25 K/min for the amorphous (Co75 Cr25 )0.8 Si5 B15 alloy. The onset temperatures of the two crystallization stages in the DTA curve with the heating rate of 10 K/min are 760 K (Tp1 ) and 848 K (Tp2 ), respectively.

Fig. 2. XRD spectra for the specimens that are devitrified at 773, 873, and 973 K.

spectra of the specimens that were annealed at 773 and 873 K for 30 min. The hexagonal close packed (HCP) (Co, Cr) was precipitated during the first crystallization, and ␴-CoCr during the second. The ␴-phase is known to be kinetically suppressed in the bulk alloy and the relevant boundaries of the binary Co–Cr phase diagram are, in fact, rather uncertain due to the sluggishness of the diffusion and low-temperature transformations [10]. The sluggishness of the crystallization of ␴-phase is evidenced by the extraordinary large value of the activation energy. It is interesting that borides such as Co2 B or Co3 B were not crystallized during the second crystallization. As boron is hardly soluble in Co lattice [11], it is expected that most boron is interstitially dissolved in the ␴-phase since the lattice parameters of the ␴-phase are a = 8.81 and c = 4.56, which are much larger than that of HCP Co. The highly boron-rich ␴-phase eventually decomposed at elevated temperature. The specimen that was annealed at 973 K was composed of Co3 B and HCP (Co, Cr) as is shown in Fig. 2. The ␴-phase disappeared and orthorhombic Co3 B was newly formed. The ␴-phase seems to decompose into Co3 B and HCP (Co, Cr). The microstructure that underwent each crystallization stage is shown in Fig. 3. Fig. 3(a) is the TEM bright field image of the specimen that was annealed at 773 K. Elongated crystals are dispersed in the amorphous matrix. The electron diffraction pattern revealed that the crystals were HCP (Co, Cr). In our previous study [7], face centered cubic (fcc) Co, HCP Co, and orthorhombic Co3 B were observed at the initial stage of crystallization of the Co68 Fe5 Cr4 Si13 B11 amorphous alloy. It is noted that HCP Co is stabilized by the addition of Cr, since it elevates the transformation temperature of fcc Co into HCP Co [11]. Fig. 3(b) is the TEM bright field image of the specimen that was annealed at 873 K. It shows that the specimen is fully crystalline and the remaining amorphous matrix in Fig. 3(a) was crystallized.

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Fig. 4. Temperature dependence of saturation magnetization (Ms at 5 kOe) were measured at 10 K/min for the (Co75 Cr25 )0.8 Si5 B15 glass and Co75 Cr25 alloy. Tp1 and Tp2 are 760 and 848 K, respectively. Tc1 and Tc2 are 830 and 1165 K.

Fig. 3. TEM bright field images of the specimen devitrified at (a) 773 K for 30 min and (b) 873 K for 30 min.

3.2. Magnetic properties The saturation magnetization of cobalt at room temperature is known to be 161 emu/g and the Curie temperature to be 1388 K [12]. The alloying of cobalt with chromium drastically reduced the magnetic properties. Fig. 4 shows the temperature dependence of the saturation magnetization (Ms ) with the heating rate of 10 K/min for Co75 Cr25 alloy and (Co75 Cr25 )0.8 Si5 B15 glass. Their saturation magnetizations were in the order of a few electromagnetic unit/gram and the Curie temperatures were about 500 K. The temperature dependence of Ms of the magnetic glass had two magnetization peaks above 760 K. The temperatures at which Ms begins to rise were 760 and 848 K and coincided with the onset temperatures of two crystallization stages in the DTA curve with the same heating rate in Fig. 1. The initial rise of Ms at Tp1 suggests that ferromagnetic crystals were nucleated in the paramagnetic amorphous ma-

trix and therefore, the Cr content in the ferromagnetic crystals should be lower than that of the remaining paramagnetic amorphous matrix, which points that nonmagnetic Cr atoms was rejected from the initial crystallization product. This is rather unexpected, considering that the binary system of Co and Cr is chemically miscible since their bond has a negative mixing enthalpy [13]. However, it should be noted that metalloid elements, B and Si, bonded to Cr or Co may have lowered the solubility of Cr in the (Co75 Cr25 )0.8 Si5 B15 glass. Cr atoms in the glass network appear to be energetically more favorable than in the form of Co–Cr solid solution during the initial crystallization. The ferromagnetic HCP (Co, Cr) phase which crystallized during the initial crystallization had the Curie temperature of 830 K (Tc1 ), whereas the HCP (Co, Cr) phase which crystallized during the second crystallization had 1165 K (Tc2 ), indicating that the Cr content in the ferromagnetic phase formed during the latter crystallization stage is lower than that in the ferromagnetic phase formed during the initial crystallization stage as inferred from the Co–Cr phase diagram [13] and the Curie temperature data [14]. The Curie temperature of the binary alloys of (Co85.7 Cr14.3 ) and (Co94.5 Cr5.5 ) are estimated to be 830 and 1165 K, respectively [14]. The significant deficiency of Cr in the HCP (Co, Cr) during the second crystallization is attributed to the precipitation of ␴-CoCr which will require the migration of Cr from HCP (Co, Cr) to ␴-phase, since ␴-phase in the Co–Cr system occurs when Cr is 50–70 at.% [11,13]. Fig. 5 shows room temperature hysteresis loops for the samples crystallized at different temperatures for 30 min. As the annealing temperature increased, Ms rose and then sharply increased after the formation of Cr-depleted HCP (Co, Cr) phase and subsequent ␴-phase. At the same time, coercivity of the material increased substantially. The increased coercivity is due to the increase of

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The remaining amorphous matrix subsequently became enriched in Cr. Nucleation of the intermetallic ␴-CoCr during the second crystallization appeared to be triggered by the rejection of Cr, thus drastically changed the magnetic properties of the crystallized alloy. Boron is incorporated into the ␴-phase, which will help to isolate the magnetic grains together with the segregation of Cr.

Acknowledgements This work was supported by the Korea Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University.

References Fig. 5. Hysteresis loops for the specimens that were annealed at different temperatures.

magnetic anisotropy that is accompanied by the formation of ferromagnetic crystals and can be partly attributed to the non-magnetic CoCr grains pinning the domain wall movement. In addition to the increased coercivity, the non-magnetic ␴-phase at the grain boundary can decouple exchange interaction between ferromagnetic grains and the segregation of boron to the grain boundary can further isolate the magnetic grains together with Cr.

4. Conclusion The crystallization of (Co75 Cr25 )0.8 Si5 B15 amorphous alloy occurred in two stages. The primary crystallization of HCP (Co, Cr) was followed by the precipitation of ␴-CoCr. The ␴-phase had the extraordinarily large activation energy of 1690 kJ/mol which proved its kinetic suppression during the crystallization. The crystallization of Cr-depleted HCP (Co, Cr) in the paramagnetic amorphous matrix during the initial crystallization was attributed to the rejection of Cr from the crystals and resulted in the rise of magnetization.

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