INFLUENCE OF CR AND MO ON STRUCTURE

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The influence of Cr, Mo and Mn on structural changes and magnetic properties of Co75,5Fe4,5Si6B14 amorphous alloy

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13th International Conference on Liquid and Amorphous Metals Journal of Physics: Conference Series 98 (2008) 062010

IOP Publishing doi:10.1088/1742-6596/98/6/062010

The influence of Cr, Mo and Mn on structural changes and magnetic properties of Co75,5Fe4,5Si6B14 amorphous alloy L Bednarska, B Kotur, S Mudry, M Kovbuz, G Haneczok1 and M Karolus1 Ivan Franko National University of Lviv, Kyryla & Mefodiya Str. 6, UA-79005, Lviv, Ukraine 1 Institute of Materials Science, University of Silesia, Bankowa St. 12, 40-007 Katowice, Poland [email protected] Abstract. The annealing of amorphous alloys, that is needed in order to improve their properties, is connected with various changes of structure and properties. The understanding of such changes is interesting both from fundamental and applied views. On that reason the influence of Mo and Cr admixtures on the main structure parameters and physical and chemical properties has been studied. For investigation of amorphous alloys the X-ray-diffraction, differential scanning calorimetry, magnetic susceptibility and High-resolution microscopy methods were used. Experimental data allowed us to estimate the changes in crystallization kinetics and to calculate the activation energies. These results were analyzed conjointly with data on X-ray diffraction and magnetic measurements.

1. Introduction Magnetic amorphous alloys have excellent soft magnetic properties such as high saturation magnetization, high permeability, low coercivity and losses, that provide their applications in antitheft security system, magnetic recording heads, transformers and electronic devices. A thermal treatment of amorphous materials below their crystallization temperature relaxes the amorphous structure-giving rise to ultra-soft magnetic properties. Thus the magnetic amorphous alloys present a new system, which crystallization process and corresponding changes in magnetic properties can be systematically studied. The activation energy of the crystallization process can be obtained from the temperature dependence of the reaction-rate constant, known as Kissinger’s method [1]. The crystallization kinetics of Co-based metallic glasses is scarcely studied while substantial amount of work exists on those of Fe-based metallic glasses. The Avrami exponent estimated in various Fe-based metallic glasses varies from 2.0 to 4.0 [2-3]. Filipecki reported that thermal stability of (Fe,Co)78Si9B13 amorphous alloy decreases with increasing of cobalt content [4]. This result suggests that although Co-rich system brings interesting magnetic properties, but the addition of Co appears to destabilize the amorphous system. In this study, we report crystallization behavior of metallic glasses using a differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and Xray diffraction (XRD) since it is known that alloying by a small percent of doped metals such as Cr, Mo, Mn and Ni with cobalt-based amorphous alloys can effectively reduce the magnetostriction of the materials to zero [5], that makes the magnetic properties much less sensitive to processing conditions [6]. The aim of this work was to study the influence of Cr, Mn, Mo as doping element in Co-Fe-Si-B-M amorphous alloys on the crystallization processes and magnetic properties. 2. Experimental details and principles The amorphous alloys Co75.5Fe4.5Si5B15 (AMA-1), Co66.5Fe4.0Mo1.5Si16.0B12.0 (AMA-2), Co67.2Fe3.8Сr3.0Si14.0B12.0 (AMA-3), Co73.6Fe3.2Mn3.2Si5.0B15.0 (AMA-4), Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 (AMA-5) in the form of 30 μm thick ribbons were prepared from elements of purity 99.99% by rapid quenching from the liquid state using the melt spinning technique. c 2008 IOP Publishing Ltd

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Differential scanning calorimetry (DSC) (Perkin-Elmer Pyris 1) was used to determine the crystallization process of as-cast samples at heating rate ranging from 5 to 20 K min-1. The obtained data were evaluated by applying the standard program Pyris. The amorphous structure and structure of AMA after heat treatment were studied by X-ray diffraction (X-Pert Philips PW 3040 diffractometer, Cu-Kα radiation with monocromator on a reflected beam, 2θ range: 10o-140o, scanning step 0.04o). This method was used for samples in the as-quenched state and annealed at elevated temperatures. In order to determine the structure of the examine material the Rietveld method was used (standard FullProf program). Highresolution electron microscopy (HREM) (JEM 3010) observations were used in order to confirm (or not) the conclusions established by DSC and XRD techniques. The thermomagnetic analysis was performed using a Faraday’ magnetic balance. The measurements of saturation magnetization for alloys in the as-quenched state were performed from room temperature up to 800 K at a heating rate of 1.5, 5 and 10 K/min in magnetic field of 1.0 T. 3. Results and discussion

Mangura Hogue S. at all [4] described the two-stages in crystallization process of AMA Co67Fe4Mo2Si15B12, therefore our investigation were aimed on studying the influence of the doping elements Cr, Mn, Ni, Mo on the first stage of AMA crystallization process. Figure 1 shows the DSC curves for the Co-based amorphous alloy with doping elements Cr, Mn, Ni, Mo. There are one-stage in crystallization process of AMA Co75.5Fe4.5Si5B15, Co66.5Fe4.0Mo1.5Si16.0B12.0, Co73.6Fe3.2Mn3.2Si5.0B15.0, Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 at heating rate of 20 K/min and heating range 298...870 K. The exothermic peaks for Co66.5Fe4.0Mo1.5Si16.0B12.0 exist at 829 K, for Co73.6Fe3.2Mn3.2Si5.0B15.0 - at 834 K and coexistence doped elements (Mo, Mn, Ni) in AMA-5 lead to shift of exothermic peak to higher temperatures (T=864 K). It is interesting that Cr addition decreased the temperatures of first stage crystallization to 810 К and the second stage to 840 K. Figure 2 shows the DSC curves for the amorphous Co67.2Fe3.8Сr3.0Si14.0B12.0 alloy at various heating rates. The apparent activation energy of each crystallization step can be estimated by the 2 Kissinger’s method, that is written as ln T p = E + const , where Tp is the peak temperature, a - the a RT p heating rate, R - the gas constant and E - the apparent activation energy. 70 V= 5 K/min V=10 K/min V=20 K/min

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Figure 1. DSC curves for the as-cast AMA Co75.5Fe4.5Si5B15 (1), Co66.5Fe4.0Mo1.5Si16.0B12.0 (2), Co67.2Fe3.8Сr3.0Si14.0B12.0 (3), Co73.6Fe3.2Mn3.2Si5.0B15.0 (4), Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 (5) at heating rate of 20 K/min Figure 2. DSC curves for the as-cast AMA Co67.2Fe3.8Сr3.0Si14.0B12.0 at different heating rates

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The activation were energies determined using the least square method are 443 and 320 mol-1 for the first and second peak, respectively.

The nature of the doping elements, such as Mo, Cr, Mn and Ni, strongly influences the amorphous alloys magnetization. Figure 3 shows the magnetization curves of AMA obtained with heating rate 10 K/min. As indicated in the figure 3a, the Cr-addition to AMA Co75.5Fe4.5Si5B15 promotes a paramagnetic state in a temperature interval 680-780 K. AMA Co66.5Fe4.0Mo1.5Si16.0B12.0 is

close to characteristics of the paramagnetic state and at 800K moM=0 mT. The Mo additions in AMA lead to the increase of Curie temperature to 743 K, in comparison with Tc of Co67.2Fe3.8Сr3.0Si14.0B12.0 (667 K) 800

1400 Co75.5Fe4.6Si16B16.7

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100 80 60 40 20 0

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Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)

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Figure 3. Magnetization curves of AMA obtained with heating rate of 10 K/min Mn compensates the magnetic moments, i.e. leads to realization of the antiferromagnetic state. Mn magnetic moments in the amorphous Co73.6Fe3.2Mn3.2Si5.0B15.0 and Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 alloys can not be fully compensated due to their disordered structure (the disorder leads to the random fluctuations of the Mn–Mn interatomic distances) and characteristic composition (more than 60% of atoms in the alloys are not Mn). Hence, those ordered Mn magnetic moments which are not compensated can give the origin of the weak paramagnetic behavior observed in Co73.6Fe3.2Mn3.2Si5.0B15.0 and Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 amorphous alloys at relatively high temperatures, while the Co subsystem is not ordered yet. Two ferromagnetic phases (at 820 and 880 K) are observed in Co73.6Fe3.2Mn3.2Si5.0B15.0 and Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 amorphous alloys. The low temperature soft-magnetic phase is formed by Co subsystem. The second magnetic phase at T=880 K (weak magnetism) is formed by noncompensated Mn magnetic moments evidently. Selected area diffraction patterns (figure 5) of Co67.2Fe3.8Сr3.0Si14.0B12.0 amorphous alloys during crystallization at 810 K illustrated the first stage crystallization – formation of b-Co (111) phase [6, 7]. The interplanar distances obtained directly from figure 5 are in good agreement with ones calculated from XRD data and in [7].

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Figure 5

Figure 4. Normalized magnetization curves of AMA Co67.2Fe3.8Сr3.0Si14.0B12.0 obtained with different heating rates Figure 5. TEM bright field images and selected area diffraction patterns of Co67.2Fe3.8Сr3.0Si14.0B12.0 amorphous alloys during crystallization at 810 K 4. Conclusions The influence of Cr, Mn, Mo as doping elements in Co-Fe-Si-B-M amorphous alloys on the crystallization processes and magnetic properties were studied. The temperature of crystallization for AMA Co75.5Fe4.5Si5B15 was found to be 820 K. The addition of Mo, Mn, Ni to these alloys leads to the shift of the crystallization temperatures to higher values (829…864 K). The additions of Cr lead to two-stages crystallization. Heating rate proportionally influences the values of heat flow and magnetic properties at phases creating. The Cr addition to AMA Co75.5Fe4.5Si5B15 promotes a paramagnetic state in a temperature interval 680-780 K. AMA Co66.5Fe4.0Mo1.5Si16.0B12.0 is close to characteristics of the paramagnetic state and at 800K moM=0 mT. Mo additions to AMA lead to the increase of Curie temperature to 743 K, in comparison with Tc of Co67.2Fe3.8Сr3.0Si14.0B12.0 (667 K). Selected area diffraction patterns of the amorphous alloys Co67.2Fe3.8Сr3.0Si14.0B12.0 during crystallization at 810 K illustrated the first stage crystallization – formation of b-Co (111) phase with interplanar distances ~ 2,026 Å. Second magnetic phase in Co73.6Fe3.2Mn3.2Si5.0B15.0 and Co73(Fe,Ni,Mo,Mn)5.7(Si0.2B0.8)21.3 amorphous alloys at T=880 K (weak magnetism) is formed by noncompensated Mn magnetic moments evidently. References [1] Garcia J A, Rivas M, Tejedor M 2005 J. Magn. Magn. Mater. 290-291 1499 [2] Du S W, Ramanujan R V, 2005 J Non-Cryst. Solids 351 3105 [3] Tufescu F M, Chiriac H 2005 Sens. Actuator A 119 305 [4] Manjura Hoque S, Khan F A, Hakim M A 2007 Mat. Lett. 61 1227 [5] Rho I C, Yoon C S, Kim C K Byun T Y, Hong K S 2002 Mat Sci Eng B96 48 [6] Bednarska L, Kovbuz M, Nosenko V, Mudry S, Korolyshyn A. 2002 Mol phys reports 36 117 [7] Mudry S, Kotur B, Bednarska L, Kulyk Yu 2004 J. Alloys Compd. 367 274

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