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Effect of cooling rate in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx alloy system on magnetic properties
Current Applied Physics 15 (2015) 326e329
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Effect of cooling rate in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx alloy system on magnetic properties Bo-Kyeong Han a, Nam-Hui Kim b, Chun-Yeol You b, Haein Choi-Yim a, * a b
Department of Physics, Sookmyung Women's University, Seoul 140-742, Republic of Korea Department of Physics, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2014 Received in revised form 2 January 2015 Accepted 5 January 2015 Available online 5 January 2015
The effect of cooling rate on the thermal stability and soft magnetic properties of [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) system was investigated. The alloys were produced into the form of ribbon and cylindrical rod by melt-spinning and injection casting, respectively. Their structure, thermal, mechanical and soft magnetic properties were investigated by x-ray diffraction, differential scanning calorimetry, universal testing machine and vibrating sample magnetometer, respectively. All of the alloys were identified as fully amorphous by X-ray diffraction. It turned out that the rod samples had exceptionally high saturation fields reaching 3.0 kOe, which is key properties for sensor application. Also, among these Fe,Co-based samples, the Fe35.25 Co35.25 B18.8 Si4.70 Nb6 ribbon exhibits the highest saturation magnetization with 142.1 emu/g. © 2015 Elsevier B.V. All rights reserved.
Keywords: Fe,Co-based Soft magnetic properties Ferromagnetic Bulk metallic glass Amorphous ribbon
1. Introduction The Fe,Co-based amorphous materials have attracted considerable interests for commercial applications due to their unique combination of good corrosion resistance, high facture strength, low cost and excellent soft magnetic properties such as high saturation magnetization and low coercivity [1e7]. These features have motivated comprehensive research activities on Fe,Co-based alloys. In the history of ferromagnetic amorphous alloy system, the alloy of FeePeC was firstly fabricated by using melt-spinning technique in 1967 [8]. A large number of Fe-based amorphous alloys have been developed: (Fe, Co, Ni)ePeB [9], (Fe, Co, Ni)-(Cr, Mo, W)eC [10], (Fe, Co, Ni)eBeSi [11,12], (Fe, Co, Ni)eZr [13] and (Fe, Co, Ni)-(Zr, Hf, Nb)eB [14,15], and investigated to improve the soft magnetic properties and thermal stability in the forms of bulk or as-spun ribbon for commercial applications [16e18]. Especially, it has been of great interest to improve the soft magnetic properties such as low coercivity, high saturation magnetization and high permeability in Fe,Co-based amorphous materials for various applications: inductors, sensors, magnetic memories, magnetic heads, electro-magnetic cores, current
* Corresponding author. E-mail address: [email protected] (H. Choi-Yim). http://dx.doi.org/10.1016/j.cap.2015.01.002 1567-1739/© 2015 Elsevier B.V. All rights reserved.
machines and motors [19e22]. In these systems, (Fe,Co)eBeSi alloy system was developed in 1974 and has been used in application because these system have good soft magnetic properties and high strength compared to other FeeBeSi systems [23e25]. So we selected the [Fe,Co]-B-Si system. Also, elements of Nb were added on FeeCoeBeSi systems because addition of Nb enhances the thermal stability. Moreover, it didn't decrease soft magnetic properties in contradistinction to B and Si [25]. In this study, we report on the effect of cooling rate in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) alloys system on thermal stability and soft magnetic properties by comparing the alloys in the form of thin ribbons and 2 mm diameter rods. Also, we studied the mechanical properties of [(Fe0.5Co0.5)0.75B0.2Si0.05]100xNbx (x ¼ 5, 6, 7, 8 at. %) alloys. 2. Experiments Alloy ingots with nominal composition of FeeCo based alloys with [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) were synthesized by arc-melting of high-purity elements under a Tigettered argon atmosphere. Each ingot was re-melted at least four times in order to properly mix the elements constituting the material. Rapidly solidified ribbons of amorphous alloys with various thicknesses were produced by using a single copper roller
B.-K. Han et al. / Current Applied Physics 15 (2015) 326e329
vacuum melt-spinning technique under a controlled argon gas atmosphere. The spinning speed of the copper wheel of the melt spinner was optimized at 33 m/s. Also, Cylindrical rods with diameter of 2 mm were produced using the technique of injecting liquid material into cooled Cu-mold. The amorphous structures of the prepared samples were examined using X-ray diffraction (XRD) with Cu-Ka radiation (l ¼ 1.54050 Å). Samples were scanned from 30 to 80 in 2theta. The thermal characterization associated with glass transition temperatures (Tg), crystallization temperatures (Tx) and supercooled liquid region (DTx ¼ Tx Tg) of the samples was carried out by using differential scanning calorimetry (DSC) under an argon atmosphere and constant heating up to 1073 K at a heating rate of 0.34 K/s. The mechanical properties such as yield stress and ultimate fracture strength at room temperature were measured by Universal Testing Machine (UTM) at a strain rate of 1.0 104/s under a compression mode using as-prepared amorphous rod specimens. The compression specimens with the diameter of 2 mm and the length of 4 mm were cut from the amorphous rods, and the ends of the rod were polished to ensure parallelism. The magnetic properties such as the saturation magnetization (Ms) and the coercivity (Hc) of those samples were measured with
Fig. 1. XRD patterns of (a) [(Fe0.5 Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbons and (b) [(Fe0.5 Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
327
the vibrating sample magnetometer (VSM) with a hysteresis loop tracer at room temperature. The prepared samples were examined for in-plane magnetic fields with maximum applied field of 1.0 T. 3. Results & discussion The X-ray diffraction patterns obtained for the amorphous ribbons and cast rods are shown in Fig. 1. The samples of [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) have a fully amorphous phase as no sharp Bragg peaks are observed and all diffraction lines show the typical broad halo pattern of an amorphous state. The DSC curves of all of the ribbon and rod samples with composition variation at a heating rate of 0.34 K/s were obtained. The Fig. 2 shows the DSC pattern obtained for the rapidly solidified [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) samples where Tg, Tx, and DTx are marked by arrows. All samples show a clear glass transition and the crystallization with wide supercooled liquid region (DTx). The thermal stability parameters such as glass transition temperature (Tg), the crystallization temperature (Tx), and supercooled liquid region (DTx) are reported in Table 1. From the DSC curve, the glass transition temperature and the crystallization
Fig. 2. DSC curves of (a) [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbons and (b) [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
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B.-K. Han et al. / Current Applied Physics 15 (2015) 326e329
Table 1 Thermal properties and magnetic properties of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbon and 2 mm diameter rods. Alloys
Fe35.63Co35.63B19.0Si4.75Nb5 Fe35.25Co35.25B18.8Si4.70Nb6 Fe34.88Co34.88B18.6Si4.65Nb7 Fe34.50Co34.50B18.4Si4.60Nb8 Fe35.63Co35.63B19.0Si4.75Nb5 Fe35.25Co35.25B18.8Si4.70Nb6 Fe34.88Co34.88B18.6Si4.65Nb7 Fe34.50Co34.50B18.4Si4.60Nb8
Thermal properties
(ribbon) (ribbon) (ribbon) (ribbon) (bulk) (bulk) (bulk) (bulk)
Magnetic properites
Tg (K)
Tx (K)
DTx (K)
Ms (emu/g)
819.0 828.5 831.9 833.7 826.3 827.1 827.9 834.0
865.1 876.7 883.4 886.5 870.7 878.4 876.1 886.8
46.1 48.2 51.5 52.8 44.4 51.3 48.2 52.8
130.9 142.1 116.1 112.7 99.3 96.0 82.1 75.1
Table 2 Mechanical properties of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods. Alloys
Fe35.63Co35.63B19.0Si4.75Nb5 Fe35.25Co35.25B18.8Si4.70Nb6 Fe34.88Co34.88B18.6Si4.65Nb7 Fe34.50Co34.50B18.4Si4.60Nb8
Mechanical properties
(bulk) (bulk) (bulk) (bulk)
Ultimate strength (MPa)
Yield strength (MPa)
3739.3 3436.5 4126.7 3770.9
3587.6 3325.1 4103.9 3770.9
temperature of the [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) amorphous ribbons were determined to be in the range of 819.0e833.7 K and 865.1e886.5 K, respectively. The supercooled liquid regions of the ribbons were in the range of 46.1e52.8 K. According to the results, for ribbons of FeeCoeBeSieNb system, with larger proportion of Nb, Tg, Tx and DTx increase gradually up to 833.7 K, 886.5 K and 52.8 K, respectively. Also, Fe34.5Co34.5 B18.4 Si4.60Nb8 has the largest DTx. It indicates that the thermal stability is improved by increasing Nb content in FeeCoeBeSieNb ribbon system. The Tg, Tx and DTx of rod samples increase gradually up to 834.0 K, 886.8 K and 52.8 K, respectively, which agree well with the data obtained from the ribbons. The mechanical properties were measured by compressive test at room temperature. Fig. 3 shows the compressive true stressestrain curves of the [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) glassy alloy rods with a diameter of 2 mm and a length of 4 mm in the room-temperature. The rod of Fe34.88Co34.88 B18.6 Si4.65Nb7 exhibits the highest ultimate strength (4126.7 MPa) and yield strength (4103.9 MPa). The acquired mechanical properties for all specimens are summarized in Table 2. It appears that the increase in Nb content caused the loss of compressive ductility. The magnetic properties of as-prepared [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) amorphous ribbons and rod samples were measured by using VSM at room temperature. The size of ribbons used for the measurement was 10 mm in length, 1 mm in width, and 0.03 mm in thickness. The magnetic field was applied to in-plane direction, and the size of bulk sample was about 2 2 2 mm cylindrical rod. The hysteresis
MH loops for the glassy ribbon and rod samples are shown in Fig. 4. We are sure that the coercivities of all samples are always less than 1 Oe, but more precise measurement is impossible due to the field measurement limitation of the VSM. It is obvious that all samples show very soft characteristics, which is expected and also reported for the amorphous FeeCoeBeSieNb alloy samples [1e7]. The values of saturation magnetization of the ribbons were confirmed to be in the range of 112.7e142.1 emu/g, and from 75.1 to 99.3 emu/g for the cast rod specimens. For both cases, the saturation magnetizations tends to increase with decreasing Nb, without loss of the ultra-low coercivity. The saturation magnetization increments are 116 and 132% for ribbon and bulk samples by decreasing Nb atomic percent from 8 to 5 %, respectively. It is easily understandable that the amount of nonmagnetic Nb decreases the saturation magnetization, however, the reason of different increment ratio between ribbons and bulks for the same composition are not clear yet. The saturation magnetization of ribbons are always larger than bulk samples of the same compositions in our experiments. The most striking features of our results are high saturation fields of bulk samples. Even though their coercivity is very small (<1 Oe), their saturation fields are almost 2.5e3.0 kOe. Such high saturation fields are distinguishable from the ribbon case. The observed high saturation field cannot be explained by demagnetization field when we consider the shape of the sample. Rather it indicates the existence of random magnetic anisotropy [26]. It is known that the random magnetic anisotropy can exist in the amorphous system [27]. More detailed analysis of the random magnetic anisotropy is beyond the scope of this work. Before we conclude, we would like to point out that the small coercivity and large saturation field guarantees a linear response of
Fig. 3. Compressive stressestrain curves of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
Fig. 4. Hysteresis loops of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbons and [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
B.-K. Han et al. / Current Applied Physics 15 (2015) 326e329
the magnetization to the external field. For large field range (2.5e2.5 kOe in Fig. 4) the magnetization is linearly proportional to the external field without hysteresis for the bulk samples. It is very important properties for the sensor allocations, therefore, it may open new way to the possible application of amorphous FeeCoeBeSieNb alloy for the sensor applications which is operated within kOe field. 4. Conclusion In this present work, the effect of cooling rate on thermal stability and soft magnetic properties of [(Fe0.5Co0.5)0.75B0.2Si0.05]100xNbx (x ¼ 5, 6, 7, 8 at. %) system was examined by comparing the specimens in two different forms: melt-spun ribbons and injection cast rods. All of the prepared samples exhibit amorphous structure in FeeCoeBeSieNb system. With increasing Nb content in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) system, thermal stability was increased. The supercooled liquid region was increased by increasing Nb content except the rod sample with Fe34.88Co34.88 B18.6 Si4.65Nb7. Also, the there was no difference in thermal properties between the specimens made by melt spinning and injection rod casting. Among the samples, the amorphous ribbon of Fe35.25 Co35.25 B18.8 Si4.70 Nb6 exhibits the highest soft magnetic properties such as a low coercivity of 0.054 Oe and a high saturated magnetization of 142.1 emu/g. Acknowledgment This research was supported by Sookmyung Women's University Research Grants (1-1203-0317). The authors gratefully acknowledge the financial support.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of cooling rate in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx alloy system on magnetic properties Bo-Kyeong Han a, Nam-Hui Kim b, Chun-Yeol You b, Haein Choi-Yim a, * a b
Department of Physics, Sookmyung Women's University, Seoul 140-742, Republic of Korea Department of Physics, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2014 Received in revised form 2 January 2015 Accepted 5 January 2015 Available online 5 January 2015
The effect of cooling rate on the thermal stability and soft magnetic properties of [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) system was investigated. The alloys were produced into the form of ribbon and cylindrical rod by melt-spinning and injection casting, respectively. Their structure, thermal, mechanical and soft magnetic properties were investigated by x-ray diffraction, differential scanning calorimetry, universal testing machine and vibrating sample magnetometer, respectively. All of the alloys were identified as fully amorphous by X-ray diffraction. It turned out that the rod samples had exceptionally high saturation fields reaching 3.0 kOe, which is key properties for sensor application. Also, among these Fe,Co-based samples, the Fe35.25 Co35.25 B18.8 Si4.70 Nb6 ribbon exhibits the highest saturation magnetization with 142.1 emu/g. © 2015 Elsevier B.V. All rights reserved.
Keywords: Fe,Co-based Soft magnetic properties Ferromagnetic Bulk metallic glass Amorphous ribbon
1. Introduction The Fe,Co-based amorphous materials have attracted considerable interests for commercial applications due to their unique combination of good corrosion resistance, high facture strength, low cost and excellent soft magnetic properties such as high saturation magnetization and low coercivity [1e7]. These features have motivated comprehensive research activities on Fe,Co-based alloys. In the history of ferromagnetic amorphous alloy system, the alloy of FeePeC was firstly fabricated by using melt-spinning technique in 1967 [8]. A large number of Fe-based amorphous alloys have been developed: (Fe, Co, Ni)ePeB [9], (Fe, Co, Ni)-(Cr, Mo, W)eC [10], (Fe, Co, Ni)eBeSi [11,12], (Fe, Co, Ni)eZr [13] and (Fe, Co, Ni)-(Zr, Hf, Nb)eB [14,15], and investigated to improve the soft magnetic properties and thermal stability in the forms of bulk or as-spun ribbon for commercial applications [16e18]. Especially, it has been of great interest to improve the soft magnetic properties such as low coercivity, high saturation magnetization and high permeability in Fe,Co-based amorphous materials for various applications: inductors, sensors, magnetic memories, magnetic heads, electro-magnetic cores, current
* Corresponding author. E-mail address: [email protected] (H. Choi-Yim). http://dx.doi.org/10.1016/j.cap.2015.01.002 1567-1739/© 2015 Elsevier B.V. All rights reserved.
machines and motors [19e22]. In these systems, (Fe,Co)eBeSi alloy system was developed in 1974 and has been used in application because these system have good soft magnetic properties and high strength compared to other FeeBeSi systems [23e25]. So we selected the [Fe,Co]-B-Si system. Also, elements of Nb were added on FeeCoeBeSi systems because addition of Nb enhances the thermal stability. Moreover, it didn't decrease soft magnetic properties in contradistinction to B and Si [25]. In this study, we report on the effect of cooling rate in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) alloys system on thermal stability and soft magnetic properties by comparing the alloys in the form of thin ribbons and 2 mm diameter rods. Also, we studied the mechanical properties of [(Fe0.5Co0.5)0.75B0.2Si0.05]100xNbx (x ¼ 5, 6, 7, 8 at. %) alloys. 2. Experiments Alloy ingots with nominal composition of FeeCo based alloys with [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) were synthesized by arc-melting of high-purity elements under a Tigettered argon atmosphere. Each ingot was re-melted at least four times in order to properly mix the elements constituting the material. Rapidly solidified ribbons of amorphous alloys with various thicknesses were produced by using a single copper roller
B.-K. Han et al. / Current Applied Physics 15 (2015) 326e329
vacuum melt-spinning technique under a controlled argon gas atmosphere. The spinning speed of the copper wheel of the melt spinner was optimized at 33 m/s. Also, Cylindrical rods with diameter of 2 mm were produced using the technique of injecting liquid material into cooled Cu-mold. The amorphous structures of the prepared samples were examined using X-ray diffraction (XRD) with Cu-Ka radiation (l ¼ 1.54050 Å). Samples were scanned from 30 to 80 in 2theta. The thermal characterization associated with glass transition temperatures (Tg), crystallization temperatures (Tx) and supercooled liquid region (DTx ¼ Tx Tg) of the samples was carried out by using differential scanning calorimetry (DSC) under an argon atmosphere and constant heating up to 1073 K at a heating rate of 0.34 K/s. The mechanical properties such as yield stress and ultimate fracture strength at room temperature were measured by Universal Testing Machine (UTM) at a strain rate of 1.0 104/s under a compression mode using as-prepared amorphous rod specimens. The compression specimens with the diameter of 2 mm and the length of 4 mm were cut from the amorphous rods, and the ends of the rod were polished to ensure parallelism. The magnetic properties such as the saturation magnetization (Ms) and the coercivity (Hc) of those samples were measured with
Fig. 1. XRD patterns of (a) [(Fe0.5 Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbons and (b) [(Fe0.5 Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
327
the vibrating sample magnetometer (VSM) with a hysteresis loop tracer at room temperature. The prepared samples were examined for in-plane magnetic fields with maximum applied field of 1.0 T. 3. Results & discussion The X-ray diffraction patterns obtained for the amorphous ribbons and cast rods are shown in Fig. 1. The samples of [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) have a fully amorphous phase as no sharp Bragg peaks are observed and all diffraction lines show the typical broad halo pattern of an amorphous state. The DSC curves of all of the ribbon and rod samples with composition variation at a heating rate of 0.34 K/s were obtained. The Fig. 2 shows the DSC pattern obtained for the rapidly solidified [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) samples where Tg, Tx, and DTx are marked by arrows. All samples show a clear glass transition and the crystallization with wide supercooled liquid region (DTx). The thermal stability parameters such as glass transition temperature (Tg), the crystallization temperature (Tx), and supercooled liquid region (DTx) are reported in Table 1. From the DSC curve, the glass transition temperature and the crystallization
Fig. 2. DSC curves of (a) [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbons and (b) [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
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B.-K. Han et al. / Current Applied Physics 15 (2015) 326e329
Table 1 Thermal properties and magnetic properties of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbon and 2 mm diameter rods. Alloys
Fe35.63Co35.63B19.0Si4.75Nb5 Fe35.25Co35.25B18.8Si4.70Nb6 Fe34.88Co34.88B18.6Si4.65Nb7 Fe34.50Co34.50B18.4Si4.60Nb8 Fe35.63Co35.63B19.0Si4.75Nb5 Fe35.25Co35.25B18.8Si4.70Nb6 Fe34.88Co34.88B18.6Si4.65Nb7 Fe34.50Co34.50B18.4Si4.60Nb8
Thermal properties
(ribbon) (ribbon) (ribbon) (ribbon) (bulk) (bulk) (bulk) (bulk)
Magnetic properites
Tg (K)
Tx (K)
DTx (K)
Ms (emu/g)
819.0 828.5 831.9 833.7 826.3 827.1 827.9 834.0
865.1 876.7 883.4 886.5 870.7 878.4 876.1 886.8
46.1 48.2 51.5 52.8 44.4 51.3 48.2 52.8
130.9 142.1 116.1 112.7 99.3 96.0 82.1 75.1
Table 2 Mechanical properties of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods. Alloys
Fe35.63Co35.63B19.0Si4.75Nb5 Fe35.25Co35.25B18.8Si4.70Nb6 Fe34.88Co34.88B18.6Si4.65Nb7 Fe34.50Co34.50B18.4Si4.60Nb8
Mechanical properties
(bulk) (bulk) (bulk) (bulk)
Ultimate strength (MPa)
Yield strength (MPa)
3739.3 3436.5 4126.7 3770.9
3587.6 3325.1 4103.9 3770.9
temperature of the [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) amorphous ribbons were determined to be in the range of 819.0e833.7 K and 865.1e886.5 K, respectively. The supercooled liquid regions of the ribbons were in the range of 46.1e52.8 K. According to the results, for ribbons of FeeCoeBeSieNb system, with larger proportion of Nb, Tg, Tx and DTx increase gradually up to 833.7 K, 886.5 K and 52.8 K, respectively. Also, Fe34.5Co34.5 B18.4 Si4.60Nb8 has the largest DTx. It indicates that the thermal stability is improved by increasing Nb content in FeeCoeBeSieNb ribbon system. The Tg, Tx and DTx of rod samples increase gradually up to 834.0 K, 886.8 K and 52.8 K, respectively, which agree well with the data obtained from the ribbons. The mechanical properties were measured by compressive test at room temperature. Fig. 3 shows the compressive true stressestrain curves of the [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) glassy alloy rods with a diameter of 2 mm and a length of 4 mm in the room-temperature. The rod of Fe34.88Co34.88 B18.6 Si4.65Nb7 exhibits the highest ultimate strength (4126.7 MPa) and yield strength (4103.9 MPa). The acquired mechanical properties for all specimens are summarized in Table 2. It appears that the increase in Nb content caused the loss of compressive ductility. The magnetic properties of as-prepared [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) amorphous ribbons and rod samples were measured by using VSM at room temperature. The size of ribbons used for the measurement was 10 mm in length, 1 mm in width, and 0.03 mm in thickness. The magnetic field was applied to in-plane direction, and the size of bulk sample was about 2 2 2 mm cylindrical rod. The hysteresis
MH loops for the glassy ribbon and rod samples are shown in Fig. 4. We are sure that the coercivities of all samples are always less than 1 Oe, but more precise measurement is impossible due to the field measurement limitation of the VSM. It is obvious that all samples show very soft characteristics, which is expected and also reported for the amorphous FeeCoeBeSieNb alloy samples [1e7]. The values of saturation magnetization of the ribbons were confirmed to be in the range of 112.7e142.1 emu/g, and from 75.1 to 99.3 emu/g for the cast rod specimens. For both cases, the saturation magnetizations tends to increase with decreasing Nb, without loss of the ultra-low coercivity. The saturation magnetization increments are 116 and 132% for ribbon and bulk samples by decreasing Nb atomic percent from 8 to 5 %, respectively. It is easily understandable that the amount of nonmagnetic Nb decreases the saturation magnetization, however, the reason of different increment ratio between ribbons and bulks for the same composition are not clear yet. The saturation magnetization of ribbons are always larger than bulk samples of the same compositions in our experiments. The most striking features of our results are high saturation fields of bulk samples. Even though their coercivity is very small (<1 Oe), their saturation fields are almost 2.5e3.0 kOe. Such high saturation fields are distinguishable from the ribbon case. The observed high saturation field cannot be explained by demagnetization field when we consider the shape of the sample. Rather it indicates the existence of random magnetic anisotropy [26]. It is known that the random magnetic anisotropy can exist in the amorphous system [27]. More detailed analysis of the random magnetic anisotropy is beyond the scope of this work. Before we conclude, we would like to point out that the small coercivity and large saturation field guarantees a linear response of
Fig. 3. Compressive stressestrain curves of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
Fig. 4. Hysteresis loops of [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) amorphous ribbons and [(Fe0.5Co0.5)0.75 B0.2 Si0.05]100-xNbx (x ¼ 5, 6, 7, 8%) 2 mm diameter rods.
B.-K. Han et al. / Current Applied Physics 15 (2015) 326e329
the magnetization to the external field. For large field range (2.5e2.5 kOe in Fig. 4) the magnetization is linearly proportional to the external field without hysteresis for the bulk samples. It is very important properties for the sensor allocations, therefore, it may open new way to the possible application of amorphous FeeCoeBeSieNb alloy for the sensor applications which is operated within kOe field. 4. Conclusion In this present work, the effect of cooling rate on thermal stability and soft magnetic properties of [(Fe0.5Co0.5)0.75B0.2Si0.05]100xNbx (x ¼ 5, 6, 7, 8 at. %) system was examined by comparing the specimens in two different forms: melt-spun ribbons and injection cast rods. All of the prepared samples exhibit amorphous structure in FeeCoeBeSieNb system. With increasing Nb content in [(Fe0.5Co0.5)0.75B0.2Si0.05]100-xNbx (x ¼ 5, 6, 7, 8 at. %) system, thermal stability was increased. The supercooled liquid region was increased by increasing Nb content except the rod sample with Fe34.88Co34.88 B18.6 Si4.65Nb7. Also, the there was no difference in thermal properties between the specimens made by melt spinning and injection rod casting. Among the samples, the amorphous ribbon of Fe35.25 Co35.25 B18.8 Si4.70 Nb6 exhibits the highest soft magnetic properties such as a low coercivity of 0.054 Oe and a high saturated magnetization of 142.1 emu/g. Acknowledgment This research was supported by Sookmyung Women's University Research Grants (1-1203-0317). The authors gratefully acknowledge the financial support.
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