Atomic structure and thermal behavior of (Co0.65,Fe0.35)72Ta8B20 metallic glass with excellent soft magnetic properties

Intermetallics 69 (2016) 21e27

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Atomic structure and thermal behavior of (Co0.65,Fe0.35)72Ta8B20 metallic glass with excellent soft magnetic properties Amir Hossein Taghvaei a, *, Jozef Bednar
cik b, Jürgen Eckert c, d a

Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz, Iran Deutsches Elektronen-Synchrotron DESY, Photon Science, Notkestraße 85, 22603 Hamburg, Germany c Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria d €t Leoben, Jahnstraße 12, A-8700 Leoben, Austria Department of Materials Physics, Montanuniversita b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 July 2015 Received in revised form 11 October 2015 Accepted 17 October 2015 Available online xxx

New soft magnetic (Co0.65,Fe0.35)72Ta8B20 metallic glass has been synthesized and its atomic structure and thermal stability have been studied using high-energy synchrotron X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. Analysis of the pair correlation functions (PDF) indicates a notable shift in position of the first PDF peak to a lower distance and the decreasing the average coordination number of the first shell, compared to the (Co0.65,Fe0.35)62Ta8B30 glassy alloy, recently produced. DSC analysis shows that the new alloy has a wide supercooled liquid region of 51 K and relatively high activation energy of crystallization of about 445 kJ/mol, calculated according to the Kissinger and Ozawa methods, which demonstrates its high thermal stability. Magnetic measurements show that the new alloy exhibits outstanding soft magnetic properties, i.e., very low coercivity of 1.2 A/m, high saturation magnetization of 93.5 Am2/kg and Curie temperature of 660 K, which are significantly larger than those of (Co0.65,Fe0.35)62Ta8B30 glassy ribbon. The influence of the annealing treatment on evolution of the soft magnetic properties of the (Co0.65,Fe0.35)72Ta8B20 glassy ribbons has been investigated. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Metallic glasses (or amorphous metals) Magnetic properties Thermal stability Pair correlation function Differential scanning calorimetry

1. Introduction Since the discovery of metallic glasses (MGs) in the binary AueSi alloy at 1960 [1], these materials have attracted an increasing attention due to their superior properties in comparison with their crystalline counterparts [2,3]. The exceptional physical, chemical and mechanical properties like ultrahigh strength, excellent corrosion and wear resistance, and very good soft magnetic behavior have made them as promising candidates in many potential applications [2,4]. With respect to the number of elements, their atomic size ratio, and their corresponding heat of mixing, MGs can be synthesized in the form of thin ribbons, wires, powders and rods with diameter of few millimeters up to even couple of centimeters, depending on their glass forming ability (GFA) [5e9]. It has been shown that a proper selection of the constituent elements not only influences the GFA, but it can systematically determine the thermal stability, physical and mechanical properties of a MG [2,4].

* Corresponding author. E-mail address: [email protected] (A.H. Taghvaei). http://dx.doi.org/10.1016/j.intermet.2015.10.012 0966-9795/© 2015 Elsevier Ltd. All rights reserved.

After the first synthesis of ferromagnetic MGs at 1967 [10], extensive experimental and theoretical studies have been conducted to develop this type of glassy alloys because of their unique magnetic properties [11]. Among ferromagnetic MGs, Co-based alloys can be used in diverse applications like high-sensitivity magnetic sensors, power transformers, magnetic shielding and magnetic heads, owing to their extreme soft magnetic properties, e.g, very low coercivity, moderate saturation magnetization, very high magnetic permeability, particularly for the amorphous wire and ribbons, which results in a giant magneto-impedance (GMI) effect [12e15]. Co-based alloys in the form of glassy ribbons or bulk metallic glasses (BMGs) have been synthesized in different alloying systems like CoeFeeMoeSieB (Vitrovac group) [12,16,17], Co(Fe,Ni)eSieNbeB [18], Co-(Fe)eNb-(Zr,Dy,Er,Tb)eB [19e21], and Co-(Fe)eTa(Y)eB [22e24]. Besides the excellent magnetic properties, some Co-based BMGs exhibit promising mechanical properties such as ultrahigh fracture strength ranging between 5 and 6 GPa, [20,22] and large plasticity [20]. Recently, the (Co0.65,Fe0.35)62Ta8B30 MG with very high thermal stability and good soft magnetic properties has been produced [23]. In addition, it has been shown that the (Co0.65,Fe0.35)62Ta8B30 BMG

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with a high relative density, ultrahigh hardness, and promising soft magnetic properties could be produced by hot consolidation of the glassy powders in the temperature range of the supercooled liquid region (SLR) [23]. Although the as-quenched (Co0.65,Fe0.35)62Ta8B30 alloy exhibits a very low coercivity of 0.8 A/m, its relatively low saturation polarization of 0.5 T and Curie temperature of 412 K can limit its potential applications [23]. Such magnetic parameters can be improved through partial replacement of B by the Fe and Co atoms. In the current research, new (Co0.65,Fe0.35)72Ta8B20 glassy ribbons was developed and its atomic structure, thermal behavior and magnetic properties in the as-quenched and annealed states were investigated. It was shown that while the new alloy has still a high thermal stability and very small coercivity of 1.2 A/m, it exhibits two times larger saturation magnetization, and notably higher Curie temperature compared to the (Co0.65,Fe0.35)62Ta8B30 glassy ribbons. 2. Experimental procedure An alloy ingot with nominal composition of (Co0.65,Fe0.35)72Ta8B20 (at.%) was prepared by arc melting of pure Co (99.9%), Fe (99.9%), Ta (99.96%) and crystalline B (99.5%) under a Ti-gettered Ar atmosphere. The obtained master alloy was re-melted at least three times in order to enhance its chemical homogeneity. Glassy ribbons (width of 2 mm and thickness of 30 mm) were produced by a singleroller Bühler melt-spinner under Ar atmosphere with a copper wheel rotating at 41 m/s. The microstructure analysis of the produced ribbons was conducted by X-ray diffraction (XRD) in a reflection mode using Co Ka radiation (l ¼ 1.7902 Å), and in a transmission geometry using a high-energy synchrotron radiation with an incident beam energy of 59.89 keV (l ¼ 0.207 Å) at the P02.1 beamline of the PETRA III electron storage ring (DESY, Hamburg, Germany). A twodimensional (2 D) detector (Perkin Elmer 1621) was employed to record the synchrotron XRD patterns at room temperature. The data analysis and integration of the 2 D XRD patterns to the Q-space (Q ¼ wave vector) were carried out by FIT2D software [25]. To achieve the coherent and elastically scattered intensities, polarization, sample absorption, fluorescence and inelastic scattering contributions were subtracted using the PDFgetX2 software [26]. The total structure factor S(Q) was calculated according to the Faber-Ziman approach [27]. The X-ray weighting factors of the iej pairs, wij, were determined as: [27].


 fi ðQ Þfj ðQ Þ wij ðQ Þ ¼ 2  dij ci cj ; 〈f ðQ Þ〉2

The MeH plots and saturation magnetization of the ribbons at room temperature were measured by a vibrating sample magnetometer (VSM). The thermomagnetic behavior and Curie temperature of the ribbons were determined by a Faraday magnetometer working at constant heating rate of 10 K/min. The Coercivity of the ribbons was measured using a Foerster Coercimat under a DC applied field of 200 kA/m, high enough to saturate the ribbons. 3. Results and discussion 3.1. Atomic structure Fig. 1 shows the total structure factor S(Q) of the as-quenched (Co0.65,Fe0.35)72Ta8B20 ribbon. In addition, the S(Q) of the (Co0.65,Fe0.35)62Ta8B30 glassy ribbons is shown for comparison. The latter and former alloys are called B30 and B20 hereafter in this work for simplicity. According to Fig. 1, the B20 ribbon has a diffuse scattering pattern, typical for a fully glassy structure with pronounced oscillations up to around Q ¼ 17 Å1 and a maximum intensity at Q ¼ 3.1 Å1. The reciprocal-space data suggest that some changes take place in the atomic structure upon substitution of B with Fe and Co. For instance, based on Fig. 1, despite very close positions of the first diffuse maximum in both alloys, the positions of the other diffraction maxima move to larger wave vectors in case of the B20 alloy (see the inset of Fig. 1). The extent and scale of structural variations (short-range or medium-range) can be further examined in the real-space through comparing the reduced pair distribution functions G(r) of both ribbons, as shown in Fig. 2. According to Fig. 2(a), one can find some differences in the G(r) curves, particularly in the length-scale smaller than 7 Å. In contrast, both glassy ribbons show relatively similar oscillations appearing on G(r) beyond r ¼ 7 Å and extending up to around 16 Å, indicating that the alloys probably have a similar type and degree of the medium-range order (MRO). The observed similarity in MRO is in line with the fact that the metal-boron MGs usually have a network-like MRO, originating mainly from the strong chemical interactions and formation of the covalent bonding in the metalboron pairs [24,28]. Such a network-like MRO is constructed through the connection of the boron-centered tri-capped trigonal prism (TTP) type polyhedra [28,29]. The TTP-type short-range order (SRO) is mostly observed in the metal-metalloid MGs in which the atomic size ratio, R*, between metalloid and solvent atoms is lower

(1)

where the c denotes the concentration, f(Q) is the scattering power and d is the delta symbol of Kronecker. The reduced pair distribution function G(r) was determined by the sine Fourier transform of S(Q) as:

GðrÞ ¼

2 p

Q Zmax

Q ðSðQ Þ  1ÞsinðQrÞdQ ;

(2)

Qmin

The radial distribution function, RDF was calculated from G(r) and the average atomic number density r0 according to:

RDFðrÞ ¼ 4pr0 r 2 þ rGðrÞ;

(3)

Thermal behavior of the ribbons at different heating rates (10e40 K/min) were evaluated by a differential scanning calorimeter (DSC, NETZSCH DSC 404) working under a high purity Ar atmosphere.

Fig. 1. Total XRD structure factors S(Q) of the as-quenched (Co0.65,Fe0.35)72Ta8B20 and (Co0.65,Fe0.35)62Ta8B30 ribbons.

A.H. Taghvaei et al. / Intermetallics 69 (2016) 21e27

23

Fig. 2. (a)Reduced pair distribution functions G(r) of the as-cast (Co0.65,Fe0.35)72Ta8B20 and (Co0.65,Fe0.35)62Ta8B30 glassy ribbons; (b) first coordination shell region of the G(r) functions.

than 0.732 [29e31]. In contrast to the medium-range scale, the extent of structural difference between B20 and B30 glassy alloys is very clear in the range of the first coordination shell (1.9 Å  r  3.5 Å) according to Fig. 2(b). In other words, the observed changes in shape and position of the first G(r) peak reveal that some structural variations take place in the SRO upon partial replacement of B by Fe/Co. For both B20 and B30 alloys, 10 overlapping atomic pairs exist in the first coordination shell. The theoretical inter-atomic bond lengths determined as a sum of Goldschmidt's radii and the X-ray weighting factors calculated according to Eq. (1) for Q ¼ 0 Å1 are listed in Table 1 for the B20 alloy and in Ref. [23] for the B30 alloy. Based on Fig. 2(b), both alloys indicate a pre-peak at r ~ 2.1 Å, corresponding to (Co,Fe)eB pairs. The main peak of the G(r) located at r ¼ 2.58 Å for the B20 alloy is predominantly formed by CoeCo and CoeFe atomic pairs due to their weighting factors (Table 1). Furthermore, a pronounced shoulder appears on the right-hand side of the main peak in the B30 alloy, which is attributed to the TaeTa pairs [23], and is not so pronounced in the B20 alloy. Another significant difference between the two G(r) functions are the positions of their strongest maxima, which are 2.61 Å and 2.58 Å for B30 and B20 alloy, respectively. Due to a significant contribution of the (Co,Fe)eCo pairs in the main G(r) peak of both alloys, the

difference in position of this peak can be attributed to the shorter (Co,Fe)eCo bond length in the B20 alloy. A similar result has been observed in FeeNbeB glassy alloys, where decreasing the B content from 30 at% to 20 at% is followed by decreased FeeFe pair separation [32]. The average coordination number, CN, of the first coordination shell was calculated by integrating the first peak of corresponding RDFs (not shown here) determined according to Eq. (3). This gives the values of 13.7 ± 0.4 and 12.7 ± 0.4 for CN of B30 and B20 alloys, respectively. It can be seen that the B20 alloy has probably a less dense atomic packing due to a smaller average CN compared to the B30 alloy. 3.2. Thermal behavior Fig. 3 shows the DSC plot of the B20 glassy alloy measured at a constant heating rate of 20 K/min. The DSC curve reveals an

Table 1 Theoretical bond inter-atomic bond lengths and the X-ray weighting factors wij determined at Q ¼ 0 Å1 for (Co0.65,Fe0.35)72Ta8B20 alloy. Atomic pair ij

rij (Å)

wij

FeeFe FeeCo CoeCo FeeB CoeB FeeTa CoeTa TaeB TaeTa BeB

2.48 2.49 2.50 2.12 2.13 2.73 2.74 2.37 2.98 1.80

0.065 0.246 0.231 0.019 0.037 0.114 0.216 0.017 0.050 0.001

Fig. 3. DSC plot of the (Co0.65,Fe0.35)72Ta8B20 glassy ribbons measured at a constant heating rate of 20 K/min.

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A.H. Taghvaei et al. / Intermetallics 69 (2016) 21e27

endothermic event with the onset temperature of 819 K corresponding to the glass transition temperature, Tg, followed by a sharp exothermic reaction characteristic of the crystallization with an onset temperature of Tx1 ¼ 870 K. As a result, the B20 glassy alloy exhibits a relatively wide supercooled liquid region, SLR, (DTx ¼ Tx1  Tg) of 51 K, which demonstrates its high thermal stability. Moreover, the DSC plot shows another crystallization event with the onset temperature of 1126 K. Compared to the DSC plot of the B30 alloy, which shows three-stages crystallization [24], the current composition indicates two crystallization events and lower Tg, Tx1 and DTx, indicating completely a different thermal behavior. In order to determine the type of the crystalline products after devitrification, the B20 ribbon was heated up to a temperature above the end of each crystallization event and immediately cooled down to room temperature. Subsequently, the XRD analysis was conducted on these samples, as shown in Fig. 4. In contrast to the B30 alloy in which both (Co,Fe)21Ta2B6 and (Co,Fe)3B2 phases formed upon primary crystallization [33], only (Co,Fe)21Ta2B6 phase appears after heating of the B20 ribbons up to T ¼ 920 K. According to Fig. 4, the annealing the B20 ribbon above the second crystallization reaction (T ¼ 1180 K) results in formation of new (Co,Fe)2B and an unknown phase. In addition, the diffraction peaks of the (Co,Fe)21Ta2B6 phase become narrower due to the growth of this phase upon temperature rise. As shown in Ref. [33], the B30 alloy after complete crystallization includes the a-(Fe,Co) phase, which is usually formed upon crystallization in most of the Fe(Co)-based MGs. The presence of the a-(Fe,Co) crystals for the crystallized B30 alloy was confirmed by the thermomagnetic measurements, which indicated that the saturation magnetization of the completely devitrified ribbon does not become zero up to 900 K due to existence of the a-(Fe,Co) phase with a Curie temperature above 900 K [33]. For the partially and fully crystallized B20 ribbon, the thermomagnetic measurements (not shown here) revealed that the saturation magnetization becomes zero before 900 K. Hence, it is concluded that the a-(Fe,Co) phase is not formed upon crystallization of the B20 alloy. It is worth to note that the DTx and thermal stability of the B20 ribbon is larger than many other Co-based glassy alloys with relatively similar metalloid content [34e36]. The high thermal stability is correlated with strong bonding nature among the constituent elements, which decreases the diffusion rate of atoms upon crystallization [32]. The strong bonding nature can be confirmed according to the large negative heat of mixing for FeeB (26 kJ/mol),

CoeB (24 kJ/mol) and TaeB (54 kJ/mol) pairs [33,37]. The (Co,Fe)21Ta2B6 phase with a complex face-centered cubic (FCC) structure has a very different atomic structure compared to the Bcentered TTP polyhedra existing in most of Metal-Metalloid MGs [38]. Hence, formation of the (Co,Fe)21Ta2B6 crystals upon crystallization requires a large extent of atomic rearrangement followed by breaking the strong TaeB atomic pairs in glassy state, which is absent in the (Co,Fe)21Ta2B6 compound [38], and consequently results in wide SLR and high thermal stability of B20 alloy. Despite formation of a common phase ((Co,Fe)21Ta2B6) upon primary crystallization of B20 and B30 alloys, the B20 alloy exhibits a lower thermal stability. It has been demonstrated that the electrons can transfer from the metalloids like B to the d shells of transition metals (Fe, Co,Ta) and create strong s-d or p-d hybridization [39,40]. Such hybrid bonding between large (L) atoms (especially Ta) and small (S) metalloid atoms create a strong LeS percolating network or reinforced backbone in the glassy phase [23]. A notable decline in Tg from 893 K for the B30 alloy [24] to 808 K for the B20 alloy may indicate a lower extension of the percolating network and consequently a weaker bonding nature in the B20 alloy. This is consistent with lower fractions of the (Fe,Co,Ta)eB pairs in the B20 alloy, as can be inferred from their corresponding X-ray weighting factors wij (Table 1) compared to those reported for the B30 alloy [23]. Moreover, a higher thermal stability of the B30 alloy can be attributed to its more dense atomic packing, which is deduced from its larger average CN of the nearestneighbors, as calculated above. Furthermore, it has been shown that the atomic packing of the glassy structure can be evaluated by calculation of atomic size distribution parameters ε according to the following equation: [41].

(4) where Ri denotes the atomic radius of element i, ci is the atomic fraction of element i and R is the average radius of atoms which is calculated as:

R¼

n X

(5)

using above equations, the ε parameter was calculated as 0.1574 and 0.1363 for B30 and B20 alloys, respectively. The smaller value of ε parameter for the B20 alloy indicates its less dense atomic packing, and consequently a lower thermal stability compared to the B30 alloy. Fig. 5 shows the heating rate dependence of the DSC curves for the B20 alloy depicted in the range of the first crystallization event. As the figure shows, by increasing the heating rate, the transformation temperatures including Tg, Tx1 and Tp1 (the peak temperature of the first crystallization event) increases, as shown in Table 2. The apparent activation energy of the crystallization Ec can be calculated according to the Kissinger equation as: [42].

b ln 2 Tp

Fig. 4. XRD patterns of the (Co0.65,Fe0.35)72Ta8B20 ribbons heated isochronally up to end of each crystallization events.

ci Ri ;

i¼1

! ¼

Ec þ C; RTp

(6)

where b is the heating rate, Tp is the crystallization peak temperature and R is the gas constant. The variation of lnðb=Tp2 Þ as a function of 1000=Tp is shown in Fig. 6 for the B20 alloy. A straight line was fitted to the data points with a slope of 445 ± 11 kJ/mol, corresponding to the Ec of the B20 alloy. It should be noted that due to existence of only one fitted straight line, the transformation

A.H. Taghvaei et al. / Intermetallics 69 (2016) 21e27

ln b ¼ 1:0516

25

Ec þ C; RTp

(7)

Fig. 5 also depicts the variation of ln b versus 1000=Tp for the B20 alloy, indicating that the Ozawa plot is a linear curve with a slope of 438 ± 10 kJ/mol, which implies that the apparent activation energies obtained from the two methods are in good agreement, suggesting that the reaction order is close to unity [33,51]. It should be mentioned that the Ec is correlated with the activation energy required for diffusion and the corresponding rearrangement of the constituent elements during nucleation and subsequent growth of crystallized phase(s). In other words, a large value of Ec demonstrates the small diffusivity of the elements in the B20 alloy as a result of the large attractive bondings and a dense random packing of atoms, which is in line with its high thermal stability. On the other hand, the B20 ribbon shows a lower Ec compared to the B30 alloy (520 kJ/mol) [33], in a good agreement with its smaller DTx. 3.3. Magnetic properties Fig. 5. DSC curves of the as-quenched the (Co0.65,Fe0.35)72Ta8B20 MG at different heating rates plotted in the range of the first crystallization peak.

Table 2 Thermal stability parameters of the (Co0.65,Fe0.35)72Ta8B20 MG at different heating rates. Heating rate (K/min)

Tg (K)

Tx1

DTx

Tp1

10 20 30 40

814 819 825 832

859 870 875 881

45 51 50 49

869 877 884 888

Fig. 7 shows the magnetic hysteresis curves of the B20 alloy in the as-quenched and heat treated states. The as-cast ribbon shows a saturation magnetization Ms of 93.5 Am2/kg and small coercivity Hc of 1.2 A/m, demonstrating a good soft magnetic properties. In addition, due to the large magnetic susceptibility and small demagnetizing factor of the ribbon, the hysteresis loop shows completely a rectangular shape. In order to observe a possible improvement in soft magnetic properties upon heat treatment, the B20 ribbon was isochronally annealed up to different temperatures for 60 s. Annealing up to T ¼ 820 K, corresponding to Tg increases the Ms to 95.5 Am2/kg, decreases the Hc to 0.4 A/m and enhances the rectangular tendency of the hysteresis curve (see the inset of Fig. 7), manifesting the improvement in soft magnetic properties. A very low Hc of the relaxed ribbon implies its very small magnetocrystalline anisotropy due to formation of a homogeneous glassy phase and a low density of quasi-dislocation dipole-type (QDD) defects, which are the important domain wall pinning sites in glassy phase [52]. Fig. 8 compares the DSC plots of the as-quenched and relaxed B20 alloy in the temperature range around Tg. It is clearly inferred that the area of the broad hump appearing below Tg is significantly reduced upon annealing at Tg, as a result of

Fig. 6. Kissinger and Ozawa plots of the (Co0.65,Fe0.35)72Ta8B20 MG. The experimental data and the corresponding fitting lines are sown by dots and solid lines, respectively.

mechanism should be constant for the entire heating rates. It is worth to note that the value of the Ec is larger than that of many other ferromagnetic glassy alloys like Co68Fe4Cr4Si13B11 (380 kJ/ mol) [43], Fe80P13C7 (432 kJ/mol) [44], Fe61Co9Zr8Mo5B17 (414 kJ/ mol) [45], Fe70Nb2Al5Ga2P11C6B4 (422 kJ/mol) [46], Fe41.54Co20.56Nb8B30 (404 kJ/mol) [47], Fe73.5xMnxCu1Nb3Si13.5B9 (337e413 kJ/mol) [48] and comparable to the well-known Co43Fe20Ta5.5B31.5 glassy alloy (452 kJ/mol) with high glassforming ability (GFA) and thermal stability [49]. Furthermore, the apparent energy barrier for devitrification of a glassy alloy can be determined according to the equation suggested by Ozawa: [50].

Fig. 7. The MeH hysteresis (Co0.65,Fe0.35)72Ta8B20 ribbon.

loops

of

the

as-quenched

and

heat

treated

26

A.H. Taghvaei et al. / Intermetallics 69 (2016) 21e27

Fig. 8. DSC plots of the as-cast and relaxed (Co0.65,Fe0.35)72Ta8B20 illustrated in the temperature range around Tg.

structural relaxation, i.e., annihilation of the quenched-in free volume and QDD defects. Variations of the Ms (normalized) with temperature for the asquenched and relaxed B20 alloy, measured at a constant heating rate of 20 K/min is shown in Fig. 9. As it can be seen, the Ms of the as-cast and relaxed ribbons decreases with increasing temperature and passes through an inflection point, and finally reaches a value around zero due to reaching a paramagnetic state. The inflection point which is assigned to the Curie temperature of the glassy phase (Tc) was precisely calculated using the Herzer approach [53], as shown in the inset of Fig. 7. Similar to the Ms (Fig. 5), it is observed that Tc increases upon annealing at Tg from 660 K to 677 K. The spin-exchange interaction between the magnetic atoms in the glassy phase can be enhanced upon annealing owing to the decrease in average inter-atomic distances caused by free volume reduction, which in turn increases the Ms and Tc [23]. It is worth to note that the B20 alloy in the relaxed-state shows a comparable Hc, but significantly larger thermal stability and saturation polarization Js (Js ¼ 4p  107 rMs, where r is the mass density) of 0.98 T with respect to the well-known zero

magnetostrictive Co-based amorphous alloys with Js of about 0.5 T [12,16,17]. Compared to the as-cast B30 ribbon (Ms ¼ 42 Am2/kg, Js ¼ 0.5 T, Hc ¼ 0.8 A/m and Tc ¼ 412 K) [23], while B20 alloy has a lower thermal stability, it shows around two times larger Js, notably higher Tc and comparable Hc in both as-quenched and relaxed state. A larger Ms (Js) and Tc of the B20 alloy compared to the B30 one can be attributed to its smaller fraction of B as a nonmagnetic element, which can decrease the magnetic moment of Fe or Co through pd hybridization [40]. In addition, the decrease in the (Co,Fe)eCo pair separation upon decreasing the B content can be followed by enhancing the exchange interaction, and consequently increasing the Ms and Tc. However, according to the inset of Fig. 7, the soft magnetic behavior disappeared after crystallization of the B20 ribbon, as can be inferred from a less rectangular shape of devitrified samples, especially for the ribbon annealed at T ¼ 1180 K. The deterioration of soft magnetic properties results from the precipitation of the (Co,Fe)21Ta2B6 and (Co,Fe)2B crystals, which can significantly enhance the magnetocrystalline anisotropy and domain wall pinning. From Fig. 7, it can be seen that the first crystallization reaction doesn't change the Ms while the second crystallization slightly decreases the Ms. The Ms of (partially) crystallized ribbon depends on the fraction of precipitated ferromagnetic borides and the remaining amorphous matrix, which has a different composition and Ms compared to the original glassy phase. In contrast to the B20 alloy, it was shown that the complete crystallization of the B30 alloy could significantly enhance its Ms probably due to formation of the a-(Fe,Co) phase with a larger Ms compared to that of glassy phase [33]. A decrease of the Ms after complete crystallization of the B20 ribbon may be attributed to the formation of unknown and most probably nonmagnetic phase (see Fig. 4). Moreover, such Ms reduction can indicate that the a-(Fe,Co) phase is not formed upon crystallization of the B20 alloy, as discussed in previous section. This could be further supported by larger value of the Hc for the B20 ribbon after complete crystallization at T ¼ 1180 K, which reaches a large value of 8.3 kA/m, manifesting the disappearance of soft magnetic properties. 4. Conclusions Atomic structure, thermal behavior and magnetic properties of new (Co0.65,Fe0.35)72Ta8B20 glassy ribbons were investigated. Considerable differences in the atomic structure, especially in the SRO scale were detected compared to the (Co0.65,Fe0.35)62Ta8B30 glassy ribbons, fabricated previously. It was shown that decreasing the B content can result in a smaller (Co,Fe)eCo pair separation and a lower average coordination number of the first coordination shell. Thermal analysis demonstrated that the (Co0.65,Fe0.35)72Ta8B20 glass shows a wide SLR of 51 K and it transforms partially to the complicated FCC (Co,Fe)21Ta2B6 phase upon crystallization with a high activation energy of 445 kJ/mol. Magnetic measurements indicated that while the new alloy in the as-quenched state has a comparable Hc with respect to the (Co0.65,Fe0.35)62Ta8B30 MG, it exhibits more than two times larger Ms and significantly higher Tc. As a result of structural relaxation after isochronal annealing up to Tg the new alloy indicated excellent soft magnetic properties, i.e., Hc ¼ 0.4 A/m, Ms ¼ 95.5 A m2/kg and Tc ¼ 677 K. Acknowledgments

Fig. 9. Variation of saturation magnetization (normalized) of the as-quenched and relaxed (Co0.65,Fe0.35)72Ta8B20 alloy as a function of temperature. The inset shows the transformed data according to the Herzer approach.

A.H. Taghvaei is grateful to Iran National Science Foundation (INSF, Grant No: 93005710) for financial support. Parts of this research were carried out at the light source PETRA III at DESY (Hamburg, Germany), a member of the Helmholtz Association (HGF).

A.H. Taghvaei et al. / Intermetallics 69 (2016) 21e27

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