Formability and magnetic properties of Dy-Co binary amorphous alloys

In this study, binary Dy-Co ribbons were synthesized by a conventional melt-spinning approach and glassy ribbons were successfully obtained within the compositional range Dy 50 Co 50 to Dy 68 Co 32 . The glass formability and magnetic properties of these amorphous alloys were examined. The compositional dependence of glass formability, Curie temperature and magneto-caloric response of the Dy x Co 100-x (x=50, 55, 60, 65 and 68) amorphous alloys, as well as the mechanism involved, were determined. © 2018 Author(s). All article content, except where other-wise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5037357


I. INTRODUCTION
Amorphous alloys have huge scientific and industrial significance because of their superior properties resulting from their unique structure induced by fabrication conditions far from the equilibrium state. 1-4 Among these alloys, transition metal (TM) and rare earth (RE) based amorphous alloys have shown application potential because they exhibit pre-eminent magnetic properties. For instance, Nd(Pr)-TM-based bulk metallic glasses (BMGs) show anomalous large coercivity; 5,6 (Tb, Dy)-TMbased amorphous alloys exhibit extraordinary magnetostriction; [7][8][9] and Gd-Co-based metallic glasses exhibit an outstanding magnetocaloric effect (MCE). [10][11][12][13][14][15][16][17][18] Gd-TM-based glassy alloys possess a second order magnetic phase transition (MPT), which leads to a broadened magnetic entropy change (−∆S m ) peak and also an ultra-high refrigeration capacity. However, the peak of −∆S m (−∆S m peak ) of the Gd-TM-based glassy alloys is usually inferior to that of the intermetallic compounds undergoing a first order MPT. Therefore, it is essential to find ways to enhance the value of the -∆S m peak of Gd-TM-based amorphous alloys. One of the valid methods to improve the MCE and formability of Gd-TM-based amorphous alloys is to replace the Gd elements by other RE metals, such as Tb, Dy, Ho, Er, Y and so on. [16][17][18] However, the addition of Tb and Dy may induce spin-glass-like behavior and make the magnetic entropy change of the amorphous alloys irreversible. 18,19 Therefore, systematic investigation of the formability and magnetic properties of Tb/Dy-TM binary amorphous alloys can be a simple but valid way to understand the influence of the addition of Tb or Dy on the formability and MCE of amorphous alloys in more detail.
In this investigation, we studied the glass formability and magnetic properties of a representative binary Dy-Co amorphous alloy system. The amorphous Dy-Co alloys were prepared as ribbons by the conventional melt-spinning method, and the best composition for glass formation in this alloy system was identified. The compositional dependence of the glass formability, Curie temperature (T c ) and −∆S m of the binary glassy alloys were constructed, and the mechanisms were investigated.
The findings are helpful for understanding the formability, T c and magnetocaloric responses of Tb/Dy-TM-based amorphous alloys.

II. EXPERIMENTAL PROCEDURES
Dy x Co 100-x (x=45, 50, 55, 60, 65, 68, 70 and 75) alloys were fabricated by arc-melting mixtures of high purity Dy (99.9 at. %) and Co (99.9 at. %) metals under a Ti-gettered argon atmosphere. Binary Dy x Co 100-x ribbons were fabricated by spinning the melting liquid on a copper wheel with a surface linear velocity of 30 m/s under a highly purified argon atmosphere. The approximate width of the as-prepared ribbon was 1 mm and the average thickness was about 40 µm. The structure state of the Dy x Co 100-x ribbons was confirmed using X-ray diffraction (XRD) analysis with Cu K α radiation (Rigaku D\max-2550). High resolution electron microscopy (HREM) observations were performed using a JEOL JEM-2010F high resolution electron microscope to ascertain the disordered microstructure of the amorphous ribbons. Specimens for HREM observation were prepared by ion-polishing under a pure argon atmosphere using the GATAN 691 precision ion-polishing system. The thermal properties of the Dy x Co 100-x amorphous ribbons were measured using differential scanning calorimetry (DSC, Perkin-Elmer DIAMOND) under a highly purified argon atmosphere at a constant heating rate of 20 K/min. The magnetic properties of the Dy x Co 100-x ribbons were acquired using a Quantum Design Physical Properties Measurement System (PPMS 6000). The temperature dependence of the magnetization (M-T ) curves were obtained under a magnetic field of 0.03 T in the heating process after cooling from room temperature to 10 K under a zero field (ZFC) or a magnetic field of 0.03 T (FC). The hysteresis loops and the isothermal magnetization (M-H) curves were obtained at different selected temperatures under a magnetic field of 5 T.

A. Glass forming ability of the Dy-Co binary alloys
The DSC curves for the Dy x Co 100-x (x=50, 55, 60, 65 and 68) amorphous ribbons are shown in Fig. 2(a). The amorphous characteristics of these ribbons are further verified by the typical endothermic glass transition and exothermic crystallization behavior in the DSC curves. The onset temperature for glass transition (T g ) and crystallization (T x ) of each amorphous ribbon, as marked clearly in Fig. 2(a), are listed in Table I. Therefore, associated with the liquidus temperatures (T l ) of the alloys acquired from the binary phase diagram, 20 we calculated the reduced glass transition temperature  (T rg ) 3 and the parameter γ (=T x /(T g +T l )) 21 of the Dy x Co 100-x (x=50, 55, 60, 65 and 68) amorphous alloys. This enabled investigation of the formability of the amorphous alloys because the above two parameters are the most widely used gauges for evaluating the formability of metallic glasses. The compositional dependence of T rg and γ of the Dy x Co 100-x amorphous ribbons is shown in Fig. 2(b). It is clear that the eutectic Dy 60 Co 40 provides the best composition for glass forming in the Dy-Co alloy system, which is in accord with the deep eutectic rule for predicting the glass formability of the alloys. 21,22 B. Magnetic properties of the amorphous alloys  Fig. 3(a). The decreasing magnetization under a very low magnetic field from 50K to 10 K indicates the typical spin glass behavior of the Dy 55 Co 45 amorphous ribbon. However, this kind of ZFC-FC curve and M-H curve can also be found in nano-structured amorphous composites as a result of their superparamagnetic behavior. Therefore, HREM observation of the Dy 55 Co 45 ribbon was undertaken to ascertain the disordered microstructure of the as-spun ribbons. Figure 4 shows the HREM image of the as-spun ribbon. The Dy 55 Co 45 as-spun ribbon is fully amorphous with only short ranged orders, and no obvious crystalline phases or small grains have been found in the amorphous matrix.   It is known that Dy(Tb)-based amorphous systems involve huge random magnetic anisotropy (RAM) due to the local random electrostatic field. According to the random anisotropy model, the RMA existing in amorphous alloys will break the rotational symmetry of the Hamiltonian and increase the hysteresis. 8,23 The hysteresis loops of the Dy 55 Co 45 amorphous ribbon were measured under a magnetic field of 5 T at 10 K, 30 K, 45 K, 50 K, 60 K and 300 K, respectively, as displayed in the lower-right inset of Fig. 3(a). The ribbon is paramagnetic at ambient temperatures and is soft magnetic with nearly zero coercivity from 50 K to 60 K, but is hard magnetic at 10 K and 45 K. The coercivity is about 0.03 T at 45 K, about 0.09 T at 30 K and about 0.46 T at 10 K, all due to the strong RMA at lower temperatures. The decreased coercivity with increasing temperature is due to the unfreezing of the magnetic moment due to the thermal fluctuation. T c of the Dy 55 Co 45 amorphous ribbon is about 73 K, and the spin freezing temperature (T f ) is about 55 K.  The M-T (including ZFC and FC) curves, M-H curves and hysteresis loops of the other Dy x Co 100-x amorphous ribbons are shown in Fig. 3(b) for x=60, Fig. 3(c) for x=65 and Fig. 3(d) for x=68. Agreeing with our preliminary work on Dy 50 Co 50 amorphous ribbons, 24 all the Dy x Co 100-x amorphous ribbons show spin-glass-like behavior. The Curie temperatures and spin freezing temperatures for the Dy x Co 100-x (x=50, 55, 60, 65 and 68) amorphous ribbons are also listed in Table I.
One of the characteristics of amorphous alloys that is superior to intermetallic compounds is that T c of the amorphous alloys can be adjusted by customizing the composition of the alloys. In our previous work, we found a linear relationship between the Gd content and T c in Gd-Co binary amorphous alloys. The compositional dependence of T c of RE-Co-based amorphous alloys was investigated. Based on the model of Rudermann-Kittel-Kasuya-Yosida indirect interaction, 16 a linear relationship between T c and the de Gennes factor (G) was found in various RE-Co-based amorphous alloys. For amorphous alloys containing only one RE element, the G factor is proportional to the molar fraction of the RE atoms. Therefore, from the viewpoint of 4f -4f indirect interaction, the compositional dependence of T c in Dy-Co amorphous alloys should be linear. However, in addition to the 4f -4f indirect interaction between the RE-RE atoms, there are two other kinds of interactions in RE-TM-based amorphous alloys: 3d-3d direct interaction between the TM-TM atoms and the 3d-4f indirect interaction between the RE-TM atoms. The influence of 3d-3d direct interaction on the Curie temperature in the RE-TM-based amorphous alloys is supposed to be similar to that of the 4f -4f indirect interaction. This is understandable because there is only (100%) 3d-3d direct interaction in a pure TM metal and no (0%) 3d-3d direct interaction in alloys free of TM elements. The contribution of the 3d-3d direct interaction is proportional to the molar fraction of the Co element in Dy-Co amorphous alloys and the compositional dependence of T c in Dy-Co amorphous alloys is still linear from the viewpoint of 3d-3d direct interaction. However, the effect of 3d-4f indirect interaction on the Curie temperature in RE-TM-based amorphous alloys is more complicated. The 3d-4f indirect interaction exists in RE-TM amorphous alloys but does not exist in either RE free or TM free alloys, which means that the compositional dependence of the 3d-4f indirect interaction in Dy-Co amorphous alloys will not be linear. As a result, the compositional dependence of T c , determined by the three kinds of interactions in RE-TM-based amorphous alloys, is not linear. Figure 5 displays the compositional dependence of T c for the Dy x Co 100-x glassy ribbons. The non-linear T c -Dy (at. %) curve indicates that the non-linear compositional dependence of the 3d-4f indirect interaction plays a significant role in dominating the Curie temperature of Dy-Co amorphous alloys. The nearly linear dependence of T c on the Gd content in Gd-Co amorphous alloys 25 is likely due to the negligible 3d-4f indirect interaction compared to the 3d-3d direct interaction and 4f -4f indirect interaction in Gd-Co binary amorphous alloys.

C. Magneto-caloric response of the amorphous alloys
The spin freezing behavior is reported to make the magnetic entropy change irreversible at temperatures well below T f and reduce the magneto-caloric properties of Dy-based bulk metallic glasses. 18,19 Therefore, we determined the temperature dependence of −∆S m from the M-H curves of the Dy x Co 100-x amorphous ribbons above T f , as shown in Fig. 6. The −∆S m peak of the Dy x Co 100-x amorphous ribbons clearly increases with the decreasing Dy concentration. However, the MCE of the Dy-Co amorphous alloys is not so significant when compared to that of the Gd-Co-based amorphous alloys. This is possibly because that Gd has no orbital momentum with a relatively low magnetocrystalline anisotropy. [10][11][12][13][14][15][16][17][18]28 The dependence of −∆S m peak on the T c of the amorphous alloys is usually described by the −∆S m peak ∝ T c -2/3 relationship, as proposed by Belo et al., according to the mean field theory approach. 26 According to the −∆S m peak values for the Dy x Co 100-x amorphous ribbons under the various magnetic fields listed in Table I  The deviation of the −∆S m peak -T c relationship of the Dy x Co 100-x amorphous alloys from the mean field prediction can also be verified by the −∆S m ∝H n relationship of each amorphous ribbon. Figure 7(b) shows the linear fitting of ln(H) vs ln(−∆S m peak ) for the Dy x Co 100-x amorphous alloys. The n value near T c is about 0.83 for x=50, 0.78 for x=55, 0.73 for x=60, 0.7 for x=65 and 0.74 for x=68, all of which are larger than the value predicted from the mean field theory, but agree well with the results predicted by the Arrott-Noakes equation, according to the typical microstructure of amorphous alloys with numerous short range orders embedded in the disordered matrix. 12,27

IV. CONCLUSIONS
In summary, the glass formability and magnetic properties of the binary Dy-Co amorphous alloys were investigated in this work. Dy-Co ribbons were fabricated by the melt-spinning method at a wheel surface velocity of 30 m/s and Dy x Co 100-x (x=50, 55, 60, 65 and 68) amorphous ribbons were obtained. The glass formability of the Dy-Co binary alloys was examined, and the best composition for glass formation was found to be the Dy 60 Co 40 alloy, which agrees well with the deep eutectic rule. The magnetic properties and the magnetocaloric response for the amorphous alloys were also investigated. It was found that the Curie temperature decreases with the augmenting Dy concentration. The compositional dependence of T c shows a non-linear relationship, indicating that the non-linear compositional dependence of the 3d-4f indirect interaction plays a vital role in dominating the Curie temperature of Dy-Co binary amorphous alloys. All the Dy x Co 100-x amorphous ribbons show spin-glass-like behavior, with the spin freezing temperatures obtained. The (−∆S m )-T curves of the Dy x Co 100-x amorphous ribbons were constructed at temperatures above T f to avoid the irreversible magnetic entropy change induced by the spin-glass-like behavior of the amorphous alloys. The dependence of −∆S m peak on the Curie temperature, as well as the magnetic field, was investigated. The deviation of the magneto-caloric behavior of the Dy x Co 100-x amorphous alloys from the mean field prediction was ascertained by both the (−∆S m peak )-T c and the H-(−∆S m peak ) relationships.