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Article

Formability and Magnetic Properties of the Binary Nd-Co Amorphous Alloys

by
Qiang Wang
,
Ding Ding
and
Lei Xia
*
Institute of Materials, Shanghai University, Shanghai 200072, China
*
Author to whom correspondence should be addressed.
Metals 2021, 11(11), 1730; https://doi.org/10.3390/met11111730
Submission received: 23 September 2021 / Revised: 25 October 2021 / Accepted: 28 October 2021 / Published: 29 October 2021
(This article belongs to the Special Issue Forming Ability and Properties of Bulk Metallic Glasses)
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Abstract

:
In this paper, binary Nd-Co alloys with compositional range from Nd72.5Co27.5 to Nd50Co50 were successfully vitrified into glassy state by a melt-spinning method. The glass formability of the metallic glasses (MGs) was studied and the best glass former in the binary Nd-Co alloys was obtained. Magnetic properties of the MGs were measured. The compositional dependence of Curie temperature of the MGs was observed. The mechanism for the spin-glass-like behaviors and high coercivity at low temperature, and their influence on the magnetic entropy change of the MGs, were investigated.

1. Introduction

Metallic glasses (MGs) are the non-crystalline metal–metal alloy systems first fabricated in the 1960s [1]. The disordered atomic arrangement in metallic glasses and the resulting unique properties have attracted extensive scientific interest in the last few decades. As a result, a large number of glass-forming alloy systems with excellent mechanical and physical properties have been developed and the critical section thickness of the glassy alloys has been improved constantly, such as the Ce-based metallic glasses with heavy-fermion behavior, the Mg-based amorphous alloy with the size up to 25 mm, and the Zr-based glassy alloys with a good strength and plasticity [2,3,4,5].
The magnetocaloric amorphous alloys, as one of the important branches of functional MGs, are ideal candidates for the key working components in a magnetic refrigerator because their broad magnetic entropy change (−ΔSm) peaks and the resulting high refrigeration capacity (RC) are suitable for constructing the flattened −ΔSm curves that are essential for the Ericsson cycle used in magnetic refrigeration [6,7]. Moreover, one of the most important advantages superior to the intermetallic compounds is that the MGs can be vitrified within a wide compositional range, and thus their physical properties, especially magnetocaloric properties, are tailorable [8,9,10,11,12]. The magnetocaloric MGs can generally be classified into two categories: the rare-earth (RE)-based and transition metal (TM)-based MGs. Although the RE-based MGs, especially the Gd-based amorphous alloys, exhibit extraordinary magnetocaloric properties [10,11,13], they are too rare to be widely used in a magnetic refrigerator. TM-based MGs, typically the iron-based MGs, are much cheaper than the RE-based MGs due to the greater reserves of TM. However, unfortunately, the iron-based MGs show lower maximum −ΔSm (−ΔSmpeak) and thus cannot match the requirement of high efficiency for a magnetic refrigerator [12,14,15,16].
An alternative with good magnetocaloric properties may be obtained in the high abundant RE elements (such as Ce, Nd, and La, which are much more abundant than the heavy RE elements)-based metallic glasses. In previous works, NdCoAl and NdFeAl (Co) amorphous alloys usually show excellent formability and can easily be vitrified into bulk sample with a critical diameter up to several millimeters [17,18,19,20,21,22]. However, the microstructure of the multicomponent Nd-based bulk metallic glasses (BMGs) is complicated and some of these BMGs possess high coercivity at room temperature [20,21,22], which is harmful to their magnetocaloric properties. Furthermore, the origin of high coercivity for these multicomponent MGs containing multiple elements is not easy to be explored due to the presence of various exchange interactions. Therefore, the systematic investigation on the formability and magnetic properties of the binary alloy system with simple compositions and electronic structure should be helpful for the understanding of the formability, the mechanism for the compositional relationship on the magnetic properties, the origin of coercivity, and its effect on the magnetocaloric properties of the Nd-based MGs.
In our preliminary work, the binary Nd50Co50 MG was successfully fabricated and showed better magnetocaloric effect and magnetostriction than the iron-based amorphous alloys [23]. In the current work, the formability and magnetic properties of binary Nd-Co MGs, and their relationship to the alloy composition were investigated systematically. The Nd-Co MGs were fabricated into the shape of ribbons by a melt-spinning method. The thermal properties of the amorphous ribbons were determined for the purpose of evaluating the formability of the MGs. Magnetic properties of the MGs were measured and their relationship to the alloy composition were investigated.

2. Materials and Methods

Nd100−xCox (x = 15, 20, 25, 27.5, 32, 35, 40, 50, 55) master alloys were prepared one by one by mixing the Nd and Co high-purity metals (purity over 99.9 at. %) and subsequently melting the mixtures at least three times in a high vacuum arc furnace using a Ti-gettered argon protective atmosphere. The master alloys were broken up and placed in quartz tubes to melt by induction coil. The ribbons of Nd100−xCox alloys were manufactured by spinning the molten liquid on a rotating copper wheel of 50 m/s. The average width of the as-spun ribbons was about 2 mm and the ribbons with an average thickness of ~40 μm and the length of above 50 mm were selected for the further investigations on formability and magnetic properties. The structural features of the Nd100−xCox ribbons were examined by a D\max-rC diffractometer (Rigaku, Tokyo, Japan) using a Cu Kα radiation at an angular step of 0.02° and a scanning speed of 8°/min. The microstructural observation of the ribbons was performed by a JEM-2010 field emission high resolution electron microscope (HREM, JEOL, Tokyo, Japan) and the observation specimens were prepared by a model 691 precision ion-polishing system (PIPS, GATAN, Berwyn, PA, USA). A DIAMOND calorimeter (Perkin-Elmer, Shelton, CT, USA) was used to measure the thermal properties of the Nd100−xCox amorphous ribbons. The magnetic measurements of the Nd100−xCox amorphous ribbons were performed by a vibrating sample magnetometer module on a PPMS 6000 system (Quantum Design, San Diego, CA, USA).

3. Results and Discussion

Figure 1 displays the X-ray diffraction patterns of the Nd100−xCox (x = 15, 20, 25, 27.5, 32, 35, 40, 50, 55) ribbons. The broadened diffractive peaks as well as the absence of obvious crystalline peaks (as shown in Figure 1a) in the patterns of the Nd72.5Co27.5, Nd68Co32, Nd65Co35, Nd60Co40, and Nd50Co50 ribbons indicate that these ribbons are fully amorphous. In contrast, the existence of visible crystalline peaks (including Nd4Co3, Nd, and NdCo2, as marked on Figure 1b) on the X-ray diffraction patterns of the Nd85Co15, Nd80Co20, Nd75Co25, and Nd45Co55 ribbons indicate their partially or even fully crystalline structure. Therefore, the binary Nd-Co alloys with compositional range from Nd72.5Co27.5 to Nd50Co50 can be vitrified into glassy ribbons with an average thickness of ~40 μm.
The Nd72.5Co27.5, Nd68Co32, Nd65Co35, Nd60Co40, and Nd50Co50 amorphous ribbons are selected for the further investigations on their formability and magnetic properties. Figure 2a illustrates the differential scanning calorimetry (DSC) traces of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) ribbons measured at a heating rate of 0.333 K/s. All the samples show typical amorphous characteristics, such as the endothermic glass transition humps (as seen in zoom-in in the left insets) and sharp exothermic crystallization peaks. The onset temperatures of glass transition (Tg) and primary crystallization (Tx) with a measurement error of ±0.1 K are listed in Table 1. Therefore, associated with the liquidus temperature (Tl) of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) alloys obtained from the Nd-Co binary phase diagram [24], as also listed in Table 1, we can evaluate the formability of the Nd100−xCox MGs by using the reduced glass transition temperature (Trg, defined as the ratio of Tg to Tl) [25] and the parameter γ (=Tx/(Tg + Tl)) [26]. According to the values of Trg and γ for the Nd100−xCox (x = 27.5, 32, 35, 40, 50) MGs listed in Table 1, it was found that both of them increased with Co content from x = 27.5 and reached the maximum values at x = 35, and then decreased with further Co addition, as shown in Figure 2b. Therefore, the best glass former in the binary Nd-Co alloys was the near eutectic Nd65Co35 alloy, which is roughly in accordance with the deep eutectic rule [27]. In order to check the amorphous structure of these ribbons, the Nd50Co50 amorphous ribbon with lower formability was selected for HREM observation. The HREM image of the Nd50Co50 sample is shown in Figure 3. No regions with obvious long-range order were observed in the disordered matrix, which confirms the amorphous atomic arrangement of the Nd50Co50 ribbon.
The dependence of the magnetization on temperature (M-T curves) of the MG samples were measured after the samples were cooled from room temperature to 10 K under two different conditions: under a magnetic field 0.03 T (field cooled, FC) and under a zero field (zero-field cooled, ZFC), respectively. Figure 4 illustrates the FC and ZFC M-T curves of the Nd100−xCox MG ribbons: (a) x = 27.5, (b) x = 32, (c) x = 35, and (d) x = 40. In agreement with our previous work on Nd50Co50 MG [23], the FC and ZFC M-T curves for each sample showed a λ-shape. That is, the ZFC curve overlapped with the FC curve from room temperature to a certain temperature below the Curie temperature (Tc) and then the two kinds of M-T curves diverged below this temperature. The λ-shaped FC and ZFC M-T curves that appear commonly in Tb (Dy)-based MGs indicate the spin-glass-like behavior in the Nd-Co binary MGs [8,28,29,30]. The Tc and the spin freezing temperature (Tf) of the Nd-Co binary MG ribbons obtained from their M-T curves with a lower ±0.05 K measurement error are summarized in Table 2. Figure 4e displays the relationship between the Tc and the Co content of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) MG ribbons. Similar to the case in the Dy-Co and Tb-Co binary MG systems [8,28], the compositional dependence of Tc in the binary Nd-Co MGs followed a non-linear relationship. The non-linear relationship between the Curie temperature and Co content in the Dy-Co, Tb-Co and Nd-Co binary MGs is supposed to be closely related to the anisotropic 3d-4f interaction between the RE (Tb, Dy and Nd)-Co atoms. In contrast, the Gd-Co binary MGs with isotropic 3d-4f interaction between Gd-Co atoms due to the half-full 4f electronic arrangement of the Gd atom showed a linear relationship between Tc and Co content [10].
The randomly oriented 3d-4f interaction between Nd-Co atoms played an important role as the random anisotropy, and the coupling of these random anisotropy gave rise to the high coercivity as well as the spin-glass-like behaviors in the Nd-Co binary MGs at low temperature. Figure 5a illustrates the hysteresis loops of the binary Nd-Co MG ribbons at 10 K. All the samples were hard magnetic at 10 K, and the coercivity (Hc) of the samples at 10 K were ~0.65 T for x = 50, ~0.2 T for x = 60, ~0.11 T for x = 65, ~0.07 T for x = 68, and ~0.05 T for x = 72.5. The coercivity of the samples was closely related to their Curie temperature, as shown in Figure 5b. The higher the Tc was, the larger the Hc at 10 K became.
The high coercivity obstructed the magnetization of the samples at low temperature and thus led to another characteristic of typical spin glass behavior, that is, the decreasing magnetization under a very low magnetic field from Tf to 10 K [8,28,30]. Figure 6 shows the isothermal magnetization (M-H) curves for other Nd100−xCox glassy ribbons: (a) x = 27.5, (b) x = 32, (c) x = 35, and (d) x = 60. Similar to the M-H curves of the Nd50Co50 MG ribbon [23], the magnetization of the Nd60Co40 sample under a low magnetic field (~0.01 T) obviously decreased with the decreasing temperature from 40 K to 10 K. This phenomenon was not so obvious in the Nd65Co35, Nd68Co32, and Nd75Co25 MGs, probably because of their low coercivity at 10 K.
The coercivity in the amorphous alloys is believed to be harmful to their magnetocaloric properties [30,31]. From the M-H curves of the Nd100−xCox MGs, we can obtain the magnetic entropy change curves of these amorphous ribbons with an error limit of less than 0.5%. Figure 7a shows the temperature dependence of −ΔSm for the Nd100−xCox (x = 27.5, 32, 35, 40, 50) glassy ribbons under the field of 1.5 T and 5 T. −ΔSmpeak of the Nd100−xCox MGs increased rapidly with the Nd concentration and reached a maximum value of 2.93 J/kgK under 1.5 T and 7.59 J/kgK under 5 T at 37.5 K in the Nd65Co35 glassy ribbon, but decreased slightly with more Nd addition. According to the mean field theory, the relationship between the −ΔSmpeak and Tc−2/3 in the RE-TM amorphous alloys usually obeys a linear relationship, which is verified in the binary Gd-based and Dy-based MGs [9,10,28]. However, the −ΔSmpeak versus Tc−2/3 plots for the Nd100−xCox amorphous alloys, as shown in Figure 7b, did not follow a linear relationship because the correlation coefficients of their linear fitting (Adj. R-Square) were only 0.86 under 5 T and 0.89 under 1.5 T, which indicates that the −ΔSmpeak and Tc−2/3 plots can hardly be linearly fitted. The deviation of the relationship between −ΔSmpeak and Tc−2/3 from the linear relationship is most likely due to the random magnetic anisotropy (RMA) and the resulting high coercivity in the Nd-Co binary amorphous alloy system [8]. The influence of spin-glass-like behaviors as well as the coercivity on the magnetic entropy change of the MGs can also be reflected by the irreversible −ΔSm at temperatures much lower than Tf. For example, similar to the RE-TM MGs that exhibit spin-glass-like behaviors, the −ΔSm of Nd50Co50 glassy ribbon dropped rapidly from Tf to 20 K and even decreased to below zero at 20 K, which resulted from the obstructed magnetization by the high coercivity of the samples at low temperature. On the other hand, with the increasing Co content to x = 50, the (−ΔSm)-T curves became broader, that is, ΔTFWHM (where ΔTFWHM is the temperature range at the half maximum of −ΔSmpeak) was larger. Therefore, although the −ΔSmpeak of Nd50Co50 MG was lowest among these ribbons, its RC (RC = −ΔSmpeak × ΔTFWHM) was larger than that of others, as listed in Table 2.

4. Conclusions

The glass formability and magnetic properties of the binary Nd-Co MGs were studied in this paper. Ribbons with an average thickness of ~40 μm showed amorphous characteristics in XRD patterns within the compositional range from Nd72.5Co27.5 to Nd50Co50. The Trg and parameter γ obtained from the DSC traces of the Nd100−x Cox MG ribbons indicated that the best glass former in the binary Nd-Co alloys was the near eutectic Nd65Co35 alloy, which roughly corresponds to the deep eutectic rule. Magnetic measurements revealed the spin-glass-like behaviors and high coercivity at 10 K of the Nd100−xCox MGs. Similar to the situations in the binary Dy-Co and Tb-Co MG systems, Tc of the Nd-Co MG ribbons showed a monotonically non-linear change with the Nd content, which is supposed to be closely related to the anisotropic 3d-4f interaction between the RE (Tb, Dy, and Nd)-Co atoms. The high coercivity, which is supposed to be due to the coupling of the randomly oriented 3d-4f interaction, obstructed the magnetization of the samples at low temperature and led to the non-linear relationship between −ΔSmpeak and Tc−2/3 as well as the irreversible −ΔSm at temperatures much lower than Tf in the binary Nd-Co amorphous alloys. The exploration of the simple binary Nd-Co system provides a foundation for the interpretation of magnetic behaviors of multicomponent Nd-based amorphous alloys.

Author Contributions

Data curation, writing—original draft, Q.W.; measurement, methodology, D.D.; conceptualization, writing—review and editing, L.X. All authors have read and agreed to the published version of the manuscript.

Funding

The work described in this paper was supported by the National Natural Science Foundation of China (Grant Nos. 51671119, 51871139, and 52071196).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This research was technically supported by the Center for Advanced Microanalysis of Shanghai University.

Conflicts of Interest

The authors declare that they have no conflict of interest to this work.

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Metals 11 01730 g001 550
Figure 1. XRD patterns of the Nd100−xCox ((a) x = 27.5, 32, 35, 40, 50; (b) x = 15, 20, 25, 55) as-spun ribbons.
Figure 1. XRD patterns of the Nd100−xCox ((a) x = 27.5, 32, 35, 40, 50; (b) x = 15, 20, 25, 55) as-spun ribbons.
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Metals 11 01730 g002 550
Figure 2. (a) DSC traces of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) MG ribbons at a heating rate of 0.333 K/s; (b) the compositional dependence of Trg and γ for the Nd-Cso MGs.
Figure 2. (a) DSC traces of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) MG ribbons at a heating rate of 0.333 K/s; (b) the compositional dependence of Trg and γ for the Nd-Cso MGs.
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Metals 11 01730 g003 550
Figure 3. The HREM image of the Nd50Co50 amorphous ribbon.
Figure 3. The HREM image of the Nd50Co50 amorphous ribbon.
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Metals 11 01730 g004 550
Figure 4. ZFC and FC M-T curves of the (a) Nd72.5Co27.5, (b) Nd68Co32, (c) Nd65Co35, (d) Nd60Co40 MG ribbons under 0.03 T; (e) the variation of the Tc with Co concentration for the Nd-Co MGs.
Figure 4. ZFC and FC M-T curves of the (a) Nd72.5Co27.5, (b) Nd68Co32, (c) Nd65Co35, (d) Nd60Co40 MG ribbons under 0.03 T; (e) the variation of the Tc with Co concentration for the Nd-Co MGs.
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Metals 11 01730 g005 550
Figure 5. (a) The hysteresis loops of the binary Nd-Co MG ribbons at 10 K, and (b) the relationship between coercivity and Tc of the samples.
Figure 5. (a) The hysteresis loops of the binary Nd-Co MG ribbons at 10 K, and (b) the relationship between coercivity and Tc of the samples.
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Metals 11 01730 g006a 550Metals 11 01730 g006b 550
Figure 6. Magnetization curves measured at various temperatures for the (a) Nd72.5Co27.5, (b) Nd68Co32, (c) Nd65Co35, and (d) Nd60Co40 MG ribbons (the error of magnetization < 0.05%).
Figure 6. Magnetization curves measured at various temperatures for the (a) Nd72.5Co27.5, (b) Nd68Co32, (c) Nd65Co35, and (d) Nd60Co40 MG ribbons (the error of magnetization < 0.05%).
Metals 11 01730 g006aMetals 11 01730 g006b
Metals 11 01730 g007 550
Figure 7. (a) The (−ΔSm)-T plots, (b) −ΔSmpeak vs. Tc−2/3 linear fitting curves of the Nd100−xCox amorphous alloys under the magnetic field of 1.5 T and 5 T.
Figure 7. (a) The (−ΔSm)-T plots, (b) −ΔSmpeak vs. Tc−2/3 linear fitting curves of the Nd100−xCox amorphous alloys under the magnetic field of 1.5 T and 5 T.
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Table
Table 1. Thermal parameters, Trg and γ of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) amorphous alloys.
Table 1. Thermal parameters, Trg and γ of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) amorphous alloys.
Nd100−xCox Amorphous RibbonTg (K)Tx (K)Tl (K)Trgγ
x = 27.53924319110.4300.331
x = 323984608580.4640.366
x = 353964758450.4690.383
x = 404054528780.4610.352
x = 5041547311100.3740.310
Table
Table 2. Tc, Tf, −ΔSmpeak and RC of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) amorphous alloys.
Table 2. Tc, Tf, −ΔSmpeak and RC of the Nd100−xCox (x = 27.5, 32, 35, 40, 50) amorphous alloys.
Nd100−xCox Amorphous RibbonTc (K)Tf (K)−ΔSmpeak (J/(kgK))RC under 5 T (J/kg)
1.5 T5 T
x = 27.527212.957.38214.02
x = 3231273.107.53199.55
x = 3535302.937.59197.34
x = 4044372.366.29182.41
x = 5088611.544.18231.99
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Wang, Q.; Ding, D.; Xia, L. Formability and Magnetic Properties of the Binary Nd-Co Amorphous Alloys. Metals 2021, 11, 1730. https://doi.org/10.3390/met11111730

AMA Style

Wang Q, Ding D, Xia L. Formability and Magnetic Properties of the Binary Nd-Co Amorphous Alloys. Metals. 2021; 11(11):1730. https://doi.org/10.3390/met11111730

Chicago/Turabian Style

Wang, Qiang, Ding Ding, and Lei Xia. 2021. "Formability and Magnetic Properties of the Binary Nd-Co Amorphous Alloys" Metals 11, no. 11: 1730. https://doi.org/10.3390/met11111730

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