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

Abstract

In this paper, binary Nd-Co alloys with compositional range from Nd_72.5Co_27.5 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 (−ΔS_m) peaks and the resulting high refrigeration capacity (RC) are suitable for constructing the flattened −ΔS_m 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 −ΔS_m (−ΔS_m^peak) 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

Nd_100−xCo_x (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 Nd_100−xCo_x 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 Nd_100−xCo_x ribbons were examined by a D\max-rC diffractometer (Rigaku, Tokyo, Japan) using a Cu K_α 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 Nd_100−xCo_x amorphous ribbons. The magnetic measurements of the Nd_100−xCo_x 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 Nd_100−xCo_x (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 Nd_72.5Co_27.5, 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 Nd_72.5Co_27.5 to Nd₅₀Co₅₀ can be vitrified into glassy ribbons with an average thickness of ~40 μm.

The Nd_72.5Co_27.5, 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 Nd_100−xCo_x (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 (T_g) and primary crystallization (T_x) with a measurement error of ±0.1 K are listed in Table 1. Therefore, associated with the liquidus temperature (T_l) of the Nd_100−xCo_x (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 Nd_100−xCo_x MGs by using the reduced glass transition temperature (T_rg, defined as the ratio of T_g to T_l) [25] and the parameter γ (=T_x/(T_g + T_l)) [26]. According to the values of T_rg and γ for the Nd_100−xCo_x (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 Nd_100−xCo_x 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 (T_c) 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 T_c and the spin freezing temperature (T_f) 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 T_c and the Co content of the Nd_100−xCo_x (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 T_c 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 T_c 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 (H_c) 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 T_c was, the larger the H_c 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 T_f to 10 K [8,28,30]. Figure 6 shows the isothermal magnetization (M-H) curves for other Nd_100−xCo_x 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 Nd_100−xCo_x 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 −ΔS_m for the Nd_100−xCo_x (x = 27.5, 32, 35, 40, 50) glassy ribbons under the field of 1.5 T and 5 T. −ΔS_m^peak of the Nd_100−xCo_x 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 −ΔS_m^peak and T_c^−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 −ΔS_m^peak versus T_c^−2/3 plots for the Nd_100−xCo_x 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 −ΔS_m^peak and T_c^−2/3 plots can hardly be linearly fitted. The deviation of the relationship between −ΔS_m^peak and T_c^−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 −ΔS_m at temperatures much lower than T_f. For example, similar to the RE-TM MGs that exhibit spin-glass-like behaviors, the −ΔS_m of Nd₅₀Co₅₀ glassy ribbon dropped rapidly from T_f 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 (−ΔS_m)-T curves became broader, that is, ΔT_FWHM (where ΔT_FWHM is the temperature range at the half maximum of −ΔS_m^peak) was larger. Therefore, although the −ΔS_m^peak of Nd₅₀Co₅₀ MG was lowest among these ribbons, its RC (RC = −ΔS_m^peak × ΔT_FWHM) 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 Nd_72.5Co_27.5 to Nd₅₀Co₅₀. The T_rg and parameter γ obtained from the DSC traces of the Nd_100−x Co_x 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 Nd_100−xCo_x MGs. Similar to the situations in the binary Dy-Co and Tb-Co MG systems, T_c 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 −ΔS_m^peak and T_c^−2/3 as well as the irreversible −ΔS_m at temperatures much lower than T_f 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.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

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 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 3. The HREM image of the Nd50Co50 amorphous ribbon.

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 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 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%).

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.

Table 1

Thermal parameters, T_rg and γ of the Nd_100−xCo_x (x = 27.5, 32, 35, 40, 50) amorphous alloys.

Nd100−xCox Amorphous Ribbon	Tg (K)	Tx (K)	Tl (K)	Trg	γ
x = 27.5	392	431	911	0.430	0.331
x = 32	398	460	858	0.464	0.366
x = 35	396	475	845	0.469	0.383
x = 40	405	452	878	0.461	0.352
x = 50	415	473	1110	0.374	0.310

Table 2

T_c, T_f, −ΔS_m^peak and RC of the Nd_100−xCo_x (x = 27.5, 32, 35, 40, 50) amorphous alloys.

Nd100−xCox Amorphous Ribbon	Tc (K)	Tf (K)	−ΔSmpeak (J/(kgK))		RC under 5 T (J/kg)
			1.5 T	5 T
x = 27.5	27	21	2.95	7.38	214.02
x = 32	31	27	3.10	7.53	199.55
x = 35	35	30	2.93	7.59	197.34
x = 40	44	37	2.36	6.29	182.41
x = 50	88	61	1.54	4.18	231.99

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