Magnetic and Magneto-Caloric Properties of the Amorphous Fe_92−xZr₈B_x Ribbons

Abstract

Magnetic and magnetocaloric properties of the amorphous Fe_92−xZr₈B_x ribbons were studied in this work. Fully amorphous Fe₈₉Zr₈B₃, Fe₈₈Zr₈B₄, and Fe₈₇Zr₈B₅ ribbons were fabricated. The Curie temperature (T_c), saturation magnetization (M_s), and the maximum entropy change with the variation of a magnetic field (−ΔS_m^peak) of the glassy ribbons were significantly improved by the boron addition. The mechanism for the enhanced T_c and −ΔS_m^peak by boron addition was studied.

1. Introduction

With the rising concerns on environmental pollution and the higher and higher cost of energy, it is an urgent need recently to develop energy-saving and environmental-friendly materials, such as new energy storage materials, magnetocaloric materials, giant impedance materials, thermoelectric materials, and magneto-strictive materials [1,2,3,4,5]. Magnetocaloric materials are the materials that exhibit an adiabatic temperature change when they experience a magnetization or demagnetization process, which is called the magnetocaloric effect (MCE). Magnetic refrigerators using the magnetocaloric alloys or compounds as working materials are believed to be more compact (because of solid refrigerant) and efficient (due to their lower energy consumption) than the traditional vapor compression/expansion refrigerator. Furthermore, the magnetic refrigerators are considered to be safer to the environment because they do not emit ozone-depleting gases [6,7]. Therefore, the magnetocaloric materials have recently attracted more and more attention, and as a result, a great number of magnetocaloric alloys or compounds have been developed in the past several decades [8,9,10].

The amorphous alloys, which exhibit higher corrosion resistance and better mechanical properties than the crystalline alloys, are considered to be suitable candidates for magnetic refrigerants because they exhibit rather broad magnetic entropy change (−ΔS_m) peak [11,12]. The broadened −ΔS_m peak and the resulted high value of refrigeration capacity (RC) lead to a rather wide working temperature range and a large amount of cooling, which is an important indicator that helps to obtain the maximum cooling capacity in the Ericsson cycle [13]. The amorphous alloys can be fabricated within a large compositional range, indicating that the Curie temperature (T_c) and properties, depending on the alloy compositions, can be easily tuned [14].

Among the amorphous alloys that have been studied so far, Gd-based amorphous alloys and some high-entropy alloys show good reversible magnetocaloric effects [15,16,17]. Usually, the large maximum −ΔS_m (−ΔS_m^peak) value of these amorphous alloys only obtained at low temperatures and their high cost limit their industrial applications. Although some amorphous alloys with reduced Gd content can achieve application conditions in the room temperature range, they still have shortcomings in terms of cost, magnetocaloric effect, and forming ability [18,19]. In contrast, the low-cost transition metal (TM)-based amorphous alloys represented by the Fe-based amorphous alloy gained more and more attention.

In previous studies, Fe_100−xB_x (x = 12–28) amorphous alloy exhibits good soft magnetic properties [20,21], However, almost all the compositions of the Curie temperatures, T_c, are above room temperature. By adding Nb, Y, Nd, or Mn elements to amorphous system, the Curie temperature is greatly reduced, but it fails to increase the magnetic entropy change value of the alloy or even deteriorate [22]. On the other hand, FeZr binary system alloys exhibit excellent soft magnetic properties near room temperature and are accompanied with a −ΔS_m^peak close to about half of the Gd [23,24]. As a result, substitution of Fe by transition metals Nb, Mn, Y, or metalloid elements B can significantly change the magnetic properties [25,26,27]. In particular, the addition of B can even make the alloy appear ferromagnetic near room temperature. For the purpose of meeting the requirements of magnetic refrigerants in a domestic refrigerator, recently, many multicomponent Fe-based amorphous alloys with excellent magnetocaloric properties have been synthesized based on the ternary Fe-Zr-B glass-forming alloys [27,28,29,30]. The lower Zr content Fe-B-Zr amorphous alloys usually have a lower MCE with the −ΔS_m^peak about 1.04 J K⁻¹ kg⁻¹ for a field change of 0–15 kOe in the Fe_94−xZr₆B_x (x = 5, 6, 8, and 10) amorphous alloys. When the Zr content is increased, the ΔS_m of the alloy becomes significantly improved: for instance, the magnetic properties, phase transitions, and MCE were systematically studied in amorphous Fe_89−xB_xZr₁₁ (x = 0–10) alloys and the T_c got enhanced with B addition and the −ΔS_m value to be about 1.73 J K⁻¹ kg⁻¹ for the Fe₇₉Zr₁₁B₁₀ sample. Meanwhile, the addition of a series of 3D elements, such as Cu, Cr, Mn, Co, Ni..., also obtained a series of FeZrB-based amorphous alloys with a good ΔS_m value near room temperature [31,32,33,34,35,36]. Recently, related studies have shown that FeZrB(Cu,Co...) amorphous alloys also have dispersed nanocrystalline particles on the amorphous matrix [37,38]. Appropriate selection and control methods of heat treatment can make nanocrystalline particles aggregate and grow, further, by selectively removing the surface nanocrystalline particles, the amorphous materials with nanoporosity on the surface can be obtained. However, after the selective dealloying treatment of nanocrystalline amorphous alloys, it is found that the magnetization of the alloys is improved. This is mainly related to the increase in the concentration of ferromagnetic atoms in the system, which also provides a way to further improve the magnetic properties of the amorphous alloys.

Through the previous study on the Fe₈₈Zr₈B₄ amorphous alloy [30,39,40], it is found that a moderate Zr content makes the alloy exhibit good magnetic properties near room temperature. However, the mechanism for their good magnetocaloric properties has not been investigated systematically. Therefore, the detailed investigation on the magnetic and magnetocaloric properties of Fe-Zr-B ternary metallic glasses may be helpful for the understanding of the tailorable magnetic and magnetocaloric properties near room temperature in the multicomponent Fe-Zr-B-based metallic glasses. In the present work, we fabricated Fe_92−xZr₈B_x (x = 3, 4, 5) amorphous samples in the shape of ribbons with an average thickness of 0.04 mm. Magnetic properties of the amorphous samples were measured and their magnetocaloric properties were obtained. The dependence of T_c as well as −ΔS_m^peak on the composition of the metallic glasses were constructed for the purpose of revealing the mechanism involved.

2. Experiments

Alloy ingots with a nominal composition of Fe_92−xZr₈B_x (x = 3, 4, 5) were prepared by arc melting a mixture of high-purity (99.95 wt%) Fe, Zr metallic pieces and Fe-B pre-alloy for at least four times in a non-consumable electrode high vacuum arc melting furnace. Fe_92−xZr₈B_x ribbons with an average thickness of ~0.04 mm were prepared by ejecting the melts from the quartz tube to the surface of a rotating copper wheel under a pure Ar atmosphere. The surface speed of the copper wheel was optimized at 30 m/s. Structure of the ribbons was checked by X-ray diffraction (XRD) using the K_α radiation of Cu on a Rigaku diffractometer (model D/max-2550) (Rigaku, Tokyo, Japan). Thermal properties about the glass transition temperature (T_g), crystallization temperature (T_x) and the liquidus temperature (T_l) of the amorphous ribbons were measured by a Netzsch DSC-404C differential scanning calorimetry (DSC) (Netzsch, Selb, Germany) under a purified argon atmosphere at a heating rate of 20 K/min. Microstructures of the amorphous ribbons were observed by a JEOL JEM-2010F (JEOL, Tokyo, Japan) high-resolution electron microscope (HREM). The specimens for HREM observations were prepared by ion-polishing under a pure argon atmosphere using the GATAN 691 precision ion-polishing system (AMETEK, Berwyn, PA, USA). Magnetic properties of the as-spun ribbons were measured by a Quantum Design Physical Properties Measurement System (Ever cool II): the temperature dependence of the magnetization (M-T) curves were obtained under a field of 0.03 T in the cooling process; hysteresis loops were measured under a field of 5 T at 10 K and 380 K, respectively; isothermal magnetization (M-H) curves were obtained at various temperatures under a field of 5 T. The heat capacity (C_p(T)) of the Fe₈₇Zr₈B₅ amorphous ribbon was also measured by PPMS near its T_c under a zero magnetic field.

3. Results and Discussion

X-ray diffraction patterns of the Fe_92−xZr₈B_x (x = 3, 4, 5) ribbons are presented in Figure 1. Only one typical broadened diffraction hump was observed between 2θ of 30° and 35° on each pattern; and the absence of visible crystalline peaks are present on the XRD curves of the ribbons. It indicates that the Fe_92−xZr₈B_x ribbons are fully amorphous structures.

The amorphous feature of the Fe_92−xZr₈B_x ribbons prepared under the linear velocity of 30 m/s can be further confirmed from their differential scanning calorimetry (DSC) trace (Figure 2). The obvious endothermic glass transition behaviors before the crystallization and the visible crystallization exothermic peak (see the small figure in Figure 2) also verify the amorphous characteristics of the ribbon. As seen from the DSC trace, the onset temperatures of glass transition (T_g) of Fe_92−xZr₈B_x (x = 3, 4, 5) are about 798 K, 805 K, 807 K; and crystallization (T_x) is about 827 K, 834 K, and 837 K, respectively.

In order to verify the above assumption more intuitively, high-resolution electron microscope (HREM) micrographs of the Fe₈₈Zr₈B₄ sample were performed and depicted in Figure 3. The HREM image reveals the fully amorphous characteristics with only short-range order in the disordered matrix. Similar features in XRD patterns and the DSC curves of these samples indicates the approximate structural features in all other samples studied.

Coercivity and saturation magnetization (M_s) of the Fe_92−xZr₈B_x (x = 3, 4, 5) glassy ribbons were obtained from the hysteresis loops measured at 10 K under 5 T. As shown in Figure 4a, the nearly zero coercivity of all the Fe_92−xZr₈B_x (x = 3, 4, 5) glassy samples indicates that the metallic glasses are soft magnetic properties at 10 K. M_s of the Fe_92−xZr₈B_x (x = 3, 4, 5) glassy ribbons obtained from their hysteresis loops are about 107.5 Am²/kg for x = 3, 109.2 Am²/kg for x = 4, and 110.7 Am²/kg for x = 5, respectively. The dependence of M_s on the boron content of the three glassy samples, as plotted in the inset of Figure 4a, shows a roughly linear relationship between M_s and x. The increasing M_s with x in the Fe_92−xZr₈B_x metallic glasses is most likely related to the improved Fe-B interactions with increasing B content [27,39]. The enhanced M_s implies the improvement of MCE by boron addition in the Fe_92−xZr₈B_x amorphous alloys because M_s or −ΔS_m depends on the ordering of magnetic moments in metallic glasses upon magnetization.

In addition, unlike the rare earth (RE)-transition metal (TM)-based (RE-TM-based) metallic glasses, the enhanced interaction by boron addition may result in the improvement of T_c in Fe_92−xZr₈B_x glassy alloys because T_c of the Fe-based metallic glass samples primarily depend on the 3d-3d direct interaction [27,29,34,35,36,37]. Figure 4b shows the variation of magnetization on the temperature (M-T curves) of the Fe_92−xZr₈B_x glassy samples measured under 0.03 T. T_c derived from the M-T curves is about 271 K for Fe₈₉Zr₈B₃, 291 K for Fe₈₈Zr₈B₄, and 306 K for Fe₈₇Zr₈B₅ amorphous ribbons. The Curie temperatures of the glassy samples were located in the working temperature range of a domestic refrigerator, which indicates that the Fe_92−xZr₈B_x glasses ribbons may be the good working medium of magnetic refrigeration when the −ΔS_m^peak of these alloys are high enough.

By measuring the isothermal magnetization (M-H) curves of the Fe_92−xZr₈B_x glassy ribbons at various temperatures, we can calculate the −ΔS_m of these amorphous alloys. Figure 5 shows the −ΔS_m plots at different temperatures under the magnetic fields of 1 T, 1.5 T, 2 T, 2.5 T, 3 T, 3.5 T, 4 T, 4.5 T, and 5 T. According to the trend, with a flat and continuously changing value, shown in the −ΔS_m-T curves, it can be seen that the Fe_92−xZr₈B_x (x = 3, 4, 5) metallic glasses exhibit the secondary magnetic phase transition features of a soft magnetic alloy. −ΔS_m^peak values of the glassy ribbons under 1 T, 2 T, 3 T, 4 T, and 5 T are listed in

Table 1. Curie temperature, T_c, and −ΔS_m^peak of the several Fe-Zr-B-based amorphous samples.

Composition	−ΔSmpeak * (J kg−1 K−1)					Tc (K)	Ref.
	1 T	1.5 T	2 T	3 T	5 T
Fe89Zr8B3	0.79	1.08	1.35	1.85	2.75	271	Present work
Fe88Zr8B4	0.88	1.20	1.50	2.06	3.04	291
Fe87Zr8B5	0.94	1.29	1.61	2.19	3.25	306
Fe88Zr9B3	0.94	1.28	1.59	2.16	3.17	286	[29]
Fe87Zr9B4	0.99	1.35	1.67	2.26	3.29	304
Fe86Zr9B5	1.02	1.39	1.72	2.3	3.34	327
Fe88Zr8B4	0.87	1.2	1.5	2.06	3.04	292	[34]
Fe87Co1Zr8B4	0.93	1.29	1.61	2.2	3.24	317
Fe86Co2Zr8B4	0.98	1.35	1.69	2.31	3.38	340
Fe87Zr8B4Sm1	0.98	1.33	1.65	2.24	3.27	308	[35]
Fe86Zr8B4Sm2	1.04	1.41	1.73	2.32	3.35	325
Fe85Zr8B4Sm3	1.09	1.47	1.81	2.44	3.55	333
Fe87Zr7B4Co2	1.01	1.38	1.72	2.34	3.42	333	[36]

* The maximum magnetic entropy change (−ΔS_m) value in the −ΔS_m-T curves.

. The −ΔS_m∝Hⁿ relationship for the three samples were constructed and the n values at different temperatures were obtained. Figure 6a illustrates the ln(−ΔS_m) vs. ln(H) plots near the Curie temperature of the three glassy samples and their linearly fitted lines. n is about 0.771 for Fe₈₉Zr₈B₃ at 270 K, 0.769 for Fe₈₈Zr₈B₄ at 290 K, and 0.766 for Fe₈₇Zr₈B₅ at 305 K. The values of n near T_c are approximately consistent in the alloys, and the alloys with fully amorphous structures exhibit 2rd magnetic phase transition [17,41,42]. The n-T curves for the Fe_92−xZr₈B_x glassy samples, seen in the inset of Figure 6a, display typical magnetocaloric behaviors of soft magnetic metallic glasses: n is nearly 1 at low temperature when the sample is ferromagnetic, then gradually reduces to a minimum value near T_c, and finally increases dramatically to a value up to 2 at the paramagnetic state [18,43].

−ΔS_m^peak of Fe_92−xZr₈B_x amorphous alloys, as predicted above, increases with boron addition: −ΔS_m^peak under 5 T at x = 5 is about 6.9% higher than the −ΔS_m^peak value at x = 4 and about 18.2% higher the −ΔS_m^peak value at x = 3. The increasing −ΔS_m^peak as well as M_s with boron addition is most likely due to the exciting of free electrons in Fe atoms to a high spin state and thus strengthening the overall magnetic moments by adding of nonmagnetic B element. Therefore, the dependence of −ΔS_m^peak on T_c of the Fe_92−xZr₈B_x metallic glasses is contrary to the −ΔS_m^peak-T_c relationship proposed by Belo et al. [44] in the RE-based glassy samples. The −ΔS_m^peak (under 5 T) vs. T_c plots in several Fe-Zr-B-based amorphous samples (listed in Table 1), and their linear fitting (dash line) is illustrated in Figure 4b. −ΔS_m^peak for these Fe-Zr-B-based glassy ribbons increases monotonically with the Cutie temperature, which is possibly because the magnetic interactions in the Fe-Zr-B-based glass is not so complicated as the situation in the RE-based amorphous samples.

It is worthy to note that the Fe₈₇Zr₈B₅ amorphous alloy exhibits a rather high −ΔS_m^peak at the temperature near 305 K. For instance, the −ΔS_m^peak under 5 T of the Fe₈₇Zr₈B₅ glassy sample reaches to 3.25 J K⁻¹ kg⁻¹ at 305 K, which is comparable to that of the Fe₈₇Zr₈B₄Sm₁ amorphous ribbon (3.27 J K⁻¹ kg⁻¹ at 308 K) and the Fe₈₇Zr₉B₄ amorphous ribbon (3.29 J K⁻¹ kg⁻¹ at 304 K), but is larger than that of the Fe₈₇Zr₆B₆Cu₁ (3 J K⁻¹ kg⁻¹ at 300 K) metallic glass [29,30,35]. In order to reveal the refrigeration efficiency of the Fe₈₇Zr₈B₅ amorphous alloy, we calculate the temperature rise under an adiabatic condition (ΔT_ad) of the sample according to

$$\Delta T_{ad}\left(T,0\to H\right)=-\frac{T}{C_p\left(T\right)}\Delta S_m\left(T,0\to H\right)$$

(1)

The ΔT_ad-T curve of the Fe₈₇Zr₈B₅ glassy ribbon is shown in Figure 7, the inset is the C_p(T)) curve. The maximum ΔT_ad for the Fe₈₇Zr₈B₅ metallic ribbon is about 0.76 K under 1.5 T, and 1.98 K under 5 T, respectively.

4. Conclusions

In summary, the Fe_92−xZr₈B_x (x = 3, 4, 5) glassy ribbons were successfully prepared. The magnetic and magnetocaloric behaviors of these glassy samples were studied. T_c of the Fe_92−xZr₈B_x glassy samples is about 271 K at x = 3, about 291 K at x = 4, and about 306 K at x = 5. It was found that T_c, −ΔS_m^peak, and M_s of the Fe_92−xZr₈B_x amorphous samples show an increase trend with the boron content. The simultaneously increasing M_s, T_c, and −ΔS_m^peak with boron content in the Fe_92−xZr₈B_x amorphous alloys is mostly likely attributed to the enhanced interaction between the Fe-B atoms by boron addition. The high −ΔS_m^peak and ΔT_ad of the Fe₈₇Zr₈B₅ metallic ribbon near 305 K indicate that the amorphous sample may be a good candidate for the magnetic refrigerants of a domestic magnetic refrigerator.