Magnetic and Magneto-Caloric Properties of the Amorphous Fe92−xZr8Bx Ribbons

Magnetic and magnetocaloric properties of the amorphous Fe92−xZr8Bx ribbons were studied in this work. Fully amorphous Fe89Zr8B3, Fe88Zr8B4, and Fe87Zr8B5 ribbons were fabricated. The Curie temperature (Tc), saturation magnetization (Ms), and the maximum entropy change with the variation of a magnetic field (−ΔSmpeak) of the glassy ribbons were significantly improved by the boron addition. The mechanism for the enhanced Tc and −ΔSmpeak by boron addition was studied.


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 (−∆S m ) peak [11,12]. The broadened −∆S 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 −∆S m (−∆S 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−x B 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 −∆S 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].  [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 Fe 88 Zr 8 B 4 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−x Zr 8 B 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 −∆S m peak on the composition of the metallic glasses were constructed for the purpose of revealing the mechanism involved.

Experiments
Alloy ingots with a nominal composition of Fe 92−x Zr 8 B 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−x Zr 8 B 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 Fe 87 Zr 8 B 5 amorphous ribbon was also measured by PPMS near its T c under a zero magnetic field. Materials 2020, 13, x FOR PEER REVIEW 3 of 10 radiation of Cu on a Rigaku diffractometer (model D/max-2550) (Rigaku, Tokyo, Japan). Thermal properties about the glass transition temperature (Tg), crystallization temperature (Tx) and the liquidus temperature (Tl) 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 (Cp(T)) of the Fe87Zr8B5 amorphous ribbon was also measured by PPMS near its Tc under a zero magnetic field.

Results and Discussion
X-ray diffraction patterns of the Fe92-xZr8Bx (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 Fe92-xZr8Bx ribbons are fully amorphous structures. The amorphous feature of the Fe92-xZr8Bx 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 (Tg) of Fe92-xZr8Bx (x = 3, 4, 5) are about 798 K, 805 K, 807 K; and crystallization (Tx) is about 827 K, 834 K, and 837 K, respectively.  The amorphous feature of the Fe 92−x Zr 8 B 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−x Zr 8 B 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 Fe 88 Zr 8 B 4 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. bvious endothermic glass transition behaviors before the crystallization and the visible rystallization exothermic peak (see the small figure in Figure 2) also verify the amorphous haracteristics of the ribbon. As seen from the DSC trace, the onset temperatures of glass transition Tg) of Fe92-xZr8Bx (x = 3, 4, 5) are about 798 K, 805 K, 807 K; and crystallization (Tx) is about 827 K, 834 , and 837 K, respectively.
Endo. Up (Arb. units) 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.  [27,39]. The enhanced Ms implies the improvement of MCE by boron addition in the Fe92-xZr8Bx amorphous alloys because Ms or −ΔSm depends on the ordering of magnetic moments in metallic glasses upon magnetization.

Temperature (K)
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 Tc in Fe92-xZr8Bx glassy alloys because Tc 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 Fe92-xZr8Bx glassy samples measured under 0.03 T. Tc 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 Fe92-xZr8Bx glasses ribbons may be the good working medium of magnetic refrigeration when the −ΔSm peak of these alloys are high enough.  [17,41,42]. The n-T curves for the Fe92-xZr8Bx 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 Tc, and finally increases dramatically to a value up to 2 at the paramagnetic state [18,43].   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].   * The maximum magnetic entropy change (−ΔSm) value in the −ΔSm-T curves.  Table 1). −ΔSm peak of Fe92-xZr8Bx amorphous alloys, as predicted above, increases with boron addition: −ΔSm peak under 5 T at x = 5 is about 6.9% higher than the −ΔSm peak value at x = 4 and about 18.2% higher the −ΔSm peak value at x = 3. The increasing −ΔSm peak as well as Ms 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 −ΔSm peak on Tc of the Fe92-xZr8Bx metallic glasses is contrary to the −ΔSm peak -Tc relationship proposed by Belo et al. [44] in the RE-based glassy samples. The −ΔSm peak (under 5 T) vs. Tc plots in several Fe-Zr-B-based amorphous samples (listed in Table 1), and their linear fitting (dash line) is illustrated in Figure 4b. −ΔSm peak for these Fe-Zr-B-based glassy ribbons increases monotonically with the Cutie temperature,  (3.27 J K −1 kg −1 at 308 K) and the Fe 87 Zr 9 B 4 amorphous ribbon (3.29 J K −1 kg −1 at 304 K), but is larger than that of the Fe 87 Zr 6 B 6 Cu 1 (3 J K −1 kg −1 at 300 K) metallic glass [29,30,35]. In order to reveal the refrigeration efficiency of the Fe 87 Zr 8 B 5 amorphous alloy, we calculate the temperature rise under an adiabatic condition (∆T ad ) of the sample according to The ∆T ad -T curve of the Fe 87 Zr 8 B 5 glassy ribbon is shown in Figure 7, the inset is the C p (T)) curve. The maximum ∆T ad for the Fe 87 Zr 8 B 5 metallic ribbon is about 0.76 K under 1.5 T, and 1.98 K under 5 T, respectively.
Materials 2020, 13, x FOR PEER REVIEW 7 of 10 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 −ΔSm peak at the temperature near 305 K. For instance, the −ΔSm 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 (ΔTad) of the sample according to The ΔTad-T curve of the Fe87Zr8B5 glassy ribbon is shown in Figure 7, the inset is the Cp(T)) curve. The maximum ΔTad for the Fe87Zr8B5 metallic ribbon is about 0.76 K under 1.5 T, and 1.98 K under 5 T, respectively.

Conclusions
In summary, the Fe92-xZr8Bx (x = 3, 4, 5) glassy ribbons were successfully prepared. The magnetic and magnetocaloric behaviors of these glassy samples were studied. Tc of the Fe92-xZr8Bx 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 Tc, −ΔSm peak , and Ms of the Fe92-xZr8Bx amorphous samples show an increase trend with the boron content. The simultaneously increasing Ms, Tc, and −ΔSm peak with boron content in the Fe92-xZr8Bx amorphous alloys is mostly likely attributed to the enhanced interaction between the Fe-B atoms by boron addition. The high −ΔSm peak and ΔTad 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.