Effect of Minor Co Substitution for Fe on the Formability and Magnetic and Magnetocaloric Properties of the Amorphous Fe 88 Ce 7 B 5 Alloy

: A small amount of Co was added to the Fe 88 Ce 7 B 5 glass forming alloy for the possi-bility of improving its glass formability and magnetocaloric effect. The Curie temperature of the amorphous Fe 88- x Ce 7 B 5 Co x ( x = 0, 1, 2, 3) ribbons increases linearly with the Co content, while the maximum magnetic entropy change ( − ∆ S mpeak ) increases to 3.89 J/(kg × K) under 5 T at x = 1 and subsequently decreases with further Co addition. The mechanism for the inﬂuence of Co addition on magnetic properties and the magnetocaloric effect of the amorphous alloys was investigated. Furthermore, a ﬂattened − ∆ S m proﬁle was designed in the amorphous laminate composed of the amorphous Fe 88- x Ce 7 B 5 Co x ( x = 0, 1, 2) ribbons. The high average − ∆ S m from ~287 K to ~320 K indicates the potential application perspective of the amorphous hybrid as a magnetic refrigerant of a domestic refrigerator. × K) under 5 T from 287 K to 320 K; these values are much higher than those of other Fe-Zr-B-based amorphous hybrids [19,34]. Furthermore, the compositions of the amorphous laminate do not contain any radioactive elements and will not bring about some health hazards. Therefore, the high − ∆ S maverage from the T cold to the T hot of the amorphous composite indicates the potential application perspective as magnetic refrigerant in a domestic air conditioner.


Introduction
With the increasing shortage of energy and the worsening environmental pollution, it is vitally urgent to develop new refrigeration technology to replace the vapor expansion/compression refrigerators because the traditional refrigeration technology is of low refrigeration efficiency and is not eco-friendly. The magnetic refrigerators based on the magnetocaloric effect (MCE) of magnetic materials are regarded as one of the potential alternatives to the traditional refrigerators because of their energy conservation (at least 30%), eco-friendliness due to their free of ozone-depleting gases, and compactness due to the use of solid refrigerants [1,2].
The MCE refers to the heating of a magnetic material upon magnetization under an adiabatic condition induced by the decrease of magnetic entropy due to the ordering of magnetic moment [3]. Materials exhibiting excellent MCE are considered to be suitable for application as magnetic refrigerants. The magnetic refrigerator generally undergoes an Ericsson cycle, and thus the magnetic refrigerant should better exhibit a table-like magnetic entropy change (−∆S m ) profile within the working temperature range of a magnetic refrigerator [4]. However, the table-like −∆S m profile can hardly be achieved in a single alloy or compound; instead, it is usually achieved in composites composed of several alloys or compounds with Curie temperatures (T c ) ranging from the cold end (T cold ) to the hot end (T hot ) of a magnetic refrigerator [5][6][7][8][9]. Obviously, the broad −∆S m hump of the alloys experiencing 2nd-order magnetic phase transition (MPT) behavior rather than the narrow −∆S m peak of 1st-order MPT alloys, and the tunable Curie temperature of the alloys, are essential for constructing the table-like −∆S m profile.
Amorphous alloys (AAs) can perfectly match the above requirements, not only because they experience a 2nd-order MPT and exhibit a broadened −∆S m hump but also due to their tailorable T c within a wide temperature range by compositional adjustment [10][11][12][13][14][15][16][17][18][19][20][21][22]. The major challenge for the AAs to be used as magnetic refrigerants is how to enhance the −∆S m as much as possible. The rare earth (RE)-based AAs, typically the Gd-based bulk metallic glasses, show outstanding glass formability as well as rather large peak value of magnetic entropy change (−∆S m peak ) at low temperature [10][11][12]. However, the RE-based metallic glasses are expensive, and the alloys with T c near room temperature (RT) usually show poor glass formability and low −∆S m peak [13]. The transition metal (TM)-based metallic glasses with T c near RT are less expensive and can be easily fabricated, but their −∆S m peak values are very low. For instance, the Fe-Zr-B-based AAs show better MCE in the iron-based metallic glasses near the ambient temperature, but most of their −∆S m peak are not higher than 3.2 J/(kg × K) under 5 T [14][15][16][17]. The minor substitution of Co for Fe can obviously improve the −∆S m peak of the Fe-Zr-B amorphous ribbons to above 3.2 J/(kg × K) under 5 T, or even to about 3.4 J/(kg × K) under 5 T at 2% (at.%) Co, but they simultaneously enhance their Curie temperature to above 330 K, which is well higher than RT [18,19].
More recently, we successfully fabricated the Fe-La/Ce-B metallic glasses and achieved better magnetocaloric properties with −∆S m peak of at least 10% larger than those of the Fe-Zr-B-based AAs near the ambient temperature [20,21]. In this paper, we selected a Fe 88 Ce 7 B 5 AA with a −∆S m peak of~3.83 J/(kg × K) under 5 T at 287 K [22] as a basic alloy and prepared the Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) amorphous ribbons. The effect of minor Co replacement for Fe on the glass formability, magnetic properties and MCE of the ternary amorphous alloy, as well as the mechanisms involved, was studied.

Materials and Methods
The master ingots with nominal Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) compositions were prepared one by one by arc-melting the mixture of raw materials several times using a nonconsumable electrode in a high vacuum furnace (Physcience Opto-electronics, Beijing, China) filled with high-purity Ar. The ingots were manufactured to be the shape of 40-µm-thickness ribbons under a high-purity Ar atmosphere by a melt-spinning method at a wheel surface speed of 50 m/s. The amorphous features of the as-spun Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) ribbons were ascertained by their X-ray diffraction (XRD) patterns measured by a Rigaku D\max-rC diffractometer (Rigaku, Tokyo, Japan) with Cu K α radiation [23]. The glass formability of the amorphous ribbons was evaluated from the thermal properties obtained from their differential scanning calorimetry (DSC) traces measured by a NET-ZSCH 404C calorimeter (Netzsch, Selb, Germany) [24] at a heating rate of 20 K/min. The temperature and field dependence of magnetization curves were measured by a vibrating sample magnetometer (VSM), which is a module of a Physical Property Measurement System (PPMS, model 6000, Quantum Design, San Diego, CA, USA) [25]. The Arrott plots were derived from the isothermal magnetization (M-H) curves to confirm the type of phase transition. The −∆S m vs. temperature curves were constructed from M-H curves according to the Maxwell equation. The −∆S m of the amorphous hybrid was calculated as where w i is the weight fraction of an amorphous ribbon.

Results and Discussion
The X-ray diffraction results of the as-spun Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) ribbons are displayed in Figure 1a. The ribbons show smooth and broad diffraction humps, indicating that all the as-spun Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) ribbons are amorphous. From the DSC traces of the three samples, as shown in Figure 1b, the endothermic glass transition hump and the exothermic crystallization peaks also ascertain the amorphous characteristics of these samples. Simultaneously, the onset temperatures of glass transition (T g ) and primary crystallization (T x ), as well as the liquid temperature (T l ) of the Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) ribbons, are listed in Table 1. Therefore, two commonly used criteria for evaluating the glass formability of amorphous alloys, namely, the reduced glass transition temperature (T rg , defined as the ratio of T g and T l ) [26] and the parameter γ (defined as the ratio of T x and (T g + T l )) [27], can be calculated accordingly to be 0.421 and 0.368 for Fe 87 Ce 7 B 5 Co 1 , 0.426 and 0.361 for Fe 86 Ce 7 B 5 Co 2 , 0.437 and 0.360 for Fe 85 Ce 7 B 5 Co 3 . Compared to the Fe 88 Ce 7 B 5 ribbon, the minor Co substitution for Fe dramatically decreases the T g , which decreases the T rg and reaches a minimum at x = 1 but obviously enlarges the supercooled liquid region (∆T x = T x − T g [28], also listed in Table 1), which makes the γ value reach to a maximum at x = 1, as illustrated in Figure 1c. Overall, both the T rg and γ of the Fe 88-x Ce 7 B 5 Co x (x = 0, 1, 2, 3) ribbons are in accordance with their glass formability: they can be quenched into amorphous ribbons easily but are not able to be vitrified into bulk amorphous samples.

Results and Discussion
The X-ray diffraction results of the as-spun Fe88-xCe7B5Cox (x = 1, 2, 3) ribbons are displayed in Figure 1a. The ribbons show smooth and broad diffraction humps, indicating that all the as-spun Fe88-xCe7B5Cox (x = 1, 2, 3) ribbons are amorphous. From the DSC traces of the three samples, as shown in Figure 1b, the endothermic glass transition hump and the exothermic crystallization peaks also ascertain the amorphous characteristics of these samples. Simultaneously, the onset temperatures of glass transition (Tg) and primary crystallization (Tx), as well as the liquid temperature (Tl) of the Fe88-xCe7B5Cox (x = 1, 2, 3) ribbons, are listed in Table 1. Therefore, two commonly used criteria for evaluating the glass formability of amorphous alloys, namely, the reduced glass transition temperature (Trg, defined as the ratio of Tg and Tl) [26] and the parameter γ (defined as the ratio of Tx and (Tg + Tl)) [27], can be calculated accordingly to be 0.421 and 0.368 for Fe87Ce7B5Co1, 0.426 and 0.361 for Fe86Ce7B5Co2, 0.437 and 0.360 for Fe85Ce7B5Co3. Compared to the Fe88Ce7B5 ribbon, the minor Co substitution for Fe dramatically decreases the Tg, which decreases the Trg and reaches a minimum at x = 1 but obviously enlarges the supercooled liquid region (ΔTx = Tx − Tg [28], also listed in Table 1), which makes the γ value reach to a maximum at x = 1, as illustrated in Figure 1c. Overall, both the Trg and γ of the Fe88-xCe7B5Cox (x = 0, 1, 2, 3) ribbons are in accordance with their glass formability: they can be quenched into amorphous ribbons easily but are not able to be vitrified into bulk amorphous samples.    Figure 2a shows the hysteresis loops under 5 Tesla of the Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) glassy samples measured at 180 K and 380 K, respectively. All these samples show soft magnetic at 180 K and almost paramagnetic at 380 K, indicating that the ferromagneticparamagnetic transition occurs within 180 K and 380 K. The saturation magnetization is approximately 144.4 Am 2 /kg for x = 1, 145.0 Am 2 /kg for x = 2 and 144.0 Am 2 /kg for x = 3, which implies the slightly fluctuation of the magnetic moment with the Co addition. The temperature dependence of magnetization curves for the Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) ribbons was measured under 300 Oe after a zero-field-cooling operation, as shown in Figure 2b. The T c , which is obtained at the minimum of dM/dT, can be found to be 305 K for Fe 87 Ce 7 B 5 Co 1 , 323 K for Fe 86 Ce 7 B 5 Co 2 , and 346 K for Fe 85 Ce 7 B 5 Co 3 . Similar to the situation in the Co substituted Fe 88 Zr 8 B 4 amorphous ribbons [19], T c of the Fe 88-x Ce 7 B 5 Co x (x = 0, 1, 2, 3) amorphous ribbons increases linearly with the Co content, as shown in the inset of Figure 2b, which is attributed to the enhanced 3d-3d interaction between 3d atoms by the Co addition [29]. As shown in Figure 3a Figure 2a shows the hysteresis loops under 5 Tesla of the Fe88-xCe7B5Cox (x = 1, 2, 3) glassy samples measured at 180 K and 380 K, respectively. All these samples show soft magnetic at 180 K and almost paramagnetic at 380 K, indicating that the ferromagneticparamagnetic transition occurs within 180 K and 380 K. The saturation magnetization is approximately 144.4 Am 2 /kg for x = 1, 145.0 Am 2 /kg for x = 2 and 144.0 Am 2 /kg for x = 3, which implies the slightly fluctuation of the magnetic moment with the Co addition. The temperature dependence of magnetization curves for the Fe88-xCe7B5Cox (x = 1, 2, 3) ribbons was measured under 300 Oe after a zero-field-cooling operation, as shown in Figure 2b. The Tc, which is obtained at the minimum of dM/dT, can be found to be 305 K for Fe87Ce7B5Co1, 323 K for Fe86Ce7B5Co2, and 346 K for Fe85Ce7B5Co3. Similar to the situation in the Co substituted Fe88Zr8B4 amorphous ribbons [19], Tc of the Fe88-xCe7B5Cox (x = 0, 1, 2, 3) amorphous ribbons increases linearly with the Co content, as shown in the inset of Figure 2b, which is attributed to the enhanced 3d-3d interaction between 3d atoms by the Co addition [29]. As shown in Figure 3a    The magnetic phase transition from ferromagnetic to paramagnetic usually results in the reduction of magnetic entropy due to the ordering of magnetic moments.  Table 2, accompanied with that of the Fe 88 Ce 7 B 5 amorphous ribbon for comparison purposes. It was found that −∆S m peak of Fe 88 Ce 7 B 5 AA was improved by adding 1% (at. %) Co but was decreased by adding more Co. As the Metals 2022, 12, 589 5 of 9 average magnetic moment of Co atoms is lower than that of the Fe atoms, the −∆S m peak of the Fe 88-x Ce 7 B 5 Co x (x = 0, 1, 2, 3) AAs should be generally decreased with the Co addition. The slightly increased −∆S m peak at x = 1 may be induced by the extra 3d-3d interaction between Co and Fe atoms [30].
According to the Arrott-Noakes equation, the relationship between the −∆S m and the external magnetic fields (H) in an amorphous alloy undergoing a 2nd-order magnetic transition can be expressed as −∆S m = A × H n , where A is a constant [31]. Figure 4d shows exponent n vs temperature curves of the Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) glassy ribbons by linearly fitting ln(−∆S m )-ln(H) plots at various temperatures. As predicted by V. Franco [31], n exponent of all the three samples is about 1 at low temperatures well below T c , subsequently decreases to a minimum (about 0.75) near T c , and finally approaches to a value of 2 at temperatures much higher than T c . The values of n near T c of these three samples, seen in the inset of Figure 4d, are 0.763 for x = 1 at 305 K, 0.758 for x = 2 at 322.5 K, and 0.756 for x = 3 at 347.5 K, all of which agree well with the results of other alloys undergoing a 2nd-order MPT [11,[15][16][17][18][19][20][21] and indicate the typical magnetocaloric effect of these AAs.  The magnetic phase transition from ferromagnetic to paramagnetic usually results in the reduction of magnetic entropy due to the ordering of magnetic moments.  Table 2, accompanied with that of the Fe88Ce7B5 amorphous ribbon for comparison purposes. It was found that −ΔSm peak of Fe88Ce7B5 AA was improved by adding 1% (at. %) Co but was decreased by adding more Co. As the average magnetic moment of Co atoms is lower than that of the Fe atoms, the −ΔSm peak of the Fe88-xCe7B5Cox (x = 0, 1, 2, 3) AAs should be generally decreased with the Co addition. The slightly in-      Figure 5a shows the −∆S m peak under 5 T of various iron-based metallic glasses with T c ranging from 280 K to 360 K (also listed in Table 2). The −∆S m peak of the Fe(Co)-Ce-B glassy alloys are comparable to or even larger than those of most iron-based metallic glasses around RT [9,[14][15][16][18][19][20][32][33][34][35][36][37]. For example, the −∆S m peak of the Fe 87 Ce 7 B 5 Co 1 amorphous ribbon (3.89 (J/(kg × K) under 5 T) is comparable to that of the Fe 83 Ce 11 B 6 glassy alloy [32], which is the largest among those metallic glasses. The −∆S m peak of the Fe 85 Ce 7 B 5 Co 3 amorphous ribbon, which is the lowest −∆S m peak value among the Fe 88-x Ce 7 B 5 Co x ribbons, is still higher than the −∆S m peak of most of those iron-based metallic glasses. On the other hand, it should be noted that the T c of Fe 88 Ce 7 B 5 (287 K) and Fe 86 Ce 7 B 5 Co 2 (323 K) glassy alloys are close to the T cold and T hot of a domestic air conditioner. Therefore, high −∆S m peak of the Fe 88-x Ce 7 B 5 Co x metallic glasses allows us to construct a specific table-like −∆S m profile within temperature interval from 280 K to 320 K in an amorphous hybrid composed of these amorphous ribbons. Figure 5b [19,34]. Furthermore, the compositions of the amorphous laminate do not contain any radioactive elements and will not bring about some health hazards. Therefore, the high −∆S m average from the T cold to the T hot of the amorphous composite indicates the potential application perspective as magnetic refrigerant in a domestic air conditioner.  Figure 5a shows the −ΔSm peak under 5 T of various iron-based metallic glasses with Tc ranging from 280 K to 360 K (also listed in Table 2). The −ΔSm peak of the Fe(Co)-Ce-B glassy alloys are comparable to or even larger than those of most iron-based metallic glasses around RT [9,[14][15][16][18][19][20][32][33][34][35][36][37]. For example, the −ΔSm peak of the Fe87Ce7B5Co1 amorphous ribbon (3.89 (J/(kg × K) under 5 T) is comparable to that of the Fe83Ce11B6 glassy alloy [32], which is the largest among those metallic glasses. The −ΔSm peak of the Fe85Ce7B5Co3 amorphous ribbon, which is the lowest −ΔSm peak value among the Fe88-xCe7B5Cox ribbons, is still higher than the −ΔSm peak of most of those iron-based metallic glasses. On the other hand, it should be noted that the Tc of Fe88Ce7B5 (287 K) and Fe86Ce7B5Co2 (323 K) glassy alloys are close to the Tcold and Thot of a domestic air conditioner. Therefore, high −ΔSm peak of the Fe88-xCe7B5Cox metallic glasses allows us to construct a specific table-like −ΔSm profile within temperature interval from 280 K to 320 K in an amorphous hybrid composed of these amorphous ribbons. Figure 5b displays the table-like (−ΔSm)-T curves under 1.5 T and 5 T for an amorphous laminate composed of 49% (wt.%) Fe88Ce7B5 + 2% (wt.%) Fe87Ce7B5Co1 + 49% (wt.%) Fe86Ce7B5Co2 glassy ribbons. The average −ΔSm value (−ΔSm average ) of the amorphous laminate is about 1.28 J/(kg × K) under 1.5 T from 280 K to 315 K, and approximately 3.48 J/(kg × K) under 5 T from 287 K to 320 K; these values are much higher than those of other Fe-Zr-B-based amorphous hybrids [19,34]. Furthermore, the compositions of the amorphous laminate do not contain any radioactive elements and will not bring about some health hazards. Therefore, the high −ΔSm average from the Tcold to the Thot of the amorphous composite indicates the potential application perspective as magnetic refrigerant in a domestic air conditioner.

Conclusions
In this work, the Fe88-xCe7B5Cox (x = 1, 2, 3) alloys were successfully fabricated to be about 40-μm-thickness amorphous ribbons, and the magnetic properties, as well as MCE of these glassy samples, were investigated. All the samples are soft magnetic at 180 K and paramagnetic at 380 K. The Tc of the Fe88-xCe7B5Cox amorphous ribbons increases linearly from 287 K when x = 0 to 305 K when x = 1, 323 K when x = 2, and 346 K when x = 3, which

Conclusions
In this work, the Fe 88-x Ce 7 B 5 Co x (x = 1, 2, 3) alloys were successfully fabricated to be about 40-µm-thickness amorphous ribbons, and the magnetic properties, as well as MCE of these glassy samples, were investigated. All the samples are soft magnetic at 180 K and paramagnetic at 380 K. The T c of the Fe 88-x Ce 7 B 5 Co x amorphous ribbons increases linearly from 287 K when x = 0 to 305 K when x = 1, 323 K when x = 2, and 346 K when x = 3, which is probably due to the enhanced 3d-3d interaction by the Co addition. The Arrott plots as well as the −∆S m = A × H n relationship of the amorphous Fe 88-x Ce 7 B 5 Co x ribbons confirm the typical magnetocaloric behaviors of 2nd-order MPT alloys. The −∆S m peak of these amorphous samples increases to 3.89 (J/(kg × K) at x = 1 and subsequently decreases with further Co addition, which may be attributed to the compromise of two factors: the decreasing −∆S m peak with Co addition due to the lower average magnetic moment of Co, and the slightly enhanced −∆S m peak due to the introduction of extra 3d-3d interaction between Co and Fe atoms by Co substitution. Based on these results, an amorphous laminate with a table-like −∆S m profile from~280 K to~320 K was achieved by mixing 49% (wt.%) Fe 88 Ce 7 B 5 + 2% (wt.%) Fe 87 Ce 7 B 5 Co 1 + 49% (wt.%) Fe 86 Ce 7 B 5 Co 2 amorphous ribbons. The high −∆S m average of the amorphous hybrid makes it a better candidate for application as a magnetic refrigerant in a domestic air conditioner.