Microstructure and Its Effect on the Magnetic, Magnetocaloric and Magnetostrictive Properties of Tb55Co30Fe15 Glassy Ribbons

In the present work, the microstructure and its effect on the magnetic, magnetocaloric, and magnetoelastic properties of the Tb55Co30Fe15 melt-spun ribbon were investigated. The ribbon exhibits typical amorphous characteristics in its X-ray diffraction examination and differential scanning calorimetry measurement. However, the magnetic properties of the ribbon indicate that the ribbon is inhomogeneous in the nanoscale, as ascertained by a high-resolution electron microscope. Compared to the Tb55Co45 amorphous alloy, the Tb55Co30Fe15 ribbon shows poor magnetocaloric properties but outstanding magnetostriction. A rather high value of reversible magnetostriction up to 788 ppm under 5 T was obtained. The mechanism for the formation of nanoparticles and its effect on the magnetocaloric and magnetostrictive properties were investigated.


Introduction
Magnetostriction, which refers to the change in shape or dimension in response to a magnetic field, is one of the inherent properties of magnetic materials that was firstly discovered in 1842 by J. P. Joule in Fe [1,2]. These magnetic materials can be applied as sensors, actuators, and energy harvesting devices because the magnetostrictive response allows these ferromagnetic materials to convert electromagnetic energy to mechanical energy. Therefore, magnetostrictive materials have evoked increasing interest in recent years, especially when the giant magnetostriction effect was found in TbDyFe alloy (known as Terfenol-D) [3,4]. The TbDyFe alloy exhibits ultrahigh magnetostriction up to 2000 ppm. However, the shortcomings inherited in the Terfenol-D alloy, such as brittleness, poor corrosion properties, and high eddy-current loss, can hardly be overcome in practical applications. The Cu addition can improve the fracture toughness of the TbDyFe alloy, but unfortunately deteriorate the magnetostriction performance of the alloy [5].
The vitrification of the TbDyFe alloy may provide a useful way to overcome the above shortcomings: by enhancing the strength of the alloy and making the glassy alloy less brittle; by improving the corrosion resistance; and by decreasing the eddy-current loss through enlarging the electric resistance. Although the magnetostriction of the amorphous (Tb 0.3 Dy 0.7 ) 40 Fe 60 thin film (λ = 400 ppm) is not as high as the Terfenol-D alloy [6], it is still comparable or higher than the Gafenal alloys [7][8][9]. However, the (Tb 0.3 Dy 0.7 ) 40 Fe 60 amorphous alloy can only be fabricated into the shape of thin film due to its poor glass forming ability (GFA). Recently, we systematically investigated the GFA and magnetic properties of binary Tb-transition metal (TM) and Dy-TM alloys and obtained excellent magnetostrictive properties in these binary amorphous ribbons. For example, the Tb 62.5 Co 37.5 fully amorphous alloy exhibits a rather high magnetostriction:~320 ppm and~470 ppm under 2 T and 5 T, respectively, at 50 K [10]; while the Dy 50 Co 50 glassy ribbon shows a higher magnetostriction:~320 ppm and~600 ppm under 2 T and 5 T, respectively, at 60 K [11]. Figure 1 shows the X-ray diffraction (XRD) pattern of the Tb 55 Co 30 Fe 15 ribbon and the DSC curve of the ribbon. The broad diffraction hump in the XRD pattern (Figure 1a), the endothermic glass transition and the sharp exothermic crystallization in the DSC trace ( Figure 1b) illustrate the typical amorphous characteristics of the Tb 55 Co 30 Fe 15 ribbon. The glass transition temperature (T g ), crystallization temperature (T x ), and liquidus temperature (T l , obtained from the DSC trace in Figure 1c) of the Tb 55 Co 30 Fe 15 ribbon are about 561 K, 606 K and 1023 K, respectively. The reduced glass transition temperature (T rg = T g /T l ) [12] of the Tb 55 Co 30 Fe 15 ribbon is therefore found to be about 0.548, and the parameter γ (=T x /(T g + T l )) [13] for the ribbon is 0.383. Compared to the Tb 55 Co 45 amorphous ribbon, the T rg of the Tb 55 Co 30 Fe 15 ribbon is over 20% higher than that of the Tb 55 Co 45 amorphous ribbon, and the γ value is about 11% higher. As the two parameters are the most commonly used glass formability gauge, the higher T rg and γ values of the Tb 55 Co 30 Fe 15 alloy indicate enhanced glass formability by 15% (at. %) Fe substitution for Co.

Results
The relationship between magnetization and temperature (M-T curves) of the Tb 55 Co 30 Fe 15 ribbon is revealed in Figure 2a. The sample was firstly cooled from room temperature to 10 K without a magnetic field, and subsequently heated to room temperature under a field of 0.03 T for the measurement of the zero-field-cooled (ZFC) M-T curve. Then, the sample was cooled from room temperature to 10 K under a magnetic field of 0.03 T and heated to room temperature under the same magnetic field for the measurement of field-cooled (FC) M-T curve. The Curie temperature (T c ) of the sample obtained from the derivative of the M-T curves is about 169 K, which is 64 K higher than that of the Tb 55 Co 45 amorphous ribbon, demonstrating the enhancement of magnetic ordering temperature by 15% (at. %) Fe substitution for Co. The shape of the FC and ZFC M-T curves of the Tb 55 Co 30 Fe 15 ribbon is similar to those of canonical spin glass systems [11,[14][15][16][17][18][19]. The spin freezing temperature The relationship between magnetization and temperature (M-T curves) of the Tb55Co30Fe15 ribbon is revealed in Figure 2a. The sample was firstly cooled from room temperature to 10 K without a magnetic field, and subsequently heated to room temperature under a field of 0.03 T for the measurement of the zero-field-cooled (ZFC) M-T curve. Then, the sample was cooled from room temperature to 10 K under a magnetic field of 0.03 T and heated to room temperature under the same magnetic field for the measurement of field-cooled (FC) M-T curve. The Curie temperature (Tc) of the sample obtained from the derivative of the M-T curves is about 169 K, which is 64 K higher than that of the Tb55Co45 amorphous ribbon, demonstrating the enhancement of magnetic ordering temperature by 15% (at. %) Fe substitution for Co. The shape of the FC and ZFC M-T curves of the Tb55Co30Fe15 ribbon is similar to those of canonical spin glass systems [11,[14][15][16][17][18][19]. The spin freezing temperature (Tf) obtained from the ZFC M-T curve is about 139 K, which is also 44 K higher than that of the Tb55Co45 amorphous ribbon. It is worth noting that the M-T curves of the Tb 55 Co 30 Fe 15 ribbon are much smoother than other metallic glasses, and the magnetization of the sample does not drop to zero even at a temperature far above the Curie temperature. On the other hand, in contrast to other fully amorphous alloys with spin-glass-like behavior [11,[14][15][16][17][18][19], the divergence between the ZFC and FC M-T curves is observed at temperatures well above the Curie temperature. Both the above phenomena indicate that the Tb 55 Co 30 Fe 15 ribbon may be inhomogeneous in the nanoscale. Figure 2b shows the hysteresis loops of the Tb 55 Co 30 Fe 15 ribbon measured at 10 K, 30 K, 50 K, 90 K, 140 K, 170 K, and 300 K. Just like other spin-glass-like metallic glasses [10,14,16,18], the ribbon is hard magnetic at low temperature with a coercivity of~1.42 T at 10 K and is nearly paramagnetic at 300 K. However, although the coercivity of the sample decreases with the increasing temperature, the sample is not soft magnetic within the temperature range from T f to T c . The nearly 0.04 T large coercivity of the sample at 170 K indicates that the Tb 55 Co 30 Fe 15 ribbon is most likely inhomogeneous in the microstructure because a fully amorphous spin-glass-like alloy should be soft magnetic at temperatures between its T f and T c [11,15,19]. It is worth noting that the M-T curves of the Tb55Co30Fe15 ribbon are much smoother than other metallic glasses, and the magnetization of the sample does not drop to zero even at a temperature far above the Curie temperature. On the other hand, in contrast to other fully amorphous alloys with spin-glass-like behavior [11,[14][15][16][17][18][19], the divergence between the ZFC and FC M-T curves is observed at temperatures well above the Curie temperature. Both the above phenomena indicate that the Tb55Co30Fe15 ribbon may be inhomogeneous in the nanoscale.  Figure 2b shows the hysteresis loops of the Tb55Co30Fe15 ribbon measured at 10 K, 30 K, 50 K, 90 K, 140 K, 170 K, and 300 K. Just like other spin-glass-like metallic glasses [10,14,16,18], the ribbon is hard magnetic at low temperature with a coercivity of ~1.42 T at 10 K and is nearly paramagnetic at 300 K. However, although the coercivity of the sample decreases with the increasing temperature, the sample is not soft magnetic within the temperature range from Tf to Tc. The nearly 0.04 T large coercivity of the sample at 170 K indicates that the Tb55Co30Fe15 ribbon is most likely inhomogeneous in the microstructure because a fully amorphous spin-glass-like alloy should be soft magnetic at temperatures between its Tf and Tc [11,15,19].
In order to ascertain the above assumption, we observed the microstructure of the Tb55Co30Fe15 as-spun ribbon. The HRTEM image of the as-spun ribbon is shown in Figure  3. One can find that there are some nanoparticles with an average diameter less than 10 nm distributed randomly in the disordered matrix. Such microstructure is similar to the structure of Nd-Fe-Al bulk metallic glasses [20,21], which is supposed to be closely related to the positive mixing enthalpy between the Nd and Fe atoms. Using the Miedema's model [22], we found that the mixing enthalpy between the Tb and Fe atoms is also positive. Therefore, the unique microstructure of the Tb55Co30Fe15 as-spun ribbon with some metastable intermediate nanoparticles embedded in disordered matrix is most likely resulted from the positive mixing enthalpy between the Tb and Fe atoms. In order to ascertain the above assumption, we observed the microstructure of the Tb 55 Co 30 Fe 15 as-spun ribbon. The HRTEM image of the as-spun ribbon is shown in Figure 3. One can find that there are some nanoparticles with an average diameter less than 10 nm distributed randomly in the disordered matrix. Such microstructure is similar to the structure of Nd-Fe-Al bulk metallic glasses [20,21], which is supposed to be closely related to the positive mixing enthalpy between the Nd and Fe atoms. Using the Miedema's model [22], we found that the mixing enthalpy between the Tb and Fe atoms is also positive. Therefore, the unique microstructure of the Tb 55 Co 30 Fe 15 as-spun ribbon with some metastable intermediate nanoparticles embedded in disordered matrix is most likely resulted from the positive mixing enthalpy between the Tb and Fe atoms.     Figure 4b. Like the situation in other spin-glass-like RE-TM metallic glasses [14,18,19], the high coercivity and the spin freezing behavior obviously undermine the magnetocaloric properties of the Tb 55 Co 30 Fe 15 ribbon at temperatures below T f , and the −∆S m value is even decreased to negative at temperatures below 50 K. On the other hand, compared to the Tb 55 Co 45 amorphous ribbon, as shown in Figure 4c, the Tb 55 Co 30 Fe 15 ribbon shows a much broader and lower −∆S m peak. Furthermore, one can find that unlike other fully amorphous alloys, the maximum −∆S m (−∆S m peak ) of the Tb 55 Co 30 Fe 15 ribbon does not appear near its Curie temperature, but appears near 145 K, which is over 20 K lower than the Curie temperature of the ribbon. The deteriorated magnetocaloric properties, as well as the deviation of the −∆S m peak temperature from T c , are considered to be induced by the existence of nanoparticles in the amorphous matrix and the resulted residual coercivity near T c in the Tb 55 Co 30 Fe 15 ribbon.  Figure 4a shows the magnetization (M-H) curves of the Tb55Co30Fe15 as-spun ribbon measured at various temperatures under 5 T. The dramatic decreasing of magnetization under a low magnetic field (<0.5 T) from 90 K to 10 K also illustrates the spin-glass-like behavior of the Tb55Co30Fe15 as-spun ribbon. Based on the isothermal M-H curves, we can derive the magnetic entropy change (−ΔSm) at different temperature ((−ΔSm)-T plots) of the Tb55Co30Fe15 ribbon under various magnetic fields, as shown in Figure 4b. Like the situation in other spin-glass-like RE-TM metallic glasses [14,18,19], the high coercivity and the spin freezing behavior obviously undermine the magnetocaloric properties of the Tb55Co30Fe15 ribbon at temperatures below Tf, and the −ΔSm value is even decreased to negative at temperatures below 50 K. On the other hand, compared to the Tb55Co45 amorphous ribbon, as shown in Figure 4c, the Tb55Co30Fe15 ribbon shows a much broader and lower −ΔSm peak. Furthermore, one can find that unlike other fully amorphous alloys, the maximum −ΔSm (−ΔSm peak ) of the Tb55Co30Fe15 ribbon does not appear near its Curie temperature, but appears near 145 K, which is over 20 K lower than the Curie temperature of the ribbon. The deteriorated magnetocaloric properties, as well as the deviation of the −ΔSm peak temperature from Tc, are considered to be induced by the existence of nanoparticles in the amorphous matrix and the resulted residual coercivity near Tc in the Tb55Co30Fe15 ribbon.  The deterioration of the magnetocaloric properties of the Tb55Co30Fe15 ribbon does not necessarily lead to its poor magnetostriction because the magnetocaloric property of RE-TM metallic glasses depends mainly on the interaction between RE and TM atoms, while the magnetoelastic properties of these alloys are dominated by the random magnetic anisotropy, which is closely related to the spin freezing behavior and hysteresis in these alloys. In addition, it is reported that the partial crystallization may be helpful for the improvement of magnetostriction of the Tb(Dy)-Fe amorphous alloy [6,23,24]. Therefore, the microstructure and the high coercivity of the Tb55Co30Fe15 ribbon may lead to a higher magnetostriction. Figure 5 shows the reversible magnetostriction (λ-H) curves of the The deterioration of the magnetocaloric properties of the Tb 55 Co 30 Fe 15 ribbon does not necessarily lead to its poor magnetostriction because the magnetocaloric property of RE-TM metallic glasses depends mainly on the interaction between RE and TM atoms, while the magnetoelastic properties of these alloys are dominated by the random magnetic anisotropy, which is closely related to the spin freezing behavior and hysteresis in these alloys. In addition, it is reported that the partial crystallization may be helpful for the improvement of magnetostriction of the Tb(Dy)-Fe amorphous alloy [6,23,24]. Therefore, the microstructure and the high coercivity of the Tb 55 Co 30 Fe 15 ribbon may lead to a higher magnetostriction. Figure 5 shows the reversible magnetostriction (λ-H) curves of the Tb 55 Co 30 Fe 15 ribbon and the Tb 55 Co 45 amorphous ribbon for comparison, both of which are measured at 50 K. As expected, high values of 520 ppm and 788 ppm are observed under 2 T and 5 T, respectively. These values are much higher than the Tb 55 Co 45 amorphous ribbon (270 ppm and 465 ppm under 2 T and 5 T, respectively), and other Tb(Dy)-Co amorphous ribbons [10,11]. The λ-H curves of the Tb 55 Co 30 Fe 15 ribbon were also measured at 90 K, 140 K, and 170 K. The magnetostriction is~395 ppm at 90 K,~197.5 ppm at 140 K and~96 ppm at 170 K. Considering the coercivity of the Tb 55 Co 30 Fe 15 ribbon decreases from~0.3 T at 50 K, to~0.14 T at 90 K,~0.065 T at 140 K, and 0.04 T at 170 K, one can find that the magnetostriction decreases monotonically with the coercivity, which ascertains the close relationship between the magnetostriction and hysteresis in the Tb 55 Co 30 Fe 15 ribbon.

Conclusions
In summary, the microstructure of a Tb55Co30Fe15 glassy ribbon and its effect on the magnetic and magnetostrictive properties were investigated. The ribbon shows typical amorphous characteristics, and its apparent glass formability is better than the binary Tb55Co45 amorphous ribbon. The Tb55Co30Fe15 ribbon shows typical spin-glass-like feature in its ZFC and FC M-T curves, with a Tc of ~169 K and a Tf of ~139 K. However, the magnetization in the M-T curves that does not go to zero at temperatures far above Tc, and the residual coercivity near Tc, indicate that the Tb55Co30Fe15 glassy ribbon may be inhomogeneous in the nanoscale. HREM observation has revealed that there are some nanoparticles with an average diameter less than 5 nm distributed randomly in the matrix. The formation of the nanoparticles is supposed to be due to the positive mixing enthalpy between the Tb and Fe atoms. It was found that the −ΔSm peak of the nano-glass ribbon is low and the −ΔSm peak does not appear near its Curie temperature. The deterioration of the magnetocaloric property of the ribbon is considered to be induced by the existence of nanoparticles in the amorphous matrix and the resulted residual coercivity near Tc. In contrast to the poor magnetocaloric properties, the nano-glass structure and the resulting high coercivity of the Tb55Co30Fe15 ribbon led to a rather high value of reversible magnetostriction up to 788 ppm under 5 T, which is almost the highest in the amorphous alloys reported in the literature [6,10,11,23,24].

Conclusions
In summary, the microstructure of a Tb 55 Co 30 Fe 15 glassy ribbon and its effect on the magnetic and magnetostrictive properties were investigated. The ribbon shows typical amorphous characteristics, and its apparent glass formability is better than the binary Tb 55 Co 45 amorphous ribbon. The Tb 55 Co 30 Fe 15 ribbon shows typical spin-glass-like feature in its ZFC and FC M-T curves, with a T c of~169 K and a T f of~139 K. However, the magnetization in the M-T curves that does not go to zero at temperatures far above T c , and the residual coercivity near T c , indicate that the Tb 55 Co 30 Fe 15 glassy ribbon may be inhomogeneous in the nanoscale. HREM observation has revealed that there are some nanoparticles with an average diameter less than 5 nm distributed randomly in the matrix. The formation of the nanoparticles is supposed to be due to the positive mixing enthalpy between the Tb and Fe atoms. It was found that the −∆S m peak of the nano-glass ribbon is low and the −∆S m peak does not appear near its Curie temperature. The deterioration of the magnetocaloric property of the ribbon is considered to be induced by the existence of nanoparticles in the amorphous matrix and the resulted residual coercivity near T c . In contrast to the poor magnetocaloric properties, the nano-glass structure and the resulting high coercivity of the Tb 55 Co 30 Fe 15 ribbon led to a rather high value of reversible magnetostriction up to 788 ppm under 5 T, which is almost the highest in the amorphous alloys reported in the literature [6,10,11,23,24].