Effects of annealing on the structure and magnetic properties of Fe80B20 magnetostrictive fibers

Introduction

Magnetostrictive materials, which are able to couple magnetic energy and elastic energy, have been extensively used for a large variety of components and devices, including sensors, transducers, resonators and actuators (1, 2). Magnetostrictive materials can be classified by structure as crystalline materials and amorphous materials (3), both of which offer unique advantages depending on the applications. Crystalline magnetostrictive materials, such as Ni, Fe, Co, Permalloy (Ni₈₀Fe₂₀), DyFe₂, TbFe₂, Terfenol-D (Tb_0.3Dy_0.7Fe_1.9) and Gafenol (Fe-Ga), exhibit strong anisotropy and high magnetostriction (4). Compared with crystalline materials, amorphous materials are isotropic and uniform due to the special noncrystalline form (5).

Iron-based amorphous magnetostrictive materials exhibit excellent soft magnetic properties, high abrasive resistance and outstanding corrosion resistance (6). To improve the glass formation ability, elements with low atomic weight, such as B and Si, are usually introduced into ferromagnetic metal matrix (7, 8). Due to the simple components and excellent magnetic property, Fe-B binary amorphous alloys in various forms have been fabricated and studied. Carabias et al fabricated Fe₈₀B₂₀ amorphous thin films with coercivity (H_C) in the range of 15~35 Oe using ion beam magnetron sputtering (9). Li et al fabricated Fe₈₀B₂₀ amorphous films with the H_C of 9.37 Oe by electrochemical deposition and made the thin films as biosensor platforms for the detection of pathogen (10). Sheng et al investigated the effect of electrochemical deposition parameters on the structures and magnetic properties of Fe₈₀B₂₀ amorphous films, and found that the samples prepared under 3mA/cm² had the lowest H_C of 6.81 Oe (11). Johnson et al fabricated Fe₇₉B₂₁ amorphous particles by sputtering and used them as biosensor platforms for the detection of pathogen (12). Ceniga fabricated Fe₈₀B₂₀ amorphous ribbons with H_C of 7.20 Oe using spinning wheel technique (13). Fiorani studied the magnetic properties of Fe₈₀B₂₀ amorphous ribbons which were prepared by the melt-spinning technique (14).

Fe₈₀B₂₀ amorphous alloys have been fabricated and studied (15-16-17) in the forms of thin films, ribbons and bulk. However, there have been few studies on Fe₈₀B₂₀ fibers. The one-dimensional Fe₈₀B₂₀ amorphous fibers have promising potential application in the realm of sensors, magnetic shielding and anti-counterfeit label. In this paper, Fe₈₀B₂₀ amorphous micro-fibers with H_C of 3.33 Oe are firstly fabricated by the single roller melt-spinning method. The effects of annealing temperature on the structure and magnetic properties of the alloy are studied. The important parameters to characterize the performance of sensors, including the resonance behavior and quality merit factor of Fe₈₀B₂₀ micro-fibers, are investigated in the present work.

Experimental procedures

The Fe₈₀B₂₀ alloy ingot was prepared by vacuum arc melting of a mixture of Fe (99.99 wt%) and B (99.99 wt%) (Purchased from Beijing Xingyou Trading Company) in argon atmosphere. The alloy was melted three to four times to achieve homogenization. Rapidly solidified fibers with a diameter of 60 μm were melt-spun with the wheel speed of 8.2 m/s. The fibers were annealed at 200°C, 300°C, 400°C, and 500°C, respectively, in vacuum below 3.0 × 10⁻⁴ Pa for 60 min. The structures of as-spun and annealed fibers were investigated by X-ray diffractometer (XRD) (PANalytical X,Pert Power) using Cu Kα radiation. The morphology of the fibers was observed by scanning electron microscopy (SEM) (HITACHI-S4800). Differential scanning calorimetry (DSC) measurements of the fibers were performed on Mettler Toledo TGA/DSC1 device under N₂ protection at the heating rate of 10°C/min, 30°C/min and 60°C/min, respectively. Vibrating sample magnetometer (VSM, Versalab) was used to examine the magnetic properties of the fibers. The resonance behavior of the fibers was characterized by an impedance analyzer (Agilent 4294A) with a home-made copper coil (10 mm in length) as shown in Figure 1. The impedance analyzer was connected with the two ends of the copper coil which was wound on a glass tube with an inner diameter of 0.5 mm. Before characterization, a fiber sample was put inside the coil with its axial direction in parallel to the glass tube axis. The impedance analyzer was an AC current source and swept over frequency through the coil. As a result, an AC magnetic field along the glass tube axis direction was obtained so that the magnetic fiber was forced to vibrate longitudinally inside the coil due to the magnetostriction effect and resulted in a change in the total impedance of the coil + fiber with frequency. Particularly, the change was significant when the sweeping frequency was near the natural/resonant frequency of the fiber so that the resonant behavior of the fiber can be easily characterized. Additional experimental details can be found elsewhere (11).

Results and discussion

The XRD results of as-spun and annealed Fe₈₀B₂₀ fibers are shown in Figure 2. Clearly, a broad diffraction peak, indicating the amorphous nature of the structure, was observed in the pattern of the as-spun fiber and the fibers annealed at 200°C, 300°C and 400°C, respectively. For the fiber annealed at 500°C, the XRD pattern shows the characteristic peak of α-Fe phase at 2θ = 43° and BFe₃ compound phase at 2θ = 45°. The result is in agreement with that of the Fe₈₀B₂₀ amorphous ribbons (18).

The SEM images of the Fe₈₀B₂₀ fibers are shown in Figure 3. The surface of the as-spun fiber is pretty smooth, and only small spots of precipitation can be observed as shown in Figure 3A. However, the annealing treatment at 200oC results in some dendrites on the fiber surface (Fig. 3B), the structure of which is believed to be Fe₃B (19). A further increase in annealing temperature leads to bigger dendrites, which is due to the increasing precipitation and accumulation of Fe₃B. The annealing treatment at 500oC produces both dendrites and cubic particles, as shown in Figure 3E. The appearance of Fe₃B phase is due to the enrichment of Boron, which is the result of precipitation of α-Fe phase.

Figure 4 shows the DSC curves of the Fe₈₀B₂₀ fibers measured under flowing nitrogen gas at a heating rate of 60°C/min from room temperature. Exothermic peaks (T_P) at 449°C are observed for the fibers annealed below 400°C but no exothermic peak (T_P) appears for the fibers annealed at 500°C. By referring to the XRD results, it can be noted that the peak represents a crystallization process. To further calculate the crystallization activation energy, the Kissinger formula is used (20):

In [ β T p 2 ] = − E R 1 T p +  constant

where E is activation energy; R is the gas constant; β is the heating rate; T_P is the peak value. The activation energy of fibers is calculated based on three values of T_P, which are obtained at the heating rate of 10, 30 and 60^oC/min, respectively. Based on the Kissinger formula, the activation energy is obtained as 221 kJ/mol, which is close to the value for Fe₇₅Si₁₀B₁₅ amorphous ribbons (21).

Figure 5 shows the hysteresis loops (M-H) of as-spun Fe₈₀B₂₀ amorphous fibers, where II and ⊥ represent the applied field H parallel or perpendicular to the fiber axis, respectively. Although the as-spun fibers have amorphous structure, a significant difference between the two M-H loops is observed. This is due to the shape anisotropy and the easy magnetization direction along the fiber axis. The H_C (II) and H_C (⊥) of as-spun fibers are found at 3.33 Oe and 12.61 Oe, respectively.

In comparison, the magnetic properties of Fe₈₀B₂₀ amorphous alloys in the forms of thin films, ribbons, bulk and as-spun fibers are summarized in Table I. It can be seen that the H_C (II) of the as-spun fiber is higher than that of the bulk but lower than that of the thin film and ribbon, while the M_r/M_s of the fibers is the lowest, which is due to the different shape anisotropy and magneto-elastic anisotropy.

The dependence of coercivity H_C (II) and H_C (⊥) on the annealing temperature is obtained from the hysteresis loop of fibers and plotted in Figure 6. The result indicates that the coercivity H_C (II) and H_C (⊥) slightly increases with the increase of annealing temperature below 449oC (the crystallization temperature), which is the result of the increase in total anisotropy (i.e., magneto-elastic anisotropy, shape anisotropy, and magneto-crystalline anisotropy). The intra-structure relaxation (22) and constant magneto-crystalline anisotropy due to the same amorphous structure below crystallization temperature, lead to the fact that there is almost no variation in shape anisotropy. Therefore, it is believed that the increase in coercivity is caused by the increasing saturation magnetostriction which results in larger magneto-elastic anisotropy. When the annealing temperature rises from 400 to 500°C, there is a dramatic increase of H_C (II) from 3.33 to 12.61 Oe, and H_C (⊥) from 21.61 to 61.52 Oe, which is due to the significant increase in magneto-crystalline anisotropy and magneto-elastic anisotropy resulting from crystallization of the fibers at 449°C.

Typical resonance behavior of an as-spun Fe₈₀B₂₀ micro-fiber is shown in Figure 7A. Characteristic/peak frequency of 630 kHz is observed in the real part of impedance spectrum, showing that the Fe₈₀B₂₀ fiber is capable of working as a resonator under magnetic field for sensor application. The quality merit factor (Q) is an important parameter to characterize the performance of an acoustic sensor. This factor is defined as:

Q = E 1 E 2

where E₁ is the energy stored in a vibration cycle and E₂ is the energy loss in one vibration cycle. Here, the value of Q is calculated as the ratio of characteristic frequency to the width at half peak height in the real part of impedance spectrum.

Figure 7B shows the Q value of samples treated at different annealing temperatures. It can be seen that the Q value increases slightly with increasing annealing temperature until the annealing temperature reaches 400°C. On the one hand, the hysteresis loss increases due to the increase of H_C and the eddy-current loss increases due to the decrease in resistivity caused by the reduction of defects. On the other hand, the elastic loss decreases due to the release of intrinsic stress. Based on the results, it can be noted that the latter dominates the effect on Q.

When the annealing temperature rises from 300°C to 400°C, the Q value decreases abruptly. It is probably because the intrinsic stress has already fully disappeared when the fibers are annealed at 300°C. In other words, there is no further release of intrinsic stress or reduction of elastic loss when the fibers are annealed at 400°C. At the same time, the hysteresis loss and the eddy-current loss continue to increase with increasing annealing temperature. In addition, abnormal eddy-current loss increases because of the magnetic domain growth. In such cases, the total loss increases which causes the decrease of Q value.

However, when the annealing temperature reaches 500°C, the value of Q dramatically increases to ~2000, which is far higher than that of the commercial product Metglas 2826 MB (~1000) (23). This is because the abnormal eddy-current loss decrease and thus E₂ significant decrease due to the fragmentation of magnetic domain caused by the crystallization when the annealing temperature reaches 500°C.

Conclusion

In this study, Fe₈₀B₂₀ amorphous fibers with the diameter of 60 μm were prepared by the single roller melt-spinning method and annealed at 200°C, 300°C, 400°C, and 500°C, respectively. XRD results indicate that the fiber structure remains amorphous when the annealing temperature is below 400°C. α-Fe phase and Fe₃B phase appear when the annealing temperature rises to 500°C, which is above the crystallization temperature of 449°C. The recrystallization activation energy is calculated to be 221 kJ/mol using Kissinger formula. The coercivity increases with increasing annealing temperature, which attributes to the increase of total anisotropy. The coercivity of as-spun Fe₈₀B₂₀ fibers is lower than those of Fe₈₀B₂₀ thin films and ribbons but higher than bulk, which is mainly due to the shape anisotropy. All the as-spun and annealed fibers exhibit good resonance behavior. The Q value firstly increases slightly with the annealing temperature increasing from RT to 300°C. It then decreases when the annealing temperature increases from 300°C to 400°C. Eventually, it increases to ~2000 at annealing temperature of 500oC.