Keywords

Introduction

With the rapid development of information technology, the demand for large capacity information storage devices is increasing rapidly. It has been paid attention to by the researchers in the past to search for high density magnetic recording devices. 1L0 ordered phase FePd alloy (KU = 107erg/cm3) with high magnetocrystalline anisotropy and high coercivity. Theoretically, The maximum magnetic energy product is 48.0 MGOe [1]. These unique and excellent comprehensive properties are considered to be one of the best materials for ultrahigh density magnetic recording media and micro-electromechanical systems [2].

In recent years, a lot of research work mainly focused on the origin of the ordered L10 phase FePd alloy with high magnetocrystalline anisotropy [3,4,5,6] and to explore a variety of physical and chemical methods for the preparation of various alloy films, such as FePd alloy film [7,8,9], FePd/Fe multilayer film, [10] nanowire [11] and nano particles [12]. In order to improve the magnetic properties of the alloy (film), doping or alloying is the main method to improve the properties of materials [13,14,15]. The results show that if the FePd alloy nanocrystalline particles added in the C element, it will significantly hinder the diffusion of Fe and Pd atoms and it can inhibit the conversion of FePd nanoparticles from disordered FCC phase to ordered FCT phase [16], the comprehensive performance of the alloy can be improved.

At present, the effects of rare earth elements on the magnetic properties of FePd alloy thin films and their role in the microstructure evolution and order-disorder phase transition have not been systematically studied. In this paper, a series of Nd–Fe–Pd thin films were prepared by DC magnetic sputtering. The effects of Nd content and annealing temperature on the structure and magnetic properties of Ndx(Fe47.5Pd52.5)100-x thin films were studied by changing the content of rare earth Nd and annealing temperature.

Experiment

The thin film samples of Ndx(Fe47.5Pd52.5)100-x (x = 2, 2.7, 3.4, 4) were prepared by ultrahigh vacuum multi target magnetron sputtering. Rare earth Nd doped FePd layer by alternating deposition method. Nd rare earth films were uniformly placed on the FePd composite target (the purities of the ingredients were better than 99.9 wt%) and the content of rare earth in FePd layer was controlled by changing the number of rare earth films. It could be considered that the addition of rare earth will not change the proportion of the FePd layer and the total thickness because the rare earth was very few in the sputtering. In the experiment, the purity of the rare earth was not less than 99.5%. Before using, the Nd target was carefully cleaned and placed on the sputtering target of excitation. Before the deposition of thin films, the “pre sputtering” ready for 30 min to remove the oxide layer and the target surface impurities. In this study, Ndx(Fe47.5Pd52.5)100-x thin films were prepared by FJL560B4 ultra-high vacuum magnetron sputtering and the vacuum degree of the sputtering system was better than 6.5 × 10−5 Pa. The sputtering gas was argon and the sputtering pressure was 2.0 Pa. The films were sealed by argon filled under vacuum condition (10−1 Pa) and annealing in muffle furnace. Annealing temperature at 500, 550 and 600 °C, the holding time was 30 min. The deposition rates measured by an AMBIOS XP-2 stylus profilometer; the structure of annealed films was determined by Bruker D8-ADVANCE X ray diffractometer (XRD) using Co Kα radiation; the elemental composition analyzed by a JSM-5600LV scanning electron microscope equipped with EDS; magnetic properties of the samples were measured and analyzed by physical parameter testing system (PPMS-9T).

Results and Discussion

Figure 1 shows X-ray diffraction patterns for Ndx(Fe47.5Pd52.5)100-x (x = 0, 2, 2.7, 3.4, 4) thin film samples annealed at 550 °C for 30 min. As can be seen from the graph, the undoped film samples are basically composed of FCC phase at the same annealing conditions. When the content of Nd is 2 at.%, the weak FCT phase diffraction peak can be observed. With the further increase of Nd content, the intensity of the diffraction peak of FCT phase increases gradually. When the Nd content increases to 4 at.%, it is obvious to see the FCT phase diffraction peak, which is an ordered FCT phase, which means that the addition of Nd element can promote the phase transition of FCC—FCT. The result is that the phase transition temperature of FePd alloy films decreases with the increase of Nd content.

Fig. 1
figure 1

XRD patterns of Ndx(Fe47.5Pd52.5)100-x films annealed at 550 °C for 30 min

Full size image

According to the diffraction data of alloy thin films, the lattice parameters of the FCC phase have been derived, and the results are shown in Table 1. Table 1 shows that the lattice parameter a increases in a near-linear fashion with increasing of Nd concentration. This indicates that the Nd atoms dissolved into the FCC phase, forming a solid solution, which leads to a cell-volume expansion resulting from the fact that the Nd atom radius is larger than those of both Fe and Pd atoms.

Table 1 Lattice parameters of Ndx(Fe47.5Pd52.5)100-x films
Full size table

We calculated the c/a ratio and the order degree S2, from the XRD data, and the results are displayed in Table 1 and Fig. 2. We can see that with the increase of Nd doping amount, the corresponding sample c/a ratio was decreased gradually; the order degree increased to a certain extent. This indicates that the addition of rare earth element Nd can enhance the driving force of phase transformation and contribute to the occurrence of phase transition in thin films. Figure 3 displays with the increase of Nd content, (002)/(200) peak intensity ratio increased gradually, it also shows that the control of the content of doping of rare earth Nd can adjust the ordered and disordered FePd phase ratio.

Fig. 2
figure 2

Relationship of order degree and the ratios of c/a with the change of Nd content

Full size image
Fig. 3
figure 3

Relationship of grain size and the ratios of I(002)/I(200) with the change of Nd content

Full size image

By using XRD data and Scherrer equation, we obtain the relationship between the grain size of the FCT phase and the content of rare earth, and the results are shown in Fig. 3. It can be seen from the figure that with the increase of the content of rare earth, the grain size is almost linearly reduced, Therefore, the rare earth element Nd has the role of grain refinement in the microstructure of the thin films. In the process of preparing samples, we can not only control the size of grain growth by changing the heat treatment conditions, but also can effectively control the size of the grains by adding rare earth elements.

Figure 4 shows the EDS data of the thin film samples, in which the samples were analyzed by surface scanning. The results showed that the films were composed of Nd2.7(Fe47.5Pd52.5)97.3.

Fig. 4
figure 4

EDS data of the Nd2.7(Fe47.5Pd52.5)97.3 film

Full size image

The XRD results obtained for Nd2.7(Fe47.5Pd52.5)97.3 films annealed at various temperature for 30 min are shown in Fig. 5. Figure 5 shows that the as deposited films were disordered FCC structure, and the (002) reflection appears in the XRD patterns when the annealing temperature is no less than 500 °C. At this point the phase transformation from FCC to FCT phase is not completed, because the (200) reflection has not fully split into two reflections ((200) and (002)). When the annealing temperature is 550 °C, the (110), (200), (002), (220) and (202) reflection can be clearly observed. It shows that the structure of the films is mainly composed of FCT phase. After the addition of rare earth element Nd, the phase transition of FePd thin film is very sensitive to the annealing temperature and time, and the rare earth element Nd can effectively reduce the phase transition temperature or shorten the annealing time greatly. We could derive the values of the average grain sizes by means of Scherrer’s formula, and we obtained the relationship between the grain size of Nd2.7(Fe47.5Pd52.5)97.3 thin film (x = 2.7) and annealing temperature. These values are included in Table 2. The grain size of FCT phase increases with the increase of annealing temperature.

Fig. 5
figure 5

XRD patterns of Nd2.7(Fe47.5Pd52.5)97.3 films annealed at different temperatures for 30 min

Full size image
Table 2 Grain size at different annealing temperatures
Full size table

Figure 6 illustrates the effect of Nd concentration on the magnetic properties of the Ndx(Fe47.5Pd52.5)100-x (x = 0, 2, 2.7, 3.4, 4) films. The obtained results are also included in Fig. 7. The inset in Fig. 6 shows that the Hc of the films first increase rapidly as the Nd content increases to x = 2.7, At the same time, the coercivity (Hc) of the film reaches the maximum, and then shows a more gradual decrease for x values from 2.7 to 3.4. Hc again decreases rapidly with further increasing of Nd content. As show in Fig. 7, we find that the relationship between remanence ratio (Mr/Ms) and the Nd content is similar to that between the coercivity (Hc) of the films and the Nd content.

Fig. 6
figure 6

Magnetic hysteresis loops of Ndx(Fe47.5Pd52.5)100-x films annealed at 550 °C for 30 min

Full size image
Fig. 7
figure 7

Hc, Mr/Ms versus Nd content

Full size image

The factors that affect the coercivity are the magnetic anisotropy constant, the exchange coupling coefficient between grains, the concentration of FCT phase and the degree of order. According to the model of Kneller and Hawig [17], exchange coupling effect of the hard magnetic phase (k) and soft magnetic phase (m) is determined by the grain size. In the system of exchange coupled nanocomposite-magnet, the critical size of the exchange-coupling can be calculated according to the following formula:

$$ b_{cm} = \pi \left( {A_{m} /2K_{k} } \right)^{1/2} $$
(1)

where Am is the exchange interaction energy of soft magnetic phase, Kk is the magnetic anisotropy constant of the hard magnetic phase. In order to obtain a strong exchange coupling magnetic phase, the grain size of soft phase should be about equal to bcm. The result shows that, when the grain size is less than 20 nm and uniform size, the exchange coupling effect between soft and hard magnetic phase can not only enhance the remanence Br, but also can increase the coercivity (Hc), so as to obtain high magnetic energy product. From the experimental data of XRD film, under the same heat treatment conditions, the more the concentration of rare earth Nd, the intensities of the FCT phase characteristic peaks (110), (200) (002) were stronger, and the splitting degrees of diffraction peak (200) and (002) were greater. This shows that the larger the volume fraction of FCT phase in the film, the higher the degree of order, the greater the anisotropy constant. When x exceeds 2.7, it is possible that the grain size is smaller than the optimum size of the exchange coupling, and lead to the coupling between soft and hard magnetic phase is weakened. Another possible reason is that the concentration of rare earth Nd exceeds the solid solution range of the FePd alloy, which makes the rare earth elements gather around the grain, which increases the distance between the grains and weakens the exchange coupling effect, which leads to further decreasing in Hc. These factors added together, the maximum value of the coercivity and remanence ratio (Hc = 3.05 kOe, Mr/Ms = 0.646) appeared at x = 2.7.

Figure 8 shows the effect of annealing temperature on the magnetic properties of the Nd2.7(Fe47.5Pd52.5)97.3. The curve of the coercivity Hc and remanence ratio Mr/Ms with the change of annealing temperature were shown in Fig. 9. Peak values for both the coercivity Hc (Hc = 3.05 kOe) and the remanence ratio (Mr/Ms = 0.646) are reached at annealing temperature of 550 °C. The rise in Hc with annealing time is mainly due to the improvement in the chemical ordering of the FCT phase, leading to an increase in the anisotropy constant Kk. The relationship between the coercivity and the anisotropy constant can be expressed as follows:

Fig. 8
figure 8

Magnetic hysteresis loops of Nd2.7(Fe47.5Pd52.5)97.3 films annealed at different temperatures for 30 min

Full size image
Fig. 9
figure 9

Hc, Mr/Ms versus annealing temperature

Full size image
$$ Hc = K_{k} /\mu_{0} M_{sm} $$
(2)

where Msm is the saturation magnetization of the soft magnetic phase and Kk is the anisotropy constant of the hard magnetic phase. The higher remanence ratio suggests that exchange-coupling occurs between the soft magnetic phase and the hard magnetic phase. When the annealing temperature is higher than 550 °C, the coercivity and remanence ratio are beginning to decline. The reason is that when the annealing temperature increases, the grain size becomes larger, and the optimum size of coupling is removed, which leads to the decrease of coupling effect. Another possibility is because when the annealing temperature is too high, rare earth elements separate out from FePd alloys and enriched in grain boundaries, and the distance between the grains become larger, which affects the exchange coupling effect between grains, while the coercivity and remanence ratio also decreased. This is why coercivity and remanence ratio decreases with increasing of annealing temperature for FePd films.

Conclusion

The addition of rare earth element Nd in FePd alloy film can significantly shorten the transition time from FCC phase to FCT phase and decrease the annealing temperature, which indicates that adding moderate amount of rare earth Nd can increase the driving force of phase transformation. When small amounts of Nd are substituted for FePd, the Nd atoms dissolve into the FCC phase, forming a solid solution. The addition of Nd leads to an increase in the degree of ordering S and a decrease in the average grain size. The function relationship between the coercivity of FePd thin films and the rare earth Nd content increases sharply and then decreases. For Ndx(Fe47.5Pd52.5)100-x (x = 0, 2, 2.7, 3.4, 4) alloys films, the maximum value of the coercivity and remanence ratio (Hc = 3.05 kOe, Mr/Ms = 0.646) are obtained in x = 2.7. With the increase of annealing temperature, the change of Nd2.7(Fe47.5Pd52.5)97.3 alloys films also first increases and then decreases, and the maximum value of Hc = 3.05 kOe has obtained at 550 °C for 30 min. Therefore, a small amount of rare earth doped (x < 2.7 at.%) and appropriate temperature conditions can improve the magnetic properties of the films.