Elsevier

Vacuum

Volume 121, November 2015, Pages 88-95
Vacuum

Microstructural evolution and magnetic properties of nanocrystalline Fe films prepared in a high magnetic field

https://doi.org/10.1016/j.vacuum.2015.07.021Get rights and content

Highlights

  • High magnetic field improves soft magnetic properties of Fe films.

  • Grain column changed into spherical particle by increasing substrate temperature.

  • High magnetic field increases the energy of formation for the Fe films.

  • Influence of high magnetic field on condensation process of film is significant.

  • Substrate thermal disturbances decrease the effect of high magnetic field.

Abstract

In this study we have explored how a high magnetic field (HMF) affects the structure and magnetic properties of Fe thin films fabricated on Si (100) substrates at various substrate temperatures by using a thermal evaporation method. At a substrate temperature of room temperature (RT), the HMF increased the width of column and the formation energy of the films. This behavior occurred because the HMF induced an excess energy-Zeeman energy which is larger than the sum of the kinetic energy and thermal energy of Fe without the HMF. This increased the particle size and decreased the roughness of the film deposited at RT and 6 T. When the substrate temperature was increased to 400 °C, the growth mode is changed from columnar (RT) to layered (400 °C). The HMF increased the grain size. The film deposited at RT and 6 T had much improved soft magnetic properties from its modified microstructure. While thermal disturbances decreased the effect of HMF on the magnetic properties of the film deposited at 400 °C. In particular, the influence of HMF on the microstructure and magnetic properties of Fe films is significant not in the evaporation stage but in the condensation process of film.

Introduction

Nanocrystalline magnetic films have received much attention because of their wide applications in magnetic recording, electronic devices, and sensing technology [1], [2], [3], [4], [5], [6], [7], [8]. Fe-based nanocrystalline soft magnetic films are the most suitable magnetic head material for the core of high-density magnetic recording because of their high saturation magnetization and low coercivity [9]. Because they can be used as spin filters and spin injectors in semiconductor devices, Fe nanocrystalline films on semiconductors are widely used in magnetoelectric and spintronic devices; they are also used as the basic element of electrical and electro-optical heterostructure devices [10], [11]. Much research has focused on optimizing the structure and quality of Fe thin films by altering processing conditions. For example, Salvdor et al. [12] found that increasing the deposition temperature improved the magneto-crystalline anisotropy and crystallinity of Fe thin films. However, increasing the deposition temperature also increases grain size and coercivity. The morphology of the Fe film strongly depends on the beam orientation relative to the sample normal [13]. The oblique deposition of Fe film obviously increased the coercivity and induced uniaxial magnetic anisotropy [14]. However, the oblique deposition will reduce the density and the saturation magnetization of the Fe film. The magnetization reversal process and coercivity of Fe films are profoundly influenced by adding an Ag buffer layer and the surface roughness of that layer [15], [16], [17]. However, adding a buffer layer complicates film fabrication. Thus, an effective and convenient preparation method is still needed for preparing high-quality Fe films with excellent soft magnetic performance.

High magnetic fields (HMFs) can influence the movement of magnetic and paramagnetic particles because of non-contact operation, enhanced orientation and magnetization energy. Ref. [18] written by our research group summarized the various effects of HMF. The results indicate that HMF is a very promising fabrication method. Materials, such as magnetic optical materials, optical crystal, optical fiber, magnetic materials, and superconducting materials, etc., with special properties can be prepared by HMF. Furthermore, HMF has also been introduced during film preparation. It can exert magnetization energy, magnetic torque and magnetic force on the grains of the film, changing the film structure and improving performance [19], [20], [21], [22]. Fe films are ferromagnetic. Therefore a HMF will strongly influence them. However, because of limitations in current preparation methods, there are few reports on Fe films prepared under HMF. Matsushima et al. [23], [24] applied HMF during electrodeposition of Fe films, finding the surface morphology changed drastically from angular to wavy. Magnetic fields have also been found to control the crystal texture of Fe films: the (110) planes were orientated in same direction of the magnetic field vector [25]. Koza et al. [26] showed that HMF strongly influenced the nucleation process and surface morphology of Fe films during electrodeposition. However, these studies only focus on how HMF affects the crystal structure and microstructure of Fe films. There is rarely reported about how HMF influences the magnetic performance. Furthermore, so far the control mechanism of HMF on the microstructure and magnetic performance is also not clear. And from the above research, HMF affects film growth via magnetohydrodynamic convection during electrodeposition. However, it is difficult to directly observe how magnetic fields influence the grain growth in films and to reveal the control mechanism of HMF.

To better study the influence of HMF, we deposited films using thermal evaporation method [27]. This technique resistively heats the source materials. The grain size, growth rate and component ratio of films can be adjusted at the atomic scale [28]. This technique allowed us to easily determine how HMF influenced the growth of magnetic nanoparticles. In this study, we prepared a nanocrystalline Fe film under HMF by using thermal evaporation. The influence of HMF on the crystal structure and microstructure were studied. We also considered the magnetic properties of the Fe films. Because the structure and properties of the film depend on the growth environment—including the substrate temperature, deposition rate, pressure, and energy [29]—The various substrate temperatures will lead to different thermal energy. It is well known that the grains will be oriented in HMF if the anisotropy energy induced by HMF is greater than the thermal energy [22]. However, there are few reports about the comparison of the effects of HMF and thermal energy on the growth mode of Fe films. Thus in this study, we studied how HMF affected the microstructure and magnetic properties of nanocrystalline Fe thin films at different substrate temperatures. And the different effects of HMF and thermal energy on the growth mode of Fe films were compared. Besides, the control mechanism of HMF on the microstructure and magnetic performance of Fe film at different substrate temperatures is also revealed.

Section snippets

Experimental details

Experimental set-up of thermal evaporation used in this study was described in the Refs. [30], [31] in detail. The magnetic flux density of center magnet can be continuously adjusted from 0 to 6 T. The orientation of HMF was upward and perpendicular to the substrate. According to calculating the geometric position of maximum magnetic flux density in the vacuum chamber, we set the n-type Si(100) substrates at the position of maximum magnetic flux density. The fixed distance of evaporation source

Microstructure

Fig. 1 shows cross-sectional TEM images of the films. When the substrate temperature was RT, the columns were stacked by nano-grains whether the HMF was applied or not, as shown in inset (ii) in Fig. 1(a) and (b). The column in RT 0 T was not completely perpendicular to the substrate. Bending occurred at 100 nm in the column, as shown by the fold dotted line in inset (ii) of Fig. 1(a). The column in the 6 T film was oriented along the direction of the HMF, as shown by inset (ii) of Fig. 1(b).

Discussion

With HMF applied or not, the evaporation rate of the atomic flux in the evaporation sources remained consistent [32], [33]. When the HMF is applied, the upward magnetic force will act on the atomic flux during evaporation in a magnetic field gradient [22], as shown schematically in Fig. 5. The evaporated atoms will increase in energy. That is to say, the evaporated atoms will not only have an average kinetic energy [36]:UKe=32kBT=3.46×1020J,where kB is the Boltzmann constant: 1.38 × 10−23 J/K,

Conclusions

Nanocrystalline Fe films at different substrate temperatures were studied with and without HMF of 6 T. The structural evolution as well as magnetic properties under different conditions was examined. It is found that HMF can effectively increase the wide of column and the particle size of Fe films at RT. However, the effect of HMF on particle size was weakened at the substrate temperature of 400 °C. The increase of substrate temperature leads to the growth mode changed from columnar (RT) to

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51101034 and 51425401) and the Fundamental Research Funds for the Central Universities (Grant Nos. N130509002 and N140902001).

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