Growth, structure, morphology, and magnetic properties of Ni ferrite films
© Dong et al.; licensee Springer. 2013
Received: 25 February 2013
Accepted: 14 April 2013
Published: 27 April 2013
The morphology, structure, and magnetic properties of nickel ferrite (NiFe2O4) films fabricated by radio frequency magnetron sputtering on Si(111) substrate have been investigated as functions of film thickness. Prepared films that have not undergone post-annealing show the better spinel crystal structure with increasing growth time. Meanwhile, the size of grain also increases, which induces the change of magnetic properties: saturation magnetization increased and coercivity increased at first and then decreased. Note that the sample of 10-nm thickness is the superparamagnetic property. Transmission electron microscopy displays that the film grew with a disorder structure at initial growth, then forms spinel crystal structure as its thickness increases, which is relative to lattice matching between substrate Si and NiFe2O4.
Ferrite films have been widely used in computer memory chips, magnetic recording media, frequency filters, and many branches of telecommunication and electronic engineering. In particular, Ni ferrite (NiFe2O4) films with spinel structure were currently of great interest due to their high magnetic permeability, high resistivity, and low losses, making itself a promising material for high-frequency applications. Many methods have been carried out to fabricate ferrites, such as molecular beam epitaxy , pulsed laser deposition [2, 3], spin-spray [4, 5], sol–gel , electrochemical deposition , direct liquid phase precipitation , hydrothermal growth [9, 10], and sputtering [11, 12]. Researches on structural and magnetic properties of ferrites have been devoted recently. Li et al.  have reported that NiZn ferrite can be fabricated under low temperature. However, the magnetic properties of NiZn ferrite films fabricated under low temperature were not as good as bulk status, usually amorphous or with high coercivity (Hc) and low saturation magnetization (Ms) . Usually, high-temperature post-heating treatments or in-situ heating was needed to obtain a better spinel structure and soft magnetic property . But heating treatment was detrimental to the electric circuit integrations, which limited the applications of ferrite films as promising materials for high-frequency devices. Therefore, it was significant to investigate the effect of growth at room temperature (RT) on the structure and magnetic properties of ferrite films.
In this work, Ni ferrite films with different thicknesses (10, 50, 100, 500, and 1,000 nm) were fabricated under RT. Structure and magnetic properties were investigated as functions of thickness. Note that the 10-nm film showed superparamagnetism, different from the other samples (ferromagnetism), which was believed to be caused by the disordered layer discovered by transmission electron microscopy (TEM).
NiFe2O4 ferrite films were deposited onto 20 mm × 20 mm Si(111) substrates attached to a water-cooling system by radio frequency magnetron sputtering with a base pressure below 5 × 10-5 Pa. The mixed gas of argon and oxygen was used as the sputtering gas at total pressure of 2.5 Pa. The sample thickness was controlled by deposition duration. The crystal structure was checked by X-ray diffraction (XRD; X’Pert PRO PHILIPS (Almelo, Netherlands) with CuKα radiation). The images of the surface microstructure were taken using a field emission scanning electron microscope (SEM; S-4800, Hitachi, Ltd., Tokyo, Japan). The magnetic properties were measured using the MPMS magnetometer based on a superconducting quantum interference device (SQUID). The micrograph of the cross-section of the 500-nm NiFe2O4 film was taken by TEM (Tecnai TMG2F30, FEI, Hillsboro, OR, USA).
Results and discussion
Figure 1c shows the in-plane hysteresis loops of the films at different thicknesses at RT. The Hc and Ms with various Ni ferrite film thicknesses are summarized in Figure 1d. Ms increases monotonically with increasing ferrite film thickness, while Hc increases sharply with the film thickness less than 100 nm and then decreases hugely at 500 nm. Note that the 10-nm film shows superparamagnetic behavior with almost zero Hc.
Generally speaking, the Ms of ferrite is related to its crystal structure. For spinel ferrite films, ferromagnetism is induced by oxygen superexchange effect between sites A and B . Therefore, the better spinel crystal structure is, the larger Ms is. In our work, according to the XRD results, the crystal structure becomes better with increasing film thickness, which results in the increase of Ms. However, Hc is attributed to many factors such as grain size, the magnetization (M) reversal process, etc.
Ni ferrite films with different thicknesses were fabricated under RT. Structure and magnetic properties of Ni ferrite films were investigated as functions of thickness: the 10-nm film exhibits superparamagnetism; Ms increases monotonically, while Hc first increases then decreases as the film thickness increases. The SEM and TEM images were taken to investigate the underlying magnetic mechanism. A disordered layer at the bottom of the ferrite layer can be seen in the TEM image; this layer may probably be responsible for the superparamagnetic behavior of the 10-nm film.
This work is supported by the National Basic Research Program of China (grant no. 2012CB933101), the National Science Fund for Distinguished Young Scholars (grant no. 50925103), the Key Grant Project of Chinese Ministry of Education (grant no. 309027), the National Natural Science Foundation of China (grant no. 11034004 and no. 50902064), and the Fundamental Research Funds for Central Universities (lzujbky-2012-31).
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