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High Coercivity Co-ferrite Thin Films

Magnetic films with high coercivity are interesting for many applications, including high density magnetic recording, MEMS, magnetic-optical devices and biosensors. Today, FePt films are being extensively investigated, because of their excellent magnetic properties. However, the high cost of FePt may limit its applications. In our research work, we have developed high coercivity Co-ferrite films and studied their coercivity mechanisms. Compared to FePt and other rare-earth magnets, Co-ferrite with the formula, CoFe2O4, has advantages such as low cost, good chemical stability (high corrosion resistance) and excellent mechanical properties.

Recently, high coercivity has been achieved in Co-ferrite thin films prepared by using Pulsed Laser Deposition (PLD). Figure 1 shows the hysteresis loop of a 33 nm thick CoFe2O4 film deposited at 550°C by PLD. The film possessed a coercivity of 12.5 kOe, the highest value we believe, that has ever been obtained. Besides the high coercivity, the film also has a high remanence ratio and a perpendicular magnetic anisotropy of approximately 6x106 erg/cm³

The film also possesses a nano-grained structure (as shown in Fig. 2), good surface smoothness and a (440) texture. These properties are certainly very promising for high-density magnetic recording media.

In our previous work, high coercivity of over 9 kOe was obtained in sputtering-derived Co-ferrite thin films. We have observed that high coercivity was associated with a unique microstructure including lattice expansion, residual strain and high density of defects. In order to understand the coercivity mechanisms, we have studied Co-ferrite powders with different particle sizes by high-energy mechanical milling. As shown in Figure 3, magnetic properties of sub-micron Co-ferrite powders are sensitive to the milling time. High coercivity values of 5-6 kOe can be achieved. Our work has shown clearly that high coercivity is generated by a high density of defects. A possible mechanism to explain high coercivity is the induced stress anisotropy plus the pinning effect arising from the presence of defects.

In summary, high coercivity of greater than 10 kOe has been achieved in Co-ferrite thin films. These films also possess other properties, such as good smoothness, nanostructure and crystallographic texture. Therefore, they are very promising for hard magnetic applications, including high-density magnetic recording and MEMS. The high coercivity is probably associated with induced stress anisotropy and pinning effect.

Contact Person : Assoc Prof J Ding

E-mail: msedingj@nus.edu.sg

Tel : 6516 4631
Fax: 6776 3604