Barium ferrite films prepared by pulsed laser deposition with a varying substrate temperature

Abstract

Barium femte is one of the candidates for high-density magnetic recording. So far, bilrium femte films have been prepared by sputtering and laser deposition [I-51. These thin films have been obtained by either applying in-situ substrate heating during film growth or through a postannealing process. For in-situ heating, substrates are heated up to a certllin temperature before deposition starting, then substrate temperature is kept constant during film growth. After that, films are cooled down to room temperature. In contrast, for post annealing, films are grown at room temperature or a low temperature. The films are then processed with a post annealing. Both methods have been widely used in spultering and laser deposition. For these two methods, the substrate temperature is always constant during film growth. Since substrate temperature is one of the key parameters to the film quality, it is of interest to investigate the influence of a varying substrate temperature on film properties. In this paper, we report an approach to fabricate barium ferrite films by laser deposition with a varying substrate temperature during film growth. The experimental setup was described in a previous paper 141. During film growth. substrate temperature varied linearly at a rate of 50 Chin and reached 800 C in about 15 minutes. Films were grown for 15 minutes at a laser fluence of about 3 Jlcm'and a repetition rate of 5 Hz. After laser deposition, films were cooled down to room temperature in an oxygen atmosphere. The profile of substrate temperature with time during film growth and after deposition is shown in Fig. I. The magnetization hysteresis loops of the barium ferrite film deposited with a varying substrate temperature are shown in Fig. 2. It is found that the magnetization hysteresis loops are similar for both magnetic fields applicd parallel and perpendicular to the substrate surface, indicating that the film exhibits almost isotropic propeaies. The perpendicular and in-plane coercivities are 4149 and 4291 Oe, respectively. Comparison of the magnetization hysteresis loops of the film with those films deposited with a post annealing suggests the similar magnetic properties such as isotropic properties and high coercivity 151. While the films deposited with insitu beating has large magnetic anirusotropy and lower coervicity [SI. Atomic force microscopy (AFM) profile of the barium ferrite film deposited with a varying substrate temperature is shown in Fig. 3. It was observed that the grains in the film show both circular and elongated shapes, which are similar to those in the film deposited with a post annealing. Since the c-axis is normal to the long axis of the grain for barium femte smcture, the c-axis is in the plane for that elongated shape grain. Therefore, a significant amount of crystals with in-plane c-axis orientation exist in the film except those with c-axis orientation normal tu the film plane. While the grains in the film deposited with in-situ heating have good crystallinity with hexagonal symmetry, indicating that the film is grown with c-axis normal to the film plane. In summary, laser dcposition of barium ferrite films with a varying substrate temperature has becn studied for the first time. The film exhibits both c-axis orientation normal to the film plane and in-plane c-axis orientation and the grains in the film show both circular and elongated shapes. The perpendicular and in-plane coercivities are 4149 and 4291 Oe and the film has almost isotropic properties. The magnetic properties, grain shape and crystalline orientation of the film with a varying substrate temperature during film growth are close to those of the film deposited with a post annealing and different to those of the film deposited with in-situ heating.