Tuning structure and roughness in exchange biased NiO/permalloy bilayers

Polycrystalline NiO thin films have been grown by pulsed laser deposition on quartz substrates. These films exhibit a strong texture, which can be tuned by changing deposition parameters such as substrate temperature or oxygen partial pressure. By varying the deposition temperature from room temperature up to 900 deg. C (220), (111), and (200) textured films are prepared. In the temperature zones separating these orientations, competition between different growth directions leads to smaller crystallites, characterized by broader diffraction lines. Surface roughness measured by atomic force microscopy is strongly correlated with these structural features. Roughness is minimum for highly textured samples (about 7Aa for 500 Aa thick films), and it exhibits two peaks in the intermediate zones, with maximum values of about 40 Aa. In order to correlate exchange bias with these structural features, 100 Aa thick FeNi layers were deposited by rf sputtering on top of the 500 Aa thick NiO films. Hysteresis loops were measured at 10 K by superconducting quantum interference device magnetometry after the samples were cooled in a 100 Oe magnetic field. Exchange bias is maximum for (111) oriented samples. No clear correlation between exchange bias and surface roughness is observed at low temperature. Exchange bias temperature dependence strongly depends upon NiO films deposition temperature. The blocking temperature, for which the exchange bias vanishes, varies between 150 K for (220) oriented samples and 250 K for (111) textured samples, and it exceeds room temperature for (200) films