Magnetic interlayer coupling in ferromagnet/insulator/ferromagnet structures

Magnetic coupling between Co50Fe50 (CoFe) and Ni81Fe19 (NiFe) thin films separated by a SiO2 layer was investigated with magnetization measurements, Mossbauer spectroscopy, and Lorentz imaging. SiO2 thicknesses varied from 0 to 1000 Angstrom. When the spacer layer was thicker than 10 A, separate reversal of the magnetic layers was observed in the hysteresis loops. The coercivity of a 300 Angstrom NiFe film separated from a 300 Angstrom CoFe film by 20 Angstrom of SiO2 was about 50 Oe, compared to 1 Oe for a free NiFe layer. The coercive field of the NiFe decreased and the magnetization reversal became sharper with increasing SiO2 thickness. The NiFe showed an enhanced coercivity even with a demagnetized CoFe layer, suggesting that domain walls contribute to the coupling. Mossbauer measurements in zero applied field confirmed that the spin dispersion of the NiFe layer resembled the CoFe dispersion in strongly coupled trilayers, but that the NiFe spins were nearly collinear with the easy axis in trilayers with small coupling. Lorentz imaging of single magnetic layer samples showed a complex, immobile domain-wall structure in the CoFe, but only ripple structure was observed in the NiFe. The Lorentz images of trilayers suggested that magnetostatic coupling between domain walls in the CoFe and induced walls and ripple structure in the NiFe resulted in the enhanced NiFe coercivity.