Magnetic interlayer coupling in ferromagnet/insulator/ferromagnet structures

Magnetic coupling between ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ (CoFe) and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (NiFe) thin films separated by a ${\mathrm{SiO}}_2$ layer was investigated with magnetization measurements, Mössbauer spectroscopy, and Lorentz imaging. ${\mathrm{SiO}}_2$ thicknesses varied from 0 to 1000 Å. When the spacer layer was thicker than 10 Å, separate reversal of the magnetic layers was observed in the hysteresis loops. The coercivity of a 300 Å NiFe film separated from a 300 Å CoFe film by 20 Å of ${\mathrm{SiO}}_2$ 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 ${\mathrm{SiO}}_2$ thickness. The NiFe showed an enhanced coercivity even with a demagnetized CoFe layer, suggesting that domain walls contribute to the coupling. Mössbauer 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.