Determination of the Saturation Magnetization from Perpendicular Magnetic Anisotropy Measurements

Over the last years the modification of magnetic parameters of thin films and multilayers by ion irradiation and implantation has become fashionable. However, especially in multilayer structures ion irradiation can lead to interfacial mixing and thus to a significant reduction of the magnetically active volume, i.e. 'dead' layers. Thus, it can be quite difficult---if not at all impossible---to determine the correct effective magnetic volume, e.g. for measurements of the saturation magnetization by SQUID. Here we show how ferromagnetic resonance (FMR) and magneto-optical Kerr effect (MOKE) measurements can be utilized to determine the saturation magnetization of irradiated Py/Ta multilayers instead and how the magnetic properties change upon irradiation.
We prepared thin films and multilayers of Py/Ta with an overall Py thickness of 20 nm and varying number of Py/Ta stacks and irradiated them with Ne ions at various fluences. FMR, MOKE, and SQUID magnetometry were used for investigation. With both, increasing ion fluences and increasing number of Py/Ta interfaces, a decrease of saturation magnetization and an increase of precessional damping can be observed. The uniaxial anisotropy of the samples is only of small magnitude and remains almost unaffected. There is, depending on the number of interfaces, a critical ion fluence at which ferromagnetic order in the multilayers vanishes.
This work is supported by DFG grant FA 314/3-1.