Determination of the Saturation Magnetization from Perpendicular Magnetic Anisotropy Measurements of Ion Irradiated Multilayers

Ion beam irradiation and ion implantation of ferromagnetic films is a smart technique to tailor their magnetic properties and structural composition of multilayers or nanostructured samples [1,2]. Metals like Ta are commonly used as seed and cap layers in spintronic devices like Giant Magneto-Resistance sensors as Ta is chemically stable. However, it is known that 12% of Ta intermixing in Py leads to magnetically dead layers of 0.6-1.2 nm in thickness [3]. These dead layers make it impossible to determine the correct magnetic volume, which is needed to obtain the saturation magnetization from the magnetic moment measured e.g. by SQUID. This is especially true for multilayer samples which typically have a large number of interfaces.
Here we present a method to determine the saturation magnetization of Py/Ta multilayers from VNA-FMR (Vector Network Analyzer Ferromagnetic Resonance) and MOKE (Magneto-optical Kerr Effect) measurements even in the case of interfacial mixing due to ion irradiation, where SQUID magnetometry fails due to the unknown magnetic volume. Three sets of Py/Ta thin film multilayer systems were sputter-deposited on a Si/SiO2 substrate: (1xPy) is a single 20 nm thick Py layer, (5×Py) a multilayer of the structure 31 nm Ta/[4 nm Py/1 nm Ta]5/2 nm Ta and (10xPy) a multilayer of 30.5 nm Ta/[2 nm Py/0.5 nm Ta]10/2.5 nm Ta. The overall Py amount was always 20 nm and the total Ta thickness including seed and cap layer corresponds to 38 nm. Finally, the films have been irradiated with Ne ions at 40 keV with ion fluences in the range of 5×1013 to 5×1016 Ne/cm2. FMR shows that the FMR frequency vs. field dependence is significantly influenced by the amount of irradiation and number of interfaces (see Fig 1). At fluences above 2.5×1015 Ne/cm2 a significant decrease of the resonance frequency can be observed for the 1xPy samples. For the three unirradiated samples the FMR frequency decreases with an increasing number of Py/Ta repetitions, i.e. increasing number of interfaces. This decrease will be even more pronounced if larger numbers of interfaces are used. This can be explained by the higher number of neighboring Ta atoms in those cases. The deleterious effect of Ta on the ferromagnetic properties is becoming much stronger reducing the effective ferromagnetic film thickness by creating magnetically dead layers close to the interface [1,3-6]. This reduction of the saturation magnetization is directly linked to the resonance frequency. This allow to determine not only the uniaxial in-plane anisotropy field K2|| but also the saturation magnetization μ0Ms from the FMR frequency vs. field dependence. However, this can only be done, if there is only shape anisotropy but no uniaxial out-of-plane anisotropy as it is the case for our Py/Ta multilayers. From polar MOKE loops μ0Ms can be obtained by determining the perpendicular anisotropy field of the samples as well. This feature proves to be very useful here since SQUID magnetometry suffers from a major drawback: It requires the exact effective Py volume to calculate μ0Ms from the magnetic moment, which becomes more and more difficult to determine due to the increasing interfacial mixing induced by the ion irradiation and thinner Py layers. In our case, polar MOKE and FMR represent suitable alternatives since they allow μ0Ms to be determined without the knowledge of any film thickness. The good agreement with the SQUID data of the non-irradiated samples supports this (see Fig 2). ...