Feedback of magnetic nanoparticles susceptibility to their magnetic dipole interaction with ferromagnetic GMR sensor

Abstract

Effect of individual magnetic nanoparticle on magnetization reversal of ferromagnetic layers is well know [1,2 ]. Here we present effect of ferromagnetic surface of the magnetoresistive sensor on magnetic susceptibility of nanoparticles (NPs), deposited on the sensor surface. Deposition of CoFe2O4 nanoparticles on CoFeB/Ta/CoFeB platform, containing two ferromagnetic nanolayers, changes blocking temperature of the NPs. Magnetic dipole interaction between the NPs and the platform effects frequency and field dependences of the blocking temperature of the NPs. Effective barrier of magnetization reversal and blocking temperature of NP, coupled with ferromagnetic platform, are higher, than in NP deposited on diamagnetic substrate. We observed feedback of magnetic susceptibility of cobalt ferrite cubic nanoparticles on magnetic dipole interaction between NPs and ferromagnetic bilaere CoFeB/Ta/CoFeB platform. The main magnetic characteristics of dynamical magnetization reversal in NPs are changed, when they are deposited on the surface of the ferromagnetic platform.Change of the blocking temperature of NPs under the magnetic dipole interaction with platform is caused by increase of the constant of effective magnetic anisotropy of the NPs up to 15-20%.Micromagnetic modeling predicts change of platform magnetization under individual NP up to 10 % that is similar with changes of the blocking temperature, effective magnetic anisotropy and other phenomenological parameters directly determined in experiments (Fig.1).The variations of the magnetizations of the two ferromagnetic layers under alternative magnetic field were visualized by micromagnetic modeling.Obtained results have principal importance for correct data collecting in lab-on-chip devices, where concentration of the NPs or magnetically labeled biology objects should be measured. Change of the AC susceptibility of NPs caused by their magnetic dipole interaction with the platform can valuably affect estimated number of nanoparticles in real devices.  Fig.1. Calculated orientations of local magnetization in top and bottom layers of the CoFeB/Ta/CoFeB platform in presence of ferromagnetic NP on its surface in external magnetic field.