Magnetic and magnetotransport properties of bi-component arrays of magnetic dots by self-assembling of polystyrene nanospheres

Abstract

Composite magnetic materials where magnetic phases presenting different coercivities are coupled by exchange interaction at the nanometer scale were introduced as a potential route to improve magnetic properties of multilayer systems for potential applications in different technological fields such as permanent magnets, information storage media and magnetic microactuators. In this context, antidot structures in which the holes are filled with another ferromagnetic material emerged of interest in magnonic applications for unique static and dynamic properties [1]. In this context, the static behaviors of magnetic hysteresis and magnetotransport response have been investigated in Ni 80 Fe 20 antidot filled with Co nanostructures. A modification in the magnetic behavior and magnetotransport properties is observed in bi-component sample (arrays of Co dots dispersed in a Ni 80 Fe 20 magnetically soft matrix) with respect to the constituting systems (Co dot arrays and Ni 80 Fe 20 antidot arrays) and can be ascribed the coupling between the two ferromagnetic materials. To synthesize such a bi-component material, a continuous Co layer having thickness ranging in the interval 20–40 nm has been deposited by sputtering on a Si/SiO 2 substrate (oxide thickness 300 nm). The exploited patterning process is polystyrene nanosphere lithography by depositing a layer of 500 nm diameter nanospheres on the continuous magnetic film [1]. The sphere diameter is then reduced by plasma etching in Ar to reach dimension around 400 nm and sputter etching is performed to remove the residual magnetic film among the spheres. At this stage, by using the nanospheres as a mask, a Ni 80 Fe 20 film having thickness around 30 nm, lower with respect to Co layer, is then deposited. The nanospheres are finally removed by sonication ending up with Co dots embedded in a Ni 80 Fe 20 matrix. The sample microstructure is visible in a Scanning Electron microscopy image shown in Fig.1a: on the top of the Co dots the nanospheres are still visible and the loss of their spherical shape is due to the etching process. Magnetisation reversal has been studied by magnetic force microscopy (MFM) at magnetisation remanence and as a function of magnetic field. Room-temperature magnetic hysteresis loops have been measured by alternated-gradient force magnetometer after each preparative step disentangling the contribution of each magnetic component and evaluating the exchange coupling effect in the final configuration. As it can be observed in the curves reported in Fig. 1 b, the hysteresis behavior of the bi-component structure displays both the features of the Ni 80 Fe 20 antidot array (low-coercivity and high magnetic permeability) and of the Co dot array (larger coercivity). In addition, the fingerprint of the presence of magnetic vortex is visible. SQUID magnetometry has been employed to measure temperature behavior of hysteresis loops in the two nanostructure arrays and in the bicomponent sample ranging in the interval 5–300 K. First-order-reversal-curves (FORCs) consisting in magnetization curves measured at varying field and magnetization values located inside the major hysteresis loop have been measured in all films in order to evaluate magnetic interactions among different magnetic phases. Bi-compo-nent magnetic materials composed by compositions having different coercivity values represent indeed a suitable class of system to be studied. This experimental procedure also allowed to put in evidence the presence of an exchange-bias occurring at low T and related to the presence of Co oxide.