Substrate conformal imprint fabrication process of synthetic antiferromagnetic nanoplatelets

Abstract

Methods to fabricate and characterize monodisperse magnetic nanoplatelets for fluid/bio-based applications based on spintronic thin-film principles are a challenge. This is due to the required top-down approach where the transfer of optimized blanket films to free particles in a fluid while preserving the magnetic properties is an uncharted field. Here, we explore the use of substrate conformal imprint lithography (SCIL) as a fast and cost-effective fabrication route. We analyze the size distribution of nominal 1.8 μm and 120 nm diameter platelets and show the effect of the fabrication steps on the magnetic properties which we explain through changes in the dominant magnetization reversal mechanism as the size decreases. We show that SCIL allows for efficient large-scale platelet fabrication and discuss how application-specific requirements can be solved via process and material engineering.

Optical Monitoring of the Magnetization Switching of Single Synthetic-Antiferromagnetic Nanoplatelets with Perpendicular Magnetic Anisotropy

“…The SQUID measurements of major and minor loops in Figure 1c show the typical SAF behavior. 1,5,6,10 At a low magnetic field, the total magnetization is 0 due to the antiferromagnetic coupling of the top and bottom CoB layers of nearly equal magnetic moments. Increasing the external field leads to an on-switch at B on of one of the CoB layers, giving B rkky = 127 mT and B c = 32 mT, calculated from the minor loop.…”

“…The NPs used in the study consist of the following film stack: Ta(4)/Pt(2)/Co 80 B 20 (0.8)/Pt­(0.4)/Ru­(0.8)/Pt­(0.4)/Co 80 B 20 (0.8)/Pt­(2)/Ta­(4) with thickness in nanometer indicated between parentheses (total thickness 15.2 nm). The stack was fabricated through magnetron sputter deposition on Si substrates and patterned via substrate conformal imprint lithography (SCIL) and a lift-off procedure into a liquid environment (see Supporting Information, Section 1 and ref ). The NPs dispersed in solution are termed “released”, and the NPs which are still attached to the substrate, i.e., before release, are termed “unreleased”.…”

“…Magnetic nanoparticles (NPs) have shown promising applications in biomedicine. − Among magnetic NPs, synthetic antiferromagnetic (SAF) systems with a large perpendicular anisotropy (PMA) are of interest in various nanoscale torque-transfer-related applications , due to their large magnetic and shape anisotropy. The SAF structure is composed of two ferromagnetic layers which are antiferromagnetically coupled by a spacer layer through the Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction. − In this case, the structure shows a 0 net magnetic moment at low applied magnetic fields.…”

“…Knowing the switching fields ( B on and B off ), which consist of the RKKY coupling field B rkky and a stochastic coercive field B c , needed to magnetically (de-)­activate the NPs is a key requirement. Several reports , have found that ensembles of PMA-SAF nanostructures are characterized by large switching field distributions (SFDs) reflecting the degree of particle-to-particle heterogeneity of their magnetic properties, whereas a well-defined B on and B off and narrow SFDs are preferred for applications. The broad SFDs are understood by considering the switching mechanism of ultrathin PMA nanostructures, assigned to stochastic thermally activated nucleation of a small magnetic domain, followed by fast domain wall propagation. − These nucleation centers, which have variable density and broaden the SFD, are crystal defects in the nanostructure and fabrication-induced defects.…”