Controlling magnetic configuration in soft-hard bilayers probed by polarized neutron reflectometry

TL;DR

This study uses polarized neutron reflectometry to investigate how layer thickness, saturation magnetization, temperature, and magnetic fields influence the magnetic configurations in hard/soft bilayer thin films with perpendicular anisotropy, informing better design of composite magnets.

Contribution

It provides experimental insights into how soft layer thickness, saturation magnetization, and external conditions affect interfacial magnetic configurations in hard/soft bilayers, supported by micromagnetic simulations.

Findings

Interfacial magnetic configuration depends strongly on soft layer thickness and saturation magnetization.

Thin soft layers exhibit rigid out-of-plane coupling, transitioning to in-plane relaxation with increased thickness.

External magnetic fields and temperature can delay the transition from out-of-plane to in-plane magnetic states.

Abstract

Hard/soft magnetic bilayer thin films have been widely used in data storage technologies and permanent magnet applications. The magnetic configuration and response to temperatures and magnetic fields in these heterostructures are considered to be highly dependent on the interfacial coupling. However, the intrinsic properties of each of the layers, such as the saturation magnetization and layer thickness, also strongly influence the magnetic configuration. Changing these parameters provides an effective method to tailor magnetic properties in composite magnets. Here, we use polarized neutron reflectometry (PNR) to experimentally probe the interfacial magnetic configurations in hard/soft bilayer thin films: L10-FePt/A1-FePt, [Co/Pd] /CoPd, [Co/Pt] /FeNi and L10-FePt/Fe, which all have a perpendicular magnetic anisotropy in the hard layer. These films were designed with different soft and hard layer thicknesses (t_soft and t_hard) and saturation magnetization (M_s^soft and M_s^hard), respectively. The influences of an in-plane magnetic field (H_ip) and temperature (T) are also studied using a L10 FePt/A1-FePt bilayer sample. Comparing the PNR results to micromagnetic simulations reveals that the interfacial magnetic configuration is highly dependent on t_soft, M_s^soft and the external factors (H_ip and T), and has a relatively weak dependence on t_hard and M_s^hard. Key among these results, for thin t_soft, the hard and soft layers are rigidly coupled in the out-of-plane direction, then undergo a transition to relax in-plane. This transition can be delayed to larger t_soft by decreasing M_s^soft. Understanding the influence of these parameters on the magnetic configuration is critical to designing functional composite magnets for applications.