Probing interfacial coupling in cobalt/metal-free phthalocyanine and cobalt/C60 heterostructures

Abstract

Exchange bias is experimentally realized when the magnetization loop is shifted from the zero magnetic field, and used in spin valve sensors to pin their hard reference layer, nullifying the subsequent effect of any variation in the sensor layers. In this regard, fundamental insights into magnetic behavior of sufficiently thin films are important for tuning their properties, and are critical for better understanding of surface phenomena in nanodevices. Here, the hysteresis loop shift phenomenon is investigated for nanoscale ferromagnetic-molecular layers using a superconducting quantum interference device (SQUID) over a wide temperature range (2–300 $K$). Two heterostructure systems consisting of cobalt ($Co$) nanolayers in contact with metal-free phthalocyanine ($H_{2}Pc$) planar and fullerene ($C60$) spherical molecules are fabricated using organic molecular beam epitaxy and thermal evaporation methods, demonstrating the effect of surface phenomena on magnetic characteristics. For two stacks of $Co$/ $H_{2}Pc$ and $Co$/ $C60$ molecular layers, SQUID measurements indicate the presence of interfacial unidirectional anisotropy (i.e., the anisotropy that is effective only in one direction at the interface between thin films) in the temperature range below $20K$. The magnitude of the strength of interactions through the shift and coupling energy is computed for both hybrid stacks, and compared with that for $Co$-magnetic and nonmagnetic transition metal $H_{2}Pc$ structures. Therefore, the influence of magnetic hardening of $H_{2}Pc$ and $C60$ molecular layers is confirmed for the bias field. By contrast, antiparallel spin alignment inside the molecular layer results from magnetic ions in the organic layer detached from the contact area, being responsible for large value of exchange bias. Accordingly, such heterostructured layers may act as pinning sites, paving the way for designing cost effective molecular spintronics and spin logic devices with faster data processing, ecofriendly, and high flexibility.