Planar Hall study in molecular exchange-bias system

Abstract

Exchange-bias as an interfacial phenomenon is extensively investigated in bilayer films of a ferromagnet (FM) and an antiferromagnet (AFM) with large internal magnetic anisotropy. This mechanism is also observable by replacing the AFM layer with a hard-FM of sufficiently strong magnetic anisotropy; a response that was recently demonstrated in the transport study of Fe/metal-phthalocyanine (MPc) bilayers [Mundlia et al., Phys. Rev. Appl. 14, 024095 (2020)]. In this bilayer system, hybridization with the molecule causes the surface-Fe to become magnetically hard and couple to the bottom soft-Fe layer via magnetic exchange-bias. In this letter, the planar-Hall study in such exchange-biased Fe/MPc devices is performed using cobalt- and vanadyl- phthalocyanine (CoPc and VOPc) molecules with their responses being sensitive to the choice of molecule and to the field-cooling conditions. For the case of zero-field or in-plane field cooling, the planar-Hall signal in Fe/VOPc devices is larger than in the Fe/CoPc devices arising due to the difference in the magnetization rotation pathway during magnetization reversal, with a possible transition through non-co-planar spin-configuration in the Fe/VOPc devices. In the case of the Fe/CoPc device, this rotation pathway could be activated by out-of-plane field-cooling resulting in more than double the increase in the planar Hall signal. We also investigate the variation in interfacial spin-disorder by field-cooling procedures and its effect on exchange-bias and planar Hall signal. The work support the spin-freezing response at the Fe/MPc interface, making this study exciting for further investigations.