Transporting
spin information using magnon currents in magnetic
insulators has garnered considerable interest as a means to advance
information technology. A material’s magnon transport properties
can be tuned indirectly by modifying the magnetic properties, on which
the former heavily depend. Molecular functionalization, in which organic
molecules form a hybrid interface with a substrate material, has been
shown to be a promising approach to modify the magnetic properties
of metallic materials. In this work, we go beyond metals and demonstrate
that the interfacial interaction between the organic molecule cobalt
phthalocyanine (CoPc) and the magnetic insulator Y3Fe5O12 (YIG) modifies the magnetic properties of YIG,
thereby allowing us to tune the transport properties of magnon currents
in this magnetic insulator. Comparing a device with a YIG/CoPc interface
to a simple YIG device, we find that the functionalized device exhibits
larger amplitudes of nonlocal signals for both electrically and thermally
excited magnon currents. Through the measurement of field-dependent
nonlocal magnon signals and ferromagnetic resonance (FMR) in the YIG/CoPc
device, we observe that the interfacial effect changes the magnetic
anisotropy of the YIG, resulting in a larger saturation field. Moreover,
the device with the CoPc overlayer exhibits a larger electrically
excited magnon diffusion length at low temperatures due to the reduced
Gilbert damping constant, as verified by FMR measurements. Our findings
give us insight into the potential use of molecular functionalization
to enhance magnon transport in magnetic insulators through the interfacial
hybridization effect.