  The spin Hall magnetoresistance (SMR) effect arises from spin-transfer
processes across the interface between a spin Hall active metal and an
insulating magnet. While the SMR response of ferrimagnetic and
antiferromagnetic insulators has been studied extensively, the SMR of a
paramagnetic spin ensemble is not well established. Thus, we investigate herein
the magnetoresistive response of as-deposited yttrium iron garnet/platinum thin
film bilayers as a function of the orientation and the amplitude of an
externally applied magnetic field. Structural and magnetic characterization
show no evidence for crystalline order or spontaneous magnetization in the
yttrium iron garnet layer. Nevertheless, we observe a clear magnetoresistance
response with a dependence on the magnetic field orientation characteristic for
the SMR. We propose two models for the origin of the SMR response in
paramagnetic insulator/Pt heterostructures. The first model describes the SMR
of an ensemble of non-interacting paramagnetic moments, while the second model
describes the magnetoresistance arising by considering the total net moment.
Interestingly, our experimental data are consistently described by the net
moment picture, in contrast to the situation in compensated ferrimagnets or
antiferromagnets.
