  Alpha-phase (a-) RuCl_3 has emerged as a prime candidate for a quantum spin
liquid (QSL) that promises exotic quasiparticles relevant for fault-tolerant
quantum computation. Here, we report spin sensitive transport measurements to
probe spin correlation in a-RuCl_3 using a proximal spin Hall metal platinum
(Pt). Both transverse and longitudinal resistivities exhibit oscillations as
function of the angle between an in-plane magnetic field and the current, akin
to previously measured spin Hall magnetoresistance (SMR) in antiferromagnet/Pt
heterostructures. The oscillations are observed from 1.5 T to 18 T, both within
and beyond the magnetic field range where the antiferromagnetic order and QSL
state are reported in a-RuCl_3. The SMR oscillations show that spins in a-RuCl3
are largely locked to an in-plane quantization axis transverse to the magnetic
field, constituting a continuous-symmetry-broken state that does not
necessarily represent a long-range order. This robust anisotropy of spin axis
uncovers critical energy scales connected with reported QSL signatures in
a-RuCl_3. Simulations suggest a predominantly antiferromagnetic correlation to
moderately high magnetic-fields, that may support the SMR oscillations. The
coupling of the spin states within a-RuCl_3 and Pt demonstrated in our
experiment opens a transport route to exploring exotic spin phases and device
functionalities of QSL materials.
