Tunable Spin Hall Magnetoresistance in All-Antiferromagnetic Heterostructures

We investigate the spin Hall magnetoresistance (SMR) in all-antiferromagnetic heterostructures \alpha-Fe_2O_3/Cr_2O_3 with Pt contacts. When the temperature is ultralow ( <  50 K), the spin current generated in the Pt layer cannot be transmitted through Cr_2O_3 (t = 4 nm), and the SMR is near zero. Meanwhile, when the temperature is higher than the spin fluctuation temperature T_\rm F (\approx  50 K) of Cr_2O_3 and lower than its Néel temperature T_\rm N (\approx  300 K), the spin current goes through the Cr_2O_3 layer and is reflected at the \alpha-Fe_2O_3/Cr_2O_3 interface; an antiferromagnetic (negative) SMR is observed. As temperature increases higher than T_\rm N, paramagnetic (positive) SMR mainly arises from the spin current reflection at the Cr_2O_3/Pt interface. The transition temperatures from negative to positive SMR are enhanced with increasing Cr_2O_3 layer thickness, accompanied by the absence of SMR signals when t = 10 nm. Such a tunable SMR builds a bridge between spin transport and structures. It also enriches antiferromagnetic spintronics.