Temperature-dependent sign reversal of tunneling magnetoresistance in van der Waals ferromagnetic heterojunctions

Magnetic tunnel junctions (MTJs) are the elemental devices for advanced spintronic technologies, where tunneling magnetoresistance (TMR) serves as one of the key performance metrics. Here, we used the topologically nontrivial magnetic insulator CrVI₆ and magnetic metal Fe₃GeTe₂ to fabricate CrVI₆/Fe₃GeTe₂ heterojunctions and Fe₃GeTe₂/CrVI₆/Fe₃GeTe₂ MTJs. In the heterojunctions, the addition of CrVI₆ led to a 180% coercive field enhancement of Fe₃GeTe₂ near the Curie temperature (T_C) of CrVI₆, which originated from the antiferromagnetic coupling between them. More importantly, a temperature-dependent TMR sign reversal in the Fe₃GeTe₂/CrVI₆/Fe₃GeTe₂ MTJ was observed, gradually transforming from a negative value at low temperature to a positive value above 60 K, close to the T_C of CrVI₆. The results demonstrate that the spin-filtering effect, with -opposite polarity of CrVI₆ to Fe₃GeTe₂, constitutes the primary mechanism for temperature-dependent TMR sign reversal. Furthermore, under combined temperature and bias voltage modulation, we observed the coexistence of both positive and negative TMR. This finding suggests the potential extension of conventional bistate MTJ operation to multi-state functionality. This research broadens the controllable degrees of freedom in MTJs and advances van der Waals magnet-based spintronic devices.