Approaches to tunnel magnetoresistance effect with antiferromagnets.

The tunnel magnetoresistance (TMR) effect is one of the representative phenomena in spintronics. Ferromagnets, which have a net spin polarization, have been utilized for the TMR effect. Recently, by contrast, the TMR effect with antiferromagnets, which do not possess a macroscopic spin polarization, has been proposed, and also been observed in experiments. In this topical review, we discuss recent developments in the TMR effect, particularly focusing on the TMR effect with antiferromagnets. First, we review how the TMR effect can occur in antiferromagnetic tunnel junctions. The Julliere model, which has been conventionally utilized to grasp the TMR effect with ferromagnets, breaks down for the antiferromagnetic TMR effect. Instead, we see that the momentum dependent spin splitting explains the antiferromagnetic TMR effect. After that, we revisit the TMR effect from viewpoint of the local density of states (LDOS). We particularly focus on the LDOS inside the barrier, and show that the product of the LDOS will qualitatively capture the TMR effect not only in the ferromagnetic tunnel junctions but also in the ferrimagnetic and antiferromagnetic tunnel junctions. This method is expected to work usefully for designing magnetic tunnel junctions.