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Altermagnetic (AM) materials have recently attracted significant interest due to their non-relativistic momentum-dependent spin splitting of their electronic band structure which may be useful for antiferromagnetic (AFM) spintronics. So far, however, most research studies have been focused on conducting properties of AM metals and semiconductors, while functional properties of AM insulators have remained largely unexplored. Here, we propose employing AM insulators (AMIs) as efficient spin-filter materials. By analyzing the complex band structure of rutile-type altermagnets MF2 (M = Fe, Co, Ni), we demonstrate that the evanescent states in these AMIs exhibit spin- and momentum-dependent decay rates resulting in momentum-dependent spin polarization of the tunneling current. Using a model of spin-filter tunneling across a spin-dependent potential barrier, we estimate the TMR effect in spin-filter magnetic tunnel junctions (SF-MTJs) that include two magnetically decoupled MF2 (001) barrier layers. We predict a sizable spin-filter TMR ratio of about 150-170% in SF-MTJs based on AMIs CoF2 and NiF2 if the Fermi energy is tuned to be close to the valence band maximum. Our results demonstrate that AMIs provide a viable alternative to conventional spin-filter materials, potentially advancing the development of next-generation AFM spintronic devices. 

Abstract Magnetic tunnel junctions (MTJs), that consist of two ferromagnetic electrodes separated by an insulating barrier layer, have non-trivial fundamental properties associated with spin-dependent tunneling. Especially interesting are fully crystalline MTJs where spin-dependent tunneling is controlled by the symmetry group of wave vector. In this work, using first-principles quantum-transport calculations, we explore spin-dependent tunneling in fully crystalline SrRuO₃/SrTiO₃/SrRuO₃(001) MTJs and predict tunneling magnetoresistance (TMR) of nearly 3000%. We demonstrate that this giant TMR effect is driven by symmetry matching (mismatching) of the incoming and outcoming Bloch states in the SrRuO₃(001) electrodes and evanescent states in the SrTiO₃(001) barrier. We argue that under the conditions of symmetry-controlled transport, spin polarization, whatever definition is used, is not a relevant measure of spin-dependent tunneling. In the presence of diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO₃/SrTiO₃/SrRuO₃(001) MTJs is strongly reduced. Under these conditions, the spin polarization of the interface transmission function becomes a valid measure of TMR. These results provide an important insight into understanding and optimizing TMR in all-oxide MTJs. 

Magnetic tunnel junctions (MTJs) that are comprised of epitaxially-grown complex oxides offer a versatile platform to control the symmetry of tunneling states and tailor magnetic anisotropy useful for practical applications. This work employs thin films of SrTiO3 as an insulating barrier deposited between two ferromagnetic SrRuO3 electrodes to form fully epitaxial MTJs and demonstrate these functionalities. Transport measurements demonstrate large tunneling magnetoresistance (TMR), significantly exceeding previously found values of TMR in MTJs based on SrRuO3 electrodes. These results are explained by perpendicular magnetic anisotropy of SrRuO3 and matching (mismatching) between symmetry and spin across the SrTiO3/SrRuO3 (001) interface for the parallel (antiparallel) MTJ magnetization state, supported by density functional (DFT) calculations. The angular varia- tion of TMR indicates that the SrRuO3 electrodes contain multiple magnetic domains, allowing the devices to exhibit at least three stable resistance states. 

Magnetic tunnel junctions (MTJs) are key components of spintronic devices, such as magnetic random-access memories. Normally, MTJs consist of two ferromagnetic (FM) electrodes separated by an insulating barrier layer. Their key functional property is tunneling magnetoresistance (TMR), which is a change in MTJ's resistance when magnetization of the two electrodes alters from parallel to antiparallel. Here, we demonstrate that TMR can occur in MTJs with a single FM electrode, provided that the counter electrode is an antiferromagnetic (AFM) metal that supports a spin-split band structure and/or a Néel spin current. Using Ru⁢O2 as a representative example of such antiferromagnet and Cr⁢O2 as a FM metal, we design all-rutile Ru⁢O2/Ti⁢O2/Cr⁢O2 MTJs to reveal a non-vanishing TMR. Our first-principles calculations predict that magnetization reversal in Cr⁢O2 significantly changes conductance of the MTJs stacked in the (110) or (001) planes. The predicted giant TMR effect of about 1000% in the (110)-oriented MTJs stems from spin-dependent conduction channels in Cr⁢O2 (110) and Ru⁢O2 (110), whose matching alters with Cr⁢O2 magnetization orientation, while TMR in the (001)-oriented MTJs originates from the Néel spin currents and different effective Ti⁢O2 barrier thickness for two magnetic sublattices that can be engineered by the alternating deposition of Ti⁢O2 and Cr⁢O2 monolayers. Our results demonstrate a possibility of a sizable TMR in MTJs with a single FM electrode and offer a practical test for using the antiferromagnet Ru⁢O2 in functional spintronic devices.