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1. Symmetry-controlled SrRuO ₃ /SrTiO ₃ /SrRuO ₃ magnetic tunnel junctions: spin polarization and its relevance to tunneling magnetoresistance

   https://doi.org/10.1088/1361-648X/ad765f  

   Samanta, Kartik; Tsymbal, Evgeny Y (September 2024, Journal of Physics: Condensed Matter)

   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diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO₃/SrTiO₃/SrRuO₃(001) MTJs is strongly reduced.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. 

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2. Tunneling magnetoresistance in magnetic tunnel junctions with a single ferromagnetic electrode

   https://doi.org/10.1103/PhysRevB.109.174407  

   Samanta, Kartik; Jiang, Yuan-Yuan; Paudel, Tula R; Shao, Ding-Fu; Tsymbal, Evgeny Y (May 2024, Physical Review B)

   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. 

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3. Electronic reconstruction in confined ${\mathrm{SrRuO}}_3$ monolayers

   https://doi.org/10.1103/PhysRevB.110.235104  

   Lamichhane, U; Sankhi, B; Kundu, N; Fabbris, G; Choi, Y; Haskel, D; McChesney, J L; Cao, Yue; Li, J; Bisogni, V; et al (December 2024, Physical Review B)

   We report the observation of an electronic reconstruction in dimensionally controlled ruthenate heterostructures synthesized by pulsed laser deposition. High structural and electronic quality of superlattices comprised of a single SrRuO3 layer inter-spaced with varying thicknesses of insulating SrTiO3 layers was verified by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, reciprocal space mapping, and x-ray absorption spectroscopy. X-ray absorption spectroscopy evidences a confinement-driven evolution of the Ru electronic configuration from the d5L to the d4 state. Significant increases of the spin-orbit coupling are observed in connection with the configuration changes supporting recent works identifying large enhancement of the magnetic anisotropy. The growth of high quality two-dimensional confined ruthenate layers under precisely controlled environments highlights the potential to directly manipulate interlayer coupling and selectively perturb the electronic state in ruthenates in analogy to superconducting Sr2RuO4. 

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4. Large magnetic anisotropy and magnetostriction in thin films of ${\mathrm{CoV}}_2{\mathrm{O}}_4$

   https://doi.org/10.1103/PhysRevMaterials.7.014408  

   Kim, Sangsoo; Thompson, Christie J.; Xin, Yan; Beekman, Christianne (January 2023, Physical Review Materials)

   Spinel cobalt vanadate CoV2O4 has been grown on (001) SrTiO3 substrates. Using torque magnetometry experiments, we find that the previously observed temperature-induced anisotropy change, where the easy axis changes from the out-of-plane [001] direction to a biaxial anisotropy with planar <100> easy axes, occurs in a gradual second-order structural phase transition. This paper characterizes this transition and the magnetic anisotropies in the (001), (100), and (-110) rotation planes, and explores their field dependence up to 30 T. Below 80 K, hysteretic features appear around the hard axes, i.e., the out-of-plane direction in (-110) and (010) rotations and the planar <110> directions in (001) rotations. This is due to a Zeeman energy that originates from the lag of the magnetization with respect to the applied magnetic field as the sample is rotated. The appearance of the hysteresis, which persists up to very high fields, shows that the anisotropy at low temperature is rather strong. Additionally, field-dependent distortions to the symmetry of the torque response in increasing applied fields shows that magnetostriction plays a large role in determining the direction and magnitude of the anisotropy. 

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5. Large and tunable magnetoresistance in van der Waals ferromagnet/semiconductor junctions

   https://doi.org/10.1038/s41467-023-41077-0  

   Zhu, Wenkai; Zhu, Yingmei; Zhou, Tong; Zhang, Xianpeng; Lin, Hailong; Cui, Qirui; Yan, Faguang; Wang, Ziao; Deng, Yongcheng; Yang, Hongxin; et al (December 2023, Nature Communications)

   Magnetic tunnel junctions (MTJs) with conventional bulk ferromagnets separated by a nonmagnetic insulating layer are key building blocks in spintronics for magnetic sensors and memory. A radically different approach of using atomically-thin van der Waals (vdW) materials in MTJs is expected to boost their figure of merit, the tunneling magnetoresistance (TMR), while relaxing the lattice-matching requirements from the epitaxial growth and supporting high-quality integration of dissimilar materials with atomically-sharp interfaces. We report TMR up to 192% at 10 K in all-vdW Fe3GeTe2/GaSe/Fe3GeTe2 MTJs. Remarkably, instead of the usual insulating spacer, this large TMR is realized with a vdW semiconductor GaSe. Integration of semiconductors into the MTJs offers energy-band-tunability, bias dependence, magnetic proximity effects, and spin-dependent optical-selection rules. We demonstrate that not only the magnitude of the TMR is tuned by the semiconductor thickness but also the TMR sign can be reversed by varying the bias voltages, enabling modulation of highly spin-polarized carriers in vdW semiconductors. 

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