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1. Symmetry-driven large tunneling magnetoresistance in ${\mathrm{SrRuO}}_3$ magnetic tunnel junctions with perpendicular magnetic anisotropy

   https://doi.org/10.1103/PhysRevMaterials.8.L091401  

   Goossens, Anouk S; Samanta, Kartik; Jaman, Azminul; Boubaker, Wissem; van_Rijn, Job_J L; Tsymbal, Evgeny Y; Banerjee, Tamalika (September 2024, Physical Review Materials)

   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. 

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2. High Tunnelling Magnetoresistance in Magnetic Tunnel Junctions with Sub-nm thick Al2O3 Tunnel Barriers Fabricated Using Atomic Layer Deposition

   https://doi.org/10.1021/acsami.0c03428  

   Acharya, Jagaran; Goul, Ryan; Wu, Judy Z. (July 2020, ACS Applied Materials & Interfaces)

   Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) is a key to obtaining high tunnelling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including an atomic-scale control on the TB thickness, conformal coating, and low defects density. Motivated by this, this work explores fabrication and characterization of spin-valve Fe/ALD-Al2O3/Fe MTJs with ALD-Al2O3 TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ~77% and ~ 90% have been obtained respectively at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlOx TBs with optimized device structures. In situ scanning tunnelling spectroscopy characterization of the ALD-Al2O3 TBs has revealed a higher tunnel barrier height (Eb) of 1.33±0.06 eV, in contrast to Eb~0.3-0.6 eV for their AlOx TB counterparts, indicative of significantly lower defect concentration in the former. This first success of the MTJs with sub-nm thick ALD-Al2O3 TBs demonstrates the feasibility of in situ ALD for fabrication of pinhole-free and low-defect ultrathin TBs for practical applications and the performance could be further improved through device optimization. 

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3. 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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4. Temperature and bias voltage dependences of magnetic tunnel junction with FeAlSi electrode

   https://doi.org/10.1063/5.0189570  

   Akamatsu, Shoma; Lee, Byung Hun; Hou, Yasen; Tsunoda, Masakiyo; Oogane, Mikihiko; Beach, Geoffrey_S D; Moodera, Jagadeesh S (February 2024, APL Materials)

   We fabricated magnetic tunnel junctions (MTJs) with FeAlSi free layers and investigated the tunnel magnetoresistance (TMR) properties. We found that the temperature and bias voltage dependences of the TMR effect in FeAlSi-MTJs were almost the same as MTJs with Fe free layers despite the low Curie temperature of FeAlSi. In the inelastic electron tunneling spectroscopy measured at low temperatures, the relatively large cutoff energy of magnon excitation at the FeAlSi and MgO interface was confirmed. In addition, we studied for the first time the exchange stiffness constant of FeAlSi films by Brillouin light scattering. The determined value of the stiffness constant of FeAlSi was 14.3 (pJ/m), which was similar to that of Fe. Both the large magnon cutoff at the interface and the stiffness constant of FeAlSi are considered to be the reason for the good temperature and voltage dependences of FeAlSi-MTJs. 

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5. 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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