Session R19: Magnetic interactions at complex oxide interfaces

Efficient bias-driven magnetization control by orbital selection at a La0.67Sr0.33MnO3 interface

Bias-driven magnetization control of a ferromagnet, which utilizes the ability of controlling the magnetic anisotropy (MA) with an electric field, is crucial for spintronic applications due to its low power consumption. However, it remains a challenge to induce a large change in the MA of ferromagnetic materials that can rotate the magnetization. In this work, using La_0.67Sr_0.33MnO₃ (LSMO)/SrTiO₃ (STO)/LSMO magnetic tunnel junctions (MTJs), we demonstrate that a drastic change in the MA of LSMO can be induced when the chemical potential (E_F) at the LSMO/STO interface is moved between bands with different orbital symmetries. By this new approach, we successfully realize a deterministic and magnetic-field-free 90°-magnetization switching of LSMO solely by applying a small electric field of 0.05 V/nm on the tunnel barrier, with a negligibly small current density of ~ 10^–2 A/cm².
The studied MTJs are grown on an STO (001) substrate by molecular beam epitaxy. We probe the orbital symmetry of the carriers and the MA of the LSMO layers at each bias V by measuring the magnetic-field-direction dependence of the tunneling conductance and that of the tunneling magnetoresistance (TMR), respectively. Using this approach, we show that, with applying V, the MA of the LSMO switches from a two-fold symmetry to a four-fold symmetry by shifting E_F from the e_g band to the t_2g band [1]. This change of MA is strong enough to rotate the magnetization direction from [110] to [1-10] without any assisting magnetic field. Our findings indicate that highly efficient magnetization control can be realized by designing materials so that the E_F lies close to the band edges of different-symmetry orbitals [2].
[1] L. D. Anh et al., Sci. Rep. 7, 8715 (2017). [2] L. D. Anh et al., Phys. Rev. Applied. 12, 041001 (2019).   

Spin-orbit coupling and local magnetism at a complex oxide interface

SrTiO₃-based thin-film heterostructures are a powerful platform for studying a wide array of electronic phases in two dimensions, at high carrier densities. This talk will discuss low-temperature electronic transport studies of NdTiO₃/SrTiO₃ interfaces, which reveal local ferromagnetic order and strong spin-orbit interaction. As the magnetic field angle is gradually tilted away from the sample normal, the data reveals an intriguing interplay between strong k-cubic Rashba-type spin-orbit coupling and a substantial magnetic exchange interaction from local magnetic regions. The resulting quantum corrections to the conduction are in excellent agreement with existing models and allow sensitive determination of the small magnetic moments (22 µ_B on average), their magnetic anisotropy and mutual coupling strength. This effect is expected to arise in other 2D magnetic materials systems and could provide a simple yet powerful tool for investigating local magnetism.

Reference: Xinxin Cai, Yilikal Ayino, Jin Yue, Peng Xu, Bharat Jalan, and Vlad S. Pribiag, Phys. Rev. B 100, 081402(R) (2019).   

Interfacial control of chiral magnetic interactions and Hall effect in Iridate-manganite superlattices

One of the most intriguing outcomes of symmetry breaking and spin-obit interactions in magnetic systems is the possibility to create non-collinear and chiral spin textures. The Dzyaloshinskii-Moriya interaction (DMI) results from strong spin-orbit coupling and broken inversion symmetry to generate magnetization rotations with fixed chirality. The discovery of magnetic skyrmions originating from strong DMI in metal thin films has led to an explosion of efforts to manipulate magnetic phases originating from interfaces. I will describe our progress to understand interface-induced magnetism in epitaxial 3d/5d iridate/manganite superlattices. Our previous work on high-quality epitaxially stabilized SrMnO₃/SrIrO₃ superlattices revealed charge-transfer induced interfacial collinear ferromagnetism and an anomalous Hall effect [1,2]. In LaMnO₃/SrIrO₃ superlattices, we find a large additional topological Hall effect arising from the interaction of charge carriers with a noncoplanar chiral spin texture induced by interfacial DMI [3]. I will describe how the interfacial atomic layer stacking and symmetry enabled by the nonmagnetic A-sites determine the competition between collinear and chiral magnetic interactions originating from the oxide interface. These results will be compared with magnetometry measurements, soft and hard x-ray experiments, which provide a comparison of the bulk and interface electronic and magnetic properties. Our findings provide insight to the manipulation of chiral magnetism from atomic-scale control of DMI at oxide interfaces.

[1] J. Nichols et al., Nat. Commun. 7, 12721 (2016).
[2] S. Okamoto et al., Nano Lett. 17, 2126 (2017).
[3] P. Bruno et al., Phys. Rev. Lett. 93, 096806 (2004).   

Spin to charge conversion in the topological insulator HgTe and in STO-based two-dimensional electron gas

While classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, a new approach called spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling in non-magnetic systems. However, the interconversion efficiency of the Hall effect is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics.

In this contribution, we first focus on strained mercury telluride, using spin pumping experiments at room temperature. We show that a HgCdTe barrier can be used to protect the HgTe topological surface states, leading to high conversion rates, with inverse Edelstein lengths up to 2.0±0.5 nm. These measurements, associated with the temperature dependence of the resistivity, suggest that these high conversion rates are due to the spin momentum locking property of HgTe surface states [1].

We then focus on the SrTiO3 (STO)-based 2D electron system, presenting experiments performed on NiFe/Al/STO heterostructures. We investigate the nature of the spin-to-charge conversion through a combination of spin pumping, magnetotransport, spectroscopy and gating experiments, finding a very highly efficient spin-to-charge conversion, with inverse Edelstein lengths beyond 20 nm. More importantly, we demonstrate that the conversion rate can be tuned in amplitude and rate by a gate voltage. We then discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve efficient spin-to-charge interconversion.

[1] P. Noel et al., Phys. Rev. Lett. 120, 167201 (2018)
[2] D. C. Vaz et al., Nature Materials 1-7 (2019)   

Symmetry Control of Emergent Magnetism in 2D Oxides and Oxide Heterostructures

The transition metal oxides exhibit a variety of interesting properties that are important for many practical applications, such as the dielectrics, magnetism, ferroelectricity, superconductivity, multiferroicity, etc. Such a rich plethora of functional properties primarily originate from the hybridized bands between the transition metal and the oxygen atoms. In artificially heterostructured oxide thin films, there exist the exciting new properties that are absent in the bulk materials, i.e. the emergent properties in oxide heterostructures. The understanding of the fundamental physics behind these emergent properties remains a huge challenge in condensed matter physics and attracts a lot of research interests in recent days. In this talk, I will introduce our recent advances in understanding the emergent magnetism in 2D oxide ferromagnets and a number of magnetic oxide heterostructures, from the symmetry aspects of the oxygen sublattice, including the octahedral rotations, tilts and the oxygen vacancies. Our study reveals the importance of the symmetry of the oxygen sublattice in shaping the emergent magnetism in low dimensional correlated oxide heterostructures.