Search

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics., In the typical spin-hall effect, spin-current, charge current, and spin polarisation are all mutually perpendicular, a feature enforced by symmetry. Here, using an anti-ferromagnet with a triangular spin structure, the authors demonstrate a spin-hall effect without a perpendicular spin alignment.

Antiferromagnetic (AF) thin films typically exhibit a multidomain state, and control of the AF N\'eel vector is challenging, as AF materials are robust to magnetic perturbations. In this paper, uniaxial N\'eel vector control is demonstrated by relying on anisotropic strain engineering of epitaxial thin films of the prototypical AF material ${\mathrm{LaFeO}}_{3}$ (LFO). Orthorhombic (011)- and (101)-oriented ${\mathrm{DyScO}}_{3}$, ${\mathrm{GdScO}}_{3}$, and ${\mathrm{NdGaO}}_{3}$ substrates are used to engineer different anisotropic in-plane strain states. The anisotropic in-plane strain stabilizes structurally monodomain monoclinic LFO thin films. The uniaxial N\'eel vector is found along the tensile strained $b$ axis, contrary to bulk LFO having the N\'eel vector along the shorter $a$ axis, and no magnetic domains are found. Hence, anisotropic strain engineering is a viable tool for designing unique functional responses, further enabling AF materials for mesoscopic device technology.

We investigate the effect of strain and film thickness on the orbital and magnetic properties of ${\mathrm{LaSrCrO}}_{3}\phantom{\rule{0.16em}{0ex}}(\mathrm{LSCO})/{\mathrm{LaSrMnO}}_{3}$ (LSMO) heterostructures using bulk magnetometry, soft x-ray magnetic spectroscopy, first-principles density-functional theory, high-resolution electron microscopy, and x-ray diffraction. We observe an antiparallel ordering of the magnetic moments between the ferromagnetic LSMO layers and the LSCO spacers, leading to a strain-independent ferromagnetic ground state of the LSCO/LSMO heterostructures for LSMO layers as thin as two unit cells. As the LSMO thickness is increased, a net ferromagnetic state is maintained, however, the average magnetic moment per Mn is found to be dependent on the magnitude of the substrate-induced strain. The differences in the magnetic responses are related to preferential occupation of the Mn ${x}^{2}\ensuremath{-}{y}^{2}$ (in-plane) $d$ orbitals for tensile strain and $3{z}^{2}\ensuremath{-}{r}^{2}$ (out-of-plane) orbitals under compressive strain, leading to competing ferromagnetic and antiferromagnetic exchange interactions within the LSMO layers. These results underscore the relative contributions of orbital, structural, and spin degrees of freedom and their tunability in atomically thin crystalline complex oxide layers.

Determining the three-dimensional (3D) crystallography of a material with subnanometer resolution is essential to understanding strain effects in epitaxial thin films. A scanning transmission electron microscopy imaging technique is demonstrated that visualizes the presence and strength of atomic movements leading to a period doubling of the unit cell along the beam direction, using the intensity in an extra Laue zone ring in the back focal plane recorded using a pixelated detector method. This method is used together with conventional atomic resolution imaging in the plane perpendicular to the beam direction to gain information about the 3D crystal structure in an epitaxial thin film of LaFeO3 sandwiched between a substrate of (111) SrTiO3 and a top layer of La0.7Sr0.3MnO3. It is found that a hitherto unreported structure of LaFeO3 is formed under the unusual combination of compressive strain and (111) growth, which is triclinic with a periodicity doubling from primitive perovskite along one of the three ⟨110⟩ directions lying in the growth plane. This results from a combination of La-site modulation along the beam direction, and modulation of oxygen positions resulting from octahedral tilting. This transition to the period-doubled cell is suppressed near both the substrate and near the La0.7Sr0.3MnO3 top layer due to the clamping of the octahedral tilting by the absence of tilting in the substrate and due to an incompatible tilt pattern being present in the La0.7Sr0.3MnO3 layer. This work shows a rapid and easy way of scanning for such transitions in thin films or other systems where disorder-order transitions or domain structures may be present and does not require the use of atomic resolution imaging, and could be done on any scanning transmission electron microscopy instrument equipped with a suitable camera.

Exchange bias occurs in field-cooled antiferromagnet/ferromagnet systems and can most often be explained in terms of uncompensated magnetic moments at the interface that are pinned in their orientation during field cooling. The presence of spin-flop coupling is often associated with spin-compensated interfaces. Here, we report exchange bias in complex oxide heterostructures of antiferromagnetic ${\mathrm{LaFeO}}_{3}$ and thin layers of ferromagnetic $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ with several intriguing features. The exchange bias does not require field cooling but can also be obtained by applying a setting field at elevated temperature. Furthermore, the exchange bias is positive for setting fields up to 3 T, and its magnitude is strongly dependent on the setting-field strength. X-ray magnetic linear dichroism measurements show a predominantly perpendicular spin configuration at the interface. We discuss the possibility of the exchange bias being driven by a net moment from spin canting in the antiferromagnet due to Dzyaloshinkii-Moriya interactions.

We have investigated the magnetodynamic properties of La0.7Sr0.3MnO3 (LSMO) films of thickness 10, 15 and 30 nm grown on (111)-oriented SrTiO3 (STO) substrates by pulsed laser deposition. Ferromagnetic resonance (FMR) experiments were performed in the temperature range 100–300 K, and the magnetodynamic damping parameter α was extracted as a function of both film thickness and temperature. We found that the damping is lowest for the intermediate film thickness of 15 nm with α≈2·10−3, where α is relatively constant as a function of temperature well below the Curie temperature of the respective films., This work was supported by the Norwegian Research Council (NFR), project number 216700. V.F acknowledge partial funding obtained from the Norwegian PhD Network on Nanotechnology for Microsystems, which is sponsored by the Research Council of Norway, Division for Science, under contract no. 221860/F40. F.M. acknowledges financial support from RYC-2014-16515 and from the MINECO through the Severo Ochoa Program (SEV- 2015-0496). J.M and F.M aslo acknowledge funding from MINECO through MAT2015-69144.

Thresholds for beam damage have been assessed for La$_{0.7}$Sr$_{0.3}$MnO$_3$ and SrTiO$_3$ as a function of electron probe current and exposure time at 80 and 200 kV acceleration voltage. The materials were exposed to an intense electron probe by aberration corrected scanning transmission electron microscopy (STEM) with simultaneous acquisition of electron energy loss spectroscopy (EELS) data. Electron beam damage was identified by changes of the core loss fine structure after quantification by a refined and improved model based approach. At 200 kV acceleration voltage, damage in SrTiO$_3$ was identified by changes both in the EEL fine structure and by contrast changes in the STEM images. However, the changes in the STEM image contrast as introduced by minor damage can be difficult to detect under several common experimental conditions. No damage was observed in SrTiO$_3$ at 80 kV acceleration voltage, independent of probe current and exposure time. In La$_{0.7}$Sr$_{0.3}$MnO$_3$, beam damage was observed at both 80 and 200 kV acceleration voltages. This damage was observed by large changes in the EEL fine structure, but not by any detectable changes in the STEM images. The typical method to validate if damage has been introduced during acquisitions is to compare STEM images prior to and after spectroscopy. Quantifications in this work show that this method possibly can result in misinterpretation of beam damage as changes of material properties.

We report the effects of nitrogen diffusion on exchange bias in MnN/CoFeB heterostructures as a function of MnN thickness and field-annealing temperature. We find that competing effects occur in which high-temperature annealing enhances exchange bias in heterostructures with thick MnN through improved crystallinity, but in thinner samples this annealing ultimately eliminates the exchange bias due to nitrogen deficiency. Using polarized neutron reflectometry and magnetic x-ray spectroscopy, we directly observe increasing amounts of nitrogen migration from MnN into the underlying Ta seed layer with increased annealing temperature. In heterostructures with thin MnN layers, the resulting nitrogen deficiency becomes significant enough to alter the antiferromagnetic state before the Ta seed layer is nitrogen saturated. Furthermore, we observe intermixing at the MnN/CoFeB interface which is attributed to the nitrogen deficiency creating vacancies in the MnN layer after annealing in a field. This intermixing of Mn with Co and Fe is not believed to be the cause for loss of exchange bias when the MnN layer is too thin but is instead a secondary effect due to increased vacancies after nitrogen migration.

In this study, we report on a shape-imposed magnetic anisotropy in micro- and nanostructures defined in antiferromagnetic (AF) LaFeO3 (LFO) thin films. Two distinct types of structures are investigated: embedded magnets created via ion implantation and free-standing magnets created via ion milling. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, we examine the impact of the structure type, AF layer thickness, and crystal geometry on the Néel vector orientation in these structures. We demonstrate a distinct shape-imposed anisotropy in embedded and free-standing structures alike and show that both parallel and perpendicular alignments of the AF spin axis with respect to structure edges can be achieved by variation of the AF layer thickness and the orientation of the structure edges with respect to the LFO crystalline axes. This work demonstrates how the fabrication procedure affects the magnetic order in thin film AF nanostructures and shows how nanoscale patterning can be used to control the orientation of the Néel vector in epitaxial oxide thin films. This is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in (citation of published article) and may be found at https://doi.org/10.1063/1.5116806

We have studied the interface magnetodynamic properties of La0.7Sr0.3MnO3/SrTiO3 (111) heterostructures by Ferromagnetic resonance spectroscopy (FMR). In addition to the bulk FMR mode, the measurements indicate a mode originating from an independently excited ferromagnetic layer at the interface. The peak-to-peak intensity of the interface mode suggests a layer thickness on the order of few unit cells. Angle resolved FMR measurements reveal a hexagonal symmetry of the magnetocrystalline anisotropy of the mode with the easy axis along the in-plane crystallographic directions matching with the (111) surface orientation of the substrate, in contrast to bulk mode symmetry which is always found to have uniaxial symmetry caused by magnetostriction. The temperature dependence of the anisotropy and a large temperature variation in the intensity ratio of interface and bulk mode indicate a coupling of the interface to the bulk mode.We have studied the interface magnetodynamic properties of La0.7Sr0.3MnO3/SrTiO3 (111) heterostructures by Ferromagnetic resonance spectroscopy (FMR). In addition to the bulk FMR mode, the measurements indicate a mode originating from an independently excited ferromagnetic layer at the interface. The peak-to-peak intensity of the interface mode suggests a layer thickness on the order of few unit cells. Angle resolved FMR measurements reveal a hexagonal symmetry of the magnetocrystalline anisotropy of the mode with the easy axis along the in-plane crystallographic directions matching with the (111) surface orientation of the substrate, in contrast to bulk mode symmetry which is always found to have uniaxial symmetry caused by magnetostriction. The temperature dependence of the anisotropy and a large temperature variation in the intensity ratio of interface and bulk mode indicate a coupling of the interface to the bulk mode.

In this study, we report on a temperature-driven antiferromagnetic (AF) spin reorientation transition in micro- and nanostructures of AF/ferromagnetic (FM) LaFeO3/La0.7Sr0.3MnO3 thin film bilayers. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, the Néel vector is shown to reorient 90° as a result of the competition between a shape-imposed anisotropy in the AF layer and interface coupling to the adjacent FM layer. We demonstrate how a temperature dependence of the AF/FM spin configuration in line-shaped nanomagnets can be tuned by variation of their linewidth. This work provides insight into the AF/FM interface exchange coupling in complex oxide heterostructures and the possibilities of spin control by nanostructuring in thin film spintronics. This is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in (citation of published article) and may be found at https://doi.org/10.1063/1.5094604

In antiferromagnetic spintronics control of the domains and corresponding spin axis orientation is crucial for devices. Here we investigate the antiferromagnetic axis in (111)-oriented LaFeO3/SrTiO3, which is coupled to structural twin domains. The structural domains have either the orthorhombic a- or b-axis along the in-plane h110i cubic directions of the substrate, and the corresponding magnetic domains have the antiferromagnetic axis in the sample plane. Six degenerate antiferromagnetic axes are found corresponding to the h110i and h112i in-plane directions. This is in contrast to the biaxial anisotropy in (001)-oriented films and reflects how crystal orientation can be used to control magnetic anisotropy in antiferromagnets. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4986555]

We observe an induced switchable magnetic moment of $1.6\ifmmode\pm\else\textpm\fi{}0.40\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$ for the nominally antiferromagnetic $\mathrm{LaFe}{\mathrm{O}}_{3}$ extending two to four interface layers into the non--charge transfer system $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}\text{/}\mathrm{LaFe}{\mathrm{O}}_{3}\text{/}\mathrm{SrTi}{\mathrm{O}}_{3}(111)$. Simultaneously a mismatch of oxygen octahedra rotations at the interface implies an atomic reconstruction of reduced symmetry at the interface, reaching two to five layers into $\mathrm{LaFe}{\mathrm{O}}_{3}$. Density functional theory of a structure with atomic reconstruction and different correlation strength shows a ferrimagnetic state with a net Fe moment at the interface. Together these results suggest that engineered oxygen octahedra rotations, affecting the local symmetry, affect electron correlations and can be used to promote magnetic properties.

Using strain, i.e. subtle changes in lattice constant in a thin film induced by the underlying substrate, opens up intriguing new ways to control material properties. We present a study of the effects of strain on structural and ferromagnetic properties of (1 1 1)pc-oriented La0.7Sr0.3MnO3 epitaxial thin films grown on NdGaO3, SrTiO3, and DyScO3 substrates. (The subscript pc denotes the pseudo-cubic symmetry.) The results show that La0.7Sr0.3MnO3 assumes a monoclinic unit cell on NdGaO3 and DyScO3 and a rhombohedral unit cell on SrTiO3. For La0.7Sr0.3MnO3 on NdGaO3 and DyScO3 a uniaxial magnetic anisotropy is found, while La0.7Sr0.3MnO3 on SrTiO3 is magnetically isotropic. The Neel model is used to explain the anisotropy of the thin films on NdGaO3 and SrTiO3, however, for La0.7Sr0.3MnO3 on DyScO3 the effect of octahedral rotations needs to be included through the single ion model. Through examination of the Curie temperature of the strained films we suggest that (1 1 1)-strain has a different effect on the Jahn-Teller splitting of e g and t 2g electron levels than what is seen in (0 0 1)pc-oriented La0.7Sr0.3MnO3 thin films.

In crystalline thin film growth a prerequisite is substrate surfaces with a stable and uniform structure and chemical composition. Various substrate treatments were used to obtain atomically smooth, step-and-terrace (111)-oriented SrTiO3 with uniform cation layers at the surface, i.e., single termination. The surface control enables subsequent layer-by-layer epitaxial growth of perovskite thin films of La0.7Sr0.3MnO3, LaFeO3, and BaTiO3. Reflection high-energy electron diffraction and electron energy loss spectroscopy revealed that a single chemically intermixed (A,A′)BO3 perovskite layer formed at the interface. As the terminating layer of (111) SrTiO3 is polar, a surface reconstruction consisting of TiOx surface layers is expected, and the intermixing at the interface can be understood as A′-cations from the film material compensating an A-cation deficient substrate surface during initial growth. This finding has important consequences for engineered interfaces between perovskite thin films and polar substrate facets.

We report on structural domains in LaFeO3 epitaxial thin films on (111) oriented SrTiO3 observed by transmission electron microscopy using low magnification dark field imaging and high resolution transmission electron microscopy techniques. The films were grown by pulsed laser deposition and had a thickness ≈ 20 nm. Three domain orientations are found, in accordance with the orthorhombic structure of LaFeO3. The domains themselves are of irregular shapes, and vary in size from tens to thousands of nm2. Regions of reduced Bragg scattering are observed as straight lines along < 100 >, hinting at a complex domain structure. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd.

Mixed-valence manganite thin films are attractive for spintronic devices. Crystalline orientation is a promising route to tailor switching mechanisms, as magnetization reversal depends on the magnetic anisotropy. Here, magnetic properties of (111)-oriented La0.7Sr0.3MnO3 thin films are elucidated by correlating macroscopic and local properties. The coercive field is an order of magnitude lower than (001)-oriented La0.7Sr0.3MnO3. Locally, a 6-fold magnetic anisotropy is observed, while macroscopically, an isotropic response is prevailing. This local coupling between the symmetry of the (111)-facet and magnetization governs the domain reversal process, demonstrating that symmetry offers a route to control magnetic properties for spintronic devices. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

We report on the stability of the La0.7Sr0.3MnO3 thin film surface when deposited on (111)-oriented SrTiO3. For ultrathin La0.7Sr0.3MnO3 films, an initial 3-dimensional morphology is observed, which becomes 2-dimensional with increasing film thickness. For even thicker samples, we show that the surface morphology evolves from 2-dimensional to 3-dimensional and that this observation is consistent with an Asaro-Tiller-Grinfeld instability, which can be controlled by the deposition temperature. This allows for synthesis of films with step-and-terrace surfaces over a wide range of thicknesses. Structural characterization by x-ray diffraction and transmission electron microscopy shows that the films are strained to the SrTiO3 substrate and reveals the presence of an elongated out-of-plane lattice parameter at the interface with SrTiO3.