Session A54: Antiferromagnetism and Emergent Magnetism in Oxides

Angstrom-Scale Mapping of the Local Magnetic Moment in Metallic Antiferromagnet Fe2As using 4D-STEM

Antiferromagnets are a promising platform for high density spintronic devices. Techniques such as Lorentz transmission electron microscopy (LTEM) have been used to probe the arrangement of spins in topological structures such as skyrmions but cannot achieve the high spatial resolution needed to observe the local magnetic structure in antiferromagnets. In our work, we apply 4-dimensional scanning transmission electron microscopy (4D-STEM) with a fast, high dynamic range pixelated detector to investigate the magnetic structure of Fe₂As, a metallic antiferromagnet. Using a combination of quantum mechanical electron scattering simulations and experimental data, we show how the weak scattering signals from magnetism can be isolated in convergent beam electron diffraction patterns.  We apply our findings to measure the direction and magnitude of the local magnetization and obtain up to 6 Å resolution in Fe₂As.  Our results indicate new applications for 4D-STEM to study the magnetic structure of antiferromagnets.   

Ultrafast electron diffraction of free standing expitaxial nickel oxide thin films

Spintronic devices incorporating antiferromagnetic materials have the potential to exhibit faster dynamics, a higher density of information storage, and more robustness to external magnetic fields than devices based on ferro or ferrimagnetic materials. However, these same advantages also make their static and dynamic properties difficult to probe, especially in thin films. To this end, we have fabricated freestanding antiferromagnetic thin films of nickel oxide (NiO) on SiN membranes to probe via ultrafast electron diffraction (UED). We show epitaxial growth of NiO on a water-soluble Sr₂CaAl₂O₆ buffer layer on SrTiO₃ substrates. After dissolving the sacrificial layer we transfer the free standing films to the SiN membranes and maintain the epitaxial character of the films. UED experiments were performed to probe the response of the lattice and potentially the magnetic order to optical excitations. We use an above-gap photoexcitation (l= 280 nm) and measure an initial lattice response on the order of picoseconds and the persistence of this state for greater than 20 ps. This work demonstrates the viability of the thin film transfer method combined with UED to probe the dynamic response of antiferromagnetic oxide thin films.   

Cavity-Enhanced Linear Dichroism in a van der Waals Antiferromagnet

The coupling of electronic states with crystal lattice underlies a wealth of condensed matter phenomena. Coupling of electronic charge with lattice vibrations or distortions underlies superconductivity, charge density wave as well as metal-insulator transitions. In contrast, coupling between electronic charge and spins has been scarcely investigated, particularly in the optical frequency domain. The recent discovery of anti-ferromagnetic (AFM) insulators of the form MPX3 (M = Fe, Mn, Ni; X= S, Se) has opened new avenues in studying spin-charge correlations via optical means. Here we report such an interaction in AFM FePS₃ crystals which induces strong in-plane anisotropy, resulting in linear dichroism (LD). We identify that this LD is characteristic of the AFM spin ordering in the crystal coincident with the Neel transition temperature (~118 K). Further, we show that the LD is tunable both spectrally and in magnitude up to near-unity (~98.4%) levels when coupled to an optical cavity. Finally, through transfer-matrix simulations, we also demonstrate the dispersion of the LD in the visible-near infrared range as a function of the cavity length and the FePS₃ thickness. We observe a close agreement between experiments and simulations and prove that the LD can be enhanced from 5% (in bulk) to 98.4% (in ~200nm thick samples) by virtue of cavity coupling and engineering the dielectric stack. Our findings open a new route to probing spin-charge coupling in magnetic materials and have implications in the design of novel birefringent nanophotonic elements.   

Gating of an Antiferromagnet with Hydrogen

Robust and energy efficient electrical manipulation of antiferromagnets (AFM), be that via currents or voltages, is a highly desirable functionality owing to a number of inherent advantages of AFMs over ferromagnets (FM). Here, we present results of magneto-ionically gating an AFM with hydrogen, in a Co/Co₉₀Ni₁₀O exchange-biased (EB) heterostructure. In this system, EB serves as a means to probe changes occurring in the AFM as well as the interface. Because domain wall creep dynamics is highly sensitive to the local properties of the heterostructure, we investigate the effect of gating on EB through measurement of domain wall velocities (in Co) using a wide field MOKE. This ensures that extracted parameters such as the EB, depend only on the local properties of the gated area. We report a reversible enhancement of the EB field by up to 100% upon gating, suggesting considerable modulation of bulk and or interfacial characteristics of the AFM. These early results show that AFMs are susceptible to ionic gating, and we believe our work can pave the way for more robust electrical control of AFMs, pushing the boundaries of the exciting field of AFM spintronics.   

Phase-controllable molecular beam epitaxy growth of tetragonal and hexagonal FeTe thin films

FeTe can crystallize in either tetragonal or hexagonal structure. The tetragonal phase is a known antiferromagnet with bi-collinear antiferromagnetic (AFM) order, and the hexagonal phase has been shown to be a ferromagnet. In this work, we demonstrate the phase-controllable molecular beam epitaxy growth of tetragonal and hexagonal FeTe thin films on epitaxial graphene/SiC(0001). For the tetragonal phase, temperature-dependent superconducting quantum interference device (SQUID) magnetometry measurements confirm the AFM ordering with a Néel temperature of about 75 K. The hexagonal phase is found to be ferromagnetic with a Curie temperature greater than 300K, with an in-plane magnetic anisotropy. The selective growth of ferromagnetic and antiferromagnetic FeTe thin films by molecular beam epitaxy provides new opportunities for spintronic applications.   

Atomic-Scale Spin-Wave Polarizer Based on a Sharp Antiferromagnetic Domain Wall

We theoretically study the scattering of spin waves from a sharp domain wall in an antiferromagnetic spin chain. While the continuum model for an antiferromagnetic material yields the well-known result that spin waves can pass through a wide domain wall with no reflection, here we show that, based on the discrete spin Hamiltonian, spin waves are generally reflected by a domain wall with a reflection coefficient that increases as the domain-wall width decreases. Remarkably, we find that, in the interesting case of an atomically sharp domain wall, the reflection of spin waves exhibits strong dependence on the state of circular polarization of the spin waves, leading to complete reflection for one polarization while permitting  partial transmission for the other, thus realizing an atomic-scale spin-wave polarizer. The polarization of the transmitted spin wave depends on the orientation of the spin in the sharp domain wall, which can be controlled by an external field or spin torque. Our utilization of a sharp antiferromagnetic domain wall as an atomic-scale spin-wave polarizer leads us to envision that  ultra-small magnetic solitons such as  domain walls and  skyrmions may enable realizations of atomic-scale spin-wave scatterers with useful functionalities.   

A spin Hall Ising machine.

Ising Machines (IMs) are physical systems designed to find solutions to combinatorial optimization (CO) problems mapped onto the IM via the coupling strengths of its binary spins. Using the intrinsic dynamics and different annealing schemes, the IM relaxes over time to its lowest energy state, which is the solution to the CO problem. Here we present the world's most miniaturized, and the first spin Hall nano-oscillator (SHNO) based Ising machine. One of the most intriguing properties of SHNOs is their ability to synchronize to each other and to an external source. We demonstrate robust phase binarization due to injection locking at twice the natural frequency of SHNOs and analyze the binarization behavior as a function of injection locking power and frequency.  We then show binarization in 1x2 and 2x2 SHNO interconnected arrays. The phase binarization manifests itself as distinct microwave output power levels, which are readily distinguished using electrical means. In addition, we use phase-resolved Brillouin Light Scattering (phase-BLS) microscopy to directly observe the individual phases of the precessing magnetization in each nano-constriction. The different states can be accessed using either different injected power levels or a detuned frequency of the injected signal. We then propose pathways to control the coupling between SHNOs in order to gain flexibility in terms of mapped problems, and realize truly miniaturized, ultra-fast, and large-scale oscillator-based Ising Machines.   

Thermosensitivity Through Exchange Coupling in Ferrimagnetic/Antiferromagnetic Nanocrystalline Composites for Spatially Resolved Thermometry

Temperature is a fundamental physical quantity critical to every scientific discipline. Noncontact measurement of temperature beyond the surface of a given 3D volume is generally not possible; however, a temperature-sensitive variant of magnetic particle imaging (MPI) hopes to remedy this metrological deficiency. In order to enable thermal magnetic imaging for near room temperature applications (200 K to 400 K), it is necessary to develop magnetic materials that exhibit a strong temperature dependent magnetization (thermosensitivity) in this range. We demonstrate that nanocrystalline composites of ferrimagnetic iron oxide embedded in antiferromagnetic CoO and Ni doped CoO produce a sharp change in magnetization with temperature; with one possibility being that this is due to the Néel temperature of the antiferromagnets. The nanocrystalline composites were synthesized by a seed mediated colloidal synthesis approach, and the structure and morphology were characterized by XRD, TEM, and STEM. Temperature dependent magnetization has been measured by DC liquid and powder magnetometry.   

Tailoring the magnetocaloric effect in oxides

Searching for new magnetocaloric materials with significant magnetic entropy change over a wide temperature range, we combine magnetic oxides with different transition temperatures. We use pulsed laser deposition (PLD) technique to growth bilayer and trilayer composites of two different oxides La₂NiMnO₆ (LNMO) and La_2/3Sr_1/3MnO₃ (LSMO) on LSAT substrates. In bilayer samples, we can affect the ratio of the cation ordering in LNMO layer by changing the arrangement of the layers. We get a cation-ordered LNMO layer with a magnetic phase transition at 230 K when it is deposited on a bare substrate. However, it shows a lower magnetic phase transition temperature at 180 K when it is grown on top a LSMO. This behavior driven by strains can be used in trilayer samples where three magnetic transition can be detected, enabling us to get a roughly temperature-independent table-top magnetic entropy change over a temperature range as large as 100 K. These results suggest a route through composites to tune the magnitude, shape and width of the temperature dependence of the magnetic entropy change. This technique allows us to tailor an appropriate material for magnetic refrigeration at room temperature by changing the arrangement of layers.   

Curved Magnetism in CrI3

Strain gradients or curvature are ubiquitous at the nanoscale and can have tremendous effects on material properties. Micromagnetic modeling has shown how curved magnets can exhibit unusually rich phase diagrams, including chiral or topological spin states. This is particularly important in relation to the recent finding of ferromagnetism in 2D monolayers, such as CrI₃, given their extreme flexibility and natural tendency to rippling. Nevertheless, a thorough, quantitative, understanding of such effects in real materials, from first principles theory, is challenging and still lacking. Here, we use non-collinear-spin density-functional theory to study the (flexomagnetic) coupling between magnetism and curvature in monolayer CrI₃. We find a crossover from a magnetization normal to the surface at small curvatures, to a cycloidal spin state at larger curvatures. We show that this cycloidal state is stabilized by curvature induced, effective anisotropy and Dzyaloshinskii–Moriya type contributions to the magnetic energy. While the latter appears to be dominated by non-relativistic effects, in line with earlier predictions, our results reveal an unexpectedly large impact of spin-orbit coupling on the curvature dependence of the former, qualitatively different from the purely geometric effect discussed in earlier theory.   

Influence of native defects and their evolution on the ferromagnetic ordering of transition metals doped ZnO thin films.

We report on the correlation between the native defects including oxygen vacancies (V_O)  and Zinc vacancies (V_Zn) and their evolution under transition metal (TM) substitution on the ferromagnetic ordering of TM-doped ZnO thin films; TM = Co, Mn, Ni, and Cu. In addition, we report on the thermal annealing effects in different atmospheres including ambient air, vacuum, nitrogen, and hydrogen on the ferromagnetic ordering of TM-doped ZnO thin films. The TM-doped ZnO thin films were successfully synthesized using spray pyrolysis technique, and characterised using x-ray diffraction, x-ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, photoluminescence spectroscopy, and ultraviolet-visible spectroscopy. All TM-doped ZnO films showed a single phase würtzite structure, indicating a successful incorporation of TM dopants into the ZnO lattice [1-3]. The doped films showed a clear ferromagnetic phase at room temperature, which is improved at low temperatures. The results clearly showed a direct correlation between the concentration of V_O and the observed ferromagnetism (FM). By increasing the Co, Mn, Ni, and Cu content, we observed a clear enhancement in the saturation magnetization and coercivity in parallel with a considerable enhancement in the concentration of V_O. The FM in the doped films became stronger after annealing in vacuum and weaker after annealing in air, in accordance with a considerable increase and decrease in the concentration of V_O that mediates the observed FM. In Cu doped ZnO, we observed a link between the concentration of V_Zn and the observed FM. The overall results provided a closer view on the ordering mechanism in TM-doped ZnO films and the role of the native defects that mediates this long-range ordering.   

Ferromagnetic Order in Relativistic Quantum Mechanics

The fully relativistic Dirac theory of the electron has been celebrated for predicting the spin magnetic moment of the electron with the correct g factor g = 2 that emerges in the weakly relativistic limit to the Dirac theory often called the Pauli equation.  In ferromagnetically ordered solids, g becomes the prefactor for the spin magnetization.  Interestingly, a spin magnetic moment is absent in the actual Dirac equation; but the interaction of the electrons with a magnetic field is entirely accounted for via the minimal coupling to the vector potential for the magnetic field (i.e., we have g = 0 in the Dirac equation).  We show [1] how the observable magnetization in ferromagnetically ordered solids becomes a purely orbital magnetization in the Dirac theory, focusing for conceptual clarity on magnetic nanostructures.

[1] R. Winkler and U. Zülicke, Phys. Rev. Research 2, 043060 (2020)