Bryan D. Esser

Exceptionally high magnetization of stoichiometric Y3Fe5O12 epitaxial films grown on Gd3Ga5O12

The saturation magnetization of Y3Fe5O12 (YIG) epitaxial films 4 to 250 nm in thickness has been determined by complementary measurements including the angular and frequency dependencies of the ferromagnetic resonance fields as well as magnetometry measurements. The YIG films exhibit state-of-the-art crystalline quality, proper stoichiometry, and pure Fe3+ valence state. The values of YIG magnetization obtained from all the techniques significantly exceed previously reported values for single crystal YIG and the theoretical maximum. This enhancement of magnetization, not attributable to off-stoichiometry or other defects in YIG, opens opportunities for tuning magnetic properties in epitaxial films of magnetic insulators.

Epitaxial growth of iridate pyrochlore Nd2Ir2O7 films.

Epitaxial films of the pyrochlore Nd2Ir2O7 have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd2Ir2O7 film with a short post-annealing provides insight into the mechanism for crystallization of Nd2Ir2O7 during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. The epitaxial relationship between the YSZ and Nd2Ir2O7 is observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.

Metallic ferromagnetic films with magnetic damping under 1.4 × 10-3

Low-damping magnetic materials have been widely used in microwave and spintronic applications because of their low energy loss and high sensitivity. While the Gilbert damping constant can reach 10−4 to 10−5 in some insulating ferromagnets, metallic ferromagnets generally have larger damping due to magnon scattering by conduction electrons. Meanwhile, low-damping metallic ferromagnets are desired for charge-based spintronic devices. Here, we report the growth of Co25Fe75 epitaxial films with excellent crystalline quality evident by the clear Laue oscillations and exceptionally narrow rocking curve in the X-ray diffraction scans as well as from scanning transmission electron microscopy. Remarkably, the Co25Fe75 epitaxial films exhibit a damping constant <1.4 × 10−3, which is comparable to the values for some high-quality Y3Fe5O12 films. This record low damping for metallic ferromagnets offers new opportunities for charge-based applications such as spin-transfer-torque-induced switching and magnetic oscillations.Owing to their conductivity, low-damping metallic ferromagnets are preferred to insulating ferromagnets in charge-based spintronic devices, but are not yet well developed. Here the authors achieve low magnetic damping in CoFe epitaxial films which is comparable to conventional insulating ferromagnetic YIG films.

HAADF/MAADF Observations and Image Simulations of Dislocation Core Structures in a High Entropy Alloy

High entropy alloys (HEAs) are a new class of multi-component alloys in which the individual elements have similar concentrations. A single-phase solid solution HEA containing 5 elements (Co, Cr, Fe, Mn, and Ni) with equiatomic composition was first discovered by Cantor [1]. Among the surprising characteristics of this fcc HEA are: strong temperature dependence of the yield strength at temperatures around and below room temperature, relatively weak strain-rate dependence over the same temperature range [3]; very large hardening rates [2,3]; and large fracture toughness at room temperature [4]. These features are linked to deformation twinning and dislocation-mediated plasticity, yet presently there is insufficient knowledge of dislocation dissociation, stacking fault energy, or core structures in this alloy. The highly planar deformation involves dislocation arrays on active slip systems (Figure 1a and 1b). This characteristic could imply the presence of short range order, low fault energy, or supplementary displacements in the wake of glide dislocations.

Characterizing Sub-lattice Occupancies in B2 Phases in High Entropy Metallic Alloys using Atomic Resolution STEM-XEDS Mapping

In the recent past, there has been considerable emphasis placed on the exploration of high entropy alloys (HEA). These alloys have been defined as ones with five or more, essentially equal atomic concentrations [1, 2]. CoCrCuFeNiAl is an example of an HEA alloy which mainly consists of two phases, namely ordered B2 and disordered bcc. Although this alloy has been the subject of much study and its microstructures characterized using a number of techniques, only recently has aberration-corrected (S)TEM coupled with x-ray energy dispersive spectroscopy (XEDS), involving large collection angles (ChemiSTEMTM), been applied [3]. In this latter study, it was found that, between the B2, consisting mainly of Al, and Ni, Co, and Fe, and disordered bcc phases, consisting mainly of Cr and Fe, there is a transition region, approximately 1.5nm in width, over which the chemical composition changes from the B2 to that of the bcc phase. The crystal structure of this interfacial region is also B2, but with a significantly different sub-lattice occupancy than that of the adjacent B2 compound. In these B2 phases with very differing compositions, and hence sub-lattice compositions, the intensities of both atomic columns in HAADF images and superlattice reflections in diffraction patterns may vary considerably [3]. The origin of these intensity differences is of interest. For example, when the difference in the intensities of atomic columns in each of the sub-lattices is small, this may be interpreted as either the average compositions of the sub-lattices being similar, and/or being a reduced degree of order of the B2 compound. It is obvious that in order to be able to understand the behavior of these alloys, it is necessary that the degree of order be known. The first of these possibilities may be assessed by making direct measurements of the sub-lattice composition, while the second possibility, degree of ordering, may be assessed by plotting these compositions on an ordering tie-line diagram [4]. The current study involves the direct measurement of sub-lattice compositions.

Giant Piezomagnetism in Mn3NiN

Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here, we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn3NiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of Mn3NiN with intrinsic biaxial strains of ±0.25% result in Néel transition shifts up to 60 K and magnetization changes consistent with theory. Films grown on BaTiO3 display a striking magnetization jump in response to uniaxial strain from the intrinsic BaTiO3 structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient α (where α = d B/d E) of 0.018 G cm/V. The latter agrees with the 1000-fold increase over Cr2O3 predicted by theory. Overall, our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature.

Structural and Magnetic Characterization of B20 Skyrmion Thin Films and Heterostructures Using Aberration-Corrected Lorentz TEM and Differential Phase Contrast STEM

Magnetic materials exhibiting topological spin textures have shown great promise for magnetoelectronic applications including ultra-high density magnetic memory. [1-4] Specifically, skyrmions are vortex-like spin textures that can form hexagonal magnetic lattices at temperatures near room temperature and small applied magnetic fields. The skyrmion phase results from the competition between exchange interactions and the Dzyaloshinskii-Moriya (DM) interaction, where the exchange interaction promotes parallel alignment between neighboring spins and the DM interaction promotes 90° alignments. DM interactions only occur in structures with broken inversion or mirror symmetry like the family of materials with the B20 crystal structure (space group P213). In addition to materials lacking in bulk inversion or mirror symmetry, superlattices can host the skyrmion phase due to their broken mirror symmetry. Recently, Ahmed et al. demonstrated the ability grow epitaxial B20 superlattices of [CrGe/MnGe/FeGe]n via molecular beam epitaxy (MBE) opening the door for tunable skyrmions through varying layer thicknesses. [5]

Electron Diffraction of Germanane

Differences in the physical phenomena exhibited by two-dimensional materials, compared with bulk materials, is the driver of significant interest in monolayers and bilayers of van der Waals layered materials. The ability to tune the properties and electronic structure of these layered materials via chemical functionalization opens numerous opportunities for novel applications and devices. For example, hydrogen terminated graphene, or graphane, has been extensively studied and used for a variety of applications. However, other two-dimensional materials that exhibit direct band gaps and high carrier mobilities are desired. Germanane, a group IV analogue of graphane, has recently been synthesized and exhibits a larger and direct band gap and higher electron mobility than that of bulk germanium [1-3]. However, while the electronic properties have been studied, the structure of the material remains largely unexplored. We have used electron diffraction in a transmission electron microscope (TEM) to investigate the structure of germanane on the nanometer scale.

Characterizing Epitaxial Growth of Nd 2 Ir 2 O 7 Pyrochlore Thin Films via HAADF-STEM Imaging and EDX

Much interest has been given to the 5d transition metal oxides because of their strong spin-orbit coupling, leading to the prediction of new exotic phases of matter, including the Weyl semimetal, topological Mott insulator, and spin liquid [1–4]. The pyrochlore iridates with the general formula of A2Ir2O7 have been predicted to show the Weyl semimetal and topological insulator phases under epitaxial strain [5,6]. These theoretical predictions have motivated the study and growth of epitaxial thin films of such pyrochlore iridates. In this work, we report the synthesis of Nd2Ir2O7 and growth of epitaxial thin films using off-axis magnetron sputtering and ex-situ post-growth annealing. Using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) and energy dispersive X-ray spectroscopy (EDX), the growth mechanisms of Nd2Ir2O7 are characterized. A thermodynamic explanation of the observed mechanisms is presented.

Structure-Properties Relations in III-Nitride Nanostructures for Optoelectronics

III-Nitride based nanowire heterostructures are useful for optoelectronics applications across the visible and ultraviolet (UV) spectral range. A remarkable range of applications for these nanomaterials have been demonstrated, including solid-state-lighting, UV LEDs, lasers, photovoltaics, and photocatalysts. Compared with their thin film counterparts, III-N nanowires exhibit several key advantages including: inherently low defect densities (zero misfit dislocations and minimal stacking faults), lattice mismatch tolerance, and greater tunability of polarization and bandgap within a single heterostructure. In particular, single crystal III-N nanowires can be grown on a variety of substrates while retaining their high optical and electronic quality. Here we discuss a few illustrative examples of correlating the atomic scale structure determined by scanning transmission electron microscopy (STEM) measurements with the functional properties of the nanowire heterostructures.