News

New publication! Tunable magnetoelastic anisotropy in epitaxial (111) Tm3Fe5O12 thin films

April 21, 2020 By Peter Meisenheimer

Abstract: Ferrimagnetic insulators with perpendicular magnetic anisotropy are of particular interest for spintronics due to their ability to mitigate current shunting in spin–orbit torque heterostructures and enable low switching energy, high-density storage magnetic devices. Rare earth iron garnet Tm₃Fe₅O₁₂ (TmIG) is one such material where prior studies have shown that the negative magnetostriction coefficient and isotropic in-plane tensile strain enable the magnetoelastic anisotropy to overcome the demagnetization energy and stabilize perpendicular magnetic anisotropy. However, the investigation of the tunability of the magnetoelastic anisotropy between thin films that possess perpendicular magnetization and quantification of the magnetoelastic constants has not been reported. Here, we quantify the evolution of magnetic anisotropy in (111)-oriented, epitaxial, 17 nm thick thin films of TmIG using a systematic variation of in-plane epitaxial strain (ranging 0.49%–1.83%) imposed by a suite of commercially available garnet substrates. Within the confines of the imposed strain range and deposition condition, the distortion from cubic symmetry is found to be approximately linear within the in-plane strain. The magnetic anisotropy field can be tuned by a factor of 14 in this strain range. The magnetoelastic anisotropy constant, B₂, is found to be approximately constant (∼2500 kJ m⁻³) and more than 2× larger than the reported bulk value (∼1200 kJ m⁻³) for a cubic distortion between 90.17° and 90.71°. B₂ is found to decrease at cubic distortions of 90.74° and larger. Our results highlight strain engineering, and its limitations, for control of perpendicular magnetic anisotropy.

The full text is available as an editor’s choice article from Journal of Applied Physics

Peter wins the Rackham Predoctoral Fellowship!

April 1, 2020 By Peter Meisenheimer

The Rackham Predoctoral Fellowship supports outstanding doctoral students who are actively working on dissertation research and writing. It seeks to support students working on dissertations that are unusually creative, ambitious and impactful.

Peter’s research thrust is to find new materials and design devices that can mitigate heating from electronics, saving a tremendous amount of energy in the long run. To this end, his research explores the frontiers of materials synthesis to engineer new, sustainable, non-volatile devices with unprecedented performance. His research centers around discovering new ferroic states in disorder-driven materials. This includes synthesis and characterization of new, magnetic, entropy-stabilized oxides and device implementation and optimization of existing composite multiferroic systems. Through these projects he has experience with many aspects of materials design, ranging from material deposition and basic characterization to nanolithography and coupled electronic measurements.

See the press release here: https://mse.engin.umich.edu/about/news/three-mse-students-win-rackham-predoctoral-awards

Sieun and Peter give talks at EMA 2020

January 31, 2020 By Peter Meisenheimer

Sieun and Peter both gave contributed talks on entropy-stabilized oxides at ACerS EMA 2020 in Orlando last week.

Tunability of native defect density through local configuration-controlled disorder in entropy-stabilized oxides
S. Chae
Abstract: Entropic stabilization has become a strategy to create new oxide materials and novel properties, however, achieving an atomistic understanding of these properties has been challenged by the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations, machine-learning algorithms, and experimental characterization to investigate the role of local configurational and structural disorder on the thermodynamics of intrinsic point defects in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. From theory, we find that the cation-vacancy formation energy decreases with increasing local tensile strain, while oxygen-vacancy formation depends on the local structural distortion associated with the local configuration of chemical species. Through relatively small changes in the mole fraction of cations, the equilibrium defect density can be tuned by over two orders of magnitude. Vacancies in ESOs exhibit deep thermodynamic transition levels leading to transport via variable range hopping. Our results motivate tuning local structural distortions by local alloy composition as an engineering principle to enable controlled defect formation in multi-component oxides.

Electronic and magnetic interplay in entropy stabilized oxide thin films
P. Meisenheimer
Abstract: A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local chemical and structural disorder resulting from different atomic sizes and preferred coordinations of the constituents. In the antiferromagnetic entropy-stabilized oxides studied here, we see that by tuning the chemistry, and thus the concentration of local structural distortions, we can either induce or reclaim a large degree of frustration in the magnetic lattice of the material. As the large dielectric response of these materials may be strongly linked to their structure, here we study the interplay of electronic and magnetic functional responses in entropy-stabilized oxides. Our results reveal that the unique characteristics of entropy stabilized materials can be utilized to engineer and enhance magnetic functional phenomena in oxide thin films, as well as offer a powerful platform for the study of defects and functional properties.

New Publication! Boron arsenide heterostructures: lattice-matched heterointerfaces and strain effects on band alignments and mobility

January 17, 2020 By Peter Meisenheimer

Abstract: BAs is a III–V semiconductor with ultra-high thermal conductivity, but many of its electronic properties are unknown. This work applies predictive atomistic calculations to investigate the properties of BAs heterostructures, such as strain effects on band alignments and carrier mobility, considering BAs as both a thin film and a substrate for lattice-matched materials. The results show that isotropic biaxial in-plane strain decreases the band gap independent of sign or direction. In addition, 1% biaxial tensile strain increases the in-plane electron and hole mobilities at 300 K by >60% compared to the unstrained values due to a reduction of the electron effective mass and of hole interband scattering. Moreover, BAs is shown to be nearly lattice-matched with InGaN and ZnSnN₂, two important optoelectronic semiconductors with tunable band gaps by alloying and cation disorder, respectively. The results predict type-II band alignments and determine the absolute band offsets of these two materials with BAs. The combination of the ultra-high thermal conductivity and intrinsic p-type character of BAs, with its high electron and hole mobilities that can be further increased by tensile strain, as well as the lattice-match and the type-II band alignment with intrinsically n-type InGaN and ZnSnN₂ demonstrate the potential of BAs heterostructures for electronic and optoelectronic devices.

Full Text available from Nature Computational Materials