Last week at MRS fall meeting, Peter gave a talk on composite multiferroic materials and Nguyen presented a poster on thin film Cr₂O₃.

Nguyen’s poster was titled: Electrical and Magnetic Properties of Thin Single Crystal Cr2O3 Films 
Abstract: Magnetoelectric materials have been of great interest due to their potential for low-power spintronic devices via the electric field switching of magnetization. Antiferromagnet Cr2O3 is one of a very few room temperature magnetoelectrics and possesses unique properties such as uncompensated surface spins and perpendicular magnetic anisotropy. [1] Since the first demonstration of the electric field control of exchange bias in bulk single crystal Cr2O3 heterostructures [2], intense effort has focused the demonstration of magnetoelectric switching using Cr2O3 thin films at room temperature. [3,4] The existence of twin domains in thin films grown on metallic electrodes, however, leads to high leakage current and dielectric breakdown fields that can only be circumvented by growing rather thick films (250-500 nm). [4,5] By using an isostructural epitaxial oxide electrode, V2O3, recent studies have shown the reduction and even possible elimination of twin domains in Cr2O3 films. [3] Dielectric and magnetoelectric switching studies of 200 nm thick films show bulk like performance, however, for next generation logic and memory the films must be scaled down. [6] Here we present an investigation of the electrical endurance and magnetic properties of very thin (30-60 nm) single crystal Cr2O3 films grown by pulsed laser deposition onto V2O3 buffered (0001) oriented Al2O3 substrates. Our results show that 60 nm single crystal thin film has bulk-like resistivity ( 10^12 cm) and significantly improved breakdown voltage (150-300 MV/m). Using magnetometry, we investigate exchange bias of thin film Cr2O3/ferromagnet heterostructure. The blocking temperature is found to be at 285 K which is higher compared to twinned films with similar or greater thickness in literature. [7] Further, Second Harmonic Generation confirms bulk magnetoelectric order of our single crystal thin film at room temperature. These results indicate the importance of crystallinity to realize bulk like properties in very thin films at room temperature.

[1] X. He, Y. Wang, N. Wu, A. N. Caruso, E. Vescovo, K. D. Belashchenko, P. A. Dowben, and C. Binek, Nat. Mater. 9, 579 (2010).
[2] P. Borisov, A. Hochstrat, X. Chen, W. Kleemann, and C. Binek, Phys. Rev. Lett. 94, 117203 (2005).
[3] T. Kosub, M. Kopte, R. Hühne, P. Appel, B. Shields, P. Maletinsky, R. Hübner, M. O. Liedke, J. Fassbender, O. G. Schmidt, and D. Makarov, Nat.
Commun. 8, 13985 (2017).
[4] T. Ashida, M. Oida, N. Shimomura, T. Nozaki, T. Shibata, and M. Sahashi, Appl. Phys. Lett. 106, 132407 (2015).
[5] C. Sun, Z. Song, A. Rath, M. Street, W. Echtenkamp, J. Feng, C. Binek, D. Morgan, and P. Voyles, Adv. Mater. Interfaces 4, 1700172 (2017).
[6] A. Mahmood, M. Street, W. Echtenkamp, C. P. Kwan, J. P. Bird, and C. Binek, Phys. Rev. Mater. 2, 044401 (2018).
[7] N. Shimomura, S. P. Pati, T. Nozaki, T. Shibata, and M. Sahashi, AIP Adv. 7, 025212 (2017). 

Peter talk was titled: Epitaxially engineered, enhanced magnetostriction in a strain-driven composite multiferroic
Abstract: Composite multiferroics, composed of a magnetostrictive ferromagnet and a piezoelectric ferroelectric, have widely been targeted for beyond-CMOS logic due to their large coupling coefficients and high operating temperature^1–3. Magnetoelectric multiferroic systems potentially offer the lowest energy dissipation per bit operation in a scalable platform, yet significant materials challenges still exist in the field. For composite multiferroics, this requires finding pathways to enhance piezomagnetic effects and coupling between layers, an effort that has seen relatively little work⁴. Here, we present a means to boost the magnetostriction of Fe_1-xGa_x alloys and magnetoelectric coupling in a Fe_1-xGa_x -(PMN-PT)composite multiferroic heterostructure through epitaxy.
In bulk, the magnetostriction coefficient of Fe_1–xGa_x alloys versus Ga composition peaks near ~18% Ga occurring due to a phase change from the disordered A2 phase to an ordered BCC phase (D0₃), which reduces the magnetostriction coefficient⁵. A distinct advantage of thin film deposition is the potential to access metastable phases through epitaxy, allowing us to promote the chemically disordered BCC (A2) phase in our film at high (22%) Ga concentrations. We demonstrate that thin film epitaxy stabilizes a chemically disordered BCC Fe_0.78Ga_0.22 alloy where the magnetostriction is enhanced by 200-300% relative to the bulk.
Transport-based magnetoelectric characterization shows 90 electrical switch of magnetic anisotropy and one of the largest converse magnetoelectric coefficients ever achieved at room temperature in a composite multiferroic. Energy dissipation per operation scales to 5.9 J cm^-2, making our devices competitive with other state-of-the-art beyond CMOS technologies⁶. This hyperactive performance is achieved through epitaxial stabilization of a disordered, metastable phase of earth-abundant and rare-earth-free magnetostrictor, Fe_0.78Ga_0.22. By epitaxially engineering our ferromagnetic layer to prevent the formation of deleterious intermetallic nanoregions, we provide a pathway to engineering new performance levels in rare-earth free magnetoelastic and magnetoelectric heterostructures.

1. Meisenheimer, P. B., Novakov, S., Vu, N. M. & Heron, J. T. Perspective: Magnetoelectric switching in thin film multiferroic heterostructures. J. Appl. Phys. 123, 240901 (2018).
2. Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nat. Mater. 6, 21–29 (2007).
3. Bibes, M. & Barthélémy, A. Multiferroics: Towards a magnetoelectric memory. Nat. Mater. 7, 425–426 (2008).
4. Shevlin, S. Multiferroics and the path to the market. Nat. Mater. 18, 191 (2019).
5. Du, Y. et al. Relation between Ga ordering and magnetostriction of Fe-Ga alloys studied by x-ray diffuse scattering. Phys. Rev. B 81, 054432 (2010).
6. Manipatruni, S. et al. Scalable energy-efficient magnetoelectric spin–orbit logic. Nature 565, 35–42 (2019).

The interplay of electronic and magnetic functionalities in entropy-stabilized oxides

Abstract: 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.

Abstract: Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with increasing band gap and significantly limit the semiconductor devices that can currently be realized. Rutile germanium oxide (r-GeO2) is a promising UWBG (4.68 eV) material, yet has not been explored for semiconducting applications. Using hybrid density functional theory, we demonstrate r-GeO2 to be an alternative UWBG material that can be ambipolarly doped.