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Research Highlights

(* for correspondence author, ^# for 91��ý students )

Xiao Wang^#, Alexandra R. Stuart, Mitchell S. Swyt, Carla M. Quispe Flores, Andy T. Clark^#, Adzo Fiagbenu^#, Rajesh V. Chopdekar, Pavel N. Lapa, Zhuyun Xiao, Dava Keavney, Richard Rosenberg, Michael Vogel, John E. Pearson, Suzanne G. E. te Velthuis, Axel Hoffmann, Kristen S. Buchanan*, and Xuemei M. Cheng*

Phys. Rev. Materials 6, 084412 (2022), Editor's Suggestion

Magnetic skyrmions are topologically protected spin textures that have attracted considerable interest recently. This paper reports the discovery of a topological spin memory effect, which provides a promising new avenue for information encryption and recovery. The authors demonstrate experimentally that antiferromagnetically (AFM)-coupled bubble skyrmion pairs stabilized in a Co/Gd/Pt multilayered film at room temperature evolve into complex in-plane spin textures as the temperature is lowered and then reform completely when warmed back up. Simulations show that Dzyaloshinskii-Moriya interactions play a key role in this spin memory effect and reveal that the topological charge is preserved throughout the spin reorientation transition and recovery.

Andy T. Clark^#, X. Wang^#, A. R. Stuart, Q. Wang, W. Jiang, J. E. Pearson, S. G. E. te Velthuis, A. Hoffmann, X. M. Cheng*, and K. S. Buchanan*

J. Magn. Magn. Mater., 563, 169951 (2022).

We report on the formation of Néel-type magnetic bubble skyrmions at room temperature in [Pt/Co/Ir]₃ multilayered thin films after an in-plane magnetic field treatment. Polar magneto-optical  (p-MOKE) microscopy images show that the dendritic magnetic configurations observed after AC  evolve into magnetic bubble skyrmions after the application and subsequent removal of an in-plane magnetic field. Micromagnetic simulations were used to systematically investigate the role of the in-plane magnetic field magnitude, misalignment of the sample, and the Dzyaloshinskii-Moriya interactions (DMI) in generating bubble skyrmions during the field treatment. The simulations show that in-plane fields slightly below the saturation field are the most effective at producing skyrmions, and, furthermore, a small field angle away from the sample plane not only leads to improved skyrmion formation but also provides a means to select the skyrmion polarity where the direction of the out-of-plane component of the field is opposite to the direction of the skyrmion cores. This field treatment scheme leads to a simple and reliable way to create magnetic bubble skyrmions in multilayered thin films with DMI.

Mengying Bian, Liang Zhu, Xiao Wang^#, Junho Choi, Rajesh  V. Chopdekar, Sichen Wei, Lishu Wu, Chang Huai, Austin Marga, Qishuo Yang, Yuguang C. Li, Fei Yao, Ting  Yu, Scott A. Crooker, Xuemei M. Cheng, Renat F. Sabirianov, Shengbai Zhang, Junhao Lin, Yanglong Hou, and Hao Zeng

Advanced Materials, 34, 2200117, (2022). 

Realizing van der Waals (vdW) epitaxy in the 1980s represents a breakthrough that circumvents the stringent lattice matching and processing compatibility requirements in conventional covalent heteroepitaxy. However, due to the weak vdW interactions, there is little control over film qualities by the substrate. Typically, discrete domains with a spread of misorientation angles are formed, limiting the applicability of vdW epitaxy. Here, the epitaxial growth of monocrystalline, covalent Cr₅Te₈ 2D crystals on monolayer vdW WSe₂ by chemical vapor deposition is reported, driven by interfacial dative bond formation. The lattice of Cr₅Te₈, with a lateral dimension of a few tens of micrometers, is fully commensurate with that of WSe₂ via 3 × 3 (Cr₅Te₈)/7 × 7 (WSe₂) supercell matching, forming a single-crystalline moiré superlattice. This work establishes a conceptually distinct paradigm of thin-film epitaxy, termed “dative epitaxy”, which takes full advantage of covalent epitaxy with chemical bonding for fixing the atomic registry and crystal orientation, while circumventing its stringent lattice matching and processing compatibility requirements; conversely, it ensures the full flexibility of vdW epitaxy, while avoiding its poor orientation control. Cr₅Te₈ 2D crystals grown by dative epitaxy exhibit square magnetic hysteresis, suggesting minimized interfacial defects that can serve as pinning sites.

Andy T Clark^#, David Marchfield,  Zheng Cao, Tong Dang^#, Nan Tang,  Dustin Gilbert,  Elise A. Corbin,  Kristen S. Buchanan, and  Xuemei M. Cheng*

APL Materials 10, 041106 (2022).

Ultrasoft magnetorheological elastomers (MREs) offer convenient real-time magnetic field control of mechanical properties that provides a means to mimic mechanical cues and regulators of cells in vitro. Here, we systematically investigate the effect of polymer stiffness on magnetization reversal of MREs using a combination of magnetometry measurements and computational modeling. Poly-dimethylsiloxane-based MREs with Young’s moduli that range over two orders of magnitude were synthesized using commercial polymers Sylgard™ 527, Sylgard 184, and carbonyl iron powder. The magnetic hysteresis loops of the softer MREs exhibit a characteristic pinched loop shape with almost zero remanence and loop widening at intermediate fields that monotonically decreases with increasing polymer stiffness. A simple two-dipole model that incorporates magneto-mechanical coupling not only confirms that micrometer-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis of ultrasoft MREs but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses.

Xuanyuan Jiang, Xiao Wang^#, Pratyush Buragohain, Andy T. Clark^#, Haidong Lu, Shashi Poddar, Le Yu^#, Anthony D. DiChiara, Alexei Gruverman, Xuemei Cheng, and Xiaoshan Xu

Phys. Rev. Materials 6, 074412 (2022).

Persistent photoresponses require optical excitations to metastable states, which are rare of ionic origin due to the indirect photon-ion interaction. In this work, we explore the photoinduced metastable proton states in the proton-transfer type molecular ferroelectric croconic acid. We observe that, after the photoexcitation, the changes of structural and ferroelectric properties relax in ∼10³s, indicating persistent photoresponses of ionic origin. In contrast, the photoconductivity relaxes within 1 s. The 10³s timescale suggests that the ionic metastable states result from proton transfer both along and out of the hydrogen bonds. This discovery unveils an ionic mechanism for the phototunability, which offers persistent opto-ferroelectric control for proton-transfer type molecular ferroelectrics.

Hannah M. Zlotnick, Andy T. Clark^#, Sarah E. Gullbrand, James L. Carey, Xuemei M. Cheng, Robert L. Mauck

Advanced Materials, 32, 48, 2005030, (2020).  

Engineering complex tissues represents an extraordinary challenge and, to date, there have been few strategies developed that can easily recapitulate native-like cell and biofactor gradients in 3D materials. This is true despite the fact that mimicry of these gradients may be essential for the functionality of engineered graft tissues. Here, a non-traditional magnetics-based approach is developed to predictably position naturally diamagnetic objects in 3D hydrogels. Rather than magnetizing the objects within the hydrogel, the magnetic susceptibility of the surrounding hydrogel precursor solution is enhanced. In this way, a range of diamagnetic objects (e.g., polystyrene beads, drug delivery microcapsules, and living cells) are patterned in response to a brief exposure to a magnetic field. Upon photo-crosslinking the hydrogel precursor, object positioning is maintained, and the magnetic contrast agent diffuses out of the hydrogel, supporting long-term construct viability. This approach is applied to engineer cartilage constructs with a depth-dependent cellularity mirroring that of native tissue. These are thought to be the first results showing that magnetically unaltered cells can be magneto-patterned in hydrogels and cultured to generate heterogeneous tissues. This work provides a foundation for the formation of opposing magnetic-susceptibility-based gradients within a single continuous material.

Binquan Luan, Tien Huynh, Xuemei Cheng, Ganhui Lan, and Hao-Ran Wang

J. Proteome Res. 19, 11, 4316–4326,(2020). 

The unprecedented pandemic of coronavirus disease 2019 (COVID-19) demands effective treatment for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The infection of SARS-CoV-2 critically depends on diverse viral or host proteases, which mediate viral entry, viral protein maturation, as well as the pathogenesis of the viral infection. Endogenous and exogenous agents targeting for proteases have been proved to be effective toward a variety of viral infections ranging from HIV to influenza virus, suggesting protease inhibitors as a promising antiviral treatment for COVID-19. In this Review, we discuss how host and viral proteases participated in the pathogenesis of COVID-19 as well as the prospects and ongoing clinical trials of protease inhibitors as treatments.

Wanjun Jiang, Xichao Zhang, Guoqiang Yu, Wei Zhang, Xiao Wang^#, Matthias Jungfleisch, Xuemei Cheng, John Pearson, Olle Heinonen, Kang L. Wang, Yan Zhou, Axel Hoffmann, and Suzanne te Velthuis

Nature Physics, 13, 162 (2017). 

The well-known Hall effect describes the transverse deflection of charged particles (electrons/holes) as a result of the Lorentz force. Similarly, it is intriguing to examine if quasi-particles without an electric charge, but with a topological charge, show related transverse motion. Magnetic skyrmions with a well-defined spin texture with a unit topological charge serve as good candidates to test this hypothesis. In spite of the recent progress made on investigating magnetic skyrmions, direct observation of the skyrmion Hall effect has remained elusive. Here, by using a current-induced spin Hall spin torque, we experimentally demonstrate the skyrmion Hall effect, and the resultant skyrmion accumulation, by driving skyrmions from the creep-motion regime (where their dynamics are influenced by pinning defects) into the steady-flow-motion regime. The experimental observation of transverse transport of skyrmions due to topological charge may potentially create many exciting opportunities, such as topological selection.

Ty Newhouse-Illige, Yaohua Liu, Meng Xu, Danielle Reifsnyder Hickey, Anirban Kundu, Hamid Almasi, Chong Bi, Xiao Wang^#, John Freeland, David Keavney, Chenjun Sun, Yiheng Xu, Marcus Rosales, Xuemei Cheng, Shufeng Zhang, K. Andre Mkhoyan, and Weigang Wang

Nature Communications, 8, 15232 (2017). 

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdO_x tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdO_x, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Le Yu^#, Zhongying Yan^#,  Zhonghou Cai, Dongtang Zhang, Ping Han,  Xuemei Cheng*, and Yugang Sun*

Nano Letters, 16(10), 6555 (2016)

We report the in situ investigation of the morphological evolution of silver nanowires to hollow silver oxide nanotubes using transmission X-ray microscopy (TXM). Complex silver diffusion kinetics and hollowing process via the Kirkendall effect have been captured in real time. Further quantitative X-ray absorption analysis reveals the difference between the longitudinal and radial diffusions. The diffusion coefficient of silver in its oxide nanoshell is, for the first time, calculated to be 1.2 × 10^–13 cm²/s from the geometrical parameters extracted from the TXM images.

Xiao Wang^#, D. J. Keavney, M. Asmat, K. Buchanan, A. Melikyan, and X. M. Cheng*

Appl. Phys. Lett. 105, 102408 (2014).  

The interactions between three magnetic vortices in a planar equilateral triangular arrangement were studied by time-resolved photoemission electron microscopy. The gyrotropic resonance frequencies of the three individual vortices in the tri-disk system are different from one another and also shifted from that of an isolated vortex by as much as 12%. A comparison with analytical calculations and numerical simulations shows that the observed frequency shifts result from the dipolar interaction between the vortices.

Wenbin Wang, Jun Zhao, Wenbo Wang, Zheng Gai, Nina Balke, Miaofang Chi, Ho Nyung Lee, Wei Tian, Leyi Zhu, Xuemei Cheng, David J. Keavney, Jieyu Yi, Thomas Z. Ward, Paul C. Snijders, Hans M. Christen, Weida Wu, Jian Shen, and Xiaoshan Xu

Phys. Rev. Lett. 110, 237601 (2013). 

The crystal and magnetic structures of single-crystalline hexagonal LuFeO3 films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO3 films are room-temperature multiferroics.

X. M. Cheng and D. J. Keavney

Reports on Progress in Physics, 75, 026501 (2012) (Invited Review Paper).

As interest in magnetic devices has increased over the last 20 years, research into nanomagnetism has experienced a corresponding growth. Device applications from magnetic storage to magnetic logic have compelled interest in the influence of geometry and finite size on magnetism and magnetic excitations, in particular where the smallest dimensions reach the important magnetic interaction length scales. The dynamical behavior of nanoscale magnets is an especially important subset of research, as these phenomena are both critical for device physics and profoundly influenced by finite size. At the same time, nanoscale systems offer unique geometries to promote and study model systems, such as magnetic vortices, leading to new fundamental insights into magnetization dynamics. A wide array of experimental and computational techniques have been applied to these problems. Among these, imaging techniques that provide real-space information on the magnetic order are particularly useful. X-ray microscopy offers several advantages over scanning probe or optical techniques, such as high spatial resolution, element specificity and the possibility for high time resolution. Here, we review recent contributions using static and time-resolved x-ray photoemission electron microscopy to nanomagnetism research.

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