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1. On the rheology and magnetization of dilute magnetic emulsions under small amplitude oscillatory shear

   https://doi.org/10.1017/jfm.2022.1019  

   Abdo, Rodrigo F. ; Abicalil, Victor G. ; Cunha, Lucas H.P. ; Oliveira, Taygoara F. ( January 2023 , Journal of Fluid Mechanics)

   A dilute magnetic emulsion under the combined action of a uniform external magnetic field and a small amplitude oscillatory shear is studied using numerical simulations. We consider a three-dimensional domain with a single ferrofluid droplet suspended in a non-magnetizable Newtonian fluid. We present results of droplet shape and orientation, viscoelastic functions and bulk emulsion magnetization as functions of the shear oscillation frequency, magnetic field intensity and orientation. We also investigate how the magnetic field induces mechanical anisotropy by producing internal torques in oscillatory conditions. We found that, when the magnetic field is parallel to the shear plane, the droplet shape is mostly independent of the shear oscillation frequency. Regarding the viscometric functions, we show how the external magnetic field modifies the storage and loss moduli, especially for a field aligned to the main velocity gradient. The bulk emulsion magnetization is studied in the same fashion as the viscoelastic functions of the oscillatory shear. We show that the in-phase component of the magnetization with respect to the shear rate reaches a saturation magnetization, at the high frequencies limit, dependent on the magnetic field intensity and orientation. On the other hand, we found a non-zero out-of-phase response, which indicates a finite emulsion magnetization relaxation time. Our results indicate that the magnetization relaxation is closely related to the mechanical relaxation for dilute magnetic emulsions under oscillatory shear. 

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2. Single‐Domain Multiferroic Array‐Addressable Terfenol‐D (SMArT) Micromagnets for Programmable Single‐Cell Capture and Release

   https://doi.org/10.1002/adma.202006651  

   Khojah, Reem ; Xiao, Zhuyun ; Panduranga, Mohanchandra K. ; Bogumil, Michael ; Wang, Yilian ; Goiriena‐Goikoetxea, Maite ; Chopdekar, Rajesh V. ; Bokor, Jeffrey ; Carman, Gregory P. ; Candler, Rob N. ; et al ( April 2021 , Advanced Materials)

   Programming magnetic fields with microscale control can enable automation at the scale of single cells ≈10 µm. Most magnetic materials provide a consistent magnetic field over time but the direction or field strength at the microscale is not easily modulated. However, magnetostrictive materials, when coupled with ferroelectric material (i.e., strain‐mediated multiferroics), can undergo magnetization reorientation due to voltage‐induced strain, promising refined control of magnetization at the micrometer‐scale. This work demonstrates the largest single‐domain microstructures (20 µm) of Terfenol‐D (Tb_0.3Dy_0.7Fe_1.92), a material that has the highest magnetostrictive strain of any known soft magnetoelastic material. These Terfenol‐D microstructures enable controlled localization of magnetic beads with sub‐micrometer precision. Magnetically labeled cells are captured by the field gradients generated from the single‐domain microstructures without an external magnetic field. The magnetic state on these microstructures is switched through voltage‐induced strain, as a result of the strain‐mediated converse magnetoelectric effect, to release individual cells using a multiferroic approach. These electronically addressable micromagnets pave the way for parallelized multiferroics‐based single‐cell sorting under digital control for biotechnology applications.

    

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3. A Magnetogram-matching Method for Energizing Magnetic Flux Ropes Toward Eruption

   https://doi.org/10.3847/1538-4357/ac874e  

   Titov, V. S. ; Downs, C. ; Török, T. ; Linker, J. A. ( September 2022 , The Astrophysical Journal)

   We propose a new “helicity-pumping” method for energizing coronal equilibria that contain a magnetic flux rope (MFR) toward an eruption. We achieve this in a sequence of magnetohydrodynamics relaxations of small line-tied pulses of magnetic helicity, each of which is simulated by a suitable rescaling of the current-carrying part of the field. The whole procedure is “magnetogram-matching” because it involves no changes to the normal component of the field at the photospheric boundary. The method is illustrated by applying it to an observed force-free configuration whose MFR is modeled with our regularized Biot–Savart law method. We find that, in spite of the bipolar character of the external field, the MFR eruption is sustained by two reconnection processes. The first, which we refer to as breakthrough reconnection, is analogous to breakout reconnection in quadrupolar configurations. It occurs at a quasi-separator inside a current layer that wraps around the erupting MFR and is caused by the photospheric line-tying effect. The second process is the classical flare reconnection, which develops at the second quasi-separator inside a vertical current layer that is formed below the erupting MFR. Both reconnection processes work in tandem with the magnetic forces of the unstable MFR to propel it through the overlying ambient field, and their interplay may also be relevant for the thermal processes occurring in the plasma of solar flares. The considered example suggests that our method will be beneficial for both the modeling of observed eruptive events and theoretical studies of eruptions in idealized magnetic configurations.

    

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4. Electrically and magnetically switchable nonlinear photocurrent in РТ-symmetric magnetic topological quantum materials

   https://doi.org/10.1038/s41524-020-00462-9  

   Wang, Hua ; Qian, Xiaofeng ( December 2020 , npj Computational Materials)

   Nonlinear photocurrent in time-reversal invariant noncentrosymmetric systems such as ferroelectric semimetals sparked tremendous interest of utilizing nonlinear optics to characterize condensed matter with exotic phases. Here we provide a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems. We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed ±kpoints. Taking$${\cal{P}}{\cal{T}}$$$PT$-symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi₂Te₄as an example, we predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks$${\cal{P}}{\cal{T}}$$$PT$symmetry and enables large NSC response in bilayer AFM MnBi₂Te₄, which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM-zMnBi₂Te₄as a tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials.

    

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5. Compressible two-dimensional turbulence: cascade reversal and sensitivity to imposed magnetic field

   https://doi.org/10.1088/1367-2630/ad0172  

   Fouxon, Itzhak ; Kritsuk, Alexei G. ; Mond, Michael ( November 2023 , New Journal of Physics)

   We study the impact of compressibility on two-dimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach numberMaincreases, from inverse atMa = 0, to direct at$Ma=\mathrm{\infty }$. Thus, at$Ma\sim 1$comparable amounts of energy flow from the pumping scale to large and small scales, in accord with previous data. For supersonic turbulence with$Ma\gg 1$the cascade is direct, as in three dimensions, which results in multifractal density field. For that regime ($Ma\gg 1$) we derive a Kolmogorov-type law for potential forcing and obtain an explicit expression for the third order correlation tensor of the velocity. We further show that all third order structure functions are zero up to first order in the inertial range scales, which is in sharp contrast with incompressible turbulence where the third order structure function, that describes the energy flux associated with the energy cascade is non-zero. The properties of compressible turbulence have significant implications on the amplification of magnetic fields in conducting fluids. We thus demonstrate that imposing external magnetic field on compressible flows of conducting fluids allows to manipulate the flow producing possibly large changes even at small Mach numbers. Thus Zeldovich’s antidynamo theorem, by which atMa = 0 the magnetic field is zero in the steady state, must be used with caution. Real flows have finiteMaand, however small it is, for large enough values ofI, the magnetic flux through the disk, the magnetic field changes the flow appreciably, or rearranges it completely. This renders the limitMa → 0 singular for non-zero values ofI. Of particular interest is the effect of the density multifractality, at$Ma\gg 1$which is relevant for astrophysical disks. We demonstrate that in that regime, in the presence of non-zeroIthe magnetic field energy is enhanced by a large factor as compared to its estimates based on the mean field. Finally, based on the insights described above, we propose a novel two-dimensional Burgers’ turbulence, whose three-dimensional counterpart is used for studies of the large-scale structure of the Universe, as a model for supersonic two-dimensional magnetohydrodynamic flows.

    

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