Emergent dynamic chirality in a thermally driven artificial spin ratchet

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice1, 2 can lead to specific collective behaviour3, including emergent magnetic monopoles4, 5, charge screening6, 7 and transport8, 9, as well as magnonic response10, 11, 12. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet13, 14, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells

Deterministic Control of Individual Nanomagnets in Strain-mediated Multiferroic Heterostructures

Controlling magnetism on the nanoscale has attracted considerable research interest for the high potential in non-volatile memory and logic applications. Using strain to control magnetization in strain-mediated multiferroic heterostructures is considered the most energy efficient approach, reducing energy dissipation by orders of magnitude. The strain-mediated multiferroic heterostructure has a ferromagnetic element on a ferroelectric substrate. Applying voltage to the ferroelectric substrate induces piezoelectric strain, which manipulates the magnetization of the ferromagnetic element through magnetoelastic effect. Nanomagnets, as information storage bits for non-volatile memory applications, need to be both individually and deterministically controlled. In the present work, two concepts are developed for this aim, one uses an electrode pattern design on a piezoelectric substrate to produce localized strain, and the other consists of architecting the shape of nanomagnets to take advantage of magnetic shape anisotropy. Patterned electrodes are designed and their effect is modeled using finite element simulations. By selectively applying voltage to electrode pairs, various strain configurations are produced between the electrodes, creating localized strain that controls individual nanomagnets. The modeling results were confirmed by experiments that used magnetization characterization techniques including magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM). By architecting the geometric shape, “peanut” and “cat-eye” shaped nanomagnets were engineered on piezoelectric substrates. These nanomagnets undergo repeated deterministic 180? magnetization rotations in response to individual electric-field-induced strain pulses. The designs were modeled using micromagnetics simulations. Both concepts provide significant contributions for next generation strain-mediated magnetoelectric memory research. This work opens a broad design space for next generation magnetoelectric spintronic devices

Shape Memory Alloy Helical Microrobots with Transformable Capability towards Vascular Occlusion Treatment

Practical implementation of minimally invasive biomedical applications has been a long-sought goal for microrobots. In this field, most previous studies only demonstrate microrobots with locomotion ability or performing a single task, unable to be functionalized effectively. Here, we propose a biocompatible shape memory alloy helical microrobot with regulative structure transformation, making it possible to adjust its motion behavior and mechanical properties precisely. Especially, towards vascular occlusion problem, these microrobots reveal a fundamental solution strategy in the mechanical capability using shape memory effect. Such shape-transformable microrobots can not only manipulate thrust and torque by structure to enhance the unclogging efficiency as a microdriller but also utilize the high work energy to apply the expandable helical tail as a self-propulsive stent. The strategy takes advantage of untethered manipulation to operate microsurgery without unnecessary damage. This study opens a route to functionalize microrobots via accurate tuning in structures, motions, and mechanical properties

Stray-Field Imaging of a Chiral Artificial Spin Ice during Magnetization Reversal

Artificial spin ices are a class of metamaterials consisting of magnetostatically coupled nanomagnets. Their interactions give rise to emergent behavior, which has the potential to be harnessed for the creation of functional materials. Consequently, the ability to map the stray field of such systems can be decisive for gaining an understanding of their properties. Here, we use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image the magnetic stray field distribution of an artificial spin ice system exhibiting structural chirality as a function of applied magnetic fields at 4.2 K. The images reveal that the magnetostatic interaction gives rise to a measurable bending of the magnetization at the edges of the nanomagnets. Micromagnetic simulations predict that, owing to the structural chirality of the system, this edge bending is asymmetric in the presence of an external field and gives rise to a preferred direction for the reversal of the magnetization. This effect is not captured by models assuming a uniform magnetization. Our technique thus provides a promising means for understanding the collective response of artificial spin ices and their interactions

Nanomagnetic encoding of shape-morphing micromachines

ISSN:0028-0836ISSN:1476-468

Chirally coupled nanomagnets

ISSN:0036-8075ISSN:1095-920