Session P40: Domain Walls

Live

The engineering of chiral coupling in thin magnetic films with inhomogeneous anisotropy [1] allows for designing synthetic magnetic systems of fundamental and technological interest [2,3]. This coupling arises from the interfacial Dzyaloshinskii-Moriya interaction, which, in combination with spin-orbit torques, leads to the local manipulation of chiral magnetic order [4]. So far, the chiral coupling between nanomagnets with patterned magnetic anisotropy has been studied only in Pt/Co/AlOx trilayers.

Here we design a system for quantifying the chiral coupling based on field- and current-induced asymmetric domain wall motion in Pt/Co/AlOx racetracks. This establishes a versatile method to characterize the chiral coupling in different materials. We further provide evidence of chiral coupling in Pt/CoB/AlOx trilayers with ultralow pinning. We show that the domain walls in such a system propagate in response to the DMI effective field even in the absence of any external driving force. This work shows that the chiral coupling can be used to locally design intrinsic fields that affect the magnetization dynamics.

[1] Luo, et al. Science 363, 1435 (2019)
[2] Luo, et al. Nature 579, 214 (2020)
[3] Dao et al. Nano Lett. 19, 5930 (2019)
[4] Hrabec, et al. Appl. Phys. Lett. 117, 130503 (2020)   

Live

Competing magnetic exchange interactions often result in noncollinear magnetic states, such as spin spirals, which break the inversion symmetry and induce ferroelectric polarization. The resulting strong interactions between spins and ferroelectric polarization give rise to electromagnons and open the door to a technologically important possibility to control magnetic order by electric fields. Here we study the effects of chiral domain walls on dielectric and magnetoelectric properties of cycloidal spirals. We use a quasi-1D model Hamiltonian with competing Heisenberg exchange interactions, leading to a spin spiral, and Dzyaloshinskii-Moriya interactions, responsible for magneto-electric coupling. The results suggest that low frequency dielectric anomalies in spiral magnets, such as TbMnO$_3$ and MnWO$_4$, may originate from hybrid magnon-polar phonon excitations associated with domain walls.   

Live

Long-range interactions between quasiparticles give rise to a ‘drag’ that affects the fundamental properties in condensed matter physics like Coulomb drag [1]. Drag typically involves the exchange of linear momentum between quasiparticles affecting transport properties.
We recently discovered a new form of drag, “Chiral Exchange Drag”, that involves the exchange of angular momentum between two current-driven magnetic domain walls (DWs) from synthetic antiferromagnets [2]. Previously we identified a powerful new torque, exchange coupling torque, in synthetic antiferromagnets that can move composite DWs very efficiently [3]. I will present our new findings that the composite DWs motions are correlated and determined by the strength of the exchange drag between component DWs. For example, when the drag is below a threshold value, the DWs move together at a constant intermediate velocity with a steady leakage of angular momentum from the faster to the slower wall. On the other hand, I will show that when the drag exceeds a threshold value, an entirely different dynamic can take place in which the faster DW’s magnetization oscillates synchronously with a precessional motion of the slower DW’s magnetization, and angular momentum is continuously transferred between them: “Chiral Exchange Drag Anomaly”. In this talk, I will discuss a new method for delivering spin angular momentum remotely to magnetic entities showing analogues with massive Dirac fermions.
[1] B. N. Narozhny, and A. Levchenko, “Coulomb drag”, Rev. Mod. Phys. 88, 025003 (2016).
[2] S.-H. Yang, C. Garg, and S. S. P. Parkin, “Chiral Exchange Drag and Chirality Oscillations in Synthetic Antiferromagnets”, Nat. Phys. 15, 543 (2019).
[3] S.-H. Yang, K.-S. Ryu, and S. S. P. Parkin, “Domain-wall velocities of up to 750 m s− 1 driven by exchange-coupling torque in synthetic antiferromagnets”, Nat. Nano. 10, 221 (2015).   

Live

It has long been known that the low-energy dynamics of spin systems are well-described by classical linear spin-wave theory. This is particularly true in the case of systems with a large spin moment, where one can expand in powers of 1/S. For large-spin antiferromagnets, linear spin wave theory predicts dispersive magnon modes which are gapped in the presence of anisotropy. Such spectra are found time and again in the literature, but can the quantum nature of spin give rise to exotic excitations that are not well described by linear spin-wave theory?
Using a path integral approach [1], we show that the movement of domain walls in an anisotropic large-spin antiferromagnetic chain can be described by solitons. In the presence of weak coupling to other chains, the frustration induced by this soliton propagation leads to a nonlinear confinement potential that can support bound states [2]. We will then show possible experimental evidence for these soliton bound states [3], demonstrate strong agreement with our theoretical model and offer insights into where one might expect to find such a phenomenon.

[1] F. D. M. Haldane, Phys. Rev. Lett. 50, 1153 (1983)
[2] H. Lane et al, Phys. Rev. B 102, 024437 (2020)
[3] C. Stock et al, Phys. Rev. Lett. 117, 017201 (2016)   

Live

Antiferromagnetic (AF) materials has been drawing increasing attention for practical applications due to their ultrafast spin dynamics. θ-MnN, a metallic AF, exhibits many properties that are interesting for spin switching applications such as a low anisotropy field and a high Neel temperature (650 K) [1]. A challenge in studying spin domains, and their switching, in AF materials is the lack of readily available experimental techniques that can be used to determine the orientation of the Neel vector. In this work, we use optical second harmonic generation to study the Neel order and spin domains in single-crystal epitaxially-grown MnN. Our results show that spins in θ-MnN are ferromagnetically aligned in planes perpendicular to the c-axis and have a large out-of-plane tilt which results in 2/m1’ magnetic point group. Different spin domains correspond to the in-plane component of the spins aligned along the two in-plane crystal cubic axes. Our measurements reveal that the sizes of the spin domains are smaller than a micron. We will also present results from nonlinear optical imaging of spin switching in MnN/Pt devices in which spin orbit torque is used to switch the Neel vector. [1] Phys. Rev. Research 2, 013347 (2020).   

Live

Neuromorphic computing attempts to mimic the neurons and synapses in a human brain in order to provide significant improvements in the computation of unstructured, real-world data. In past research, we have proposed three separate leaky integrate-and-fire (LIF) neurons that provide the leaking, integrating, and firing characteristics without the use of any additional circuitry [1]-[3]. These neurons are therefore able to significantly reduce the area and energy overhead of neuromorphic circuits. To improve the biomimicry of these neurons and to better match the neurons to different neuromorphic schema and algorithms, it is desirable for neuron leaking to implement specific mathematical functions in addition to exhibiting the three basic LIF neuronal functionalities. By varying the shape of the devices, it is possible to implement a variety of leaking characteristics. In this work, we will discuss the implementation of linear and sigmoidal leaking characteristics.

[1] Hassan, et al., JAP, 2018.
[2] Brigner, et al., JxCDC, 2019.
[3] Brigner, et al., TED, 2019.   

Live

GdFeO₃ is a typical material showing magnetoelectric (ME) effects. It has three order parameters, antiferromagnetic moments of Fe and Gd ions and electric polarization, and they interact with each other. Tokunaga et al. [1] pointed out the presence of composite domain walls (DW), and discussed ME effects related to DW motion as well as DW splitting induced by impurity pinning.
We have studied the dynamics of composite DW in GdFeO₃ driven by either magnetic or electric field based on the time-dependent Ginzburg-Landau equations [2]. One of the two major findings is about the scaling of DW mobility with field strength. The scaling differs between magnetic and electric field drives, and this is due to a parasitic nature of electric polarization. The second finding is the instability of a composite DW, and it splits into a pair of other types of DWs when a driving field exceeds a critical value. This originates in the deformation of DW internal structure, and thus is an
intrinsic dynamical instability. Phase diagram of DW stability is determined in the space of applied field and strength of order parameter interaction.

[1] Y. Tokunaga et al., Nature Mater 8, 558 (2009)
[2] K. Kawahara and H. Tsunetsugu, arXiv:2008.08600, in press in J. Phys. Soc. Jpn.   

Live

We perform micromagnetic simulations to show that energy efficient [1] strain control of domain wall (DW) in a perpendicularly magnetized racetrack can realize a multi-state synapse well suited to neuromorphic computing-based classification tasks. In conjunction with a fixed current exerting spin orbit torque (SOT), the racetrack is strained to modulate its anisotropy by applying voltage across the piezoelectric substrate on which the racetrack is patterned to control the translation of the DW to different positions in the racetrack. Simulations that include edge roughness and thermal noise showed that 5-state and 3-state synapse are possible in a 500 nm long and 50 nm wide racetrack [2]. Such limited state DW synapse is attractive to implement quantized neural network which is proven to achieve near equivalent accuracy to full-precision network [3] even in the presence of device variability [4]. Preliminary experiments with such racetracks will also be presented.

[1]. M. A. Azam et al., Nanotechnology (2020)
[2]. W. A. Misba et al., https://arxiv.org/abs/2010.10076
[3]. I. Hubara et al., J. Mach. Learn. Res (2017)
[4]. V. Joshi et al., Nat. Commun. (2020)   

Live

The control of domain walls (DW) is central to nearly all magnetic technologies, particularly for information storage and spintronics. Creative attempts to increase storage density need to overcome volatility due to thermal and quantum fluctuations of nanoscopic domains and heating limitations. In contrast, topological defects, such as solitons, skyrmions, and merons, maybe much less susceptible to fluctuations, owing to topological constraints, while also being controllable with relatively low current densities. Here we present, the first evidence for soliton/soliton and soliton/antisoliton DW in a thin lamella of the hexagonal chiral magnet Mn_1/3NbS₂ that respond asymmetrically to small magnetic fields and exhibit pair-annihilation. This is important because it suggests that it may be possible to control the occurrence of soliton pairs and to make use of small fields or, more interestingly, small currents to control these nanoscopic magnetic domains. Specifically, our data suggest we can stabilize a state of either soliton/soliton or soliton/antisoliton pairs by tuning the balance between intrinsic exchange interactions and long-range magnetostatics in restricted geometries.   

Live

The ferrimagnetic spinels Mn3O4 and MnV2O4 are known for their anomalous magnetoresponsive behaviors, including strong magnetoelastic and magnetodielectric couplings at low temperatures. At the same temperatures, crystals of these materials display a real-space separation of the volume into nanometer length-scale stripe domains, reminiscent of those in ferroelectrics. These stripe patterns demonstrate a remarkable sensitivity to environmental stress, and are controllable by applied fields. In this talk, we present new small angle neutron scattering and bulk data on single crystalline MnV2O4 which allow us to directly correlate anomalous inverse magnetoelastic and magnetocapacitance effects with the onset and field response of these mesoscale features. Neutron diffraction and small-angle X-ray scattering meanwhile reveal a strong intertwining of these effects with lattice degrees of freedom, collectively establishing strain as an important tuning parameter for anomalous behavior.   

Live

Drag effects in domain-wall dynamics in metallic ferromagnets have been observed in a wide range of materials, and traditionally have been attributed to eddy current back-action. Recently, drag effects have been observed in the electrically-insulating disordered Ising ferromagnet, LiHo_xY_1-xF_4, which cannot sustain eddy-currents. It was hypothesized that these drag effects were instead driven by disorder-induced random-field pinning. We test this hypothesis by measuring the statistics and form of magnetic avalanches due to domain-wall motion in the pure compound, LiHoF_4, free from random-field pinning. Following the approach derived from the ABBM model, we examine the average scaled shape of Barkhausen events and test for drag effects by characterizing the skewness. We find that shorter Barkhausen events approach a symmetric lineshape, indicating a drag-free response, whereas longer avalanches do retain a weak drag signature. Furthermore, the dynamics of the system can be tuned by enhancing the quantum tunneling rate via an applied magnetic field transverse to the Ising axis. Tuning from a classical regime to a quantum regime dominated by tunneling reveals the structure of the free-energy surface and barriers to domain reorientation, with applications to quantum annealing.   

Live

Recent experiments show striking unexpected features when sequences of alternating magnetic field pulses are applied to ferromagnetic samples: domains show area reduction and domain walls change their geometrical properties. In this work, we use a simple scalar-field model which only considers two preferential values for an order parameter, short-range exchange, disorder, and temperature. By proposing a numerical protocol that mimics the experimental one, we show that domains described by the model are also subject to area reduction under sequential field application. We study two domain geometries common in ferromagnetic films: a bubble and a stripe domain. In both cases, the area reduction as a function of the number of alternating field cycles follows a linear combination of an exponentially and a linearly decreasing functions, as reported for the experiments. We characterize the domain walls geometry by computing its roughness exponents. The obtained roughness exponents are indistinguishable from the ones observed experimentally under alternating fields and also when the standard protocol to measure velocities is numerically emulated.   

On Demand

We propose a four-terminal domain wall-magnetic tunnel junction (DW-MTJ) neuron that enables the first-ever CMOS-free purely spintronic perceptron with unsupervised learning. The proposed leaky integrate-and-fire neuron has a ferromagnetic DW track coupled to a free layer of a binary MTJ by an electrically insulated layer. Current through the ferromagnetic track performs integration by moving the DW. Intrinsic leaking occurs by moving the DW in the opposite direction of integration due to either dipolar magnetic field, anisotropy gradient, or shape variation. When the DW passes underneath the electrically-isolated MTJ, it fires by switching between the anti-parallel resistive state and parallel conductive state. Additionally, by exploiting stray magnetic field interactions, these neurons perform lateral inhibition.
In a crossbar perceptron, the DW track of each four-terminal neuron is connected to the analog three-terminal DW-MTJ synapses. Finally, an unsupervised learning algorithm results from the feedback between the neuron MTJ and the third terminals of the analog synapses, providing best results of 98.11% accuracy on the Wisconsin breast cancer clustering task.