Torques appear between charge carrier spins and local moments in regions of ferromagnetic media where spatial magnetization gradients occur, such as a domain wall, owing to an exchange interaction. This phenomenon has been predicted by different theories 1-7 and confirmed in a number of experiments on metallic and semiconductor ferromagnets 8-19. Understanding the magnitude and orientation of such spin-torques is an important problem for spin-dependent transport and currentdriven magnetization dynamics, as domain-wall motion underlies a number of emerging spintronic technologies 20,21. One outstanding issue concerns the non-adiabatic spin-torque component β, which has an important role in wall dynamics, but no clear consensus has yet emerged over its origin or magnitude. Here, we report an experimental measurement of β in perpendicularly magnetized films with narrow domain walls (1-10 nm). By studying thermally activated wall depinning, we deduce β from the variation of the Arrhenius transition rate with applied currents. Surprisingly, we find β to be small and relatively insensitive to the wall width, which stands in contrast to predictions from transport theories 2,5-7. In addition, we find β to be close to the Gilbert damping constant α, which, in light of similar results on planar anisotropy systems 15 , suggests a universal origin for the non-adiabatic torque. The adiabatic torque, which accounts for transport processes in which the conduction spin follows the local spatial magnetization variation by remaining in either the majority or minority state, is well understood and has been reproduced by a number of different transport theories. In contrast, the non-adiabatic contribution, characterized by a dimensionless parameter β (ref. 22), remains the subject of much debate. Various mechanisms have been put forward to explain its origin, such as momentum transfer 2,7 , spinmistracking 4,6 or spin-flip scattering 3. It is predicted that large nonadiabatic effects should appear in narrow domain walls because of large magnetization gradients 2,5,6 , whereby the wall width becomes comparable to important transport scales such as the spin-diffusion length 2 or the Larmor precession length 6 , which are of the order of a few nanometres in ferromagnetic transition metals. The presence of a non-adiabatic term is of fundamental importance, because its existence implies that current-driven wall motion is possible for any finite current in a perfect system, even in the absence of an applied magnetic field. Difficulty in characterizing β experimentally therefore stems in part from being able to distinguish between extrinsic sources of wall pinning, due to structural defects, for example, from the intrinsic finite threshold current predicted 2 for β = 0. We have studied current-driven domain wall dynamics in two different pseudo spin-valve systems based either on CoNi

The microscopic magnetization variation in magnetic domain walls in thin films is a crucial property when considering the torques driving their dynamic behaviour. For films possessing out-of-plane anisotropy normally the presence of Néel walls is not favoured due to magnetostatic considerations. However, they have the right structure to respond to the torques exerted by the spin Hall effect. Their existence is an indicator of the interfacial Dzyaloshinskii–Moriya interaction (DMI). Here we present direct imaging of Néel domain walls with a fixed chirality in device-ready Pt/Co/AlOx films using Lorentz transmission electron and Kerr microscopies. It is shown that any independently nucleated pair of walls in our films form winding pairs when they meet that are difficult to annihilate with field, confirming that they all possess the same topological winding number. The latter is enforced by the DMI. The field required to annihilate these winding wall pairs is used to give a measure of the DMI strength. Such domain walls, which are robust against collisions with each other, are good candidates for dense data storage.

We demonstrate that an antiferromagnet can be employed for a highly efficient electrical manipulation of a ferromagnet. In our study, we use an electrical detection technique of the ferromagnetic resonance driven by an in-plane ac current in a NiFe/IrMn bilayer. At room temperature, we observe antidampinglike spin torque acting on the NiFe ferromagnet, generated by an in-plane current driven through the IrMn antiferromagnet. A large enhancement of the torque, characterized by an effective spin-Hall angle exceeding most heavy transition metals, correlates with the presence of the exchange-bias field at the NiFe/IrMn interface. It highlights that, in addition to the strong spin-orbit coupling, the antiferromagnetic order in IrMn governs the observed phenomenon.

Using localized surface plasmon resonances (LSPR) to focus electromagnetic radiation to the nanoscale shows the promise of unprecedented capabilities in optoelectronic devices, medical treatments and nanoscale chemistry, due to a strong enhancement of light-matter interactions. As we continue to explore novel applications, we require a systematic quantitative method to compare suitability across different geometries and a growing library of materials. In this work, we propose application-specific figures of merit constructed from fundamental electronic and optical properties of each material. We compare 17 materials from four material classes (noble metals, refractory metals, transition metal nitrides, and conductive oxides) considering eight topical LSPR applications. Our figures of merit go beyond purely electromagnetic effects and account for the materials’ thermal properties, interactions with adjacent materials, and realistic illumination conditions. For each application we compare, for simplicity, an optimized spherical antenna geometry and benchmark our proposed choice against the state-of-the-art from the literature. Our propositions suggest the most suitable plasmonic materials for key technology applications and can act as a starting point for those working directly on the design, fabrication, and testing of such devices.

Titanium oxynitride (TiON) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained, and their optical properties are presented. The films grown when the level of residual oxygen in the background vacuum was between 5 nTorr to 20 nTorr exhibit double epsilon-near-Zero (2-ENZ) behavior with ENZ1 and ENZ2 wavelengths tunable in the 700-850 and 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual oxygen in the background vacuum was above 2 × 10 Torr exhibit nonmetallic behavior, while the layers deposited when the level of residual oxygen in the background vacuum was below 5 × 10 Torr show metallic behavior with a single ENZ value. The double ENZ phenomenon is related to the level of residual oxygen in the background vacuum and is attributed to the mixture of TiN and TiON and TiO phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiO, and TiON compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behavior is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of TiO and TiN phases suggesting that double ENZ TiON films are formed by inclusions of TiN within a TiO matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges. Materials with ENZ properties are advantageous for designing the enhanced nonlinear optical response, metasurfaces, and nonreciprocal behavior.

The antiferromagnetic ground state and the metamagnetic transition to the ferromagnetic state of CsCl-ordered FeRh epilayers have been characterized using Hall and magnetoresistance measurements. On cooling into the ground state, the metamagnetic transition is found to coincide with a suppression in carrier density of at least an order of magnitude below the typical metallic level that is shown by the ferromagnetic state. The carrier density in the antiferromagnetic state is limited by intrinsic doping from Fe/Rh substitution defects, with approximately two electrons per pair of atoms swapped, showing that the decrease in carrier density could be even larger in more perfect specimens. The surprisingly large change in carrier density is a clear quantitative indication of the extent of change at the Fermi surface at the metamagnetic transition, confirming that entropy release at the transition is of electronic origin, and hence that an electronic transition underlies the metamagnetic transition. Regarding the nature of this electronic transition, it is suggested that an orbital selective Mott transition, selective to strongly-correlated Fe 3d electrons, could cause the reduction in the Fermi surface and change in sign of the magnetic exchange from FM to AF on cooling. Gesellschaft antiferromagnet (AF) that exhibits a first-order transition to the α phase, become a ferromagnet (FM) around T N = 350 → 400 K [2-4]. On heating, the metamagnetic transition between the two different but both fully ordered magnetic states (type II AF [5] and FM) is accompanied by an isotropic 1% volume expansion [6,7], a large entropy release [8], and a large drop in the resistivity [3]. The metamagnetic transition temperature is highly sensitive to the composition x in Fe x Rh 1−x [9, 10] and chemical doping [11], T N decreases with increased applied magnetic field [4,12] and increases with the application of pressure [13]. Neutron diffraction [5,14] and more recently x-ray magnetic circular dichroism (XMCD) measurements [15] indicate that part of the 3.3µ B magnetic moment centred on the Fe in the AF phase is transferred to the Rh in the FM phase, with µ Fe ∼ 2.2µ B and µ Rh ∼ 0.6µ B . The Curie temperature for the high-temperature FM phase is ∼670 K [3], comparable to the Curie temperature of alloys with x > 0.53 [9].The transition also occurs in epitaxially-grown and polycrystalline FeRh thin films [4, 10,16,17], albeit usually with a wider temperature hysteresis (∼15 K instead of the ∼5 K usual for bulk specimens). Thin films of FeRh have been of much recent interest because of a potential application in heat-assisted high-density magnetic recording, taking advantage of the extremely fast switching (within a few ps) from AF to FM that can be achieved with ultrashort laser pulses [18,19]. Further research on FeRh thin films is now focussed at tuning the transition towards specific applications, by growing epitaxially on a variety of substrates and in heterostructures [4,[20][21][22].There is an ongoing debate about the origin of this ...

Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here, we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite MnNiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of MnNiN with intrinsic biaxial strains of ±0.25% result in Néel transition shifts up to 60 K and magnetization changes consistent with theory. Films grown on BaTiO display a striking magnetization jump in response to uniaxial strain from the intrinsic BaTiO structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient α (where α = d B/d E) of 0.018 G cm/V. The latter agrees with the 1000-fold increase over CrO predicted by theory. Overall, our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature.

We have studied the anomalous Hall effect (AHE) in strained thin films of the frustrated antiferromagnet Mn 3 NiN. The AHE does not follow the conventional relationships with magnetization or longitudinal conductivity and is enhanced relative to that expected from the magnetization in the antiferromagnetic state below T N = 260 K. This enhancement is consistent with origins from the noncollinear antiferromagnetic structure, as the latter is closely related to that found in Mn 3 Ir and Mn 3 Pt where a large AHE is induced by the Berry curvature. As the Berry-phase-induced AHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of the chemically flexible Mn 3 AN structure.