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The establishment of chiral coupling in thin magnetic films with inhomogeneous anisotropy has led to the development of novel artificial systems of fundamental and technological interest. The chiral coupling itself is enabled by the Dzyaloshinskii-Moriya interaction (DMI) enforced by the patterned non-collinear magnetization [1-4]. Here, we create a domain wall track with out-of-plane (OOP) magnetization coupled to OOP stripes on each side via narrow parallel strips with in-plane magnetization as shown in Fig.1 (a). Since the chiral coupling prefers antiparallel OOP magnetization configuration over the parallel case, the regions of noncollinear magnetization in a single magnetic layer can be used to bias the domain wall velocity, as shown in series of Kerr images in Fig.1 (b). To tune the chiral torques, the design of the magnetic racetracks can be modified by varying the width of the tracks or the width of the transition region between noncollinear magnetizations, reaching effective chiral magnetic fields of up to 8 mT. Furthermore, we show how the magnitude of the chiral torques can be estimated by measuring asymmetric domain wall velocities. We also demonstrate spontaneous domain wall motion propelled by intrinsic torques even in the absence of any external driving force, which is of high importance for domain wall motion-based neuromorphic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/630ddf8c04b523dac1399ba71d81db43.jpg)

Figure 1: (a) Schematic of a DW racetrack with OOP magnetization delimited by two strips with IP anisotropy. The energy landscape of the DW racetrack is monotonous, except for local DW pinning sites. (b) Sequence of Kerr images of asymmetric DW motion in a racetrack with wOOP = 950 nm. The dashed lines correspond to the position of the IP magnetized strip while the full lines correspond to the boundary of the magnetic track. 80 current pulses are applied between each image; the pulse duration is 50 ns and the current density is 3.6×1011 A/m2.

2022 Joint MMM-INTERMAG

Engineering of intrinsic chiral torques in magnetic thin films based on the Dzyaloshinskii-Moriya interaction

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

technical paper

Single pulse all-optical switching (SP-AOS) using 40 fs light pulses was discovered in films of the amorphous rare earth-transition metal (RE-TM) alloy a-Gd(FeCo)3 [1]. The switching takes less than 5 ps and relies on the different demagnetisation times of the Gd and Fe,Co subnetworks. Replacing Gd by another RE was expected to yield more materials that exhibit ultrafast switching, but it did not happened. Indeed, amorphous RE-TM alloys other than Gd(FeCo)3 have only been switched transiently by a single pulse [2] or slowly using many polarised pulses [3]. Here, we report stable SP-AOS in amorphous Dy-Co and Tb-Co films with perpendicular anisotropy. Room-temperature coercivity is 30-50 mT, and compensation temperatures Tcomp are below 300 K. The example of a-DyCo3 is shown in Fig 1. The behavior is surprising. A single 200 fs pulse creates a non-uniform 40 µm dark spot on a uniformly-magnetized light film. Domain structure on a scale of ~ 3 µm is observed. After two pulses, the darker domains switch back, and the multidomain state will toggle for a few more pulses before leaving the sample in a nonuniform mid-gray state. Images are obtained on a strip of material irradiated with 1-5 pulses, using an off-site Kerr microscope. Other samples show a variety of behavior. The switching of a-TbCo4 shown in Fig. 2 is also granular, but the lighter and darker spots are aggregated into lighter and darker rings with unequal numbers of small dark and light domains. The contrast between rings again disappears after a few pulses, but the small domains persist. This is quite unlike the uniform toggling of large domains, comparable in size to the laser spot, that are seen in SP-AOS of ferrimagnetic a-Gd(FeCo)3 and crystalline Mn2RuxGa. The key difference is probably the random uniaxial anisotropy for the non S-state rare ions and temperature-dependence, which governs that of the coercivity. The influence of stray dipolar fields on a µm scale and possible of radial heat waves in the substrate will be considered in the context of ultra-fast SP-AOS in these a-RE-TM materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/d2f69edc767f147428037a22f4910d14.jpg)

SP-AOS in a 10 nm a-DyCo3 alloy film with perpendicular magnetization. Scale bar 40 μm.

![](https://assets.underline.io/uploads/markdown_image/1/image/952962fa921f2d2bdfcf19a77cdb0149.jpg)

SP-AOS in 20 nm a-TbCo4 with perpendicular magnetic anisotropy.

Single pulse all-optical magnetic switching in amorphous DyCox and TbCox

The ability to control and tune magnetic dissipation is a key concept of emergent spintronic technologies. Magnon-magnon processes constitute a major dissipation channel in nanomagnets [1-3], redefine their response to spin-torques [4], and hold promise for manipulating magnetic states on the quantum level. Controlling these processes [5,6], while being imperative for many spintronic applications [4], has remained difficult to achieve. We identify two avenues to realize this goal: by breaking the symmetry of the static magnetic state or by breaking the symmetry of the magnon modes. Here we focus on the latter approach due its particular technological relevance. As a vehicle for our study, we employ magnetic tunnel junction nanopillars that consist of a free layer and a synthetic antiferromagnet (SAF) [7]. In the normal state of the SAF, its stray field is small at the position of the free layer – the fields from the two constituent layers of the SAF largely cancel out [8]. However, once we trigger SAF’s spin-flop transition, their spins become canted and generate a stray field which is nonuniform at nanoscale – a prerequisite for breaking the symmetry of free layer’s magnons. By controllably switching the SAF between its normal and spin-flop states, we achieve to toggle three-magnon processes in the free layer. We tune the magnon interaction by at least one order of magnitude and thus engineer two distinct dissipative states. Our results open up an avenue for manipulating magnon processes by external stimuli at nanoscale, tuning the nonlinear response in magnetic neuromorphic networks, and controlling large excitation regimes in spin-torque devices. They furthermore show promise for engineering coherent magnon coupling in hybrid quantum information systems. This work was supported by the National Science Foundation through Grant No. ECCS-1810541.

Controlling Magnon Interaction by a Nanoscale Switch

Ferromagnetic [Pt/Co]n multilayers that are composed of n stacked [Pt/Co] bilayers have been widely investigated over the past decades, mainly due to the easy tunability of various magnetic properties such as the potentially strong perpendicular magnetic anisotropy (PMA). More recently, it has been demonstrated that the addition of heavy metals (HM) such as Ir or W can lead to the presence of sizeable chiral exchange interactions in [Pt/Co/HM]n systems [1]. As a consequence, topological spin textures (e.g., skyrmions) are commonly reported in these material systems even at room temperature, thus making them promising candidates for future spintronics applications [2]. In the present work, we use additional manganese layers to break the spatial inversion symmetry of conventional [Pt/Co]n stacks and thereby, in combination with spin-orbit interaction, introduce chiral exchange interactions. The investigated [Pt/Co/Mn]n samples were fabricated by magnetron sputtering. Structural characterization of the multilayer stacks was done by performing X-ray reflectometry (XRR) measurements, while magnetic hysteresis curves were acquired in a vibrating sample magnetometer (VSM). In addition, real-time imaging of magnetic domains as a function of the external magnetic field was conducted using a home-built magneto-optical Kerr effect (MOKE) microscope. Our results indicate a complex and sensitive dependence of various magnetic energy terms on the individual layer thicknesses, number of multilayer repetitions as well as on deposition parameters. This is particularly reflected in the shape of hysteresis loops or the types of spin textures seen in our magneto-optical imaging experiments. For instance, we discuss that the presence of defects leads to a very slow dynamics of dendritic-type domain groth in some of the investigated multilayers. Finally, we also elaborate on various approaches towards the possible observation of magnetic skyrmions in [Pt/Co/Mn]n multilayers. This work is supported by the Deutsche Forschungsgemeinschaft (DFG) through the research fellowship LO 2584/1-1 and the NSF through the University of Illinois at Urbana-Champaign Materials Research Science and Engineering Center under Grant No. DMR-1720633. .

Tuning the Magnetic Properties of Ferromagnetic [Pt/Co/Mn]n Multilayers

The recent reports on magnetodynamic properties of soft magnetic thin films attain the lowest damping factor [1-3]. The damping is required for spin-wave propagation and spin manipulation investigation where the reduced spin-wave dissipation leads to a larger decay length. The low damping thin films are important for microwave applications [4], spin transfer switching devices, and other application in magnon spintronics. In order to achieve the lowest damping, the FeGaB has chosen with Al2O3 in this study. Ta(10nm)/FeGaB(15nm)/Al2O3(3nm)/Ta(3nm) thin film were deposited on Si/SiO2(190nm) substrate by sputtering and was annealed at 300oC for 3 hours in the Ar gas. The static magnetization results inform that saturation and remanent magnetization has increased for annealed thin film shown in Fig.1 (a). After annealing, the dipolar field influence has reduced, the magnetic moment fluctuation has not observed. Fig. 1 (b) shows the annealed films have monotonously increased. The dynamics properties are studied with temperature variation with 100 K to 300 K and exciting frequency from 2 to 20 GHz. Fig.2 (a) illustrates the absorption spectrum of both films at 100K, the spectrum has shifted to lower fields for annealed films. The unannealed thin film have existed the small hump at higher magnetic films before resonance peak, which may be impurities and surface defects of the thin film. But, the hump has disappeared for annealed films in Fig. 2 (b). The absorption spectrum provides linewidth and resonance fields (Hr) for excitation frequency. The linewidth –frequency plot provides damping factor and inhomogeneous linewidth [5]. The damping factor of both films are decreasing with decreasing temperature as Fig.2 (c). Fig.2 (d) shows the annealed film has larger effective magnetization value than that of unannealed thin film.

![](https://assets.underline.io/uploads/markdown_image/1/image/456a20cfd7f6097c2f6a3fe240b5f535.jpg)

Fig.1 (a) Normalized M-H loop for unannealed and annealed thin films at 300K. Insets shows M-H loop at different temperatures; (b) Coercive fields (Hc) versus temperature.

![](https://assets.underline.io/uploads/markdown_image/1/image/f9b3d8de83b131c23c6946adfb17f683.jpg)

Fig.2 (a) Absorption spectrum of both thin films at 100K, (b) absorption of annealed thin film at 20GHz with different temperatures, (c) damping factor (α) versus temperature, (d) effective magnetization versus temperature.

Temperature dependence static and dynamic properties of soft magnetic thin film FeGaB/Al2O3 for microwave applications

Magnetization dynamics in laterally confined nanodots have potential applications in spin-torque nano oscillators,1 magnetic logic and sensors2. The ground state of nanodot shows vortex at remanence and it exhibits rich spin dynamics spectra3. With the application of magnetic field, the vortex and saturated magnetization states are observed at low and high field regions respectively4. In our present work, we have studied the magnetization dynamics in high aspect ratio single and trilayer nanodots. Circular Permalloy (Py) nanodots (diameter = 120nm) are fabricated over a large area using deep ultraviolet lithography technique. The structures consist of single layer nanodots with thicknesses (L) 20 and 50 nm and a trilayer with two Py layers separated by a non-magnetic layer: Py(20)/Pd(10)/Py(50). Numbers within the brackets are in nm. The dynamic properties are characterized using the ferromagnetic resonance (FMR) technique. The simulated hysteresis loops are shown in Fig. 1(a). The FMR absorptions show rich spectra as a function of magnetic field and thickness. For instance, we discuss the experimental results at an excitation frequency of 10 GHz as shown in Fig. 1(b). We observed two modes for L = 20 nm nanodots at resonance fields 130 and 220 mT and a single mode appears at 210 mT for L = 50 nm nanodots. We performed micromagnetic simulations to understand the experimental results. The simulated mode profiles reveal that the high intense mode corresponds to the center mode (CM) and the low intense mode corresponds to the edge mode (EM) arising due to the inhomogeneous effective field. The CM is found to be sensitive to the thickness of the nanodot. On the other hand, in the FMR spectra of Py(20)/Pd(2)/Py(50), two resonant modes appear at 200 mT and 250 mT. These two center modes arise due to the in-phase and out-of-phase precession of spins in magneto-dynamically coupled layers. These results can be useful for nanoscale magnonic applications.

![](https://assets.underline.io/uploads/markdown_image/1/image/2e01f4ef86e02401510bbe909771a373.jpg)

Fig 1: (a) Simulated hysteresis loops for single and trilayer nanodot arrays. The remanent states are shown in the inset. (b) Experimental FMR spectra of single and trilayer nanodot arrays.

Magnetization Dynamics of Single and Trilayer Permalloy Nanodots

Tailoring and control of electromagnetic properties of synthetic magnetic structures is advantageous for high frequency applications, such as shielding materials in magnetic recording, where characterisation in the gigahertz frequency range is necessary to determine parameters for simulations and aid component design. In such applications it is desirable to have a high saturation magnetisation, low coercivity, high permeability and close to zero magnetostriction. Meeting almost all these requirements, NiFe is the material of choice for many high frequency applications. We present a systematic study of the ferromagnetic resonance (FMR) and spin wave properties of NixFe100-x alloy thin superlattice films of the form n[NixFe100-x/NiyFe100-y], where n is the number of bilayers. Films were fabricated by planar magnetron sputtering and the effects of deposition conditions were investigated. Magnetic and material properties were measured using FMR [1], vibrating sample magnetometer and x-ray diffraction, with the aim to determine any relation between microstructure and the static and dynamic magnetic properties. Layer compositions within the bilayers have been chosen with particular interest in fcc/bcc layer combinations such as Ni80Fe20/Ni20Fe80, which may have benefits for some static properties such as high magnetisation and low magnetostriction [2]. Studies on single layers have shown dramatic changes in FMR response across the fcc-bcc boundary near x = 40, figure 1. However, for bilayer films it was found that increasing the thickness of the bcc Ni20Fe80 layer in the structure increased the resonant frequency but did not significantly reduce the FMR signal, figure 2.

![](https://assets.underline.io/uploads/markdown_image/1/image/2d9c99cff6d1c85880be60efcc3b4eb1.jpg)

Fig. 1: FMR spectra of 100nm NixFe100-x single layer thin films with x varying from 10 to 60. Note that all measurements were performed at an applied field H=253Oe. XRD is inset.

![](https://assets.underline.io/uploads/markdown_image/1/image/80e55db23ea15b0120ab8fa67c4beb0e.jpg)

Fig. 2: FMR spectra of Ni80Fe20(60-t nm)/Ni20Fe80(t nm) thin films with t varying from 50 to 10. Note that all measurements were performed at an applied field of 253Oe. XRD is inset.

Ferromagnetic Resonance of Tuneable Low Magnetostrictive NixFe100-x Superlattice Structures for High Frequency Application

I Introduction Accurate iron loss prediction is of great significance to the optimal design of electrical equipment[1]. At present, the application of silicon steel lamination in complex excitation greatly increases the difficulty of accurately predicting iron loss, especially eddy current loss. To solve the above problems, various eddy current loss prediction methods have been proposed[2]-[3], but these methods lack the physical background. A physical-based improved parametric magneto-dynamics (IPMD) model is proposed. For the sake of calculating the eddy current loss more accurately, the Tellinen (TLN) hysteresis model is used in the implementation of the IPMD model. II Improved Parametric Magneto-Dynamic Model To solve the problem of inaccurate calculation caused by nonuniform magnetic field distribution of silicon steel lamination at high frequency, the IPMD model based on one-dimensional Maxwell diffusion equation is proposed in this paper. In the IPMD model, the influence of hysteresis on magnetic field distribution is considered by coupling the TLN model, and the influence of skin effect at high frequency is considered by using the nonlinear segmentation method. The segmentation method divides the analysis area of silicon steel lamination into several slices and realizes the uniform expression of the magnetic field on a single slice. According to the characteristics of slow change of magnetic field of inner slice and fast change of magnetic field of outer slice, the thickness of outer slice is appropriately reduced and the thickness of inner slice is appropriately increased, as shown in Fig.1. By using the piecewise constant function to approximate the magnetic field distribution of silicon steel lamination, the eddy current loss can be accurately predicted by the IPMD model, as shown in Fig.2. III Conclusion The IPMD model proposed in this paper can accurately predict the eddy current loss in a wide frequency range while considering the physical background, which has practical significance for the study of eddy current loss characteristics of soft magnetic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/3f29b24d3766d42592f6f99f46519de4.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/d51ad658646887115e6a672e465bac31.jpg)

An Improved Eddy Current Loss Model Over Broadband Frequency Range

An Improved Eddy Current Loss Model Over Broadband Frequency Range

Recently, ferromagnetic resonance (FMR) in insulating YIG film was shown to be driven by microwave current in the Pt film deposited on the top of YIG film and detected as a dc voltage in the Pt layer due to spin Hall effect and/or spin rectification[1,2]. Here, we demonstrate an alternative method to study FMR in ceramic YIG. A polycrystalline pellet was made from YIG nanopowder by sintering in the air at 900oC for 12 hr. Magnetization at 300K, shown in the main panel of Fig.1(a), affirms the soft ferromagnetic nature of YIG. The upper inset in (a) illustrates the sketch of our experimental setup. Since the resistance of YIG could not be measured by the impedance analyzer, a layer of silver paint was coated on the top surface of the sample and at two ends to allow radio-frequency current to flow. The sample bridges the signal line and the ground plane of the sample stage, which is connected to the impedance analyzer via a rigid coaxial cable. A microwave signal is injected into the Ag paint layer through the signal line. Resistive and reactance of the Ag-paint were measured while sweeping frequency at different values of the dc magnetic field,H. The lower inset shows magnetoresistance MR(b) and magnetoreactance MX(c) for five frequencies (f) as H decreased from 3 kOe to 0 Oe. The MR of the Ag-paint for 3 GHz exhibits a prominent peak at H = Hr around which MX crosses zero. Both the amplitude of the peak in MR and Hr decreases as f decreases. We argue that the peak in MR is caused by FMR. The inset shows the dependence of Hr on f and we are able to fit the data with Kittel’s relation for in-plane FMR. The observed electrical detection of FMR in YIG with colloidal silver particles having weak spin-orbit coupling than Pt deserves an urgent understanding. R. M. thanks the Ministry of Education, Singapore for support (Grant no. R144-000-442-114).

![](https://assets.underline.io/uploads/markdown_image/1/image/ae1fde754d75a41639f3df99d02d023b.jpg)

Fig. 1 (a) M-H isotherm of YIG sample at 300 K(main panel) and the schematic diagram of the experimental setup (upper inset). Field dependence of (b) magnetoresistance MR)and (c) magnetoreactance MX at selected frequencies of current excitation (bottom inset). Inset in (b) shows the frequency dependence of the resonance field (symbols) and Kittel's fit for FMR.

Electrically detected ferromagnetic resonance in polycrystalline YIG covered with a layer of Ag paint

The prospect of thermal imaging and control in magnetic particle imaging (MPI) is an important advancement for non-invasive medical diagnostics and therapeutics. Current MPI systems focus mainly on imaging from measurement of nanoparticle concentrations. Accurate derivation of temperature relies on detailed understanding of magnetization dynamics in the presence of AC drive fields and variable temperature [1]. Here, we report measurements of nanosecond magnetization dynamics using both time and frequency domain techniques. It has been reported that the imaging resolution of MPI is critically impacted by nanoparticle relaxation dynamics [2]; specifically, spin relaxation results in image blurring. The dynamic response (e.g. Néel and Brownian relaxation) of magnetic nanoparticles in magnetic fields is intricate, depending strongly on the particles’ inherent magnetic and structural properties, inter-particle interactions, and the local environment. We have developed an arbitrary-wave magnetic particle spectrometer with magnetic field amplitude up to 10 mTPk for characterizing magnetization dynamics over a broad temperature range (200 K to 350 K). We have characterized the dynamic AC susceptibility of nanoparticles ranging in diameter from 10 nm to 70 nm and in composition and observed peaks in the imaginary component (χ’’) at frequencies from as low as 50 Hz to 50 MHz. Figure 1 show the χ’’ peak frequency for 11.5 nm cobalt-doped ferrite nanoparticles at different temperatures. This peak reveals the effective relaxation timescale (τ ~ 1 μs). Additionally, we used pulsed excitation to characterize the magnetization dynamics with and without magnetic fields. Figure 2 shows a magnetization response of 7.5 nm cobalt-doped ferrites to a magnetic field pulse with 20 ns rise time. Monte Carlo simulations are used for analyzing both time and frequency domain measurements. This knowledge will eventually inform strategies for design and synthesis of magnetic nanoparticles with properties targeted for accurate and sensitive thermal imaging using MPI.

![](https://assets.underline.io/uploads/markdown_image/1/image/c09e0bfc2893d134e13e9ccf078bc358.jpg)

The imaginary component of AC susceptibility for 11.5 nm ferrite nanoparticles

![](https://assets.underline.io/uploads/markdown_image/1/image/cab02aa8ffbaedf3c9a626601584bbd6.jpg)

Pulsed magnetic field (black). Magnetization step-response of 7.5 nm cobalt-doped ferrite (red).

Magnetization dynamics of magnetic nanoparticles for thermal and magnetic particle imaging

New physics emerge from the interplay between spin waves and magnetic topological objects. Excitation modes permit to exploit the full potential of topological objects and increase the range of applications, in particular in the field of insulator-based spintronics. Recently, the number of proposals leveraging excitation modes of topological objects is steadily increasing, such as in microwave generation and detection, spin-wave guides, parametrons and in neuromorphic spintronic devices. In this talk I will present three ways to exploit excitations of magnetic topological objects. First, I will show that domain walls in thin films behave as flexible strings with an extra internal degree of freedom1. A direct application is the use of domain walls as spin-wave guides and hosts of magnetic domain wall skyrmions. Moreover, by considering closed domain wall strings, I will derive analytically deformations, instabilities2,3, and excitation modes of skyrmions4. Second, I will propose the use of topological objects as tunable nanoscale in-materio frequency multipliers for spintronic systems5. I will show that the excitation of topological objects by fractions of the eigenfrequencies presents advantages compared to the well-known parametric excitation as it requires less energy input and does not generate instabilities away from the excited topological objects. Third, I will discuss a mechanism to controllably manipulate the direction and speed of domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source6. In this case, the coupling of spin wave modes inside the non-collinear domain wall allows for a highly tunable spin-wave induced domain wall motion with a frequency dependent domain wall velocity.

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