United States

Spin-Hall oscillators (SHOs) are promising spintronic devices to realize dc controlled GHz frequency signals in nanoscale devices for a variety of applications1,2, including for neuromorphic computing3. However, traditional SHOs have high auto-oscillation threshold currents and those based on magnetic tunnel junctions require a complex etching process. Here, we demonstrate that by combining a metallic ferromagnetic nanowire with low damping and high-quality ferromagnetic insulators, specifically epitaxial lithium aluminum ferrite thin films4, SHO characteristics are dramatically improved. Our magnetic bilayers SHOs are composed of Pt(5nm)/Py(5nm)/(Li0.5Al1.0Fe1.5O4 (LAFO) or Li0.5Al0.5Fe2O4 (LFO)) (x nm) layers with varied thicknesses x (including x=0, i.e. just Pt/Py layers) and two different compositions of insulator. The Pt/Py layers are patterned into 400 nm wide x 400 nm long nanowires between two Au contact pads as shown in Fig. 1a. Ferromagnetic resonance (FMR) measurements of the unpatterned heterostructure and spin-torque ferromagnetic resonance (ST-FMR) of micron-scale wires show that Py and LAFO are strongly ferromagnetically coupled and thus precess coherently. In the nanowires, we observe two prominent auto-oscillation modes in all samples, which we associate with bulk and edge spin-wave modes. Increased oscillator quality factor and maximum emission power (Fig. 1b) as well as lower threshold currents are obtained in samples containing LAFO and LFO. In addition, the maximum emission power and quality factor increases with the LAFO thickness. The performance can be further enhanced by replacing LAFO with LFO which possesses higher saturation magnetization. The improved characteristics are associated with the excitation of larger spin-precession angles and volumes in LAFO and LFO samples. We further find that the presence of LAFO enhances the auto-oscillation amplitude of spin-wave edge modes, consistent with our micromagnetic modeling.&lt;br&gt;&lt;br&gt;
**References** &lt;br&gt;1 V.E. Demidov, S. Urazhdin, H. Ulrichs, V. Tiberkevich, A. Slavin, D. Baither, G. Schmitz, and S.O. Demokritov, Nat. Mater. 11, 1028 (2012).
2 Z. Duan, A. Smith, L. Yang, B. Youngblood, J. Lindner, V.E. Demidov, S.O. Demokritov, and I.N. Krivorotov, Nat. Commun. 5, 1 (2014).
3 J. Grollier, D. Querlioz, K. Y. Camsari, K. Everschor-Sitte, S. Fukami, and M. D. Stiles, Nature Electronics 3, 360 (2020)
4 X.Y. Zheng, L.J. Riddiford, J.J. Wisser, S. Emori, and Y. Suzuki, Appl. Phys. Lett. 117, 1 (2020).
&lt;br&gt;&lt;br&gt;

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MMM 2022

Spin Hall oscillators based on ferromagnetic metal/lithium aluminum ferrite bilayers with enhanced output power and quality factor

Spin-Hall oscillators (SHOs) are promising spintronic devices to realize dc controlled GHz frequency signals in nanoscale devices for a variety of applications1,2, including for neuromorphic computing3. However, traditional SHOs have high auto-oscillation threshold currents and those based on magnetic tunnel junctions require a complex etching process. Here, we demonstrate that by combining a metallic ferromagnetic nanowire with low damping and high-quality ferromagnetic insulators, specifically epitaxial lithium aluminum ferrite thin films4, SHO characteristics are dramatically improved. Our magnetic bilayers SHOs are composed of Pt(5nm)/Py(5nm)/(Li0.5Al1.0Fe1.5O4 (LAFO) or Li0.5Al0.5Fe2O4 (LFO)) (x nm) layers with varied thicknesses x (including x=0, i.e. just Pt/Py layers) and two different compositions of insulator. The Pt/Py layers are patterned into 400 nm wide x 400 nm long nanowires between two Au contact pads as shown in Fig. 1a. Ferromagnetic resonance (FMR) measurements of the unpatterned heterostructure and spin-torque ferromagnetic resonance (ST-FMR) of micron-scale wires show that Py and LAFO are strongly ferromagnetically coupled and thus precess coherently. In the nanowires, we observe two prominent auto-oscillation modes in all samples, which we associate with bulk and edge spin-wave modes. Increased oscillator quality factor and maximum emission power (Fig. 1b) as well as lower threshold currents are obtained in samples containing LAFO and LFO. In addition, the maximum emission power and quality factor increases with the LAFO thickness. The performance can be further enhanced by replacing LAFO with LFO which possesses higher saturation magnetization. The improved characteristics are associated with the excitation of larger spin-precession angles and volumes in LAFO and LFO samples. We further find that the presence of LAFO enhances the auto-oscillation amplitude of spin-wave edge modes, consistent with our micromagnetic modeling. 
**References** 1 V.E. Demidov, S. Urazhdin, H. Ulrichs, V. Tiberkevich, A. Slavin, D. Baither, G. Schmitz, and S.O. Demokritov, Nat. Mater. 11, 1028 (2012).
2 Z. Duan, A. Smith, L. Yang, B. Youngblood, J. Lindner, V.E. Demidov, S.O. Demokritov, and I.N. Krivorotov, Nat. Commun. 5, 1 (2014).
3 J. Grollier, D. Querlioz, K. Y. Camsari, K. Everschor-Sitte, S. Fukami, and M. D. Stiles, Nature Electronics 3, 360 (2020)
4 X.Y. Zheng, L.J. Riddiford, J.J. Wisser, S. Emori, and Y. Suzuki, Appl. Phys. Lett. 117, 1 (2020).
 

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technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 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 tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Implementation of spin transfer torque magneto-resistive random access memory (STT-MRAM) in memory chips requires that the write margin of the MRAM cell △Vwr, defined as the difference between breakdown voltage Vbd and write voltage Vwr for the specified endurance, write error rate and write speed, is sufficiently large in order to accommodate resistance variations arising from external chip circuitry (transistors, metal lines etc). Optimization of MRAM cell design and process as well as MRAM materials development aims at accomplishing these by either reducing Vwr, enhancing Vbd, or both [1]. In this talk we will show that △Vwr can be increased by optimizing the MgO cap layer that is already present in a perpendicular MRAM cell, whose original intended function has been to increase perpendicular magnetic anisotropy energy of the free storage layer (FL) by adding additional Fe/MgO interface, and thus improve the MRAM retention [2]. By increasing the thickness of the MgO cap layer to make its resistance-area product RA close to that of the main MgO barrier, we show that △Vwr can be increased substantially for the given write speed and endurance, without affecting thermal stability of the cell in a significant way. For example, for MRAM cells with diameters 50 - 60 nm we show that Vwr at 50% switching probability for 50 ns pulse width increases by about 0.28 V for thick cap (see Fig. 1(a)) while Vbd obtained by endurance measurements under bipolar stress at the same pulse width increases by almost 0.5 V, resulting in △Vwr improvement of approximately 0.2 V. We will discuss physical mechanisms underlying this finding and its technological importance. 
**References** [1] D. Apalkov et al., Proc. of IEEE 104, 1796 (2016).
[2] G. Jan et al., APEX 5, 093008 (2012). 
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Enhancing write margin of perpendicular MRAM cells using thick MgO cap layer

Ferrimagnets (FIMs) can function as high-frequency antiferromagnets while being easy to detect as ferromagnets, offering unique opportunities for ultrafast device applications. While the physical behavior of FIMs near the compensation point has been widely studied, there lacks a general understanding of FIMs that allows us to freely vary the ratio of sublattice spins between the FM and the AFM limit. Such a generic picture can not only reveal the unique features not visible near the compensation point but also unifies the spin dynamics in FIMs with that in FM and AFM materials. Here we investigate the physical properties of a two-sublattice FIM manipulated by static magnetic fields and current-induced torques. By continuously varying the ratio of sublattice spins, we clarify how the dynamical chiral modes in an FIM are intrinsically connected to their Ferro- and Antiferromagnetic counterparts, which reveals unique features not visible near the compensation point. 
**References** [1]M. Guo, H. Zhang, and R. Cheng, Manipulating Ferrimagnets by Fields and Currents, Phys Rev B 105, 064410 (2022).
[2]C. Kim, S. Lee, H.-G. Kim, J.-H. Park, K.-W. Moon, J. Y. Park, J. M. Yuk, K.-J. Lee, B.-G. Park, S. K. Kim, K.-J. Kim, and C. Hwang, Distinct Handedness of Spin Wave across the Compensation Temperatures of Ferrimagnets, Nat Mater 1 (2020).
[3]J. Barker and G. E. W. Bauer, Thermal Spin Dynamics of Yttrium Iron Garnet, Phys Rev Lett 117, 217201 (2016).
[4]S. K. Kim, K.-J. Lee, and Y. Tserkovnyak, Self-Focusing Skyrmion Racetracks in Ferrimagnets, Phys Rev B 95, 140404 (2017).
 
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Spin Dynamics of Ferrimagnets Jointing Ferro and Antiferromagnets

Since the first all-optical switching experiments carried on GdFeCo ferrimagnet [1-2], the all-optical switching represents an ultrafast and easy method to manipulate magnetization without any applied field. Single pulse helicity independent all-optical switching (HI-AOS) has been demonstrated in ferrimagnetic GdFeCo alloy, Gd/FM bilayers where FM is a ferromagnet, and recently in a ferrimagnetic Heusler alloy and Tb/Co multilayer-based electrode [2-5].
In this work, we demonstrate that robust toggle HI-AOS has been observed in ferrimagnetic Tb based multilayers (Fig.1a). The properties of single pulse HI-AOS in Tb-based multilayers mainly differ in two ways compared to GdFeCo alloys. First, the shape of the state diagram of Tb-based multilayers in which the toggle switching can be observed at a pulse duration of 10 ps is completely different from that of GdFeCo alloy [6] (Fig.1d). The threshold switching fluence (Fswitch) is independent of the laser pulse duration of at least up to 10 ps (Fig. 1b). Moreover, the multidomain fluence (Fmulti), above which a multidomain state is observed in the center of the spot, increases slightly for pulse durations below 3 ps and then remains constant for longer pulse durations. This indicates that the mechanisms responsible for the Tb-based multilayers switching differ from those of Gd-based alloys. Second, the observed equilibrium state after the first laser pulse excitation shows a complex structure with rings of opposite magnetization directions when the fluence increases. This ring structure was observed in all Tb-based multilayer films we have studied so far. The number and the size of the rings depend on the sample composition (thickness of the Tb and transition metal layers) as well as on the annealing condition.
Acknowledgments: UFO, COMRAD 
**References** [1] C. D. Stanciu, F. Hansteen, A. V. Kimel, Phys. Rev. Lett. 99, 047601, 2007.
[2] T. A. Ostler, J. Barker, R. F. L. Evans, Nat. Commun. 3, 666, 2012.
[3] M. Beens, M. L. M. Lalieu , A. J. M. Deenen, Phys. Rev. B 100, 220409(R) , 2019.
[4] L. Avilés-Félix, A. Olivier, G. Li, C. S. Davies, Sci. Rep. 10, 5211, 2020.
[5] C. Banerjee, N. Teichert, K. Siewierska, Nat. Commun., 11, 4444, 2020.
[6] J. Wei, B. Zhang, M. Hehn, Phys. Rev. Applied 15, 054065, 2021. 
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Robust toggle switching in Tb based multilayers by a single

Spin current generation lies at the heart of modern spintronics. Pure spin current can be generated via spin pumping, wherein the magnetization precession of ferromagnet produces a spin current into a neighboring non-magnetic layer.[1] So far, the spin pumping has mainly been used as a tool for quantifying magnetic parameters, including the spin Hall angle, spin diffusion length, spin mixing conductance, etc.,[2-4] because the efficiency of spin current generation is not large enough for use in spintronic applications, such as current-induced magnetization switching.[5] In this study, we propose a novel method to generate a large spin current via spin pumping. We utilize a FeRh which undergoes a magnetic phase transition from an antiferromagnet (AFM) to ferromagnet (FM) around 370 K.[6] The magnetic phase transition of FeRh from AFM to FM accompanies the change of total angular momentum from zero to finite. The change of angular momentum generates a spin current into a neighboring non-magnetic layer, and is converted to the charge current through the inverse spin Hall effect (ISHE). We find that the ISHE voltage is generated exclusively during the phase transition. Furthermore, the measured signal is found to depend on the sign of spin Hall angle and the direction of magnetization of FeRh, confirming that the observed signal indeed originates from the phase transition-induced spin pumping and ISHE. The generated spin current density is at least 3 orders of magnitude higher than those in previous spin pumping reports,[7,8] and is comparable to that generated by the spin Hall effect.[5] Our work provides a novel way to generate spin current, which could be utilized in potential spintronics applications.

Generation of Large Spin Current Burst during Magnetic Phase Transition of FeRh

A study of the effect of a nonuniform perpendicular magnetic anisotropy on the magnetic configurations and on the ferromagnetic resonance (FMR) spectra in two systems
was performed by micromagnetic simulations, by applying a short magnetic pulse [1-3].
First system corresponds to magnetic nanorings of different widening with radial distribution of the perpendicular magnetic anisotropy (RPMA) [1]. Both the resonant frequencies and the
number of peaks depend on the lower energy magnetization configuration [4] (figure 1) which in turn is a function of anisotropy gradient. Also, we compare the diagram between nanorings
with and without RPMA showing that the effects of the RPMA are relevant even for the narrowest ring of 10 nm wide [1].
The second system corresponds to nanodisks with radial distribution but with the center of the distributed anisotropy displaced from the center of the nanodisk (figure 2). This break on the
symmetry of the RPMA with respect to the disk center, allows several new magnetic configurations such as skyrmionium-like magnetic states and therefore the resonance spectra of FMR
have several new modes and peaks compared to non-breaking symmetry of RPMA.
The results show that by controlling the RPMA strength K of the anisotropy gradient, and its symmetry, several new resonance modes and magnetic configurations can be obtained for
instance, vortex, meron and knot states in nanorings and skyrmionum in nanodisks.
Authors acknowledge DICYT Grant 042131EM, PAI77190042, Fondecyt 1201491 and Basal AFB 180001. 
**References** 
[1] Saavedra, E. et al. Effect of nonuniform perpendicular anisotropy in ferromagnetic resonance spectra in magnetic nanorings. Sci. Rep. 11, 14230 (2021). 
[2] McMichael, R.D. and Stiles, M.D. Magnetic normal modes of nanoelements. J. Appl. Phys. 97, 10J901 (2005). 
[3] Baker, A. et al. Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations. J. Magn. Magn. Mater. 421, 428 (2017). 
[4] Castro, M.A. et al. New magnetic states in nanorings created by anisotropy gradients. J. Magn. Magn. Mater. 484, 55–60 (2019). 

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Figure 1: a) Minimal energy configurations as a function of the RPMA strength K: I, II, and III correspond to vortex, knot and meron states respectively. b) Imaginary component of the
dynamic susceptibility for R1 = 50 nm c) Spatial distribution of the amplitude for vortex (K =300 kJ/m3), knot (K = 350 kJ/m3) and meron (K = 450 kJ/m3), respectively.

Effect of nonuniform perpendicular anisotropy in ferromagnetic resonance spectra in magnetic nanorings and nanodisks

Strain-induced modification of magnetic properties in hybrid piezoelectric/ferromagnetic heterostructures is being intensively investigated due to its potential for low-power
spintronics applications based on magnetic domain wall (DW) motion ([1], [2]).
In this study, we present PMN-PT(011)/Ta/CoFeB/MgO-based devices where both out-of-plane and in-plane strain configurations have been exploited to control DW dynamics. In PMNPT (011), the application of a gate voltage along the out-of-plane (OOP) direction of the sample leads to a non-volatile polarisation state which can be reverted back to the un-poled state by
annealing at temperatures above the Curie temperature ([3]), which was confirmed in our system by piezoresponse force microscopy. In this configuration, we observe a non-volatile
increase in the coercivity in 10µm wires in the poled state of about 90% with respect to the unpoled state. This response can be linked not only to a strain-control of magnetic anisotropy but
also to a strain-controlled change in the homogeneity of the magnetic landscape. The multiple polarisation directions of the domains in the un-poled state of the PMN-PT can translate into
a wider anisotropy distribution in the magnetic system, compared to the poled state. This can significantly affect DW nucleation/depinning fields, which opens the door to using strain to
control magnetic disorder. We also combine this non-volatile effect with local, volatile, in-plane control of DW dynamics, dominated by strain-induced changes in magnetic anisotropy.
In conclusion, we present a multifunctional strain-controlled device where both disorder and anisotropy can be manipulated. We show that multiple strain configurations in
piezoelectric/ferromagnetic hybrid structures can open new opportunities for the electrical
manipulation of DW dynamics. 
**References** [1] Na Lei, Thibaut Devolder, Guillaume Agnus, et al., Nature Communications, 4, 1-4 (2013)
[2] P. M. Shepley, A. W. Rushforth, M. Wang, et al., Scientific Reports, 5, 1-5 (2015)
[3] V.S. Kathavate, B. Praveen Kumar, I. Singh, et al., Ceramics International, 46, 12876-12883 (2020)

Exploring multiple strain geometries in PMN PT/Ta/CoFeB/MgO magneto

Ferrimagnetic (FiM) alloys as GdFeCo, formed by rare-earth (RE:Gd) and transition-metal (TM:FeCo) sublattices antiferromagnetically coupled, constitute the most promising platform to develop the next generation for ultrafast spintronics devices. These alloys combine the advantages of ferromagnetic and antiferromagnetic materials, and their properties can be easily manipulated by tunning the relative composition of the RE and TM. For instance, except at the magnetization compensation point, they present a finite magnetization which can be detected with the same techniques as done in ferromagnets. At the same time, FiM are almost insensitive to external magnetic fields and their negligible magnetostatic interaction makes them ideal candidates to store information with high density. They have recently raised strong interest owing to their ultrafast magneto-optical switching properties [1], and the possibility to drive series of domain-walls (DWs) with high speed under current pulses due to spin-orbit torques (SOTs) [2]. We have developed an advanced micromagnetic model (mM) to explore both the local all-optical switching (AOS) under ultrashort laser pulses with duration of a few fs, and the subsequent domain and DW dynamics by injection of current pulses along a heavy metal (HM) underneath the FiM. The scheme is shown in Fig. 1(a). Starting with a uniform state where both sublattices are anti-parallel aligned along the z-axis, a laser pulse is applied. A proper selection of the fluence and duration of the laser pulse [3] results in a reversed domain flanked by two homochiral DWs, which are subsequently driven along the FiM strip upon injection of the current pulses along the HM [4]. Fig 1(b)-(c) shows an example of the laser-induced writing and current-driven shifting of a sequence magnetic domains (bits). In the talk, we will describe the details of the mM and how it can be used to infer the potential of these FiM/HM stacks to develop DW-based memory devices. 
**References** 
[1] See for instance T. A. Ostler et al. Nat. Commun. 3, 1666 (2012). 
[2] L. Caretta et al. Nature Nanotechnology. 13, 1154 (2018). 
[3] V. Raposo et al. Phys. Rev. B 105, 104432 (2022). 
[4] E. Martinez at al. J. Magn. Magn. Mater. 491, 165545 (2019). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b0094215f4561838a4b53be007b025e2.png) 
Fig.1. (a) Scheme of the HM/FiM stack with the laser. (b) Temporal sequence of laser (top) and current (bottom) pulses to write a 01101000 bit sequence. (c) Snapshots of the resulting bit
sequence.

All optical writing and current driven shifting of bits in ferrimagnetic strips: a micromagnetic study

With the recent progress in pulsed high-brightness photocathodes, Lorentz imaging approaches in ultrafast transmission electron microscopy (UTEM) now offer a pathway for the investigation of magnetization dynamics with nanometer spatial and femtosecond temporal resolution [1,2]. Extending the available frequencies in field-driven ultrafast Lorentz microscopy to the GHz range, we developed an in-situ radiofrequency (RF) excitation sample holder for transmission electron microscopy based on a two-dimensional microwave cavity (Fig. 1a,b). As a first test case, we investigated ferromagnetic resonances in permalloy stripe arrays (Fig. 1f) which support localized magnonic modes identified by the back-reflected RF power (Fig. 1d,g). For ultrafast Lorentz microscopy, the RF excitation is phase-locked to the nanolocalized photoemission of ultrashort electron pulses from a Schottky field emitter, using high harmonics of an amplified laser system as a master clock for their synthetization [3]. Phase-resolved ultrafast Lorentz micrographs are expected to yield high-contrast FMR mode mapping. A simulated micrograph is shown in Fig. 1c. With these developments, we aim to establish ultrafast Lorentz microscopy as a versatile lab-scale imaging tool for femtosecond and picosecond transient states in ultrafast magnetism and magnonics.

Ultrafast Lorentz Microscopy with Magnetic Field Excitation at Microwave Frequencies

Plasmonic surface lattice resonances, visible in periodic arrays of nanodisks, arise from radiative coupling between the localized surface plasmon resonance[1-3] of individual particles and
diffracted orders[4]. Here, we study the impact of these optical modes on the magneto-optical properties[5,6] and energy absorption efficiency[7] of [Co/Gd/Pt]N nanodisks by measuring
the response of different arrays to optical excitations as function of the light wavelength, the disk diameter, and the array period. We demonstrate that surface lattice resonances allow AllOptical single pulse switching[8,9] of the nanodisks using much less energy than for the thin film, with an energy absorption enhanced by more than 400 %. Besides, these optical modes
increase the magneto-optical Faraday effect by more than 2000 %. The influence of the disk diameter and array period on the amplitude, width and position of the resonances is in
qualitative agreement with theoretical calculations and opens the way to design magnetic metasurfaces for all-optical magnetic recording. 
**References** 
[1] Liu, T.-M., et al. Nanoscale Confinement of All-Optical Magnetic switching in TbFeCo – Competition with Nanoscale Heterogeneity. Nanoletters, 15:6862 – 6868, 2015. 
[2] Savoini, M., et al. Highly efficient all-optical switching of magnetization in GdFeCo microstructures by interference-enhanced absorption of light. Physical Review B, 86:140404, 2012. 
[3] Cheng, F., et al. All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances. Nanoletters. 20:6437 – 6443, 2020. 
[4] Kataja, M., et al. Surface lattice resonances and magneto-optical response in magnetic nanoparticle arrays. Nature Communications, 6:1 – 8, 2015. 
[5] Freire-Fernández, F., et al. Surface-plasmon-polariton driven narrow-linewidth magneto-optics in Ni nanodisk arrays. Nanophotonics, 9:113 – 121, 2019. 
[6] Freire-Fernández, F., et al. Magnetoplasmonic properties of perpendicularly magnetized [Co/Pt]N nanodots. Physical Review B, 101:054416, 2020. 
[7] Kataja, M., et al. Plasmon-induced demagnetization and magnetic switching in nickel nanoparticle arrays. Applied Physics Letters, 112:072406, 2018. 
[8] Radu, I., et al. Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins. Nature, 472:205 – 208, 2011. 
[9] Lalieu, M. L. M., et al. Deterministic all-optical switching of synthetic ferrimagnets using single femtoseccond laser pulses. Physical Review B, 96:220411, 2017. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ce745f3e9cc680537e51e8ee5aa93ef2.png) 
Fig. 1. Magnetic [Co/Gd/Pt]N metasurface.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/018ffca0228f5b63139eb1c00164c594.png) 
Fig. 2. a) Ratio of the All-Optical switching threshold fluence measured on the film and metasurfaces with P = 500 nm and different array diameters. b) Average Faraday readout-sensitivity
among 6 samples, defined as the ratio of the Faraday angle of [Co/Gd/Pt]N metasurfaces on resonance and in the continuous [Co/Gd/Pt]N films at the same wavelength, scaled with the
packing density.

Energy efficient single pulse switching of magnetic nanodisks using surface lattice resonances

We report on the study of the cycloidal antiferromagnetic state in bulk single crystals of the multiferroic bismuth ferrite using a combination of real-space and reciprocal magnetic imaging techniques [1]. We first discuss the origin of the magnetic stray field allowing us to image the cycloidal modulation of the antiferromagnetic state. This stray field is produced by the small uncompensated moment arising from the spin density wave which is locked to the cycloid [2]. We then use scanning NV center magnetometry to image the magnetic texture, with additional resonant X-ray diffraction measurements to obtain a larger scale insight. We observe the unexpected coexistence inside a single ferroelectric domain of magnetic domains hosting different cycloid wavevectors. We show that the direction of these wavevectors is not strictly locked to the preferred crystallographic axes as continuous rotations bridge different wavevectors. Making profit of the nanoscale spatial resolution of scanning NV-center magnetometry, we show that topological defects form at the junction between the magnetic domains. These defects are identical to those found in a broad variety of lamellar physical systems with rotational symmetries (liquid crystals, magnetic helix [3], ferromagnetic garnets, copolymers, etc.). The presence of these objects in such a multiferroic antiferromagnet offers new opportunities in terms of robustness and electrical control towards their use in spintronic devices. 

**References:** 

[1] Finco et al, Phys. Rev. Lett. 128, 187201 (2022)
[2] Ramazanoglu et al, Phys. Rev. Lett. 107, 207206 (2011)
[3] Schönherr et al, Nat. Phys. 14, 465 (2018)

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Spin Hall oscillators based on ferromagnetic metal/lithium aluminum ferrite bilayers with enhanced output power and quality factor

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