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Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting the potential for low-power data processing and storage devices. Magnetoionic approaches appear particularly promising, electrolyte- and ionic-conductor-based devices being shown capable of on/off voltage control of ferromagnetism and voltage tuning of magnetic anisotropy [1-3]. A clear limitation, however, is that such demonstrations involve either electrically tuning a known ferromagnet, or electrically inducing ferromagnetism from another magnetic state, e.g., antiferromagnetic. In this work, we report proof-of-principle that ferromagnetism can be voltage-induced even from a diamagnetic, i.e., zero-spin state, suggesting that useful magnetic phases could be electrically-induced in “non-magnetic” materials [4]. We use ionic-liquid-gated diamagnetic semiconducting FeS2 as a model system, showing that as little as +1 V gate bias induces a highly reversible insulator-metal transition, driven by electrostatic inversion of the surface from a p-type semiconductor to an n-type metal. Anomalous Hall effect measurements then reveal the onset of robust electrically-tunable surface ferromagnetism at Curie temperatures up to 25 K, with intriguing high temperature spin correlations. Density-functional-theory-based modelling explains the induced magnetism in terms of Stoner-type ferromagnetism induced solely via filling of a narrow Fe eg band. Work supported primarily by the NSF MRSEC.References: [1] Navarro-Senent, Quintana, Menendez, Pellicer, Sort, APL Mater. 7, 030701 (2019) [2] Molinari, Hahn, Kruk, Adv. Mater. 31, 1806662 (2019) [3] Leighton, Nat. Mater. 18, 13, (2019) [4] Walter, Voigt, Day-Roberts, Heltemes, Fernandes, Birol and Leighton, in press (2020)

MMM 2020

Voltage-induced Ferromagnetism in Diamagnetic FeS2

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The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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We present successful fabrication and demonstration of magnetic vortex core tip (VC) for magnetic force microscopy (MFM). The design of VC probe is based on permalloy disk with diameter of 330nm and thickness of 55nm fabricated on intentionaly blunted tip (see the Fig.1). The ground state of this permalloy disk is the magnetic vortex state with out-of-plane magnetic moment determined by well spatially localized vortex core. The novelty of this design is creation of spatially well localized magnetic out-of-plane field in mechanically blunt tip. This design offers two advantages: First, due to large area of the disk, VC probe has higher wear resistance and longer lifetime than sharp commercial MFM probes. Second, it has very good stability of the out-of-plane magnetic moment, because vortex core is almost independent on the edge imperfections. However, VC probe has also some limitations: the sample should be flat and it should not produce quasi-uniform field in large volume nearby the tip, because the polarization affects vortex core position. We analyze the permalloy tip polarization effect by micromagnetic calculations of MFM images of periodic magnetic strips. By this approach we identify the limits on samples for this new kind of MFM probe.References: J. Soltys, J. Feilhauer, I. Vetrova, J. Tobik, K. Bublikov, T. Scepka, J. Fedor, J. Derer, and V. Cambel, Appl. Phys. Lett. 116, 242406 (2020).

New Vortex Core Probe for Magnetic Force Microscopy

Rich variation of magnetic texture attracts attention in connection with topology of electronic structures. The temperature range slightly below Curie temperature (Tc) is interesting, because the contributing micromagnetic energies (demagtization, exchange, Dzyaloshinsky-moriya and anisotropy) change rapidly with temperature. Recently we reported bubble domain formation immediately below Tc within a kagome ferromagnet Co3Sn2S2 (Tc=176 K) in which anomalous high-temperature, low-field magnetic phase occurred [1]. Self-reorganization of domain walls (DWs) occurred even at zero field, and the domain evolution was dependent not only on temperature and field but also on temperature variation rate [2]. To examine such domain evolution associated with slow spin dynamics, we developed a method to record Lorentz images continuously at a fixed time interval when a specimen is heated/cooled at a constant rate, not like an usual manner to wait until thermal drift settles down. We applied it to examine domain evolution within another uniaxial kagome magnet, Fe3Sn2 (Tc= 657 K), in the temperature range at 303-673 K under no external field. A psuedo-three-dimensional (x-y-time (temperature)) image was constructed from a series of drift-corrected Lorentz images. Fig. 1 shows a cross-section image including time-axis, exhibiting time(temperature) evolution of the magnetic domains. The stripe domain stricture was very stable between room temperature (RT) and 620 K at which DW contrast disappeared due to reduced magnetization both on heating and cooling across Tc, although interesting temperature-dependent texture such as skyrmionic bubble was observed for the alloy below RT. It was also found that the domain structure was nearly independent of cooling/heating rate. The differece in domain evolution between Fe3Sn2 and Co3Sn2S2 is explained in terms of DW creep associated with thermal depinning in Co3Sn2S2.References: [1] M. A. Kassem et al., Phys. Rev. B 96, 014429 (2017). [2] Akira Sugawara et al., Phys. Rev. Mater. 3, 104421 (2019)

Magnetic texture evolution within uniaxial kagome ferromagnets at constant heating/cooling rates

In wide-range magnetometry using optically detected magnetic resonance of NV- centers, we demonstrate more than order-of-magnitude speedup with sequential Bayesian experimental design as compared with conventional frequency-swept measurements. The NV- center is a quantum defect with spin 1 and coherence time up to several milliseconds at room temperature. Zeeman splitting of the NV- energy levels allows detection of the magnetic field via photoluminescence. NV-center is an excellent platform for magnetometry with potential spatial resolution down to few nanometers and demonstrated sensitivity down to nT/Hz1/2. We used a commercially available type IIa single crystal diamond grown by chemical vapor deposition (CVD). Green laser with 520 nm wavelength was used to optically excite NV- center, and a microstrip line was used to supply microwaves to manipulate spin state of the NV- center. For each data point, photons were counted for 50 ms. Measurement of each data point was followed by 50 ms measurement of a reference data point with microwaves switched off. The combination of laser power, microwave power and counting time produced measurements with a signal-to-noise ratio on the order of 1. Moreover, the magnetic field was slightly misaligned from the NV- center axis to produce a three-dip signal. Such experimental setup showcased ability of sequential Bayesian experiment design to locate and measure complex multiple-peak signal even in extremely noisy data and showed its broad dynamical range for sensitivity. Using the measurement described above, we compare conventional magnetic field measurements of fluorescence that use pre-determined sweeps of the microwave frequency with the measurement using a Bayesian inference methodology. In sequential Bayesian experiment design, the frequency of each measurement is determined in real-time from utility predictions based on the accumulated experimental data. We report more than order-of-magnitude speedup in magnetic field and hyperfine splitting determination with sequential Bayesian experimental design, compared with the conventional NV- magnetometry measurement.

Center Magnetometry Using Sequential Bayesian Experiment Design

Antiferromagnetic thin films attract significant interest for future low-power spintronic devices. They are largely insensitive to external magnetic fields, exhibit domains with negligible cross-talk and can be switched at THz frequencies [1]. Furthermore, in multiferroics where antiferromagnetism can coexist with ferroelectricity, it is possible to control the magnetic state electrically - a particularly appealing feature for future device applications. In this talk, we show how the Qnami ProteusQTM -- a commercial room temperature scanning nitrogen-vacancy magnetometer (SNVM) -- can be used to quantitatively image antiferromagnetic spin textures with proven DC field sensitivity ≤ 2 μT/√Hz. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 103 nm, indicative of a type II spin cycloid. [2] In addition, we take advantage of the quantitative nature of SNVM to extract an average magnetic moment density of 0.08 ±0.02 μB /Fe in the BiFeO3 film. These findings agree well with reported vibrating sample magnetometer results (MS = 0.06 μB /Fe) on similar, strained BiFeO3 thin films. [3] Our results shed light on future design requirements for reconfigurable nanoscale spin textures in such multiferroic systems.References: [1] V. Baltz, et al., Rev. Mod. Phys. 90, 015005 (2018) [2] A. Haykal, et al., Nat. Commun. 11, 1704 (2020) [3] W Eerenstein, et al., Science 307, 1203a (2005)

Antiferromagnetic Spin Cycloidal Orders Imaged with a Commercial Scanning Nitrogen-Vacancy Magnetometer

Chiral magnetic structures induced by Dzyaloshinskii-Moriya interaction (DMI) [1] have been proposed as the cornerstone of new technology applications such as high-density data storage devices or neuromorphic computing [2], due their room temperature stability and efficient current induced motion. Recent studies of the amplitude of magnetic satellites of a crystal Bragg peak by Zhang et al. [3] showed that x-ray resonant magnetic scattering (XRMS) is a powerful tool to access to the relevant topological parameters of these magnetic structures. In this work, we use small angle reflectivity XRMS to reveal directly the chiral properties of FM multilayers with tailored magnetic chiralities driven by spin-orbit-related effects at interfaces [4,5]. We show that it can straightforwardly and unambiguously determine the main characteristics of chiral magnetic distributions in perpendicularly magnetized multilayers [4]: its chiral nature, the quantitative winding sense (clockwise or counterclockwise), and its type, i.e. Néel (cycloidal) or Bloch (helical). Moreover we prove that this approach combined with micromagnetic simulations reveals hybrid chiral spin texture in multilayers [5]. Finally, we studied the in plane field dependence intensity of the dichroism and the appearance of second order diffraction peaks, usually forbidden on these systems. An asymmetric change of the intensity of the magnetic asymmetry lobes and a hysteretic behavior of these ones with the external field suggest a complex interplay between the hybrid domain wall state existing in these systems and DMI fields. European Union Grant FLAG-ERA SoGraph (ANR-15-GRFL-0005), French ANR grant TOPSKY (ANR-17-CE24-0025), DARPA TEE program grant (MIPR#HR0011831554) and EU grant SKYTOP (H2020 FET Proactive 824123) are acknowledged for their financial support.References: References [1] I. Dzyaloshinskii, J. of Phys. and Chem. of Solids 4, 241 (1958), T. Moriya, Phys. Rev. 120, 91 (1960) [2] A. Fert, N. Reyren and V. Cros, Nat. Rev. Mat. 2, 17031 (2017) [3] S. L. Zhang, G. van der Laan, and T. Hesjedal, Nat. Commun. 8, 14619 (2017) [4] J.-Y. Chauleau et al., Phys. Rev. Lett. 120, 037202 (2018) [5] W. Legrand et al., Sci. Adv. 4, eaat45 (2018)

Field dependence chirality in ferromagnetic multilayers probed by dichroism in x-ray resonant magnetic scattering

Complex oxides offer possibilities to engineer and control spin textures via their many degrees of freedom and sensitivity to epitaxial strain, and have been proposed for use in energy efficient magneto-electric spin-orbit logic architectures.1 Additionally, recent interest in antiferromagnetic spintronics has surged due to the possibility of obtaining ultrafast magnetization dynamics with no stray field2. However, lack of stray field makes imaging of antiferromagnetic domain structure difficult though conventional magnetic imaging techniques. This talk will discuss the use of X-ray photoemission electron microscopy (PEEM) with both x-ray linear and circular dichroism to spatially and chemically resolve the shape and voltage-controlled ferroic order in heterostructures containing antiferromagnetic (AFM), ferroelectric(FE), and ferromagnetic (FM) layers of including (La,Sr)FeO33,4, Ni/CoFeB/PMN-PT5, and CoFe/(La,Bi)FeO36. Dimensional confinement can be used to tailor the easy axis spin orientation in (La,Sr)FeO3, as edge-induced anisotropy in microstructures can induce nearly-single domain structures in wires,3 and in the limit of few unit-cell thick layers, the antiferromagnetic spin axis lies in the plane of the film in contrast to thicker layers where the spin axis cants out of plane.4 By changing the ratio of Ni to CoFeB in artificial multiferroic heterostructures, the ground state domain configuration and magneto-electric coupling behavior are strongly modified, and this coupling can be visualized in situ with PEEM.5 Furthermore, we show that the energy barrier for magneto-electric switching energy density can be scaled through careful engineering in a BiFeO3-based system towards 10 µJ cm-2 at a switching field of 200 mV.6 Thus, coupling at these interfaces allows for mutual control of the domain structure via a variety of pathways, including shape anisotropy and magneto-electric interactions, and thus offers additional degrees of freedom towards low-energy AFM-based spintronics.References: 1. S. Manipatruni et al, Nature 565 35 (2018). 2. V. Baltz et al, Rev. Mod. Phys 90, 015005 (2018). 3. A. D. Bang, RVC et al, Appl. Phys. Lett 115 112403 (2019). 4. M.S. Lee, RVC et al, J. Appl. Phys 127, 203901 (2020). 5. Z. Xiao, RVC et al, ACS Appl. Mat. 12, 6752 (2020). 6. B. Prasad, RVC et al, Adv. Mat. 2001943 (2020).

Imaging Shape and Voltage Control of Spin Textures in Ferroic Heterostructures

Although it is well documented that the vortex core in a micromagnet can switch polarisation in response to magnetic field pulses[1], the temperature dependence of this process has not yet been addressed by experiments. Using scanning transmission x-ray microscopy, we investigate the magnetic vortex dynamics in a La0.7Sr0.3MnO3 microplatelet at temperatures ranging from 150 K up to TC at 350 K[2]. The time-resolved images reveal qualitatively different dynamic regimes as function of temperature and applied field, as the relative strengths of the micromagnetic energy terms strongly vary over this temperature range. By explicitly accounting for the temperature dependence of the magnetic parameters in our micromagnetic simulations[3], we found an excellent agreement with the experiments over the full measurement range. In line with previous models, the simulations reveal that the vortex core switches polarisation when it reaches a critical velocity[4] that mainly depends on the strength of the exchange interaction. It is therefore strongly temperature dependent, thus explaining our observation of the different dynamical regimesReferences: [1] B. Van Waeyenberge, A. Puzic, H. Stoll, et al., Nature Vol. 444, p.461 (2006). [2] E. Digernes, J. Leliaert, M. Weigand, et al., submitted manuscript [3] A. Vansteenkiste, J. Leliaert, M. Dvornik, et al., AIP Adv. Vol.4, p.107133 (2014). [4] K. Y. Guslienko, K.-S. Lee, and S.-K. Kim, Phys. Rev. Lett. Vol. 100, p. 027203 (2008)

Direct Observation of Temperature Dependent Vortex Dynamics in a La0.7Sr0.3MnO3 Micromagnet

Fluorescent defects in diamond present a versatile platform for compact quantum magnetometers, [1] and optical fiber technology enables in situ laser excitation and signal collection from these sensors. [2-4] The advent of microstructured fibers has allowed novel analytical techniques employing differential magnetic field measurements, common-mode noise rejection, [5] and high-resolution magnetic field imaging [6] to be used in environments without easy optical access. In this work, we demonstrate a microresolution stereomagnetometer probe by attaching a high nitrogen–vacancy (NV) concentration microdiamond to the tip of a dual-core photonic-crystal fiber (PCF) and simultaneously interrogate two regions of the diamond separated by the 6 μm core-to-core distance (Fig. 1 (a)). This probe was then used to characterize the magnetic field emanating from a 40 μm diameter current carrying wire in the presence of a large static bias solenoid by extracting the center wavelength of each peak in the optically detected magnetic resonance (ODMR) spectrum of the diamond fluorescence recollected by the PCF. [5] The magnitude and direction of the inhomogeneous wire field source was discriminated from a homogeneous background bias field with a magnitude more than 1000 fold larger than the localized wire field (Fig. 1 (b)). This stereomagnetometer was found to have a shot-noise limited sensitivity of 2 nT/√Hz and a 6 μm spatial resolution while imaging the vector field around the wire. [5]References: [1] L. Bougas, A. Wilzewski, Y. Dumeige, et al., Micromachines 9 (6), 276 (2018) [2] Y. Ruan, D. A. Simpson, J. Jeske, et al., Scientific Reports 8 (1), 1268 (2018) [3] S. M. Blakley, I. V. Fedotov, S. Ya. Kilin, et al., Optics Letters 40, 3727-3730 (2015) [4] S. M. Blakley, I. V. Fedotov, L. V. Amitonova, et al., Optics Letters 41, 2057-2060 (2016) [5] S. M. Blakley, I. V. Fedotov, J. Becker, et al., Applied Physics Letters 113 (1), 011112 (2018) [6] I. V. Fedotov, S. M. Blakley, E. E. Serebryannikov, et al., Optics Letters 41, 472-475 (2016)

Fiber-Optic Magnetic Vector Gradient Imaging with Fluorescent Defects in Diamond

Since it was first demonstrated in 1987, magnetic force microscopy (MFM) has become a truly widespread and commonly used characterization technique that has been applied to a variety of research and industrial applications. Some of the main advantages of the method includes its high spatial resolution (typically ∼50 nm), ability to work in variable temperature and applied magnetic fields, versatility, and simplicity in operation, all without almost any need for sample preparation. However, the technique has historically provided only qualitative information, and the number of available modes was typically limited, thus not reflecting the experimental demands. We present the recent progress and development of MFM as well as a summary of the current state-of-the-art techniques and objects for study. Aspects including quantitative MFM, the accurate interpretation of the MFM images, new instrumentation, probe-engineering alternatives, and applications of MFM to new (often interdisciplinary) areas of the materials science, physics, and biology are discussed. We outline the importance of the technique in emerging fields including skyrmions, 2D-materials, and topological insulators.References: O. Kazakova, et al., ‘Frontiers of Magnetic Force Microscopy’ J Appl. Phys. 125, 060901 (2019) H. Corte-León, et al., ‘Magnetic imaging using geometrically constrained nano-domain walls’ Nanoscale 11, 4478 (2019) V. Panchal, et al., ‘Calibration of multilayered magnetic force microscopy probes’ Sci Reports 7, 7224 (2017) F. Moro, et al., ‘Room temperature uniaxial magnetic anisotropy induced by Fe-islands in the InSe semiconductor van der Waals crystal’, Advanced Science, 5, 1800257 (2018) H. Corte-León, et al., ‘Comparison and validation of different magnetic force microscopy calibration schemes’ Small 20, 1906144 (2020) X. Hu, et al., ‘Round Robin Comparison on Quantitative Nanometer Scale Magnetic Field Measurements by Magnetic Force Microscopy’ J. Magn. Magn Mater. 511, 166947 (2020)

Frontiers of magnetic force microscopy INVITED

Antiferromagnets attract a great interest for spintronics owing to the robustness of their magnetic textures and their fast dynamics. However, since they exhibit no net magnetization, antiferromagnets are challenging to work with. NV-center magnetometry, which provides a sensitivity of a few µT/Hz-1/2 combined with a nanoscale spatial resolution, has emerged in the last years as a powerful technique to investigate them [1]. Here we introduce a new imaging mode of the NV-center magnetometer which does not rely on the measurement of the static magnetic stray field but on the detection of magnetic noise originating from spin waves inside the non-collinear antiferromagnetic textures of interest. The presence of magnetic noise accelerates the NV spin relaxation. As a consequence, the emitted photoluminescence is reduced, allowing a simple detection of the noise sources [2]. We demonstrate this new technique on synthetic antiferromagnets (SAF) [3] consisting of two ferromagnetic Co layers antiferromagnetically coupled through a Ru/Pt spacer. We first image domain walls and prove that we perform noise-based imaging by measuring a shorter NV spin relaxation time above an antiferromagnetic domain than above a domain wall. Calculations of the spin waves dispersion both in the antiferromagnetic domains and in the domains walls as well as maps of simulated magnetic noise intensity enable us to conclude that the noise which we probe arises from spin waves channelled in the domain walls. Going further, we tune the composition of the SAF stacks in order to stabilize spin spirals or antiferromagnetic skyrmions. In both cases, our relaxometry-based technique is able to image the non-collinear structures, demonstrating its efficiency and opening new avenues of exploration in the characterization of complex structures in magnetically-compensated materials. This work was performed in collaboration with the UMR CNRS/Thalès and the Center for Nanoscience and Nanotechnology (C2N) in Palaiseau, France. It has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 846597 and from the DARPA TEE Program.References: [1] I. Gross et al, Nature, Vol 549, p 252 (2017) [2] A. Finco et al, arXiv:2006.13130 (2020) [3] W. Legrand et al, Nature Materials, Vol 19, p 34 (2020)

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