The vast multi-principal element alloy (MPEA) composition space is promising for discovery of new material phases with unique combinations of properties.1 We have explored the potential to achieve high magnetic anisotropy materials in MPEA FeCoNiMnCu thin films. Thin films of FeCoNiMnCu sputtered on thermally oxidized Si/SiO2 substrate at room temperature exhibit soft magnetic properties with a coercivity on the order of 10 Oe . After post-deposition rapid thermal annealing (RTA), the films exhibit a single face-centered cubic (FCC) phase from grazing incidence X-ray diffraction (GIXRD), as shown in Fig. 1, as well as a continuous surface morphology measured by atomic force microscopy. Magnetometry measurements reveal a coercivity of 400 Oe, a nearly 40-fold increase compared to as-grown samples. First-order reversal curve (FORC) analysis2 of the RTA sample reveals a distinct feature in the high coercivity regime of the FORC distribution, as shown in Fig. 2, indicating the presence of a single magnetic phase with much enhanced coercivity. Additionally, we have explored inclusion of Pt to facilitate uniaxial structure and enhance magnetic anisotropy. We find that RTA of the FeCoNiMnCuPt films induces ordering into an L10 intermetallic phase. A significant enhancement of coercivity to over 2 kOe after RTA has been observed, compared to 5 Oe before RTA, along with the development of an out-of-plane magnetic easy-axis, reflecting the formation of a high anisotropy L10 phase. These results demonstrate the potential in achieving high magnetic anisotropy materials using the multi-principal element approach. 

This work has been supported by the NSF (ECCS-2151809). 
**References:** 
1. J.W. Yeh, S.K. Chen, S.Y. Chang et al., Adv. Eng. Mater., 6, 299-303, (2004). 
2. D.A. Gilbert, J.W. Liao, K. Liu et al., APL Mater., 2, 086106, (2014). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e950d258e4e85c804cb517bbdc6f1322.png) 
Fig. 1. Grazing-incidence XRD for FeCoNiMnCu films (bottom) as-grown and (top) after RTA. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ca2a4a7de03b99275018c61a1b745243.png) 
Fig. 2. (left ) Family of first-order reversal curves (FORC) and (right) FORC distribution of an FeCoNiMnCu film after RTA.

Phase Stabilization and Coercivity Enhancement in Multi principal Element FeCoNiMnCu Thin Films

Magnetic skyrmions are topologically nontrivial spin textures with envisioned applications in energy-efficient magnetic information storage [1,2]. Toggling the presence of magnetic skyrmions via writing/deleting processes is essential for spintronics applications, which usually requires a magnetic field, current injection, electric field, laser pulse, or thermal excitation [3,4]. A key challenge is to directly image such ultrasmall spin textures in order to confirm their topological character. 

Using spin-polarized low-energy electron microscopy (SPLEEM), we have demonstrated a new, field-free method to write/delete magnetic skyrmions at room temperature, via hydrogen chemisorption/desorption cycles on Ni/Co/Pd/W(110) [5]. The spin structures of the written skyrmions are resolved to be hedgehog Néel-type via magnetization vector mapping using SPLEEM (Fig. 1). Supported by Monte-Carlo simulations, the skyrmion creation/annihilation is attributed to the hydrogen-induced magnetic anisotropy change on ferromagnetic surfaces. The roles of hydrogen and oxygen on magnetic anisotropy and skyrmion deletion on other magnetic surfaces are also demonstrated. 

In another study, we focus on the interfacial Dzyaloshinskii-Moriya interaction (DMI), which stabilizes magnetic chirality with preferred handedness. Experimentally, controlling the handedness is crucial to tune the efficiency of current-induced manipulation of spin texture. This is conventionally achieved by stacking asymmetric multilayers where the thickness of each layer is at least a few monolayers. We observed an ultrasensitive chirality switching in (Ni/Co)n multilayer induced by capping only 0.22 monolayer of Pd [6]. Using SPLEEM, we monitor the gradual evolution of domain walls from left-handed to right-handed Néel walls and quantify the DMI induced by the Pd capping layer (Fig. 2). We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of < 1 attojoule during the skyrmion chirality switching. 

These results open up new opportunities for designing energy-efficient skyrmionic and magneto-ionic devices. They also illustrate the effectiveness of SPLEEM in resolving magnetization vector in chiral spin textures with high spatial resolution.

This work has been supported in part by the NSF (DMR-2005108), SRC/NIST SMART Center and US DOE. 

References [1] R. Wiesendanger, Nat. Rev. Mater. 1, 16044 (2016).
[2] A. Fert, et al. Nat. Rev. Mater. 2, 17031 (2017).
[3] W. Jiang, et al. Phys. Rep. 704, 1-49 (2017).
[4] X. Zhang, et al. J. Phys.: Condens. Matter 32, 143001 (2020).
[5] G. Chen, et al. Nat. Commun. 13, 1350 (2022).
[6] G. Chen, et al. Nano Lett., in press; arXiv: 2206.12069.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e3d647dc58c4efce282d51103355d0ce.png)

Pulling Magnetic Skyrmions out of Thin Air

Networks of interconnected magnetic nanowires offer an exciting platform to explore 3-dimensional (3D) nanomagnetism, where their structure, topology and frustration may be used as additional degrees of freedom to tailor the materials properties. We have achieved quasi-ordered metallic networks over cm-scale areas. Using a cost-effective multiple azimuthal angle ion-tracking and etching method with suitable fluence, interconnected nanoporous polycarbonate membranes were first fabricated. A subsequent electrochemical deposition process led to interconnected metallic nanowire networks, with density as low as 40 mg/cm3 (Fig. 1) [1]. New magnetization reversal mechanisms in cobalt networks (Fig. 1d) are captured by the first-order reversal curve (FORC) method, which demonstrate the evolution from strong demagnetizing dipolar interactions to intersections-mediated domain wall pinning and propagation, and eventually to shape-anisotropy dominated magnetization reversal (Fig. 2). By varying the applied magnetic field strength, orientation, and sequence, it is in principle possible to selectively address a certain subset of the nanowire networks, encode digital information into their magnetic state, and propagate it through the networks for potential 3D magnetic memory and logic applications. Such network of magnetic nanowires also has the potential to implement repeatable multi-state memristors in neuromorphic circuits, due to the expected discrete nature of domain wall motion through the intersecting networks. Interestingly, another random configuration of metallic nanowire networks has found applications in deep-submicron particulate filtration, relevant to combatting COVID-19 and air pollution, due to the extremely large surface areas and the excellent mechanical strength [2]. This work has been supported by the NSF (ECCS-1611424 and ECCS-1933527), SRC/NIST nCORE SMART center, DTRA, U.C. Davis and Georgetown University.

![](https://assets.underline.io/uploads/markdown_image/1/image/235e34862100e61fc1216607d02eed81.jpg)

Figure 1: (a) Side-view and (b) top-view SEM images of an interconnected copper network, tracked at five angles as illustrated by the schematics. Inset in (a) shows that at an intersection the wires are physically connected. Top-view SEM images of (c) a copper network and (d) a cobalt network, tracked at three angles. This produces intersections in planes that contain the three irradiated angles (which are highlighted with white dashed lines).

![](https://assets.underline.io/uploads/markdown_image/1/image/46695a346d2d61a65f1ffb4d1ef4c0ad.jpg)

Figure 2. FORC diagrams shown in 2D contour plots for (a-c) a single-angle irradiated sample, and (d-i) three-angle irradiated sample. The schematic diagrams in the lower right corner of panels (a-i) indicate the orientation of the wires (black lines) and membrane (blue box) relative to the applied field shown by the large black arrow.

3D Nanomagnetism in Interconnected Magnetic Nanowire Networks

Three-dimensional (3D) magnetic assemblies can accommodate unique topological features and exotic spin textures, along with embedded magnetic frustrations [1]. Recently, we have realized interconnected cobalt nanowire networks using multiple angle ion-tracking and electrodeposition; such networks exhibit new magnetization reversal behaviors, offering an interesting platform for 3D information storage [2]. Here we report magnetoresistance (MR) studies of such interconnected networks, to explore how the magnetic configurations can be correlated with spin transport. When a sizable collection of connected nanowires is measured, typical anisotropic magnetoresistance behavior with sharp minima near the coercive fields is found, suggesting that the current is largely bypassing the high resistance intersections. However, when only a few interconnected nanowires are measured, the MR curve exhibits multiple kinks and local minima, indicative of domain wall pinning by the intersections (Fig. 1). More interestingly, during the positive to negative field sweep, a significant MR minimum is observed at positive field, which is the largest among all the MR minima. These features are not seen in parallel nanowires and are a direct consequence of the intersections. Micromagnetic simulations using an equivalent resistor model based on two intersecting nanowires reproduces the experimental observations, including the multiple kinks and a large MR minimum in positive field. Furthermore, small angle neutron scattering has been used to characterize magnetic correlations in the networks under different applied magnetic field configurations. These results show that the intersections in the networks play an important role in pinning / depinning of domain walls, which may be utilized for 3D magnetic memory and memristive devices. Work supported by the SRC/NIST nCORE SMART center and NSF.

![](https://assets.underline.io/uploads/markdown_image/1/image/33fe181bcbcc9e0f263230fd717c8566.jpg)

Figure 1: Magnetoresistance of a few interconnected Co nanowires showing multiple kinks, indicative of domain wall pinning and depinning during magnetization reversal.

Intersection-Mediated Magnetoresistance in 3D Nanowire Networks

Due to the long spin diffusion length and the high mobility, monolayer graphene is an intriguing material element for spintronic applications [1]. For example, if a monolayer graphene can be magnetized, Dirac electrons and holes will carry opposite spins, leading to a good spin filter or a convenient generator of pure spin currents. However, a prestine monolayer graphene does not have net magnetization. Previous experiments have demonstrated that when a monolayer graphene is transferred to the surface of a magnet, the p orbitals on the carbon atoms can overlap with the magnetic atoms in the adjacent layer, leading to an induced magnetization in the graphene layer [2]. This is known as the proximity effect, and has been reported in different types of ferromagnetic heterostructures. Compared to ferromagnets, antiferromagents are better candidates to magnetize graphene. Due to the zero net magnetization, an antiferromagnet does not respond to external magnetic noise, and therefore devices using antiferromagnetic heterostructures have good scalbility and stability. Also, antiferromagents can switch almost three orders of magnitude faster than conventional ferromagnets, which may lead to high-speed information processing. Here, we show that a monolayer graphene can indeed be magnetized through the proximity effect provided by a thin film of hexagonal chromium selenide (Fig. 1), a semiconducting antiferromagnet [3]. The induced spin splitting energy is experimentally measured to be 134meV at 2K. Such exchange splitting is experimentally confirmed by the shift of quantum oscillations in the graphene layer. The magnitude of the splitting energy is obtained by its modulation through field cooling, with the exchange splitting energy increasing driven by positive cooling fields and decreasing by negative cooling fields (Fig. 2). The magnitude of the exchange coupling and the Laudau-level indices are confirmed by a machine-learning search for the best fit in a large parameter space. Such spin splitting is also supported by polarized neutron reflectometry and magneto-optic Kerr measurements. Acknowledgements: The transport measurement and theoretical modelling in this work were supported by Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under award #SC0012670. We are also grateful for support from the National Science Foundation (NSF) (DMR-1411085 and DMR-1810163) and the ARO programme (contract no. W911NF-15-1-10561). Research was performed in part at the NIST Center for Nanoscale Science and Technology. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF grant no. OCI-1053575. Specifically, it used the Bridges system (supported by NSF award no.ACI-1445606) at the Pittsburgh Supercomputing Center (PSC). Certain commercial equipment, instruments or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

![](https://assets.underline.io/uploads/markdown_image/1/image/17681d6778d9ccff54b5964943012fc1.jpg)

Fig. 1. The schematic of the graphene/CrSe heterostructure. Hexagonal CrSe has a layered AFM structure. Each layer contains a non-colinear spin texture with a non-zero magnetization along z. Adjacent layers cancel the moments of each other, resulting in a net antiferromagnet.

![](https://assets.underline.io/uploads/markdown_image/1/image/d699ad42c2ba992a04094c15c97d5021.jpg)

Fig. 2. Quantum oscillations are shifted by field cooling in the heterostructure. The dips corresponding to each Landau level are denoted by dashed lines.

Large Exchange Splitting in Monolayer Graphene Magnetized by an Antiferromagnet

KOB: Hard Magnetic Materials IV: Rare-Earth Free Hard Magnetic Materials

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EOA - 3D Nanomagnetism and Nanowire Arrays

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Gen Yin

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Phase Stabilization and Coercivity Enhancement in Multi principal Element FeCoNiMnCu Thin Films

Pulling Magnetic Skyrmions out of Thin Air

3D Nanomagnetism in Interconnected Magnetic Nanowire Networks

Intersection-Mediated Magnetoresistance in 3D Nanowire Networks

Large Exchange Splitting in Monolayer Graphene Magnetized by an Antiferromagnet

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