United States

Patterned FePt media solutions, such as Bit-Patterned Media (BPM) and Embedded Hardmask Patterning (EMP), have been proposed as alternative methods to improve Heat-Assisted Magnetic Recording areal density. Understanding etching mechanism of FePt material is important to warrent the low damage to the magnetic properties caused by micro-fabrication process. In this research, Reactive Molecular Dynamics (MD) modeling is carried out to study methanol (MeOH) plasma etch on L1&lt;sub&gt;0&lt;/sub&gt;-FePt media. The atomistic-level etch simulation process is divided into two sequential MD simulation steps: the radical-based reactive treatment step, followed by the ion-based physical etch step. Carbon-monoxide (CO) and H&lt;sub&gt;2&lt;/sub&gt; molecules are identified as the main etch precursors in methanol plasma interacting with substrate. Reactive interaction between CO/H&lt;sub&gt;2&lt;/sub&gt; radicals and Fe/Pt atoms is studied using bond-order ReaxFF force field with atomic charge calculation. Bond energy calculation shows the positively charged Fe and Pt and the negatively charged C atoms in CO molecules can form ionic carbonyl bonds that are shown more stable than Fe-Pt metallic bonds. As the result, it is found that the number of Fe(CO) and Pt(CO) carbonyl bonds are dominated among the new bonds produced after the reactive-treatment process. In the following physical etch step simulated with Ar or He ion bombardment, in comparison with FePt surface without CO/H&lt;sub&gt;2&lt;/sub&gt; treatment, the sputter yield of FePt can significantly improve by 2-4X on the samples with the CO/H&lt;sub&gt;2&lt;/sub&gt;-treatment step.&lt;br&gt;
**References** &lt;br&gt;
[1] H. Wang, H. Zhao, P. Quarterman, and J.-P. Wang, Appl. Phys. Lett., vol. 102, no. 5, p. 052406, 2013&lt;br&gt;
[2] Patent: US10,347,467B2, J.-P. Wang, P. Quarterman and J. Zhu&lt;br&gt;
[3] LAMMPS - A flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales, Comp. Phys. Comm. (accepted 09/2021),
DOI:10.1016/j.cpc.2021.108171&lt;br&gt;
[4] J. Zhu, P. Quarterman, J.-P. Wang, AIP Advances 2017, 7 (5) , 056507. https://doi.org/10.1063/1.4977223&lt;br&gt;
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a01666868c95347f3bf6c30ec19d4fec.png)&lt;br&gt;
Figure 1. Number and types of carbonyl bonds created on FePt surface after CO/H&lt;sub&gt;2&lt;/sub&gt; exposure

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/535be17232038eb114b6084ff7a87b38.png)&lt;br&gt;
Figure 2. FePt sputter yield ( with Ar ions) comparison on CO/H&lt;sub&gt;2&lt;/sub&gt;-treated and non-treated surface

MMM 2022

Reactive Molecular Dynamics Modeling of Plasma Etching of L10 FePt Magnetic Media

Patterned FePt media solutions, such as Bit-Patterned Media (BPM) and Embedded Hardmask Patterning (EMP), have been proposed as alternative methods to improve Heat-Assisted Magnetic Recording areal density. Understanding etching mechanism of FePt material is important to warrent the low damage to the magnetic properties caused by micro-fabrication process. In this research, Reactive Molecular Dynamics (MD) modeling is carried out to study methanol (MeOH) plasma etch on L10-FePt media. The atomistic-level etch simulation process is divided into two sequential MD simulation steps: the radical-based reactive treatment step, followed by the ion-based physical etch step. Carbon-monoxide (CO) and H2 molecules are identified as the main etch precursors in methanol plasma interacting with substrate. Reactive interaction between CO/H2 radicals and Fe/Pt atoms is studied using bond-order ReaxFF force field with atomic charge calculation. Bond energy calculation shows the positively charged Fe and Pt and the negatively charged C atoms in CO molecules can form ionic carbonyl bonds that are shown more stable than Fe-Pt metallic bonds. As the result, it is found that the number of Fe(CO) and Pt(CO) carbonyl bonds are dominated among the new bonds produced after the reactive-treatment process. In the following physical etch step simulated with Ar or He ion bombardment, in comparison with FePt surface without CO/H2 treatment, the sputter yield of FePt can significantly improve by 2-4X on the samples with the CO/H2-treatment step. 
**References** 
[1] H. Wang, H. Zhao, P. Quarterman, and J.-P. Wang, Appl. Phys. Lett., vol. 102, no. 5, p. 052406, 2013 
[2] Patent: US10,347,467B2, J.-P. Wang, P. Quarterman and J. Zhu 
[3] LAMMPS - A flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales, Comp. Phys. Comm. (accepted 09/2021),
DOI:10.1016/j.cpc.2021.108171 
[4] J. Zhu, P. Quarterman, J.-P. Wang, AIP Advances 2017, 7 (5) , 056507. https://doi.org/10.1063/1.4977223 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a01666868c95347f3bf6c30ec19d4fec.png) 
Figure 1. Number and types of carbonyl bonds created on FePt surface after CO/H2 exposure

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/535be17232038eb114b6084ff7a87b38.png) 
Figure 2. FePt sputter yield ( with Ar ions) comparison on CO/H2-treated and non-treated surface

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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Voltage-driven ion transport modulation of magnetism (magneto-ionics) represents a significant breakthrough to enhance energy efficiency in magnetically-actuated devices. Particularly, voltage-driven oxygen ion motion in Co3O4 allows for voltage-controlled On−Off ferromagnetism, since Co3O4 (paramagnetic: Off) can be partially reduced to Co (ferromagnetic: On) by voltage actuation. This can be afterwards reversed by applying a voltage of the opposite polarity (1). The way the electric field is applied has a strong impact in the room temperature magneto-ionic motion. Specifically, in electrolyte-gated 100 nm-thick paramagnetic Co3O4 films, the oxygen magneto-ionic motion can be sped up by an order of magnitude (from 1000 to 100 s) applying the electric field through an electrochemical capacitor configuration (i.e., using, as a working electrode, an underlying conducting buffer layer beneath the Co3O4 film) rather than using an electric-double-layer transistor-like configuration (i.e., placing the electric contact at one side of the Co3O4 film as a working electrode). This is due to the better uniformity and larger strength of the electric field in the capacitor design (1,2). However, ion motion at room temperature is still too slow for applications. By reducing the Co3O4 film thickness down to 15 nm, sub-10 s ON-OFF ferromagnetism can be achieved in electrolyte-gated films (3). Remarkably, cumulative magneto-ionic effects can be generated by applying voltage pulses at frequencies as high as 100 Hz, which is the frequency at which dynamic effects of biological synapses occur. Magneto-ionics in Co3O4 can then be used to emulate synapse functionalities, including plasticity, potentiation (magnetization increase) and depression (i.e., depletion of magnetization), voltage-controlled threshold activation of ferromagnetism (i.e., spike-magnitude dependence), and learning and forgetting capabilities. The former by means of spike-rate-dependent plasticity of the generated magnetization upon pulsed voltage actuation. These features demonstrate Co3O4 magneto-ionics can be used for the design of neuromorphic-like systems (3). 

**References:** 
(1) A. Quintana, E. Menéndez, M. O. Liedke, M. Butterling, A. Wagner, V. Sireus, P. Torruella, S. Estradé, F. Peiró, J. Dendooven, C. Detavernier, P. D. Murray, D. A. Gilbert, K. Liu, E. Pellicer, J. Nogues and J. Sort, Voltage-controlled ON−OFF ferromagnetism at room temperature in a single metal oxide film, ACS Nano 12 (2018) 10291–10300 
(2) J. de Rojas, A. Quintana, A. Lopeandía, J. Salguero, J. L. Costa-Krämer, L. Abad, M. O. Liedke, M. Butterling, A. Wagner, L. Henderick, J. Dendooven, C. Detavernier, J. Sort and E. Menéndez, Boosting room-temperature magneto-ionics in a non-magnetic oxide semiconductor, Advanced Functional Materials 30 (2020) 2003704 
(3) S. Martins, J. de Rojas, Z. Tan, M. Cialone, A. Lopeandía, J. Herrero-Martín, J. L. Costa-Krämer, E. Menéndez and J. Sort, Dynamic electric-field-induced magnetic effects in cobalt oxide thin films: towards magneto-ionic synapses, Nanoscale 14 (2022) 842–852

On Off Ferromagnetism in Co3O4 by Voltage

Magnetic skyrmions are candidates for information carriers in applications for racetrack memories and Brownian computers because of their topologically protected properties. The circuits to confine the skyrmions in the 1D track with the magnetic wire or the magnetic energy wall play a key role in realizing such novel devices1,2. The ratchet using magnetic energy gradient is also required for improving calculation speed in ultralow-energy-consumption operation3. Because the focused-ion-beam (FIB) technique can vary the dose of ions locally, FIB effectively makes the circuit with the energy gradient4. This study aims to verify the effect of the FIB irradiation on the confinement and diffusion of the skyrmions and to structure the magnetic-energy gradient driving the skyrmions.
We fabricated the stacking structure as Ta(5.9)|Co16Fe64B20(1.2)|Ta(0.2)|MgO(2.2)|SiO2(2.9) and microfabricated the surface of the sample by the FIB of the Ga+ source. The potential condition for skyrmion creation can be controlled spatially by adjusting the dose of ions. Fig.1 shows the observations of the irradiated region indicated with the white dash line by the magneto-optical Kerr effect (MOKE) microscope. The skyrmions in the unirradiated region appear at about 333K, and in the irradiated region at about 295.5K. Fig.2(a) shows the mean squared displacement (MSD) of the skyrmions in the irradiated region. Fig.2(b) shows the temperature dependence of the diffusion coefficient before and after FIB irradiation. FIB irradiation increases the activation energy. However, the size of the diffusion coefficient after irradiation is as large as that in the sample before irradiation as far as a higher temperature is utilized. We will discuss the possibility of sputtering and of Ga+ doping and the motion in the energy gradient. This work was supported by ULVAC, Inc., JSPS KAKENHI Grant Number JP20H05666, Japan and JST, CREST Grant Number JPMJCR20C1, Japan, and Nanotechnology Platform of MEXT, Grant Number JPMXP09 S-21-OS-0053. 
**References** [1] S. Woo, et al. Nat. Mater. 15, 501 (2016)
[2] Y. Jibiki, S. Miki, et al. Appl. Phys. Lett. 117, 082402 (2020)
[3] S. Miki, et al. J. Phys. Soc. Jpn. 90, 114703 (2021)
[4] K. Fallon, et al. small 16, 1907450 (2020) 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d39a2a8539e87d2efebfe4fcc418a73f.png)

Spatial control of the nucleation and dynamics of magnetic skyrmions by FIB irradiation

Antiferromagnets (AFM) exhibit a host of properties which make them attractive materials for applications. Although they are robust against perturbation by external
magnetic fields, magnetic phases do manifest in bipartite AFMs depending on the applied field and the material properties. The phases are surprisingly complex even in the small particle
limit where the individual lattices are uniformly magnetized. While previous work [1,2,3,4,5] has examined similar systems and determined some of the equilibrium states, this work
comprehensively identifies all critical energy points (local minima, local maxima, saddle points) under the macrospin model [6] where the energy is composed of an applied field (h0)
aligned with a uniaxial anisotropy (hk) mediated by a spatially homogeneous AFM exchange (he). Both easy axis (hk>0) and easy plane (hk<0) cases are considered.
In this analysis eleven candidate magnetization configurations are identified as critical energy states. Six of these are shown to be local energy minima (metastable) for some range of
material and field parameters, illustrated in the Fig. 1 phase diagram. Note the bi- and tristable regions in this diagram that can support hysteretic behavior. The lowest energy (ground)
states, representing long-term stable phases in the presence of thermal effects, are presented in Fig. 2. Saddle points identified by this analysis determine the energy barrier between local
minima and can be used to estimate the time scale for thermally activated switching.
The transition between phases driven by changing h0 is also studied, including in particular the well-known spin-flip and spin-flop behaviors [7]. Consideration of energy levels in this analysis shows that of the eleven candidate phases alluded to above, the five which are never local energy minima also do not occur during adiabatic phase transitions. 
**References** 1) F. B. Anderson and H. B. Callen, Phys. Rev., 136, p. A1068 (1964).
2) N. Yamashita, J. Phys. Soc. Jpn., 32, p. 610 (1972).
3) A. N. Bogdanov and U. K. Rößler, Appl. Phys. Lett., 89, p. 163109 (2006).
4) A.-V. Plamadă, D. Cimpoesu and A. Stancu, Appl. Phys. Lett., 96, p. 122505 (2010).
5) H.-F. Li, npj Comput. Mater., 2, p. 16032 (2016).
6) V. Baltz, A. Manchon, M. Tsoi, Rev. Mod. Phys., 90, p. 015005 (2018).
7) N. Ntallis and K.G. Efthimiadis, Comput. Mater. Sci., 97, p. 42 (2015). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/91d94e67a526421cf301142398fc5fd1.png) 

Fig. 1: Phase diagram for energetically stable (local energy minima) configurations in an AFM. SFO (spin-flopped) is an easy-axis canted phase. All parameters and fields are scaled by he, denoted by a bar on top. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/fba2235f3ad5831dd388d3ee7e41df19.png) 

Fig. 2: Phase diagram for energetically preferred configurations (global energy minima). All parameters and fields are scaled by he, denoted by a bar on top.

Energetics of the Spin Flop and Spin

The key to realizing high area recording density capability (ADC) for heat assisted magnetic recording (HAMR) is to achieve small grain size (and pitch) in the granular L10 FePt media with a sufficiently high grain aspect ratio [1]. Media such as FePt-C, FePt-SiOx, and FePt-TaOx, or multilayers made of a combination of them, appear not able to reach the desired goal because of their failure to achieve either high grain aspect ratio, small grain size, or high L10 ordering, respectively [2, 3]. In this study, using the co-sputtering technique, we demonstrate the fabrication of FePt-BN granular media with much reduced FePt grain size and high grain aspect ratio. In particular, an underlayer stack of Ta(5nm)/Cr(50)/MgO(9) is first deposited on Corning LotusTM NXT glass followed by co-sputtering of FePt and BN at an elevated substrate temperature along. A RF bias is applied to the substrate during the FePt-BN deposition with a short and optimized delay. Figure 1C shows the order parameter as a function of substrate temperature for the FePt-BN granular film media, calculated from XRD measurements. The volumetric concentration of BN is at 38%. Figure 1A on the left of the figure shows a cross-section TEM micrograph of the FePt-BN layer formed at a substrate temperature of 725 oC. Figure 1B shows a cross-section TEM picture of FePt-BN/FePt-SiOx multilayer granular film with temperature and composition graded over the grain height for producing tall grains [4] for comparison. The grain size of single layer FePt-BN film (Figure1 A) is around 4 nm with an aspect ratio of 2 while the grain size of the FePt-BN/FePt-SiOx multilayer film is around 7nm and grain height 11nm. The small grain size with a relatively high grain aspect ratio in the single layer FePt-BN film is promising for enabling high area recording density capability. 
**References** 
[1]Y. Kubota et al., “Heat-Assisted Magnetic Recording’s Extensibility to High Linear and Areal Density,” IEEE Transactions on Magnetics, vol. 54, no. 11, Nov. 2018. 
[2] A. Perumal, Y.K. Takahashi, K. Hono, “L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution”, Appl. Phys. Express 1 101301 (2008). 
[3] B. S. D. C. S. Varaprasad, M Chen, YK Takahashi, K Hono “L1-Ordered FePt-Based Perpendicular Magnetic Recording Media for Heat-Assisted Magnetic Recording” IEEE
transactions on magnetics 49 (2), 718-722 (2013) 
[4] C. Xu, B. Zhou, T. Du, B.S.D.C.S. Varaprasad, D.E. Laughlin, J.-G. Zhu, Understanding the growth of high-aspect-ratio grains in granular L10-FePt thin-film magnetic media, APL Materials. 10 (2022). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b46f142fcddcff5ebe95523b1fdfc98e.png) 
Figure 1 (A) Out of plane micrographs of FePt-BN (8 nm) (B) Out of plane micrographs of FePt-BN/FePt-SiO 2 (11 nm) and (C) Order parameter Vs set temperature of FePt-BN sample

FePt BN Granular HAMR Media with High Grain Aspect Ratio and High L10 Ordering on Corning Lotus™ NXT Glass

L10 ordered FeNi alloy is considered as a promising candidate for rare-earth-free and environment-friendly permanent magnet. Highly ordered single-phase L10 FeNi is challenging to synthesize due to its low chemical order-disorder transition temperature. Recently, a nonequilibrium synthetic route utilizing a nitrogen topotactic reaction was considered as a valid approach although the detailed phase transformation mechanism is missing.[1] In this work, we investigated the formation mechanism of the tetragonal FeNiN precursor phase during the nitridation of FeNi nanopowders. [2] 
A low oxygen induction thermal plasma system was applied to synthesize the FeNi nanopowders (NPs). The processed FeNi NPs were annealed under a hydrogen gas flow at 1 L/min for 2 h at 400 °C. Then, the NPs were nitrided at 350 °C under ammonia with gas flowing at 2 L/min for 16 h. The detailed microstructure analysis was performed with an atomic resolution analytical electron microscope JEM ARM200F. 
High-resolution TEM observation reveals intensive nanotwins in the nitrided FeNi NPs which results in a distorted lattice Interestingly Fe segregates at the TBs was confirmed with EDS mapping. Thus, the nanotwins region may provide preferential nucleation sites for the FeNiN product phase in the Fe2Ni2N parent matrix Furthermore, detailed microstructure characterization shown in Fig.1 revealed that the growth of the FeNiN product phase followed a massive transformation with high index irrational orientation relationships and ledgewise growth motion characteristics detected at the FeNiN/Fe2Ni2N migrating interface. Based on the results, we delineated a potential formation route of the FeNiN precursor phase in the FeNi NPs during nitridation. 
**Acknowledgment:** 
This work was partially supported by the project Development of Magnetic Material Technology for High Efficiency Motors which was commissioned by Japan's New Energy and Industrial Technology Development Organization (NEDO). 
**References:** 1) G oto, S. et al. Synthesis of single-phase L10 FeNi magnet powder by nitrogen insertion and topotactic extraction.
Sci Rep 7, 13216, doi:10.1038/s41598 017 135622 (2017). 
2) Wang, J., Hirayama, Y., Liu, Z. et al. Massive transformation in FeNi nanopowders with nanotwin assisted
nitridation. Sci Rep 12, 3679, doi.org/10.1038/s41598-022-07479-8 (2022). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/7799f041271182e5315509ee10844611.png) 
Fig. 1. ABF-STEM images of nitrided FeNi NPs with terraces/ledges demonstrating the ledgewise growth motion of the FeNiN/Fe2Ni2N interface (a & b). (c) schematizes the representative high-index orientation relationship present in (b).[2]

Nanotwins assisted nitridation in FeNi nanopowders synthesized by induction thermal plasma

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

The interplay between geometry and topology of the order parameter is crucial properties in soft and condensed matter physics, including cell membranes [1], nematic crystals [2,3], superfluids [4], semiconductors [5], ferromagnets [6] and superconductors [7]. Until recently, in the case of magentism, the influence of the geometry on the magnetization vector fields was addressed primarily by the design of the sample boundaries, aiming to tailor anisotropy of the samples. With the development of novel fabrication techniques allowing to realize complex 3D architectures, not only boundary effects, but also local curvatures can be addressed rigorously for the case of ferromagnets and antiferromagnets. It is shown that curvature governs the appearance of geometry-induced chiral and anisotropic responses [6-8]. Here we provide experimental confirmations of the existence of local and non-local curvature-induced chiral interactions of the exchange and magnetostatic origin in conventional soft ferromagnetic materials. Namely, we will present the experimental validation of the appearance of exchange-driven Dzyaloshinskii-Moriya interaction interaction (DMI, local effect) for the case of conventional achiral yet geometrically curved magnetic materials [9,10]. This curvature induced DMI is predicted to stabilize skyrmions [11] and skyrmionium states [12]. Furthermore, we will address the impact of nonlocal magnetostatic interaction on the properties of curvilinear ferromagnets, which enables the stabilization of topological magnetic textures [13,14], realization of high-speed magnetic racetracks [15] and curvature-induced asymmetric spin-wave dispersions in nanotubes [16]. Furthermore, symmetry analysis demonstrates the possibility to generate a fundamentally new chiral symmetry breaking effect, which is essentially nonlocal [13]. Thus, geometric curvature of thin films and nanowires is envisioned as a toolbox to create artificial chiral nanostructures from achiral magnetic materials.

Local and Nonlocal Curvature induced Chiral Effects in Nanomagnetism

The degree of magnetic order that can be achieved in thermally artificial kagome spin ice depends on the superparamagnetic blocking temperature of the individual nanomagnets [1] and the predicted phase transition temperature of the system [2]. The interplay between these two temperature scales dictates the observable magnetic order in experiments and in this talk, I will discuss how phase transition temperatures can be independently tuned from superparamagnetic blocking temperature.

We find that by placing nanomagnetic bridges asymmetrically at the vertices of artificial kagome ice we can control the degree of frustration in the system, which allows for the independent tuning of the phase transition temperature [3]. We demonstrate that by carefully varying the widths of the bridges, we obtain various degrees of magnetic ordering and observe two magnetic phases. We then utilize the thermally active system to show that we can image the phase transition using temperature-dependent X-ray photoemission electron microscopy. 
**Reference:** 
[1] K. Hofhuis, A. Hrabec, H. Arava, N. Leo, Y.L. Huang, R.V. Chopdekar, S. Parchenko, A. Kleibert,
S. Koraltan, C. Abert, C. Vogler, D. Suess, P.M. Derlet, & L.J. Heyderman, Physical Review B 102, 180405(R) (2020) 
[2] G. Moller & R. Moessner, Physical Review B 80, 140409(R) (2009). 
[3] K. Hofhuis, S. H. Skjærvø, Z. Luo, S. Parchenko, A. Kleibert, P. M. Derlet, & L. J. Heyderman, Nature Physics 18, 699-705 (2022)

Phase Transitions in Thermally Active Artificial Kagome Spin Ice

Artificial spin-ice systems permit the design and study of a wide family of emergent physical phenomena resulting from the combination of a very large number of interactions between simple elements. They are of great interest since the arrangement of elements, as well as their interactions, can be tuned by means of relatively simple lithography.
An important challenge in the study of artificial spin-ice systems is the large number of elements involved in each system, in combination with their small physical size. Techniques such as magnetic force microscopy or X-ray microscopy are usually needed to study them. In addition, due to the dynamic and/or stochastic nature of the emerging phenomena, also a large number of states of the system need to be probed in order to have a complete physical picture of the system, or to reduce enough the uncertainty in measurements of stochasticity parameters. Finding the right balance between spatial resolution, speed and availability of experimental facilities is therefore an exciting endeavour.
In this talk, we will discuss our experience with the optimization of full-field Kerr microscopy at high resolution and throughput (number of images), presenting the technique as an attractive option for the study of artificial spin-ice systems. We will present the key technical components necessary to perform this type of experiments, and, in particular, how to produce very large datasets volumes that can subsequently be employed to extract complex physical parameters via statistical analysis. See, for example, the use of automatic switch detection in Figures 1 and 2, in which 16 triangular honeycomb lattices are imaged in parallel (Figure 1) and the switching of vertical elements is automatically extracted from differential Kerr contrast (Figure 2). We will use our recent demonstration of the tunability of stochastic domain-wall decisions in honeycomb lattices using >200,000 images [1] as an example to guide the discussion.
We anticipate a great interest from the artificial spin-ice community, as the size of individual elements in artificial spin-ice networks often falls within the resolution limit of the technique (≈150nm). This could open the door to exploring statistical physics and new device applications with a level of detail hardly reachable until now. 

**References:** 

[1] Sanz-Hernández, D., Massouras, M., Reyren, N., Rougemaille, N., Schánilec, V., Bouzehouane, K., Hehn, M., Canals, B., Querlioz, D., Grollier, J. and Montaigne, F., 2021. Tunable stochasticity in an artificial spin network. Advanced Materials, 33(17), p.2008135. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e3ab4d92881d104156283d8c1efa8c58.png)

High throughput Immersion Kerr Microscopy of Artificial Spin Ice: a route towards new discoveries and applications

Metamagnetic Heusler compounds display a first order structural transformation associated with large changes of magnetization and/or magnetic order. Their properties can
be tuned by suitable changes in composition making them a versatile class of multifunctional materials, thanks to the strong interplay between thermal, mechanical and magnetic degrees of
freedom [1]. In addition, their hierarchically interrelated twin-with-twin microstructure, in the low temperature martensitic phase, and the strong spin-lattice coupling allow to control their
magnetic and multifunctional properties from the atomic to the micro-scale by tuning growth conditions and applying external fields [2, 3].
In my talk I will report on our recent results on nano/microscale materials obtained by different fabrication methods (e.g. epitaxial thin films, patterned and free-standing structures) [3-6].
Thin films and micro/nanostructures are of particular interest not only for the realization of new-concept devices, but also for providing insights into the magnetostructural coupling at the
different length scales. The talk will focus on microstructure engineering and microstructure related effects on the martensitic transformation, also in view of the possible exploitation of
this class of materials in energy related and smart applications.
In particular, by an accurate magnetic and structural investigation at different length-scales, I will demonstrate how growth temperature and simple post-growth treatments, i.e. postannealing
at low T, magnetic field cooling and mechanical stress, are suitable to manipulate the twin variant configuration in epitaxial thin films. X-type variants with out-of-plane
magnetic easy axis or Y-type variants with in-plane magnetic easy-axis can be selected, as well as their geometrical distribution, in films with mixed X/Y-type microstructure [3].
Taking advantage from the possibility to manipulate microstructure we have studied the role of specific martensitic configurations on the martensitic transition path. By advanced magnetic
force microscopy imaging in a wide temperature (260–350 K) and magnetic field range (up to 14 T) we have directly observed the nucleation and the self-accommodation of the martensitic
twinning configurations under zero-field, isofield and isothermal conditions. We have found that between the two possible twinning configurations, the Y-type, which nucleates first, shows
a significantly smaller thermal hysteresis as well as a sharper phase transition with respect to X-type microstructure, for all the three investigated conditions [4].
The effects of lateral size, shape and geometry on the properties of patterned epitaxially grown Ni-Mn-Ga films will also be discussed. In particular, I will present the characterization of
arrays of microstructures, obtained by means of different lithographic techniques, with lateral sizes down to 200 nm range and having different shapes and orientations with respect to the
substrate edges [5].
The last part of my talk will be devoted to free-standing nanostructures in view of their possible exploitation as micro/nano actuators exploiting microstructure-controlled actuation
mechanisms by the combined application of temperature and magnetic field [6]. 

**References:** 

[1] M. Acet, L. Mañosa, A. Planes, Handbook of Magnetic Materials (Ed: K. H. J. Buschow), Elsevier, Amsterdam 2011, pp. 231–289 
[2] P. Ranzieri et al., Adv. Mater. 2015, vol. 27, p. 4760 
[3] M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 187, p. 135–145 (2020) 
[4] M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 23, 117356 (2021) 
[5] M. Takhsha Ghahfarokhi, et al. Appl. Mater. Today vol. 23, p. 101058 (2021). 
[6] M. Campanini et al., Small, vol. 14, p. 1803027 (2018) 

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