United States

Ferromagnetic thin films with perpendicular magnetic anisotropy (PMA) provide an excellent support to develop media for high-density magnetic recording. To optimize
the density of magnetic domains and control the magnetic response of the magnetic film to an applied magnetic field, further understanding of the domain pattern formation and evolution
throughout the magnetization process is necessary. Here we study the in-situ morphology of magnetic domains in [Co/Pt]N multilayered thin films with PMA via magnetic force
microscopy (MFM). We found that the domain pattern evolves between three distinct topologies: bubble pattern, short stripe pattern, and maze pattern. Previous studies1-4 have
demonstrated that the morphology of these patterns at remanence depends on the number of bilayer repeats, layer thickness, and maximum of the previously applied field. Here we extend
the remanence study to explore the evolution of the domain patterns under in-situ magnetic field. Domain density and interdomain distance are key metrics for quantifying morphological
changes in the domain patterns, as illustrated in Figure 1. Using MFM, we mapped the domain density and interdomain distance in response to the in-situ and maximum applied magnetic
field up to 7.2 kOe. We completed these studies for layer repeats of N from 20 down to 10 and Co thicknesses from 30 Å down to 10 Å. We found three main types of in-situ phase
transitions, which result in the previously observed remanent phase transitions Additionally, we found that the domain density is maximized with in-situ applied field when N = 20 with Co
thicknesses of 30 Å.&lt;br&gt;

**References:**&lt;br&gt; 

1. A. S. Westover, K. Chesnel, K. Hatch, P. Salter and O. Hellwig, Journal of Magnetism and Magnetic Materials 399, 164-169 (2016).&lt;br&gt;
2. K. Chesnel, A. S. Westover, C. Richards, B. Newbold, M. Healey, L. Hindman, B. Dodson, K. Cardon, D. Montealegre, J. Metzner, T. Schneider, B. Böhm, F. Samad, L. Fallarino and
O. Hellwig, Physical Review B 98 (22), 224404 (2018).&lt;br&gt;
3. Fallarino, A. Oelschlägel, J. A. Arregi, A. Bashkatov, F. Samad, B. Böhm, K. Chesnel and O. Hellwig, Physical Review B 99 (2), 024431 (2019).&lt;br&gt;
4. A. Gentillon, C. Richards, L. A. Ortiz-Flores, J. Metzner, D. Montealegre, M. Healey, K. Cardon, A. Westover, O. Hellwig and K. Chesnel, AIP Advances 11 (1), 015339 (2021).&lt;br&gt;


![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/3c9444db7ca08250e56aea7951c4926c.png)

&lt;br&gt;
Fig. 1 Domain density plot for [Co(10 Å)/Pt(7 Å)]20 as a response to the in-situ applied magnetic field, Hin-situ, and for a maximum applied magnetic field of 6.4 kOe. &quot;Reversed domains&quot;
refers to domains where the magnetization is anti-aligned with the applied field while &quot;parallel domains&quot; refers to domains where the magnetization is aligned with the applied field.
ROB-08

MMM 2022

Mapping In situ Morphological Phase Transitions of Magnetic Domains in [Co/Pt]N under applied magnetic field

Ferromagnetic thin films with perpendicular magnetic anisotropy (PMA) provide an excellent support to develop media for high-density magnetic recording. To optimize
the density of magnetic domains and control the magnetic response of the magnetic film to an applied magnetic field, further understanding of the domain pattern formation and evolution
throughout the magnetization process is necessary. Here we study the in-situ morphology of magnetic domains in [Co/Pt]N multilayered thin films with PMA via magnetic force
microscopy (MFM). We found that the domain pattern evolves between three distinct topologies: bubble pattern, short stripe pattern, and maze pattern. Previous studies1-4 have
demonstrated that the morphology of these patterns at remanence depends on the number of bilayer repeats, layer thickness, and maximum of the previously applied field. Here we extend
the remanence study to explore the evolution of the domain patterns under in-situ magnetic field. Domain density and interdomain distance are key metrics for quantifying morphological
changes in the domain patterns, as illustrated in Figure 1. Using MFM, we mapped the domain density and interdomain distance in response to the in-situ and maximum applied magnetic
field up to 7.2 kOe. We completed these studies for layer repeats of N from 20 down to 10 and Co thicknesses from 30 Å down to 10 Å. We found three main types of in-situ phase
transitions, which result in the previously observed remanent phase transitions Additionally, we found that the domain density is maximized with in-situ applied field when N = 20 with Co
thicknesses of 30 Å. 

**References:** 

1. A. S. Westover, K. Chesnel, K. Hatch, P. Salter and O. Hellwig, Journal of Magnetism and Magnetic Materials 399, 164-169 (2016). 
2. K. Chesnel, A. S. Westover, C. Richards, B. Newbold, M. Healey, L. Hindman, B. Dodson, K. Cardon, D. Montealegre, J. Metzner, T. Schneider, B. Böhm, F. Samad, L. Fallarino and
O. Hellwig, Physical Review B 98 (22), 224404 (2018). 
3. Fallarino, A. Oelschlägel, J. A. Arregi, A. Bashkatov, F. Samad, B. Böhm, K. Chesnel and O. Hellwig, Physical Review B 99 (2), 024431 (2019). 
4. A. Gentillon, C. Richards, L. A. Ortiz-Flores, J. Metzner, D. Montealegre, M. Healey, K. Cardon, A. Westover, O. Hellwig and K. Chesnel, AIP Advances 11 (1), 015339 (2021). 


![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/3c9444db7ca08250e56aea7951c4926c.png)

 
Fig. 1 Domain density plot for [Co(10 Å)/Pt(7 Å)]20 as a response to the in-situ applied magnetic field, Hin-situ, and for a maximum applied magnetic field of 6.4 kOe. "Reversed domains"
refers to domains where the magnetization is anti-aligned with the applied field while "parallel domains" refers to domains where the magnetization is aligned with the applied field.
ROB-08

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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Spin-transfer torque magnetoresistive random-access memory (STT-MRAM) is a promising non-volatile memory technology for eliminating the classical von Neumann bottleneck to build a high-speed, energy-efficient memory hierarchy.1 One of the challenges that STT-MRAM needs to overcome, notably for automotive applications, is the robust and reliable operation in the temperature range of -40 °C to 150 °C.2 Therefore, it is vital to investigate STT-MRAM device characteristics as a function of temperature.
We investigated thermal stability factor (Δ), tunneling magnetoresistance (TMR) ratio, critical switching current (IC), and the other device parameters of STT-MRAM devices from -25 °C to 150 °C. Four different device wafers (W1-4) with increasing storage layer (SL) thicknesses and three different device diameters D1, D2, and D3 (with D1<D2<D3) were studied. The temperature-dependent behavior of Δ and TMR ratio were analytically modeled to understand their thermal variation. Figure 1 shows the experimental values of Δ, and the fitting of its thermal variation by macrospin and domain-wall model for device D2 in W1-4. Extrapolating the thermal variation of Δ, blocking temperatures (Tb) of different types of memory cells were calculated as shown in Fig. 1. It was observed that the magnetization reversal most likely occurs by domain wall motion at lower temperatures, which changes to multinucleation of domains at larger temperatures making Δ invariable with the diameter (not shown here). Figure 2 shows the thermal variation of practical switching efficiency (κ), calculated by considering Δ at different temperatures and IC at -25 °C. Similar to the variation of IC (not shown here), the κ at -25 °C increases with the thickness of SL. This improvement is believed to be due to the decrease in damping constant. This temperature-dependent investigation is important to understand the retention, writing, and reading performance of STT-MRAM cells for high-temperature applications. 
**References** 1 B. Dieny, I.L. Prejbeanu, K. Garello, P. Gambardella, et al., Nat. Electron. 3, 446 (2020).
2 K. Lee, R. Chao, K. Yamane, V.B. Naik, et al., Tech. Dig. - Int. Electron Devices Meet. IEDM 2018-December, 27.1.1 (2019).
3 X. Feng and P.B. Visscher, J. Appl. Phys. 95, 7043 (2004).
 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/22bcf7f98b6fe62f9a9b383439671b72.png)

Thermal Variation of Thermal Stability Factor and Switching Efficiency of STT MRAM Devices

Quantum computation and cryogenic electronics have gained considerable importance in this last decade. Currently, a bottleneck for advancing it further is the power consumption of conventional electronic components, which is not adapted to the needs of cryogenic circuits. Perpendicular Spin-Transfer-Torque Magnetic Random Access Memory (p-STT-MRAM) has potential to comply with cryo-electronics requirements of low power. Previous works done on cryogenic temperature switching of STT-MRAM have assessed its characteristics and the Write Error Rate for different device diameters [1]. The conditions to write an STT-MRAM are promising (0.7V at 9K with 200ns pulses, corresponding to a writing energy of 11.2 pJ) [2]. However, the full integration of MRAMs into quantum processors demands a much lower operating voltage (below 0.1V).Therefore, we focused our research on engineering MTJs for low anisotropy to reduce the critical switching voltage VC and consequentially the write energy per bit Ebit. Our approach consists in changing the interface of the storage layer by adding an insertion layer as in Mg+Ox/X/FeCoB/W/Y/W, where X can be wedges of Mg, FeCoB and Y is permalloy (Py). This type of device can show high thermal stability at cryogenic temperatures (i.e.4K), while having no retention at 300K. This strategy allows the reduction of the storage layer anisotropy by at least one order of magnitude.
Figure 1 shows a current pulse phase diagram of a junction with Mg insertion. Upon changing the operation temperature the write current density JC for junctions with different insertion layers is varying as reported in Figure 2. Device diameter, composition and thickness play an important role in modulating the anisotropy. Tuning these parameters it is possible to bring the switching currents close to 20µA for a 30nm diameter device at 10K (red curve), corresponding to Ebit=1.2pJ. This can be further improved by modulating the required current density with composition as shown in the figure 2. 
**References** [1] L. Rehm, G. Wolf, B. Kardasz, M. Pinarbasi, and A. D. Kent, "Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures", Appl. Phys. Lett. Vol. 115, p.182404 (2019).
[2] Lili Lang, Yujie Jiang, Fei Lu, Cailu Wang, et al, "A low temperature functioning CoFeB/MgO-based perpendicular magnetic tunnel junction for cryogenic nonvolatile random access memory", Appl. Phys. Lett. Vol. 116, p.022409 (2020) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/7104e850bea9e89cf9c614e4c15db138.png)

Control of Interface Anisotropy for Spin Transfer Torque in Perpendicular Magnetic Tunnel Junctions for Cryogenic Temperature Operation

We study the Hall response of topological defects, such as merons and antimerons, to spin currents in two-dimensional magnets with in-plane anisotropy in the vicinity of the Berezinskii-Kosterlitz-Thouless (BKT) transition [1]. We use a combination of Monte Carlo and spin dynamics simulations to study the transition from spin superfluidity to conventional spin transport, the universal jump of the spin stiffness, and the exponential growth of the transverse vorticity current. We accompany our results by phenomenological description. We show how the spin and vorticity currents can be modulated by changes in density of free topological defects, e.g., by tuning the in-plane magnet across the BKT transition by changing the exchange interaction, magnetic anisotropy, or temperature. We further show how spin and vorticity currents can be controlled by employing connection between vorticity and spin, e.g., we demonstrate spin diode effect.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0021019.
 
**References** [1] Edward Schwartz, Bo Li, and Alexey A. Kovalev, Phys. Rev. Research 4, 023236 (2022)

Spin and vorticity transport in two dimensional magnets with easy

The emergence of the Internet of Things (IoT) devices, one of the crucial building blocks of modern technologies, shows of late a remarkable interest in the field of
microwave-based technologies, e.g., RF energy harvesting, radars, miniaturized antennas, ultra-low field magnetic sensors etc. [1-5]. The voltage-controlled microwave oscillators typically
consume a large amount of power to produce microwave frequencies with moderate frequency stabilities. The conventional batteries cannot be used for powering such power-hungry IoT
devices and prohibitory cost for replacement in wireless sensor nodes (WSN), particularly in inaccessible locations and their sustainability due to the overall pollution they are likely to
produce. Hence, researchers are looking to invent state-of-the-art microwave oscillators [5] for such cutting-edge applications. In this work, we report the domain wall movement in the
patterned array of rectangular magnetostrictive nanomagnets/piezoelectric heterostructures caused by surface acoustic waves (SAW) for microwave generation. A surface acoustic wave
launched on the substrate produces periodic strain within the patterned nanomagnets, which, in turn, stimulates the magnetization precession of the nanomagnets resulting in different
magneto-dynamical resonance modes in the array of nanomagnets with a rich spin wave (SW) texture. The generated SWs (magnons) further interact with the EM wave radiation (photons)
at the SAW frequency [6]; this phonon-magnon-photon coupling generates a 0.56 GHz microwave frequency with a 13.9 MHz linewidth and Q-factor of 40. The generated non-volatile
spin textures of the nanowires are also useful in energy-efficient logic and low-power computing applications [7].
References Acknowledgement: Both, AS and SR gratefully acknowledge the financial support for this work from the "EU-H-2020" project, “EnABLES-JRA”, Project ID: 730957. 
**References:** [1] T. Nan, H. Lin, Y. Gao, Nature Communications 8, 296 (2017). [2] J.-S. Kim, M.-A. Mawass, A. Bisig, Nat. Commun. 5, 3429 (2014). [3] T. Ono and Y. Nakatani, Appl.
Phys. Express 1, 61301 (2008). [4] R. Sbiaa, M. Al Bahri, and S. N. Piramanayagam, J. Magn. Magn. Mater. 456, 324 (2018). [5] S. Bhatti and S. N. Piramanayagam, Phys. Status Solidi –
Rapid Res. Lett. 13, 1800479 (2019). [6] R. Fabiha, J. Lundquist, S. Majumder, Advanced Science 9, 2104644 (2022). [7] V. Sampath, N. D’Souza, D. Bhattacharya, Nano Letters 16,
5681 (2016).

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/81ec945974ab857770bf501861dcb71d.png) 

(a)Initial magnetization configuration of NWs; l, w, are length, width of each NWs along the x, y directions, respectively, and s is the spacing between the NWs in xy plane.(b)The
multidomain state with transverse stress.(c)The multidomain structures in (b) vanished upon launching of SAW.(d)Final spin structure after completion simulation, retaining the domain
structure in (c). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/8d4208019c2915b0806c267be89cdcb5.png) 
Power distribution as a function of spacing between the neighbouring NWs along x, y, z directions in (a), (b), (c), respectively.

Stress Engineered Microwave Generation induced by Phonon

Magnetic random access memories (MRAM) store information on magnetization direction instead of electric charge. All optical switching (AOS) of a magnetic tunnel junction provides an efficient way to enable writing at speeds orders of magnitude faster than electrical alternatives based on spin torque methods, allowing for write operations at THz frequencies. The [Tb/Co] multilayer is particularly interesting, since its large perpendicular magnetic anisotropy would allow data retention at small sizes and could be integrated into a magnetic tunnel junction [1-2]. This work reports magnetization reversal driven by femtosecond light pulses on a [Tb/Co] based perpendicular magnetic tunnel junction patterned to sub-100nm lateral dimensions. This optical switchable electrode was integrated into an MgO based MTJ having synthetic anti-ferrimagnet reference layer element, where the AOS magnetization direction can be read directly from the resistance state of the junction. The optically switchable electrodes are [Tb/Co]x5 multilayers coupled to a 1.4nm FeCoB layer through a 0.2nm W insertion layer. Patterned AOS-MTJ nanopillars fabricated with diameters down to 80 nm showed TMR ratios up to 74% and a resistance area product of 112 Ω μm2. Figure 1) shows the hysteresis loop for a [Tb 0.6nm/ Co 1.4nm]x5 multilayer. It was possible to observe a toggle switching between low and high resistance states, corresponding to parallel and anti-parallel alignment of the two electrodes, after each 50 fs long laser pulse applied on the device. A systematic study on the switching probability will be compared to existing results of AOS experiments on continuous films. There it was possible to demonstrate that reliable and robust toggle switching can be achieved for specific Tb and Co thickness in the range of 0.6-0.9 nm of Tb and 1.3-1.5 nm of Co. The magnetization reversal is reliable after a 150.000 pulse sequence. Single shot AOS is achievable using laser pulse durations from 50 fs up to 10 ps and fluences 3-6.5 mJ/cm2. These results pave the way towards an AOS ultra-fast and energy-efficient memory. 
**References** 
[1] A. Olivier et al., Nanotechnology 31, 425302 (2020). 
[2] L. Avilés-Félix et al., Scientific Reports 10, 1 (2020). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/dca0d23bf1cc210d5479056d84ef1c36.png) 
Figure 1 a) AOS-MTJ stack illustration b) R-H loop in perpendicular field c) AOS field-free toggle switching between P and AP states.

Field free all optical switching in Tb/Co multilayer magnetic tunnel junction

The application of Nd-Fe-B magnet with substitution of Nd by cheap and abundant Ce elements is seriously limited due to the intrinsically inferior magnetic properties of Ce-Fe-B [1-3]. Recently, dual-main-phase (DMP) sintered magnet was successfully developed [4], which allowed special distribution of heterogeneous main phases (Ce-rich and Nd-rich R2Fe14B main phases), contributing remarkably to balance the utilization of high-abundance rare earths and the performance. This work demonstrated a novel design of quasi-trivalent DMP Ce magnets using the strategy of two main phases with giant magnetic anisotropy energy gap (MAEG). The maximum energy product of DMP Ce magnet with 40% weight percentage of Ce over total rare earth is optimized to be 40.57 MGOe, as shown in Fig. 1, which is the energy-product record. The superior magnetic properties are closely corelated to the average valence of Ce. The average valence of Ce in the DMP magnets was quasi-trivalent, proved by XPS spectroscopy, as shown in Fig. 2, which was lower than that in the single-main-phase magnets.
Furthermore, with an isostructural heterogenous configuration in the DMP magnets could manipulate electron migration and induce magnetic Ce3+ in the main phase, thereby improving the remanence. It was possible to control the Ce valence for the design of high-performance Ce magnets using main phases with diversity of MAEG. This would undoubtedly provide ideas for researching and developing new rare earth materials with a synergistic composition and structure. Compared with Nd-Fe-B magnets, the new quasi-trivalent DMP Ce magnets would save 30-40% Nd and remarkably reduce the production cost of magnets with much higher Ce content. 
**References:** [1] J. F. Herbst, Rev. Mod. Phys. 63, 819 (1991). 
[2] D. Li, Y. Bogatin, J. Appl. Phys. 69, 5515 (1991). 
[3] S. X. Zhou, Y. G. Wang, R. Høier, J. Appl. Phys. 75, 6268, (1994). 
[4] M.G. Zhu, W. Li, Y.K. Fang, et al. J. Appl. Phys. 109, 07A706 (2011). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/820e12846854022ea880456a41a440c1.png) 
Fig. 1 Demagnetization curve of the DMP-Ce-0.4 magnet at room temperatures. The energy-product record of 40.57 MGOe is reported for DMP-Ce-0.4 magnet. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/570ea6fc6a5760005e128776d2926bec.png) 
Fig. 2 Semi-quantitative fitting of the Ce 3d core-level spectra for the DMP-Ce-0.4 magnet using the ordinary least squares approach, where the standard XPS spectra of Ce3+ and Ce4+ were sourced from the Avantage software database.

Novel Quasi trivalent Dual

We are studying the effect that illumination by coherent resonant x-rays may have on magnetic domain memory (MDM) in a [Co/Pd] / IrMn multilayers [1-3]. MDM is the
ability of the magnetic domains to retrieve their exact same domain topology upon field cycling. In the case of [Co/Pd] / IrMn multilayers, MDM is induced by exchange couplings
between the Co/Pd ferromagnetic layer and the IrMn antiferromagnetic layer. We found that under high dose of x-ray illumination, the material may lose its existing MDM. To investigate
this potential effect, we have used both x-ray resonant magnetic scattering (XRMS) along with magneto-transport measurements [4,5] to track the exchange bias while the sample is
illuminated with x-rays, as illustrated in Figure 1. Magneto-transport is here used to measure the hysteresis loop of the multilayered material and observe exchange bias. A loss of exchange
bias would indicate that the x-rays illumination dose may alter the strength of the exchange couplings and ultimately the amount of MDM. Knowing if a loss of exchange bias has occurred
requires collecting magneto-transport data as well as XRMS data and correlating the observed changes under various dose of x-ray illumination. The data has been collected at different
angles, different configurations, and different temperatures. These configurations consist of Hall effect measurements as well as magnetoresistance measurements. Also, data was collected
using various sets of electrical contacts: a set of inner contacts around the x-ray transmission window and as set of outer contacts for reference. The outer contacts were located at a distance
of ~1 cm whereas the spacing between inner contacts was ~100 microns. These measurements have been carried at BYU in order to better understand the shape of the data observed in-situ
at the APS 

**References:** 

[1] K. Chesnel, A. Safsten, M. Rytting, and E.E. Fullerton, Nature Communications 7, (2016). 
[2] K. Chesnel, B. Wilcken, M. Rytting, S.D. Kevan, and E.E. Fullerton, New Journal of Physics 15, 023016(2013). 
[3] K. Chesnel, J. Nelson, B. Wilcken, and S.D. Kevan, Journal of Synchrotron Radiation 19, 293 (2012). 
[4] C. Hurd, Hall Effect in Metals and Alloys (Springer, 2012). 
[5] L.J. van der Pauw, A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shape (1958). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/4b50c3a1949acb220aa816766ae3c2d9.png)

The effect of x ray illumination on magnetic domain memory in [Co/Pd] / IrMn multilayers

Magnetic skyrmions [1] are topologically stabilized, magnetic quasi-particles, which exhibit interesting physical phenomena and are also potential candidates for efficient neuromorphic computation schemes. On the micro-meter scale, they are detectable with Kerr-microscopy, using the magneto-optical Kerr-effect. However, depending on the material stack, temperature, the growth procedure and other influences, those measurement might suffer from noise, low-contrast, intensity gradients or defects, therefore manual data treatment is normally necessary to generate data that can be evaluated easily.
Our approach is to use a convolutional neural network, based on the U-Net [2], to detect the position and shape of the skyrmions in our measurement data. This work was motivated by reports that machine learning has been successfully applied to (micro-)magnetic problems [3]. We are tuning the U-Net with various techniques to optimize the predictions and identify skyrmions and defects fast and with high reliability. A well-trained neural network is shown to minimize manual treatment of data. The approach is also easily extendable to other magnetic structures. 

**References:** 

[1] Everschor-Sitte et al., Journal of Applied Physics 124, 240901 (2018) 
[2] Ronneberger et al., MICCAI 2015. Lecture Notes in Computer Science, vol 9351. Springer, Cham. (2015) 
[3] Alexander Kovacs et al., Journal of Magnetism and Magnetic Materials 491, 165548 (2019) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b905ba9abf4990b3c3c3e3a184f5069f.png)

Skyrmion detection with U Net

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

The frustrated interactions in nanomagnetic arrays can lead to the emergence of complex behaviour. Here we consider nanomagnets with dimensions on the crossover from single ferromagnetic domain (macrospin) and vortex states [1-3], in both ordered and disordered array geometries. Magnetic force microscopy reveals that there is a ratchet-like transformation of macrospins into vortices, during minor loop magnetic reversals. Simulations show that interactions with neighbouring magnets play an important role, giving cooperative growth of macrospin and vortex chains. This gradual evolution of linkages is neuromorphic in character, and interesting for both neurally-inspired computation [1] and self-assembly of magnonic circuits[3]. 

**References:** 

[1]Gartside, Jack C., et al. "Reconfigurable Training and Reservoir Computing via Spin-Wave Fingerprinting in an Artificial Spin-Vortex Ice." Nature Nanotechnology (2022)
[2]Gartside, Jack C., et al. "Reconfigurable magnonic mode-hybridisation and spectral control in a bicomponent artificial spin ice." Nature Communications 12.1 (2021): 1-9.
[3]Stenning, Kilian D., et al. "Magnonic bending, phase shifting and interferometry in a 2d reconfigurable nanodisk crystal." ACS nano 15.1 (2020): 674-685.

Mapping In situ Morphological Phase Transitions of Magnetic Domains in [Co/Pt]N under applied magnetic field

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Thermal Variation of Thermal Stability Factor and Switching Efficiency of STT MRAM Devices

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