United States

Fluctuations are ubiquitous in nanometer-scale systems, spanning orders of magnitude in space and time. Real-space access to fluctuating states is impeded by a dilemma
between spatial and temporal resolution. Averaging over an extended period of time (or repetitions) is key for the majority of high-resolution imaging experiments, especially in weak
contrast systems. If, by lack of better knowledge, averaging is indiscriminate, it leads to a loss of temporal resolution and to motion-blurred images. We present coherent correlation imaging (CCI) – a high-resolution, full-field imaging technique that realizes multi-shot, time-resolved imaging of stochastic processes. The key of CCI is
the classification of camera frames that correspond to the same physical state (Fig. 1) even at low photon count, where imaging is not possible. CCI combines a correlation-based similarity
metric with powerful classification algorithm developed for genome research [1] realizing informed, non-sequential signal averaging while maintaining single frame temporal resolution. We apply CCI to study previously inaccessible magnetic fluctuations in a highly degenerate magnetic stripe domain state. Our material is a Co-based chiral ferromagnetic multilayer with
magnetic pinning low enough to exhibit stochastically recurring dynamics that resemble thermally-induced Barkhausen jumps near room temperature. CCI reconstructs sharp, high-contrast
images of all domain states by phase retrieval [2] and, unlike previous approaches, also tracks the time when these states occur. The spatiotemporal imaging reveals an intrinsic transition
network between the states and unprecedented details of the magnetic pinning landscape (Fig. 2).&lt;br&gt;
**References** &lt;br&gt;
[1] G. Sherlock, et. al., Current Opinion in Immunology 12, 201-205 (2000)
[2] Flewett, S. et al., Optics Express 20, 29210–29216, 2012&lt;br&gt;
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/575255bbb0f7b8891a22722531593fa6.png)&lt;br&gt;
Principle of time-resolved coherent correlation imaging. Top: Sequence of camera frames showing Fourier-space coherent scattering patterns. Coherent correlation imaging classifies
scattering frames by their underlying domain state, as indicated by the colors. Bottom: Real-space images reconstructed from an informed average of same-state frames.&lt;br&gt;

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c06609f11a71267f63b9f1a35eff95c7.png)&lt;br&gt;
Map of attractive (blue dots) and repulsive (red areas) pinning sites. The background shows the position of the domain walls and their relative occurrence observed in the experiment.

MMM 2022

Coherent Correlation Imaging: Resolving fluctuating states of matter

Fluctuations are ubiquitous in nanometer-scale systems, spanning orders of magnitude in space and time. Real-space access to fluctuating states is impeded by a dilemma
between spatial and temporal resolution. Averaging over an extended period of time (or repetitions) is key for the majority of high-resolution imaging experiments, especially in weak
contrast systems. If, by lack of better knowledge, averaging is indiscriminate, it leads to a loss of temporal resolution and to motion-blurred images. We present coherent correlation imaging (CCI) – a high-resolution, full-field imaging technique that realizes multi-shot, time-resolved imaging of stochastic processes. The key of CCI is
the classification of camera frames that correspond to the same physical state (Fig. 1) even at low photon count, where imaging is not possible. CCI combines a correlation-based similarity
metric with powerful classification algorithm developed for genome research [1] realizing informed, non-sequential signal averaging while maintaining single frame temporal resolution. We apply CCI to study previously inaccessible magnetic fluctuations in a highly degenerate magnetic stripe domain state. Our material is a Co-based chiral ferromagnetic multilayer with
magnetic pinning low enough to exhibit stochastically recurring dynamics that resemble thermally-induced Barkhausen jumps near room temperature. CCI reconstructs sharp, high-contrast
images of all domain states by phase retrieval [2] and, unlike previous approaches, also tracks the time when these states occur. The spatiotemporal imaging reveals an intrinsic transition
network between the states and unprecedented details of the magnetic pinning landscape (Fig. 2). 
**References** 
[1] G. Sherlock, et. al., Current Opinion in Immunology 12, 201-205 (2000)
[2] Flewett, S. et al., Optics Express 20, 29210–29216, 2012 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/575255bbb0f7b8891a22722531593fa6.png) 
Principle of time-resolved coherent correlation imaging. Top: Sequence of camera frames showing Fourier-space coherent scattering patterns. Coherent correlation imaging classifies
scattering frames by their underlying domain state, as indicated by the colors. Bottom: Real-space images reconstructed from an informed average of same-state frames. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c06609f11a71267f63b9f1a35eff95c7.png) 
Map of attractive (blue dots) and repulsive (red areas) pinning sites. The background shows the position of the domain walls and their relative occurrence observed in the experiment.

technical paper

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LaMn3+O3 is an AFM insulator with TN = 140 K but transforms into a FM insulator with Co2+ or Ni2+ substituted at the Mn-site in La2CoMnO6 and La2NiMnO6 (1), accompanied by a change of Mn valence from 3+ to 4+. However, LaMn0.96Mo0.04O3 is a metallic ferromagnet at low temperatures (2). The FM Curie temperature TC decreased only by 7 K upon 6% Mo6+: d0 substitution in La0.67Sr0.33Mn1-xMoxO3 (3). It was stated that Mo6+ doping induced Mn2+ (d5) which interacted with Mn3+(d4) through Zener’s double exchange giving rise to FM and metallic behavior. But, studies on Mo-doped manganites are yet rare. Here, we report the physical characterization of LaMn0.94Mo0.06O3 prepared by solid-state route. Fig.1(a) shows the temperature dependence of magnetization (M) and inverse susceptibility (Χ-1) measured under H = 1kOe. Fig. 1(b) shows the dc resistivity (ρ) and thermopower (S). Tc obtained from the inflection point of dM/dT is 237 K whereas the Χ-1 deviates from the Curie-Weiss behavior below 260 K. This is possibly due to Griffiths phase, i.e. nano-sizes FM clusters preformed above TC in the PM matrix. The ρ shows an IM transition with a peak at 235 K. However, S shows a peak around 258 K, which is about 20 K above the TC, suggesting that S is sensitive to nucleation of FM nano domains or short-range FM correlation. Fig. 1(c) shows ESR signal as a derivative microwave absorption as a function of dc magnetic field at selected temperatures near FM transition. The spectrum at 300 K is typical of ESR. While the spectrum at 300 K can be fitted to a single Lorentzian curve, the spectrum at 250 K reveals another low-intensity peak at a lower field (~1900 Oe) in addition to the ESR signal around 3200 Oe. The intensity of the ESR-signal decreases dramatically at 245 K but the intensity of the low-field peak increases. At 230 K, we do not see the ESR signal. These results indicate that the low field peak is due to FMR, but this feature start appearing above TC confirms that FM metallic nano domains are preformed above TC in the PM state and increase in size with lowering temperature and percolate leading to long-range FM and metallicity. 

**References:** (1) G. Blasse, Ferromagnetic interactions in non-metallic perovskites, J. Phys. Chem. 26, 1969 (1965);S. Vasala and M. Karppinen, A2B’B’’O6 perovskites: A review, Prog. Soild State Chem. 42, 1 (2015). 

(2) W. J. Lu et al., Induced ferromagnetism in Mo-substituted LaMnO3, Phys. Rev. B, 73, 174425 (2006). 

(3) L. Chen et al., Critical behavior of Mo-doping La0.67Sr0.33M1-xMoxO3 perovskite, Physica B, 404,1879 (2009). 

R. M. thanks the Ministry of Education, Singapore for supporting this work (Grant no. R144-000-442-114)


(a) T-dependence of M and X-1; (b) T-dependence of ρ and S; 1(c) ESR at selected T

Magnetism and Electron Spin Resonance (ESR) in Mo doped LaMnO3

In recent work, the time evolution of the magnetization reversal caused by spin-transfer torque in small perpendicular MRAM cells was investigated experimentally and theoretically [1, 2]. If the magnetization reversal is incoherent, that is, it does not follow a macro-spin model, intermediate states in between magnetization up or down can be found. These states can last up to tens of ns for cell sizes of the order of 50nm, thus lengthening the reversal time.
We investigated time resolved current measurements for our MRAM cells with sizes smaller than 20nm in the expectation that no longer any phenomena of this kind should be observed for very small cells that are below the “single domain limit”. However, it turns out that even MRAM cells smaller than 20nm (see Fig. 1) show typical indications of domain wall pinning, albeit with shorter pinning times. Fig. 2 shows a clear trend that the reversal times – which have been carefully separated from the incubation times - decrease with size. The reversal times are stochastic in nature and for each size a substantial range for the reversal times exists. Even for the smallest cells, we can show that the observed features are not an artifact caused by noise. In the full paper, details of the measurements and interpretations of the data will be given. 
**References** [1] P. Bouquin et al., Phys. Rev Appl, 15, 024037 (2021)
[2] I. Volvach et al., Appl. Phys. Lett. 116, 192408 (2020) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a1de406ff602175d2c4c91219a9b3ebf.png)

Evidence for Domain Wall Reversal in very Small Perpendicular MRAM Cells

Bismuth (Bi) or Bi-based heterostructures has attracted much interest as candidates for spin current generator. Recently, helicity-dependent (HD) photocurrent in Bi or Bi/Cu(or Ag) thin films was reported by using pulse laser-induced terahertz (THz) emission and transport measurement[1, 2]. It is believed that spin current in Bi induced by circularly-polarized laser via photo-spin conversion effect is converted into charge current via spin-charge conversion effect. However, details of photo-spin conversion in Bi have yet to be clarified because the photo-spin and the spin-charge conversion effects are observed simultaneously, thus, one cannot distinguish two effects. Here, we studied THz emission induced by spin-current generation using the photo-spin conversion effect in Bi and the laser-induced demagnetization in an adjacent ferromagnetic Co layer in Bi/Co films to separate two spin-related conversion effects and investigate the photo-spin conversion in Bi.
The samples, glass sub./Bi(tBi)/Co(5)/MgO(2)/Ta(2) (thickness is in nm), were prepared by a DC/RF magnetron sputtering. Laser-induced THz emission in Bi/Co films was investigated by THz time-domain spectroscopy (THz-TDS) with 120-fs Ti: Sapphire laser[3,4]. Figure 1 and Figure 2 show the Bi thickness tBi dependence of THz peak values induced by linearly-polarized laser and HD-THz peak values, respectively. Here, HD-THz is difference between signals obtained with left- and right-circularly polarized laser. We found different tBi dependence as shown in Fig. 1 and Fig. 2, which is possibly due to the difference in spin-relaxation length of two experiments. In the presentation, we will discuss the physics behind the different THz signals obtained in two experiments and quantitative consideration of the photo-spin conversion in Bi. This work is partially supported by KAKENHI (19K15430, 21H05000). K. I. acknowledges Grant-in-Aid for JSPS Fellow (22J22178) and GP-Spin at Tohoku University. S. M. acknowledges CSRN in CSIS at Tohoku University. 
**References** [1] Y. Hirai et al. Phys. Rev. Appl. 14, 064015 (2020)
[2] H. Hirose et al. Phys. Rev. B 103, 174437 (2021)
[3] H. Idzuchi, et al., AIP Advances 11, 015321 (2021)
[4] Y. Sasaki et al. Appl. Phys. Lett. 111, 102401 (2017)
 
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Laser induced terahertz wave emission from Bi/Co structure

Domain walls (DWs) in thin films with perpendicular magnetic anisotropy (PMA) are promising information carriers for the next generation of data storage and logic
operation devices.
[1,2] However, controlling DW motion efficiently remains unsolved. Here, we experimentally demonstrated an enhanced DW velocity in a PMA film using both standing
and travelling surface acoustic waves (SAWs). Two interdigitated transducers (Fig.1a), centre frequency 47.93 MHz (Fig. 1b), were patterned on opposite side of a
Ta(5)/Pt(2.5)/Co(0.5)/Ta(5) film (thicknesses in nm). The film was dc magnetron sputtered onto a lithium niobate substrate. Kerr microscopy was used to measure the DW velocity by
measuring the DW displacement due to a pulsed field. Results showed that DW velocity increased from 7±1 to 34±1 µm/s with an increasing magnetic field from 10.9 to 14.8 Oe without
SAWs (Fig.1c). A significant DW velocity increase can be observed in the presence of the standing SAW (Fig.1c) with applied SAW power from 15.5 to 20.5 dBm. An up to 24-fold DW
velocity increase (870±10 µm/s at 14.8 Oe) can be found in the presence of standing SAWs at 20.5 dBm compared to that without SAW at the same field. A less significant DW increase
(372±6 µm/s at 14.8 Oe) can also be observed with the application of travelling SAWs (Fig.1d). SAWs locally and periodically lower and raise the anisotropy of the thin film due to the
magnetoelastic coupling effect, temporarily assisting the DW to overcome pinning energy barriers during its motion.
[3,4] This process is possibly an accumulative effect, which can explain
the velocity difference between results for the travelling and standing SAW. This study demonstrates the potential for SAWs to efficiently manipulate magnetic DWs. 
**References** [1] Marathe et al. Vacuum, 14, 329, (2017) 
[2] Li et al., J. Appl. Phys, 115, 17E307 (2014) 
[3] Shepley Sci. Rep. 5, 7921 (2015) 
[4] Shuai et al. Appl. Phys. Lett. 120, 252402 (2022) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/5e83d1bb0c46b6c5625753505ad56110.png) 

Fig. 1 (a) Experimental set-up; (b) S-parameters of the SAW device; (c) Domain wall velocity against the applied field with standing SAWs (various applied SAW power) and without
SAW. (d) Domain wall velocity against the applied field without SAW and with standing/travelling SAWs (applied SAW power of 20.5 dBm).

Surface Acoustic Waves Effect on Domain Wall Motion

Hybrid magnetic layer systems consisting of a heavy metal, an ultrathin ferromagnet (F), and an antiferromagnet (AF) are interesting material systems that potentially exhibit both the interface-driven exchange bias (EB) effect and the asymmetric exchange interaction, known as Dzyaloshinskii-Moriya interaction (DMI) [1]. The DMI is essential for stabilizing chiral spin-structures like skyrmions, which are promising candidates for magnetic memory technologies [2]. Due to the DMI and the resulting Néel domain walls with chirality set by DMI, tilted magnetic moments are present at the domain edges [3]. Experimentally, this effect has been observed as asymmetric domain propagation while applying in-plane and perpendicular magnetic fields simultaneously [4]. While the DMI and EB can be modified independently from each other [5], only few studies so far have focused on the interplay between chiral DMI and the unidirectional EB anisotropy, affecting the magnetic domain texture and the resulting magnetization reversal. 
We report on a systematic study of the magnetization reversal in perpendicularly exchange biased Ti/Au/Co/NiO/Au micro stripes by high-resolution Kerr microscopy. Thereby, the magnetization reversal process is observed to be asymmetric with respect to the two branches of the hysteresis loop. We are able to quantify this as an asymmetry of the nucleation density formed in the two field branches as a function of the structure width. Additionally, a local asymmetry in the domain nucleation and domain wall movement within each stripe is observed. This phenomenon is investigated by field-cooling and the application of additional in-plane magnetic fields during the magnetization reversal. Additionally, XMCD/XMLD-PEEM experiments were performed to reveal the corresponding domain patterns in the F and AF. The results pave the way in the understanding of the interplay between chiral DMI and the unidirectional EB anisotropy in micro- and nanostructures. 
**References:** 
[1] F. Hellman et al., “Interface-induced phenomena in magnetism,” Rev. Mod. Phys., vol. 89, no. 2, pp. 1–79, 2017, doi: 10.1103/RevModPhys.89.025006. 

[2] A. Manchon et al., “Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems,” Rev. Mod. Phys., vol. 91, no. 3, 2019, doi: 10.1103/RevModPhys.91.035004. 

[3] S. Rohart and A. Thiaville, “Skyrmion confinement in ultrathin film nanostructures in the presence of Dzyaloshinskii-Moriya interaction,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 88, no. 18, pp. 1–8, 2013, doi: 10.1103/PhysRevB.88.184422. 

[4] P. Kuswik et al., “Asymmetric domain wall propagation caused by interfacial Dzyaloshinskii-Moriya interaction in exchange biased Au/Co/NiO layered system,” Phys. Rev. B, vol. 97, no. 2, pp. 1–7, 2018, doi: 10.1103/PhysRevB.97.024404. 

[5] P. Kuswik, M. Matczak, M. Kowacz, F. Lisiecki, and F. Stobiecki, “Determination of the Dzyaloshinskii-Moriya interaction in exchange biased Au/Co/NiO systems,” J. Magn. Magn. Mater., vol. 472, no. April 2018, pp. 29–33, 2019, doi: 10.1016/j.jmmm.2018.10.002. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/fdc9f6be29f2cd8a09625f05a98c3f93.png) 
Fig. 1: Asymmetric nucleation in 10 μm wide Ti/Au/Co/NiO/Au stripes. The Kerr images show that for the decreasing field branch a)-c) the domains are formed preferentially at one edge, smaller ones are formed equally at both edges for the increasing field branch d)-f).

Chirality induced asymmetric magnetization reversal in perpendicularly exchange biased micro stripes

In our previous work, we developed the highly sensitive thin film sensor and indicated the successful measurement of Magnetocardiogram signals without the magnetic
shielding [1]. But the sensitivity was poor around the ferromagnetic resonance frequency (in GHz bands) since the sensor had the severe reflection loss because of the impedance
mismatching [2]. In this research, we have improved the impedance matching of the sensor by employing the slits in the magnetic thin-film and evaluated the appropriate slit width. As
shown in Fig. 1, the proposed sensor composes of CoNbZr, SrTiO, and Cu/Cr films. The CoNbZr film was annealed in rotating and static magnetic fields to induce transverse magnetic
anisotropy. Several sensors having 6, 10, 26, 36, and 50 mm slit widths were fabricated and evaluated from the phase and the amplitude of the transmission coefficient (S21) measured by
the network analyzer (Advantest Corp., R3767CG). In the measurement, the DC magnetic field of 0-20 Oe was applied to the longitudinal direction of the coplanar by a Helmholtz coil.
The amplitude of S21 was improved about five times larger by employing the narrow slits. Although the changes of the amplitude and the phase by the magnetic field were small as the
narrow slit, the maximum amplitude of S21 became large. Fig. 2 shows the sensor sensitivity versus the slit width. The sensitivity of the sensor was evaluated by assuming it to be
proportional to the product of the phase and the amplitude gradients as the magnetic field and the carrier signal strength [3]. The appropriate slit width was around 10 mm for the coplanar
line type thin film magnetic field sensor and the sensitivity will enhance more than 10 times higher than the sensor without slits [1]. However, it is expected that the sensor can get higher
sensitivity by using amplitude modulation. 

**References:** 

[1] S. Yabukami et al, J. Magn. Soc. Jpn., Vol. 38, pp. 25-28 (2014) [2] T. Ishihara et al, Journal of Magnetic of Japan, vol. 6 (2022) [3] N. Horikoshi et al, Journal of
Magnetic of Japan, vol. 29, pp. 472 (2005) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/100387c7642046f3efe5cb725ebc980a.png)

Evaluation of Coplanar Line Type Thin Film Magnetic Field Sensor with Narrow Slits.

Corrado C. Capriata, Bengt G. Malm
Division of Electronics and Embedded Systems, KTH Royal Institute of Technology, Stockholm, Sweden
Abstract Body Two-dimensional synchronized arrays of Nanoconstriction Spin-Hall nano-oscillators (NC-SHNO) [1] have several promising features like higher power, better stability,
and applications in neuromorphic computing [2-3]. For all these applications, the synchronization of the oscillator array is crucial. Oscillators are synchronized with the neighboring ones
via dipolar and exchange coupling. In a recent study [4], NC-SHNO arrays were simulated under an injection locked condition and a one-dimensional array was investigated in [5]. Our
previous study [6] demonstrated how local variations of the exchange coupling can induce multiple oscillation modes in single devices and cause variability in the oscillation frequency. In
this study, the aim is to expand the modeling to free-running 2x2 and 4x4 synchronized arrays using the MuMax3 [7] micromagnetic simulator.
Here, we illustrate results from the 2x2 array with 100 nm pitch and 50 nm width (Fig.1 inset). The exchange coupling is reduced to 15 % at a line in between the columns or rows. The
presence of grains is equivalent to the coexistence of multiple lines and it is an expansion towards a real thin film representation. In this case, the exchange is randomly reduced to 10-30 %
and assigned to each grain.
Modifying the exchange coupling inside the array results in a minor shift of the synchronized frequency at low and medium currents while the array is much more disturbed at high
currents, Fig.1. The phase and power analysis, Fig.2, shows that the shape of the oscillating volume is disturbed by the presence of grains. At high current (3 mA), the excited mode
becomes propagating while at lower currents it is localized and well phase synchronized. In general, disturbed oscillators can be synchronized column-wise or diagonally, the extreme case
is only one device of the array delivering output power. Similar results have been found for the 2x2 array with 200 nm pitch and 120 nm width, highlighting that the phase synchronization
is not always guaranteed or can be broken by local variations in the exchange coupling strength in some operating conditions.

 
**References** 

[1]: V. E. Demidov, et al. "Nanoconstriction-based spin-Hall nano-oscillator", Appl. Phys. Lett. 105, 172410 (2014) 
[2]: A. Smith, et al. “Dimensional crossover in spin Hall oscillators”, Phys. Rev. B 102, 054422 – Published 17 August 2020. 
[3]: M. Zahedinejad, et al. “Two-dimensional mutually synchronized spin Hall nano-oscillator arrays for neuromorphic computing”, Nat. Nanotechnol. 15, 47–52 (2020). 
[4]: A. Houshang, et al. “Phase-Binarized Spin Hall Nano-Oscillator Arrays: Towards Spin Hall Ising Machines”, Phys. Rev. Applied 17, 014003 – Published 3 January 2022. 
[5]: T. Kendziorczyk, at al. “Mutual synchronization of nanoconstriction-based spin Hall nano-oscillators through evanescent and propagating spin waves”, Phys. Rev. B 93, 134413 –
Published 11 April 2016. 
[6]: C. C. M. Capriata, et al. "Impact of Random Grain Structure on Spin-Hall Nano-Oscillator Modal Stability", IEEE Electron Device Letters, vol. 43, no. 2, pp. 312-315, Feb. 2022. 
[7]: J. Leliaert, et al. "Adaptively time stepping the stochastic Landau-Lifshitz-Gilbert equation at nonzero temperature: Implementation and validation in MuMax3", AIP Advances 7,
125010 (2017).
Frequency stability of 2x2 NC-SHNO array under different conditions. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/2f252ea22a6e3226a7d5ad27d3b9e0cf.png)

Grain Structure Influence on Synchronized Two Dimensional Spin

One of the most promising magnetic systems studied in the last years are the 3D nanostructures with curved geometry where novel and interesting magnetization textures and dynamics are involved [1-2]. One of these nanostructures, cylindrical nanowires, present multiple topologically non-trivial magnetization structures such as Bloch-point domain walls, faster and more stable as compared to those in planar nanowires, vortices, and skyrmion tubes [2-3]. 
This talk presents different ways to manipulate the magnetization configuration and the domain wall dynamics in cylindrical nanowires. The control and stabilization of domains and domain walls have been achieved in multi-segmented structures [4] by alternating the magnetic anisotropy or by imprinting notches along the nanostructures length. The domain wall dynamics, under the influence of magnetic field or current pulses, was investigated by XMCD-PEEM and micromagnetic simulations. The results indicate that large current densities induce domain wall nucleation while smaller currents move domain walls preferably against the current direction but also along it. The pinning of domain walls has been observed not only at geometrical constrictions but also induced by the Oersted field which creates chiral domain walls. 
**References:** [1] R. Streubel et al, J. Phys. D: Appl. Phys. 2012, 49, 363001. 
[2] A. Fernández-Pacheco et al., Nat Commun 2017, 8, 15756. 
[3] J.A. Fernandez-Roldan et al., Nanoscale 2018, 10, 5923. 
[4] C. Bran et al., ACS Nano 2020, 14, 12819–12827.

Nucleation and Motion of Domain Walls in Cylindrical Nanowires

In this talk, we first report the roadmap and development of advanced perpendicular magnetic tunnel junctions based on low-damping magnetic materials with crystalline anisotropy that can be scaled down to sub-3-nm diameter with 10 years’ retention time [1]. Then, we will present the perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields. Traditional voltage-controlled magnetic anisotropy only linearly lowers the energy barrier of ferromagnetic layer via electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here we propose and demonstrate a bipolar electric field effect switching of 100-nm p-MTJs through voltage-controlled exchange coupling (VCEC) [2]. The switching current density, ~1.1×105 A/cm2, is one order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition and generates a field-like interlayer exchange coupling torque, which cause the bidirectional magnetization switching. We will further report our recent results to switch the p-MTJs using the interplay of SOT and VCEC, which lowers the VCEC switching current density to ~5×103 A/cm2[3]. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications. This technology can be used directly for the future spin-orbit-torque (SOT) MRAM [3] and spin logic devices [4]. 
**References** [1] D. Zhang, et al, Phys. Rev. Applied, (2018) vol. 9, 044028 
[2] D. Zhang, et al, Nano Letters (2022), doi.org/10.1021/acs.nanolett.1c03395 
[3] B. Zink, et al, Advanced Electronic Materials, (2022), DOI: 10.1002/aelm.202200382 
[4] M. Mankalale, et al, IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, 3 (2017) 27-36; 10.1109/JXCDC.2017.2690629 

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Perpendicular Magnetic Tunnel Junctions with Crystalline Anisotropy and Energy efficient Switching of Synthetic Antiferromagnet pMTJs by voltage

Magnetic skyrmions are compact chiral spin textures that exhibit a rich variety of topological phenomena and hold potential for the development of high-density memory devices and novel computing schemes driven by spin currents. In this invited talk, we will demonstrate the room-temperature interfacial stabilization and current-driven control of skyrmion bubbles in the ferrimagnetic insulator Tm3Fe5O12 coupled to Pt, showing the current-induced motion of individual skyrmion bubbles (1). The ferrimagnetic order of the crystal together with the interplay of spin–orbit torques and pinning determine the skyrmion dynamics in Tm3Fe5O12 and result in a strong skyrmion Hall effect characterized by a negative deflection angle and hopping motion. Further, we will show that the velocity and depinning threshold of the skyrmion bubbles can be modified by exchange coupling Tm3Fe5O12 to an in-plane magnetized Y3Fe5O12 layer, which distorts the spin texture of the skyrmions and leads to directional-dependent rectification of their dynamics. This effect, which is equivalent to a magnetic ratchet, is exploited to control the skyrmion flow in a racetrack-like device. 

**References:** 
(1) S. Vélez et al., Nature Nanotechnology (2022). https://doi.org/10.1038/s41565-022-01144-x 

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