The synthetic antiferromagnetic (SAF) nanoplatelets (NPs) with perpendicular magnetic anisotropy (PMA) are of particular interest for torque-related applications due to their large anisotropy [1]. However, previous research indicates that the coercivity (Hc) increases significantly for small structures with PMA due to thermally activated-magnetization reversal [2]. As the size of NPs drops, larger field is required, and its distribution increases, to activate the NPs which is undesirable for applications. This raises a question of how the magnetization reversal characteristics of the SAF-PMA system scale with size. In this work, we study the magnetization reversal process of PMA-SAF systems of different sizes and materials (CoB/Pt and CoFeB/Pt). The basic thin-film stack is shown in the insert of Fig 1. PMA-SAF nanoplatelets with different sizes are fabricated through electron beam lithography (EBL) and measured using polar magneto-optic Kerr effect (MOKE) magnetometry. One example of the fabricated nanoplatelets is shown in Fig 1. An increase of Hc and its distribution (indicated by the error bar in Fig 2.) is observed with decreasing size as shown in Fig 2. An overall smaller Hc is observed in CoFeB/Pt system compared to CoB/Pt system which can be attributed to the decrease of anisotropy and the different defect-nucleation distribution of the materials. A model based on a local anisotropy distribution and thermal activation is established to simulate the data [3]. The model can accurately predict the Hc behavior, giving further insights into how the size, materials, and anisotropy affect the underlying reversal process. In this talk, we present the size-depended magnetization reversal process of SAF NPs, discuss the physics based on the simulation, and propose the methods to control the switching distribution. This will pave the way to tune the activating magnetic field of PMA-SAF NPs for applications.

Size dependent Magnetization Reversal Characteristics of Synthetic Antiferromagnetic Nanoplatelets

Spin-orbitronics and single pulse alloptical switching (AOS) of magnetization are two major successes of the fast advancing field of nanomagnetism in recent years, with high potential for enabling novel fast and energy-efficient memory and logic platforms. Fast current induced domain wall motion and single shot AOS have been individually demonstrated in different ferrimagnetic alloys. However, the stringent requirement for their composition control makes them challenging material for wafer scale production. Here, we demonstrate simultaneously fast current-induced domain wall motion and energy efficient AOS in a synthetic ferrimagnetic system based on [Co/Gd]2 multilayers. We firstly show AOS is present in its full composition range. We find current driven domain wall velocities over 2 km/s at room temperature conditions is achieved by compensating the total angular momentum through layer thickness tuning. Furthermore, analytical modelling of the current-induced domain wall motion reveals that Joule heating needs to be treated transiently to properly describe the current-induced domain wall motion for our sub-ns current pulses. Our studies establish Co/Gd based synthetic ferrimagnets to be a unique material platform for domain wall devices with access to ultrafast single pulse AOS [1]. 
**References** [1] Pingzhi Li, Thomas J Kools, Bert Koopmans and Reinoud Lavrijsen. Ultrafast racetrack based on compensated Co/Gd-based synthetic ferrimagnet with all-optical switching. arXiv preprint arXiv:2204.11595, 2022.

Ultrafast racetrack based on compensated Co/Gd based synthetic ferrimagnet with all

Synthetic antiferromagnetic nanoplatelets (NPs) with perpendicular magnetic anisotropy (PMA) are promising candidates for nano-torque-related applications [1]. To
manipulate the NPs, a good understanding of the homogeneity in magnetic and structural properties is required. Most techniques measure these properties based on ensemble techniques
[2], however, an efficient and easy magnetic characterization technique on single particle level, down to the nanometer range, is currently lacking.
Here, we present a characterization method based on photothermal magnetic circular dichroism (PT MCD), which measures the differential absorption of left and right circularly polarized
light of an individual magnetic nanoparticle due to the polar Kerr effect [3]. In this way the hysteresis loop on a single 120 nm diameter SAF NP is obtained (see Fig 1b). When compared
to SQUID (of ~106 particles) as shown in Fig. 1c, the loop exhibits comparable behavior i.e., the antiferromagnetic state at zero applied field and a sharp magnetization switch at a large
field. We notice that the values of the switching fields (BL and BH as schematically represented in Fig. 1b) are different from the SQUID measurement, which is due to the increased
temperature of the NP during the PT MCD measurement. The statistics of the switching fields by PT MCD are shown in Fig 1d where a difference in the distribution is observed for the BH
and BL switch, which we attribute to a change in the dominant magnetic reversal mechanism of the NPs [4].
Our results will pave the way to a deeper understanding of torque based applications using PMA-SAF NPs and introduce the PT MCD method to the magnetic-nanoparticle community [5]. 

**References:** 

[1] Mansell, R., et al. Scientific reports 7.1 (2017): 1-7. 
[2] Wasielewski, M. R., et al. Nature Reviews Chemistry 4.9 (2020): 490-504. 
[3] Spaeth, Patrick, et al. Nano letters 22.9 (2022): 3645-3650. 
[4] Thomson, T., et al. Physical review letters 96.25 (2006): 257204. 
[5] Adhikari, S, Li, J., et al. In-Preparation (2022) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/fd4c3052abd43b278aa35d7c73eb20e3.png)

Magnetization Switching of Single 120 nm Diameter Synthetic Antiferromagnetic Nanoplatelets with Perpendicular Magnetic Anisotropy

The utilization of magnetic torques at the nano and microscale induced via an externally applied magnetic field is of particular interest for applications such as micromixing [1] and mechanical cancer cell destruction [2]. Over the past several years, synthetic antiferromagnetic (SAF) nanoplatelets with perpendicular magnetic anisotropy (PMA) have been identified as one of the most promising vehicles due to their high tunability, easy-axis/hard-plane PMA efficiently transferring the full magnetic torque to their environment [3]. However, the fabrication of these nanoplatelets, especially in the submicron regime with a high yield is still a challenge and efficient characterization methods of the magnetic properties and mechanical behavior in a fluid environment are still lacking. In this work, we present a novel fabrication method based on substrate conformal imprint lithography (SCIL) [4] and discuss the size effect on the magnetic properties. The basic thin film stack is shown in the insert of Fig. 1. Platelets with 1.89 ± 0.02 µm dimeter (Fig. 1) and 102 ± 1 nm diameter were successfully produced with a yield of ~108 and ~ 1010 particles per 2 inch wafer. MOKE and SQUID were used to measure the hysteresis loop. A dramatically increase of the coercivity (Hc) after patterning is observed (see Fig. 2) rendering the particles ferromagnetic at zero applied field destroying their favorable properties. To define where the increase comes from, a systematic size study is conducted on PMA-SAF nanoplatelets fabricated through electron beam lithography (EBL). We find that Hc and its distribution increase with decreasing size (see Fig. 2). A model based on local anisotropy distribution and thermal activation was established to fit the data [5]. The fit can accurately describe the Hc behavior giving insight on how to engineer the particles. In this talk we will discuss the physics, our current methods to circumvent the Hc issue and give an outlook on single-platelet mechanical behavior characterization methods [6].

![](https://assets.underline.io/uploads/markdown_image/1/image/1f533b6fc3b9014d316dd90edc92ea86.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/cd16737989cc4310aba5cad6a49c1556.jpg)

Substrate Conformal Imprint Lithography Defined Synthetic Antiferromagnetic Nanoplatelets for Torque Related Applications

Substrate Conformal Imprint Lithography Defined Synthetic Antiferromagnetic Nanoplatelets for Torque Related Applications

Flexibility for interface engineering, and access to all-optical switching of the magnetization, make synthetic ferrimagnets (SFiMs) an interesting candidate for advanced spintronic devices like racetrack memory [1,2]. However, the absence of compensation at room temperature (RT) in Co/Gd bilayers has hampered the efficiency with which their magnetic order can be manipulated, as the highest efficiency of spin-orbit torques and current induced domain wall motion (CIDWM) are generally achieved under compensation [3,4]. In order to combine the favourable properties of SFiMs with compensation at RT, Co/Gd/Co/Gd quadlayer structures are investigated. We present a systematic study of the conditions for magnetization compensation and perpendicular magnetic anisotropy (PMA) in Co/Gd/Co/Gd quadlayers. These conditions were investigated on a sample with the middle Co and Gd films wedged orthogonally with respect to each other, as shown in Fig. 1a. By scanning the polar magneto-optical Kerr effect (MOKE), a magnetostatic phase diagram like in Fig. 1b can be obtained. Three regimes are distinguished: In-plane magnetization (teal), and out-of-plane (OOP) Co (red) and Gd (blue) dominated. The boundaries between these regimes provide valuable insight into the balance between shape anisotropy and PMA, and the conditions for compensation. A simple model based on partial intermixing at Co/Gd interfaces has been developed which explains the experiments well, and provides a tool to qualitatively and quantitatively understand these phase diagrams. This work provides a simple framework to experimentally characterise, and describe basic magnetostatic properties in these promising heterostructures. The compensated SFiMs investigated in this research, carry the promise of breaking the elusive 1000 m/s barrier for CIDWM, and exhibiting ultrafast exchange-driven dynamics, both relevant properties for future spintronic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/97d2d2073b964b56a371f90f9d4c02dc.jpg)

Characterization of double wedge Co(0.65)/Gd(0-3)/Co(0-2)/Gd(1.5) SFiMs. a): Schematic of wedge orientations in the multilayer. b): Magnetostatic phase diagram measured with polar MOKE. Yellow dashed line indicates compensation at RT. Green lines indicate a toy model fit to the boundaries between the three regimes.

Tuning Room Temperature Compensation in Synthetic Co/Gd Ferrimagnetic Quadlayers for Advanced Spintronics Devices

Skyrmions are topologically protected chiral spin textures that have shown promise as data carriers in future spintronic applications. They can be stabilized by the interfacial Dzyaloshinskii-Moriya interaction (iDMI) in material systems with inversion asymmetry and spin-orbit coupling, such as Ir/Co/Pt multilayers [1]. The ability to locally (at the nanoscale) tune such interface interactions, and hence the skyrmion energy, could lead to novel ways to optimise the nucleation and control of skyrmions. In this contribution we investigate the local tuning of the iDMI and the perpendicular magnetic anisotropy (PMA) using Ga+ ion irradiation, as well as its effect on skyrmion nucleation. Ga+ ion irradiation affects the coordination of atoms at the interfaces (Fig. 1a), which is expected to decrease both PMA and iDMI [2]. From Fig. 1b, we find that the reduction in PMA and iDMI are correlated, suggesting that both effects depend similarly on the interface quality. This reduces the energy cost of domain walls but does not affect their chirality. Hence, we expect that this will also facilitate the nucleation of more complicated chiral textures, such as skyrmions. To confirm this, we studied for the first time the current driven nucleation of skyrmions [3] in identical devices with and without Ga+ ion irradiation. We find that the ion irradiation indeed affects skyrmion nucleation, see Fig. 2. The critical current density required for nucleation is reduced by almost 30% and the number of skyrmions is doubled by irradiation. Thus, we argue that ion irradiation can be used to facilitate and control the nucleation of skyrmions in novel devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/3e984d72283a4626aa9f30b40e9bb2d9.jpg)

Fig. 1 a) Illustration: Ion irradiation causes intermixing at interfaces (left to right). b) Plot of the strength of the iDMI Ds against the strength of the interface PMA Ks, both Ds and Ks decrease when the Ga+ dose is increased, as indicated by the arrow.


![](https://assets.underline.io/uploads/markdown_image/1/image/920cf7b2d1c34d1c6fbb082d4a8f6f88.jpg)

Fig. 2 Number of skyrmions nucleated in our devices after applying 1000 50 ns pulses with current density J. In an irradiated device the number of skyrmions is increased and the current density required for nucleation is lowered. Black lines are fits to the data with the error function. Top: MFM scans of the devices.

Facilitating Skyrmion Nucleation in Ir/Co/Pt Multilayers With Ga+ Ion Irradiation

Co/Gd based synthetic ferrimagnets have received considerable attention owing to the coexistence of both pronounced spin-orbitronic effects and all-optical switching (AOS), allowing for novel, hybrid devices [1,2]. Previous studies [3,4] have shown that spin-orbit torque (SOT) driven domain wall motion in these 3d-4f ferrimagnets system is most efficient at angular momentum compensation. Following this notion, in this study, we explore the stack engineering of Co/Gd based synthetic ferrimagnets to achieve high SOT driven domain wall motion at room temperature[5]. Here, we experimentally demonstrate that magnetization compensation can be achieved and tuned by varying layer thicknesses in Co/Gd(x)/Co/Gd quadlayer systems (see inset of Fig. 1), which shows AOS in the full range of Gd thickness[5]. Fig. 1 (a) shows the coercivity and polar magneto-optical Kerr effect (MOKE)-signal in such a quadlayer system where the middle Gd layer is wedged. The sign change of the MOKE signal and divergence of the coercivity indicate a compensated system. The CIDWM velocity as a function of Gd thickness of this stack has been investigated, after structuring a series of domain wall conduits from another wedge sample grown in the same batch. In Fig. 1 (b) it can be observed that for low to moderate current densities the optimum in the velocity can be found at compensation. As the current density is increased the optimum in the velocity is found to shift towards larger Gd thicknesses, likely due to the difference in temperature dependence of the magnetization in Co and Gd leading to a shift in the compensation thickness due to joule heating during the applied current pulses. Our study shows a significant improvement of the room temperature domain wall velocity in synthetic ferrimagnetic systems through stack engineering. Further studies to explore the role of the weak exchange link provided by the proximity induced Gd magnetization profile are in progress.

![](https://assets.underline.io/uploads/markdown_image/1/image/abbc4d28d2f8fc22cfb8a86256cef998.jpg)

Fig. 1: Coercivity, polar MOKE signal (a) and current-driven domain wall velocity (b) as a function of Gd thickness in a Pt(4)/Co(0.6)/Gd(x)/Co(0.7)/Gd(1.5) multilayer system (thicknesses in nm in parenthesis), and x indicates Gd layer thickness.

![](https://assets.underline.io/uploads/markdown_image/1/image/439e7460be2e15697bd7be423caa409c.jpg)

Fig. 1 (a) The XRD pattern and (b) the FC magnetic moment as a function of temperature at 100 Oe, of LNMO films. The sample numbers represent the annealing time in minutes.

Fast Current-Induced Domain Wall Motion in Compensated Co/Gd Quadlayers

The development of small and simple room-temperature devices capable of generating and/or receiving resonant signals in a "THz-gap" still stands a significant challenge. However antiferromagnetic spintronics promises to be the right direction for the research and development of THz technology. Recently [1], [2], it has been theoretically proposed to use active AFM generators for the detection of external THz-frequency signals via the mechanism of injection-locking of such a signal to the oscillations generated by a DC-current-driven AFM/HM THz generator. An alternative way to develop quasi-passive AFM/HM-based detectors [3] is to use the fact that resonance eigenfrequencies of the AFM dynamic modes (standing AFMR modes) lie in the THz frequency range. It has been shown theoretically in [3], that a dielectric AFM having bi-axial anisotropy, such as NiO, can be used for the resonance quadratic rectification of a linearly-polarized AC spin current of THz-frequency, and could have a sensitivity in the range of 102-103 V/W. In this work [4], we propose the concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal bilayer. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in a bilayer. An additional bias DC current in the HM layer can be used to vary the effective anisotropy of the AFM, and, therefore, to tune the AFMR frequency. The proposed hetero-structure works as a resonance-type quadratic detector which can be tuned by the bias current in the range of at least 10% of the AFMR frequency. In addition, we discuss experimental results obtained during spin-pumping experiments with antiferromagnetic/heavy metal bilayer (NiO/Pt). The results were obtained in the HFML-FELIX laboratory where we have access to intense THz radiation free-electron laser source as well as high DC magnetic fields up to 38T.

**References:**

[1] O. Gomonay, T. Jungwirth, and J. Sinova. Physical Review B 98.10 (2018) [2] R. Khymyn, V. Tiberkevich, and A. Slavin, Book of abstracts “Intermag – 2018”, p. 795, (2018) [3] R. Khymyn, V.Tiberkevich, and A.Slavin, AIP Advances 7.5 (2017) [4] A. Safin, V. Puliafito, M. Carpentieri, G. Finocchio, S. Nikitov, P. Stremoukhov, A. Kirilyuk, V. Tyberkevych, and A. Slavin Appl. Phys. Lett. 117, 222411 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8912bbefc6eefba483c89d307904df9b.jpg)
Schematic view of the THz-frequency resonance detector based on the AFM-Pt structure.

 Electrically Tunable Detector of THz-Frequency Signals Based on an Antiferromagnet

MOA: Nanocomposite Magnetic Materials

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Presentations

Size dependent Magnetization Reversal Characteristics of Synthetic Antiferromagnetic Nanoplatelets

Ultrafast racetrack based on compensated Co/Gd based synthetic ferrimagnet with all

Magnetization Switching of Single 120 nm Diameter Synthetic Antiferromagnetic Nanoplatelets with Perpendicular Magnetic Anisotropy

Substrate Conformal Imprint Lithography Defined Synthetic Antiferromagnetic Nanoplatelets for Torque Related Applications

Tuning Room Temperature Compensation in Synthetic Co/Gd Ferrimagnetic Quadlayers for Advanced Spintronics Devices

Facilitating Skyrmion Nucleation in Ir/Co/Pt Multilayers With Ga+ Ion Irradiation

Fast Current-Induced Domain Wall Motion in Compensated Co/Gd Quadlayers

Electrically Tunable Detector of THz-Frequency Signals Based on an Antiferromagnet

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