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1. Tailoring magnon modes by extending square, kagome, and trigonal spin ice lattices vertically via interlayer coupling of trilayer nanomagnets

   https://doi.org/10.1088/1361-648X/ad5d3f  

   de_Rojas, Julius; Atkinson, Del; Adeyeye, Adekunle O (July 2024, Journal of Physics: Condensed Matter)

   Abstract In this work high-frequency magnetization dynamics and statics of artificial spin-ice lattices with different geometric nanostructure array configurations are studied where the individual nanostructures are composed of ferromagnetic/non-magnetic/ferromagnetic trilayers with different non-magnetic thicknesses. These thickness variations enable additional control over the magnetic interactions within the spin-ice lattice that directly impacts the resulting magnetization dynamics and the associated magnonic modes. Specifically the geometric arrangements studied are square, kagome and trigonal spin ice configurations, where the individual lithographically patterned nanomagnets (NMs) are trilayers, made up of two magnetic layers of ${\text{Ni}}_{81}{\text{Fe}}_{19}$of 30 nm and 70 nm thickness respectively, separated by a non-magnetic copper layer of either 2 nm or 40 nm. We show that coupling via the magnetostatic interactions between the ferromagnetic layers of the NMs within square, kagome and trigonal spin-ice lattices offers fine-control over magnetization states and magnetic resonant modes. In particular, the kagome and trigonal lattices allow tuning of an additional mode and the spacing between multiple resonance modes, increasing functionality beyond square lattices. These results demonstrate the ability to move beyond quasi-2D single magnetic layer nanomagnetics via control of the vertical interlayer interactions in spin ice arrays. This additional control enables multi-mode magnonic programmability of the resonance spectra, which has potential for magnetic metamaterials for microwave or information processing applications. 

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2. Optimizing time-resolved magneto-optical Kerr effect for high-fidelity magnetic characterization

   https://doi.org/10.1063/5.0275720  

   Kim, Yun; Huang, Dingbin; Lyu, Deyuan; Sun, Haoyue; Wang, Jian-Ping; Crowell, Paul A; Wang, Xiaojia (July 2025, Applied Physics Letters)

   Spintronics has emerged as a key technology for fast and nonvolatile memory with great CMOS compatibility. As the building blocks for these cutting-edge devices, magnetic materials require precise characterization of their critical properties, such as the effective anisotropy field (Hk,eff, related to magnetic stability) and damping (α, a key factor in device energy efficiency). Accurate measurements of these properties are essential for designing and fabricating high-performance spintronic devices. Among advanced metrology techniques, time-resolved magneto-optical Kerr effect (TR-MOKE) stands out for its superb temporal and spatial resolutions, surpassing traditional methods like ferromagnetic resonance. However, the full potential of TR-MOKE has not yet been fully fledged due to the lack of systematic optimization and robust operational guidelines. In this study, we address this gap by developing experimentally validated guidelines for optimizing TR-MOKE metrology across materials with perpendicular magnetic anisotropy and in-plane magnetic anisotropy. While Co20Fe60B20 thin films are used for experimental validation, this optimization framework can be readily extended to a variety of materials such as L10-FePd with easy-axis dispersion. Our work identifies the optimal ranges of the field angle to simultaneously achieve high signal amplitudes and improve measurement sensitivities to Hk,eff and α. By suppressing the influence of inhomogeneities and boosting sensitivity, our work significantly enhances TR-MOKE capability to extract magnetic properties with high accuracy and reliability. This optimization framework positions TR-MOKE as an indispensable tool for advancing spintronics, paving the way for energy-efficient and high-speed devices that will redefine the landscape of modern computing and memory technologies. 

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3. Giant bulk spin–orbit torque and efficient electrical switching in single ferrimagnetic FeTb layers with strong perpendicular magnetic anisotropy

   https://doi.org/10.1063/5.0087260  

   Liu, Qianbiao; Zhu, Lijun; Zhang, Xiyue S.; Muller, David A.; Ralph, Daniel C. (June 2022, Applied Physics Reviews)

   Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin–orbit torque in strong-PMA ferrimagnetic Fe 100− x Tb x single layers that are integration-friendly (composition-uniform, amorphous, and sputter-deposited). For sufficiently thick layers, this bulk torque is constant in the efficiency per unit layer thickness, [Formula: see text]/ t, with a record-high value of 0.036 ± 0.008 nm −1 , and the damping-like torque efficiency [Formula: see text] achieves very large values for thick layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe 100− x Tb x layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm 2 . Surprisingly, for a given layer thickness, [Formula: see text] shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin–orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices. 

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4. Effect of electron–electron interaction on magnitude of quantum oscillations of dissipative resistance in magnetic fields

   https://doi.org/10.1063/5.0127286  

   Abedi, Sara; Vitkalov, Sergey; Bykov, A. A.; Bakarov, A. K. (December 2022, Journal of Applied Physics)

   Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields [Formula: see text] tilted from the normal to the 2D electron layer at different temperatures [Formula: see text]. The in-plane component ([Formula: see text]) of the field [Formula: see text] induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled ([Formula: see text]) 2D systems yields both difference frequency of DOS oscillations, [Formula: see text], and strength of the electron–electron interaction, described by parameter [Formula: see text], in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both [Formula: see text] and [Formula: see text]. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields. 

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5. The 2022 magneto-optics roadmap

   https://doi.org/10.1088/1361-6463/ac8da0  

   Kimel, Alexey; Zvezdin, Anatoly; Sharma, Sangeeta; Shallcross, Samuel; de Sousa, Nuno; García-Martín, Antonio; Salvan, Georgeta; Hamrle, Jaroslav; Stejskal, Ondřej; McCord, Jeffrey; et al (September 2022, Journal of Physics D: Applied Physics)

   Abstract Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of today’s magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton–Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light–matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17). Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future. 

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