Tunable perpendicular magnetic anisotropy (PMA) in thin film magnetic devices provides an isotropic in-plane permeability, necessary for integrated microwave and mm-wave devices and an opportunity to break inversion symmetry that allows implementations of non-reciprocal devices like circulators and isolators. In ferrimagnetic alloys like GdxCo1-x, this is usually achieved by changing the composition such that the magnetic properties lie on either side of the composition-dependent magnetic compensation at room temperature, as shown in Fig. 1. Different compositions achieved in this work by changing the sputtering power of the Gd target show that magnetic compensation window lies between x = 28-32, and the effective anisotropy diverges about the compensation point. 

One less explored pathway is the incorporation of oxygen, which until now has been ruled out as a detriment to the ferrimagnetic properties [1]. Here, we show that preferential oxygenation of GdxCo1-x through a reactive sputtering process, such that the cation (Gd and Co) to oxygen anion ratio is 1:1, increases the effective anisotropy energy density by two orders of magnitude compared to the corresponding alloys without oxygen. X-ray photoelectron spectroscopy reveals that strong PMA with an optimal oxygen flow results in a Co-deficient stoichiometry of Gd21Co28O51, which suggests that strong ferrimagnetic order likely arises from a superexchange-like coupling between Gd and Co via the non-magnetic O [2]. This can produce a spontaneous magnetization in the out-of-plane direction due to weaker exchange interactions within the alloy in a heavy-metal/ferrimagnet heterostructure for PMA. Even greater anisotropy fields on the order of 11 kOe are achieved in a super-lattice with 10 repetitions of the GdxCo1-x heterostructure unit-cell, which corresponds to an operating frequency of 30 GHz. Therefore, not only does oxygenation increase PMA and provide tunable anisotropy to amorphous Gd-Co films, but the oxygen incorporation itself also increases the resistivity of the film, making it even more suitable for that frequency range. 
**References:** [1] K. Ueda, A. J. Tan, and G. S. D. Beach, AIP Advances, vol. 8, no. 12, p. 125204, Dec. 2018. 
[2] R. J. Gambino and J. J. Cuomo, Journal of Vacuum Science and Technology, vol. 15, no. 2, pp. 296–301, Mar. 1978. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0641d84a745d89c9edb1bbc17e16653a.png) 
Fig. 1. Saturation Magnetization and Effective Anisotropy Energy Density for varying GdxCo1-x compositions for x = 18-39.

Role of Composition and Oxygen in Ferrimagnetic GdxCo1 x Alloys for Microwave/mm

Anisotropic assemblies of magnetic nanoparticles (NPs) are highly powerful for designing micro-actuators in various applications, from robotics to biomedicine. Despite great potential, the challenge remains of how to controllably fabricate them at the micro scale due to two limitations: i) the lack of high-remanence nanoscale magnetic materials, which would dictate the resolution and displacement of the actuator; and ii) the lack of robust method for the large-scale synthesis of such materials. In this work, iron cobalt (Fe-Co) magnetic nanochains are prepared with a novel strategy, where the nanochains are formed from dipole-dipole alignment of individual Fe-Co NPs in the absence of an external magnetic field. As opposed to the widely studied field-guided assembly, our spontaneous assembly reported here enables high-throughput fabrication of anisotropic Fe-Co chains. Experiments and simulations show that dipolar magnetism plays a key role in the resulting structures of the Fe-Co chains, whose morphology, width, and length can be tuned by varying the amounts of precursors and fluid rotating speed during the synthesis. The randomly-oriented Fe-Co chains show a remanent magnetization of 28 emu/g, increasing by a factor of 2 compared to the Fe-Co NPs with equal side lengths. Furthermore, this higher remanence in the nanochains is experimentally shown to be advantageous in the development of magnetic soft actuators with a much larger mechanical deformation response to an externally applied field. To demonstrate this point, anisotropic magnetic elastomer composites were fabricated and their mechanical deformation behavior tested in both bending and twisting actuation modes. Both cases showed, very large, visible deformations in low magnetic fields (&lt; 200 Oe). Thus, by developing high-remanence magnetic chains with enhanced yields, composite materials can be made to maximize their effectiveness as micro-actuators or as other device technologies.

Exploring Magnetic Nanochain Anisotropy for Flexible Composite Micro Actuators

Developments in the design and fabrication of the high saturation magnetization nanoparticles in the single-domain size regime are highly desirable due to their extensive applications in biomedicine, magnetic resonance imaging, microelectromechanical systems, radio frequency electronics, and soft robotics. In this work, we present for the first time an in-depth investigation of different-sized iron cobalt (Fe65Co35) nanoparticles covering magnetization configurations from flower to vortex to multi-domain states. A finite element micromagnetic model was used to predict magnetic properties of Fe65Co35 nanoparticles ranging in size from 10 to 100 nm. The simulation results reveal that the flower-to-vortex transition of the Fe65Co35 nanoparticles occurs at around 20 nm, corresponding to the particle size with the highest coercivity. A decrease in remanence is expected for larger particle sizes, where simulations showed the most dramatic drop for particles between 40 and 60 nm. Furthermore, the simulated behaviors were verified experimentally. By systematically varying the reaction parameters, Fe65Co35 nanocubes with tunable edge lengths were synthesized. Scanning electron microscope (SEM) images of as-synthesized samples indicate a narrow size distribution. The measured hysteresis loops confirmed the maximum coercive field for cubes of size 17 nm and the sudden decrease in remanence for samples within the range of 40–60 nm. By combining chemical synthesis and micromagnetic calculations, we have experimentally unveiled the size-dependent spin configurations in Fe65Co35 nanoparticles. These results open the door to a more accurate design and control of spin texture, remanence, and coercivity in high magnetization nanoparticles which are vital building blocks of new biomedical and electronic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/53e8432506b4365fa494f1cd89ca89bd.jpg)

Fig. 1 SEM images of Fe65Co35 nanocubes with various edge lengths, (a) 17 nm, (b) 34 nm, (c) 40 nm, (d) 62 nm, (e) 77 nm, (f) 92 nm, and (g) 97 nm. The scale bars are 200 nm.

![](https://assets.underline.io/uploads/markdown_image/1/image/fe1f235133cb8a4f69fbfa4a62a75798.jpg)

Fig. 2 (a) The simulated spin configuration for a cube of each respective size. (b) Simulated hysteresis loops of a 5×5×5 sphere array with different sizes. (c) Measured hysteresis loops of as-synthesized Fe65Co35 nanocubes at 300 K.

Yulan Chen

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