Vortex-based soft magnetic composite with ultrastable permeability up to gigahertz frequencies

Soft magnetic materials with stable permeability up to hundreds of megahertz (MHz) are urgently needed for integrated transformers and inductors, which are crucial in the more-than-Moore era. However, traditional frequency-stable soft magnetic ferrites suffer from low saturation magnetization and temperature instability, making them unsuitable for integrated circuits. Herein, we fabricate a frequency-stable soft magnetic composite featuring a magnetic vortex structure via cold-sintering, where ultrafine FeSiAl particles are magnetically isolated and covalently bonded by Al 2 SiO 5 /SiO 2 /Fe 2 (MoO 4 ) 3 multilayered heterostructure. This construction results in an ultrastable permeability of 13 up to 1 gigahertz (GHz), relatively large saturation magnetization of 105 Am 2 /kg and low coercivity of 48 A/m, which we ascribe to the elimination of domain walls associated with almost uniform single-vortex structures, as observed by Lorentz transmission electron microscopy and reconstructed by micromagnetic simulation. Moreover, the ultimate compressive strength has been simultaneously increased up to 337.1 MPa attributed to the epitaxially grown interfaces between particles. This study deepens our understanding on the characteristics of magnetic vortices and provides alternative concept for designing integrated magnetic devices. The development of higher frequency transformers and inductors requires soft magnetic materials with stable permeability up to GHz frequencies, however, such soft magnetic materials are current lacking. Here Bai et al demonstrate a soft magnetic composite featuring isolated magnetic vortices, which endow the material with a stable permeability over a GHz range of frequency.