Magnetic needles enable tunable microwave absorption from S to Ku band via collective orientation

Magnetic microwave absorbing materials (MAMs) with light weight, small thickness, and high absorptivity are intensively investigated to meet the requirements of low-frequency microwave radiation control. The collective orientation of shape-anisotropic magnetic particles in a composite generates electromagnetic anisotropy, enabling continuous adjustment of electromagnetic parameters and selective microwave absorption. Asymmetric CoFe-based magnetic needles (CoFe-MNs) with a high aspect ratio are oriented in a matrix using an external magnetic field. The orientationally aligned CoFe-MNs have a high possibility to form long-range conductive networks via direct contacts or polarization coupling, enhancing microwave attenuation capability via conductive losses. The collective orientation of CoFe-MNs along the magnetic field of a transverse electromagnetic wave increases the permittivity moderately, and optimizes the impedance matching in the S band. The composite with a thickness of 4.8 mm has the maximum reflection loss of − 48.3 dB at 2.6 GHz. By contrast, the composite consisting of randomly oriented CoFe-MNs is capable of effectively absorbing microwaves in the X and Ku band. The shape anisotropy and the collective orientation of magnetic particles are the new features of magnetic MAMs, enabling the tunable microwave absorption from the S to the Ku band.