  The subwavelength focusing based on surface plasmon polaritons (SPP) has been
widely explored in tapered metallic structures. However, the efficiency of
energy localization is relatively weak, largely due to high propagation loss
and strong back reflection. Here, we propose a straight-tapered 3-dimensional
(3D) one-way surface magnetoplasmon (SMP) waveguide with the ending surface of
$\sim 10^{-4}\lambda_0 \times 10^{-4}\lambda_0$ to achieve energy focusing in
the microwave regime. Due to low propagation loss of SMP, we achieve huge
magnetic field enhancement in such an ultra-subwavelength area, by five orders
of magnitude. Instead of using an external static magnetic field, our proposed
SMP waveguide relies on remanence, which is very convenient for operating
practical 3D applications. These results show promising applications in
magnetic-field enhancing or quenching fluorescence, luminescence or
nonlinearity of 2D materials, novel scanning near-field microwave microscopy
and energy storage.
