  We use finite element simulations in both the frequency and the time-domain
to study the terahertz resonance characteristics of a metamaterial (MM)
comprising a spiral connected to a straight arm. The MM acts as a RLC circuit
whose resonance frequency can be precisely tuned by varying the characteristic
geometrical parameters of the spiral: inner and outer radius, width and number
of turns. We provide a simple analytical model that uses these geometrical
parameters as input to give accurate estimates of the resonance frequency.
Finite element simulations show that linearly polarized terahertz radiation
efficiently couples to the MM thanks to the straight arm, inducing a current in
the spiral, which in turn induces a resonant magnetic field enhancement at the
center of the spiral. We observe a large (approximately 20 times) and uniform
(over an area of $\sim 10~\mu m^{2}$) enhancement of the magnetic field for
narrowband terahertz radiation with frequency matching the resonance frequency
of the MM. When a broadband, single-cycle terahertz pulse propagates towards
the metamaterial, the peak magnetic field of the resulting band-passed waveform
still maintains a 6-fold enhancement compared to the peak impinging field.
Using existing laser-based terahertz sources, our metamaterial design allows to
generate magnetic fields of the order of 2 T over a time scale of several
picoseconds, enabling the investigation of non-linear ultrafast spin dynamics
in table-top experiments. Furthermore, our MM can be implemented to generate
intense near-field narrowband, multi-cycle electromagnetic fields to study
generic ultrafast resonant terahertz dynamics in condensed matter.
