  Magnetic Johnson noise is an important consideration for many applications
involving precision magnetometry, and its significance will only increase in
the future with improvements in measurement sensitivity. The
fluctuation-dissipation theorem can be utilized to derive analytic expressions
for magnetic Johnson noise in certain situations. But when used in conjunction
with commercially available finite element analysis tools, the combined
approach is particularly powerful as it provides a practical means to calculate
the magnetic Johnson noise arising from conductors of arbitrary geometry and
permeability. In this paper, we demonstrate this method to be one of the most
comprehensive approaches presently available to calculate thermal magnetic
noise. In particular, its applicability is shown to not be limited to cases
where the noise is evaluated at a point in space but also can be expanded to
include cases where the magnetic field detector has a more general shape, such
as a finite size loop, a gradiometer, or a detector that consists of a
polarized atomic species trapped in a volume. Furthermore, some physics
insights gained through studies made using this method are discussed.
