  Many magnetic equilibrium states and phase transitions are characterized by
fluctuations. Such magnetic fluctuation can in principle be detected with
scattering-based x-ray photon correlation spectroscopy (XPCS). However, in the
established approach of XPCS, the magnetic scattering signal is quadratic in
the magnetic scattering cross section, which results not only in often
prohibitively small signals but also in a fundamental inability to detect
negative correlations (anticorrelations). Here, we propose to exploit the
possibility of heterodyne mixing of the magnetic signal with static charge
scattering to reconstruct the first-order (linear) magnetic correlation
function. We show that the first-order magnetic scattering signal reconstructed
from heterodyne scattering now directly represents the underlying magnetization
texture. Moreover, we suggest a practical implementation based on an absorption
mask rigidly connected to the sample, which not only produces a static charge
scattering signal but also eliminates the problem of drift-induced artificial
decay of the correlation functions. Our method thereby significantly broadens
the range of scientific questions accessible by magnetic x-ray photon
correlation spectroscopy.
