  Understanding high-temperature superconductivity requires a prior knowledge
of the nature of the enigmatic pseudogap metallic state, out of which the
superconducting state condenses. In addition to the electronic orders involving
charge degrees of freedoms recently reported inside the pseudogap state, a
magnetic intra-unit-cell (IUC) order was discovered in various cuprates to set
in just at the pseudogap temperature, T*. In nearly optimally doped
YBa$_2$Cu$_3$O$_{6.85}$, polarized neutron scattering measurements, carried out
on two different spectrometers, reveal new features. The order is made of
finite size planar domains, hardly correlated along the c-axis. At high
temperature, only the out-of-plane magnetic components correlate, revealing a
strong Ising anistropy, as originally predicted in the loop current model.
Below T*, a correlated in-plane response develops, giving rise the apparent
tilt of the magnetic moment at low temperature. The discovery of these two
regimes put stringent constraints on the intrinsict nature of IUC order,
tightly bound to the pseudogap physics.
