  A fundamental difference between antiferromagnets and ferromagnets is the
lack of linear coupling to a uniform magnetic field due to the staggered order
parameter. Such coupling is possible via the Dzyaloshinskii-Moriya (DM)
interaction but at the expense of reduced antiferromagnetic (AFM)
susceptibility due to the canting-induced spin anisotropy. We solve this
long-standing problem with a top-down approach that utilizes spin-orbit
coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM
responses to sub-Tesla external fields by exploiting the extremely strong
two-dimensional critical fluctuations preserved under a symmetry-invariant
exchange anisotropy, which is built into a square-lattice artificially
synthesized as a superlattice of SrIrO3 and SrTiO3. The observed field-induced
logarithmic increase of the ordering temperature enables highly efficient
control of the AFM order. As antiferromagnets promise to afford switching speed
and storage security far beyond ferromagnets, our symmetry-invariant approach
unleashes the great potential of functional antiferromagnets.
