Magnetic Mushy Sea Ice Like Phase in Kagom\'e Antiferromagnets

We report magnetization and neutron scattering measurements down to 60 mK on a new family of Fe based kagom\'e antiferromagnets, in which a strong local spin anisotropy combined with a low exchange path network connectivity lead to domain walls intersecting the kagom\'e planes through strings of free spins. These produce unfamiliar slow spin dynamics in the ordered phase, evolving from exchange-released spin-flips towards a cooperative behavior on decreasing the temperature. A domain structure of independent magnetic grains is obtained that makes up a magnetic analog of the mushy phase of sea ice or earth magma, namely porous solid media with fluid flowing inside interstices.