In a dense ensemble of physically nonpercolating FM nanoparticles embedded in a $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Al}}_2{\mathrm{O}}_3{]}_{10}$ multilayer the long-discussed superferromagnetic (SFM) state has been evidenced by imaging homogeneously magnetized SFM domain patterns with x-ray photoemission electron microscopy and magneto-optical Kerr microscopy. Competing dipolar and exchange interactions give rise to extremely rough domain walls similarly as in hard multigrain magnets. The SFM state is characterized by extreme magnetic softness and higher order tunneling conductivity due to atomically small intercalated particles promoting the SFM order.

• Received 25 September 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.176601