Abstract
We report observations of spin 12 Heisenberg antiferromagnetic order in nanoparticle ensembles of Gd1.89Ni0.02Zn0.03Fe0.55O4 and Gd1.39Ni0.10Zn0.02Fe0.89O4. The staggered molar susceptibility, χm(T), of the ensembles varies as 1/T as T→TN≈17 K. Below TN, the χmT is directly proportional to the temperature before showing another upturn. An excellent agreement between the experimental χm(T) and theoretical models (Bonner-Fisher and Eggert) confirms spin 12 antiferromagnetic coupling within the linear chains of electron spins of the ensembles. The exchange constant is JkB=17K. The antiferromagnetic exchange interaction is also signified by a constriction at the middle of the hysteresis loop, at H=0, which becomes narrow at 5 K. We argue that the origin of the exchange interaction is in the quantum interference of the Bloch waves with the composite energy states of the domain walls.
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