SPIN-GLASS PROPERTIES OF A PLANAR ANTIFERROMAGNET

Small-angle neutron scattering is used to define the magnetic state of a powder sample of minnesotaite at points around its hysteresis loop. The silicate [idealized formula [Si4] {Fe2+3}O 10(OH)2] has a planar antiferromagentic ground state giving a magnetic 00 (1)/(2) reflection, but this state is destroyed to the profit of a ferromagnetic state (001 reflection) in those grains where the component of the applied field exceeds Hs =0.7 T at 4.2 K, never to be re-established in an isothermal cycle. Likewise, the planar antiferromagnetic ground state is destroyed, and the susceptibility peak eliminated, on field cooling. The existence of spin-glass magnetic properties in a material that does not have a highly degenerate, frustrated ground state derives from the irreversibility of the antiferromagnetic→ferromagnetic transition.