Small-angle neutron scattering evidence for the absence of long-range magnetic order in amorphous Fe91Zr9

Abstract

Small-angle neutron scattering (SANS) studies of an amorphous Fe91Zr9 alloy have shown that there is no transition to long-range magnetic order in contrast to previous bulk magnetization results which indicated a transition to a ferromagnetic state at 220 K followed by a reentrant spin-glass state near 60 K. The scattering line shape is Lorentzian in Q down to below 150 K and yields a spin correlation length which reaches a maximum of only 27 Å at the transition T C=210 K and exhibits an essentially constant plateau of 23 Å down to helium temperature, with no evidence of a second transition in the 60–80 K range. The line shape departs from Lorentzian below about 120 K and can be represented by a Lorentzian plus Lorentzian-squared cross section as appropriate for a system in which long-range order has been replaced by a frozen cluster configuration. The absence of ferromagnetic order in Fe91Zr9 at Fe concentrations far above the percolation threshold suggests the presence of a very broad distribution of exchange fields arising from the random Fe site coordinates.