Magnetic excitations in amorphous Co4P

Abstract

Neutron scattering techniques have been used to study the magnetic excitations in the amorphous ferromagnet Co4P. Distinct spin waves with a narrow line width have been measured near momentum transfer Q=O using a three axis spectrometer. The measurements around Q=O can ve performed only over a very restricted range in Q since the spin wave branch is quite steeply rising and at momentum transfers larger than 0.03 Å−1 the spin wave energy is too high to conserve energy in the scattering process so that the scattering triangle cannot be closed. However, measurements were possible over a range of 0.005 to 0.03 Å−1. These measurements could be well fit with quadratic dispersion law E=DQ2 if D has a value of about 185 meV Å2. Polarized neutron time‐of‐flight measurements were able to show additional low‐lying spin wave excitations near the Q equal to the first peak in the static structure factor S(Q). The polarized beam technique is especially valuable for these measurements as only spin flip excitations are measured and the time‐of‐flight technique permits measurements at a large number of momentum transfers simultaneously. The spin wave excitations at these larger values of Q again closely follow a quadratic disersion law can be reasonably well fit by the relation E = Δ + Dq2 where q is measured from the Q equal to the first peak in S(Q), D is kept at 185 meV Å2 and an energy gap Δ of about 35 meV is introduced. The gap is result of the postional disorder of the system and the excitation spectrum appears in some respects similar to the phonon excitation spectrum for liquid 4He, the higher Q excitations being analogous to the roton excitations in the liquid.