Neutron Diffraction Studies of Pd, Ni, FeMn, and Cu(Mn) Single Crystals

The possibility of antiferromagnetism in Pd at 4.2°K was investigated by using neutron diffraction to survey along lines in reciprocal lattice space from the origin through the (002), (111), and (220) positions and along lines from the (111) position to the (002), (001), and (110) positions. The sensitivity (except near nuclear peaks or near the origin) was sufficient to detect a magnetic induction wave of amplitude equal to 200 G (depending on form factor and distance from the origin a 200 G amplitude corresponds to from 0.02 to 0.03 μB). Despite several strange ``peaks'' in the patterns, no evidence of antiferromagnetism was inferred from the data. The possibility that Ni possesses a helical component to its ferromagnetism (similar to the familiar conical spin arrangement in holmium) was investigated by neutron diffraction with a field applied along the (111) direction of a Ni crystal at 4.2°K. The line from the origin of reciprocal space through the (111) nuclear peak was surveyed with the same sensitivity as the Pd experiment and with the same qualifications one could conclude that any such conical spin arrangement must have a cone angle of less than three degrees. The same survey as made for Pd was carried out in Cu(5%Mn) and the results were similarly negative as far as demonstrating antiferromagnetism. Cu(15%Mn) shows very broad peaks which could be indexed as (1½0) and (½10) and these vectors added or subtracted with respect to any primitive reciprocal lattice vector. The temperature independence of these reflections from 4.2° to 420°K points to the likelihood that they arise from an atomic ordering rather than from a magnetic ordering. This ordering is just detectable in the Cu(5%Mn) specimen. The antiferromagnetism of FeMn was investigated as this material appears (from resistivity and susceptibility data) not to possess ``localized moments'' above its Néel temperature. The temperature dependence of the spontaneous magnetization mode (a longitudinally polarized wave with (π00) wave vector) does not show a sharp break at the Néel temperature. This is due to the excitation of sizeable fluctuations in the magnetization modes. The fluctuating modes for a wide range of wave vectors are observed over a wide temperature range above and below the Néel temperature.