We have used elastic neutron scattering to examine the effect of hydriding on the sublattice magnetization of the Laves‐phase compounds RFe2 for R=Ho and Er. On addition of hydrogen or deuterium a stable phase of composition about RFe2D3.5 is formed. The pure compounds RFe2 are ferrimagnetic, with the R moment equal to the free ion value and Fe moment of ∼1.6 μB. The bulk Curie temperatures Tc of the hydrides and deuterides are considerably lower than the corresponding RFe2 values (e.g., 440 K for ErFe2D3.5 versus 574 K for ErFe2). The 10 K R moment is also significantly reduced in the deuterides (4.4 and 5.2 μB versus 9 and 10 μB for Er and Ho in RFe2) and drops steadily with increased temperature, reaching zero below Tc in the case of Er. The Fe moment remains nearly constant with temperature until just below Tc where it drops rapidly to zero; in ErFe2D3.5 and HoFe2D3.5 its magnitude is 1.6 μB, and 1.9 μB, respectively. These results indicate that the exchange interactions involving the R atoms are markedly weakened in the hydrides. The distortion of the local anisotropy field due to random site occupancy of H and the weak exchange coupling of R produces a ’’fanning’’ of the R moment which reduces the spatially averaged moment measured in the neutron experiment.

Effect of hydrogen on sublattice magnetization of Laves‐phase rare earth iron compounds

The results of a 169Tm and 57Fe Mossbauer study of the cubic Laves compound TmFe2 over the temperature range 1.3-550K are presented and discussed. The new results are used, in conjunction with existing NMR, Mossbauer and magnetic anisotropy data for TmFe2 and Tm metal, to deduce a value of P4f=536+or-14 MHz for the saturation value of the first excited state of the 169Tm nucleus. Estimates are also given for the exchange field mu BBex(T=0K)/kB=153+or-3K acting on the Tm3+ ion in TmFe2, the quadrupole moment of the I=3/2 state of the 169Tm nucleus, Q=-1.36+or-0.11 b, and the lattice contribution to the nuclear quadrupole interaction in Tm metal, Pc=-54.8+or-5 MHz (for Q=-1.20+or-0.07 b) and Pc=-61+or-8 MHz (for Q=-1.36+or-0.11 b). In addition, estimates are given for the various transferred and parent hyperfine fields in TmFe2.

https://iopscience.iop.org/article/10.1088/0305-4608/12/4/019/pdf

A Mossbauer study of the cubic Laves phase intermetallic compound TmFe2

High resolution neutron diffraction techniques were used to establish the hydrogen (deuterium) site occupation and sublattice magnetization of a series of Ho 6 Fe 23 D x compounds ( x = 1.5, 8.2, 12.1 and 15.7). The f.c.c. structure of the parent compound was maintained except for the D 12.1 sample which exhibited a tetragonal distortion. The f.c.c. lattice was reestablished for the higher D 15.7 composition. Diffraction and inelastic hydrogen vibration spectra data demonstrate conclusively that the deuterium atoms in the D 1.5 material fully occupy the octahedral a site with the residual in the tetrahedral f site. At the D 8.2 and higher concentration level, the a site is empty and the deuterium atoms partially occupy only the tetrahedral sites. The 4 K holmium magnetic moment was near the free-ion value of 10 μ B for all compounds and was oriented antiparallel to all four iron site moments (b, d, f 1 and f 2 ). For the cubic materials, the magnetization on the iron sites varied with deuteration in the range 0.8–2.4 μ B , with the D 15.7 compound showing the highest iron moment for all sites. A spin reorientation is observed in the tetragonal material between room temperature and 4 K with the holmium and iron b and n 1 moments rotating out of the basal plane.

Deuterium site occupation and magnetism in Ho6Fe23Dx, compounds

Results of a polarized neutron experiment at 4.2 K on a single crystal of HoFe2 are reported. The magnetic moment of the iron atoms is 1.88(10) μB. In comparison with the LuFe2 results, 0.25 μB of the Fe moment is induced by interactions with holmium. Extrapolating the Ho magnetic amplitudes to sinϑ/λ=0 one finds a magnetic moment of 9.15(10) μB. This value is inconsistent with that deduced from exchange and crystal field parameters already known in this compound.

Polarized neutron study of HoFe2

Neutron scattering in intermetallics

Spin wave and crystal field excitations in single crystal HoFe2 have been studied using inelastic neutron scattering. At 10 K [100] is the easy magnetic direction and at this temperature the acoustic spin wave mode exhibits a 0.60 meV gap due to anisotropy. The acoustic mode is degenerate at the zone boundary with a flat optic mode of energy 8.3 meV. The lack of dispersion in this mode is a consequence of the small (<.01 meV) Ho‐Ho exchange energy. A third mode is a highly dispersive optic mode which represents excitations of the iron sublattice spins and has a stiffness parameter almost identical to iron metal. A nearest neighbor linear spin wave model has been applied which represents the data of the three lowest modes well. This model predicts the remaining three allowed modes to be at energies above 200 meV. There is no measurable anisotropy in the spin wave modes with varying propagation directions. At room temperature, higher states of the Ho crystal field multiplet become populated and weaker dispersionless excitations between these levels are observed. The observed excitations are broadened beyond instrumental resolution because of contributions from several levels close together in energy. Crystal field calculations including exchange have been performed and are compared to the energies of the observed optic mode transitions.Spin wave and crystal field excitations in single crystal HoFe2 have been studied using inelastic neutron scattering. At 10 K [100] is the easy magnetic direction and at this temperature the acoustic spin wave mode exhibits a 0.60 meV gap due to anisotropy. The acoustic mode is degenerate at the zone boundary with a flat optic mode of energy 8.3 meV. The lack of dispersion in this mode is a consequence of the small (<.01 meV) Ho‐Ho exchange energy. A third mode is a highly dispersive optic mode which represents excitations of the iron sublattice spins and has a stiffness parameter almost identical to iron metal. A nearest neighbor linear spin wave model has been applied which represents the data of the three lowest modes well. This model predicts the remaining three allowed modes to be at energies above 200 meV. There is no measurable anisotropy in the spin wave modes with varying propagation directions. At room temperature, higher states of the Ho crystal field multiplet become populated and weaker dispe...

N. C. Koon

Papers

Magnetic excitations in HoFe2