Rare-earth Spin Research Articles

The frequency spectrum of hydrogen bonded ferroelectrics

Substitution of small quantities of Co for Fe in the rare earth orthoferrites has a marked effect on magnetocrystalline anisotropy and can reorient the net moment of these canted spin antiferromagnets from the c-axis to the a-axis of the orthorhombic cell. In the latter orientation, the moments of the Yb, Tm, Er and Ho compounds are markedly changed and a directed L+2S contribution from the rare earth to the net moment is evident. The rare earth spin tends to align parallel to the net Fe-moment.

Ferrimagnetism of the Rare-Earth-Cobalt Intermetallic Compounds R2Co17

Saturation moments, Curie temperatures, and supporting crystallographic data are reported for a series of ferrimagnetic intermetallic compounds of the stoichiometry R2Co17, where R is any of the rare-earth elements Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The saturation moment of Y2Co17 is 27.2, that of Lu2Co17 is 27.6 Bohr magnetons per formula unit, indicating a cobalt moment of 1.61 μB in this series. Substitution of Pr and Nd for R raises the saturation to 32.8 μB/F.U. for Pr2Co17; all other lanthanides lower it (minimum: 5.7 μB/F.U. for Ho2Co17). Except for Sm2Co17, the values agree well with the concept that the Co moments couple antiparallel with the rare-earth spin sublattice. For Ce and Pr, a valence>3 and electron transfer from R to Co must be assumed. The Curie temperatures lie between 795° and 940°C. The variation of Tc with the atomic number suggests that two coupling mechanisms are active: direct Co-Co exchange interaction and indirect exchange between R and Co via polarized conduction electrons.

Alignment of rare earth moments in dilute Pd-Fe alloys

Magnetization measurements were made on the ternary alloy system Pd 0.96Fe 0.03R.E. 0.01 for which R.E. = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm and Yb in order to determine the relative alignment of the rare earth moments in the ferromagnetic Pd-Fe matrix. Those rare earths to the left of Gd were aligned parallel and those to the right of Gd antiparallel to the Pd and Fe moments. This is consistent with an antiferromagnetic interaction between the rare earth spin and the d-band of the host matrix since for the former J = L− S and for the latter J = L + S. Neutron magnetic disorder scattering measurements were also made on the Dy alloy and these showed only partial alignment of the available Dy moments. This suggests that the interaction between the rare earths and the Pd-Fe matrix depends on the local environment of the rare earth atoms.

Magnetization of Cubic Laves Phase Compounds of Rare Earths with Cobalt

These compounds are strongly magnetic. For the heavier rare earths, the gross magnetic moment appears to arise from opposed alignment of the rare earth and the cobalt atoms. If we assume the rare earths to have characteristic localized magnetic moments $\mathrm{Jg}$, the moments on the cobalt atoms are about 1.7 \ensuremath{\mu}B in the Tb, Dy, Ho, and Er compounds, a little less in the Gd compound, and zero in the Pr, Nd, and Sm compounds. The basic coupling mechanism is that of indirect exchange through the conduction electrons between rare-earth spins.

Far Infrared Spectra of Magnetic Materials

The far infrared spectral region (∼10−100 cm−1) corresponds to kT for T=15−150°K or βH for H=105−106 oe. Materials with characteristic temperatures or fields of these orders of magnitude may have interesting far infrared spectra. We have studied far infrared resonance spectra in antiferromagnetic and in ferromagnetic rare-earth iron garnets. Antiferromagnets have a resonance frequency which depends on the exchange and anisotropy fields, HE and HA. If HE is found from χ⊥, HA can be found from ω0, and compared with theory. We have done this with FeF2, MnO, and NiO, which have resonances for T≈0 at frequencies of 52.7 cm−1, 27.5 cm−1, and 36.6 cm−1, respectively. As the temperature is raised, these frequencies fall, reaching zero at TN. When a magnetic field is applied along the easy axis of FeF2, there is a first-order Zeeman splitting, from which a g value of 2.25 was determined. In MnO and NiO, however, there is an easy (111) plane. In this case the resonant mode is nondegenerate and hence the Zeeman effect is second-order and unobservably small. The exchange coupling between the rare earth and iron ions in garnets is typically of order 20 cm−1 (30°K), while the iron ions are coupled strongly together, corresponding to their Curie temperature of about 550°K. Because anisotropy breaks down selection rules, transitions in which a single rare-earth ion spin flips in the exchange field of the iron may be observed at far infrared frequencies. We also observe Kaplan-Kittel exchange resonances, in which the entire sublattice of rare-earth ions precess together and induce a corresponding precession of the iron sublattice. Since the frequency of such a resonance depends on the rare earth sublattice magnetization, it is quite temperature dependent, in contrast to the single ion absorptions. To obtain quantitative agreement with experiment, the Kaplan-Kittel theory must be generalized to take account of anisotropy energy. In YbIG at 2°K, with the magnetization along the [111] easy direction, we find single ion resonances at 23.4 and 26.4 cm−1 and an exchange resonance at 14.1 cm−1. The latter rises in frequency as the temperature is raised, whereas the former frequencies are nearly constant up to 60°K, where the intensity becomes too low for observation. The corresponding frequencies at low temperatures in ErIG are 18.2, 21.6, and 10.0 cm−1, whereas in SmIG the exchange resonance occurs at 33.5 cm−1 and decreases with increasing temperature. The small temperature dependence of the single ion exchange splittings suggests a rare earth-rare earth coupling (perhaps via spin waves in the iron sublattice) of magnitude ∼4% of the iron-rare earth coupling.