H. S. Li

Evidence in rare-earth (R)-transition metal (M) intermetallics for a systematic dependence of R-M exchange interactions on the nature of the R atom

In rare earth (R)–transition metal (M) compounds, large R‐M magnetic interactions can occur, which give rise to higher values of the ordering temperature (TC,TN) for compounds with magnetic R elements than for compounds with R nonmagnetic, i.e., La, Lu, Y. Due to the localized character of the 4f shell, these R‐M interactions are indirect, mediated by the 5d, 6s conduction electrons. The highest value of the ordering temperature is obtained for Gd compounds, and as a first approximation it is reasonable to write the interaction energy as ER‐M=−nRMMSRMSM, where MSR and MSM are the rare‐earth and transition metal spin moments, respectively. The molecular field coefficient nRM is generally assumed to be a constant throughout a given series, owing to the similarities of band structure for all R elements. In this paper, the molecular field coefficient nRM has been obtained for a number of series of rare earth‐transition metal compounds. The analysis reveals that nRM is not a constant going across a given serie...

A new approach to the analysis of magnetisation measurements in rare-earth/transition-metal compounds: application to Nd2Fe14B

A new method for the determination of crystal field and exchange parameters in rare-earth/transition-metal compounds, involving the analysis of magnetisation curves, is described. Rather than trying to reproduce directly the magnetisation curves by computer diagonalisation using crystal field and exchange parameters, the method uses experimentally determined anisotropy constants. These constants are parameters which reproduce the magnetisation curves but to which no special physical significance is attached. The energy expansion in terms of these anisotropy constants allows one to avoid long iterative computing. Application of this method to Nd2Fe14B results in an excellent description of the complex magnetic behavior of this compound.

3d magnetism in R-M and R2M14B compounds (M = Fe, Co; R =rare earth)

Abstract In this paper, 3d magnetism in the R 2 M 14 B-type (M = Fe, Co) compounds is examined in conjunction with the magnetic properties of binary RM compounds. The decrease of the transition metal magnetic moment μ M as a function of the amount of rare earth alloyed is discussed in terms of the Friedel interpretation of the Slater-Pauling curve for transition metal alloys. The exchange interactions in the Co alloys are mainly dependent on the value of the Co magnetic moment; in Fe alloys, they are essentially determined by local environment effects. In Y-Fe and Lu-Fe compounds, it is shown that the molecular field coefficient n FeFe between Fe atoms decreases as the Fe coordination number increases. From the increase in T c with x in R 2 (Fe 1− x Co x ) 14 B and R 2 (Fe 1− x Co x ) 17 alloys it appears that the FeCo exchange interactions are as large as the CoCo ones.

Analysis of high‐field magnetization measurements on R2Fe14B single crystals (R=Tb, Dy, Ho, Er, and Tm)

Single crystals of the compounds R2Fe14B some 1–4 mm in size have been grown for a study of the anisotropy of the magnetization curves. These curves for crystals with R=Tb, Dy, Ho, Er, and Tm were measured at the Service National des Champs Intenses, Grenoble, between 4.2 and 275 K, with fields of 0–18 T being applied along the [100], [110], and [001] directions. Magnetization curves for all five compounds are analyzed in terms of the exchange and crystal field interactions (including terms up to sixth order, which may differ at 4 f and 4g sites) following the analysis previously developed for Nd2Fe14B. Molecular field coefficients representing the exchange interactions between R and Fe spins decrease from light to heavy R compounds as previously deduced from analysis of Curie temperatures. The CEF parameters are approximately the same across the series. In particular, the A20 terms are constant to within 10%.

Exchange and CEF interactions in R2Fe14B compounds

The magnetic properties of R ions in R2Fe14B compounds (R=rare earth, Th) are determined by a combination of R‐Fe exchange and crystalline electric field interactions. A comparison of the ordering temperatures of all compounds in the series shows that the exchange interactions between R and Fe moments are not simply proportional to the spin moments but there is a decrease across the series, with interactions for compounds with light R elements enhanced by a factor of 2 compared to those with heavy R elements. This decrease is related to the decrease of 4f‐5d exchange interactions within the R ions. The second‐order and fourth‐order terms of the crystalline electric field potential of the environment have been obtained in a number of compounds from analysis of their magnetic properties (Nd, Er, Tm, Yb). Scaling the values obtained for other compounds (Pr, Sm, Ho,...) results in a fairly good description of their magnetic properties.

Magnetic properties of new ternary R sub 6 Ga sub 3 Fe sub 11 compounds

Three new rare earth–iron ternary compounds with the La6Ga3Co11 structure (I4/mcm), Pr6Ga3Fe11, Nd6Ga3Fe11, and Sm6Ga3Fe11, are found to be ferromagnetic with Curie temperatures of 320, 397, and 462 K, respectively. Large anisotropies have been observed from the magnetization curves and the anisotropy field is larger than 7 T at room temperature. First‐order magnetization processes were observed for all samples throughout the temperature range of 4.2–300 K. Point charge calculations give A20=1070 K a−20 at the 16l site and −283 K a−20 at the 8f site.

A study of exchange and crystalline electric field interactions in Nd2Co14B: Comparison with Nd2Fe14B

Abstract The intrinsic magnetic behaviour of Nd2Co14B is interpreted in terms of exchange and crystal field interactions. The ‘spin-tilt’ transition away from the c axis at 37 K cannot be explained simply by a reduction in the exchange field acting on the Nd3+ ion relative to isomorphous Nd2Fe14B; a significant alteration of the crystal field is required. In particular, we find evidence for an important reduction in the strength of the fourth and sixth-order crystal field terms in Nd2Co14B relative to Nd2Fe14B. A correlation between the low temperature ‘spin-tilt’ transition and occupancy of the 4c transition metal site through the Nd2(Fe1−xCox)14B series suggests that this site in the Nd-B planes may be responsible for the changes in the Nd3+ crystal field.

A 155Gd Mössbauer study of exchange interactions in Gd2Fe14B

Abstract The temperature dependence of hyperfine interactions at 155Gd nuclei in Gd2Fe14B has been studied in the temperature range from 4.2 to 120 K. The results indicate that the Fe-Gd exchange interactions at the two Gd sites may be different. The frequency spectrum of Gd vibrations has been derived from the temperature dependence of the absorption intensity.

Magnetization curves of a Tb/sub 3/(Fe,V)/sub 29/ single crystal

A Tb/sub 3/(Fe,V)/sub 29/ single crystal has been grown using the Czochralski method. Magnetization measurements along the hard axis of the crystal are presented. At low temperatures, a first-order magnetization process of type II occurs at large critical fields (/spl mu//sub 0/H/sub c/=10.6 T at T=5 K). The data were analysed using the first-principle model in terms of exchange and crystal-field interactions. The second order crystal-field coefficients of two rare earth sites are estimated to be A/sub 20/=-36K.a/sub 0//sup -2/ for the 4i site, and A/sub 20/=+16K.a/sub 0//sup -2/ for the 2a site.

Intrinsic Magnetic Properties of Iron-Rich Compounds with the Nd2Fe14B or ThMn12 Structure

Intrinsic magnetic properties including magnetic ordering temperatures, spin reorientation transitions, transition metal moments, 57Fe hyperfine fields, anisotropy fields and magnetization curves in high applied fields are compared for the R2Fe14B and R(Fe11Ti) series of compounds. Exchange interactions, transition-metal anisotropy and rare-earth crystal field parameters are deduced for the two series. Single-crystal magnetization curves of eight R2Fe14B compounds and Nd2Co14B are included in the analysis, as well as data on some solid solution series. At room temperature, is similar in Sm(Fe11Ti) and Nd2Fe14B but it may be increased by rare earth substitution only in the latter case. The theoretical upper limit on energy product μ0Ms2/4 is 259 kJ m-3 for Sm(Fe11Ti), half that of Nd2Fe14B, mainly on account of the lower iron moment in the ThMn12 structure.