J. P. Gavigan

Intrinsic magnetic properties of the iron-rich ThMn12-structure alloys R(Fe11Ti); R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu

Magnetic properties of the series of ThMn12-structure intermetallic compounds R(Fe11Ti) have been determined for rare earths from Nd to Lu plus Y. The highest Curie temperature (607 K) is for R=Gd, and R-Fe exchange interactions are much stronger for light rare earths than for heavy ones. The temperature dependence of the iron sublattice magnetisation and anisotropy are determined for the Y and Lu compounds. Spin reorientation transitions are found as a function of temperature for the rare earths with a negative second-order Stevens coefficient alpha J(Nd, Tb, Dy), and a set of crystal-field parameters is derived to account for the transitions in a consistent way. A sharp increase in magnetisation observed for Sm(Fe11Ti) below 130 K in a field of about 10 T applied perpendicular to the easy direction indicates that J-mixing may be important for Sm3+. Compared with R2Fe14B, the iron anisotropy in R(Fe11Ti) is greater, and the rare-earth anisotropy is much weaker at low temperature, with the opposite sign for the rare-earth crystal-field coefficient A20. The average iron moment is 1.7 mu B in R(Fe11Ti) at 4.2 K; Mossbauer spectra are analysed to yield the average moments on each site. Limits set by the intrinsic magnetic properties on the performance of magnets made from these families of alloys are discussed.

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%.

3d magnetism in R2Fe14B compounds

Abstract An investigation of the magnetic properties of R 2 Fe 14 B compounds with R = Y, La, Ce, Gd, Lu and Th has been carried out. In all compounds the value of the Fe moment is in the range 2.0–2.2μ B , close to that of metallic Fe. This shows that effects of electron transfer and hybridization are weaker than in binary R-Fe compounds. The variations of the Curie temperatures of the different compounds is interpreted in terms of the dependence of the magnetic interactions on distance and on R. In all compounds, except Th 2 Fe 14 B, the temperature dependence of the anisotropy exhibits a maximum at T / T c ≃ 0.4. This effect is ascribed to competition between contributions from different Fe atomic sites and/or to a change in the crystal field interactions associated with the magnetovolume anomaly which occurs in R 2 Fe 14 B compounds below T c .

Magnetic properties of Pr2(Fe1-xCox)14B compounds

Abstract Lattice parameters, Curie temperatures, magnetization, anisotropy and Mossbauer spectra have been measured for a series of Pr2(Fe1-xCox)14B compounds with 0 ⩽ x ⩽ 1. The Curie temperature increases rapidly with Co addition and the room temperature spontaneous magnetization is maximum for x = 0.2. These results are discussed in terms of band structure modification and preferential site occupancy, the occurence of which has been established by Mossbauer spectroscopy. The Pr contribution to the anisotropy field is nearly constant up to x = 0.7 and then doubles as x tends to 1. There is a corresponding anomaly in the c a ratio.

Dependence of the R-R (R = Rare Earth) Exchange Interactions on the Nature of the R Atom in Intermetallics

In rare-earth (R) intermetallics, the molecular field coefficient nRR describing exchange interactions between R spin moments decreases, in a given series, by almost an order of magnitude from compounds with light R to those with heavy R elements. Simultaneously a decrease is observed in the transferred hyperfine field at nonmagnetic sites. These results are compared to the recent observation in R-transition metal (M) compounds of a decrease of the molecular field coefficient nRM across a given series of compounds (M fixed, R varied). These behaviours are coherently understood by considering that both R-R and R-M indirect exchange interactions in R intermetallics occur via 5d conduction electrons and that the 4f-5d exchange at R sites decreases from Pr to Tm.

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.

Spin rotation in R2Fe14BHx

We have studied by 57Fe Mossbauer spectroscopy and low field magnetization on oriented powders the temperature and hydrogen dependence of the magnetic structure of R2Fe14B compounds with R=Dy, Ho, and Y. We show that the iron sublattice may be canted by an angle ≂15° at high temperature. Hydrogen increases this canting at low temperature. It increases the spin reorientation temperature measured by bulk magnetization for Ho2Fe14B and induces a spin reorientation in the Dy alloys. These effects show that the strength of the crystal field terms of fourth (and maybe sixth) order compared to the terms of second order is increased by hydrogen.

High field magnetization measurements on Tb2Fe14B and Er2Fe14B single crystals

Abstract High field magnetization measurements between 4.2 and 275 K have been performed on single crystals of Tb 2 Fe 14 B and Er 2 Fe 14 B. The results obtained have been accounted for by a set of exchange and crystal field parameters. The latter were obtained by a direct scaling from values obtained in a detailed study of Nd 2 Fe 14 B.