S. Sinnema

Magnetic anisotropy in Pr2(Fe1−xCox)14B compounds

Abstract We have measured the concentration dependence of the Curie temperature, magnetization, anisotropy field and lattice parameters in Pr 2 (Fe 1− x Co x ) 14 B for 0 ⩽ x ⩽ 1. The Curie temperature strongly increases with x . The saturation magnetization at 4.2 K decreases with x after remaining approximately concentration independent up to x = 0.2. The room temperature anisotropy field first decreases with x but after passing through a minimum at about x = 0.7 strongly increases with x . The experimental results are compared with results obtained previously for Nd 2 (Fe 1− x Co x ) 14 B and are discussed in terms of crystal-field effects.

Exchange interactions and basal-plane magnetocrystalline anisotropy in R2Co17 intermetallics

Abstract The transition fields that recently have been observed in the basal-plane magnetisation curves of Ho 2 Co 17 can successfully be described in a two-sublattice model and provide a direct method to evaluate the molecular field acting on the holmium moment. Its value of 63.5 T corresponds, in a molecular-field approximation, to a value of 0.924x10 -22 J for the coupling between the holmium and cobalt spins. The basal-plane magnetocrystalline anisotropy of the holmium moment is found to be 2.2x10 -22 J. On the basis of these numbers the molecular field acting on the rare-earth moment and the basal-plane anisotropy are calculated for the whole series of R 2 Co 17 compounds.

High-field magnetisation studies on the Ho2(Co,Fe)17 intermetallics

High-field magnetisation measurements have been performed on single-crystalline samples of hexagonal Ho2Co17, Ho2Fe17 and Ho2Co14Fe3 in fields up to 40 T. In this field interval the magnetisation curves of Ho2Co17 and Ho2Co14Fe3 exhibit pronounced magnetic transitions in the hexagonal plane. The experimental data are analysed in a two-sublattice model. Within this analysis, the anisotropy constants, the sublattice magnetic moments and the 4f-3d exchange parameter are derived for the different compounds. In particular the 4f-3d exchange parameter can be determined accurately from the observed values for the transition fields.

Field-induced first-order moment reorientation transitions in R2T17 ferrimagnets

The basal-plane anisotropy in a hexagonal easy-plane ferrimagnetic system yields discontinuities in the magnetisation curve for external fields applied in the easy plane. At the discontinuities first-order moment reorientation (FOMR) transitions occur. The lowest transition field depends strongly on the difference between the values of the sublattice magnetisations and is very sensitive to the intersublattice coupling. The transition field provides a direct method for evaluating the exchange coupling between the 3d and 4f spins. The lowest transition field in the series of the easy-plane R2T17 compounds (where R=rare-earth metal and T=Fe, Co) occurs for Ho2Co17 and amounts to 21 T.

High-field magnetization studies on single crystalline Dy2Co17

Abstract Magnetization measurements have been performed at 4.2 K along the different crystallographic directions of hexagonal Dy 2 Co 17 in fields up to 35 T. The experiments reveal a magnetic transition at 28 T for the field along the easy a -axis in the hexagonal plane. Similar transitions have been observed before in Ho 2 Co 17 and could be analysed in a two-sublattice model. The transition field provides an accurate value for the 4f-3d exchange coupling parameter.

Crystal growth of R2T17 and R2Fe14B intermetallics (R = rare earth, T = Co, Fe)

Abstract Bulk single crystals of R2Co17 (R = Nd, Pr, Gd, Tb), R2Fe17 (R = Y, Dy, Er) and R2Fe14B (R = Y, Nd) have been grown from the melt using a modified “tri-arc” Czochralski method. The crystals were characterized using standard X-ray techniques. The measured densities were compared with the calculated ones obtained from the lattice parameters. High-field magnetization measurements were performed on some of the as-grown crystals.

High field and temperature dependent magnetisation measurements on some RE/sub 2/Co/sub 17/ single crystals

Single crystals of Pr/sub 2/CO/sub 17/ and Nd/sub 2/Co/sub 17/ have been grown using a tri-arc Czochralski method. High-field magnetization measurements up to 35 T have been performed on Pr/sub 2/Co/sub 17/ and Nd/sub 2/Co/sub 17/ at 4.2 K. The experimental curves show very large anisotropy; for fields along the c axis in Pr/sub 2/Co/sub 17/ and along the a axis in Nd/sub 2/Co/sub 17/ the magnetization does not reach the saturation value within this field range. Furthermore, the hard axis shows in both compounds a nonzero spontaneous magnetization indicating a cone structure with cone angles of about 26 degrees and 7 degrees for Pr/sub 2/Co/sub 17/ and Nd/sub 2/Co/sub 17/, respectively. Low-field (up to 6 T) temperature-dependent magnetization measurements (4.2-300 K) show that in both compounds the cone angle remains visible up to 300 K, but for Pr/sub 2/Co/sub 17/ the cone angle is drastically reduced above 150 K. These two RE/sub 2/TM/sub 17/ compounds (RE stands for a rare-earth metal and TM stands for a transition metal) with a cone structure are compared with other RE-TM compounds with a cone structure. >

Multiple field-induced transitions in Er2Fe17 in fields up to 50 T

Abstract High-field magnetisation measurements in fields up to 50 T have been performed on single-crystalline Er 2 Fe 17 . The a -axis curve reveals two transitions and the b -axis curve one. By applying a two-sublattice model, all transitions can be related to exchange-driven changes in the mutual orientation of the sublattice magnetisations.

Effective anisotropy constants in rare earth-3d intermetallics

Abstract It is shown that an intrinsic ferro- or ferrimagnetic arrangement in rare earth-3d intermetallics can easily be disturbed by external magnetic fields of relatively low values. All analyses of magnetisation curves assuming a rigid rotation of two sublattice magnetizations provide values for effective anisotropy constants which are strongly different from the values for the intrinsic magnetocrystalline anisotropy parameters. Magnetisation curves of Nd2Co17 and Sm2Co17 are used to illuminate these points.

Strength of the R-T exchange coupling in R2T17 compounds

Abstract The R-T exchange-coupling parameter J RT has been deduced for a number of ferrimagnetic R 2 T 17 compounds (Dy 2 Co 17 , Ho 2 Co 17 , Er 2 Co 17 and Ho 2 Co 14 Fe 3 ) from high-field magnetisation measurements on single-crystalline spheres that are allowed to orient themselves in the applied magnetic field. For J RT values ranging from 0.942 to 1.020×10 −22 J have been deduced.