J.H.P. Colpa

Magnetocrystalline anisotropy and RCo exchange interaction in monocrystalline RCo4Al (R = Y, Gd and Ho)

We report on the magnetic properties of the RCo4Al (R; Y, Gd, Ho) monocrystalline samples. Al substitution for Co in RCo5 results in remarkable changes of the magnetic properties not only of the Co sublattice but also of the R sublattice. These changes include a decrease of the magnetocrystalline anisotropy energy of the Co sublattice, a decrease of the RCo exchange interaction and a remarkable modification of the crystal field parameters of the R sublattice. The effects of Al substitution in RCo5 are compared with those of B substitution.

Study of the specific heat of a CeCoAl4 single crystal in high magnetic fields

Abstract The specific heat of a CeCoAl 4 single crystal has been studied in magnetic fields up to 17.5 T. In zero field, this compound has been found to order antiferromagnetically below the Neel temperature of about 13.5 K. The effect of the magnetic field is different when it is applied along the different crystallographic axes. Along the a -axis, with increasing magnetic field, the Neel temperature slightly decreases and reaches a value of 12.7 K at 17.5 T. Along the b -axis, the Neel temperature decreases more strongly with increasing magnetic field. At 16 T, we found a value of T N of about 6.9 K. At 17.5 T, along the b -axis, we observed a rather broad peak in the specific heat which is associated with a transition from paramagnetism to ferromagnetic order. Along the c -axis, a drastic change in the specific heat occurs at a field between 7–8 T. At 7 T, the value of T N is about 9 K. At fields higher than 8 T, the specific heat is evidently characterised by a transition from paramagnetism to ferromagnetic order. The critical values of the magnetic field above which ferromagnetic order is formed at low temperature turn out to be consistent with the measurements of the magnetisation curves at 4.2 K.

Competing exchange interactions and their relevance for the magnetisation process in RMn6-xCrxSn6 powders (R=Y, Gd, Tb, Dy, Ho, Er)

Abstract The free-powder magnetisation of RMn6-xCrxSn6 compounds has been measured for compounds with R = Y, Gd, Tb, Dy, Ho, Er in fields up to 38 T, and interpreted in terms of simple model, which is also outlined in this paper. From the measurements, estimates for the R-3d mean-field coupling constant (nRT) could be derived for the cases where R = Gd, Tb, Dy, Ho, Er. In turn, the nRT values can be related to the microscopic spin-coupling constant (JRT). In the case of YMn6Sn6 the high-field measurement presents evidence for a very weak antiferromagnetic coupling between the Mn layers. Furthermore, values for the Mn moments (μMn) were also derived from the magnetisation measurements. The estimated μMn values are of the order of 2.0μB.

On the magnetic three-sublattice model with two equivalent sublattices

Abstract The magnetic 3-sublattice model is analysed for the case that the 3 sublattices are coupled by a Heisenberg type of exchange interaction, and 2 of the 3 sublattices are equivalent and coupled by equal exchange parameters with the third sublattice. The possible magnetic configurations of the 3-sublattice system in a magnetic field are derived, and a classification is given of the combinations of sublattice magnetizations and exchange parameters according to the type of zero-temperature magnetization curve to which the combination corresponds. Finally it is proved that all phase transitions are of second-order type.

Magnetocrystalline anisotropy and exchange interaction of single-crystalline RCo4M (R: Y, Gd, Ho; M: Al, B)

Abstract The RCo 4 M compounds (R: rare earth; M: Al, B) can be obtained from RCo 5 by substituting M for Co. For the first time, large single crystalline samples of RCo 4 M (R: Y, Gd, Ho) have been grown. Some differences have been found between the effects of Al and B substitution. In particular, a first-order magnetisation process associated with the Co sublattice is reported for RCo 4 B (R: Gd, Y).

Molecular field at the rare-earth sites in Nd2Co17

Abstract Neutron-scattering measurements on a single crystal of Nd 2 Co 17 have been performed at 4.2 K. The non-dispersive mode has been interpreted as resulting from an excitation of the spatially localized rare-earth ions. In combination with the crystal-field parameters derived previously from high-field magnetization measurements the non-dispersive mode yields a fairly accurate value for the molecular field at the position of the Nd ions.

Dipole-dipole anisotropy in Y2Co17 and Gd2Co17

Abstract The dipole-dipole contribution to the anisotropy constant K 1 in ellipsoidal single crystals of Y 2 Co 17 and Gd 2 Co 17 is calculated. The results are expressed in terms of closed formulae in which demagnetization factors occur explicitly. The formulae are used to show that the relatively large difference of K 1 experimentally found in the two materials can largely be ascribed to the additional dipole-dipole interactions present in Gd 2 Co 17 .

Magnetocrystalline anisotropy and magnetic structure of Nd2Co17

Abstract A set of the crystal-field parameters relevant for the rhombohedral symmetry (R 3 m) has been determined for the Nd ion from magnetic isotherms measured on single crystalline Nd 2 Co 17 in external fields up to 35 T. The magnetic structure of Nd 2 Co 17 is planar in the b-c plane. As an effect of the crystal-field interactions the moments are tilted out of the hexagonal plane with polar angle of 82°.

Magnetic excitations in Dy2Co17

Abstract Inelastic neutron-scattering measurements have been performed on the ferrimagnet Dy 2 Co 17 in order to investigate the low-energy part of the magnetic excitations. The results are interpreted in terms of Heisenberg-exchange and anisotropy parameters using linear spin-wave theory. The resulting parameters are compared with those derived from high-field magnetization data measured earlier.

Magnetic properties of monocrystalline GdCo4B

Abstract A study of the magnetic properties of monocrystalline GdCo 4 B samples is reported. A novel property, a first-order magnetisation process (FOMP), has been found in a wide temperature range. Compared to YCo 4 B, an additional positive contribution to the overall anisotropy is present in GdCo 4 B. This contribution is likely to be associated with the Gd sublattice.