F.E. Kayzel

Exchange and anisotropy in 3d-4f compounds

Abstract High-performance permanent magnets are characterised by large values of their remanence and coercive force, which lead to high values for the energy product ( BH ) max , and by a good thermal stability which allows the magnet to be used at elevated temperatures. These technical requirements are directly related to the intrinsic properties of the materials of which the permanent magnets are composed. A high remanence can be obtained for materials possessing a high saturation magnetization. The thermal stability is largely related to the value of the Curie temperature, while the coercive force depends on the magnetic anisotropy. In the 3d-rich rare-earth-transition-metal compounds, high values for the Curie temperature and for the saturation magnetization can be reached for the transition-metal elements iron and cobalt, whereas a large and uniaxial anisotropy can be achieved for the rare-earth elements Nd, Pr or Sm in their proper crystallographic structures. Through an exchange coupling between the transition-metal and rare-earth magnetic moments, the 4f moment with its large 4f anisotropy is coupled to the 3d moment, resulting in excellent magnetic properties for compounds like SmCo 5 , Sm 2 Co 17 and Nd 2 Fe 14 B. To study the basic interactions in rare-earth magnets in full detail, applied magnetic fields are required of the same order of magnitude as the effective exchange and anisotropy fields which frequently reach values of 100 T or more in the 3d-4f intermetallics. The 3d-4f exchange interaction can be studied effectively for the heavy rare-earth compounds on finely powdered material, a technique that fails for the light rare-earth compounds that are most attractive for applications. High-magnetic-field studies on single-crystalline samples reveal both the exchange interactions and the magnetic anisotropy. Examples will be discussed for compounds belonging to several rare earth-cobalt/iron series with different stoichiometries.

High field magnetization in Pr(Ni1-xCox)5 single crystals

Abstract Magnetization measurements in fields up to 38 T performed at low temperature on single crystals of the hexagonal Pr(Ni 1- x Co x ) 5 compounds for x = 0, 0.05, 0.10 and 0.20 are presented. In PrNi 5 we observe highly original behaviour predicted by the knowledge of the Crystalline Electric Field parameters and arising from the existence of a non-magnetic singlet ground state; namely transitions associated with the field induced “anticrossing” and “crossing” of the two lowest states along the [100] and [120] directions, respectively. The measurements performed on the other compounds have allowed us to study the dependence of this behaviour on Co substitutions.

Crystalline electric field and high field magnetization in ErNi5 single crystal

Abstract High field magnetization measurements up to 380 kOe along the three symmetry axes of a single-crystalline sample of hexagonal ErNi 5 , especially the curves along [100] and [120] show us for the first time the strong field induced anisotropy between the two axes in the basal plane. A quantitative analysis of this experiment together with the other data such as paramagnetic susceptibility, specific heat and inelastic neutron scattering allowed us to determine unambiguously the crystalline electric field (CEF) parameters and the paramagnetic exchange coefficient. This leads to a better understanding of the CEF anisotropy in the basal plane. The Er 3+ CEF interactions are compared with those found in other RNi 5 compounds.

Ground state of Er3+ ions in ErNi5 as studied by high field magnetization

Abstract By magnetization studies of single crystalline ErNi5 in magnetic fields up to 35 T, the crystal field and exchange interactions have been evaluated. The ground state of the Er3+ ion is found to be a Γ9 doublet with a dominant |±15/2〉 contribution. The derived parameters describe the inelastic neutron scattering and specific heat results available in the literature very well.

Magnetostriction in pulsed high magnetic fields of RE2Fe17 single crystals

Abstract Magnetostriction on single crystals of Y2Fe17 and Er2Fe17 has been measured in pulsed high magnetic fields up to 15 T in the temperature range between 5 and 300 K. We have observed large jumps in the magnetostriction on Er2Fe17, related to first order magnetization processes induced when the magnetic field is applied along the hard magnetic c-axis.

The specific heat of NdNi/sub 5/

The specific heat of NdNi/sub 5/ measured on a single-crystalline specimen from 1.5 K to 250 K has been analyzed in terms of two electronic subsystems, i.e. conduction-electron and f-electron subsystems. The conduction-electron subsystem is found in LaNi/sub 5/ to be characterized by a value of the Sommerfeld coefficient /spl gamma/ of 36 mJ/K/sup 2/mol f.u. The contribution of the f-electron subsystem to the specific heat shows a /spl lambda/-type of peak at T/sub c/ of 7.4 K and a non-pronounced Schottky-type of peak above T/sub c/. The f contribution is well described within a single-ion Hamiltonian that includes the crystalline-electric-field (CEF) and exchange interactions of the Nd/sup 3+/ ions. The exchange parameter and the full set of the CEF parameters associated with the hexagonal symmetry have been evaluated. The ground state /spl Gammasub 8/ for the Nd/sup 3+/ ion in NdNi/sub 5/ is highly anisotropic with the magnetic moment lying within the hexagonal plane. This ground-state results from higher-order charge multipolar interactions. >

The Gd anisotropy in GdCo5

Abstract High-field magnetization curves of single-crystalline GdCo5 have been measured at 4.2 K up to 35 T in order to clarify the presence of an extra Gd contribution to the magnetocrystalline anisotropy. The 3d-4f exchange interactions and the Co sublattice anisotropy have been evaluated in GdCo5 and Gd2Fe17. No significant Gd contribution to the anisotropy has been revealed.

Thermal expansion of the non-Fermi liquid system Th1−xUxRu2Si2 (x ≤ 0.07)

Abstract We have studied non-Fermi liquid properties of the tetragonal diluted uranium alloys Th1−xUxRu2Si2 (x ≤ 0.07), by means of thermal expansion measurements in the temperature range 0.5–100 K. As the temperature is lowered below about 10 K, the volume effect αv(T)/T (=(αc + 2αa)/T) is found to exhibit the tendency to diverge logarithmically, similar to other quantities of the system: C/T ∼ − ln T, ϱ ∼ − ln T ( or T 1 2 ) . Interestingly, the sign of the anomaly in αv(T) is negative, opposite to the usual Kondo shrink behavior. As a possible interpretation, we discuss the two-channel Kondo model, proposing c-f hybridization effects dominated by the virtual f2-f3 charge fluctuations.

Magnetization measurements on RE2Fe17 single crystals

Abstract Magnetization measurements on single crystals of RE 2 Fe 17 (RE  Er, Ho and Y) have been performed in the temperature range 1.5–300 K and applied magnetic fields up to 12 T, along the hard (HMD) and easy (EMD) magnetization directions. In the Er 2 Fe 17 compound a FOMP has been observed for temperatures of 1.5–110 K.

High magnetic field studies in amorphous Fe72−xYxHo8B20 alloys

Abstract High field magnetic studies on amorphous Fe 72- x Y x Ho 8 B 20 alloys have been performed at 4.2 K for fields up to 35 T. The results are interpreted in terms of a model originally proposed for crystalline materials. The critical field at which the antiferromagnetic coupling becomes unstable is well predicted by the model. The exchange integral J Ho-Fe increases with a decrease in Fe moment.