Crystal Electric Field Interaction Research Articles

Single crystal growth and anisotropy of CeRuPO

We report on the single crystal growth of the ferromagnetic Kondo lattice system CeRuPO using a Sn-flux method. Magnetic susceptibility and electrical resistivity measurements indicate strong anisotropy of this structurally layered compound. They evidence that the magnetic moments order ferromagnetically along the c-direction of the tetragonal unit cell, whereas the crystal electric field (CEF) anisotropy favors the ab-plane. Therefore, CeRuPO presents the unusual case within rare earth systems, where the anisotropy of the interionic exchange interaction overcomes the single ion anisotropy due to the CEF interaction.

Structural and magnetic properties in the ruthenate Bi2.67Pr0.33Ru3O11

AbstractAn unreported praseodymium substituted phase of the Bi‐Ru‐O system with formula Bi2.67Pr0.33Ru3O11was prepared. Its crystal structure and magnetic properties were investigated. Pr substituted occurs only on one of the two Bi sites. The magnetism of the compound is dominated by a Pr3+ contribution, that appears to interact antiferromagnetically. A non linear variation of the reciprocal magnetic susceptibility is observed that appears to account for crystal electric field interactions on the Pr3+ ions. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transition temperatures of rare-earth compounds studied with perturbation theory

The perturbation theory is employed in present Letter to study the transition temperatures of rare-earth magnets. To simplify the model, it is assumed that the ground and first excited crystal-electric-field (CEF) levels are both nonmagnetic singlets, the Heisenberg exchange interaction is only comparable to the energy gap ( Δ) between them, and other higher levels are well separated from the first excited one, so their contributions to the bulk magnetization are negligible in low-temperature region. By treating the Heisenberg exchange as a perturbation to the CEF interaction near the critical point, explicit expressions for the Curie and Neél temperatures ( T C and T N ) are derived in terms of the Heisenberg exchange constant ( J ), the gap Δ and the matrix element of the total spin moment in the ordering direction between the two lowest CEF levels. For completeness and to verify the correctness of the theory, we have also derived the analytical expressions for the magnetic moments at zero temperature for both rare-earth ferromagnet and antiferromagnet, the conditions for the magnetic orderings, and calculated the spontaneous magnetizations and magnetic specific heats at finite temperatures numerically for a hexagonal ferromagnetic Pr compound with the model. This approach can be generalized to real systems to evaluate their transition temperatures and to investigate the mechanisms of magnetic ordering.

Magnetic Moment of Fe and Magnetocrystalline Anisotropy of Fe-rich Intermetallic Compounds Under Pressure

Magnetization and magnetocrystalline anisotropy of Fe-rich R -Fe intermetallics ( R =Y and rare earths) have been studied under pressure up to 1 GPa in temperatures from 5 K to 350 K using a miniature CuBe pressure cell and a SQUID magnetometer. A decrease of Fe-moment under pressure depends strongly on local environment parameters such as a local atomic volume, local coordination of Fe-atoms and hybridization. The pressure-induced changes of magnetocrystalline anisotropy energy are caused by both, the pressure decrease of magnetization M S and the pressure effect on the crystal electric field interactions. The pressure experiments allow to distinguish both the effects.

Analysis of the magnetic behaviour of Ce 1− xGd xCo 2 compounds

Experimental data on the spontaneous magnetization m s( T) of Ce 1− x Gd x Co 2 compounds were analysed using a mean-field phenomenological model which takes into account the exchange interactions between atoms of the three magnetic species (Gd, Ce, Co) as well as the crystal electric field interaction on the Ce ions. This model allowed us to obtain, through a numerical fitting procedure to the m s( T) curves, the composition dependence of the Gd–Co and Co–Co exchange interaction strengths ( A Gd–Co and A Co–Co, respectively) and of the Co magnetic moment m Co. Both A Gd–Co and A Co–Co increase considerably with the Ce content, while m Co decreases linearly from x=1 down to the composition x=0.15; below the latter value of x, the magnetic ordering is suppressed. The intermediate valence of Ce, as suggested by the composition dependence of the room temperature lattice parameter, being also compatible with the obtained thermal variation of the Ce contribution to the total magnetic moment, is expected to play an important role in the enhancement of the A Gd–Co and A Co–Co interaction strengths, through a net reinforcement of the 3d–5d hybridization.

Study of the crystal electric field interaction in single crystals

Magnetization measurements along the main symmetry directions of the tetragonal structure, [100], [110] and [001], have been performed on several single crystals (R = Y, Tb and Ho), at applied high magnetic fields up to 12 Tesla and temperatures ranging from 4.2 to 300 K. From the measurements on the yttrium compound the second- and fourth-order anisotropy constants and the spontaneous magnetization of the Fe sublattice have been determined. The complex magnetic behaviour has been explained using a single-ion model for the crystal electric field (CEF) interaction and a mean-field model for the exchange interaction. A reliable set of CEF parameters and mean exchange field have been obtained for the and intermetallic compounds. All the experimental features observed, the first-order magnetization process under magnetic field in and spin reorientation transition (SRT) in , have been explained using such CEF and exchange parameters. A study of the character of the SRT observed in the intermetallic compound has been undertaken by measuring the parallel and perpendicular components of the magnetization to the applied magnetic field. The experimental results obtained have been explained considering the coexistence of two magnetic phases for temperatures close to the spin reorientation one.

Effect of pressure on the magnetocrystalline anisotropy of (ErxR1-x)2Fe14B intermetallics

A systematic study of the effect of pressure up to 1 GPa on the spin reorientation transition (SRT) phenomena has been performed on several series of pseudoternary compounds (ErxR1-x)2Fe14B for R identical to Nd, Gd, Dy and Y using a low-field AC susceptibility technique. The variation in the SRT temperature TSR under pressure has been used as an indication of the effect of the pressure on the crystal electric field (CEF) at the rare-earth ion site. The authors have observed that the pressure effect on the Er2Fe14B compound ( Delta TSR/ Delta p=-20 K GPa-1) was not affected by the substitution of Er by Gd and Y ions. A completely different high-pressure behaviour has been observed for the (ErxDy1-x)2Fe14B and (ErxNd1-x)2Fe14B compounds, in which the variation in TSR under pressure decreases for increasing content of Dy and Nd. This different behaviour has been attributed to the strong influence of pressure on the CEF interactions. Using these results the authors have determined the variation in the CEF parameters under applied pressure.

Interpretation of neutron magnetic scattering in the Kondo lattices YbPd2Si2 and YbAgCu4

We present an interpretation of published neutron inelastic scattering spectra in the Kondo lattices YbPd2Si2 and YbAgCu4 obtained in terms of the Anderson impurity model, describing the hybridisation of the 4f Yb electrons with the band electrons, and also including the crystal electric field interaction. In YbPd2Si2, the tetragonal crystal field parameters were determined. In YbAgCu4 the crystal field interaction was taken to exist by analogy with the isoelectronic compound YbAuCu4 where it has been identified. Both compounds can be described by a Kondo temperature,T0=60 K and a Yb valency very close to 3.

Pressure effects on the spin reorientation transitions in rare-earth intermetallics

A systematic study of the influence of pressure on the spin reorientation transition temperature ( T SR) has been carried out on several series of pseudoternary compounds (Er x R 1− x ) 2Fe 14B for R-Dy, Nd and Y. The experimental results indicate that the pressure has a strong influence on the anisotropic properties of the studied compounds. The effect is really important in those compounds in which the crystal electric field interaction is mainly originated by second-order terms. On these bases we have used a simple single-ion two-dimensional mean-field crystal electric field model in order to study the variation of the CEF parameter with pressure in compounds which show competing magnetic anisotropies.

Spin reorientation phenomena in RFe 10V 2 intermetallic compounds (RDy and Nd).

A high Curie point and a high uniaxial anisotropy near room temperature are the most interesting features of the new iron rich R Fe 12-xM x alloys (R=rare earth, MTi, V, Cr…) in order to be used as starting materials for permanent magnets. When R is a rare earth ion with planar anisotropy spin reorientation transition (SRT) phenomena will take place due to the variation with temperature and competition of the anisotropy of the R and Fe magnetic sublattices. We have studied the SRT phenomena in RFe 10V 2 (RDy and Nd) using two different experimental techniques. A.C. initial magnetic susceptibility measurements have allowed us to determine the SRT temperature associated with the critical fluctuation of the SRT angle, being T SR=140 K and T SR=112 K for DyFe 10V 2 and Nd 2Fe 10V 2 respectively. On the other side we have measured the angular dependence of the parallel and perpendicular component of the magnetization to the applied magnetic field, which has permitted to obtain the SRT angle. From the aforementioned experiments we have obtained that at low temperatures a cone magnetic structure is stabilized in both compounds and from that we can conclude the relevance of the high order term in the crystal electric field interaction.

Magnetoelastic behaviour and the spin-reorientation transition in HoAl2

The linear thermal expansion, AC susceptibility and magnetostriction have been measured on a single crystal of HoAl2. The authors' results clearly show two phase transitions associated, respectively, with establishment of magnetic order and with spontaneous spin reorientation. The critical exponent gamma SR of the susceptibility for T>or approximately=TSR has been determined. From magnetostriction measurements along the major symmetry directions of the crystal, they have obtained the possible deformation modes compatible with the point symmetry of the cubic crystal structure. A complete study of the magnetoelastic behaviour has allowed them to determine the reduced magnetoelastic coupling coefficients M2gamma /Cgamma , M2epsilon /Cepsilon , Mlalpha /Calpha (l=4,6) and Dalpha /Calpha . The model used is based on a single-ion origin for the crystal electric field interaction of the rare-earth ion, and they have taken into account the spontaneous reorientation of the magnetic moment in the molecular field, which represents the exchange interaction.

Temperature dependence of hyperfine fields at rare-earth nuclei in iron and nickel

The temperature dependence of the hyperfine fields [${H}_{0}(T)$] at $^{146}\mathrm{Nd}$ and $^{150}\mathrm{Sm}$ nuclei recoil implanted into polarized Fe and Ni were measured from 4 to 650 K with the ion-implantation perturbed-angular-correlation technique. Experimental results for ${H}_{0}(4)$ are: FeNd, 3.04(71) MOe; NiNd, 1.67(39) MOe; FeSm, 3.14(35) MOe; NiSm, 1.77(17) MOe. The hyperfine magnetic fields induced at the implanted nuclei by the unquenched orbital angular momentum of the partially filled ionic $4f$ shell [${H}_{4f}(T)$] were deduced and the results analyzed in terms of the interactions of the respective rare-earth ionic states with the internal fields of the hosts. Interaction models incorporating pure exchange interactions, exchange and crystal electric field interactions, various implant final-site symmetries, and ionic-excited-state mixing were least-square fit to ${H}_{4f}(T)$ over various intervals of the observed temperature range. Crystal-electric-field effects are shown to be important in all cases except, perhaps, FeSm. The results of each least-squares-fit analysis are presented and discussed.