Crystal field in nitrogenated rare-earth intermetallics

The crystal-field in Sm2Fe17N3- δ and Sm(Fe11Ti)N1- δ due to interstitial nitrogen has been investigated. Intrinsic parametrization in the superposition model allows separation of the crystal field created by a neighboring nitrogen atom from a purely geometrical factor, which is different for Sm2Fe17N3- δ and Sm( Fe11Ti) N1- δ Using published magnetic data, values for the intrinsic parameter A2 per nitrogen atom of A2=200± 60 Ka0-2 and A2=270±60 Ka0-2 for Sm2Fe17N3- δ and Sm(Fe11Ti)N1- δ, respectively, are obtained. Because of charge penetration, which is discussed in the form of an explicit crystal-field weight function, it is not possible to interpret A20 or à 2 as crystal-field parameters independent of the 4f ion

Magnetization and \u3csup\u3e57\u3c/sup\u3eFe hyperfine fields in Y\u3csub\u3e2\u3c/sub\u3eFe\u3csub\u3e17\u3c/sub\u3eZ\u3csub\u3e3- δ\u3c/sub\u3e (Z=H, C, or N) interstitial compounds

Measured magnetization and 57Fe hyperfine fields at T∼0 K for Y2Fe17 and Y2Fe17Z3- δ , with Z=H, C, or N and δ∼0.5, are analyzed to determine the influence of the interstitial atoms on the 3d magnetism. All are weak ferromagnets with a nearly-full 3d subband, although when Z=N, the magnetic moment of 38.1μB is very close to the fully saturated value of 39.4μB. On a local scale, the order of hyperfine fields Bhf4f (=35 T)\u3e Bhf6g (=33 T)\u3e Bhf12j (=30 T)\u3e Bhf12k (=28 T) is different from that of the local magnetic moments given by several recent band calculations for Y2Fe17, μ4f (=2.5μB)\u3eμ12j (=2.2μB)\u3eμ12k (=2.0μB)\u3eμ6g (=1.9 μB), reflecting a large 4s-transferred hyperfine field at 6g sites which have very short Fe-Fe distances. The volume of samples with Z=C and N are identical, and so the chemical effects of the interstitials can be distinguished; both the local magnetic moment and hyperfine fields are systematically smaller in the carbide than the nitride by 5% and 12%, respectively

Contact-induced spin polarization in carbon nanotubes

Motivated by the possibility of combining spintronics with molecular structures, we investigate the conditions for the appearance of spin-polarization in low-dimensional tubular systems by contacting them to a magnetic substrate. We derive a set of general expressions describing the charge transfer between the tube and the substrate and the relative energy costs. The mean-field solution of the general expressions provides an insightful formula for the induced spin-polarization. Using a tight-binding model for the electronic structure we are able to estimate the magnitude and the stability of the induced moment. This indicates that a significant magnetic moment in carbon nanotubes can be observed.Comment: To appear in Phys. Rev. B (2003

Volume contraction at the Jahn-Teller transition of LaMnO3_3

We have studied the volume collapse of LaMnO$_3$ at the Jahn- Teller (JT) transition temperature T$_{JT}$=750 K which has recently been found in high temperature powder x- ray and neutron diffraction experiments. We construct a model Hamiltonian involving the pseudospin of Mn$^{3+}$ e$_g$ states, the staggered JT distortion and the volume strain coordinate. We show that the anharmonic coupling between these primary and secondary order parameters leads to the first order JT phase transition associated with a comparatively large reduction of the unit cell volume of $\Delta$V/V$\simeq$ 10$^{-2}$. We explain the temperature dependence of JT distortions and volume strain and discuss the volume change as function of the anharmonic coupling constant. A continuous change to a second order transition as function of model parameters is obtained. This behaviour is also observed under Ba doping.Comment: 5 pages, 4 figure

Field dependence of the electronic phase separation in Pr0.67Ca0.33MnO3 by small angle magnetic neutron scattering

We have studied by small angle neutron scattering the evolution induced by the application of magnetic field of the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in a crystal of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$. The results are compared to magnetic measurements which provide the evolution of the ferromagnetic fraction. These results show that the growth of the ferromagnetic phase corresponds to an increase of the thickness of the ferromagnetic ''cabbage'' sheets

Metal-insulator transition in EuO

It is shown that the spectacular metal-insulator transition in Eu-rich EuO can be simulated within an extended Kondo lattice model. The different orders of magnitude of the jump in resistivity in dependence on the concentration of oxygen vacancies as well as the low-temperature resistance minimum in high-resistivity samples are reproduced quantitatively. The huge colossal magnetoresistance (CMR) is calculated and discussed