Element and orbital-specific observation of two-step magnetic transition inNpNiGa5: X-ray magnetic circular dichroism study

Abstract

X-ray magnetic circular dichroism ͑XMCD͒ experiments were performed at the Np M 4,5 and the Ga K absorption edges of NpNiGa 5 to investigate the temperature-dependent changes of magnetic properties of Np 5f and Ga 4p electron states. By the sum-rule analysis of the Np M 4,5 XMCD data, the orbital magnetic moment L and the spin magnetic moment S were estimated for the Np 5f 3 and 5f 4 electronic configurations and their comparison to the previous magnetization and neutron-scattering experiments suggests that the 5f 4 configuration is more likely than the 5f 3 configuration in NpNiGa 5 . It was found that ͉ L / S ͉ tends to increase from the high-temperature low-moment ordered state to the low-temperature high-moment ordered state. The result of the Ga K XMCD indicates that the Ga 4p electrons are magnetically polarized and the temperature and magnetic-field dependences of the Ga 4p orbital moment are proportional to those of the magnetization measurements.

Magnetic and electronic properties of NpFeGa5

“…It should, however, be stressed that this is also the case for NpNiGa 5 [10] and NpRhGa 5 [8]. Thus this sheds some doubt on a possible electronic transition towards a more localised state at low temperature as suggested previously [27].…”

X-ray magnetic circular dichroism experiments and theory of transuranium Laves phase compounds

“…The spectral shape of the XMCD signal has an asymmetric s shape with two peaks -a negative and a positive peak. However this positive peak is most pronounced in NpFe 2 and quasi absent for NpOs 2 , as found in NpNiGa 5 [32]. This asymmetric spectral shape, like for the M 5 -edge of U, depends strongly on the hybridization, Coulomb, exchange, and crystal-field interactions.…”

“…Using atomic theory, Thole and Carra [40][41][42] have derived sum-rules that relate integrated intensities of XAS and XMCD to the ground-state expectation values of standard operators, such as the orbital L z and spin S z magnetic moments of the absorbing atom. Moreover, atomic multiplet theory has been used to calculate the atomic core-level spectra at the M 4,5 absorption edges, which is associated with the transitions f n → d 9 f n+1 ( [43], [32]). However, there was no attempt to calculate the spectral shape of both the XAS and XMCD for neptunium using band-structure calculations.…”

“…Such experiments have been performed on ferromagnetic uranium compounds with the NaCl structure [22][23][24][25], the Laves phase ferromagnet UFe 2 [26], heavy-fermion materials where the moment is induced with a magnetic field [27,28], UTAl ferromagnets [29], and even multilayers containing uranium [30,31]. Up to now, only two XMCD studies have been reported on the M 4,5 absorption edges of a transuranium material, NpNiGa 5 , by Okane et al [32] and work at the ESRF on Np 2 Co 17 [33].…”

“…, [32]). However, there was no attempt to calculate the spectral shape of both the XAS and XMCD for neptunium using band-structure calculations.…”