Low-temperature magnetic properties ofNpNi5

Abstract

We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound NpNi5. By combining macroscopic and shell-specific techniques, we determine the 5f -shell occupation number n f close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below TC = 16 K (µS = -1.88 µB and µL = 3.91 µB). The apparent coexistence of ordered and disordered phases observed in the Mössbauer spectra is explained in terms of slow relaxation between the components of a quasi-triplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment (3 Tz / Sz = +1.43) is positive and large, suggesting a localized character of the 5f electrons. The angular part of the spin-orbit coupling ( ℓ · s = -5.55) is close to the value of -6.25 calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.

Synchrotron Radiation Techniques and their Application to Actinide Materials

“…For the Ni analogue, a nontrivial situation was observed below the ordering temperature T N where a large ordered moment (2.25 µ B ) appears in one Np site while the other carry only an induced moment of about 0.2 µ B . Experiments were also performed on the Haucke intermetallic compound NpNi 5 (Hen et al, 2014), showing a positive and large ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment, as predicted for localized 5f electrons, and an expectation value of the angular part of the spin-FIG. 23 The ratio 3 Tz / Sz between the expectation values of the magnetic dipole term and the spin moment along the quantization direction as a function of the 5f occupation number n f .…”

Magnetic polarization of the americium $J = 0$ ground state in AmFe$_{2}$

“…The XMCD spectra are composed of a small down-up feature at the M 5 edge and a large (negative) peak at M 4 . This is the typical spectral shape characteristic of a dominating orbital moment with an oppositely oriented spin, as observed for lighter actinide (An) compounds [26,27] and in particular for the Np- x-ray magnetic circular dichroism (XMCD) spectra as a function of photon energy through the Am M5 and M4 edges in AmFe2. The experiment was conducted at room temperature in an applied field of 3 tesla.…”

“…2-3). This gives 3 T z = −0.57( 5) and a ratio The values for the r ratio for different An compounds (UGe 2 [35][36][37], US [38], USe, UTe [39], USb 0.5 Te 0.5 [40], UPd 2 Al 3 [41], UNi 2 Al 3 [42], UCoAl, UPtAl [43], URhAl [44], NpNi 5 [27], Np 2 Co 17 [26], NpOs 2 , PuFe 2 [14] and PuSb [29]) have been derived from data reported in the literature and are shown in Fig. 5, where they are compared with theoretical values calculated in IC, as well as in the two limit coupling schemes LS and jj.…”

Site-selective magnetic order of neptunium inNp2Ni17

“…A clear lambda-type anomaly is observed at the transition temperature T N = 17.5 K. The vibrational contribution has been estimated from specific heat measurements performed on ThNi 5 (a compound with a closely related crystallographic structure), subtracting the electronic term as detailed in Ref. [22] and rescaling to account for the different number of atoms in the unit cell. At lower temperatures, Schottky peaks of nuclear and electronic origin contribute to the observed temperature dependence (lower inset in Fig.…”

“…Note the similarity between the Np (2b) f -projected DOS shown in Fig. 8(c) and the DOS for Np (1a) atom in NpNi 5 [22].…”