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Structural, Electronic and Magnetic Characteristics of Np2Co17


  I. Halevy [1,2,3]  ,  A. Hen [2,4]  ,  I. Orion [2]  ,  E. Colineau [4]  ,  R. Eloirdi [4]  ,   J.-C. Griveau [4]  ,  P. Gacz´ynski [4]  ,  F. Wilhelm [5]  ,  A. Rogalev [5]  ,  J.-P. Sanchez [6]  ,  M. L. Winterrose [3]  ,  N. Magnani [7]  ,  A. B. Shick [4,8]  ,  R. Caciuffo [4]  
 [1] Physics Department, Nuclear Research Center Negev, P.O. Box 9001, Beer-Sheva, Israel
[2] Nuclear Engineering Department, Ben Gurion University, Beer-Sheva, Israel
[3] California Institute of Technology, W. M. Keck Laboratory 138-78, Pasadena, CA 91125, USA
[4] European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany
[5] European Synchrotron Radiation Facility (ESRF), B.P.220, F-38043 Grenoble, France
[6] SPSMS, UMR-E CEA/UJF-Grenoble 1, INAC, F-38054 Grenoble, France
[7] Lawrence Berkeley National Laboratory, Chemical Sciences Division, 1 Cyclotron Road, Berkeley, CA 94720-8175, USA
[8] 8 Institute of Physics, Academy of Sciences of the Czech Republic,Na Slovance 2, CZ-182 21 Prague, Czech Republic

A previously unknown neptunium-transition-metal binary compound, Np2Co17, has been synthesized and characterized by means of powder x-ray diffraction, 237NpMössbauer spectroscopy, SQUID magnetometry, and x-ray Magnetic Circular Dichroism (XMCD) . The compound crystallizes in the Th2Ni17-type hexagonal structure, with room temperature lattice parameters a = 8.3107(1) Å and c = 8.1058(1) Å. Magnetization curves indicate the occurrence of ferromagnetic order below a TC > 350 K. Mössbauer spectra suggest a Np3+ oxidation state and give an ordered moment of μNp =1.57(4) μB and μNp = 1.63(4) μB for the Np atoms located, respectively, at the 2b and 2d crystallographic positions of the P63/mm c space group. Combining these values with a sum rule analysis of the XMCD spectra measured at the neptunium M4,5 absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment (μS = -1.88(9) μB, μL = 3.48(9) μB). The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment (μmd/μS = -0.04) is negative, and one order of magnitude smaller than expected for a localized ion in intermediate coupling (IC) approximation. The expectation value of the angular part of the spin-orbit interaction operator is, on the other hand, in excellent agreement with the IC estimate. The ordered moment averaged over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is μCo ≃ 1.6 μB. The experimental results are discussed against the predictions of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction.

 

 

I. Halevy,  A. Hen,  I. Orion,  E. Colineau,  R.  Eloirdi,  J.-C. Griveau,

P. Gacz´ynski,  F. Wilhelm, A. Rogalev,  J.-P. Sanchez,  M. L. Winterrose,

 N. Magnani,  A. B. Shick and  R. Caciuffo

Structural, Electronic and Magnetic Characteristics of Np2Co17

(submitted to PRB Dec 2011)