The Effect of Carbon Concentration on the Magnetic Properties and Hyperfine Interactions of Neptunium Monocarbide D

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THE EFFECT OF CARBON CONCENTRATION ON THE MAGNETIC PROPERTIES AND HYPERFINE INTERACTIONS OF NEPTUNIUM MONOCARBIDE D. Lam, M. Mueller, A. Paulikas, G. Lander

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D. Lam, M. Mueller, A. Paulikas, G. Lander. THE EFFECT OF CARBON CONCENTRA- TION ON THE MAGNETIC PROPERTIES AND HYPERFINE INTERACTIONS OF NEP- TUNIUM MONOCARBIDE. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-917-C1-919. 10.1051/jphyscol:19711326. jpa-00214359

HAL Id: jpa-00214359 https://hal.archives-ouvertes.fr/jpa-00214359 Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Collaque C 1, supplkmet~tat1 no 2-3, Tome 32, FPvrier-Mars 1971, page C 1 - 917

THE EFFECT OF CARBON CONCENTRATION ON THE MAGNETIC PROPERTIES AND HYPERFINE INTERACTIONS OF NEPTUNIUM MONOCARBIDE (I) by D. J. LAM, M. H. MUELLER, A. P. PAULIKAS and G. H. LANDER Argonne National Laboratory, Argonne, Illinois U. S. A.

RCumB. - Les propriktes magnetiques des monocarbures de Neptunium NpC0.96, NpCo, 89 et NpCo, 82 de structure NaCl ont etC etudites par des mesures d'aimantation, d'effet Mossbauer et de diffraction neutronique. Les resultats mon- trent une grande influence de la concentration en carbone sur le couplage des ions magnetiques, mais trts peu d'influence sur la valeur du moment magnktique du Neptunium. Abstract. - Neptunium monocarbide alloys NpC0.96, NpC0.89 and NPCO.?~,wifh the NaCI-type structure, were investigated by measuring magnetization, the Mossbauer effect, and neutron d~ffract~onto determine then magnetlc properties as a function of carbon concentration. Results indicate that the carbon concentration has a pronounced effect on the coupling of the magnetic ions but has little effect on the magnitude of the magneticmoment in neptunium monocarbide.

1. Introduction. - Monocarbides of neptunium 3. Results. - The magnetic susceptibility versus and plutonium with the NaCI-type structure exist temperature curves above 220 OK for the only in the carbon-deficient substoichiometric range. are shown in figure 1. Since the three NpC,.,, samples Lallement et al. [I] suggested that the vacancies in PuC greatly disturb the nearest-neighbor plutonium atoms and decrease their magnetic moment to practi- cally zero. In contrast with plutonium, neptunium is a much more favorable element for neutron diffraction and, in addition, the 59.6 keV transition of 237Npis at present the most favorable for the study of Mossbauer effect in the actinide series. The magnetic properties of NpC, with carbon concentration in the range of 48-39 at. %, were investigated previously by measuring magnetic susceptibility [2, 31, neutron diffraction [4], and the Mossbauer effect [5]. However, these studies were not sufficiently systematic and did not extend to low carbon conccntrations. In this paper, we present the preliminary results on neptunium monocarbides with different carbon concentrations. For each composition, the samples used for all measurements were obtained from a single arc-melted button to minimize errors caused by uncertainties of stoichiometry and impurity content.

2. Experimental. - Five samples were prepared : - - one NPC,.,~, one NpC,,,,. and three NpC,,,,. The composition of the samples was later confirmcd o (IIIIIIIIIIIII~IIIIII by chemical analysis and X-ray diffraction. 225 250 275 300 ' 350 Magnetic susceptibility and magnetization measu- TEMPERATURE (OK) rements were made for all fivc samples by the method Fro. 1. - Magnetic susceptibility versus temperature curves reported previously [2]. Mossbauer measurements were for the < NpC > compounds. performed utilizing the 59.6 keV gamma ray of 237Np obtained from the alpha decay of 241Amin the Th-Am have similar temperature dependence, only one curve alloy. The method and apparatus has been described is shown. A maximum in the susceptibility (;o versus by Kalvius [6]. Mossbauer spectra were obtained temperature (T) curve is clearly evident for the NpC,.,, at 4.2 OK for three [7] samples, each with and NpC,.,, samples. The temperature dependence a fixed amount of 237Np isotope, but with different of magnetization for these compounds is shown in carbon concentrations. One of the NpC,.,, samples figure 2. The curves were obtained after the samples was used for the neutron diffraction study. had been cooled from 150 to 4.2 OK in a zero external magnetic field and measured at a magnetic field (1) This work was performcd under the auspices of the United strength of 10.7 kOe. The difference in the magneti- States Atomic Energy Commission. zation between the three compounds in the ttmpe-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711326 C1 -918 D. J. LAM, M. H. MUELLER, A. P. PAULIKAS AND G. H. LANDER

The hyperfine spectrum obtained at 4.2 OK for consists of two superimposed magnetic hyperfine spectra with different magnetic hyperfine fields. The ratio of the integrated intensities of these two spectra varies with carbon concentration, as shown in figure 3. The calculated ratios (T,/To)

TEMPERATURE (OK) FIG. 2. - Magnetization as a function of tem~eraturefor the < NpC > compounds measured in an external magnetic field CARBON CONCENTRATION of 10 700 Oe. FIG. 3. - The ratio of the integrated intensities of the two hyperfine spectra in < NpC > as a function of carbon concen- rature range from 100 to 200OK is probably due to tration. The squares indicate the experimental results, and the the inability to saturate the samples with the 10.7 kOe circles are calculated ratios. magnetic field. A 16-fold decrease in magnetization is observed between 150 and 4.2 OK for the NpC,.,, sample and for all of the NpC,.,, samples. Cooling between neptunium atoms with one and zero vacancies a sample of NpC,.,, from 150 to 4.2 OK in an external as nearest neighbors, based on a random distribution magnetic field of 10.7 kOe completely eliminated of vacancies in the carbon sublattice, are also shown the sharp drop in magnetization. When the sample in figure 3. Assuming that the hyperfine spectra are due was cooled in an external field of 1.10 kOe, the magne- to Np atoms with one and zero nearest-neighbor tization measured at 4.2 OK is only slightly smaller vacancies, and that the recoiless fraction is the same (- 5 a/,) than the 150 OK value. at both sites, figure 3 shows that the distribution of The neutron-diffraction results indicate that NPC,.~, vacancies at the carbon site is not random. begins to order antiferromagnetically (Type I) at about 260 OK, the magnetic moment per Np is about 4. Discussion. - The results are summarized in 0.5 p, at 2400K, and becomes ferromagnetically Table I.. The isomer shifts can be considered to be ordered below 2300K. Cooling in a zero external identical for all three alloys within experimental field, however, does not alter the magnetic ordering uncertainty. The hyperfine field is relatively constant between 150 and 4.2 OK. A ferromagnetic moment and the difference in hyperfine field between the two of 2.2 f 0.5~~per neptunium atom is observed neptunium sites in each alloy is about 13 %. at both temperatures. Lallement et al. [I] suggested that the vacancies

TABLE1 Experimental results obtained for neptunium monocarbide alloys Composition N~~o.96 N~Co.s9 - - Carbon Content (wt %) 4.64 4.38 Lattice Constant (A) 4.999 7 f 0.000 3 4.992 2 f 0.000 6 Tc (OK) 225 f 5 225 f 5 TN (OK) 310 f 5 270 f 5 Isomer Shift (b) (0) (") - 7.3 + 1.0 - 7,3 +_ 1.0 (mm/sec) ( (1) td) - 7.8 f 1.0 - 6.5 + 1.0 4 420 f 50 4 350 + 50 ,ff (kOe) ( j0; t:; 3 820 f 50 3 710 f 50 (") From neutron diffraction results. (b) The isomer shift is measured with respect to the Am-Th alloy. (') (0) Neptunium with no vacancy as nearest neighbor. (d) (1) Neptunium with one vacancy as nearest neighbor. THE EFFECT OF CARBON CONCENTRATION ON THE MAGNETIC PROPERTIES C 1 - 919 in PuC greatly disturb the nearest-neighbor plutonium temperature of depends strongly on the atoms and decrease their magnetic moments to practi- carbon concentration of the sample, whereas the cally zero. If this mechanism is operative in , Curie temperature is independent of carbon then one of the neptunium sites in the carbide should concentration. The drastic decrease in magnetization be nonmagnetic. Instead, both sites are strongly of NpC,.,, and NpC,.,, may be explained by magnetic. In addition, the average magnetic moment assuming that the spins are aligned antiparallel of 2.2 + 0.5 ,u,/Np atom for the NpC,.,, alloy, below 100 OK, resulting in a small net magnetization deduced from the neutron-diffraction experiment, at 4.2 OK. However, neutron-diffraction experiments is close to the value of 2.1 + 0.1 pB/Np atom for a performed on the same samples showed no change NpC,.,, sample previously studied [4]. These results of magnetic ordering between 150 and 4.2 OK. show that the vacancies have little effect on the value of the magnetic moment. However, the effect of 5. Acknowledgements. - The authors greatly carbon concentration on the magnetic ordering of appreciate the able technical assistance of Mr. A. W. is unexpectedly complicated. The Ntel Mitchell, Mr. J. M. Pasteris,and Mr. R. L. Hitterman.

References [I] LALLEMENT(R.), COSTA(P.) and PASCARD(R.), J. [5] DUNLAP(B. D.), BRODSKY(M. B.), KALVIUS(G. M.), Phys. Chem. Solids, 1965, 26, 1255. SHENOY(G. K.) and LAM(D. J.), J. Appl. Phys., [2] Ross (J. W.) and LAM(D. J.), J. Appf. Phys., 1967, 1969, 40, 1495. 39, 1451. [6] KALVIUS(G. M.), Mossbauer Effect Methodology, [3] DE NOVION(C. H.) and LORENZILLI(R.), J. Phys. ed. by I. J. Gruverman, 1 (Plenum Press, New Chem. Solids, 1968, 29, 1901. York, 1965), p. 163. [4] LANDER(G. H.), HEATON(L.), MUELLER(M. H.) [7] The symbol < NpC > is used to denote the phase and ANDERSON(K. D.), J. Phys. Chem. Solids, without implying stoichiometry. 1969. 30, 733.