THE EFFECT OF CARBON CONCENTRATION ON THE MAGNETIC PROPERTIES AND HYPERFINE INTERACTIONS OF NEPTUNIUM MONOCARBIDE

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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

To cite this version:

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�

JOURNAL DE PHYSIQUE Collaque C 1, supplkmet~t at1 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, ^{8 9} et NpCo, ^{8 2} 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~on to 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 mono- carbide.

1 . Introduction. - Monocarbides of neptunium and plutonium with the NaCI-type structure exist only in the carbon-deficient substoichiometric range.

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 diffrac- tion and, in addition, the 59.6 keV transition of 237Np is 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 measu- rements were obtained from a single arc-melted button to minimize errors caused by uncertainties of stoi- chiometry and impurity content.

2. Experimental. - Five samples were prepared : one NPC,.,~, one NpC,,,,. and three NpC,,,,.

The composition of the samples was later confirmcd by chemical analysis and X-ray diffraction.

Magnetic susceptibility and magnetization measu- rements were made for all fivc samples by the method reported previously [2]. Mossbauer measurements were performed utilizing the 59.6 keV gamma ray of 237Np obtained from the alpha decay of 241Am in the Th-Am alloy. The method and apparatus has been described by Kalvius [6]. Mossbauer spectra were obtained at 4.2 O K for three <NpC> [7] samples, each with a fixed amount of 237Np isotope, but with different carbon concentrations. One of the NpC,.,, samples was used for the neutron diffraction study.

(1) This work was performcd under the auspices of the United

States Atomic Energy Commission.

3. Results. - The magnetic susceptibility versus temperature curves above 220 O K for the <NpC>

are shown in figure 1. Since the three NpC,.,, samples

Fro. 1. - Magnetic susceptibility versus temperature curves

for the < NpC > compounds.

o

have similar temperature dependence, only one curve is shown. A maximum in the susceptibility (;o versus temperature (T) curve is clearly evident for the NpC,.,, and NpC,.,, samples. The temperature dependence of magnetization for these compounds is shown in figure 2. The curves were obtained after the samples had been cooled from 150 to 4.2 OK in a zero external magnetic field and measured at a magnetic field strength of 10.7 kOe. The difference in the magneti- zation between the three compounds in the ttmpe-

- -

( I I I I I I I I I I I I I ~ I I I I I I

225 250 275 300

350

TEMPERATURE (OK)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711326

C 1 - 9 1 8 D. J. LAM, M. H. MUELLER, A. P. PAULIKAS A N D G. H. LANDER

The hyperfine spectrum obtained at 4.2 OK for

<NpC> 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 ( O K )

FIG. 2. - Magnetization as a function of tem~erature for the

< NpC > compounds measured in an external magnetic field of 10 700 Oe.

rature range from 100 to 200OK is probably due to the inability to saturate the samples with the 10.7 kOe 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 a sample of NpC,.,, from 150 to 4.2 O K in an external magnetic field of 10.7 kOe completely eliminated the sharp drop in magnetization. When the sample was cooled in an external field of 1.10 kOe, the magne- tization measured at 4.2 OK is only slightly smaller (- 5 a/,) than the 150 OK value.

The neutron-diffraction results indicate that NPC,.~, begins to order antiferromagnetically (Type I) at about 260 OK, the magnetic moment per Np is about 0.5 ^p, a t 2400K, and becomes ferromagnetically ordered below 2300K. Cooling in a zero external field, however, does not alter the magnetic ordering between 150 and 4.2 OK. A ferromagnetic moment of 2.2 f 0 . 5 ~ ~ per neptunium atom is observed at both temperatures.

CARBON CONCENTRATION

FIG. 3. - The ratio of the integrated intensities of the two hyperfine spectra in < NpC > as a function of carbon concen- tration. The squares indicate the experimental results, and the

circles are calculated ratios.

between neptunium atoms with one and zero vacancies as nearest neighbors, based on a random distribution of vacancies in the carbon sublattice, are also shown in figure 3. Assuming that the hyperfine spectra are due to Np atoms with one and zero nearest-neighbor vacancies, and that the recoiless fraction is the same at both sites, figure 3 shows that the distribution of vacancies at the carbon site is not random.

4. Discussion. - The results are summarized in Table I.. The isomer shifts can be considered to be identical for all three alloys within experimental uncertainty. The hyperfine field is relatively constant and the difference in hyperfine field between the two neptunium sites in each alloy is about 13 %.

Lallement et al. [I] suggested that the vacancies TABLE 1

Experimental results obtained for neptunium monocarbide alloys

Composition N ~ ~ o . 9 6 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

,ff (kOe) ( j0; t:; ^{4 420} 3 820 ^f f 50 ^{50 4 350} 3 710 ⁺ f ⁵⁰ 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 atoms and decrease their magnetic moments to practi- cally zero. If this mechanism is operative in <NpC>, then one of the neptunium sites in the carbide should be nonmagnetic. Instead, both sites are strongly magnetic. In addition, the average magnetic moment of 2.2 + 0.5 ,u,/Np atom for the NpC,.,, alloy, deduced from the neutron-diffraction experiment, is close to the value of 2.1 + 0.1 pB/Np atom for a NpC,.,, sample previously studied [4]. These results show that the vacancies have little effect on the value of the magnetic moment. However, the effect of carbon concentration on the magnetic ordering of

<NpC> is unexpectedly complicated. The Ntel

temperature of <NpC> depends strongly on the carbon concentration of the sample, whereas the Curie temperature is independent of carbon concentration. The drastic decrease in magnetization of NpC,.,, and NpC,.,, may be explained by assuming that the spins are aligned antiparallel below 100 OK, resulting in a small net magnetization at 4.2 OK. However, neutron-diffraction experiments performed on the same samples showed no change of magnetic ordering between 150 and 4.2 OK.

5. Acknowledgements. ^- The authors greatly appreciate the able technical assistance of Mr. A. W.

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.