Neutron diffraction study of NpAs single crystal

Journal of the Less-Common

Metals, 121 (1986)

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NEUTRON DIFFRACTION STUDY OF NpAs SINGLE CRYSTAL* P. BURLETt,

S. QUEZEL, M. KUZNIETZ tf, D. BONNISSEAU and J. ROSSAT-MIGNOD

Centre d%tudes Nucleaires, DRF/SPh

- MDN, 85 X, 38041 Grenoble Cddex (France)

J.-C. SPIRLET and J. REBIZANT European Institute for Transuranium Elements, Postfach 2266, D-7500 Karlsruhe (F.R.G.) 0. VOGT Laboratorium fiir FestkGrperphysik, CH-8093 Ziirich (Switzerland)

Eidgeniissische

Technische

Hochschule

Ziirich,

summary A single crystal of NpAs has been studied by neutron diffraction. In zero applied magnetic field NpAs orders at TN = 173 K with an incommensurate phase characterized by a value k = 0.232 f 0.003 of the wavevector k = [0012] at TN, varying to 12= 0.236 + 0.003 at T = 155 K. At T,, = 154 K a first-order locking transition occurs to the commensurate (4+, 4-) phase with h x l/4. At To = 138 K another first-order transition leads to the type-1 phase with Iz = 1, persisting down to low temperatures. From measurements in a magnetic field applied along the [ liO] direction we find that the incommensurate and (4+, 4-) phases have a collinear, single-k structure while the type-1 phase has a triple-k structure. These results are compatible with a tetragonal distortion of the cubic structure at TN and a return to a cubic symmetry at To. Preliminary results on the magnetic field behaviour are also reported.

1. Introduction Neptunium monoarsenide NpAs, with the NaCl-type crystallographic structure (a = 5.84 A), was previously found by magnetization, Miissbauer effect, and neutron diffraction measurements on powder samples to order antiferromagnetically at TN = 175 K [ 11. X-ray diffraction measurements showed a tetragonal distortion of the cubic lattice occurring at TN down to T = 142 K, where NpAs returned to a cubic symmetry [l, 21. In the tetragonal region the powder neutron diffraction indicated a transition at TN to *Paper presented at Actinides 85, Aix en Provence, September 2 - 6, 1985. +Centre National de la Recherche Scientifique. -On leave from Nuclear Research Centre - Negev, P.O. Box 9001, 84 190 Beer-Sheva, Israel. 0022-5088/86/$3.50

@ Elsevier Sequoia/Printed in The Netherlands

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a commensurate structure, consisting of a stacking of ferromagnetic (001) planes according to a (4+, 4-) sequence (k = [ OO;] , m#). In the temperature range from about 160 K down to about 142 K, still in the tetragonal region, the (4+, 4-) phase was observed to coexist with another commensurate phase, i.e. the type-1 ordering, consisting of ferromagnetic (001) planes coupled antiferromagnetically (+-+-) (k = [OOl], mkllk). At the return to cubic symmetry (T = 142 K) only the type-1 ordering existed down to low temperatures. The ordered magnetic moment was found to be about 2.5 pg at 4.2 K and to vary smoothly through all the transitions up to TN. More recent magnetization measurements on polycrystalline NpAs showed antiferromagnetic ordering at TN = 173 K with subsequent transitions at T = 157 K and T = 142 K, the latter being of first order [3]. The tetragonal distortion in NpAs is in agreement with the antiferromagnetic (4+, 4-) structure which is characterized by a tetragonal symmetry. The return to cubic symmetry is difficult to understand with the collinear description of the low temperature type-1 structure; however, it would be compatible with a triple-k nature of this structure [4]. In such a case the magnetic structure involves all the members of the star of the wavevector: kl = [loo], kz = [OlO], k3 = [OOl], and the cubic symmetry is retained. In such a multiaxial structure the magnetic moments are along all (111) directions of the cubic cell. Neutron diffraction measurements on powder samples cannot distinguish between the collinear, single-k ordering and the triple-k ordering. Magnetization measurements on NpAs single crystals have recently been performed [5], but these cannot give clear evidence for the change in the easy axis. Only neutron diffraction measurements on a single crystal of NpAs, submitted to an applied magnetic field large enough to move the magnetic domains, can prove the assumption of a triple-k ordering. 2. Experimental details We have investigated a single crystal of NpAs (mass, 21 mg), prepared at the European Institute for Transuranium Elements in Karlsruhe and encapsulated in an aluminium can for neutron diffraction. The measurements have been made with the DN3 double-axis diffractometer in the Silo& reactor at the Centre d’Etudes Nucleaires de Grenoble. The crystal was aligned with a vertical [IlO] axis. The measurements were made in a regular cryostat for the experiment in zero applied magnetic field (H = 0), and in a cryomagnet assembly for the experiment in which the vertical field was applied along the [ liO] direction of the crystal. 3. Results Reciprocal-space scans have been performed in zero applied magnetic field along cubic directions (e.g. from [llO] to [ill]) at decreasing tem-

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peratures. We have found that NpAs orders at TN = 173 K with an incommensurate phase characterized by a value 12= 0.232 * 0.003 of the wavevector k = [OOk] at TN, varying to Iz = 0.236 f 0.003 at T - 155 K. The absence of third-order harmonics at this temperature is indicative of a longitudinal sine-wave modulation along the cubic axes, i.e. with a modulation of the magnetic moments. NpAs undergoes a first-order locking transition at T,, = 154 K to the commensurate (4+, 4-) phase, with k = 0.247 f 0.003 = a. Some third-order harmonics (with k = $ ) develop but with reduced intensities, indicating partial squaring-up only (1(3k)/l(k) is observed to be only half or less the value in the fully squared-up case). NpAs undergoes another first-order transition at T,, = 138 K to the commensurate type-1 phase (k = l), which persists down to low temperatures. The overall temperature dependence of the k value in NpAs is shown in Fig. 1.

f-

1.00 a990

I Tc $ 0250 p3 j : TN / f++i / / /

2 0.240 0.230 a2200

type-1 (*-) 1 , 50 100

-

i4*piInc lPara / i i 150 200

Temperature (K)

Fig. 1. Temperature dependence of the k value of the wavevector k = [OOk] (in reciprocal lattice units) in NpAa in zero applied magnetic field. All magnetic phases observed are indicated.

In all three phases of NpAs the magnetic peaks corresponding to the three equivalent k-vectors are observed with equal intensities, a situation compatible with a (single-domain) triple-k ordering as well as with a multidomain collinear single-k ordering. To differentiate between these two possibilities we cooled the crystal in a magnetic field of 6 kOe parallel to the [ liO] direction. Such an alignment has been chosen since only a field along (110) direction can unambiguously differentiate between single-k, double-k or triple& ordering. We have limited the field value to 6 kOe because in higher fields ferrimagnetic field-induced phases are reached at high temperatures [5]. The intensities of the three magnetic peaks corresponding to the three possible wavevectors kl, k2, k3 have been followed on cooling the crystal through TN. In the incommensurate phase the intensity of the [ 1,1,1 - k] reflection, belonging to k3, is much higher than that of the [l, 1 - k, l] reflection, belonging to k2, while the intensity of the [ 1 - 12, 1, l] reflection, belonging to kl, is almost zero. This behaviour confirms the collinear nature of the incommensurate phase. In the (4+, 4-) phase the zero intensity

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of the [ 1,1 - k, l] and [ 1 - k, 1, l] reflections again confirms the collinear single-k nature of the ordering, Below To = 138 K the reflections corresponding to k,, kz, k3 ([Oil], [ 1011, [ llO], respectively) are observed with equal intensities. The behaviour of the domain population in the vicinity of Z’,, is shown in Fig. 2. The abrupt change from the monodomain, high-temperature state to an equipartition of domains is unlikely, and it is therefore clear that in the low temperature range, with the type-1 phase, the ordering is indeed triple-k. The change of the nature of the ordering at T, is accompanied by a jump of the ordered magnetic moment of the neptunium by about lo%, from (1.6 rt 0.2) &$ at T = 141 K to (1.8 + 0.2) pg at T = 13’7 K. Such a jump could not be seen in the powder measurements [ 11. We deduce from our measurements at T = 4.2 K an orderedmagnetic moment ~~(4.2 K) = (2.2 C 0.2) pa, which is lower than the powder vafue of about 2.5 &g [l]. I

1

NpAs

Temperature

I

I,

I

El1 CliOlz6kOe

(K1

Fig. 2. Temperature

dependence of the intensity (in relative units) of the magnetic peaks in the vicinity of the transition from the (4+, [l,l,l--kl, [1,1--k,ll, [I-k,l,ll 4-) phase (k = i) to the type-1 phase (k = 1) in NpAs in a magnetic field of 6 kOe applied along the [ l?O] direction. The single-k-triple-k transition can clearly be seen.

We have carried out preliminary measurements to study the magnetic field behaviour of NpAs. The field variation (with inncreasingfield) of the [ 1101 reflection at T = 4.2 K and of the [loll and [ 0111 reflections at 2’ = 100 K show no change in the magnetic ordering up to a critical field of about 60 kOe. At this critical field the [llO] intensity drops slightly (by about 20%) whereas the ]lOl] and [Oil] reflections vanish. From the measurement of the [Ill] reflection above the critical field a net magnetization of (1.3 + 0.2) &g at T = 4.2 K is deduced. We conclude that the triple-k type-1 structure is stable up to the critical field, above which some ferrimagnetic phase is reached. These experiments are not sufficient to tell precisely the nature of this phase, as the two Fourier components observed: type-1 antiferromagnetic (k = [OOlf and rnk = (1.0 Z!L 0.2) pB at 4.2 K) and ferromagnetic (k = 0 and m, = (1.3 + 0.2) pg at 4.2 K), cannot, when combined, give the neptunium moment in any realistic model. More experiments are needed to clarify the nature of the high-field phase. However, the preliminary results are

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in good agreement with the magnetization measurements (51, which for a field parallel to the [llO J direction give a critical field of 62 kOe and a ferromagnetic component of 1.4 j&gper neptunium atom.

4. Discussion Our neutron diffraction study of a NpAs single crystal allows the precise determination of the magnetic phases encountered in zero applied magnetic field. At TN = 173 K an incommensurate phase (k = + - E(T)) develops corresponding to a sinusoidal modulation of the magnetic moments of the ferromagnetic (001) planes. At Tic = 154 K a commensurate phase (k = [OO$]) appears, corresponding to a (4+, 4-) sequence of fe~oma~etic (001) planes, followed at T,, = 138 K by the type-1 antiferromagnetic phase. The collinear nature of the incommensurate and the (4+, 4-) phases, as well as the triple& nature of the type-1 phase is proved, in good agreement with the tetragonal distortion at TN and the return to the cubic symmetry at To. The magnetic behaviour of NpAs is quite similar to that of the uranium monopnictides. Indeed USb orders in a triple-k type-1 structure at any temperature below TN [63 ; UAs and UP order at TN first in a single-k type-1 structure and undergo at To first-order transitions to the double-k type-IA [ 71 and type-1 [8] respectively. The similarity is even stronger with some UAs, -XSeX solid solutions in which incommensurate structures are observed at high temperatures transforming to triple-k commensurate (type-IA) structures at low temperatures [ 91. The multi-k structure cannot be accounted for with only bilinear exchange interaction, and higher terms must be taken into account. Furthermore, single-ion-anisotropy terms are not in themselves able to account for the observed behaviour, since they cannot explain the occurrence of single-k ordering at TN and a transition to a multi-k ordering at lower temperature. Higher order anisotropic exchange interactions must be considered. Theoretical efforts are undertaken [ 101 to understand these quite unusual magnetic properties on the basis of the interaction resulting from the mixing off electrons with band electrons.

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330 5 J. Rebizant, J.-C. Spirlet, K. Mattenberger and 0. Vogt, in J. Schoenes (ed.), Proc. 148mes Journkes des Actinides, Davos, April 2 - 3, 1984, Laboratorium fiir FestkSrperphysik, ETH Zurich, 1984, pp. 17 - 23. 6 J. Rossat-Mignod, P. Burlet, S. Quezel and 0. Vogt, Physica B, 102 (1980) 237. 7 J. Rossat-Mignod, P. Burlet, H. Bartholin, R. Tchapoutian, 0. Vogt, C. Vettier and R. Lagnier, Physica B, 102 (1980) 177. J. Rossat-Mignod, P. Burlet, S. Quezel, 0. Vogt and H. Bartholin, in R. P. Guertin, W. Suski and Z. Zoi’nierek (eds.), Crystalline Electric Field Effects in f-Electron Magnetism (Proc. 4th Znt. Conf. Wrocibw, Poland, 1981), Plenum, New York, 1982, pp. 501- 517. 8 P. Burlet, S. Quezel, J. Rossat-Mignod and R. Horyn, Solid State Commun., 55 (1985) 1057. Met., 121 9 M. Kuznietz, P. Burlet, J. Rossat-Mignod and 0. Vogt, J. Less-Common (1986) 217 - 221. 10 B. R. Cooper, R. Siemann, D. Yang, P. Thayamballi and A. Banerjea, in A. J. Freeman and G. H. Lander (eds.), Handbook on the Physics and Chemistry of the Actinides, Vol. 2, North-Holland, Amsterdam, 1985, pp. 435 - 500.