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Neutron diffraction with polarization analysis was performed on a single crystal of CuO, oriented with its alpha * and c* axes in the diffraction plane. It was verified that in the commensurate phase I(0-212.7 K; k=(1/2, 0, -1/2)), the magnetic moments were indeed along b*, but it was found that in the incommensurate phase II (212.7-232.5 K; k=(0.506, 0, -0.483)) there also exists a component of the magnetic moment along b* in disagreement with an earlier study by Forsyth et al. (1988). The data are in agreement with the very recent results of Brown et al. (1991). The authors propose a helimagnetic arrangement in the incommensurate phase II, with the magnetic moments rotating in the plane (b*; 0.506 alpha *+1.517c*). They present a short account of their experimental results, a study of the stability of the magnetic structure in phase II and the symmetry analysis of the possible magnetic space groups.

No abstract available

The paper deals with three topics: i) the use of complex representations and base vectors in primitive cells, illustrated here on the example of magnetic structures in rhombohedral space groups; ii) the symmetry lowering device which, connected with representation analysis, allows to deal with cases where the irreducible representations of the paramagnetic space group G do not describe properly the magnetic structure; cases considered are Tb3Fe5O12 and Mn3GaC (first-order transitions). iii) a modified projection operator formula which does not contain inverse operations on the site vector r. The formula was successfully applied to the preceding topics which were inspired by a recent paper of Izyumov and Naish.

Magnetic properties of the compound SrTb2Fe2O7 (space group P4 2 mnm ) have been investigated by magnetic measurements of susceptibility and magnetization and by neutron diffraction. The Neel temperature of the iron sublattice is 628 K. At 295 K the configuration of the Fe3+ spins is Gx− ± Ay− (antiferromagnetic). At 4.2 K the configuration of Tb3+ spins is represented by the modes Fx+, Cy+, Az− coupled in the representation Eg with the mode Az− of the Fe3+ spins. The values of the magnetic moments corresponding to the different modes are obtained from the magnetic structure refinement. The value of the Fx+ ferromagnetic mode of Tb3+ is 2.6 μB/ion at 4.2 K. At room temperature the orientation of Fe3+ spins perpendicularly to the c axis seems to be due to a site anisotropy, the easy direction coinciding with the axis of the FeO5 bipyramids. The reorientation of Fe3+ spins along the c axis observed at 4.2 K can be explained by a strong Fe3+Tb3+ coupling which overcomes the Fe3+ one ion anistropy. The low site symmetry (m) of Tb lifts the degeneracy completely and gives rise to a pseudodoublet (formed by two singlets of small separation) associated with an Ising-like behavior. In fact the Tb3+ spins are in the m planes, containing the c axis.

The vocable of superlattice means to physicists and chemists a structure closely related to a parent structure and having cell dimensions superior to that of the parent structure. The superlattice belongs to a subgroup of the parent structure because increasing cell dimensions means a loss of translational symmetry. Space groups which only differ by their translational symmetry are called isomorphic and are listed in the new International Tables for Crystallography. The theoretical part of the paper establishes the most general similarity matrix S which relates the well known CaF 2 structure to its rhombohedral superlattices. Two versions are given. S R and S H , according to the so called rhombohedral and hexagonal descriptions of rhombohedral structures. The determinant of the similarity matrix is directly related to the chemical formula, for instance |S|=19 in Ca 13 Y 6 F 44 and |S|=14 in Pb 8 Y 6 F 32 O where the cation numbers are respectively 19 and 14. Cell and coordinate relations are established between the superlattice and the CaF 2 structure. In the experimental part we specify the cation order in Pb 8 Y 6 F 32 O and Na−Zr 6 F 31

Fe2SiS4(a = 12,407 A, b = 7,198 A, c = 5,812 A, space group Pnma) has the olivine structure; the ferrous ions are located in two kinds of sites: one half in planes of mirror symmetry m, the other half in centers of symmetry. 1. The magnetic study of this compound by means of magnetization measurements and Mossbauer effect, indicates from 127 K an antiferromagnetic arrangement along oy for the m sites, and an induced partial order of the same kind for the 1 sites. At 33 K, the Fe2+ spins in 1 sites are completely ordered and at the same time takes place a rearrangement of the magnetic structure. The observed complex model is analogous to that of Fe2GeS4, i.e. antiferromagnetic along 0x and ferrimagnetic along Oz.

Series expansions in Fourier space are proposed for potentials, fields and field gradients. The lattice potential is shown to be harmonic on lattice points. A correction proposed for potential and field calculations by Weenk and Harwig has no serious basis. The oxygen lattice contribution to the field gradient on the Fe-site in α-Fe 2 O 3 turns out to be five times the contribution calculated with nearest neighbors only.

Similarity operators are applied to the study of displacive transitions. The example considered is the transition from the hexagonal NiAs to the orthorhombic MnP type. The number of displacement modes is superior to that deduced from representation theory of the wave vector group.

Similarity operators are shown to relate the lattice vectors and origins of the structure and the superstructure, the symmetry operators of the space group G of the structure to those of the subgroup H of G of the superstructure, and finally those atomic positions which are conserved in the superstructure. In the monoclinic and orthorhombic fluorite-related structures of composition K0.5−xLn0.5+xF2+2x (Ln = lanthanide and Y) considered here we find that the cation sublattice is conserved. The origin of the superstructure does not necessarily coincide with the origin of the structure, but is a point of high site symmetry.

In this study of metallic perovskites Mn 3 MX (M=metal, X=C, N) we emphasize the influence of the (outer) electron concentration on the sequences of crystallographic transitions, on the magnetic ordering temperatures and on the constant (Pauli-type like) part of the magnetic susceptibility. A great variety of observed magnetic structures and transitions (including spin rotations) are described for carbides and nitrides. The Labbe-Jardin tight binding model which invokes a p-d II hybridization between N and Mn orbitals having their energy levels near to the Fermi surface provides the possibility of great variations of the density of states versus temperature and is in qualitative agreement with experiment. It also explains that small substitutions on X have a more drastical effect than those can M.

Resume La structure magnetique de l'acetate de manganese Mn(CH3COO)2, 4H2O a ete resolue par diffraction des neutrons. L'acetate de manganese cristallise dans le groupe spatial P2 1 c avec Z = 6. Les atomes de manganese (en position 2a et 4e) sont dans des plans (100)′. En dessous de TN = 3.18 K ce compose est antiferromagnetique en champ applique nul. Le vecteur de propagation est k = [ 1 2 00] et les plans (100) sont ferrimagnetiques. Le groupe magnetique est P 2a 2 1 C .

The solid solutions Cr 1−x V x N show at low temperatures a progressive establishment of an orthorhombic distorsion and an antiferromagnetic order (of the fourth kind) for x 1 3 accompagnied by a (cubic) fraction which remains disordered at the lowest temperatures : No ordering occurs for x > 1 3 . Assuming a Gaussian distribution of local concentrations in otherwise homogeneous cells (cellular model), the mean magnetic moment, measured by neutron diffraction can be related to a constant moment of Cr provided the local vanadium concentration is smaller than 0.26. The cell size contains 20 metal atoms.

In an attempt to determine the magnetic structures of the heavy rare earth manganites of perovskite type, we have studied first the antiferromagnetic order of manganese in YMnO 3 . The Neel temperature is about 42 K, the Mn 3+ ordering is a helix and derives from an A mode. The propagation vector of the helical structure is along the b axis: k=[0 k y 0] with k y = 0.0786. The Mn 3+ ions carry a magnetic moment of only 3.10 ± 0.1 μ B at 4.2 K. We present a phase diagram of helical and collinear modes in terms of exchange integrals.

Resume Dans le formiate de manganese dihydrate, en-dessous de fN = 3,7°K les spins des ions A en 0 0 0 et 0 1 2 1 2 s'ordonnent antiferromagnetiquement tandis que les ions B en 1 2 0 1 2 et 1 2 1 2 0 ne presentent pas d'aimantation au-dessus de 1,5°K. Des experiences de diffraction des neutrons entre 1,5°K et 0,4°K montrent que dans ce domaine de temperature les spins des ions B ‘suivent’ antiferromagnetiquement ceux des ions A. Les ions B se comportent comme un reseau d'ions paramagnetiques soumis a un champ proportionnel a l'aimantation des ions A. Ce champ est d'origine dipolaire.

Above a temperature ΘT the perovskite type compounds ZnMn3C and GaMn3C0.935 are ferromagnetic as GaMn3C studied previously. Below ΘT the magnetic structures of ZnMn3C and GaMn3C0.935 are tetragonal noncollinear, with a ferromagnetic component along the tetragonal Oz axis and an antiferromagnetic component in the Oxy plane while GaMn3C is a collinear antiferromagnet below ΘT. The two compounds ZnMn3C and GaMn3C0.935 show with respect to GaMn3C the common feature of a reduction of the number of electrons, but obtained in substantially different ways, by the substitution Ga → Zn in ZnMn3C, by the production of vacancies in GaMn3C0.935. By NMR we have observed resonances on 69Ga and 71Ga which can be attributed to neighborhoods of zero, one and two C-vacancies in GaMn3C0.935 resonances on 67Zn in ZnMn3C and resonances on 55Mn. There is a good agreement between the symmetry changes observed by neutron diffraction and NMR, between the thermal variation of the ferromagnetic components, determined by the same techniques and by magnetic measurements.

The spin rotation, mainly due to the thermal variation of the anisotropy constants K1 and K2, and the thermally induced poly domain structure in hexagonal cobalt is directly checked by neutron diffraction.

Resume Dans Mn(DCOO) 2 , 2D 2 O isomorphe de Mn(HCOO) 2 , 2H 2 O, les spins des atomes Mn en 000 et 0 1/2 1/2 s'ordonnent antiferromagnetiquement a T N = 3,7°K selon l'axe binaire b . A T 0 = 1,7°K les spins tournent pour se trouver dans le plan ac , l'angle o avec a etant de 63° ± 5°. La transition a 1,7°K est du type ‘transition d'anisotropie’. Elle est de premier ordre. L'origine de l'anisotropie n'est pas dipolaire. Les spins des atomes Mn en 1/2 0 1/2 et 1/2 1/2 0 restent encore paramagnetiques.

Resume La structure magnetique de Mn3GaC, dont la temperature de transition est Tt = 164°K, est colineaire et ressemble a celle de MnO. Elle peut etre decrite par une succession de plans ferromagnetique a spins alternant dans la direction [111], axe d'antiferromagnetisme. Au-dessus de Tt, la phase devient ferromagnetique dans une transition de premier ordre, avec une temperature de Curie Tc = 248°K. Les moments magnetiques deduits de l'etude par diffraction neutronique sont: M = 1,8 ± 0,1μB dans l'etat antiferromagnetique a 4,2°K, M = 1,2 ± 0,1μB dans l'etat ferromagnetique a 170°K, et M =1,3μB par mesure magnetique extrapolee a 0°K.

Canting of the rare earth moments at low temperature for DyIG, ErIG and YbIG is evidenced. The canting angles and the moments are refined and compared with theoretical predictions.

Resume La structure du type perovskite Mn3GaN montre un ordre antiferromagnetique des spins de manganese en trois sousreseaux: S1 en 0 1 2 1 2 selon [0 1 1 ], S2 en 1 2 0 1 2 selon [ 1 0 1], S3 en 1 2 1 2 0 selon [1 1 0]. S est de 1, 17 a 4, 2°K. L'axe de difficile aimantation est Ox pour S1, Oy pour S2, Oz pour S3. Les integrales d'echange sont: J1 negative, J2 positive. La structure magnetique appartient a la representation tridimensionnelle γ5g du groupe cubique.

The solid solutions MnxCr1−x S show a considerable magnetic short range order above TN. The analysis of the diffused intensity for the composition x = 0.33 allows the estimation of spin correlations and of exchange integrals J1 ⋍ -4.5°K, J2 ⋍ + 6°K which are compatible with the observation below TN of a long range order of the first kind, i.e. with the positive second neighbour interactions.

Resume Les configurations magnetiques sont directement reliees aux vecteurs propres d'une matrice. Tout reseau de Bravais j , occupe par des ions magnetiques fournit a l'energie d'echange une contribution − 2λ j . Les parametres scalaires λ j sont invariants dans les operations de symetrie du groupe cristallographique. Lorsque les ions magnetiques n'occupent qu'un seul reseau de Bravais ( j = 1), λ 1 s'identifie avec la transformee de Fourier de l'integrale d'echange et la methode matricielle se reduit a celle de Villain. Dans le cas d'un ensemble { A } de n atomes equivalents, la recherche des λ j est ramenee a celle des valeurs propres d'une matrice. Quand la maille chimique est conservee, les vecteurs propres qui decrivent les configurations des spins forment la base de representations irreductibles ce qui relie la methode matricielle a celle de D zyaloshinsky et T urov . Un mode trivial est le mode ferromagnetique correspondant a la representation identite. Dans les sytemes couples { AB } les vecteurs propres du probleme matriciel sont des combinaisons lineaires des vecteurs propres de { A } et de { B }. Le ferrimagnetisme de Neel apparait comme le resultat du couplage de deux modes “triviaux” ferromagnetiques de { A } et de { B }. La stabilite des solutions s'exprime par des conditions d'inegalite entre integrales d'echange. L'etude se limite a la determination de modes fondamentaux, c'est-a-dire de modes complets a un seul vecteur de propagation. Les applications suivantes sont etudiees : (a) le cube centre (exemple d'un reseau de Bravais) (b) les structures apparentees au corindon telles que Fe 2 O 3 -α, Cr 2 O 3 , les ilmenites (exemple de n reseaux de Bravais equivalents) (c) les spinelles cubiques et (d) les grenats (exemples de systemes couples).

Resume Une methode simple, basee sur l'hypothese de voisinages equivalents permet l'enumeration de plus de modes de spins que les methodes de G ersch et K oehler et de Fourier-Villain.

Resume Nous avons determine par diffraction de rayons-X, entre 5 et 360K, les variations des parametres a, b et c de la maille. La variation thermique de l'aimantation a ete precisee par diffraction neutronique. Grâce a ces mesures, nous avons calcule dTN/dP = 0, 73. 10-9 degre cm2 dyne-1, en bon accord avec les mesures experimentales directes. Le modele de Bean et Rodbell est generalise par l'introduction d'une energie anisotrope de deformation, laquelle permet la justification theorique de la nature de premier ordre de la transition a TN

Resume Une interpretation des courbes de susceptibilite magnetoelectrique, en fonction de la temperature, de LiMnPO 4 est faite a l'aide des theories de l'influence du champ inducteur sur l'energie d'anisotropie de l'ion metallique, sur l'interaction d'echange et sur le facteur de Lande. L'introduction de termes biquadratiques est egalement envisagee.

Resume Nous avons etudie la structure magnetique de TbO2. Le terbium a dans ce compose la structure electronique du gadolinium 3+ (L = O, S = 7 2 ) TbO2 est antiferromagnetique, avec une temperature de Neel de 3K. Les moments sont repartis dans le plan (111) selon une direction indeterminee. Le terbium est porteur d'un moment magnetique de 6.25 ± 0.10 microB a 1.5K.

In the frame of a given crystallographic symmetry a matrix ζ(k) is constructed, the eigenvectors and eigenvalues of which are directly related to the spin configuration and magnetic exchange energy, respectively. For Bravais lattices the matrix equation reduces to Villain’s equation.

Resume La structure magnetique, observee a 4, 2°K de BiMn2O5 (groupe Pbam) correspond a un vecteur de propagation k = [ 1 2 0 1 2 ] . Les representations irreductibles associees a k dans le groupe Pbam permettent de deduire tous les couplages magnetiques possibles et de proposer des modeles de structure verifies par le calcul.

Resume Dans ErCrO3 les spins de Cr s'ordonnent antiferromagnetiquement a TN1 = 133° K dans un arrangement GX appartenant a la representation Γ4 et restent selon Ox jusqu'a 20° K au moins. A TN2 = 16, 8δ K Er s'ordonne dans un arrangement CZ appartenant a Γ1, representation identite. En meme temps le spin de Cr tourne de 55° dans le plan Oxy par couplage avec Γ1. L'application d'un champ magnetique selon Oz fait disparaitre la composante selon Oy des spins de Cr et apparaitre un metamagnetisme de Er. NdCrO3 a une structure magnetique voisine. A 4,2° K les moments de Er et Nd sont de 5,2 et 1,3μB respectivement.

It is shown by neutron diffraction and Mossbauer spectrometry that the formation of a long range magnetic order in zinc ferrite depends sensitivily on the content of divalent iron. The experimental data favour a non collinear model of the magnetic structure. Zinc ferrite is a normal spinel with the tetrahedral A sites occupied by Zn2+ ions and the octahedral B sites occupied by Fe3+ ions. Any magnetic order therefore is due to the nearest or more distant B-B interactions of the Fe3' ions. Measurements of the susceptibility [I 1: of the specific heat [Z], studies by neutror diffraction [3] and Mossbauer spectrometry [4] have shown that below 10 OK an antiferromagnetic order appears. The neutron diffraction patterns of J. M. Hastings and L. M. Corliss [3] and recently also one of M. K. Fayek et al. [5] have only shown broad magnetic lines, typical for short range order. From these experiments there was some doubt that a long range order can exist in ZnFez04 [6]. We show first that the formation of the magnetic order depends very sensitively on the stoichiometry of the sample, especially from the Fez+ content. Figure 1 (left) shows Mossbauer spectra of a sample with a content of 0.002 Fez+ per molecule. We observe well shaped Zeeman spectra, indicating a long range FIG. 1. Mossbauer spectra of two samples of ZnFezO4 with different Fez+-content. 57Co source embedded in Cu at room temperature. (*) Zentralinstitut fiir Forschung und Entwickiung der Fried. Krupp GmbH, Essen. (**) CNRS and CEN-G., Grenoble. (***) Laboratoire de Spectromktrie Physique, Grenoble. (****) Laboratoire Central des TkICcommunications, Paris. magnetic order up to temperatures just below the Nee1 point. Figure 1 (right) also shows Mossbauer spectra of ZnFez04 with a content of 0.01 Fe2+ per molecule. In this sample line broadenings occur and at 10 OK we observe a superparamagnetic behaviour. This is in agreement with neutron diffraction results (Fig. 2). The sample with small ~ e ' + content shows FIG. 2. Liquid helium temperature neutron diffraction pattern of two samples of ZnFezO4 with different Fez+-contents. The indices are based on the cubic unit cell (a = 8.43 A). Neutron count rates in arbitrary units. well resolved and strong magnetic peaks, whereas the sample with a content of 0.01 Fez+ shows only broad lines, thus indicating short range order. These observations may explain the published results mentioned above. Recently, B. Boucher et al. [7] independently confirmed our experimental results. Their neutron diffraction pattern of a sample with small Fez+ content agrees very well with our measurements. It may be mentioned that the neutron diffraction patterns of the two samples of figure 2 show no difference at room temperature. The results of our investigation of the sample with low Fe2+ content have been reported in [B]. The Mossbauer measurements have shown a deviation of the Hi(T) vs. T-plot from the Brillouin B (5) behaviour and the occurrence of a quadrupolar shift below the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711105 NEUTRON DIFFRACTION AND M~SSBAUER STUDIES OF ZINC FERRITE C 1 321 Neel point, indicating a non-collinear magnetic structure. The magnetic peaks of the liquid helium temperature neutron diffraction diagram (Fig. 2) can be indexed in a tetragonal cell a = b = 8.43 A and c = 16.86 A, as suggested by M. K. Fayek et al. 151. The selection rule of the magnetic lines h, f h, = 2 n f 1 and h, = 2 n -t 1 indicates a propagation vector of the magnetic mode k = [lo 41. In order to determine the magnetic structure, we have used the theory of magnetic modes as given by representation analysis [9]. If we look .for the symmetry operation of highest order which leaves k invariant modulo a vector of the reciprocal F-centered spinel lattice, we find the rotation inversion 4. By inspection of the irreducible representations of the space groupe F a, we find a non-collinear arrangement as shown in figure 3. Collinear spin arrangements non parallel to Oz would require a spin Hamiltonian of an order higher than two. An essential feature of the model are parallel z-components of groups of four neighbouring iron spins. Our model agrees with the first of the three models given by B. Boucher et al., but they predict zero z-components from the experimental evidence. We obtained the best agreement between observed and calculated magnetic intensities for s, = s,,. An arrangement along the c-axis can be ruled out, but for s, = 0 the R-factor increases only slightly, so that a clear decision between the model s, = s, and s, = 0 cannot be made on the basis of the present experimental data. We observed a low magnetic moment of 4.2 pB, FIG. 3. Magnetic structure of ZnFe204. Open circles have + components and full circles components along z-axis. Arab number indicate the height in eights of the chemical unit cell 001 and -& 5 0 are antitranslations. extrapolated to OOK, which may be explained by covalency effects and partially by competitions of short range order scattering (Fig. 2). Evidence that the electron configuration of Fe3 + changes when crossing the Neel temperature is also given by the observation of a discontinuity of the isomeric shift of 0.03 mm/s. X-ray investigations have shown that any detectable tetragonal distorsion should be smaller than 0.01 A.

Resume La structure magnetique de Tb3Fe5O12 ayant un caractere rhombo-edrique a basse temperature, nous avons cherche et trouve une deformation rhomboedrique qui commence un peu en dessous de 200 K et se sature a 7′30″ vers 16 K. Le groupe magnetique est R 3 c′ . Les composantes a 4,2 K du moment du terbium (8,5 μB), paralleles et perpendiculaires a l'axe [111] sont 7,35 et 4,25μB respectivement.

Resume Alors que l'etude par effet Mossbauer de solutions solides (Mn 2 O 3 ) 1−x (Fe 2 O 3 ) x suggere l'existence de deux transitions magnetiques dans Mn 2 O 3 , l'une vers 80°K, l'autre vers 25°K, la variation thermique des intensites des raies magnetiques (100) (110) et (111) etudiees par diffraction neutronique montre dans Mn 2 O 3 l'existence d'une transition a 90°K et d'une autre vers 50°K (et non pas 25°K).

In Bertaut & Dulac [Acta Cryst. (1972), 28, 580-588], for clerical reasons an error has crept in so that the numerical moment values of y and z components of Cr1 are to be interchanged. One should read in the Abstract and in relation (8.9): Sz(Cr1) = -0.73 (instead of -0.45); sy(Cr1) = 0.45 (instead of + 0.73).

The matrices which transform spin vectors under the operations of a crystallographic group G form a representation Γ of the group. The reduction of Γ gives rise to possible spin configurations and to their irreducible representations. The method is illustrated by the analysis of the magnetic structure, observed by neutron diffraction, of Cr2WO6, a super lattice type of rutile, a noncolinear antiferromagnet with TN = 69°K, θ = - 196°K, S(Cr) = 1.33.

The oxygen parameters of some iron garnets are refined from neutron diffraction diagrams at 400 K. The magnetic moments of the rare earth atoms and of the octahedral and tetrahedral iron atoms are determined at room temperature.

Resume Fe 2 GeS 4 cristallise dans le groupe d'espace Pnma ; les parametres de la maille sont: a = 12,467 A , b = 7,213 A , c = 5,902 A ; sa structure cristalline est isotype de l'olivine. L'etude du magnetisme par la methode d'extraction montre que Fe 2 GeS 4 est paramagnetique jusqu'a 108°K, puis antiferromagnetique jusqu'a 69°K, et enfin antiferro-et ferrimagnetique. La diffraction des neutrons a permis d'etablir les structures magnetiques du compose a 85 et a 4,2°K, et de preciser la modification de l'anisotropie intervenue a 69°K.

Resume UOS quadratique est antiferromagnetique avec une temperature de Neel de T N = 55°K et une temperature paramagnetique Θ p = −51°K. Dans la region antiferromagnetique, il y a une forte anisotopie uniaxiale. Chaque atome d'uranium est porteur d'un moment de 2 μ B environ, dirige selon l'axe c . Les interactions U-O-U sont fortement negatives. La succession des moments selon c est + + − −. La maille magnetique est deux fois la maille chimique selon c . p ]Dans la region paramagnetique, l'inverse de la susceptibilite magnetique en fonction de la temperature possede deux regions rectilignes auxquelles correspondent deux constantes de Curie C 1 = 0,9 (aux basses temperatures) et C 2 = 1,2 (pour T > 450°K).

Resume Nous avons resolu par diffraction neutronique la structure magnetique du formiate de fer Fe (HCOO)2, 2H2O. A toute temperature inferieure a TN = 3,7 δK les ions A (OOO O 1 2 1 2 ) s'ordonnent selon un mode AxFyAz. A basse temperature (O,4°K) les ions B ( 1 2 O 1 2 et 1 2 1 2 O) s'alignent sur les ions A. Jusqu'a des champs de 20 koe aucune reorientation de spins n'a ete observee. Les proprietes du formiate de fer sont comparees a celles du formiate de manganese.

Resume La deformation quadratique observee dans les solutions solides (CuO) χ (NiO) 1−χ n'est pas due a un ordre des cations, mais a un effet Jahn-Teller que devient cooperatif au-dessus d'une concentration critique X>X c ≅ 0,25, meme pour une repartition statistique des cations.

Resume Mn2GeS4 est trouve etre isotype de l'olivine qui appartient au groupe rtho rhombique Pnma - D162h. A 4,2°K, nous observons un ordre antiferromagnetique collineaire correspondant a un couplage de modes Cy des moments de Mn2+ sur les sites 4a et 4c. L'ordre magnetique correspond a la representation identite, c'est-a-dire le groupe de Shubnikov est encore Pnma.

Resume Le FeS tetragone ( a = 3, 768 A , c = 5, 039 A , groupe P 4/ nmm ) a du FeII dans un milieu tetraedrique. Etudie par diffraction de neutrons et par l'effet Mossbauer, il n'indique aucun signe d'ordre magnetique a des temperatures aussi basses que 1, 7°K. Ce comportement, ajoute a l'absence de toute conductibilite metallique, indique une forte co-valente. De la chaleur au-dessus de 160°C transforme, avec une contraction du volume, la variete tetragone instable en une forme hexagonale, d'une facon exothermique.

The series of chromites TCrO 3 (T = rare earth and Y) has been synthetized and investigated magnetically and by neutron diffraction. The Neel temperatures of the chromium spin ordering decrease with decreasing rare earth radius from θ N1 (La) = 282°K to θ N1 (Lu) = 112.4° K. The chromium spin configuration has a strong antiferromagnetic and a weak ferromagnetic component. The latter induces a measurable ferromagnetic polarization on the rare earth which sometimes shows occurrence of compensation temperatures (Gd, Tm). At low temperatures ( 3 .

Resume L'etude par diffraction neutronique des solutions solides Mn x Cr 1− x S au-dessus de la temperature de Neel et l'etude de la structure magnetique de Mg 0.5 Cr 0.5 S permettent d'evaluer les integrales d'echange J 1 entre premiers voisins et J 2 entre seconds voisins relatives aux liaisons Mn-S-Mn, Cr-S-Cr et Mn-S-Cr. Trois types d'approximations ont ete utilises: un modele semi-classique d'ising, la theorie semi-classique de Stanley et Kaplan (serie haute temperature) et l'approximation “random phase” de Lines (fonctions de Green). Le fait remarquable est le caractere fortement positif de l'integrale d'echange J 2 (Cr-S-Cr) a 180° de l'ordre de 15°K.

The Ruderman-Kittel-Kasuya-Yosida interaction between localized magnetic spins on the basis of perfectly free conduction electron is calculated as a function of the Fermi wave vector of the conduction electrons. The expression of the interaction according to us converges rapidly, furthermore is strongly related to the detailed crystal structure. The q-dependence of exchange interaction j (q) between conduction- and localized magnetic electron is also taken into account. The result of the calculation is compared successfully with the change of magnetic structures, with θ p and T c or T N of compounds having the CsCl structure, but less successfully with gadolinium compounds having the NaCl structure. These facts are discussed in conjunction with the difference of the RKKY interaction in the cscl and the NaCl structure.

Resume La mesure de la susceptibilite magnetique de Cr 0,75 V 0,25 N met en evidence trois anomalies respectivement a 30, 90 et 227 K. Des resultats de diffraction de neutrons, de diffraction de rayons X et de mesure de chaleur specifique confirment la presence d'une transition antiferromagnetique vers 227 K. Les deux autres anomalies (a 30 et 90 K) peuvent avoir leur origine dans d'autres phenomenes, par exemple dans l'existence d'impuretes non identifiees dans l'echantillon.