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We have grown high quality superlattices of Dy with nonmagnetic Lu and find that the 2.4% epitaxial compression nearly doubles the ferromagnetic ${\mathit{T}}_{\mathit{c}}$ of Dy with little change in the N\'eel temperature. A helimagnetic phase exists over a narrow temperature range. Below ${\mathit{T}}_{\mathit{c}}$ in superlattices, 300 \AA{} orthorhombic domains form despite epitaxial constraints, each with a magnetostrictive distortion comparable to that of bulk Dy. For the thinnest intervening Lu layers, individual ferromagnetic Dy blocks have parallel alignment; the remaining samples show antiparallel alignment, coherent over many bilayer periods.

Superlattices (SL's) and films of rare-earth elements have been grown by means of molecular-beam epitaxy with both the c-axis and b-axis as the growth direction. Long range order spanning many bilayers is observed for both Er-Y and Dy-Y SL's grown along the c-axis, but not for Dy-Y SL's with b-axis growth. This is explained in terms of the anisotropy of the effective exchange interaction. Suppression of the low-temperature ferromagnetic transition is found in all SL's and in Er films grown on Y. This is analyzed in terms of the changes in magnetoelastic energy imposed by epitaxy. Preliminary results on Er films grown on Lu support this picture. The suppression of ferromagnetism permits the study of modulated phases to lower temperatures and higher magnetic fields, with new intermediate phases appearing that are stabilized by the magnetic field.

The ability to tailor-make crystalline metallic superlattices with selected orientations has opened a new field of magnetism. We review work on rare-earth/yttrium and /lutetium superlattices and films, with emphasis on the effect of epitaxy on the phase diagram and on the propagation of magnetic order through the nonmagnetic component.

The magnetic behavior of single crystals and powders of ${\mathrm{BiPbSr}}_{2}$${\mathrm{MnO}}_{6}$ and ${\mathrm{BiPbCa}}_{2}$${\mathrm{MnO}}_{6}$ has been measured between 300 and 4 K. These analogues of the high-temperature superconducting bismuth cuprates have no structural modulation, and consequently do not have the sharp peak in magnetization found in modulated ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{MnO}}_{6+\mathrm{\ensuremath{\delta}}}$. Instead, their magnetic behavior is consistent with antiferromagnetism (below transition temperatures near 150 K in crystals and 200 K in powders) with the spins mainly oriented normal to the ${\mathrm{MnO}}_{2}$ planes, as confirmed by neutron diffraction. They also show weak ferromagnetism, with a remanent moment in the ${\mathrm{MnO}}_{2}$ planes; this is interpreted as small canting (less than 0.1\ifmmode^\circ\else\textdegree\fi{}) of the spins toward the ${\mathrm{MnO}}_{2}$ planes.

The diffusion coefficient of hydrogen in amorphous silicon deposited at 77 K has been measured by monitoring the decay of the low-angle neutron scattering intensity during annealing of Si:H/Si:D/Si:H/Si:D/... multilayers. Diffusion coefficient values ranging from 1 10-19 cm2/s to 5 10-19 cm2/s have been obtained for annealing temperatures between 250 °C and 300 °C. These very low values confirm recent experiments of Shinar et al. and show that the motion of hydrogen atoms may be impeded by microstructures in amorphous silicon.

Crystalline erbium thin films and Er/Y superlattices with varying Er-layer thicknesses have been grown by molecular-beam epitaxy. The magnetic and structural properties of these samples have been analyzed by x-ray-scattering, bulk-magnetization, and neutron-diffraction techniques. From a comparison of the data for the two systems, the importance of interfacial strain relative to artificial modulation in shaping the magnetic behavior has been determined. Though the basic nature of the erbium magnetic order is not qualitatively altered in either the thin films or superlattices, the conical ferromagnetic phase is suppressed in all of the samples considered. The enhanced critical fields exhibit a systematic dependence on Er-layer thickness. These effects appear to follow directly from the epitaxial basal-plane strain which is measurable in films over 14 000 \AA{} thick. This strain, along with a ``clamping'' of the Er thermal expansion to the Y lattice, leads to a reduction of the magnitude of the magnetoelastic energy that drives the ferromagnetic transition. The dependence of the magnetoelastic energy on the epitaxial strain is described by a model which accounts for the elastic coupling of the erbium lattice to the yttrium.

The spin dynamics of Nd0.6Sr0.4MnO3 have been determined along [qqq], [qq0], and [00q] propagation directions using inelastic neutron scattering. At T=13 K, the low-q Mn–Mn mode energies varied quadratically with wave vector q, but exhibited marked softening on approach to the zone boundaries when compared with a Heisenberg nearest-neighbor exchange calculation. At the larger q’s the modes also show evidence of increasing spin wave energy width. The temperature dependence of the spin wave stiffness parameter D was measured up to Tc=273 K and showed conventional behavior. The 0 K value of D=150 meV A2 yields D(0)/kTc=6.3. No anomalous E=0 central mode intensity was observed on approach to Tc. At low T additional modes were observed and are ascribed to Nd–Mn exchange interactions. These modes have q=0 spin wave gap of 2.1 meV compared to 1.1 meV for the Mn–Mn excitations.

Etude par diffraction de rayons X et de neutrons, par microscopie optique, analyse de la perte de poids, et par mesures d'aimantation et de resistivite. La solubilite de Au est voisine de 10 at.%, en dessous de laquelle tous les echantillons sont thermiquement stables jusqu'a la temperature de decomposition (950 o C). Au se substitue aux sites de la chaine Cu(1) exclusivement et possede un etat de valence probable de Au 3+ . Le principal effet du dopage par Au est une dilatation uniaxiale substantielle du reseau, alors que la structure orthorhombique est preservee. Contrairement a tous les autres dopants connus, sur les sites Cu, Au eleve legerement la valeur de T c

Les spectres de diffraction de neutrons des couches monocristallines de type zinc-Blende revelent l'etablissement d'un ordre antiferromagnetique de type-III (AFM-III) pour x=0.7 et 0.75 a une temperature de Neel precise. La phase (AFM-III) presente un ordre a grande distance, avec des longueurs de correlation d'environ 400 A, et la transition est du second ordre. Leger contraste avec des etudes anterieures des semiconducteurs magnetiques A 1-x III Mn x B VI dilues en volume, dans lesquelles seules les correlations AFM-III d'ordre a courte distance sont observees

Inelastic neutron scattering studies of AII1−xMxBVI diluted magnetic semiconductor (DMS) alloys with low and high concentations of the magnetic component (M) are reviewed. Measurements of inelastic scattering from isolated pairs of magnetic ions makes possible a direct determination of the ion–ion exchange parameters. This method has been used for JNN determination in Zn1−xMnxB (B=S, Se, Te) and Zn1−xCoxB DMS alloy families. Inelastic scattering experiments on DMS alloys with high magnetic ion concentrations (Zn1−xMnxTe and Cd1−xMnxTe with x=0.4–0.7) reveal that at low T the magnetic excitation spectra of these systems are dominated by magnonlike modes despite the absence of long‐range magnetic order. A theoretical description of these excitations can be obtained using the numerical method developed by Ching and Huber. The model results reproduce remarkably well the dispersion and the line‐shape characteristics of the excitation modes seen in the experiments.

Neutron diffraction studies on Dy/Y superlattices grown with the c axis at an angle of approximately 35° to the film plane reveal a coexistence of helical antiferromagnetic order and ferromagnetic order. The coexistence of magnetic phases has been determined to arise from the development during growth of surface facets of the Y buffer layer, which was grown on bcc (211) Ta or Nb substrates. One of the superlattice facets produces an isotropic expansion of the Dy basal plane which favors the helical antiferromagnetic spin configuration, while the other facet causes an anisotropic contraction of the Dy basal plane that favors ferromagnetism. The net result of the growth anomaly is the apparent coexistence of the two magnetic states as observed by neutron scans made with the scattering vector oriented parallel to the nominal c crystal axis of the superlattice.

Dy/Er superlattices, grown on sapphire, exhibit a succession of magnetic phases. Below the ordering temperature of dysprosium, the helix of dysprosium propagates coherently through paramagnetic erbium. At low temperature, dysprosium becomes ferromagnetically ordered in each layer but the layers are antiferromagnetically stacked. A polarized neutron scattering study of the low-temperature phase is presented. It shows that the antiferromagnetic peaks are never purely spin-flip.

Magnetic exchange coupling in metallic superlattices of [Dy‖Er], grown by molecular‐beam epitaxy, has been examined by neutron diffraction. In a superlattice of 65‐A layers of Dy and 55‐A layers of Er, distinct ordering temperatures were observed for the Dy and Er layers. Basal plane helical ordering of the Dy layers was initiated at temperatures close to TN for bulk Dy (178 K) with a long‐range interaction, preserving both phase and chirality, mediated through the Er layers. At a temperature near 70 K, the helical coupling of the Dy spins is replaced by an antiferromagnetic coupling of adjacent ferromagnetically ordered Dy layers coexisting with a remanent helical order. At a temperature well below TN of bulk Er, c‐axis ordering of the Er spins occurs in the superlattice, and in measurements at 10 K there is evidence of basal plane moment ordering in the Er.

Thin dysprosium c‐axis films (40–400 A) were grown coherently between 500‐A lutetium layers by molecular beam epitaxy. Bulk magnetization measurements show that these sandwich structures order magnetically at TN≂178 K (=TN of elemental Dy) and undergo ferromagnetic transitions at temperatures which range from 100 K (400 A Dy) to 175 K (40 A Dy), significantly enhanced from the bulk TC=85 K. The Dy basal plane lattice parameters in the films were determined by room‐temperature x‐ray diffraction. We observe a change in these values that correlates with the rise in TC, which suggests that this rise is due to epitaxial strain. The relatively small low‐temperature magnetic susceptibility displayed by these samples indicates the presence of a large anisotropy in the basal plane. We address both the issues of the susceptibility and the high ferromagnetic transition temperature.

We report neutron diffraction studies of Zn 1- x Mn x Te epitaxial layers for x =0.71, 0.84, 0.94 and 1 (pure MnTe). Molecular beam epitaxy (MBE) was used to grow the crystals on a GaAs (100) surface, resulting in zinc blende (ZB) crystallographic structure of the epilayers for all compositions. The film thickness were in the micrometer range, and could be characterized as bulk-like in terms of the strain and magnetic properties. The quality of the crystal was remarkably good, giving an indication that the NiAs form of MnTe grown by conventional techniques may be metastable at room temperature.

Several superlattices containing alternate layers of the rare earth elements dysprosium and lutetium were grown by molecular beam epitaxy. Neutron diffraction shows that these samples develop a helical phase in the (≈40 A) Dy layers at TN = 178 K which propagates coherently across the nonmagnetic Lu (20–55 A thick). The Dy layers order ferromagnetically at temperatures which vary from 140 to 160 K (the ferromagnetic phase in the bulk material appears at 85 K). Below TC the ferromagnetic Dy layers may be either aligned or anti-aligned. The transition is accompanied by a distortion of the superlattice basal plane comparable to that which occurs in bulk Dy. We also observe an approximately 80 K increase in TC in thin Lu/Dy(< 150 A)/Lu films. We discuss how the observed high TC may be related to the elastic coupling of the Dy to Lu.

Small‐angle neutron scattering was used to study the magnetic properties of intracellular magnetite crystals (magnetosomes) in the bacterium, Aquaspirillum magnetotacticum, grown in pure culture. An average of 20 single‐domain magnetite (Fe3O4) particles of diameter 400–500 A are arranged in a chain that longitudinally traverses each cell. The net magnetic dipole moment carried by each chain is sufficient to orient the bacterium in the geomagnetic field. A contrast variation series, performed on a nonmagnetic variant, established that a 30% D2O/H2O ratio in the suspension buffer resulted in the minimization of the scattering from the bacterium itself, thereby enhancing that from the magnetosomes. Measurements of A. magnetotacticum in 30% D2O and an applied field of 25 G confirmed that the scattering from the magnetite particles in bacteria grown under iron‐rich conditions is of ferromagnetic character, while that from magnetosomes in bacteria grown under iron‐starved conditions is characteristic of superp...

International audience; We present results on magnetic ordering in a Dy(60 Å)/Er(60 Å) superlattice made up of two magnetic rare earths exhibiting in-plane and perpendicular anisotropies. Neutron diffraction reveals that the helical magnetic coherence of dysprosium propagates through paramagnetic erbium layers, without inducing spin ordering in erbium. Below the ordering temperature of dysprosium individual ferromagnetic dysprosium layers show antiparallel stacking through magnetic erbium. This antiferromagnetic phase of dysprosium is irreversibly destroyed by a 5.5-kOe external magnetic field and replaced by a ferromagnetic order between dysprosium layers. Magnetization measurements indicate that this irreversibility could arise from an energy barrier between parallel and antiparallel stacking of ferromagnetic dysprosium layers.

We have investigated magnetic order in superlattices of Dy and Sc grown along the hcp c axis by molecular beam epitaxy (MBE) techniques. Our neutron diffraction experiments reveal that individual Dy layers order ferromagnetically below Tc∼150 K. The magnetic coherence length along the growth direction is less than the Dy‐layer thickness. Previous studies of rare‐earth superlattices with Y or Lu as spacer layers have shown that magnetic coherence propagates through sufficiently thin nonmagnetic interlayers. This arises from the long‐range exchange interaction that originates from nesting features in the Fermi surface of the spacer material. The lack of coupling in Dy/Sc superlattices reflects the very different Fermi surface of Sc, with much weaker nesting than Y and Lu.

The magnetic coupling in a [Dy(35 A )/Er(11 A )] × 90 superlattice has been studied by neutron scattering experiments and magnetization measurements. Basal plane helical ordering, coherent through initially paramagnetic Er layers, appears in the Dy layers for a temperature close to the ordering temperature of bulk Dy. At low temperature a ferromagnetic arrangement between the Dy layers is observed, instead of antiferromagnetic stacking in superlattices with thicker Er layers.

The asymmetry of spin-coupling through nonmagnetic yttrium interlayers in b -axis Gd/Y MBE superlattices grown on [101¯0] Y substrates has been examined using SQUID magnetometry and neuron scattering. For c -axis Gd/Y superlattices, the Gd layers align ferromagnetically or antiferromagnetically across the intervening layers depending in an oscillatory manner on the Y thickness with interlayer coupling strengths of approximately 1 kOe. Neutron scans for b -[Gd 43 A |Y 52 A ] 85 demonstrate that the Gd moments anti-align across the Y in zero field. The Gd layers in samples b -[Gd 47 A |Y 42 A ] 50 and b -Gd 60 A |Y 26 A ] 80 with thinner Y interlayers are aligned at temperatures less than 90 K. The saturation fields obtained for these samples are 80, 35 and 20 Oe at 150 K, respectively. This result suggests that the antiferromagnetic coupling mechanism is much weaker than that for the c -axis superlattices. We will compare models for the coupling based on RKKY and dipolar interactions.

Epitaxial (1102) Dy/Y rare earth superlattices and a thick (1102) Dy film, grown on sapphire with Y/Ta buffer layers, have been prepared by molecular beam epitaxy. Neutron diffraction and SQUID magnetization measurements on a single 200 nm thick (1102) Dy layer showed nearly bulk behavior. However, for the (1102) Dy/Y superlattices we found different magnetic behavior depending on the relative Dy to Y‐layer thickness. The superlattices exhibit both ferromagnetic and helical phases, but with the Neel and Curie temperatures significantly different from bulk Dy. These results differ from previous findings in Dy/Y superlattices grown along [0001].

Co p Mg1−p O offers an excellent prototypical example for the study of antiferromagnetism in diluted fcc lattices. The system retains the same crystallographic structure (NaCl type) over the entire 0≤p≤1 range. The Co2 + ions form a randomly diluted fcc spin lattice with predominantly next‐nearest‐neighbor antiferromagnetic (AF) exchange coupling. No significant changes in J are expected with magnetic dilution, since the lattice parameter changes only slightly (∼1%) between p=1 and p=0. The χ(p,T) data1 for the system reveal a distinct crossover effect at p≊0.45. A similar behavior of the χ(p,T) curve was observed in an isostructural system Eu p Sr1−p Te, and was interpreted as an AF→SG (SG=spin glass)transition.2 Recently, we have reported low‐T neutron‐diffraction data3 from Co p Mg1−p O samples with various p, showing a breakdown of the type‐II AF long‐range order (LRO) in the system at p≊0.47. In this paper we present the results of measurements of the temperature dependence of magnetic peak intensities and widths which provide a further evidence for a sharp LRO‐SRO (short‐range order) phase boundary in the system. However, neutron‐diffraction data alone cannot answer the question of whether the SRO phase is a SG state. In order to obtain a closer understanding of the LRO‐SRO transition in type‐II antiferromagnets, we have carried out Monte Carlo simulations on diluted fcc spin arrays. Since Co p Mg1−p O is an Ising‐like system (due to significant crystal‐field anisotropy), and some other materials of current interest (e.g., Eu p Sr1−p Te) are Heisenberg systems, we discuss this question in context of simulation data obtained from both Heisenberg and Ising spin arrays.

Single-crystal multilayer films with alternate heavy rare earth and yttrium layers have been shown by neutron diffraction to exhibit long-range magnetic order. Analysis of the neutron results on Dy¦Y and Er¦Y superlattices shows that the phase of the modulated magnetic structures in the dysprosium and erbium is preserved across the intervening yttrium nonmagnetic layer and corresponds to a “pseudo turn-angle” near 51° in the yttrium, which is in accord with theoretical calculations from the band structure. The ferromagnetic transitions occurring in the pure elements are completely suppressed in the multilayers as a result of epitaxial “clamping” by the yttrium layers, which inhibits the development of sufficient magnetostrictive strain to induce the phase transition. The temperature of the intermediate transitions in Er¦Y multilayers is also modified by magnetostriction, and evidence is found for different turn-angles for c axis and basal plane moment components.

Small angel neutron and X-ray scattering were used to study the magnetic and structural properties, respectively, of intracellular magnetite crystals (magnetosomes) in the bacterium, Aquaspirillum magnetotacticum, grown in pure culture. An average of twenty magnetite particles of diameter 400–500 A are arranged in a chain that longitudinally traverses each cell. Behaving like magnetic dipoles, the bacteria orient in the geomagnetic field. Small angle X-ray scattering measurements revealed that bacteria grown in the presence of large amounts of iron contained magnetosomes with diameters averaging approximately 400 A. Those grown under iron-starved conditions contained particles with slightly smaller diameters. The contrast variation technique was used for the small angle neutron scattering measurements in order to establish that a 30% D2O/H2O ratio in the solvent resulted in the maximization of the scattering from the bacterium itself, thus enhancing that from the magnetosomes. Measurements in 30% D2O and an applied field of 25 G confirmed that the scattering from the larger magnetic particles is of ferromagnetic character whereas that from the smaller particles is consistent with super-paramagnetic particles. The static spin correlations were found to be of relatively long range, with dynamical fluctuations corresponding to a distance scale of approximately 50 A.

Powder neutron diffraction measurements have been carried out on the intermetallic compound DyFe 3 at 4 and 295 K. A modified Rietveld profile refinement program was used to fit the neutron data. It was found that a non-collinear magnetic structure model in the a - c plane, gives a satisfactory fit to the data measured at 4 and 295 K. At 4 K, the iron moments on the three crystallographically non-evuivalent sites lie at 253.6° to the c avis, having a magnitude of 2.2 μ B ; the dysprosium moments on the two non-evuivalent sites lie at 71.8° and 51.5° to the c -avis, having magnitudes of 9.8 μ B and 9.6 μ b , respectively. At 295 K, the moments of iron, Dy 1 , and Dy 11 are reduced to 1.9 μ B , 6.9 μ B , and 6.5 μ B , respectively, and all these moments lie closer to the basal plane than at the lower temperature. The magnetivation per formula unit of the refined structure, 3.8 μ B , is in good agreement with the evperimental value. The orientation of the iron moments determined from the neutron data is compared with the results of a Mossbauer spectrum study of the compound.

The magnetism of epitaxial Er films up to 1 pm thick differs from bulk Er in that no ferromagnetic transition is observed. Above a critical field that depends on temperature and film thickness the magnetization saturates at the bulk value. Both neutron scattering and magnetization data show lock-in transitions, but at lower temperatures than in the bulk. We have demonstrated in recent years [I] and in a paper on Er/Y at this conference [2], that rareearth multilayers, grown with crystalline perfection by molecular-beam epitaxy, have properties distinctly different from the bulk elements. Here we show that many of the property changes are consequences of the 4 epitaxy itself, rather than of the multilayer structure. $47 Er films of various thicknesses were grown epitaxia ally by first growing a layer of (110) Nb on sapphire, then a 300 A film of (0001) Y, and finally Er films o ranging in thickness from 400 A to 9500 A. All of the samples show excellent crystalline Bragg peaks, with cand a-axis lattice parameters within 0.6 % of those of bulk Er. Nonetheless, even the thickest films are magnetically distinct from the bulk. The Er samples are not lattice matched to Y (2 % difference). Dislocations formed during film growth permit the samples to relax toward bulk lattice parameters. However, the films remain clamped to the substrate, as a result of which no magnetoelastic deformation is possible in the growth plane without irreversible damage to the sample. Bulk Er orders in a sinusoidal c-axis -modulated (CAM) structure at 89 K, with a phase advance of approximately 52.5' per Er plane. Below 50 K, a basal plane spiral appears, the c-axis modulation becomes more square-wave-like and the CAM and spiral undergo a series of phase transition to unique lock-in states [3]. The lines in figure 1 indicate the phaseadvance per Er layer for the lock-in states, with the x's indicating the temperatures at which they first appear on cooling (nomenclature of Ref. [3]). At 20 K, the c-component of the magnetization becomes ferromagnetic, while the spiral remains. While some features of bulk Er are observed in epitaxial Er films, there are significant differences. Most notably, no c-axis ferromagmetic phase is observed to the lowest temperatures studied (4.2 K). The results of a neutron scattering study of the CAM structure in two representative thin films of 1750 A (circles) and I I I I I 10 20 30 40 50 Temperature (K) Fig. 1. Lock-in states us. temperature for bulk Er (x), 1750 A (circles), and 9500 A (triangles). 9500 A (triangles) are shown in figure 1. Both samples show a variation of wave vector with temperature, and each seems to pass through several of the lock-in states of bulk Er. However, the CAM structure persists below 10 K in each case. Each lock-in state appears at a lower temperature in the thin films than in the bulk and, further, the films fail to follow the entire sequence of lock-in states observed in the bulk. The presence of lock-in transitions is also evident in magnetization data. In figures 2a and 2b, we show, respectively, the magnetization of a 1750-A and a 9500-A film, each cooled in a field of 2 kOe. Both films show a small maximum at TN = 85 K, indicating the onset of the CAM phase, followed by oscillations in magnetization. The saturation magnetization of both samples is 230 f 10 emu/g, comparable to that of bulk Er, and appears abruptly above a temperature-dependent critical field. The number, amplitude, and position in temperature of the oscillations depend sensitively on film thickness and growth conditions. A 4000 A film grown under slightly different conditions (and with an ;axis lattice parameter identical to bulk Er) shows only a smooth increase in magnetization under the same measurement conditions. 'permanent address: Kamerlingh Onnes Laboratorium, Leiden, the Netherlands. 2 ~ a t i o n a l Bureau o f Standards, Gathersburg, M D 20877, U.S.A. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888764 C8 1686 JOURNAL I IE PHYSIQUE Temperature (K) Temperature (K) Fig. 2. Field-cooled magnetization (2 kOe) for 1750 A (a) and 9500 A (b) films. Neutron scattering data were taken on the 9500 Afilm in a 2 kOe field at temperatures corresponding to the extrema in figure 2b. The minima at 10 K and 32.K are consistent with the 112 and 3/11 lockin states, while the maximum at 20 K has a phase advance close to the 5/19 state. The observed phase advances appear to be systematically 0.3' larger than those of the lock-in states reported by Gibbs et al. The maximum in magnetization observed at 38 K is puzzling. It corresponds to a phase advance of 50.5', which is not one of the expected lock-in states. What is more, this state appears to be particularly stable in an applied field. At 30 K, the structure changes from the 48.4'-phase advance to the 50.5' advance at an applied field near 10 kOe; at still higher applied fields (> 35 kOe), the CAM structure is destroyed and the full bulk magnetic moment is observed. The so-called antiferromagnetic next nearest neighbour Ising (ANNNI) model was explicitly invented to explain the behaviour of Er. More recently, the model has extended to include a magnetic field [4]. The already rich phase structure of the ANNNI model becomes even more complicated in the presence of a field. We have considered a simple model of Ising spins on a cubic lattice with ferromagnetic coupling Jo between neighboring spins on a horizontal plane, J1, between neighboring spins on successive planes, and an antiferromagnetic interaction J2, between spins on nextneighbor planes. The ratio p = 52 controls the magnetic phase diagram. For p 0.5, the ground state is the antiferromagnetic (22) state (two up, two down). We have solved the coupled equations for a 264-layer system and calcu1ate.d the Gibbs free energy for all possible starting configurations. In figure 3, we plot the magnetization of the lowest energy state as a function of temperature at various magnetic fields for p = 0.55. At low fields (triangles), the magnetization has a maximum at 0.4 TN. Doubling the field (x's) leads to a ferromagnetic low-temperature state without intermeTemperature (arb. units) Fig. 3. Lowest energy states in the ANNNI model for p = 0.55 at fields of h = 0.1 (triangles); and h = 0.2 (x's). diate phases. This behaviour is qualitatively, and even semiquantitatively, similar to that observed in Er films. Why, if the ground state of ANNNI systems is antiferromagnetic, does Er have a ferromagnetic ground state? We have, in the ANNNI model, ignored the important contribution of magnetostriction, which possibly decreases p and thus favors ferromagnetism in Er. The minimization of the magnetoelastic energyrn bulk Er results in c-axis contraction and concomitant expansion in the basal plane. However, the clamped epitaxial condition restricts the magneto-elastic energy gain with the result that exchange energy is more important in epitaxial Er films. The ferromagnetic phase can be induced by an applied field in a completely reversible process. This suggests that no plastic deformation takes place. The critical internal fields required to induce ferromagnetism at 10 K decreases from 8 kOe for a 860-A film to 3 kOe for a 9500 A film. While we have not yet determined whether this decrease is continuous or occurs as a step at a well defined thickness, it is clear that films up t o 1 pm in thickness are still mostly clamped and behave differently from bulk Er. Further studies are underway to understand the relationship t o bulk behaviour.

The compound Th 6 Mn 23 D 16 was studied at 295 K by neutron diffraction profile refinement methods. The pressure isotherm of Th 6 Mn 23 D x was measured at 295 K and exhibits a plateau between x = 17 and x = 24. The site occupancies and atomic positions were determined and are compared with results for Y 6 Mn 23 D x . In both the thorium and yttrium deutendes some of the deuterium atoms are located in the tetrahedral f sites of space group Fm 3 m . However, the octahedral a site, which is fully occupied in the yttrium compound, is virtually empty in the thorium compound. The remaining deuterium atoms occupy the trigonal i site in Th 6 Mn 23 D 16 , and not the tetrahedral j site as in Y 6 Mn 23 D 18 . Low temperature scans (4 K) of Th 6 Mn 23 D 16 revealed that the compound undergoes a crystallographic phase change from f.c.c. symmetry to orthorhombic symmetry.

The results of a thermodynamic study of the dehydrogenation of R6Mn23 (R ≡ Gd, Dy, Ho, Er) are reported and correlated with earlier work on R6Fe23 hydrides (R ≡ Y, Er, Ho, Lu). The enthalpy and entropy changes (ΔH, ΔS) associated with the desorption of hydrogen from particular hydride phases have been calculated from hydrogen pressure-composition isotherms and the van't Hoff expression. The thermodynamic work indicates four hydride phases for the iron-containing systems and three for the manganese-containing systems. A Lundin-type plot of the free energy of hydrogen desorption vs. the interstitial hole radius for the R6Fe23Hx system illustrates the dependence of hydride stability on the lattice dimensions. The relative stabilities of the different hydrogen sites that are occupied for these R6T23-H systems were established by correlating thermodynamic and published structural information. In general, the affinity of an interstitial hole for hydrogen was found to follow R6 >R3T >RT3, where T ≡ Fe or Mn (spatially defined by RxTy). Attention is drawn to the importance of H-H nearest-neighbor interactions in regard to the structures of these materials. Isotope effects are observed for the Ho6Fe23-H(D) systems.

Small angle neutron scattering (SANS) and polarized-beam spin rotation measurements show that amorphous Fe/sub 80/Zr/sub 10/ does not exhibit conventional long range ferromagnetic order below T/sub C/. The SANS measurements show the existence of two characteristic length scales; one provides evidence of relatively large spin clusters, while the other is characteristic of the spin dynamics. 4 refs., 3 figs.

The incommensurate magnetization waves (IMW) occuring at the wave vector Q = (1, 0.5 − δ, 0) 2π a and equivalent positions in reciprocal space in Cu 1− x Mn x have been studied as a function of temperature, wavevector, frequency and composition, using high-resolution, unpolarized neutron scattering techniques. We find that the elastic component of the scattering cross section at these incommensurate wavevectors approaches zero in the vicinity of the “spin glass” freezing temperature, T f , closely resembling that expected for an order parameter on approach to the critical point. We suggest that the interaction between Mn atoms is long-range and intimately connected to a spin density wave instability.

Neutron diffraction studies have demonstrated the existence of helical magnetic order in a metallic superlattice of dysprosium and yttrium produced by molecular beam epitaxy. Analysis of the magnetic satellite peak widths indicates that phase coherence of the helix exists over at least five bilayer cells, each consisting of 14 atomic planes of dysprosium and of non-magnetic yttrium. The multilayer comprises 64 bilayer cells. Results are given for the temperature dependence of the magnetic intensity and for the effect of an applied magnetic field which continuously converts the helical ordering to ferromagnetism.

High resolution neutron diffraction techniques were used to establish the hydrogen (deuterium) site occupation and sublattice magnetization of a series of Ho 6 Fe 23 D x compounds ( x = 1.5, 8.2, 12.1 and 15.7). The f.c.c. structure of the parent compound was maintained except for the D 12.1 sample which exhibited a tetragonal distortion. The f.c.c. lattice was reestablished for the higher D 15.7 composition. Diffraction and inelastic hydrogen vibration spectra data demonstrate conclusively that the deuterium atoms in the D 1.5 material fully occupy the octahedral a site with the residual in the tetrahedral f site. At the D 8.2 and higher concentration level, the a site is empty and the deuterium atoms partially occupy only the tetrahedral sites. The 4 K holmium magnetic moment was near the free-ion value of 10 μ B for all compounds and was oriented antiparallel to all four iron site moments (b, d, f 1 and f 2 ). For the cubic materials, the magnetization on the iron sites varied with deuteration in the range 0.8–2.4 μ B , with the D 15.7 compound showing the highest iron moment for all sites. A spin reorientation is observed in the tetragonal material between room temperature and 4 K with the holmium and iron b and n 1 moments rotating out of the basal plane.

Neutron diffraction studies of the Y 6 (Fe 1− x Mn x ) 23 system reveal the presence of substantial preferential atomic ordering of iron and manganese atoms on the four transition metal crystallographic sites. Throughout the entire compositional range, manganese atoms prefer to occupy the f 2 site and iron atoms the f 1 site. Neutron diffraction profile refinements show no long-range magnetic ordering occurring in the compositional range from x = 0.35 to x = 0.75. The average iron moments on each of the sites are reduced in the iron-rich ternaries and remain ferromagnetically coupled. The manganese atoms are nonmagnetic. In the manganese-rich ternaries the iron atoms have no spontaneous moments and the manganese magnetic moments are decreased dramatically from their values in Y 6 Mn 23 . In the 0.35 ⩽ x ⩽ 0.75 range detailed scans reveal broadened low intensity peaks at the (100) and (300) forbidden lattice reflections, indicating weak short-range (about 30 A) antiferromagnetic cluster spin ordering. No ferromagnetic or ferrimagnetic clusters were observed in this region from small angle neutron scattering studies.

On observe l'apparition d'une phase antiferromagnetique en spirale (par decroissance de la temperature) puis d'une phase ferromagnetique

The magnetic properties of amorphous (Fe x Ni 100−x ) 75 P 16 B 6 Al 3 have been investigated in the critical concentration region ( x c ≈ 17) via neutron scattering. For x > x c a transition to ferromagnetism is first observed followed at lower temperatures by an evolution to spin-glass behavior. Associated with this low temperature state are two distinct time scales corresponding to a resolution-limited elastic peak and to excitations. The temperature dependence of the elastic component is directly related to the spin-glass order parameter and indicates that there is a region of temperature where ferromagnetism and spin-glass order coexist.

A re-analysis for earlier magnetostriction measurements on dysprosium is presented. Domain effects have been corrected for using the phase theory. The re-analysed results for the temperature dependence of the magnetostriction constants and of the magnetocrystalline anisotropy energy are in reasonable agreement with theoretical predictions and the results of other experimental workers. For T to 0K it is found that lambda gamma ,2=(10.6+or-0.2)*10-3 and K66=(-1.4+or-0.2)*106 Jm-3.

Determination de la structure magnetique des superreseaux Er/Y par diffraction de neutrons et magnetometrie SQVID sur des echantillons monocristallins obtenus sur des substrats de saphir par epitaxie par faisceau moleculaire

The magnetic ordering of highly perfect single‐crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 A) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long‐range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 A−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long‐range coupling is a conduction band spin‐density wave in bot...

On montre par diffraction de neutrons l'existence au-dessous de T N =3,25 K d'une structure sinusoidalement modulee avec un vecteur de propagation suivant l'axe C

A low-angle neutron scattering study has been carried out on Fe/Si multilayers. It is shown that the flipping ratio of the satellites is not constant, which can be explained by the presence of a non-magnetic interlayer between Fe and Si. A simple step-model analysis shows that the thickness of the Fe-Si alloyed interlayer is about 15A, which is consistent with previous Mossbauer measurements.

The inverse susceptibility—temperature curves of MnxHg1—xTe (x = 0.05, 0.11, 0.17, 0.235) showed a Curie-Weiss behaviour at high temperatures and a downward deviation at low temperatures. The paramagnetic Curie temperatures and effective numbers of magnetic carriers also deviated from a linear dependence on x at low values of x. These results were explained in terms of a cluster model at low x values and a molecular field model at high x. Die reziproken Suszeptibilitats-Temperatur-Kurven von MnxHg1 -xTe (x = 0,05;0,11; 0,17; 0,235) zeigen ein Curie-Weiss-Verhalten bei hohen Temperaturen und eine abwarts fallende Abweichung bei tiefen Temperaturen. Die paramagnetischen Curie-Temperaturen und die effektive Anzahl der magnetischen Trager zeigen ebenfalls eine Abweichung von der linearen Abhangigkeit von x bei kleinen x-Werten. Diese Ergebnisse werden mit einem Cluster-Modell fur kleine x-Werte und einem Molekularfeld-Modell fur grose x-Werte erklart.

The "anomalous" or magnetization-dependent contribution to the Hall effect in single-crystal dysprosium has been investigated from 340 down to 4\ifmmode^\circ\else\textdegree\fi{}K. In the paramagnetic region, with H applied along $〈11\overline{2}0〉$ directions, the Hall resistivity, defined by ${\ensuremath{\rho}}_{H}\ensuremath{\equiv}{R}_{0}B+{R}_{s}4\ensuremath{\pi}{M}_{s}$, is linear in $H$ and exhibits a temperature dependence principally due to that of the magnetic susceptibility. The resulting anomalous coefficient ${R}_{s}=\ensuremath{-}18\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ \ensuremath{\Omega} cm/G and normal coefficient ${R}_{0}=\ensuremath{-}0.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ \ensuremath{\Omega} cm/G are both temperature-independent above 190\ifmmode^\circ\else\textdegree\fi{}K. Below the N\'eel temperature (178\ifmmode^\circ\else\textdegree\fi{}K), the anomalous coefficient changes sign, passes through a maximum near 120\ifmmode^\circ\else\textdegree\fi{}K (where ${R}_{s}=+21\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ \ensuremath{\Omega} cm/G), and decreases essentially to zero at 30\ifmmode^\circ\else\textdegree\fi{}K. The temperature dependence of ${R}_{s}$ below 105\ifmmode^\circ\else\textdegree\fi{}K is well represented by the first power of the magnetic (spin disorder) resistivity, ${\ensuremath{\rho}}_{m}$. Such a dependence is in agreement with the $f$-spin-$s$-orbit interaction model due to Maranzana, considering only scattering from the thermally disordered spin system as calculated by Irkhin and Abelskii. The contributions to ${\ensuremath{\rho}}_{H}$ predicted by other mechanisms are evaluated and discussed. The second independent component of ${\ensuremath{\rho}}_{H}$ allowed in hcp symmetry and characterized by H applied along $〈0001〉$ was determined in the paramagnetic range. High-temperature values of ${R}_{s}$ and ${R}_{0}$ are -23\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ \ensuremath{\Omega} cm/G and -3.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ \ensuremath{\Omega} cm/G, indicating appreciable anisotropy between $c$-axis and basalplane components. Results could not be continued into the ordered temperature range, because of the enormous magnetic anisotropy.

We have performed neutron diffraction measurements on the compound UNiSn above and below its magnetic transition temperature. Below the transition, the magnetic structure is FCC type 1 antiferromagnetism with a uranium moment of (1.42±0.03)μ B parallel to the [100] axis. A small amount of second phase U 3 Ni 3 Sn 4 was present in the sample and its structure was also analysed as the Y 3 Au 3 Sb 4 structure, with space group 1 43d.

Neutron diffraction has been used to study the long‐range helical magnetic order in a single crystal of the dilute alloy Y0.97Dy0.03. In zero applied magnetic field, the propagation vector is along the c axis and of magnitude 0.28(2π/c), with the magnetic moments lying in the basal plane of the hexagonal lattice. The Dy develops the full free‐ion moment and exhibits a temperature dependence similar to that of the Brillouin function for J=15/2. At T=6 K, a magnetic field applied in the basal plane causes almost no shift in the value of the modulation wave vector and as the magnetic field is increased, the peak scattered intensity of the magnetic satellite drops continuously, with the helical phase being completely suppressed in a field of 3 T. On reducing the field, there is hysteresis in the peak intensity, with only 79% of the original intensity being recovered at H=0. This residual intensity grows slowly with time, increasing to approximately 82% of the original value over a period of 1 h. This behavior...

First results of inelastic neutron scattering studies of quasiharmonic magnons in Zn1−xMnxTe are reported. Measurements carried out on two single‐crystal samples with x=0.65 and x=0.41 at T=4.2–5.0 K revealed broad excitation modes, which were in excellent agreement with the results of theoretical modelling. Results for x=0.65 yield JNN=(7.9±0.2) K which is the first determination of the nearest‐neighbor exchange constant in Zn1−xMnxTe in this x region. Data taken at higher temperatures showed only weak renormalization of the energy spectra.