Search

Polarization-analyzed small-angle neutron-scattering methods are used to determine the spin arrangements and experimental length scales of magnetic correlations in ordered three-dimensional assemblies of $\ensuremath{\sim}7.4$-nm-diam core-shell ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}/{\mathrm{Mn}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}$ nanoparticles. In moderate to high magnetic fields, the assemblies display a canted magnetic structure where the canting direction is coherent from nanoparticle to nanoparticle, in contrast to the less extended, more single-particle-like behavior for similar ferrite assemblies. The observed magnetic scattering is modeled by assuming that the interparticle dipolar coupling combined with Zeeman effects in a field leads to nanoparticle domains with preferred net spin alignments relative to packing symmetry axes. Over a range of fields and temperatures, the model qualitatively explains the observed scattering anomalies in terms of clusters that vary in area and thickness, highlighting the complex structures adopted in real, dense nanoparticle systems. The clusters often have a strong two-dimensional magnetic character which is attributed to structural stacking faults and the resulting influence of interparticle dipolar interactions for these magnetically soft nanoparticles.

Neutron diffraction studies of antiferromagnetic semiconductor superlattices are reviewed. Mn-VI/II-VI multilayers (e.g. MnSe/ZnTe, MnSe/ZnSe) exhibit a variety of strain-induced effects in spin ordering and phase transition phenomena. In MnTe/CdTe systems, in which the thickness of the non-magnetic CdTe spacers can be very thin (∼ 6 A) and comparable with the range of the Mn-Mn exchange interactions, the measurements clearly show the onset of interlayer coupling. Pronounced interlayer coupling has been observed in experiments on another system, EuTe/PbTe, with much thicker (30–40 A) non-magnetic layers - arguably, this coupling is maintained by dipolar fields.

We report neutron-diffraction studies of antiferromagnetism in various forms of epitaxially grown zinc-blende (ZB) MnTe: in semibulk (\ensuremath{\sim}1 \ensuremath{\mu}m thick) single-crystal films of pure MnTe, in its magnetically diluted derivative ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te with 0.695x1, and in strongly strained very thin (30--300 \AA{}) single-crystal MnTe layers in MnTe/ZnTe superlattices. ZB Mn chalcogenides are unique exmples of fcc Heisenberg antiferromagnets (AF) with dominant nearest-neighbor interactions. Such a lattice is one of the basic models of topologically frustrated spin systems. Only ZB MnS can be obtained through natural crystallization (and only in a fine powder form, which seriously limits the scope of possible studies on this system). The single-crystal forms of MnTe obtained using molecular-beam epitaxy have made it possible to study the influence of strain on a frustrated fcc antiferromagnet. We observe that such built-in strain strongly affects the domain structure as well as the phase-transition behavior. Furthermore, high-resolution x-ray diffraction reveals pronounced magnetostriction effects in the MnTe films. Both neutron as well as x-ray data indicate a rather unusual effect of a strong temperature shift in the relative populations of two inequivalent AF domain states, and a magnetostriction mechanism underlying this phenomenon is proposed. Finally, the data obtained on ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te films complement the results of previous magnetic studies on bulk forms of this material with x\ensuremath{\le}0.68.

The authors point out that new zinc-blende structural modifications of MnSe and MnTe grown by molecular beam epitaxy offer unique examples of strongly frustrated FCC Heisenberg antiferromagnets. These systems can be obtained either in the form of strained superlattices with significant strain-induced anisotropy in the Mn-Mn exchange, or of nearly isotropic semi-bulk MnTe single films. Neutron diffraction studies show that the strain changes the phase transition order and the domain structure normally seen in unstrained lattices, and in some cases (MnSe/ZnTe superlattices) even produces a transition to an entirely new incommensurate helical spin structure. >

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.

A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.

Neutron-diffraction studies of ZnTe/MnSe strained-layer superlattices show the formation of an incommensurate helimagnetic phase below 70 K. The incommensurate ordering is characterized by a wave vector that can be tuned by both strain and dimensionality. Furthermore, the incommensurate helical pitch varies smoothly with temperature, without locking in at any commensurate values.

Neutron diffraction data from MnSe/ZnTe, a MBE-grown superlattice system containing zinc-blende (ZB) MnSe layers subjected to a considerable tensile stress, indicate the formation of an incommensurate helical AF structure which is clearly different from the Type III FCC AF order seen in unstrained ZB Mn chalcogenides, and in MnSe/ZnSe and MnTe/ZnTe superlattices with compressive strain. Energy considerations show that such an arrangement results from strain-induced anisotropy in the nearest-neighbor exchange.

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 effect of adding Cl impurity on the long-range antiferromagnetic coupling in [ZnTe|MnTe] magnetic semiconductor superlattices has been investigated by neutron diffraction. Superlattices consisting of five atomic planes of ZnTe alternated with ten atomic planes of MnTe[(ZnTe)5|(MnTe)10] have been prepared by molecular-beam epitaxy both with and without Cl added to the MnTe and/or ZnTe layers during deposition. This was motivated by the fact that Cl can form either shallow or deep donor levels in II–VI semiconductors, and that such electronic states can serve to modify magnetic interlayer exchange. Unchlorinated samples showed magnetic correlation lengths of around 225 A (≈5 bilayers) at low temperatures, and the addition of Cl increased the correlation length at 15 K by nearly a factor of 2 to approximately 450 A.

Low-angle inelastic neutron scattering was used to study the temperature and wave vector dependence of the spin waves in La0.67Ca0.33MnO3 perovskite-based colossal magnetoresistance material. At low q the spin waves show Heisenberg ferromagnetic dispersion (E=Dq2+Δ) where D is the spin stiffness, q is the wave vector, and Δ is the energy gap. However, the temperature renormalization of the spin stiffness D is anomalous, and as T increases toward Tc, D does not show the expected power law collapse, but rather exhibits a sudden sharp drop suggestive of a first-order phase transition. Detailed neutron measurements of the order parameter in zero applied field showed a similar first-order-like transition. However, no temperature hysteresis was observed in either D or in the magnetization.

Continuing the analysis of epitaxial erbium thin films and Er/Y superlattices, we report the effects of basal-plane strain on the modulated spin structure as determined from bulk magnetization and neutron-diffraction measurements detailed in a previous paper [Phys. Rev. B 43, 3123 (1991)]. The phase angle of the c-axis-modulation spin order is larger than that of bulk Er in even the thickest films and is virtually independent of temperature in the superlattices. The sequence and stability of c-axis commensurate states in bulk Er are altered in all samples considered. In the superlattices, the c-axis-modulation net moment state with four spins up followed by three spins down dominates the temperature and field phase diagram. In addition, an additional intermediate spin configuration with a net moment of half the saturation moment develops in the superlattice with the thinnest Er interlayers. A phenomenological calculation of the exchange integral demonstrates that epitaxial strain and lattice clamping can lead to an enhancement of the phase angle. Specifically, additional commensurate phases may arise due to strain-induced variations of the nearest-neighbor and next-nearest-neighbor spin interactions, as shown in the context of the axial-next-nearest-neighbor Ising model.

We have carried out a study of the low-temperature magnetization and spin-wave dispersion in the amorphous metallic ferromagnet Fe78B13Si9. The temperature dependence of the magnetization was measured with a vibrating sample magnetometer and the spin-wave measurements were carried out using inelastic neutron scattering techniques. The magnetization is found to obey the usual law if M s (T) = M s (0)(1 − BT 3 2 − CT 5 2 …) , while the spin waves follow the conventional ferromagnetic dispersion relation Eq = D(T)q2 + Δ + …. The spin wave stiffness constant D(T) is found to be substantially larger than the value inferred from the magnetization data, similar to the behavior observed in other Invar-type materials.

By neutron diffraction we show that superlattices of Dy and Y grown by molecular-beam epitaxy along the hcp {ital b} axis exhibit little magnetic coupling between successive Dy layers, even for Y spacers as thin as 9 atomic planes (26 A). Previous studies of Dy/Y superlattices grown along the hcp {ital c} axis established that long-range three-dimensional helimagnetic ordering takes place even through Y spacer layers as thick as 120 A. This highly anisotropic coupling behavior is shown to have its origin in nearly-two-dimensional nesting features of the Y and Dy Fermi surfaces. Nesting along the {ital c} axis gives rise to sharp peaks along {bold c} in the wave-vector-dependent magnetic susceptibility, and causes the exchange coupling to exhibit long-range oscillations in real space. The lack of nesting features along the {ital b} axis leaves a rapid exponential decay of the exchange interaction with spin separation. From magnetic measurements by superconducting-quantum-interference-device magnetometry on {ital b}-axis superlattices and films, we deduce that the first-order ferromagnetic transition of Dy is suppressed, and that the critical field required to produce the ferromagnetic alignment is much higher than the {ital c}-axis counterpart. This difference arises from anisotropy of the energy balance of the system.more » The magnetic coherence in {ital b}-axis superlattices and films is anisotropic and exhibits an unusual temperature dependence.« less

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.

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 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

The National Institute of Standards and Technology has under development a major Cold Neutron Research Facility which, upon completion, will make available approximately 15 new neutron‐scattering instruments located on neutron guides on the reactor cold source. This facility, which includes the National Science Foundation Center for High Resolution Neutron Scattering, will be operated as a user facility, which is open via a proposal system to all scientists. This paper briefly reviews the types of cold neutron instruments that have particular relevance to magnetic problems. A discussion is given of neutron scattering from magnetic systems with examples of problems appropriate for the enhanced energy and wave‐vector resolution of cold‐source instruments. Included is a review of new experimental results and techniques that will be available, including reflectometry and grazing‐angle diffraction, as well as more conventional techniques such as triple‐axis inelastic scattering, small‐angle scattering, and time‐of‐flight spectroscopy.

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.

Amorphous Fe90−x Ni x Zr10 is a system that exhibits a relatively high degree of magnetic exchange frustration, which becomes stronger as the system approaches the composition of amorphous pure iron. Thus, while samples with moderate amounts of iron (x≥5) are ferromagnetic, the samples with the highest concentration of iron (x≤1) behave like reentrant spin glasses. We have performed a detailed neutron scattering study of the spin‐wave excitations in this system for x=1, 5, 10, 20. In all cases, well‐defined spin‐wave excitations were observed below a transition temperature T c that decreased from 455 K (for x=20) to 250 K (for x=1). For x=5, 10, 20 the spin‐wave stiffness coefficient follows the temperature dependence expected for a conventional ferromagnet but the spin‐wave excitations broaden considerably at low temperatures. For x=1 the spin‐wave stiffness coefficient softens at low temperatures and an elastic component of the scattering, associated with the development of a spin‐glass order parameter, appears below T≊0.28T c . A coexistence of propagating spin‐wave excitations and spin freezing phenomena is observed below this temperature down to T=0.09T c . These results are discussed in terms of the relevant current theories.

The layered oxides ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{MnO}}_{6+\mathit{y}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{MnO}}_{6+\mathit{y}}$, the Mn analogs of the superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6+\mathit{y}}$, have anomalously sharp peaks in their magnetic susceptibility as a function of temperature at 120 and 100 K, respectively. We have studied the crystallographic and magnetic structure of these compounds by x-ray, electron, and neutron diffraction on both powders and single crystals and correlated the structure with the magnetic properties. In the magnetically ordered state, nearest-neighbor Mn moments are antiparallel and point normal to the ${\mathrm{MnO}}_{2}$ layers. As in the superconducting Cu analog, the crystallographic structure of these compounds is distorted; because of the flexing of the atomic slabs associated with the distortion, not all the Mn in the crystal are crystallographically identical, so the magnetic moment of Mn can vary between these different lattice sites. As a result, the moments on adjacent sites do not exactly cancel, and each layer of ${\mathrm{MnO}}_{2}$ is a ferrimagnet. A magnetic field can induce a transition from an antiferromagnetic phase, where the net moments of different layers are opposed, to a ferrimagnetic phase, where the net moments are aligned. A simple mean-field theory mimics the shape of the susceptibility versus temperature, and the deficiencies of the mean-field theory suggest the importance of fluctuations and domains.

Elastic constraints on rare-earth superlattices and films grown by molecular beam epitaxy strongly perturb their magnetic structure compared to the bulk materials. Continuing studies of Dy/Y and Er/Y superlattices and Dy and Er thin films by neutron scattering and SQUID magnetometry have provided new insight into the interplay of magnetoelastic and exchange interactions in rare earths. The most striking result of the epitaxial clamping to the substrate and intervening non-magnetic layers is the suppression of the ferromagnetic transitions found in bulk Dy and Er. A quantitative calculation of the magnetoelastic energies confirms their importance in driving these phase transitions. The helical and linear spin density wave orderings are also significantly perturbed. For example, in the Dy supperlattices the temperature dependence of the interlayer turn angle is weakened as the elastic clamping is increased. This temperature dependence is completely suppressed in the Er superlattices where the turn angle is typically fixed at the commensurate lock-in value 2π 7 . In the thin films, however, the turn angles are closer to the bulk values. There is also a 50% reduction of the basal plane ordering temperatures compared to bulk Er. For Er the temperature dependence of the exchange is also crucial for explaining these effects.

The exchange parameter ${J}_{\mathrm{NN}}$ for nearest-neighbor (NN) ${\mathrm{Co}}^{2+}$ ions in a new II-VI diluted magnetic semiconductor system ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$S has been directly determined using inelastic-neutron-scattering techniques. The ${\mathrm{Co}}^{2+}$ ions in the system have a magnetic ground state with quenched orbital moment, so that the Co-Co interaction can be described, to a very good approximation, as a Heisenberg exchange between two S=(3/2 spins. Inelastic-scattering spectra obtained from polycrystalline samples with x=0.01\char21{}0.06 reveal distinct maxima corresponding to transitions between the energy levels of isolated NN Co-Co pairs. The energy of the first excited level E=2${J}_{\mathrm{NN}}$ has been found to be E=8.19\ifmmode\pm\else\textpm\fi{}0.05 meV, which yields ${J}_{\mathrm{NN}/{k}_{B}=47.5\ifmmode\pm\else\textpm\fi{}0.6}$ K, in good agreement with the result obtained from recent magnetic-susceptibility studies. The observed sequence of the excitation energies shows a slight deviation from the Land\'e interval rule, indicating biquadratic effects in the Co-Co exchange.

Magnetic semiconductor superlattices consisting of x monolayers of ZnTe alternated with y monolayers of MnTe [(ZnTe)x|(MnTe)y] have been grown in the zinc-blende structure by molecular beam epitaxy. For relative thin ZnTe nonmagnetic interlayers (3⩽x⩽6), neutron diffraction data show long range order within the MnTe layers and also an interlayer magnetic coupling across the semiconducting ZnTe interlayer extending over multiple bilayers. For the x=5, y=10; and x=4, 5, y=20 superlattices at low temperature (10 K), the spins in adjacent MnTe layers couple in an inphase antiferromagnetic structure with identical spin orientations in all MnTe layers. As the temperature is raised, this ordering slowly transforms into an antiphase coupling in which alternate MnTe layers have spin directions reversed. For increasing ZnTe layer thickness, a systematic reduction in the magnetic correlation range is observed, with the correlation range reducing to approximately one MnTe layer for x=6 and 7.

The magnetic structure and dynamics in the colossal magnetoresistance (CMR) class perovskite La0.53Ca0.47MnO3 have been studied by elastic and inelastic neutron scattering. This composition is near the 0.5 Ca transition from a metallic ferromagnet to an insulating antiferromagnet. Powder neutron diffraction on these samples showed coexisting ferromagnetic and antiferromagnetic phases at low temperature and a splitting of the lattice parameters of the antiferromagnetic phase near TN, reflecting the onset of the charge ordered state. Inelastic scattering measurements of the ferromagnetic excitations exhibited well-defined spin waves of resolution width below Tc. The spin waves exhibited conventional Heisenberg behavior with dispersion of the form E=Dq2+Δ with spin stiffness D(T=0 K)≈135 meV A2 and an energy gap Δ≈0.1 meV. The value of the spin wave stiffness D is similar to that found for other ferromagnetic perovskite materials, and it renormalized with temperature in a manner consistent with the magnetiza...

We have used complementary neutron and x-ray reflectivity techniques to examine the depth profiles of a series of as-grown and annealed Ga{sub 1-x}Mn{sub x}As thin films. A magnetization gradient is observed for two as-grown films and originates from a nonuniformity of Mn at interstitial sites, and not from local variations in Mn at Ga sites. Furthermore, we see that the depth-dependent magnetization can vary drastically among as-grown Ga{sub 1-x}Mn{sub x}As films despite being deposited under seemingly similar conditions. These results imply that the depth profile of interstitial Mn is dependent not only on annealing, but is also extremely sensitive to initial growth conditions. We observe that annealing improves the magnetization by producing a surface layer that is rich in Mn and O, indicating that the interstitial Mn migrates to the surface. Finally, we expand upon our previous neutron reflectivity study of Ga{sub 1-x}Mn{sub x}As, by showing how the depth profile of the chemical composition at the surface and through the film thickness is directly responsible for the complex magnetization profiles observed in both as-grown and annealed films.

This article presents experiments on the determination of the length scales of magnetic correlation in e-Co nanoparticle assemblies using SANS (small angle neutron scattering). In this experiment a 0.5 nm neutron beam was passed through a sealed sample containing a dense assembly of the nanoparticles, and the scattered intensity was collected with two-dimensional detector. Data were collected with magnetic fields ranging from 0 to 5 T, which were applied perpendicular to the neutron beam. The temperature varied between 5 and 275 K.

Owing to the coexistence of ferromagnetic and antiferromagnetic exchange coupling in an exchange-coupled Laves-phase superlattice composed of DyFe{sub 2} and YFe{sub 2} layers, the field dependence of the magnetization depth profile is complex. Using an approach that combines micromagnetic simulation and analysis of neutron scattering data, we have obtained the depth dependence of magnetization across the DyFe{sub 2}/YFe{sub 2} interfaces. We find that the exchange interaction across the interface is reduced compared to the exchange interaction of the constituent layers, thereby compromising the ability of this system to resist magnetization reversal in large applied fields.

The magnetic properties of MBE-grown Dy/Y superlattices (SLs) have been studied by neutron diffraction. The SL growth direction is at an angle of approximately 35° to the c -axis instead of being along one of the principal axis directions. The results confirm the coexistence of helical antiferromagnetic order and ferromagnetic order up to the Neel temperature T N ≈ 180 K. This is the first time that ferromagnetism has been observed in Dy/Y superlattices.

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.

We used neutron powder diffraction to obtain the local and long-range structure of ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ at room temperature and $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. By combining Rietveld and pair distribution function analysis of the total neutron scattering data, we have analyzed the structure of the compound using two competing models describing the low temperature phase: first the charge-ordered\char21{}orbital-ordered model and second the $\mathrm{Mn}\mathrm{Mn}$ dimer model. These structural models fit the $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ neutron powder diffraction pattern equally well using a Rietveld analysis. Therefore, pair distribution function analysis is used to probe the local and medium-range structure revealing a system with two distinctly distorted Mn octahedra and Mn ions with nonintegral valence states. The distorted octahedra differ with the structural model for the Zener polaron-type $\mathrm{Mn}\mathrm{Mn}$ dimer picture proposed for ${\mathrm{Pr}}_{0.60}{\mathrm{Ca}}_{0.40}\mathrm{Mn}{\mathrm{O}}_{3}$ and order in a similar checkerboard configuration associated with the CE-type antiferromagnetic structure. Therefore, locally the charge difference and structural ordering between the two Mn is appreciable enough to describe the system at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as ``partially charge ordered.''

We have studied the depth-dependent magnetic and structural properties of as-grown and optimally annealed ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films using polarized neutron reflectometry. In addition to increasing the total magnetization, the annealing process was observed to produce a significantly more homogeneous distribution of the magnetization. This difference in the films is attributed to the redistribution of Mn at interstitial sites during the annealing process. Also, we have seen evidence of significant magnetization depletion at the surface of both as-grown and annealed films.

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.

We report on the doping dependence of the order of the ferromagnetic metal to paramagnetic insulator phase transition in La1-xCaxMnO3. At x=0.33, magnetization and specific heat data show a first order transition, with an entropy change (2.3 J/mol K) accounted for by both volume expansion and the discontinuity of M approximately 1.7mu(B) via the Clausius-Clapeyron equation. At x=0.4, the data show a continuous transition with tricritical point exponents alpha=0.48+/-0.06, beta=0.25+/-0.03, gamma=1.03+/-0.05, and delta=5.0+/-0.8. This tricritical point separates first- (x 0.4) transitions.

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.

We report neutron diffraction studies of MnTe epitaxial layers and MnTe/ZnTe strained superlattices prepared on (100) GaAs substrates by molecular beam epitaxy. In these systems MnTe grows in the Zinc-Blende (ZB) phase, in contrast to bulk MnTe, found in the NiAs form. The magnetic interactions are short range and antiferromagnetic, and result in a fcc type-III antiferromagnetic ordering (AFM-III). The renormalization group theories predict a fluctuation-induced first order phase transition for such system in the absence of strain. Such transitionsis indeed reported in previous work on bulk β-MnS, and is observed in our MnTe epilayers. The epilayers seem to be virtually strain-free, with all three AFM-III domains present. This picture is complicated, however, by our discovery of a substantial magnetostriction in MnTe. This magnetostriction, observed by precise measurement of the lattice constant, seems to result in the relative population shifts between the domains. The superlattices with the MnTe layer thickness of 30 and 60 A exhibit a single AFM-III domain that has the lowest strain-determined energy, consistent with observed small tetragonal distortion of the MnTe lattice, resulting from the ZnTe and MnTe lattice mismatch. In the presence of strain the dimensionality of the order parameter is reduced from 6 to 2, in which case the theory predicts the continuous phase transition. This is indeed observed experimentally.

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.

New zinc‐blende Mn‐based magnetic semiconductors offer unique insights into the magnetic order and critical behavior of frustrated antiferromagnets. Molecular beam epitaxy grown samples exist both in the form of strained superlattices, with strain induced anisotropies (MnSe/ZnSe and MnTe/ZnTe), and in the form of near‐isotropic bulk MnTe. The tetragonally distorted strained superlattices exhibit a second‐order phase transition, consistent with symmetry arguments favoring such behavior. In contrast, the bulk‐like epitaxial layers of MnTe have a first‐order phase transition, associated with magnetostriction, unknown to an unaccounted for by the previous studies.

We report neutron diffraction studies of zinc‐blende strained MnSe/ZnSe superlattices. Such systems are examples of a fcc Heisenberg antiferromagnet of the third kind. The results show clearly long‐range order in the MnSe layer plane, and no magnetic coupling across the passive layers. The strain affects the relative domain population. Measurements of the diffracted intensity versus T show that the AF phase transition in the MnSe layers is of second order with TM = 107 ± 5 K.