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K L Krycka,
R A Booth,
C R Hogg,
Y Ijiri,
J A Borchers,
W C Chen,
S M Watson,
M Laver,
T R Gentile,
L R Dedon,
S Harris, J J Rhyne,
S A Majetich
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ABSTRACT: 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.
Physical Review Letters 05/2010; 104(20):207203. · 7.37 Impact Factor
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ABSTRACT: Small-angle neutron scattering measurements of Co nanoparticle assemblies reveal three characteristic length scales associated with the interparticle and intraparticle magnetic orders. The first length scale stemming from particle size and separation does not vary with applied field. In contrast, the magnetic correlation length increases from 71±9 nm in zero field at 5 K to greater than 1000 nm in fields larger than 0.2 T. The random-field length scale decreases from 37±8 nm when H = 0 to 9.1±0.3 nm in H = 0.2 T, and the contribution of this term is less significant in large fields.
Applied Physics Letters 04/2008; 92(15):152503-152503-3. · 3.84 Impact Factor
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ABSTRACT: 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 Dy and Er is preserved across the intervening Y non-magnetic layers and corresponds to a "pseudo turn-angle" near 51 degrees in the Y, which is in accord with theoretical calculations from the band structure. This suggests that the exchange coupling is via a conduction band spin density wave in both Y and Dy, stabilized by the 4f spins of the Dy. The magnetic coherence length, determined from the neutron linewidth, exhibits a 1/r dependence on the Y layer thickness for a fixed number of Dy planes. The ferromagnetic transitions occurring in the pure elements are completely suppressed in the multilayers due to epitaxial "clamping" by the Y layers which exhibits 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.
Physica Scripta 02/2007; 1989(T29):31. · 1.20 Impact Factor
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ABSTRACT: 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.
Physica Scripta 01/2007; 1991(T35):163. · 1.20 Impact Factor
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ABSTRACT: This article presents experiments on the determination of the length scales of magnetic correlation in epsiv-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.
Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 06/2006
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B. J. Kirby,
J. A. Borchers, J. J. Rhyne,
K. V. O'Donovan,
S. G. E. te Velthuis,
S. Roy,
Cecilia Sanchez-Hanke,
T. Wojtowicz,
X Liu,
W. L. Lim,
M. Dobrowolska,
J. K. Furdyna
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ABSTRACT: We have used complementary neutron and x-ray reflectivity techniques to examine the depth profiles of a series of as-grown and annealed Ga[1-x]Mn[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[1-x]Mn[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[1-x]Mn[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.
03/2006;
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ABSTRACT: Aggregates of monodisperse iron-based nanoparticles were investigated by small-angle neutron scattering. The field dependence of the scattering intensity showed marked differences for particles depending on size and degree of oxidation. The angular dependence of the intensity indicated magnetic regions within an oxidized sample with spins perpendicular to the applied field, which dominated the scattering at the diffraction peak. The unexpected results are interpreted in terms of an iron core that is exchange coupled to an iron oxide shell.
Applied Physics Letters 06/2005; 86(24):243102-243102-3. · 3.84 Impact Factor
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ABSTRACT: Annealing can increase the Curie temperature and net magnetization in uncapped Ga1−xMnxAs films, effects that are suppressed when the films are capped with GaAs. Previous polarized neutron reflectometry (PNR) studies of uncapped Ga1−xMnxAs revealed a pronounced magnetization gradient that was reduced after annealing. We have extended this study to Ga1−xMnxAs capped with GaAs. We observe no increase in Curie temperature or net magnetization upon annealing. Furthermore, PNR measurements indicate that annealing produces minimal differences in the depth-dependent magnetization, as both as-grown and annealed films feature a significant magnetization gradient. These results suggest that the GaAs cap inhibits redistribution of interstitial Mn impurities during annealing.
Applied Physics Letters 02/2005; 86(7):072506-072506-3. · 3.84 Impact Factor
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ABSTRACT: Annealing can increase the Curie temperature and net magnetization in uncapped (Ga,Mn)As films, effects that are suppressed when the films are capped with GaAs. Previous polarized neutron reflectometry (PNR) studies of uncapped (Ga,Mn)As revealed a pronounced magnetization gradient that was reduced after annealing. We have extended this study to (Ga,Mn)As capped with GaAs. We observe no increase in Curie temperature or net magnetization upon annealing. Furthermore, PNR measurements indicate that annealing produces minimal differences in the depth-dependent magnetization, as both as-grown and annealed films feature a significant magnetization gradient. These results suggest that the GaAs cap inhibits redistribution of interstitial Mn impurities during annealing. Comment: 12 pages, 3 figures, submitted to Applied Physics Letters
09/2004;
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ABSTRACT: We have studied the depth-dependent magnetic and structural properties of as-grown and optimally annealed Ga[1-x]Mn[x]As films using polarized neutron reflectometry. In addition to increasing 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. Comment: 5 pages, 3 figures
11/2003;
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ABSTRACT: 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 Å (≈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 Å. © 2000 American Institute of Physics.
Journal of Applied Physics. 04/2000; 87(9):6460-6462.
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ABSTRACT: Epitaxial superlattices of Dy and Y, grown by molecular beam epitaxy with modulation along the HCP b axis, fail to develop long-range magnetic order along b even for Y spacer layers of only nine atomic planes. The difference between these samples and c-axis superlattices reflects on highly anisotropic magnetic response of the Y interlayers.
Journal of Physics Condensed Matter 12/1998; 1(34):5997. · 2.55 Impact Factor
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ABSTRACT: Magnetic semiconductor superlattices consisting of x monolayers of ZnTe alternated with y monolayers of MnTe [( ZnTe )<sub>x</sub>|( MnTe )<sub>y</sub>] 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. © 1998 American Institute of Physics.
Journal of Applied Physics 07/1998; · 2.17 Impact Factor
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ABSTRACT: The complex magnetic structure of Nd exhibits a new magnetic phase when grown epitaxially, either as a stabilized double hexagonal close-packed alloy, or as part of a Nd/Y superlattice. In the alloy and in those superlattices with small Nd/Y ratios, the incommensurate b axis modulated structure exhibited by elemental Nd at TN=19.9 K is absent. In its place, incommensurate helimagnetic order, similar to that in dilute yttrium alloys of the heavy lanthanides, appears below approximately 30 K. Evidence for coherent propagation of the magnetic spiral through the Y spacers is observed in some Y-rich superlattices. As the Nd:Y ratio in the superlattices is increased, the b axis modulated phases reappear and coexist with the helical phase. Thick epitaxial Nd films, nevertheless, exhibit only bulklike magnetic structure. We discuss these observations in terms of band structure modifications.
Phys. Rev. B. 08/1997; 56(9).
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ABSTRACT: 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
IEEE Transactions on Magnetics 12/1993; · 1.36 Impact Factor
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ABSTRACT: Thin dysprosium c‐axis films (40–400 Å) were grown coherently between 500‐Å lutetium layers by molecular beam epitaxy. Bulk magnetization measurements show that these sandwich structures order magnetically at T N ≂178 K (=T N of elemental Dy) and undergo ferromagnetic transitions at temperatures which range from 100 K (400 Å Dy) to 175 K (40 Å Dy), significantly enhanced from the bulk T C =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 T C , 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.
Journal of Applied Physics 06/1993; · 2.17 Impact Factor
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ABSTRACT: 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 T c ∼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.
Journal of Applied Physics 06/1993; · 2.17 Impact Factor
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Magnetics Conference, 1993. INTERMAG '93., Digest of International; 05/1993
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ABSTRACT: 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.
Journal of Applied Physics 12/1991; · 2.17 Impact Factor
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ABSTRACT: Neutron diffraction studies of ZnSe/MnSe strained-layer superlattices
with individual MnSe layer thicknesses ranging from 3 to 15 monolayers
show a second-order transition to type-III antiferromagnetic order. The
magnetic reflections indicate a single orientation of the type-III
domain, with the tetragonal axis perpendicular to the layer plane and
the spins lying within the layer plane.
Physical review. B, Condensed matter 08/1991; 44:4701-4704.
