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ABSTRACT: Magnetization noncollinearity in ferromagnet-superconductor (F/S)
heterostructures is expected to enhance the superconducting transition
temperature (Tc) according to the domain-wall superconductivity theory, or to
suppress Tc when spin-triplet Cooper pairs are explicitly considered. We study
the proximity effect in F/S structures where the F layer is a Sm-Co/Py
exchange-spring bilayer and the S layer is Nb. The exchange-spring contains a
single, controllable and quantifiable domain wall in the Py layer. We observe
an enhancement of superconductivity that is nonmonotonic as the Py domain wall
is increasingly twisted via rotating a magnetic field, different from
theoretical predictions. We have excluded magnetic fields and vortex motion as
the source of the nonmonotonic behavior. This unanticipated proximity behavior
suggests that new physics is yet to be captured in the theoretical treatments
of F/S systems containing noncollinear magnetization.
Physical Review Letters 04/2013; 110(17):177001. · 7.37 Impact Factor
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Yaohua Liu,
C. Visani,
N. M. Nemes,
M. R. Fitzsimmons,
L. Y. Zhu,
J. Tornos,
M. Zhernenkov,
A. Hoffmann,
C. Leon,
J. Santamaria, S. G. E. te Velthuis
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ABSTRACT: We examine the anomalous inverse spin switch behavior in
La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO)/YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO)/LCMO
trilayers by combined transport studies and polarized neutron reflectometry.
Measuring magnetization profiles and magnetoresistance in an in-plane rotating
magnetic field, we prove that, contrary to many accepted theoretical scenarios,
the relative orientation between the two LCMO's magnetizations is not
sufficient to determine the magnetoresistance. Rather the field dependence of
magnetoresistance is explained by the interplay between the applied magnetic
field and the (exponential tail of the) induced exchange field in YBCO, the
latter originating from the electronic reconstruction at the LCMO/YBCO
interfaces.
Physical Review Letters 05/2012; 108(20):207205. · 7.37 Impact Factor
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ABSTRACT: The evolution of the magnetic structure of multilayer [Fe (35 Å)/Gd (50 Å)5] with variation in temperature and an applied magnetic field was determined using a complementary approach combining polarized
neutron and X-ray resonant magnetic reflectometry. Self-consistent simultaneous analysis of X-ray and neutron spectra allowed
us to determine the elemental and depth profiles in the multilayer structure with unprecedented accuracy, including the identification
of an inhomogeneous intralayer magnetic structure with near-atomic resolution.
Bulletin of the Russian Academy of Sciences Physics 04/2012; 74(10):1471-1473.
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ABSTRACT: We demonstrate that delta doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AFM) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Theoretical consideration of these additional carriers shows that they cause a local enhancement of ferromagnetic double exchange with respect to AFM superexchange, resulting in local canting of the AFM spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy, and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AFM state as was predicted by de Gennes [Phys. Rev. 118, 141 (1960)] but had remained elusive.
Physical Review Letters 10/2011; 107(16):167202. · 7.37 Impact Factor
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ABSTRACT: An understanding of the structure of ultrathin polymer films on solid substrates has scientific importance in applications as well as in fundamental studies of polymer diffusion or adsorption. We present studies of the organization of dewetted droplets of polymers on a silicon surface using a new neutron scattering technique, spin-echo resolved grazing incidence scattering (SERGIS), that has the potential to address at the same time the droplet-droplet correlations and the chemical configuration inside each droplet. For the seminal experiments, the polarized neutron reflectometer EVA at the Institut Laue-Langevin, Grenoble, France, was equipped with a spin-echo setup, and measurements were taken on surface structures previously characterized by different techniques. The dewetted polymers used in our studies were pure polystyrene, a mixture of polystyrene and polyparamethylstyrene, and a diblock copolymer of the two homopolymers. Even with a provisional setup SERGIS, we were able to determine the correlation between the droplets, providing results in excellent agreement with those obtained by atomic force microscopy and grazing incidence small-angle X-ray and neutron scattering. In addition, it was confirmed that the correlation function for diblock copolymer droplets is more complex than for polymer mixtures, exhibiting partial ordering of the copolymer within each droplet.
The Journal of Physical Chemistry B 05/2011; 115(19):5754-65. · 3.70 Impact Factor
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ABSTRACT: The depth profile of the intrinsic magnetic properties in an Fe/Sm-Co bilayer
fabricated under nearly optimal spring-magnet conditions was determined by
complementary studies of polarized neutron reflectometry and micromagnetic
simulations. We found that at the Fe/Sm-Co interface the magnetic properties
change gradually at the length scale of 8 nm. In this intermixed interfacial
region, the saturation magnetization and magnetic anisotropy are lower and the
exchange stiffness is higher than values estimated from the model based on a
mixture of Fe and Sm-Co phases. Therefore, the intermixed interface yields
superior exchange coupling between the Fe and Sm-Co layers, but at the cost of
average magnetization.
Physical Review B 05/2011; 83(17):174418. · 3.69 Impact Factor
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C Visani,
J Tornos,
N M Nemes,
M Rocci,
C Leon,
J Santamaria, S G E Te Velthuis,
Yaohua Liu,
A Hoffmann,
J W Freeland,
M Garcia-Hernandez,
M R Fitzsimmons,
B J Kirby,
M Varela,
S J Pennycook
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ABSTRACT: We have analyzed the interface structure and composition of La 0.7 Ca 0.3 MnO 3 /YBa 2 Cu 3 O 7 /La 0.7 Ca 0.3 MnO 3 trilayers by combined polarized neutron reflectometry, aberration-corrected microscopy, and atomic column resolution electron-energy-loss spectroscopy and x-ray absorption with polarization analysis. We find the same stacking sequence at both top and bottom cuprate interfaces. X-ray magnetic circular dichroism experiments show that both cuprate interfaces are magnetic with a magnetic moment induced in Cu atoms as expected from symmetric Mn-O-Cu superexchange paths. These results supply a solid footing for the applicability of recent theories explaining the interplay between magnetism and superconductivity in this system in terms of the induced Cu spin polarization at both interfaces [J. In recent years there has been a surge of interest on the interplay between ferromagnetism and superconductivity in artificial thin-film hybrids. 1,2 This interplay originates from the ferromagnet (F)/superconductor (S) proximity effect by which Cooper pairs penetrate into the ferromagnet, directly experiencing the exchange interaction. 3–11 As a result, the effect is short range, and the relevant length scale becomes shorter with increasing spin polarization of the ferromagnet, and should vanish in the limit of full spin polarization, i.e., a half metal. Superconductivity is also suppressed in the superconducting layer over length scales given by the superconducting coherence length. Recent studies on La 0.7 Ca 0.3 MnO 3 (LCMO)/YBa 2 Cu 3 O 7 (YBCO) epitaxial het-erostructures demonstrate superconductivity suppression of the superconducting critical temperature over length scales much larger (one to two orders of magnitude) than the coherence length of the cuprate. 12,13 This can hardly be explained in terms of a (singlet) proximity effect given the high spin polarization of the manganite and the subnanometer coherence length of the cuprate. Other explanations in terms of (self-) diffusion of spin-polarized quasiparticles 14 or induction of a triplet superconductivity component 15 also do not account for the long length scale of the superconductivity suppression in the cuprate. The strong electronic and orbital reconstruction occurring at this interface gives rise to an induced negative spin polarization of the cuprate interface as found in Refs. 16 and 17, and very recently confirmed by Ref. 18. This may arise due to a canting of the antiferromagnetic Cu sublattice induced by the interfacial (antiferromagnetic) Mn-O-Cu superexchange interaction. Recent theoretical calculations 19 have shown that Cu spin polarization, which does not originate from a proximity effect but from orbital reconstruction, accounts semiquantitatively for most experimental observations. Fur-thermore, this mechanism also explains the inverse spin-switch behavior observed in F/S/F structures where superconductivity is enhanced when the F layer magnetizations are parallel. 20 The balance between applied magnetic field and (the ex-ponential tail of the) effective field due to the interfacial antiferromagnetic superexchange interaction cancel out when the layers are parallel, giving rise to a small shift of the resistance curve toward higher transition temperatures. The applicability of the orbital reconstruction mechanism depends critically on the existence of a superexchange path linking Mn atoms and planar Cu atoms in the cuprate at both interfaces. This implies certain symmetry in the orbital and electronic reconstruction at both interfaces, a situation which cannot be taken for granted in epitaxial layers. In fact, it has been recently pointed out that in superlattices (A-B-A-B-A. . .) mirror interfaces (A-B and B-A) need not be identical. 21,22 While preliminary results of interface analysis of LCMO/YBCO samples grown by sputter deposition (similar to those reported in this work) suggest symmetric interfaces,
Physical Review B 01/2011; 105:60405-256804. · 3.69 Impact Factor
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ABSTRACT: We report a memory concept utilizing ferromagnet/superconductor/ferromagnet La <sub>0.7</sub> Ca <sub>0.3</sub> MnO <sub>3</sub>/ YBa <sub>2</sub> Cu <sub>3</sub> O <sub>7</sub>/ La <sub>0.7</sub> Ca <sub>0.3</sub> MnO <sub>3</sub> thin film hybrid structures. The orientation of the magnetic field with respect to the ferromagnetic easy axis has a strong effect on superconductivity as indicated by a strong variation in the magnetoresistance (MR). MR can be controlled by rotating a small magnetic field applied in the plane of the film in a way that is determined by the in-plane biaxial magnetic anisotropy. The proposed memory device has the advantages of superconducting detection elements (fast response and low dissipation), small (100–150 Oe) writing fields, and resistance read-out without need for applied field.
Applied Physics Letters 08/2010; · 3.84 Impact Factor
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C Visani,
N M Nemes,
M Rocci,
Z Sefrioui,
C Leon, S G E Te Velthuis,
A Hoffmann,
M R Fitzsimmons,
F Simon,
T Feher,
M Garcia-Hernandez,
J Santamaria
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ABSTRACT: We report on the magnetic anisotropy controlled modulation of the superconductivity in La 0.7 Ca 0.3 MnO 3 / YBa 2 Cu 3 O 7− / La 0.7 Ca 0.3 MnO 3 ferromagnet/superconductor/ferromagnet hybrids with biaxial easy axes. The magnetoresistance MR is determined by the local misalignment of the magnetizations in the two layers and exhibits a positive MR plateau for antiparallel alignment along the easy axes and negative MR peaks at the coercive field near the hard axes. This evidences the importance of spin-dependent interfacial scattering effects as opposed to stray fields in the MR behavior of superconducting oxide inverse spin switches.
Physical Review B 03/2010; 25. · 3.69 Impact Factor
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S J May,
P J Ryan,
J L Robertson,
J-W Kim,
T S Santos,
E Karapetrova,
J L Zarestky,
X Zhai, S G E te Velthuis,
J N Eckstein,
S D Bader,
A Bhattacharya
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ABSTRACT: The disorder inherent to doping by cation substitution in the complex oxides can have profound effects on collective-ordered states. Here, we demonstrate that cation-site ordering achieved through digital-synthesis techniques can dramatically enhance the antiferromagnetic ordering temperatures of manganite films. Cation-ordered (LaMnO3)m/(SrMnO3)2m superlattices show Néel temperatures (TN) that are the highest of any La(1-x)Sr(x)MnO3 compound, approximately 70 K greater than compositionally equivalent randomly doped La(1/3)Sr(2/3)MnO3. The antiferromagnetic order is A-type, consisting of in-plane double-exchange-mediated ferromagnetic sheets coupled antiferromagnetically along the out-of-plane direction. Through synchrotron X-ray scattering, we have discovered an in-plane structural modulation that reduces the charge itinerancy and hence the ordering temperature within the ferromagnetic sheets, thereby limiting TN. This modulation is mitigated and driven to long wavelengths by cation ordering, enabling the higher TN values of the superlattices. These results provide insight into how cation-site ordering can enhance cooperative behaviour in oxides through subtle structural phenomena.
Nature Material 11/2009; 8(11):892-7. · 32.84 Impact Factor
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J de la Venta,
V Bouzas,
A Pucci,
M A Laguna-Marco,
D Haskel, S G E te Velthuis,
A Hoffmann,
J Lal,
M Bleuel,
G Ruggeri,
C de Julián Fernández,
M A García
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ABSTRACT: X-ray magnetic circular dichroism (XMCD) and Small Angle Neutron Scattering (SANS) measurements were performed on thiol capped Au nanoparticles (NPs) embedded into polyethylene. An XMCD signal of 0.8 x 10(-4) was found at the Au L3 edge of thiol capped Au NPs embedded in a polyethylene matrix for which Superconducting Quantum Interference Device (SQUID) magnetometry yielded a saturation magnetization, M(S), of 0.06 emu/g(Au). SANS measurements showed that the 3.2 nm average-diameter nanoparticles are 28% polydispersed, but no detectable SANS magnetic signal was found with the resolution and sensitivity accessible with the neutron experiment. A comparison with previous experiments carried out on Au NPs and multilayers, yield to different values between XMCD signals and magnetization measured by SQUID magnetometer. We discuss the origin of those differences.
Journal of Nanoscience and Nanotechnology 11/2009; 9(11):6434-8. · 1.56 Impact Factor
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Physical Review B 01/2009; 80:052403. · 3.69 Impact Factor
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ABSTRACT: Superlattices of (LaMnO3){2n}/(SrMnO3){n} (1<or=n<or=5), composed of the gapped insulators LaMnO3 and SrMnO3, undergo a metal-insulator transition as a function of n, being metallic for n<or=2 and insulating for n>or=3. Measurements of transport, magnetization, and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in LaMnO3 and suppressed in SrMnO3. The modulation is consistent with a Mott transition driven by the proximity between the (LaMnO3)/(SrMnO3) interfaces. The insulating state for n>or=3 obeys variable range hopping at low temperatures. We suggest that this is due to states at the Fermi level that emerge at the (LaMnO3)/(SrMnO3) interfaces and are localized by disorder.
Physical Review Letters 06/2008; 100(25):257203. · 7.37 Impact Factor
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ABSTRACT: Polarized neutron reflectivity measurements of a ferromagnetic [(LaMnO3)11.8/(SrMnO3)4.4]6 superlattice reveal a modulated magnetic structure with an enhanced magnetization at the interfaces where LaMnO3 was deposited on SrMnO3 (LMO/SMO). However, the opposite interfaces (SMO/LMO) are found to have a reduced ferromagnetic moment. The magnetic asymmetry is accompanied by a corresponding asymmetry in the lateral structural roughness of the two interfaces observed via electron microscopy, with enhanced ferromagnetism present at the interfaces that are atomically smooth over tens of nanometers. This result demonstrates that atomic-scale roughness can destabilize interfacial phases in complex oxide heterostructures.
Phys. Rev. B. 05/2008; 77(17).
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ABSTRACT: The magnetization reversal in exchange biased FeF2/Co films was studied using polarized neutron reflectometry and magneto-optical imaging (MOI). After field cooling in a field H perpendicular to the c axis (the easy axis of FeF2) through the FeF2 Néel temperature, the magnetization curve measured with H parallel to the c axis showed a double loop, one with a positive exchange bias field (HE) and one with a negative HE. This behavior suggests that the antiferromagnet (AFM) is split into two types of domains with HE in opposite directions along the c axis. However, the observation of spin-flip reflectivity in the field region separating the two loops indicates that some rotation of the Co magnetization also occurred during reversal. Concurrently, small inhomogeneous domains with sizes much less than 30 μm were observed using MOI. This implies that small AFM domains with opposing HE created during cooling and the resulting frustration of the interface exchange interaction with the ferromagnet are responsible for the perpendicular magnetization during reversal.
Journal of Applied Physics 03/2008; 103(7):07C114-07C114-3. · 2.17 Impact Factor
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ABSTRACT: The magnetic hysteresis curves of Ni Fe (t)/ Fe Mn (15 nm )/ Ni Fe (5 nm ) exchange-biased trilayers, with t ranging from 3.8 to 18 nm , show two clearly separated loops, which were interpreted as corresponding to the independent magnetic reversal of the two ferromagnetic (FM) NiFe layers. Polarized neutron reflectometry allowed the determination of the evolution of these two layer magnetizations at various fields along the hysteresis curve. For increasing t , the behavior of the magnetization evolved from one where both layer magnetizations were always collinear to the applied field to one where rotation of the magnetizations was observed at various fields. Measurements on the thickest field-annealed sample additionally showed that, during the reversal, the magnetizations of the two NiFe layers are close to being perpendicular to each other. This magnetic configuration is discussed in terms of the FM/antiferromagnetic (AFM) interactions and of a possible coupling between the two FM layers across the thick AFM FeMn spacer.
Journal of Applied Physics 06/2007; · 2.17 Impact Factor
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ABSTRACT: We studied the transition between the antiferromagnetic and the surface spin-flop phases of a uniaxial antiferromagnetic [Fe(14 \AA)/Cr(11 \AA]$_{\rm x20}$ superlattice. For external fields applied parallel to the in-plane easy axis, the layer-by-layer configuration, calculated in the framework of a mean-field one-dimensional model, was benchmarked against published polarized neutron reflectivity data. For an in-plane field $H$ applied at an angle $\psi \ne 0$ with the easy axis, magnetometry shows that the magnetization $M$ vanishes at H=0, then increases slowly with increasing $H$. At a critical value of $H$, a finite jump in $M(H)$ is observed for $\psi<5^{\rm o}$, while a smooth increase of $M$ $vs$ $H$ is found for $\psi>5^{\rm o}$. A dramatic increase in the full width at half maximum of the magnetic susceptibility is observed for $\psi \ge 5^{\rm o}$. The phase diagram obtained from micromagnetic calculations displays a first-order transition to a surface spin-flop phase for low $\psi$ values, while the transition becomes continuous for $\psi$ greater than a critical angle, $\psi_{\rm max} \approx 4.75^{\rm o}$. This is in fair agreement with the experimentally observed results.
04/2007;
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ABSTRACT: Biconcave cylindrical lenses are used to focus beams of x rays or neutrons using the refractive properties of matter. In the case of neutrons, the refractive properties of magnetic induction can similarly focus and simultaneously polarize the neutron beam without the concomitant attenuation of matter. This concept of a magnetic refractive lens was tested using a compound lens consisting of 99 pairs of cylindrical permanent magnets. The assembly successfully focused the intensity of a white beam of cold neutrons of one spin state at the detector, while defocusing the other. This experiment confirmed that a lens of this nature may boost the intensity locally by almost an order of magnitude and create a polarized beam. An estimate of the performance of a more practically dimensioned device suitable for incorporation in reflectometers and slit-geometry small angle scattering instruments is given.
Review of Scientific Instruments 04/2007; · 1.37 Impact Factor
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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: We studied the magnetic properties of La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_{7-\delta}$ superlattices. Magnetometry showed that with increasing YBa$_2$Cu$_3$O$_{7-\delta}$ layer thickness the saturation magnetization per La$_{0.7}$Ca$_{0.3}$MnO$_3$ layer decreases. From polarized neutron reflectometry we determined that this magnetization reduction is due to an inhomogenous magnetization depth profile arising from the suppression of magnetization near the La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_{7-\delta}$ interface. Electron energy loss spectroscopy indicates an increased 3d band occupation of the Mn atoms in the La$_{0.7}$Ca$_{0.3}$MnO$_3$ layers at the interface. Thus, the suppression of ferromagnetic order at the La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_{7-\delta}$ interface is most likely due to charge transfer between the two materials.
08/2005;
