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The anomalous Hall effect (AHE) is an intriguing transport phenomenon occurring typically in ferromagnets as a consequence of broken time reversal symmetry and spin-orbit interaction. It can be caused by two microscopically distinct mechanisms, namely, by skew or side-jump scattering due to chiral features of the disorder scattering, or by an intrinsic contribution directly linked to the topological properties of the Bloch states. Here we show that the AHE can be artificially engineered in materials in which it is originally absent by combining the effects of symmetry breaking, spin orbit interaction and proximity-induced magnetism. In particular, we find a strikingly large AHE that emerges at the interface between a ferromagnetic manganite (La_(0.7)Sr_(0.3)MnO_3) and a semimetallic iridate (SrIrO_3). It is intrinsic and originates in the proximity-induced magnetism present in the narrow bands of strong spin-orbit coupling material SrIrO_3, which yields values of anomalous Hall conductivity and Hall angle as high as those observed in bulk transition-metal ferromagnets. These results demonstrate the interplay between correlated electron physics and topological phenomena at interfaces between 3d ferromagnets and strong spin-orbit coupling 5d oxides and trace an exciting path towards future topological spintronics at oxide interfaces. The anomalous Hall effect (AHE) occurs in ferromagnets caused by intrinsic and extrinsic mechanisms. Here, Yoo et al. report large anomalous Hall conductivity and Hall angle at the interface between a ferromagnet La_(0.7)Sr_(0.3M)nO_3 and a semimetallic SrIrO_3, due to the interplay between correlated physics and topological phenomena.

The anomalous Hall effect (AHE) is an intriguing transport phenomenon occurring typically in ferromagnets as a consequence of broken time reversal symmetry and spin-orbit interaction. It can be caused by two microscopically distinct mechanisms, namely, by skew or side-jump scattering due to chiral features of the disorder scattering, or by an intrinsic contribution directly linked to the topological properties of the Bloch states. Here we show that the AHE can be artificially engineered in materials in which it is originally absent by combining the effects of symmetry breaking, spin orbit interaction and proximity-induced magnetism. In particular, we find a strikingly large AHE that emerges at the interface between a ferromagnetic manganite (La0.7Sr0.3MnO3) and a semimetallic iridate (SrIrO3). It is intrinsic and originates in the proximity-induced magnetism present in the narrow bands of strong spin-orbit coupling material SrIrO3, which yields values of anomalous Hall conductivity and Hall angle as high as those observed in bulk transition-metal ferromagnets. These results demonstrate the interplay between correlated electron physics and topological phenomena at interfaces between 3d ferromagnets and strong spin-orbit coupling 5d oxides and trace an exciting path towards future topological spintronics at oxide interfaces., The anomalous Hall effect (AHE) occurs in ferromagnets caused by intrinsic and extrinsic mechanisms. Here, Yoo et al. report large anomalous Hall conductivity and Hall angle at the interface between a ferromagnet La0.7Sr0.3MnO3 and a semimetallic SrIrO3, due to the interplay between correlated physics and topological phenomena.

A Kerr microscopy method for the quantitative measurement of the size of magnetic objects that are smaller than the resolution limit is proposed. It can be applied to domain walls, bubble domains, and magnetic skyrmion-bubble hybrid microstructures. The method is based on the integral contrast, determined by proper line scans across the object, which turns out to be independent of the resolution of the microscope after normalization to the maximum domain contrast.A Kerr microscopy method for the quantitative measurement of the size of magnetic objects that are smaller than the resolution limit is proposed. It can be applied to domain walls, bubble domains, and magnetic skyrmion-bubble hybrid microstructures. The method is based on the integral contrast, determined by proper line scans across the object, which turns out to be independent of the resolution of the microscope after normalization to the maximum domain contrast.

We report on the interplay between ferromagnetism and superconductivity in trilayers La{sub 0.7}Ca{sub 0.3}MnO{sub 3}/YBa{sub 2}Cu{sub 3}O{sub 7}/La{sub 0.7}Ca{sub 0.3}MnO{sub 3} made of half metallic manganite and high temperature superconductor cuprate. Samples with a fully oxygenated cuprate show a magnetic field interval where the magnetizations of the manganite are aligned antiparallel. A considerable magnetoresistance accompanies the switching between magnetization configurations (parallel vs. antiparallel) of the manganite moments. Suppression of the free carrier density of the cuprate which occurs upon oxygen depletion, results in deep modifications in the shape of the normal state hysteresis loops indicating that there may be a magnetic coupling mediated by free carrier density of the cuprate. This result outlines the importance of quasiparticle transmission in the interplay between ferromagnetism and superconductivity in this kind of samples.

In artificial multiferroics hybrids consisting of ferromagnetic La_(0.7)Sr_(0.3)MnO_(3) (LSMO) and ferroelectric BaTiO_(3) epitaxial layers, net Ti moments are found from polarized resonant soft x-ray reflectivity and absorption. The Ti dichroic reflectivity follows the Mn signal during the magnetization reversal, indicating exchange coupling between the Ti and Mn ions. However, the Ti dichroic reflectivity shows stronger temperature dependence than the Mn dichroic signal. Besides a reduced ferromagnetic exchange coupling in the interfacial LSMO layer, this may also be attributed to a weak Ti-Mn exchange coupling that is insufficient to overcome the thermal energy at elevated temperatures.

We report large magnetoresistance in ferromagnet/superconductor/ferromagnet structures made of La 0.7 Ca 0.3 MnO 3 and YBa 2 Cu 3 O 7 at temperatures along the resistive transition. We find that the magnetoresistance phenomenon is independent on the orientation of electric current versus field. Furthermore, the effect is also independent on the sweep rate of the magnetic field. This excludes interpretations in terms of spontaneous vortices or anisotropic magnetoresistance of the ferromagnetic layers and supports the view that the magnetoresistance phenomenon originates at the spin-dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor.

We have found large magnetoresistance in ferromagnet/superconductor/ferromagnet heterostructures made of La0.7Ca0.3MnO3 and YBa2Cu3O7. It originates at an increase of the width of the resistive transition when the magnetizations of the ferromagnetic layers are aligned antiparallel. We find that the shape and height of the magnetoresistance peaks are not modified when the angle between current and magnetic field is changed from parallel or perpendicular. Furthermore, we find that the temperature shift of the resistance curves is independent of the current values. This favors the view that the magnetoresistance phenomenon originates at the spin dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor, and rules out interpretations in terms of spontaneous vortices or anisotropic magnetoresistance of the ferromagnetic layers.

The heat management bottleneck drives the search for alternative approaches for energy-efficient information technologies. Temporal and spatial magnetic spin textures are of great interest, since their dynamics can occur with minimal dissipation. Typical examples include spin waves (magnons), which are the fundamental quasi-particle excitations of magnetically ordered systems, and skyrmions, which are spatial spin textures that that are stabilized by their distinct topology, and can behave as independent quasi-particles. In bulk helimagnetic systems it has been demonstrated at low temperatures that skyrmions can be efficiently manipulated via electric currents with ultra-low threshold current densities.

Controlling the magnetic properties of solids by electric fields has been an interesting research subject, not only because of the intriguing correlation between the electric and magnetic orders in solid-state systems, but also the potential applications in ultra-low energy spintronic devices. In the past, research has mostly been carried out with multiferroic materials and magnetic semiconductors. Recently, more effort was focused on 3d transition ferromagnetic metals. Especially in heavy metal/ferromagnet/oxide (HM/FM/oxide) structures where the magnetic anisotropy has an interfacial origin, electric fields can cause a marked change in the magnetic anisotropy energy. This voltage-controlled magnetic anisotropy (VCMA) can be understood by the electric field induced charge transfer among different d orbitals of the FM. The order of this effect is around 100 fJ/Vm and it vanishes with the removing of the electric fields.

Nanoscale Fe(1 1 0) layers have been fabricated on α -Al 2 O 3 using DC magnetron sputtering with Au(1 1 1) or Ag(1 1 1) as a buffer layer. Atomic force microscopy shows that Au(1 1 1) grows as a continuous layer and Ag(1 1 1) grows as nanoislands of irregular shapes and sizes. Magneto-optic Kerr effect shows that a 12 nm Fe(1 1 0) layer deposited on Au(1 1 1) at 295 K and on Ag(1 1 1) at 640 K is ferromagnetic at room temperature. In-plane magnetic anisotropy is observed for the Fe(1 1 0) layer on Au(1 1 1). The layer thickness, the surface coverage and the magnetization of the films have been determined by polarized neutron reflectometry.

We report progress in the development and application of neutron spin filters based on transmission through polarized 3 He gas. Tests and application of these devices are underway at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) and the Intense Pulsed Neutron Source (IPNS). The results of application to polarization analysis for diffuse reflectometry is discussed, along with an example of the issues in choosing the flipping ratio for such applications. The status of our development of polarized gas production using both spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping will be presented. For SEOP, we currently obtain 70–75% 3 He polarization in cells up to 500 cm 3 in volume.

The magnetization processes in exchange-biased MnPd∕Fe bilayers have been investigated using polarized neutron reflectivity and vibrating sample magnetometry. The measurements show that by breaking the symmetry of the intrinsic cubic fourfold anisotropy of the Fe film the induced unidirectional anisotropy radically changes the magnetization processes. If the exchange bias is large the induced unidirectional anisotropy is able to pull the net magnetization of the sample to the bias direction after saturation along any of the magnetic hard Fe⟨110⟩ directions. However, if the exchange bias is small it will only give rise to a difference in net magnetization along the magnetic easy axes adjacent to the saturation direction. Thus, the net magnetic moment along the bias direction depends on the relative magnitudes of the cubic and the unidirectional anisotropies.

The recently proposed spin-echo resolved grazing-incidence scattering (SERGIS) uses the well-known neutron spin echo effect for encoding the momentum transfer in reflectometry. By the application of tilted magnetic-field borders, SERGIS measures the scattering angle in grazing incidence experiments in absence of any geometrical beam-defining tool, such as slits. The main difficulty in such set-ups is the realization of geometrically flat field borders. The possibility of the application of neutron resonance spin echo (NRSE) for such a purpose is discussed, where the field borders are defined by current sheets. Prototype SERGIS experiments performed on holographically made optical gratings at a NRSE triple-axis spectrometer are shown.

The magnetic coupling between two permalloy layers with a Cr separating layer was studied using LMOKE, polarized neutron reflectivity and XRD. The coupling between two permalloy layers is antiferromagnetic-like when the Cr thickness is near 2.0 nm along the easy axis. A strong biquadratic coupling term has been found from the polarized neutron reflectivity study. The strong biquadratic coupling may be caused by the rough interface between the permalloy and Cr layers. The missing GMR effect of this system might be due to this strong biquadratic coupling of permalloy layers.

Polarized neutron reflectometry (PNR) has long been applied to measure the magnetic depth profile of thin films. In recent years, interest has increased in observing lateral magnetic structures in a film. While magnetic arrays patterned by lithography and submicron-sized magnetic domains in thin films often give rise to off-specular reflections, micron-sized ferromagnetic domains on a thin film produce few off-specular reflections and the domain distribution information is contained within the specular reflection. In this paper, we will first present some preliminary results of off-specular reflectivity from arrays of micron-sized permalloy rectangular bars. We will then use specular reflections to study the domain dispersion of an exchange-biased Co/CoO bilayer at different locations of the hysteresis loop.

Structural relaxation in a ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$ epitaxial film, which exhibits strong spin-orbit coupling, leads to depth-dependent magnetism. We combine two depth-resolved synchrotron x-ray techniques---two-dimensional reciprocal space mapping and x-ray magnetic circular dichroism---to quantitatively determine this effect. An 800-nm-thick film of ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$, grown with tensile epitaxial strain on ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}(225\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/{\text{(LaAlO}}_{3}{)}_{0.3}({\text{Sr}}_{2}{\mathrm{AlTaO}}_{6}{)}_{0.7}$, relaxes away from the ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}{\text{/SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}$ interface to its bulk lattice parameters, with much of the film being fully relaxed. Grazing incidence x-ray diffraction of the film elucidates the in-plane strain relaxation near the film-substrate interface, while depth-resolved x-ray magnetic circular dichroism at the Re L edge reveals the magnetic contributions of the Re site. The smooth relaxation of the film near the interface correlates with changes in the magnetic anisotropy. This provides a systematic and powerful way to probe the depth-varying structural and magnetic properties of a complex oxide with synchrotron-source x-ray techniques.

Elimination of a magnetic dead-layer at the LSMO/STO interface is achieved by interface engineering through incorporation of a single La0.33Sr0.67O layer. Controlled growth of interfacial atomic stacking in combination with local magnetic probing by polarized neutron reflectometry enables controlled interface engineering and results in optimal interfacial magnetization in perovskite-type correlated oxide heterostructures.

In thin layered Fe/Co (0 0 1), grown on MgO (0 0 1), both Fe and Co crystallize in the body-centered cubic (BCC) structure, as seen in a series of superlattices where the layer thickness of the components is varied from two to twelve atomic monolayers. These superlattices have novel magnetic properties as observed by magnetization and polarized neutron reflectivity measurements. There is a significant enhancement of the magnetic moments of both Fe and Co at the interfaces. Furthermore, the easy axis of the system changes from [1 0 0] for films of low cobalt content to [1 1 0] for a Co content exceeding 33%. No indication of a uniaxial anisotropy component is found in any of the samples. The first anisotropy constant (K1) of BCC Co is found to be negative with an estimated magnitude of 110 kJ/m3 at 10 K. In all cases, the magnetic moments of Fe and Co have parallel alignment.

We demonstrate that magnetic properties of ultra-thin Co films adjacent to Gd2O3 gate oxides can be directly manipulated by voltage. The Co films can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to an oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. Consequently, an unprecedentedly large change of magnetic anisotropy energy up to 0.73 erg/cm2 has been realized in a nonvolatile manner using gate voltages of only a few volts. These results open a new route to achieve ultra-low energy magnetization manipulation in spintronic devices., 12 pages, 9 figures, including supplemental materials. Phys. Rev. Lett., in press (Editors' Suggestion, featured in Physics)

We have determined the depth-resolved magnetization structures of a series of highly ordered Sr$_{2}$CrReO$_{6}$ (SCRO) ferrimagnetic epitaxial films via combined studies of x-ray reflectometry, polarized neutron reflectometry and SQUID magnetometry. The SCRO films deposited directly on (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ or SrTiO$_{3}$ substrates show reduced magnetization of similar width near the interfaces with the substrates, despite having different degrees of strain. When the SCRO film is deposited on a Sr$_{2}$CrNbO$_{6}$ (SCNO) double perovskite buffer layer, the width the interfacial region with reduced magnetization is reduced, agreeing with an improved Cr/Re ordering. However, the relative reduction of the magnetization averaged over the interfacial regions are comparable among the three samples. Interestingly, we found that the magnetization suppression region is wider than the Cr/Re antisite disorder region at the interface between SCRO and SCNO.

Specular reflectometry of polarized-neutrons was developed in the 1980s as a tool for measuring magnetic depth profiles in flat films, which were laterally uniform. When the lateral uniformity breaks down in an assembly of domains, off-specular grazing incidence scattering takes place. This review discusses this new frontier of reflectometry, describing the advances that are taking place in linking the observations of the scattering at grazing incidence with the size, the statistics, and the magnetic orientation of the domains. The article discusses also the progress made in linking the domain distribution thus found with the transport properties of these nanomagnetic systems.

Polarized neutron reflectivity measurements on magnetic films, containing a magnetic collinear structure, yield simple and transparent information on the direction of the magnetization. Yet the phe ...

Polarized neutron reflectometry (PNR) has an important role in solving non-collinear magnetic structures in thin films. In principle PNR provides the unambiguous determination of multi-axial magnetic depth profiles in systems of scientific and technological importance. However, only a limited number of problems have been solved unimpeachably, because the magnetic depth profiles were obtained by fitting procedures yielding solutions whose main features were not evident in the raw data. A recent formulation of reflectometry, and the concurrent development of “total polarimetry” are now stimulating the design of neutron polarization and polarization analysis geometries capable of determining directly if and how a magnetic system contains a non-collinear structure in the film ( x – y ) plane. In the simplest of these geometries the neutrons are polarized and analyzed along the z -axis perpendicular to the film plane.

The depolarization of a polarized neutron beam after transmission through a granular magnetic medium is related to the size and orientation of the ferromagnetic domains. Unfortunately, this relationship is simple only for a limited number of cases. In order to investigate the interpretation of neutron depolarization data, calculations for a three-dimensional model system of granular magnetic domains are presented. A comparison is made between the equations generally used to approximate the average domain size and the actual average domain size of the model system. Calculations are performed for several configurations of monodisperse spherical domains, as well as for a polydisperse distribution of nonspherical domains. While there is a good agreement between the actual domain size and that derived from the calculated neutron depolarization at low volume fractions of magnetic domains, at increasing volume fractions there is an increasing discrepancy. This discrepancy is partially due to magnetic correlation...

The evolution of the magnetic structure of multilayer [Fe (35 A)/Gd (50 A)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.

Neutron scattering techniques are becoming increasingly accessible to a broader range of scientific communities, in part due to the onset of next-generation, high-power spallation sources, high-performance, sophisticated instruments and data analysis tools. These technical advances also advantageously impact research into magnetism and magnetic materials, where neutrons play a major role. In this Current Perspective series, the achievements and future prospects of elastic and inelastic neutron scattering, polarized neutron reflectometry, small angle neutron scattering, and neutron imaging, are highlighted as they apply to research into magnetic frustration, superconductivity and magnetism at the nanoscale.

Electric-field control of magnetism has remained a major challenge which would greatly impact data storage technology. Although progress in this direction has been recently achieved, reversible magnetization switching by an electric field requires the assistance of a bias magnetic field. Here we take advantage of the novel electronic phenomena emerging at interfaces between correlated oxides and demonstrate reversible, voltage' driven magnetization switching without magnetic field. Sandwiching a non-superconducting cuprate between two manganese oxide layers, we find a novel form of magnetoelectric coupling arising from the orbital reconstruction at the interface between interfacial Mn spins and localized states in the CuO 2 planes. This results in a ferromagnetic coupling between the manganite layers that can be controlled by a voltage. Consequently, magnetic tunnel junctions can be electrically toggled between two magnetization states, and the corresponding spin' dependent resistance states, in the absence of a magnetic field., We acknowledge financial support by the Spanish MICINN through grants MAT2011-27470-C02 and Consolider Ingenio 2010–CSD2009-00013 (Imagine), by CAM through grant S2009/MAT-1756 (Phama) and by the ERC starting Investigator Award, grant #239739 STEMOX (G.S.-S.). Work at Argonne National Laboratory (Y.L. and S.G.E.t.V.) was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Research (J.F.) and X-ray experiments carried out at the Advanced Photon Source and X-ray experiments carried out at Advanced Light Source were supported by DOE, Office of Science, BES. We thank Masashi Watanabe for the Digital Micrograph PCA plug-in. Research at ORNL was sponsored by the US Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SO) and through the Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, DOE-BES (MV). Research (MRF) and neutron scattering experiments at the Lujan Center for Neutron Scattering, the Los Alamos National Laboratory were supported by DOE, Office of Science, BES. the Los Alamos National Laboratory is operated by the Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.

We report a strong effect of interface-induced magnetization on the transport properties of magnetic tunnel junctions consisting of ferromagnetic manganite La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and insulating cuprate PrBa$_{2}$Cu$_{3}$O$_{7}$. Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits a sudden anomalous decrease at low temperatures. Interestingly, this anomalous behavior can be attributed to the competition between the positive spin polarization of the manganite contacts and the negative spin-filter effect from the interface-induced Cu magnetization., 5 pages, 4 figures, with supplemental materials (2 figures). Physical Review Letters, in press

The magnetic behavior of exchange biased Co/CoO polycrystalline thin films has been investigated using magnetometry and magneto-optical (MO) imaging. For CoO layer thicknesses of about 30 A, these films exhibit a strong training effect below the blocking temperature of 130 K. A sharp initial reversal of the magnetization of the FM Co layer after field-cooling is followed by S-shaped magnetization loops with reduced coercive fields. The MO images show that during the initial magnetization reversal the remagnetization front moves from the edge of the sample into the homogenously magnetized film, leaving behind an irregular pattern of domains of the order of 10 μm. These domains, once generated during the first reversal, do not expand or move on subsequent magnetization loops and can be erased only by heating above the blocking temperature. This suggests that the domains are related to domains in the antiferromagnetic CoO layer.

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. © 2013 American Physical Society., Work at Argonne and use of the Center for Nanoscale Materials are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The work of F. S. B. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02.

et al., We examine the anomalous inverse spin switch behavior in La 0.7Ca 0.3MnO 3(LCMO)/YBa 2Cu 3O 7-δ (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. © 2012 American Physical Society., Research at Argonne National Laboratory was supported by the U. S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Grant No. DE-AC02-06CH11357. Work at UCM was supported by Spanish MICINN Grant No. MAT 2011 27470, Consolider Ingenio CSD2009-00013 (IMAGINE), CAM S2009-MAT 1756 (PHAMA). This work has benefited from the use of the Lujan Neutron Scattering Center at LANSCE, which is funded by the U. S. Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE through Contract No. DE-AC52-06NA25396

The strain in Co layers of multilayers with largely different structural mismatch (Co/Ni, Co/Ag, and Co/Cu) has been measured with nuclear magnetic resonance and compared with a model for strain in multilayers that is based on the model of Van der Merwe and Jesser. For the Co/Ni multilayers, the measured strain depends on both the Co and the Ni layer thicknesses over a large range and follows the behavior expected from the model in the coherent regime. The strain in the Co/Ag multilayers is proportional to 1/tCo and independent of the thickness of the Ag layers. This behavior resembles that for the incoherent regime. For Co/Cu multilayers the results indicate that the multilayers are in the transition region from coherent to incoherent behavior.

A polarized 3He neutron-spin filter was used to analyze polarized neutrons reflected from a 1000-A-thick Fe film immersed in a magnetic field. The spin filter analyzed both the specularly reflected component and the off-specular component caused by the phenomenon of Zeeman splitting of surface-scattered neutrons observed by Felcher et al. and investigated in more detail in subsequent experiments. The sample of polarized 3He was polarized by metastability-exchange optical pumping and compressed into a glass cell at the Indiana University Cyclotron Facility. The polarized gas was transported by car in a battery-powered solenoid holding field to the neutron reflectometry instrument POSY I at the Intense Pulsed Neutron Source at Argonne National Laboratory. Using the large solid angle of the polarized 3He spin filter, we were able to simultaneously analyze both components to the scattering and verify (as expected) that the polarization of the specular component was unchanged upon reflection and that the polarization of the off-specular component was reversed. To our knowledge this work represents the second experiment to employ a polarized 3He neutron-spin filter in polarized neutron reflectometry.

In epitaxial Fe/Cr superlattices the coupling between the Fe layers oscillates between antiferromagnetic (AFM) and ferromagnetic as a function of the Cr layer thickness tCr. The period of the oscillation is the same for superlattices grown with (211) and (100) orientations. We measured the coupling of Fe/Cr(110) superlattices consisting of three identical Fe layers. The magnetization curves were characterized by two to four levels, M/Ms=±1 or ± $\frac\Box1\Box\Box3\Box$ . Polarized neutron reflectometry identified the direction of the magnetization of each Fe layer and showed that the levels M/Ms=± $\frac\Box1\Box\Box3\Box$ were not always due to AFM alignment of the central layer, but rather it was found that, in spite of the structural similarity, the bottom Fe layer had a different coupling strength from the top Fe layer. The magnetic coupling must be sensitive to small structural differences at the interface. Furthermore, the measurements indicate an unexpected periodicity for Fe/Cr(110) superlattices.

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., Comment: 16 pages, 6 figures and 1 table

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.

We demonstrate that delta-doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AF) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Delta-doped carriers are inserted into a manganite superlattice (SL) by a digital-synthesis technique. Theoretical consideration of these additional carriers show that they cause a local enhancement of ferromagnetic (F) double-exchange with respect to AF superexchange, resulting in local canting of the AF 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, AF state as was predicted by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained elusive.

7 páginas, 7 figuras.-- et al., We report on the magnetic anisotropy controlled modulation of the superconductivity in La0.7 Ca0.3 MnO3/ YBa 2 Cu3O7-δ / La0.7 Ca 0.3 MnO3 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. © 2010 The American Physical Society., We thank A. Goldman for fruitful discussions within the framework of the joint U.S.-Spain NSF Materials World Network under Grant No. 709584. This work was supported by the U.S. Department of Energy, Basic Energy Science, under Contracts No. DE-AC02-06CH11357 and No. DEAC02NA25396, by Spanish MICINN under Grant No. MAT2008-06517, Consolider Ingenio 2010 Grant CSD2009-00013 Imagine, and “Ramon y Cajal” program, by CAM under Grant S2009/Mat-1756 Phama , and by OTKA under Grants No. F61733, No. K68807, and No. PF63954, and the “Bolyai” program of the Hungarian Academy of Sciences.

3 páginas, 2 figuras.-- et al., We report a memory concept utilizing ferromagnet/superconductor/ferromagnet La0.7Ca0.3MnO3/YBa2Cu 3O7/La0.7Ca0.3MnO3 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. © 2010 American Institute of Physics., Work was supported by the U.S. Department of Energy, Basic Energy Science under Contract Nos. DE-AC02-06CH11357 and DE-AC02NA25396, by Spanish MICINN under Contracts “Ramon y Cajal,” Grant Nos. MAT2008-06517 and CONSOLIDER INGENIO 2010 CSD2009-00013 IMAGINE , by CAM under PHAMA Grant No. S2009/Mat-1756, and by OTKA Grant Nos. K68807 and PF63954 and the “Bolyai” program of the Hungarian Academy of Sciences.

The reversal of the direction of the exchange bias in a “double-superlattice” system which consists of an Fe/Cr antiferromagnetic (AF) superlattice which is ferromagnetically coupled with an Fe/Cr ferromagnetic (F) superlattice through a Cr spacer layer, is observed. Magnetometry and polarized neutron reflectometry show that a switch in the bias direction occurs at a field (∼447 Oe) well below the field (14 kOe) necessary to saturate the AF superlattice and well below the field (2 kOe) where the AF superlattice initiates a spin–flop transition. The switching of the exchange bias cannot be explained in terms of a model of uniform rotation, but rather by breakdown into domains and reversal of the AF layers. The transparency of magnetic behavior of the double superlattice may be useful in understanding the behavior of traditional exchange bias systems.

Exchange bias of a partially oxidized thin film of ferromagnetic Co was studied by magnetization measurements and polarized neutron reflectivity (PNR). The magnetization curve shows strong effects of training with cycling of the magnetic field. Reflectivity measurements with the field parallel to the cooling field showed the onset of spin-dependent diffuse scattering—off the specular reflection—after a training cycle. Such scattering, of the Yoneda type, is due to misaligned Co domains possibly close to the Co/CoO interface. Subjecting the field cooled Co/CoO pair to a field perpendicular to the cooling field causes a rotation of the magnetization. The PNR measurements confirmed earlier susceptibility studies by indicating that the rotation of the magnetization is reversible in fields up to 400 Oe. The rotation of the magnetization of Co is uniform across the film thickness.

Recent magnetometry measurements in modulation-doped La{sub 1-x}Ca{sub x}MnO{sub 3} suggested that a net magnetization extends from the ferromagnetic layers into the adjacent antiferromagnet layers. Here we test this hypothesis by polarized neutron reflectometry, which allows us to determine the depth resolved magnetization profile. From fits to the reflectivity data we find that the additional magnetization does not occur at the ferromagnetic/antiferromagnetic interfaces, but rather in a thin region of the first antiferromagnetic layer adjacent to the interface with the substrate.

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 Neel 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.

A unified approach combining polarized neutron and resonant x-ray magnetic reflectometry has been applied to determine the magnetic structure in an ${[\text{Fe}(35\text{ }\text{\AA{}})/\text{Gd}(50\text{ }\text{\AA{}})]}_{5}$ multilayer as a function of temperature and magnetic field. Simultaneous self-consistent refinement of neutron and x-ray data made it possible to resolve the element-specific magnetization profile in the multilayer with unprecedented accuracy. It is shown that the small number of bilayer periods together with the asymmetric termination (Fe-top, Gd-bottom) lead to unique low-temperature magnetic phases characterized by significant twisting of Fe and Gd magnetic moments and nonuniform distribution of vectorial magnetization within Gd layers. A twisted magnetic state was found to be stable at small magnetic fields and at a low temperature of 20 K, which is well below the compensation temperature of this artificial ferrimagnetic system.

The layer-by-layer magnetization of a “double-superlattice” Fe/Cr(211) exchange-bias junction was determined by polarized neutron reflectometry. An n-layered [Fe/Cr]n antiferromagnetic (AF) superlattice is coupled with an m-layered [Fe/Cr]m ferromagnetic (F) superlattice, to provide a controlled exchange bias. In low magnetic fields, the magnetizations of the two superlattices are collinear. The two magnetized states (along or opposite to the bias field) differ only in the relative orientation of the F and adjacent AF layer. At higher fields, the AF moments flop to the direction perpendicular to the applied field. The structure, thus determined, explains the magnitude of the bias field.

Superlattices of $({\mathrm{LaMnO}}_{3}{)}_{2n}/({\mathrm{SrMnO}}_{3}{)}_{n}$ ($1\ensuremath{\le}n\ensuremath{\le}5$), composed of the gapped insulators ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{SrMnO}}_{3}$, undergo a metal-insulator transition as a function of $n$, being metallic for $n\ensuremath{\le}2$ and insulating for $n\ensuremath{\ge}3$. 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 ${\mathrm{LaMnO}}_{3}$ and suppressed in ${\mathrm{SrMnO}}_{3}$. The modulation is consistent with a Mott transition driven by the proximity between the $({\mathrm{LaMnO}}_{3})/({\mathrm{SrMnO}}_{3})$ interfaces. The insulating state for $n\ensuremath{\ge}3$ obeys variable range hopping at low temperatures. We suggest that this is due to states at the Fermi level that emerge at the $({\mathrm{LaMnO}}_{3})/({\mathrm{SrMnO}}_{3})$ interfaces and are localized by disorder.