Publications

Abstract: Magnetic nanoparticles are increasingly used in nanotechnologies and biomedical applications, such as drug targeting, MRI, bio-separation. Magnetite (Fe3O4) nanoparticles stand to be effective in these roles due to the non-toxic nature of magnetite and its ease of manufacture. To be more effective in these applications, a greater understanding of the magnetic behavior of a collection of magnetite nanoparticles is needed. This research seeks to discover the local magnetic ordering of ensembles of magnetite nanoparticles occurring under various external fields. To complete this study, we use x-ray resonant magnetic scattering (XRMS). Here we discuss the modeling of the magnetic scattering data using a one-dimensional chain of nanoparticles with a mix of ferromagnetic, anti-ferromagnetic, and random orders. By fitting the model to the experimental data, we extracted information about the magnetic correlations in the nanoparticle assembly.

Abstract: We present a study of the magnetic properties of [Co(3.0 nm)/Pt(0.6nm)]N multilayers as a function of Co/Pt
bilayer repetitions N. Magnetometry investigation reveals that samples with N  15 exhibit two characteristic
magnetization reversal mechanisms, giving rise to two different morphologies of the remanent domain pattern.
For applied magnetic field angles near the in-plane field orientation, the magnetization reversal proceeds via
a spontaneous instability of the uniform magnetic state resulting in perpendicular stripe domains. Conversely,
for field angles close to the out-of-plane orientation, the reversal occurs via domain nucleation and propagation
leading to a mazelike domain pattern at remanence. Our measurements further enable the characterization of the
N-dependent energy balance between the magnetic anisotropy and magnetostatic energy contributions, revealing
a gradual disappearance of the domain nucleation process during magnetization reversal for N < 14. This leads
to the exclusive occurrence of an instability reversal mechanism for all field orientations as well as alignedlike
stripe domains at remanence. Furthermore, a detailed study of the influence of the magnetic history allows the
determination of a range of material properties and magnetic field strengths, where a lattice of bubble domains
with remarkably high density is stabilized. These modulations of the ferromagnetic order parameter are found
to strongly depend on N, in terms of center-to-center bubble distance as well as of bubble diameter. Moreover,
such Co/Pt multilayers could be utilized to engineer field reconfigurable bubble domain lattices, which resemble
magnonic crystals.

Abstract: We report a morphological transition in the magnetic domain pattern exhibited by perpendicular anisotropy ferromagnetic 
[
Co
/
Pt
]
50
multilayer films at room temperature and remanence. We found that the remanent magnetic domain morphology and the associated domain density, defined as the number of domains of a given magnetization direction per area, strongly depend on the magnetic history. When the magnitude of the previously applied external field approaches a specific value, typically 75–95% of the saturation field, the magnetic pattern, which generally forms a maze of interconnected stripe domains, decays into a shorter stripe pattern, and the domain density increases. We mapped out this morphological transition as a function of the previously applied field magnitude as well as the Co thickness. We found that a Co thickness close to 30 Å yields the highest domain density with the formation of a pure bubble domain state. Three-dimensional micromagnetic simulations confirm the formation of a pure bubble state in that parameter region and allow an estimation of the perpendicular anisotropy (here 
2
×
10
5
J
/
m
3
for an input magnetization of 1080 kA/m), as well as the interpretation of distinct features of the samples’ hysteresis loop based on the corresponding domain pattern.

Abstract: Understanding the correlations between magnetic nanoparticles is important for nanotechnologies, such as high-density magnetic recording and biomedical applications, where functionalized magnetic particles are used as contrast agents and for drug delivery. The ability to control the magnetic state of individual particles depends on the good knowledge of the magnetic correlations between particles when assembled. Inaccessible via standard magnetometry techniques, nanoscale magnetic ordering in self-assemblies of Fe3O4 nanoparticles is here unveiled via X-ray resonant magnetic scattering (XRMS). Measured throughout the magnetization process, the XRMS signal reveals size-dependent inter-particle magnetic correlations. Smaller (5 nm) particles show little magnetic correlations, even when packed close together, yielding to magnetic disorder in the absence of an external field, i.e., superparamagnetism. In contrast, larger (11 nm) particles tend to be more strongly correlated, yielding a mix of magnetic orders including ferromagnetic and anti-ferromagnetic orders. These magnetic correlations are present even when the particles are sparsely distributed. 

Abstract: The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.

Abstract: We have studied the influence of magnetic history on the topology of perpendicular magnetic domains in a thin ferromagnetic film made of [Co(8 Å)/Pt(7 Å)]50 multilayers. More specifically, we have followed the morphological changes in the domain pattern when applying a magnetic field perpendicular to the layer, throughout minor and major magnetization loops, and in the resulting remanent state. We carried out this study by using MFM microscopy with in-situ magnetic field. We find that the morphology of the magnetic domain pattern is greatly influenced by the magnetic history of the material and that some features, such as the degree of bubbliness (i.e., the extend of bubble domain formation) and density of isolated domains can be enhanced by fine tuning the magnetic field path within the major hysteresis loop towards different remanent states. In particular, we see how hysteresis is correlated to irreversible changes in the domain morphology. More interestingly, we find that the magnetic domain morphology at remanence can be changed from a labyrinthine stripe state to a state with increased bubble domains by fine tuning the magnitude of the field previously applied to the material. These results agree well with other findings, such as the magnetic reversal behavior and magnetic memory effects in Co/Pt multilayers, and provide opportunities for potential technological applications.

Abstract: Zinc oxide nanomaterials were synthesized with small amounts of magnetic ions to create dilute magnetic semiconductors (DMS), by using a low temperature sol–gel method. Conditions were controlled such that a range of amounts of Co, Ni and Mn were incorporated. The incorporation could be tracked by color changes in the powders to blue for Co, green for Ni and yellow for Mn. XRD measurements showed the ZnO has the wurtzite structure with crystallites 8–12 nm in diameter. Nanoparticles were observed by SEM and TEM and TEM showed that the lattice fringes of different nanoparticles align. Nanoparticle alignment was disrupted when high concentrations of metal dopants were incorporated. Magnetic measurements showed a change in behavior from diamagnetic to paramagnetic with increasing concentration of metal dopants.

Abstract: The magnetic behaviour of 5 to 11 nm magnetite (Fe3O4) nanoparticles (NPs) was measured at various temperatures (Ts) from 20 to 400 K. The particles were fabricated via thermal decomposition of an iron precursor, and involved a coating of the particles with oleic acid. The particle size distribution was analysed by XRD measurements and TEM imaging. Magnetization loops, measured at various Ts, indicate a superparamagnetic behaviour at high T and the occurrence of hysteresis at low T, with a stronger coercivity for the larger particles. Zero-Field-Cooling (ZFC) and Field Cooling (FC) curves indicate a superparamagnetic behaviour, with a blocking temperature varying significantly with the particle size. Namely, the peak temperature, Tmax, increases from 30 K to 170 K when the particle size increases from 5 nm to 11nm. Magnetic couplings between particles appear stronger for larger particles.

Abstract: The orbital and spin contributions to the magnetic moment of Fe in Fe3 O4 nanoparticles were measured using X-ray magnetic circular dichroism (XMCD). Nanoparticles of different sizes, ranging from 5 to 11 nm, were fabricated via organic methods and their magnetic behavior was characterized by vibrating sample magnetometry (VSM). An XMCD signal was measured for three different samples at 300 K and 80 K. The extracted values for the orbital and spin contributions to the magnetic moment showed a quenching of the orbital moment and a large spin moment. The calculated spin moments appear somewhat reduced compared to the value expected for bulk Fe3 O4 . The spin moments measured at 80 K are larger than at 300 K for all the samples, revealing significant thermal fluctuations effects in the nanoparticle assemblies. The measured spin moment is reduced for the smallest nanoparticles, suggesting that the magnetic properties of Fe3 O4 nanoparticles could be altered when their size reaches a few nanometers.

Abstract: Using a combination of resonant soft x-ray scattering, magnetometry, x-ray reflectivity, and microscopy techniques we have investigated the magnetic properties and microstructure of a series of perpendicular anisotropy Co/Pt multilayer films with respect to structural disorder tuned by varying the sputtering deposition pressure. The observed magnetic changes in domain size, shape, and correlation length originate from structural and chemical variations in the samples, such as chemical segregation and grain formation as well as roughness at the surface and interfaces, which are all impacted by the deposition pressure. All samples exhibited short-range "liquid-like" positional ordering over significant portions of their major hysteresis loops, while only the lowest disorder samples showed evidence of a random "gas-like" distribution of magnetic domains, present just after nucleation as well as prior to saturation. The structural and chemical disorder induced by the higher deposition pressure first leads to an increase in the number of magnetic point defects that limit free domain wall propagation. Then, as the sputtering pressure is further increased, the domain wall energy density is lowered due to the formation of local regions with reduced magnetic moment, and finally magnetically void regions appear that confine the magnetic domains and clusters, similar to segregated granular magnetic recording media.

Abstract: Strong magnetic domain memory is achieved in [Co/Pd] IrMn exchange-biased ferromagnetic thin films when zero-field-cooled (ZFC) below their blocking temperature T-B. By mapping out the amount of memory throughout the entire magnetization cycle, from nucleation to saturation, at different temperatures below and above T-B, we discover how microscopic morphological changes in the magnetic domain patterns correlate with the macroscopic magnetic hysteresis, in the presence or absence of exchange couplings. Our unique inter-field correlation maps show that in the ZFC state, the film exhibits the highest amount of domain memory, exceeding 90%, when domain patterns are compared at the same field value, in the coercive region of the magnetization loop. However, domain patterns also cross-correlate surprisingly well when measured at different field values, on a wide field range centered about the coercive region. The shape and symmetry of the correlation maps provide further insights into the microscopic morphological changes in the domain patterns and the amount of reversibility in the reversal process, at the nanoscale.

Abstract: The occurrence of magnetic domain memory has been observed in ferromagnets, either induced by structural defects or by exchange couplings. Being able to quantify the amount of memory as a function of length scale, field and temperature is both of fundamental and technological importance. A technique has been refined to statistically quantify the magnetic domain memory in ferromagnetic thin films by using coherent soft-X-ray scattering metrology. This technique, based on cross-correlating magnetic speckle patterns, provides a unique way to map out the behavior of domain memory. Here, the details of our correlation method and the necessary treatment of the X-ray scattering images to extract spatial and field dependences in the memory information are reviewed. The resulting correlation maps, measured on [Co/Pd]IrMn multilayers, show how magnetic domain memory evolves at various spatial scales, as a function of the field magnitude throughout magnetization cycles, but also as a function of field cycling and of temperature. This technique can easily be applied to a wide variety of systems presenting memory effects, in soft and hard matter, and also to dynamical studies.

Abstract: Magnetic domain memory (MDM) is observed in a Co/Pd ferromagnetic film when subject to exchange couplings with an IrMn antiferromagnetic layer. Our study uncovers the spatial and field dependence of the MDM at the nanoscopic scale. We found the degree of MDM to be extremely high (over 92%) at the scale of the domain periodicity, about 400 nm, and in the coercive region of the magnetization cycle, at about 1000 Oe. Furthermore, we observed an unusual spatial oscillation in the MDM, revealing a superstructure in the memory correlation at about 1.5 mu m.

Abstract:

The occurrence of memory effects in the formation of magnetic domains is both of fundamental and technological interest. We have probed the amount of domain memory in ferromagnetic thin films by using soft x-ray speckle cross-correlation metrology. We have found that a very strong domain memory (over 90%) can be induced in the ferromagnetic layer when subjected to exchange couplings with an antiferromagnetic layer. We show here the variation of the degree of memory as function of magnetic field through magnetization loop and the persistence of this memory through repeated field cycling.

Abstract: We show the possibility of creating magnetic domain memory in thin ferromagnetic films by inducing spatially varying exchange coupling interactions. We evidence this phenomenon in a perpendicular exchange bias film made of [Co/Pd] IrMn multilayers. Our coherent x-ray magnetic scattering speckle correlation study shows that the film exhibits no memory at room temperature but acquires a very high degree of magnetic memory, above 80% with subsequent field cycling when the sample is zero-field cooled below the blocking temperature of the IrMn layers (T < 275 K).

Abstract: We motivate the use of coherent soft X-ray beams to study materials that exhibit complex nanoscale behaviors. A new beamline and magnetic scattering end station that has been constructed and commissioned at the ALS will be described. Finally, we present some initial results that indicate the performance of the beamline.

Abstract: We present the magnetization evolution of perpendicular anisotropy TbFe and [Co/Pt](50) thin films either in direct contact resulting in antiferromagnetic interfacial coupling or separated by a thick decoupling Pt layer. Magnetometry and x-ray magnetic circular dichroism spectroscopy determine the spatially averaged magnetic properties. Resonant magnetic x-ray small-angle scattering and magnetic soft x-ray transmission microscopy probed the domain configurations and correlations in the reversal processes. While the Co/Pt multilayer reverses by domain propagation, the TbFe magnetization reversal was dominated either by coherent magnetization reversal processes or by lateral domain formation depending on the interface exchange coupling. In the presence of lateral domains, dipolar field induced domain replication phenomena were observed.

Abstract: We explore a number of novel effects near the orbital-order phase transition in a half-doped manganite, Pr0.5Ca0.5MnO3. To probe the unusual short-range orbital order in this system, we have performed coherent soft x-ray resonant scattering measurements in a Bragg geometry to measure dynamics. Near the transition temperature, we observe a small fluctuating component in the scattered signal that is correlated with three effects: a rapidly decreasing total signal and orbital domain size, as well as an abrupt onset of a broad background intensity that we attribute to the thermal production of correlated polarons. Our speckle results suggest that the transition is characterized by a competition between a pinned orbital domain topology that remains static and mobile domain boundaries that exhibit slow, temporal fluctuations.

Abstract: The phenomenon of stripe domain nucleation is deeply investigated both theoretically and experimentally in FePd films by the rigorous micromagnetic theory of domain nucleation and x-ray resonant magnetic scattering. The critical domain width and the nucleation field are determined by measuring the magnetic satellite peak position and integrated intensities in a wide temperature interval up to 400 degrees C (0.9T(c)) at varying in-plane magnetic fields for each temperature value. We develop and demonstrate a procedure that allows us to determine directly from the micromagnetic treatment the exchange stiffness constant A and the first order anisotropy constant K-u as a function of temperature. The proposed procedure, based on linearized micromagnetic equations at the critical field, is valid for magnetic films with perpendicular magnetic anisotropy, and is therefore effective to measure A and K-u in a technologically relevant class of materials.

Abstract: We discuss specular reflectivity and off-specular scattering of neutrons and X-rays from magnetic films. Both these techniques are capable of providing information about the morphology of the chemical and magnetic roughness and the magnetic domain structure. The use of neutrons with polarization analysis enables the spatial distribution of different vector components of the magnetization to be determined, and the use of resonant magnetic X-ray scattering enables magnetization in a compound system to be determined element-selectively. Thus both these methods provide powerful and complementary new probes for studying magnetism at the nanoscopic level in a variety of systems such as those exhibiting exchange bias, giant magnetoresistance, spin injection, etc. We shall illustrate with an example of both techniques applied to an exchange bias system consisting of a single crystal of antiferromagnetic FeF2 capped with a ferromagnetic Co film, and discuss what has been learned about how exchange bias works in such a system.

Abstract: We have used the unique spatial sensitivity of polarized neutron and soft x-ray beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned, thus providing a means to establish exchange bias.

Abstract: We have studied bilayers and trilayers of FePd thin-film alloys. where each of the constituting layers has a different magnetic-anisotropy, as controlled by the growth conditions. The competition between the magnetocrystalline anisotropy and the shape anisotropy in these films lends to the formation of stripe domains with a period of similar to 100 nm, which has been imaged by magnetic force microscopy (MFM). The average magnetic anisotropy has been obtained from the in-plane and perpendicular magnetic field dependence, measured using vibrating sample magnetometry (VSM). We measured the soft x-ray resonant magnetic scattering (SXRMS) at the Fe L(3) edge using sigma linearly polarized light, which is sensitive to the magnetization profile in the layers. The magnetic configuration of the layer systems was modeled using micromagnetic software (GL-FFT, @CNRS). The results of the micromagnetic modeling were used for a numerical simulation of the reflectivity scan and the magnetic rod scans of the SXRMS. This allowed us to determine parameters, such as the lateral roughness, the magnetic period, the magnetic correlation length, and the magnetic layer thickness. The good agreement obtained with the experimental results demonstrates that SXRMS Provides in-depth information that cannot be obtained from either MFM or VSM.

Abstract: Using coherent x-ray speckle metrology, we have measured the influence of disorder on major loop return point memory (RPM) and complementary point memory (CPM) for a series of perpendicular anisotropy Co/Pt multilayer films. In the low disorder limit, the domain structures show no memory with field cycling-no RPM and no CPM. With increasing disorder, we observe the onset and the saturation of both the RPM and the CPM. These results provide the first direct ensemble-sensitive experimental study of the effects of varying disorder on microscopic magnetic memory and are compared against the predictions of existing theories.

Abstract: Resonant magnetic x-ray scattering from dense assemblies of 9-nm-diameter epsilon-Co and hcp-Co superparamagnetic particles is reported. For lower anisotropy epsilon-Co assemblies remanent scattering is significantly enhanced compared to a random orientation model, indicating that preferred intermoment orientations with antiferromagnetic character exist with spatial frequencies ranging over several nearest neighbors. This interaction-mediated collective behavior is consistent with dipolar energies and exists well into the superparamagnetic regime, revealing that such thermally activated motion is highly correlated.

Abstract: We present a study of the prototypical NiO(111) antiferromagnet by nonresonant surface x-ray magnetic scattering. Direct access to the antiferromagnetic surface and bulk spin ordering is demonstrated. Our data support a first order antiferromagnetic to paramagnetic transition. A quantitative determination of the magnetization profile is proposed. It is shown that the NiO(111) surface spins remain ordered at higher temperatures than in the bulk and that the blocking temperature in exchange coupled ferromagnetic-NiO interfaces is most likely related to an S-domain structure loss occurring 25 K below the Neel temperature.

Abstract: An FePd thin film sample, showing magnetic stripe domains as imaged by magnetic force microscopy, has been measured by soft X-ray resonant magnetic scattering in reflection geometry. Illumination with coherent radiation, produced by inserting a 20 mum pinhole in front of the sample, leads to a magnetic speckle pattern in the scattered intensity that gives access to the domain morphology. Application of an in-plane magnetic field for a few seconds gives a strong change in the observed intensity fluctuations, which indicates a large degree of variation between the two patterns taken before and after field exposure. From the speckle pattern we calculate a degree of coherence of beta = 0.5 for the incident beam.

Abstract: We have used coherent soft x-ray resonant magnetic scattering to locally track reversal processes in magnetic nanostructures. Coherent illumination of a limited number of nano-objects in a CoPt nanoline grating produces a specific speckle pattern, whose evolution under in situ magnetic field reveals the true local magnetic ordering. While each nanoline behaves as a single macrospin whose direction depends on the dipolar coupling with neighbors, the global reversal of the line array is successively governed by two effects: first, by the random distribution of defects, followed by the dipolar coupling favoring antiferromagnetic ordering.

Abstract: We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the stripe domain critical width of FePd epitaxial films in a wide temperature range. A detailed quantitative analysis of our data in the framework of the rigorous theory of domain nucleation in thin films, allowed to determine the temperature dependence of both the exchange stiffness A and the anisotropy constant K-u. The proposed approach can be applied to thin magnetic films with perpendicular magnetic anisotropy, thus providing a way to measure A in a relevant class of magnetic materials. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: Soft X-ray resonant magnetic scattering (SXRMS) was performed on a FePd alloy thin film at the L-3-edge of Fe. This film exhibits perpendicular magnetic anisotropy giving rise to periodic alternation of up and down magnetisation domains with closure domains. Rocking curves performed in transverse geometry allowed us to measure the magnetic periodicity and correlation length of domains. Micromagnetic simulations of the FePd layers and SXRMS calculations were made to analyse the asymmetry ratio of magnetic satellite intensities, hence allowing us to quantify the magnetic anisotropy. (C) 2003 Published by Elsevier B.V.

Abstract: We recorded X-ray resonant magnetic scattering (XRMS) patterns from a 40 nm thin FePd film that exhibits magnetic stripes with similar to100 nm period. Measurements were performed using a CCD camera in reflection geometry for different light polarizations at the Fe L-3 resonance. We observed dichroic effects using circular polarization. By combining the scattering patterns obtained with both helicities we obtain the symmetric and antisymmetric part of the pattern. The experimental results are in agreement with a simple theoretical model. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: FePd thin-film samples with different perpendicular magnetic anisotropies have been studied with magnetic force microscopy (MFM), micromagnetic calculations and soft x-ray resonant magnetic scattering (SXRMS). The competition between perpendicular magnetic anisotropy (PMA) and shape anisotropy leads to the formation of highly ordered stripe domain patterns with a magnetization component perpendicular to the film plane. The magnetic stripes with a period of ∼100 nm, which are seen in the MFM images and can be modeled by micromagnetic calculations, give rise to magnetic peaks in the diffraction pattern. Closure domains occur in samples with a low to medium PMA, while a high PMA inhibits their formation. The in-plane magnetization component of the closure domains is not observable with MFM. In the presence of closure domains the interference between the scattering amplitudes in SXRMS from perpendicular and in-plane magnetized domains gives rise to a circular dichroism in the transverse geometry, where the scattering plane is along the stripes. We also recorded the magnetic speckle pattern from an 8 μm FePd wire using coherent x-rays and CCD detection. A high degree of coherence was obtained as evidenced from the observed intensity fluctuations. The speckle pattern can in principle provide information about the local disorder of the magnetic stripe domains.

Abstract: We have recorded magnetic speckle patterns of unprecedented quality on a tiny sample using soft-x-ray resonant magnetic scattering in reflection geometry. This geometry is well suited to the study of epitaxial thin films. The microscopic object consisted of an FePd wire with transversal stripe domains of alternatingly up and down magnetization. Sharp magnetic superstructure peaks reflect the domain periodicity, whereas the magnetic speckles give access to the domain morphology. A high degree of coherence has been obtained (beta=0.85) as evidenced from the strong observed intensity fluctuations.

Abstract: Periodic arrays of silicon nanolines, covered by a Co/Pt multilayer, with perpendicular magnetization, have been studied by soft x-ray resonant magnetic scattering at the Co L-3 edge. At the resonance, magnetic signals appear both on top of the structural diffraction peaks, characteristic of the grating, and between these peaks. These superstructure satellites reveal an antiferromagnetic order, generated by the interline dipolar coupling. Their intensities are strongly sensitive to the magnetic history, and can be enhanced through specific demagnetization processes. By applying an in situ magnetic field, the evolution of the magnetic signal has been monitored through the entire hysteresis loop. The magnetic contribution of the structural superlattice peaks can be quantified by their asymmetry ratio, whose angular variation stems from the scattering factor. The change of the purely magnetic satellites with the magnetic field is completely reproducible and characterizes the modifications of the magnetic configuration during the reversal process. A model of Ising macrospins, from which the distribution of the magnetic reversal fields can be deduced, is shown to be in agreement with the measured results.

Abstract: Arrays of silicon lines covered by a [Co(5)Angstrom /Pt-18 Angstrom] multilayer have been studied by soft X-ray Resonant Magnetic Scattering (XRMS). The magnetic peaks, which appear at the Co L-3 resonance, reveal the magnetic periodicity in the array, resulting from the magnetic coupling between the top of the lines, with a varying degree of influence from the trenches. In agreement with MFM images, the evolution of the magnetic peak intensities shows that small interline spacings and deep trenches favor antiferromagnetic order, which can be further reinforced by a specific demagnetization process.

Abstract: FePd thin-film samples with perpendicular magnetic anisotropy (PMA) have been studied with x-ray resonant magnetic scattering, both at the Fe and at the Pd L-3 edges. In these samples the competition between PMA and shape anisotropy leads to the formation of highly ordered striped domain patterns with a magnetization component perpendicular to the film plane. These striped domains give rise to magnetic satellite peaks in the diffraction pattern. Magnetic diffraction rod scans of these satellites were analyzed to obtain information about the magnetic depth profile of the films. It was found that flux closure occurs in samples with a low to medium PMA, while a high PMA impedes the formation of closure domains. Data analysis gives a depth of the closure domains extending to 85 Angstrom, with approximately half the magnetic moment aligned in plane.

Abstract: We show that the magnetization profile of magnetic patterns in thin films can be obtained by using circular dichroism to recover the phase relation in X-ray resonant magnetic scattering. This is demonstrated for single-crystalline FePd layers with striped magnetic domain pattern where we obtain unambiguous evidence for the presence of magnetic flux closure domains. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract: Depending on the growth conditions, FePd thin films can display a perpendicular magnetic anisotropy associated with chemical order. In competition with the shape anisotropy, this can lead to striped magnetic domains, with moments perpendicular to the film plane. Under these circumstances, magnetic flux closure should occur. The striped domains were studied with soft x-ray resonant magnetic scattering using circularly polarized light to demonstrate the presence of closure domains. Magnetic depth profiling was performed both at the Fe and Pd L-3 edge, by measuring the magnetic diffraction peak intensities versus angle of incidence theta. (C) 2000 American Institute of Physics. [S0021-8979(00)65408-7].

Abstract: We present an inelastic neutron scattering investigation of the low-frequency vibrational dynamics of liquid nitrogen-quenched CsC60. The low-frequency vibrational signatures of the different phases are discussed on the basis of the sequence of phases transitions stated by Kosaka et al. The librational modes of C60− monomer and (C60)22− dimer are observed below 150 K (monomer ordered phase) and around 210 K (dimer phase) respectively. Above 240 K the appearance of a quasi-elastic component agrees with the existence of an orientationally disordered phase.

Abstract: We report new results on the vibrational dynamics of polymer and quenched CSC 60 phases. Both the splitting and the structure of new activated lines of polymer CsC 60 are in agreement with the lowering of C 60 molecular symmetry from I h to D 2h. An inelastic neutron investigation on polymer and quenched CsC 60 is reported. It allows one to identify the vibrational signature of the low-temperature ( ${\rm T} < 150$ K) ordered monomer phase on quenched CsC 60.

Abstract:

Tabletop extreme ultraviolet (EUV) sources based on high harmonic generation (HHG) have been used as a powerful tool for probing magnetism. Obtaining magnetic information via magneto-optical contrast often requires the energy of the light to be tuned to magnetic resonance energies of the magnetic element present in the material; therefore, it is essential to calibrate the HHG spectrum to well defined absorption energies of materials. We have designed and assembled a HHG based EUV source for studying transition metal magnetic materials at their resonant M-absorption edges (35-75 eV of photon energy). One material of interest is iron, for which the iron M2,3 edge is 52.7 eV (23.5 nm wavelength) according to CXRO. We prepared and characterized a thin sample of iron for absorption spectroscopy and calibration of the absorption edge with beamline 6.3.2 at the Advance Light Source (ALS) in Lawrence Berkeley National Laboratory. This well characterized sample was capped with gold to prevent oxidation. From these measurements we extracted the absorption part of the index of refraction β spectrally and confirmed that the absorption edge of iron is 52.7 eV. With this information, we can better calibrate the HHG spectrum of our tabletop EUV source. Calibration of the HHG spectrum was achieved using model fitting the HHG spectrum using the grating equation and law of cosines while taking account into the results of the ALS data. We have determined that driving wavelength of the HHG process to be 773 nm. We also conclude that the chirp of the driving laser pulse can cause an energy shift to a HHG spectrum.

Abstract:

We report on magnetic orderings of nanospins in self-assemblies of Fe₃O₄ nanoparticles (NPs), occurring at various stages of the magnetization process throughout the superparamagnetic (SPM)-blocking transition. Essentially driven by magnetic dipole couplings and by Zeeman interaction with a magnetic field applied out-of-plane, these magnetic orderings include a mix of long-range parallel and antiparallel alignments of nanospins, with the antiparallel correlation being the strongest near the coercive point below the blocking temperature. The magnetic ordering is probed via x-ray resonant magnetic scattering (XRMS), with the x-ray energy tuned to the Fe−L₃ edge and using circular polarized light. By exploiting dichroic effects, a magnetic scattering signal is isolated from the charge scattering signal. We measured the nanospin ordering for two different sizes of NPs, 5 and 11 nm, with blocking temperatures TB of 28 and 170 K, respectively. At 300 K, while the magnetometry data essentially show SPM and absence of hysteresis for both particle sizes, the XRMS data reveal the presence of nonzero (up to 9%) antiparallel ordering when the applied field is released to zero for the 11 nm NPs. These antiparallel correlations are drastically amplified when the NPs are cooled down below TB and reach up to 12% for the 5 nm NPs and 48% for the 11 nm NPs, near the coercive point. The data suggest that the particle size affects the prevalence of the antiparallel correlations over the parallel correlations by a factor ∼1.6 to 3.8 higher when the NP size increases from 5 to 11 nm.

Abstract:

Optimizing magnetic thin films for nanotechnologies often requires imaging nanoscale magnetic domain patterns via magnetic microscopy. The finite size of the image may however significantly affect the characterization of the observed magnetic states. We evaluated finite image size effects on the characterization of a variety of stripe and bubble domain patterns exhibited by ferromagnetic Co/Pt multilayers with perpendicular magnetic anisotropy, where the domain size (stripe width and bubble diameter) is around 100 nm. If the image size is too small, below ∼5 μm, it may cause a significant underestimation of average domain size and overestimation of domain density by up to a factor 5 when reducing the image size from about 20 μm to about a 1 μm. Using a criterion based on how the excess density evolves with image size, we found that to obtain reliable statistical estimates of domain density and average domain size, the image needs to be large enough, and include at least about 100 stripes or about 2500 bubbles.

Abstract:

Thermoelectric materials hold tremendous promise for energy applications, but developing economically and environmentally viable high-performance thermoelectrics remains challenging. Recently, the paramagnon drag effect was discovered, in which local thermal fluctuations of the magnetization known as paramagnons drag charge carriers and impart to them a magnetic contribution to thermopower. This is significant because it opens a new avenue for optimizing thermoelectric properties in magnetic semiconductors. In this work, neutron scattering is used to visualize the nanoscale magnetism responsible for paramagnon drag in the antiferromagnetic semiconductor MnTe. First-principles calculations quantitatively reproduce the short-range magnetic correlations observed above the ordering temperature. These results shed light on how magnetism enhances thermoelectricity and provide a template for studies of other magnetic semiconductors as potential high-performance thermoelectrics.

Abstract:

We measured the local magneto-transport (MT) signal with an out-of-plane magnetic field, including magneto-resistance (MR) and Extraordinary Hall effect (EHE), in exchange-biased [Co/Pd]IrMn thin multilayers that are micro-structured with a 100 μm window. We found that when measured locally around the window, the MT signal deviate from the expected behavior. We studied possible causes, including film micro-structuration, electrical contact geometry as well as magnetic field angular tilt. We found that tilting the magnetic field direction with respect to the normal direction does not significantly affect the MT signal, whereas the positioning and geometry of the contacts seem to highly affect the MT signal. For comparison purposes, we carried these MT measurements using the Van-der-Pauw method on a set of four microscopic contacts directly surrounding the window, and on another set of micro-contacts located outside the window, as well as a set of four contacts positioned several millimeters away of each other at the corners of the wafer. If the contacts are sufficiently far apart, the EHE and MR signals have the expected shape and are not significantly affected by the presence of the window. If, on the other hand, the contacts are micro-positioned, the shape of the EHE signal is drastically deformed, and may be modeled as a mix of the standard EHE and MR signals measured on the outer contacts. Furthermore, if the micro-contacts are located directly around the window, the deformation is amplified, and the weight of the MR signal in the mix is further increased by about 40 %. This suggests that the electron path in the Hall geometry is disturbed by both the proximity of the electrodes and by the presence of the window, which both contribute to the deformation for about two-third and one third, respectively.

Abstract:

Magnetic nanoparticles offer unique potential for various technological, biomedical, or environmental applications thanks to the size-, shape- and material-dependent tunability of their magnetic properties. To optimize particles for a specific application, it is crucial to interrelate their performance with their structural and magnetic properties. This review presents the advantages of small-angle X-ray and neutron scattering techniques for achieving a detailed multiscale characterization of magnetic nanoparticles and their ensembles in a mesoscopic size range from 1 to a few hundred nanometers with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged determination of structural properties, such as particle morphology or particle arrangement in multilayers and 3D assemblies. Additionally, the magnetic scattering contributions enable retrieving the internal magnetization profile of the nanoparticles as well as the inter-particle moment correlations caused by interactions within dense assemblies. Most measurements are used to determine the time-averaged ensemble properties, in addition advanced small-angle scattering techniques exist that allow accessing particle and spin dynamics on various timescales. In this review, we focus on conventional small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic scattering, and neutron spin-echo spectroscopy techniques. For each technique, we provide a general overview, present the latest scientific results, and discuss its strengths as well as sample requirements. Finally, we give our perspectives on how future small-angle scattering experiments, especially in combination with micromagnetic simulations, could help to optimize the performance of magnetic nanoparticles for specific applications.

Abstract:

The magnetic properties of Fe₃O₄ nanoparticle assemblies have been investigated in detail through a combination of vibrating sample magnetometry (VSM) and muon spin relaxation (μSR) techniques. Two samples with average particle sizes of 5 and 20 nm, respectively, were studied. For both samples, the VSM and μSR results exhibit clear signatures of superparamagnetism at high temperature and magnetic blocking at low temperature. The μSR data demonstrate that the transition from the superparamagnetic to the blocked state occurs gradually throughout the sample volume over an extended temperature range due to the finite particle size distribution of each nanoparticle batch. The transition occurs between approximately 3 and 45 K for the 5-nm nanoparticles and 150 and 300 K for the 20-nm nanoparticles. The VSM and μSR data are further analyzed to yield estimates of microscopic magnetic parameters including the nanoparticle spin-flip activation energy EA, magnetic anisotropy K, and intrinsic nanoparticle spin reversal attempt time τ₀. These results highlight the complementary information about magnetic nanoparticles that can be obtained by bulk magnetic probes such as magnetometry and local magnetic probes such as μSR.

Abstract:

65th Annual Conference on Magnetism and Magnetic Materials, (Online, November 2020).

Abstract:

Due to their non-toxicity and their ability to be functionalized, magnetite (Fe3O4) nanoparticles (NP) are good candidates for a variety of biomedical applications. To better implement their applications, it is crucial to well understand the basic structural and magnetic properties of the NPs in correlation with their synthesis method. Here, we show interesting properties of Fe3O4 NPs of various sizes ranging from 5 to 100 nm and the dependence of these properties on particle size and preparation method. One synthetic method based on heating Fe(acac)3 with oleic acid consistently gives 5 ± 1 nm NPs. A second method using the thermal decomposition of Fe(oleate)3 in oleic acid led to larger NPs, greater than 8 nm in size. Increasing the amount of oleic acid caused the average NP size to slightly increase, from 8 to 10 nm. Increasing both the reaction temperature and the reaction time caused the NP size to drastically increase from 10 to 100 nm. Powder x-ray diffraction and electron-microscopy imaging show a pure single crystalline Fe3O4 phase for all NPs smaller than 50 nm and spherical in shape. When the NPs get larger than 50 nm, they notably tend to form faceted, FeO core-Fe3O4 shell structures. Magnetometry data collected in various field-cooling conditions show a pure superparamagnetic (SPM) behavior for all NPs smaller than 20 nm. The observed blocking temperature, TB, gradually increases with NP size from about 25 K to 150 K. In addition, the Verwey transition is observed with the emergence of a strong narrow peak at 125 K in the magnetization curves when larger NPs are present. Our data confirm the vanishing of the Verwey transition in smaller NPs. Magnetization loops indicate that the saturating field drastically decreases with NP size. While larger NPs show some coercivity (Hc ) up to 30 mT at 400 K, NPs smaller than 20 nm show no coercivity (Hc =0), confirming their pure SPM behavior at high temperature. Upon cooling below TB, some of the SPM NPs gradually show some coercivity, with Hc reaching 50 mT at 5 K for the 10 nm NPs, indicating emergent interparticle couplings in the blocked state.