Session D13: Focus Session: Magnetic Nanostructures-Characterization (Scanning Probe, X-ray and Neutron)

STM Studies of $\mbox{Mn}_{12}\mbox{-Ph}$ on Highly Oriented Pyrolytic Graphite

$\mbox{Mn}_{12}\mbox{-Ph}$ displays tunneling of quantized magnetization below 3K. In other $\mbox{Mn}_{12}$ ligand variants this magnetic behavior can alter the electronic behavior of the molecule making it a good candidate for a molecular logic gate. $\mbox{Mn}_{12}\mbox{O}_{12}\mbox{(}\mbox{C}_6\mbox{H}_5\mbox{COO)}_{16}$ ($\mbox{Mn}_{12}\mbox{-Ph}$) has a $\mbox{Mn}_{12}$ core and 16 Phenyl ligands and is deposited onto the surface of highly oriented pyrolytic graphite (HOPG). The samples are then studied via scanning tunneling microscopy in air at 300K and in ultra high vacuum at 300K and 4.2K. At 300K, film formation is studied to optimize samples for subsequent low-temperature studies. Isolated objects are observed via STM on the surface, clearly distinct from the underlying graphite lattice. Topographic data are analyzed in an attempt to correlate apparent features to the internal molecular structure of $\mbox{Mn}_{12}\mbox{-Ph}$. Voltage spectra of locations thought to be associated with the molecular core are compared to other locations thought to be the HOPG and Phenyl. Spectroscopic data indicate a bias voltage dependence at locations associated with the internal molecular structure thought to be related to the metallic-core of the molecules.   

Room-temperature spin-polarized scanning tunneling microscopy of antiferromagnetic Mn$_{3}$N$_{2}$(001) nanopyramids

Antiferromagnets play a key role in spintronic applications owing to the exchange bias effect. As devices miniaturize in size and dimension, novel magnetic structures dramatically different from the bulk often emerge. Here we apply spin-polarized scanning tunneling microscopy (SP-STM) at room temperature to study the local magnetization of antiferromagnetic nitride nanostructures. Mn$_{3}$N$_{2}$(001) thin films have been grown on MgO(001) substrates using molecular beam epitaxy and transferred~\textit{in situ}~to a home-built SP-STM~for magnetic imaging. Results show that the surface consists of alternating chemically in-equivalent atomic terraces. Using SP-STM with~\textit{dI/dV}~mapping, different layers can be clearly discriminated due to their different conductances. These differences in conductance are a result of not only the different chemical environments, but also the spin ordering and broken symmetry at the surface. Contrary to expectations, a layer-wise alternating surface anisotropy in these nanopyramids is observed. The presented study enables further investigations of the interplay between growth defects and the formation of intriguing antiferromagnetic domains.   

Atomic Force Microscopy Incorporated with Magnetic Sample Modulation: a new approach to detect the magnetic nanomaterials

A new imaging strategy using atomic force microscopy (AFM) for detecting magnetic nanomaterials with much higher spatial resolution and sensitivity than the traditional magnetic force microscopy (MFM) technique is developed [1,2]. This AFM-based imaging mode is referred to as magnetic sample modulation (MSM), since the flux of an AC-generated electromagnetic field is used to induce physical movement of magnetic nanomaterials on surfaces during imaging. The AFM is operated in contact mode using a soft, nonmagnetic tip to detect the physical motion of the sample. By slowly scanning an AFM probe across a vibrating area of the sample, the frequency and amplitude of vibration induced by the magnetic field is tracked by changes in tip deflection. Thus, the AFM tip serves as a force and motion sensor for mapping the vibrational response of magnetic nanomaterials. The investigations are facilitated by nanofabrication methods combining particle lithography with organic vapor deposition and electroless deposition of iron oxide to prepare designed test platforms of magnetic materials at nanometer length scales. Examples of detecting magnetic nanoparticles and magnetic biospecies at single molecular level will be presented [3,4]. \\[4pt] [1] Li et al. Analytical Chemistry, 2009, 81, 4792-4802   

Uncompensated Magnetization in Antiferromagnets, and New Classification of Exchange Bias Systems

Exchange bias (EB) is typically observed in a bilayer consisting of a ferromagnet (FM) and an antiferromagnet (AF) as a horizontal shift of the FM hysteresis loop. It is attributed to exchange coupling across the interface. Several experimental findings demonstrate, and most models agree that uncompensated magnetization (UM) in the AF plays an important role in EB. However, the origin of UM remains unknown for most EB systems. Using magnetometry and polarized neutron reflectivity (PNR) we observe UM in antiferromagnet-only, (110)-FeF$_2$ epitaxially grown on MgF$_2$, thin-film samples. The PNR reveals the spatial distribution of the UM. This UM exhibits the so-called ``\textit{intrinsic exchange bias}'': a shift of the hysteresis loop of UM. This effect is similar to the ``classical'' EB observed in bilayers, except that here, it is observed in a single layer material. The surface is responsible for the macroscopically broken time-reversal symmetry, uncompensated magnetization (UM) in a nominally compensated antiferromagnet [1], and, ultimately, for a new magnetic state. In this magnetic state, zero remanent magnetization cannot be obtained isothermally, because the origin (M(H=0)=0) is outside of the major hysteresis loop. Using symmetry group arguments [1] and results of \textit{ab-initio} calculations [2], we argue that it is an equilibrium state. Below $T_N$, the UM in FeF$_2$ is coupled to the bulk antiferromagnetic order parameter as supported by several experimental results, including high value of EB field, its temperature dependence, and the absence of the training effect. Based on the proposed origin of the UM and experimental observations for different EB systems, we discuss a new classification of exchange bias systems into two categories, explaining differences in the observed properties.\\[4pt] This work is done in collaboration with K. E. Badgley (TAMU), M. Zhernenkov (LANL and ANL), M. R. Fitzsimmons (LANL), M. Erekhinsky, I. K. Schuller (UCSD), K. D. Belashchenko (UNL), and A. H. Romero (CONACyT), and supported by Texas A\&M University, TAMU--CONACyT Collaborative Research Grant Program, DOE, AFOSR, and NSF-9976899. PPPROALMEX-DAAD-CONACyT bi-national program. TACC at UT--Austin is acknowledged for providing HPC resources. \\[4pt] [1] K. D. Belashchenko, Phys. Rev. Lett. \textbf{105}, 147204 (2010).\\[0pt] [2] S. L\'{o}pez-Moreno \textit{et al.}, Phys. Rev. B (submitted).   

Modeling Scanning SQUID Magnetometry Images of Magnetic Dipoles

Scanning superconducting quantum interference devices (SQUIDs) with sub-micron sized pick-up loops are the most sensitive detectors of local magnetic flux and can have spin sensitivities down to 100 mu{\_}B/sqrt(Hz). ~This makes them the ideal candidate for detecting magnetic dipole signals from individual nanomagnets. However, because the image kernel of the SQUID is not usually well known, quantitative analysis of magnetometry images can often be difficult. By using similarly measured SQUID magnetometry of superconducting vortices, we show that it is possible to fit images of magnetic dipoles by combining images of two monopoles.~This fitting technique allows us to extract the magnetic moment as well as the information on the spatial extent of the imaged dipole.~To quantify the statistical errors of the fit and the systematic errors of the measurement, we fabricated and measured nanomagnet bars of different lengths This analysis technique, in conjunction with scanning SQUID microscopy, can be used to study individual nanomagnets in a wide variety of fields, ranging from biology to condensed matter physics.~   

Chemical Segregation in GdFeCo: An X-ray view on Magnetic Coercivity

The magnetic coercivity in intermetallic alloys is known to be dominated by microscopic inhomogeneities. These control the characteristics of magnetic switching as they provide nuclei for magnetic domain formation, and the pinning sites governing domain wall propagation. However, such regions exist on nanometer length scales with weak magnetic contrast to their surroundings; their characterization has therefore remained illusive. Here we demonstrate how resonant x-ray scattering is intrinsically sensitive to magnetic changes in a segregated phase. We utilizes the fact that magnetic scattering asymmetry directly probes regions where this phase segregation occurs. Our measurements on GdFeCo show strongly temperature dependant magnetic canting in the segregated regions due to local changes in magnetic anisotropy. Understanding the origin and importance of these chemically segregated regions will allow a better understanding of the magnetic switching process in GdFeCo.   

Using X-Ray Diffraction Microscopy for Imaging Magnetic Domain Structures of Magnetic Thin Films

We report the application of iterative phase retrieval from magnetic x-ray diffraction for imaging magnetic domain structures of magnetic thin films. Using coherent x-ray scattering at the x-ray photon energy corresponding to the L$_{3,2}$ absorption edges of the 3d material Co, we demonstrate that linearly polarized soft x rays can be used to obtain the element specific information about both the amplitude and the phase of magnetic domain structures existing in thin films. We successfully recovered an image of the magnetic structure of an amorphous terbium-cobalt thin film with a spatial resolution of about 75 nm and could differentiate between the magnetization directions, finding qualitative agreement with soft x-ray microscopy images recorded with Fresnel zone plate optics having better than 25 nm spatial resolution.   

Magnetic order and fluctuations in Fe$_{3}$O$_{4}$ nanoparticles via coherent X-ray magnetic scattering

Magnetite (Fe$_{3}$O$_{4})$ particles exhibit a superparamagnetic behavior when their sizes are in nanometer scale. Such nanoparticles could potentially be used for applications in the medical field. We are interested in investigating the magnetic order and fluctuation dynamics in self-assemblies of such nanoparticles. Our Fe$_{3}$O$_{4 }$nanoparticles are prepared by an organic route and range from 5 nm to 50 nm in size. They are deposited on membrane where they self-assemble. We have been studying the magnetic order using X-ray resonant magnetic scattering (XRMS) at the SSRL synchrotron facility in Stanford. This unique technique, combined with X-ray Magnetic Circular Dichroism (XMCD), provide information about the spatial distribution of the particles and their magnetic order (1). In addition, the use of coherent light at the SSRL beamline, combined with the application of magnetic field in-situ at different temperatures, allows for studying local magnetic disorder (2) and dynamics of fluctuations near the blocking temperature. \begin{enumerate} \item J.B.Kortright et al., PRB \textbf{71}, 012402 (2005) \item K. Chesnel et al., PRB \textbf{83}, 054436 (2011) \end{enumerate}   

Magnetic x-ray scattering, transport and MFM study of strongly correlated La$_{1-x}$Sr$_{x}$MnO$_{3}$ nanowires

Artificial patterning is a promising new approach to studying strongly correlated materials, since a boundary acts as a perturbation that can tip the balance among various competing ground states. We have fabricated large, periodic arrays of 80 nm wide nanowires from epitaxially grown La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) thin films. Their electronic and magnetic properties were studied with resonant soft x-ray scattering (RSXS), transport measurements and magnetic force microscopy (MFM). RSXS measurements revealed a series of structural diffraction peaks that arise from the periodic wire structure. Below the Curie temperature we also observed a series of magnetic superlattice reflections, indicating collective ordering of the magnetic moments into a pattern with a spatial period of five wires. Transport measurements also showed anomalous ``telegraph'' switching noise at temperatures below 15K, and MFM revealed unusual domain formation. We interpret these results as arising from unusual, boundary-induced magnetic domains interacting via long-ranged, classical magnetic dipole coupling.   

Local structure and magnetic properties of ultrathin Mn films grown on Si(001)

We report on the structural and magnetic properties of ultrathin Mn layers deposited onto Si(001) by molecular beam epitaxy (MBE) at low temperature. X-ray absorption fine structure (XAFS) studies reveal that the structure of the silicide layer that forms depends on the growth temperature of the capping layer. A capping layer grown at 200 $^{\circ}$C on 0.35 monolayer (ML) Mn results in a metastable MnSi phase with a B2-like (CsCl) structure, whereas a cap grown at room temperature on 0.5 ML followed by annealing at 200 $^{\circ}$C produces a lower coordinated MnSi phase with a B20-like structure. Increasing the Mn thickness from 0.5 to 4 monolayers does not trigger a structural transformation but drives the structure closer to MnSi-B20. Using SQUID magnetometry, we show that the sample with B2-like structure has the largest Mn magnetic moment of 0.33$\mu _{B}$/Mn at T=2 K, and a Curie temperature $T_{C}$ above 250 K. MnSi-B20 layers showed lower moments and much lower $T_{C}$'s, in-line with those reported for MnSi-B20 thin films.   

Probing the surface magnetic properties via Auger-photoelectron coincidence spectroscopy

Auger-photoelectron coincidence spectroscopy (APECS) via a dichroic effect is a suitable tool to study complex systems such as magnetic thin films and multilayers. We present clear evidence for such a dichroic effect in the M$_{3}$VV Auger line shape of Fe films on Cu(001) measured by angle resolved APECS showing final state spin selectivity (triplet vs. singlet components). Using the Fermi Golden rule and the density functional theory formalism, the Auger spectrum and its angular distributions are computed. For magnetic systems, the spin dependence of the Auger matrix elements allows one to work out the individual multiplet contributions to the Auger spectrum. For an accurate interpretation of experiments we also take into account the valence hole-hole interaction affecting the Auger line shape by the Cini-Sawatzky theory but considering a spin dependent on-site interaction U. The calculated angular distribution, in case of a non-spherical ionized core level (e.g. l=1, m=0), follows a non-trivial behavior in agreement with experiment.   

Polarized small angle neutron scattering of MnO/Mn$_{3}$O$_{4}$ nanocrystals

Monodisperse magnetic nanoparticles are of great interest for biomedical and data storage applications, particularly in cases where the core and shell can be carefully controlled to alter properties like magnetic anisotropy. However, it is often difficult to determine the underlying moment arrangements and correlations in these systems. Here, we focus on manganese (II) oxide/manganese (II,III) oxide core/shell nanoparticles, using polarized small angle neutron scattering (SANS) to probe the magnetic intra and interparticle interactions. The 30nm diameter particles with 4-5nm shell were prepared by solution chemistry methods and self-assembled into 3D nanocrystals. SANS measurements were conducted in magnetic fields from remanence-1T and temperatures from 10-300K. Magnetic and structural scattering components were separated using an algorithm previously described in [1]. The magnetic signature depended on the field and temperature history of the sample. Modeling work has been done to further quantify the interparticle length scales and the effects of crystal packing. This work was supported in part by NSF grants DMR-0454672, -0704178, -0804779, -1104489, and DOE grant DE-FG02-08ER40481. [1] K.L. Krycka, et al. Phys. Rev. Lett. 104, 207203 (2010).