Session H32: Focus Session: Magnetic Imaging

Measuring Spin Dependent Hot Electron Transport Using Spin-Polarized Ballistic Electron Emission Microscopy

Spin polarized ballistic electron transport has been studied in Fe/Si(001) Schottky diodes using ballistic electron emission microscopy. Spin dependent scattering of polarized ballistic electrons injected from an Fe coated Au tip into the Fe films has been shown to affect the BEEM current. The spin dependent attenuation lengths were determined by measuring this effect with Fe thickness and found to be 1.8~$\pm$~0.2~nm for the minority spin electrons and 2.5~$\pm$~0.3~nm for the majority spin electrons at a tip bias of 1.5~eV. In addition, the attenuation lengths were measured as a function of tip bias, which indicated that the Fe/Si(001) interface band structure has an effect on the hot electron transport through the diode. Applications of the SP-BEEM technique to other systems will also be discussed.   

Magnetic properties of single and bilayer MnGa on GaN(0001) investigated by spin-polarized STM

We investigated the magnetic properties of ultrathin GaMn layers grown on GaN(0001) by spin-polarized scanning tunneling microscopy (SP-STM) using an Fe coated W tip. The GaN films are grown by plasma-assisted MBE on 6H-SiC(0001), and exhibit a metallic pseudo-1x1 surface structure, consisting of 2.3 ML Ga on top of the Ga-terminated GaN. At room temperature, Mn deposition on this surface resulted in the formation of GaMn islands, with the second layer islands begin to nucleate before the first layer is completed. With a Fe coated W tip, contrast between odd and even layers is observed, indicating layered antiferromagnetic magnetization of the GaMn layers. When Mn is deposited on the pseudo (1x1) at 200 C, a GaMn (3x3) structure is observed. First principles calculations show that Mn substitution of Ga leads to virtual bound states with bandwidth of 1.5 eV, indicating significant Mn-Ga interactions. As a result, top layer Ga atoms form covalent-like bonds. The Mn and the librated Ga atoms from the ``1x1'' form the (3x3) structure, with the adatom on the T$_{4}$ site.   

Localized spectroscopic and topographic studies of heterostructures of OSE/M (OSE: organic semiconductor, M: metal) using scanning tunneling microscopy (STM) and atomic force microscopy (AFM).

We employ STM with AFM to study the charge transport and domain structures of OSE/M heterostructures fabricated under differing growth conditions [OSE: sublimated tris(8-hydroxyquinoline) aluminum (Alq$_{3})$, M: paramagnetic Au or ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO)]. Specifically, using STM in the point contact mode we are able to determine the work function of the heterostructures by measuring the differential conductance versus bias voltage. In addition, we can compare the Alq$_{3}$ resistivity variations for heterostructures prepared under different Alq$_{3}$ annealing conditions and with Au or LCMO as the metal. In contrast, using STM in the tunneling mode we can determine the ballistic charge transport length by varying the Alq$_{3}$ thicknesses in the OSE/M heterostructures. Moreover, conductance maps for biased voltages above the Alq$_{3}$ band-gap provide spatially resolved information for the local conductance channel and the surface quality of the Alq$_{3}$ film, the latter further compared with the surface morphology taken with AFM. This work was supported by NSF under the Center for Science and Engineering of Materials at Caltech.   

Current-induced magnetization switching with a spin-polarized scanning tunneling microscope

In present data storage applications magnetic nanostructures are switched by external magnetic fields. Due to their non-local character, however, cross-talk between adjacent nanomagnets may occur. An elegant method to circumvent this problem is magnetization switching by spin-polarized currents, as observed in GMR\,[1] as well as in TMR\,[2] studies. However, the layered structures of these devices do not provide any insight to the details of the spatial distribution of the switching processes. Spin-polarized scanning tunneling microscopy (SP-STM) is a well-established tool to reveal the magnetic structure of surfaces at spatial resolution down to the atomic scale. Besides, SP-STM takes advantage of a perfect TMR junction consisting of an isolating vacuum barrier separating two magnetic electrodes, which are represented by the foremost tip atom and the sample. Our experiments demonstrate that SP-STM serves as a tool to manipulate the switching behavior of uniaxial superparamagnetic nanoislands\,[3]. Furthermore, we show how SP-STM can be used to switch the magnetization of quasistable magnetic nanoislands at low temperature ($T=31$\,K). Besides its scientific relevance to investigate the details of current-induced magnetization switching (CIMS), this technique opens perspectives for future data storage technologies based on SP-STM. \par \noindent [1] J.~A.~Katine \textit{et al.}, Phys.~Rev.~Lett. \textbf{84}, 3149 (2000). \par [2] Y.~Liu \textit{et al.}, Appl.~Phys.~Lett. \textbf{82}, 2871 (2003). \par [3] S.~Krause \textit{et al.}, Science \textbf{317}, 1537 (2007).   

Bloch line `crystallization' as intrinsic pinning mechanism in ferrimagnetic YIG films

The present intense drive to develop current-switched magnetic storage media has lead to a renewed interest in ferrimagnetic garnet films which, for several decades, were the focus of devices exploiting manipulation of magnetic `bubbles'. In such uniaxial materials, the appearance of Bloch lines in structured domain walls strongly influences their \textit{dynamic} properties in an applied magnetic field. Here we show that the \textit{static} magnetic properties of garnet films can also be profoundly influenced due to \textit{crystallization} of Bloch lines into a square lattice along adjacent domain walls. This rigid lattice \textit{intrinsically} pins domain walls and suppresses the expected expansion/contraction of magnetic domains in an applied field. Even in the pinned regime, ultra-sensitive scanning Hall probe measurements reveal the \textit{nanoscale motion} of magnetic blocks in the walls comprising an integer number of Bloch-lines. Although the estimated displacements ($\sim $2-25 nm) are very much smaller than the domain period, we observe highly correlated motion across many domain walls, driven by the strongly interacting Bloch line lattice.   

Evolution of magnetic domain reversal with temperature in Co/Pt multilayers observed by magneto-optical Kerr imaging

The nucleation and evolution of magnetic domain structures with temperature and magnetic field in Co(4 {\AA})/Pt($t_{Pt})$ multilayers with perpendicular anisotropy have been studied by magneto-optical imaging techniques. Relatively large Pt layer thicknesses t$_{Pt}$ = 43 {\AA} and 63 {\AA} are chosen for this study because the interlayer coupling strength in the multilayers varies from weak at room temperature to strong at low temperature. Kerr imaging during the magnetization reversal processes shows the transformation of domain patterns with temperature, which correlates directly with enhancement of interlayer exchange coupling with decreasing temperature, as well as the conversion from domain- wall-propagation dominant reversal at room temperature to nucleation-dominant reversal at low temperatures. The enhanced interlayer coupling at low temperatures leads to the entire multilayer switching as a single ferromagnet; while at higher temperatures, when the interlayer coupling weakens, quasi-independent layer-by-layer magnetic reversal is observed. The transformation from propagation- to nucleation-dominant magnetic reversal can be understood by the competition between activation energies for domain nucleation and propagation, Zeeman energy and thermal energy.   

Adding depth sensitivity to photoelectron microscopy using the standing wave/wedge method

Photoelectron microscopy (PEEM) is by now a well-established technique for studying many types of multilayer or multicomponent structure, including samples of relevance to spintronics, semiconductor technology, and polymer-based materials. The lateral resolution in such microscopes is typically 20 nm, but with the prospect of going down to ca. 1 nm in the near future. However, resolution perpendicular to the surface is not inherent in PEEM measurements, and we here discuss a novel method for providing this at sub-nm resolution, by exciting the photoelectrons with a standing wave created by soft x-ray reflection from a multilayer substrate, and growing one layer of the sample in a wedge form. This standing wave/wedge method has been demonstrated for the first time in measurements with a PEEM located at BESSY in Berlin.   

Study of the correlation between structural and magnetic properties of MnAs/Si

Ferromagnetic MnAs, has been widely studied because of its ferromagnetic properties and structural compatibility with conventional semiconductors. Magnetic properties of MnAs grown on Si(001) vary depending upon the growth conditions. To understand the variations, we carried out experiments using X-ray diffraction, atomic force microscopy (AFM) and magnetic force microscopy (MFM), together with magnetization measurements. For this study, MnAs was grown by molecular beam epitaxy (MBE) on Si(001) and Si(111) substrates. The surface structure of MnAs is correlated with the magnetic properties. For samples with no in-plane anisotropy, both AFM and X-ray diffraction measurements show the coexistence of MnAs with orthogonal orientations. The magnetic domains are very different from those observed in MnAs grown on GaAs (001). Significant differences in surface morphology are observed between MnAs layers grown on Si(001) and Si(111) because of the different orientations of MnAs .   

Magnetic domains in Nd$_{2}$Fe$_{14}$B on various length scales

Detailed knowledge about the structure of ferromagnetic domains provides a link between microscopic physics and macroscopic magnetic response. In the well known, and already widely used compound, Nd$_{2}$Fe$_{14}$B, the dimension, shape and arrangement of magnetic domains are still in discussion due to lack of suitable methods to study magnetic domain structures in the bulk and due to the geometric complexity observed on the surface. Here, we demonstrate that domain patterns revealed by quantitative Kerr and Faraday microscopy, exist well below the surface as detected by small angle neutron scattering. At room temperature, the easy-axis magnetic anisotropy yields very complex structures of domains on various length scales. In contrast, the cone-like magnetic anisotropy below 135~K reduces the complexity of the domain arrangement to a more regular and anisotropic structure of much larger domains. As a consequence the bulk magnetization increases due to the significant volume reduction of the domain walls. -- The support by U.S. DOE (DE-AC02-07CH11358), DFG (SFB463) and the Alfred P. Sloan foundation is acknowledged.   

Collective Dynamics and Slow Relaxation of Charge/Spin Density Wave domains in Antiferromagnetic Chromium

We present coherent x-ray diffraction and x-ray microscopy measurements of slow fluctuations and relaxation of charge- and spin-density wave domains in antiferromagnetic Chromium. Intensity fluctuations of the coherent x-ray speckle of incomensurate charge density wave satellite, combined with time-resolved x-ray microscopy measurements, reveal the collective nature of the uncharacteristically slow domain wall and phase defect fluctuation as well as non-equilibrium charge- and spin-density wavevector relaxation. The observed dynamics of pinned charge- and spin-density wave condensate in Chromium is similar to other examples of elastic media in presence of quenched disorder, ranging from dynamics of vortex lattices in disordered superconductors and sliding friction to snow avalanches and earthquakes. A particularly interesting analogy is dynamics of soft matter undergoing jamming transition, which show similar compressed exponential relaxation behavior.   

Color properties, hydrogen bonding and magnetic interactions in (TBA)$_{3}$[Ni(NCS)$_{5}$]

We investigated the optical and vibrational properties of (TBA)$_{3}$[Ni(NCS)$_{5}$] a pentacoordinate Ni compound, and compared the results with the more traditional hexacoordinate analog (TEA)$_{4}$[Ni(NCS)$_{6}$]. Based upon electronic structure calculations, color properties of this high spin complex can be understood in terms of the crystal field splitting of the d-orbitals and their strong hybridization with the ligands. Temperature dependent vibrational studies show an additional splitting and softening of some of the modes at low temperature, which indicates enhanced hydrogen bonding between sulfur centers and organic ligands at low temperature as well as weak structural phase transitions.   

Ferrofluid Photonic Dipole Contours

Understanding magnetic fields is important to facilitate magnetic applications in diverse fields in industry, commerce, and space exploration to name a few. Large electromagnets can move heavy loads of metal. Magnetic materials attached to credit cards allow for fast, accurate business transactions. And the Earth's magnetic field gives us the colorful auroras observed near the north and south poles. Magnetic fields are not visible, and therefore often hard to understand or characterize. This investigation describes and demonstrates a novel technique for the visualization of magnetic fields. Two ferrofluid Hele-Shaw cells have been constructed to facilitate the imaging of magnetic field lines [1,2,3,4]. We deduce that magnetically induced photonic band gap arrays similar to electrostatic liquid crystal operation are responsible for the photographed images and seek to mathematically prove the images are of exact dipole nature. We also note by comparison that our photographs are very similar to solar magnetic Heliosphere photographs.   

Magnetic Reversal Time in Open Long Range Systems

Topological phase space disconnection has been recently found to be a general phenomenon in isolated anisotropic spin systems. It sets a general framework to understand the emergence of ferromagnetism in finite magnetic systems starting from microscopic models without phenomenological on-site barriers. Here we study its relevance for finite systems with long range interacting potential in contact with a thermal bath. We show that, even in this case, the induced magnetic reversal time is exponentially large in the number of spins, thus determining {\it stable} (to any experimental observation time) ferromagnetic behavior. Moreover, the explicit temperature dependence of the magnetic reversal time obtained from the microcanonical results, is found to be in good agreement with numerical simulations. Also, a simple and suggestive expression, indicating the Topological Energy Threshold at which the disconnection occurs, as a real energy barrier for many body systems, is obtained analytically for low temperature.