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In the last years, diameter-modulated (D-M) ferromagnetic nanowires (NWs) have been intensively studied to evaluate their efficiency to control the motion of domain walls (DWs) along these one-dimensional nanostructures by the application of magnetic field or the injection of electrical current, which is essential for spintronic applications in the field of information storage, sensors and logical operations [1]. Preliminary studies in D-M NWs have been performed using theoretical and experimental procedures [2-4] in individual and isolated NWs and they have provided a first approach of the spin configuration in these systems, obtaining a non-trivial interpretation. In this work, we have exploited the potential of electron holography technique (high spatial resolution, high sensitivity and quantitative capability in volume) for achieving a full picture of the magnetic distribution in cylindrical D-M FeCoCu NWs. These NWs were prepared by filling self-assembled cylindrical D-M nanochannels of anodic aluminum oxide templates. The D-M geometry of the polycrystalline NWs consist of alternating segments of small (100 nm) and large (144 nm) diameters, with segment lengths ranging between 1000 to 300 nm. At remanence, the high-shape anisotropy of the NWs induces a single-domain state where the spins are mainly oriented along the NW axis, with the possibility to create a small closure domain in large-diameter tips. The transition zones where the diameter is varied induce a complex demagnetizing field where the stray field follows a flux-closure configuration around the large-diameter segments and a magnetic coupling between them around small-diameter segments (See Fig 1). The complex configuration of the demagnetizing field can be understood if we treat the D-M transition zones as magnetic charges. The interpretation of the magnetic distribution by EH experiments was compared with micromagnetic simulation finding a very good agreement (see Fig 2). In addition, In-situ Loren

The comprehension of the magnetic configuration in FeCoCu nanowires with a diameter-modulated cylindrical geometry will allow controlling the domain wall motion in this low-dimensional system under the application of magnetic fields and/or the injection of current pulses. Here we perform a quantitative magnetic characterization of isolated diameter-modulated FeCoCu nanowires by combining nanoscale magnetic characterization techniques such as electron holography, magnetic force microscopy, and micromagnetic simulations. Local reconstructions of the magnetic distribution show the diameter-modulated geometry of the wires induces the formation of vortex-like structures and magnetic charges in the regions where the diameter is varied. Vortex-like structures modify the axial alignment of the magnetization in large-diameter segments. Moreover, the magnetic charges control the demagnetizing field distribution, promoting a flux-closure stray field configuration around large-diameter segments and keeping the demagnetizing field parallel to the NW's magnetization around small diameter segments. The detailed description of the remanent state in diameter-modulated cylindrical FeCoCu nanowires allows us to provide a clear explanation of the origin of bright and dark contrast observed in magnetic force microscopy images, which have the same feature of magnetic domain walls. This work establishes the primary knowledge required for future magnetization reversal studies with the aim of searching efficient modulated geometries that allow an optimum and controlled domain wall propagation.

Iron nanostructures grown by focused electron beam induced deposition (FEBID) are promising for applications in magnetic sensing, storage and logic. Such applications require a precise design and determination of the coercive field (H C), which depends on the shape of the nanostructure. In the present work, we have used the Fe2(CO)9 precursor to grow iron nanowires by FEBID in the thickness range from 10 to 45 nm and width range from 50 to 500 nm. These nanowires exhibit an Fe content between 80 and 85%, thus giving a high ferromagnetic signal. Magneto-optical Kerr characterization indicates that H C decreases for increasing thickness and width, providing a route to control the magnetization reversal field through the modification of the nanowire dimensions. Transmission electron microscopy experiments indicate that these wires have a bell-type shape with a surface oxide layer of about 5 nm. Such features are decisive in the actual value of H C as micromagnetic simulations demonstrate. These results will help to make appropriate designs of magnetic nanowires grown by FEBID.

The investigation of three-dimensional (3D) ferromagnetic nanoscale materials constitutes one of the key research areas of the current magnetism roadmap and carries great potential to impact areas such as data storage, sensing, and biomagnetism. The properties of such nanostructures are closely connected with their 3D magnetic nanostructure, making their determination highly valuable. Up to now, quantitative 3D maps providing both the internal magnetic and electric configuration of the same specimen with high spatial resolution are missing. Here, we demonstrate the quantitative 3D reconstruction of the dominant axial component of the magnetic induction and electrostatic potential within a cobalt nanowire (NW) of 100 nm in diameter with spatial resolution below 10 nm by applying electron holographic tomography. The tomogram was obtained using a dedicated TEM sample holder for acquisition, in combination with advanced alignment and tomographic reconstruction routines. The powerful approach presented here is widely applicable to a broad range of 3D magnetic nanostructures and may trigger the progress of novel spintronic nonplanar nanodevices.

Remanent state and magnetization reversal processes of a series of cobalt antidot arrays with a fixed hole diameter (d ¿ 55 nm) and an array periodicity (p) ranging between 95 and 524 nm were studied by in situ Lorentz microscopy (LM) as a function of the magnetic field. At remanence, defocused LM images showed the periodicity dependence of the magnetic states inside the lattice. A remarkable transition was observed in the type of domain structures as a function of p: for the large periodicities (p > 300 nm), conventional 90° and 180° domain walls were formed, whereas in small-period antidot arrays (p lE 160 nm) magnetic superdomain walls (SDWs) were nucleated to separate regions with different average magnetization direction, the so-called magnetic superdomains. In the SDW regime, a low-frequency Fourier filtering method was implemented to allow a quantitative analysis of the LM images by the transport of intensity equation method. In situ LM experiments under applied magnetic fields were performed to study the reversal magnetization process in a particular array (p = 160 nm), and clear differences were observed as a function of the magnetic field orientation. The switching process under magnetic fields parallel to the horizontal antidot rows occurs in two stages: the system first nucleates and propagates horizontal SDWs, parallel to the field. Then, at higher magnetic fields, vertical SDWs, perpendicular to the field, appear before saturation. When the magnetic field is applied at 45° with respect to the antidot rows, both horizontal and vertical SDWs are nucleated and propagated simultaneously. All the experiments were successfully correlated with micromagnetic simulations. The current study sheds new light on the magnetization reversal processes of antidot arrays and opens new possibilities of exploiting the potential of high-resolution in situ LM and new data analysis procedures to probe magnetization processes in nanomagnetism, particularly in periodic arrays

A generalized procedure for the in situ application of magnetic fields by means of the excitation of the objective lens for magnetic imaging experiments in Lorentz microscopy and electron holography is quantitatively described. A protocol for applying magnetic fields with arbitrary in-plane magnitude and orientation is presented, and a freeware script for Digital Micrograph¿ is provided to assist the operation of the microscope. Moreover, a method to accurately reconstruct hysteresis loops is detailed. We show that the out-of-plane component of the magnetic field cannot be always neglected when performing quantitative measurements of the local magnetization. Several examples are shown to demonstrate the accuracy and functionality of the methods.

Direct observation of domain wall (DW) nucleation and propagation in focused electron beam induced deposited Co nanowires as a function of their dimensions was carried out by Lorentz microscopy (LTEM) upon in situ application of magnetic field. Optimal dimensions favoring the unambiguous DW nucleation/propagation required for applications were found in 500-nm-wide and 13-nm-thick Co nanowires, with a maximum nucleation field and the largest gap between nucleation and propagation fields. The internal DW structures were resolved using the transport-of-intensity equation formalism in LTEM images and showed that the optimal nanowire dimensions correspond to the crossover between the nucleation of transverse and vortex walls.

We report detailed structural characterization and magneto-optical Kerr magnetometry measurements at room temperature in epitaxial Co2MnSi thin films grown on MgO(001) and Cr(001) buffered MgO single crystals prepared by sputtering. While Co2MnSi/Cr//MgO(001) films display the expected cubic anisotropy, the magnetization curves obtained for Co2MnSi// MgO(001) samples exhibit a superimposed in-plane uniaxial magnetic anisotropy. The evolution of magnetization with film thickness points to a relevant interfacial Co2MnSi-buffer layer (Cr or MgO) contribution which competes with magnetic properties of bulk Co2MnSi, resulting in a drastic change in the magnetism of the whole sample. The origin of this interfacial magnetic anisotropy is discussed and correlated with our structural studies. © 2013 American Institute of Physics.

The accurate characterization and control of the magnetic configuration in nanostructures, namely domain walls (DWs) or vortex structures, by external parameters such as the magnetic field is essential for applications such as information storage, sensing or magnetic logic. Transmission electron microscopy (TEM) allows the observation of the magnetic nanostructures with nanometer-range spatial resolution by Lorentz microscopy (LM). In this work, we performed in situ characterization and magnetic-field manipulation of DWs by LM in Co nanowires (NWs) in a FEI Titan Cube 60-300 TEM. These NWs were fabricated by focused electron beam induced deposition (FEBID) with purity higher than 90% on Si3N4 membranes. We determined the nucleation and propagation field of DWs by direct observation of the magnetic structure by LM of curved L-shape Co NW with varying width (w=125-1000 nm) and thickness (t=5-30 nm), demonstrating that for specific dimensions they are good DW propagators. Focal series in LM were acquired in order to map the in-plane magnetic induction by solving the Transport-of-Intensity Equation. The nucleation process gives rise to transversal DW in the thinner NWs (t < 13 nm). Above this value, the crossover to complex structures was investigated, including multiple vortex walls on the thicker NWs.

Progress in nanofabrication of nanomaterials is pushing the scientific community to search for and develop new tools to measure physical properties at the nanoscale. In the case of magnetic nanomaterials, the measurement of the magnetic properties of individual nanomagnets is of the utmost importance to understand the magnetization processes underway and to correlate the local magnetic behavior with the macroscopic properties expected to lead to new technological applications. Spintronics is a very active area in magnetism for the development of magnetic nanostructures for devices, such as magnetic tunnel junctions (MTJs). Though great advances have been done on the magnetic and magnetotransport macroscopic behavior of MTJs, a great physical insight would be gained by direct visualization of magnetization switching processes with nanometer resolution. High spatial resolution techniques are required for local characterization of magnetization processes in nanostructures. Transmission Electron Microscopy (TEM) techniques such as Electron Holography (EH) allows the quantitative imaging of the magnetization configurations of ferromagnetic (FM) materials with unprecedented nanometer spatial resolution. Furthermore, EH can be combined with in situ TEM experiments applying external constraints such as magnetic and electric fields, temperature, etc. In this work, we use a TEM cryo-holder to image by EH the magnetization states of FM materials whose Curie temperature (TC) is below room temperature while varying in situ an external magnetic field applied on the thin sample We will present the EH study of the magnetization switching of LaxCa1-xMnO3 (LCMO) and LaxSr1-xMnO3 (LSMO) based thin film MTJs (whose TC = 180 and 300 K, respectively) as a function of the temperature and magnetic field applied thanks to the objective lens. The interlayer coupling/decoupling of magnetic electrodes in MTJs will be analyzed by performing hysteresis loops at 100 K, mapping the magnetic config