Ordered magnetic nanostructures: fabrication and properties

The mechanisms of magnetization reversal in small magnetic particles have been much discussed in the last decades and prompted intense research activities, motivated in particular by applications in magnetic recording technology [1]. However, experiments were performed, in general, on large assemblies of particles, and the dispersion of morphologies, compositions, orientations, and separations of the magnetic entities limited the interpretation of the results. Furthermore, interactions between particles were difficult to take into account. Single particle studies were possible only in few cases [2]. Recently, insights into the magnetic properties of individual and isolated particles were obtained with the help of near field magnetic force microscopy [3], electron Lorentz microscopy or holography [4], and micro-SQUID (superconducting quantum interference device) magnetometry [5]. It is now possible to make a clear link between experiments performed on nanometer-sized single objects (particles, wires, etc.) and the numerical calculations based on the Brown micromagnetic equations [6]. We report the first study of isolated nanoscale wires with diameters smaller than 100 nm, for which singledomain states could be expected. The cylindrical geometry, with its large shape anisotropy, is well suited for comparison with theory. We obtained unique insight into the process of magnetization reversal by measuring histograms of the switching field as a function of the orientation of the wires in the applied field, their diameter, and the temperature. Furthermore, we measured the probability of switching as a function of the applied field and the temperature. Our studies reveal that the magnetization reversal proceeds by a distortion of the magnetization followed by a nucleation and a propagation process. The observed behavior illustrates the fundamental importance of the study of single, isolated magnetic particles in comparing models and experiments. We developed planar microbridge dc SQUID [7], made of Nb (thickness 20 nm), which were shown to be able to detect 10 4 mB [8]. The SQUID is made of a thin (20 nm) Nb layer in order to prevent flux trapping. The experimental setup allows measurements of hysteresis loops in magnetic fields of up to 0.5 T and temperatures below 6 K, with a time resolution of 100 ms. Ni wires were produced by electrochemically filling the pores of commercially available nanoporous tracketched polycarbonate membranes of thicknesses of 6 to 10 mm [9]. The pore size was chosen in the range of 30 to 100 nm [10]. In order to place one wire on the SQUID detector, we dissolved the membrane in chloroform and put a drop on a chip of some hundreds of SQUID’s. Magnetization measurements were performed on SQUID’s with a single isolated wire. Scanning electron microscopy (SEM) (Fig. 1) was used to determine the position and morphology of the wire. The surface roughness was around 5 nm, corresponding to our SEM resolution.

/pdf/nucleation-of-magnetization-reversal-in-individual-nanosized-2rtp7e65lw.pdf

Nucleation of Magnetization Reversal in Individual Nanosized Nickel Wires.

The effect of pulsed currents on magnetization reversal were studied on single ferromagnetic nanowires of diameter about 80 nm and 6000 nm length. The magnetization reversal in these wires occurs with a jump of the magnetization at the switching field Hsw, which corresponds to unstable states of the magnetization. A pulsed current of about 107 A/cm2 was injected at different values of the applied field close to Hsw. The injected current triggered the magnetization reversal at a value of the applied field distant from the switching field by as much as 20%. This effect of current-induced magnetization reversal is interpreted in terms of the action of the spin-polarized conduction electrons on the magnetization.

/pdf/current-induced-magnetization-reversal-in-magnetic-nanowires-42ib7rtpcq.pdf

Current-induced magnetization reversal in magnetic nanowires

Static and dynamic aspects of the magnetization reversal in nanowire arrays are investigated. The arrays have been produced by electrodeposition of ferromagnetic metals (Fe, Co, and Ni) into porous anodic alumina templates, with diameters as small as 5 nm. The crystal structures of the nanowires are bcc (Fe) and fcc (Ni) and a mixture of fcc and hcp (Co), with grain sizes of a few nanometers. Magnetic properties as a function of temperature are investigated. The temperature dependence of coercivity can be understood in terms of thermal activation over an energy barrier with a $\frac{3}{2}$-power dependence on the field. Coercivity as a function of diameter reveals a change of the magnetization reversal mechanism from localized quasicoherent nucleation for small diameters to a localized curlinglike nucleation as the diameter exceeds a critical value determined by the exchange length. The quasicoherent limit is described by a model that yields explicitly real-structure-dependent expressions for coercivity, localization length, and activation volume.

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1048&context=physicssellmyer

Structure and magnetic properties of ferromagnetic nanowires in self-assembled arrays

The use of magnetic nanowires is demonstrated as a method for the application of force to mammalian cells. Magnetic separations were carried out on populations of NIH-3T3 mouse fibroblast cells using ferromagnetic Ni wires 350 nm in diameter and 35 μm long. Separation purities in excess of 90% and yields of 49% are obtained. The nanowires are shown to outperform magnetic beads of comparable volume.

/pdf/cell-manipulation-using-magnetic-nanowires-4ugoewhldg.pdf

Cell manipulation using magnetic nanowires

In a decagonal quasicrystal with composition AI70Co15Ni15 two different types of superstructure have been found by electron microscopy techniques. One of these superstructures has previously been observed. The other newly characterized superstructure causes a ring-like zone of low diffraction intensities and, additionally, reflections at positions having half-integer indices. Both types of superstructure have a doubled period of 8·2 A along the unique tenfold axis, which is suggested here to be due to Al/TM (TM= Co, Ni) ordering on different layers. The main structural elements are columns with a diameter of approximately 20 A. During irradiation with a 300 keV electron beam both superstructures are transformed to a ‘basic’ decagonal structure with a 4·1 A period, in which the wheel-like contrast features with 20 A diameter are absent. For the more disordered basic decagonal structure no conspicuous tiling can be found which can be superimposed onto the electron micrographs, while the new superst...

Two different superstructures of the decagonal Al-Co-Ni quasicrystal

The decagonal Al70Mn17Pd13 quasicrystal was studied by high-voltage high-resolution transmission electron microscopy (HRTEM) at 800 KV with a point-to-point resolution of 1.4 A. Images normal to the tenfold plane revealed a non-periodic arrangement of identical decagonal clusters with a diameter of 20 A located at the vertices of a tiling. From images normal to the twofold axes it is inferred that the structure in the direction of the tenfold axis is a periodic stacking of layers with a period of 12.5 A. Starting from the structure originally determined for decagonal Al-Mn by Steurer in 1991 a structure model including Al, Mn and Pd atoms was proposed. Comparison of the simulated images with the real HRTEM images allowed the determination of the most probable positions for the replacement of Pd atoms. High-resolution images normal to the tenfold axis revealed a triangular pattern near the centres of all decagonal clusters, indicating that the structure at the decagonal clusters is reconstructed i...

The structure and its reconstruction in the decagonal Al70Mn17Pd13 quasicrystal

Faceted microholes in icosahedral Al-Mn-Pd quasicrystals form by condensation of thermal vacancies, typically during annealing in the temperature range above approximately 700oC Prolonged annealing at temperatures between 750 and 830oC leads to the growth of large and highly faceted microholes through a classical Ostwald ripening process The indexing of all facets of such a microhole is presented The idealized morphology is found to correspond to an icosahedral polyhedron with 302 facets

Highly faceted growth shape of microvoids in icosahedral Al-Mn-Pd

Study of Co, Ni and Co/Cu nanowires: Magnetic flux imaging by off-axis electron holography

Reference CIME-ARTICLE-1995-003doi:10.1103/PhysRevLett.75.4630View record in Web of Science Record created on 2007-02-15, modified on 2017-05-12

Flux quantization in magnetic nanowires imaged by electron holography.

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