Top PDF Magnetic nanowires and nanotubes

as a function of type, selection of circulation for BPWs and vortex walls in tubes, plateau of speed and emission of spin waves. Wartelle et al. ( 2018b ) reported the first hint of dynamics, by monitoring the inner structure of DWs before and after application of a pulse of magnetic field. They showed experimentally that, although initially not predicted, the type of wall can change during motion. This raises new frontiers in their understanding. This said, there is an intriguing similarity with glass-coated amorphous microwires ( Vazquez 2015 ). This is a large and long-standing family of wires, with various types of anisotropy. Their diameter is rather in the range of micrometers, so, not expected to be a textbook case for the micromagnetic and one-dimensional predictions. Still, wall speed in the range 1 − 10 km/s was measured is such wires with the so-called inductive Sixtus-Tonk method, e.g., for FeSi and FeNiSi compositions ( Varga et al. 2005 ; 2006 ; 2008 ). Interestingly, the topics and communities of microwires on one side, and nanowires and nanotubes on the other side, are closing the gap, starting to shed light on the two branches. For instance, Stupakiewicz et al. ( 2014 ) used vectorial Kerr microscopy to elucidate the domain configuration under the influence of either axial or azimuthal field, in 100 µm- diameter wires, highlighting well-defined helical domains, very similar to those in magnetostrictive ( Staˇ no et al. 2017b ) or angle-deposited-wire ( Zimmermann et al. 2018 ) tubes. Visualization of the DWs is key in understanding their dynamics. For instance, Chizhik et al. ( 2016 ) evidenced the impact of the tilt of 180 ◦ walls with respect to the normal to the wire axis: mobility is enhanced for tilted walls, and explained simply by the geometry of motion. The recent availability of glass-coated wires with sub-micrometer diameters, and their structural and magnetic comparison, provides an experimental playground to bring both topics ever closer together ( Ov´ ´ ari and Chiriac 2014 ). This leaves exciting challenges for the future.

Actually, carbon nanotubes represent a new type of nanomaterial that does not exist in traditional bulk form. The same holds for semiconductor nanowires. Random carbon nanotube networks [4] are the “thin-film” equivalent form of CNTs. Although devices made from such films exhibit stunning characteristics [5] (especially considering their relatively simple processing conditions), such characteristics (carrier mobility, subthreshold slope, . . .) are still degraded compared to those of counterpart devices obtained with individual SWNT specimens (compare device results from Ref. [1] with those of Ref. [5]). Also, such random films are not amenable to ultra large scale integration (ULSI)- type manufacturing methods because they are not continuous at the nanometric scale. Hence, new paradigms and fabrication schemes have to be found, in order to allow the use of such nano-materials (or rather collections of such nano-objects) and be able to compete with the organisation and complexity levels reached by silicon MOS technology.

Abstract. A  comparative  study  of  the  low  temperature  conductivity  of  an  ensemble  of  multiwall  carbon  nanotubes  and  semiconductor  nanowires  is  presented.  The    quasi  one‐dimensional  samples  are  made  in  nanoporous  templates    by   electrodeposition  and  CVD    growth.    Three    different  structures    are  studied  in  parallel:  multiwall  carbon  nanotubes,  tellurium  nanowires, and  silicon nanowires. It  is shown that  the Coulomb  blockade regime  dominates the  electronic transport  below  50  K,    together  with  weak  and  strong  localization  effects.  In  the  Coulomb  blockade  regime,  a  scaling  law  of  the  conductance measured  as  a  func‐  tion  of  the  temperature  and  the  voltage  is  systematically    observed.  This  allows  a  single  scaling  parameter α to  be  defined. This  parameter  accounts  for  the  specific realization  of  the  “disorder”,  and  plays  the  role  of  a  fingerprint  for  each  sample.  Correlations  between  α and  the  conductance  measured  as  a  function of temperature and  voltage,  as  a  function  of the  perpendicular magnetic  field, and  as  a  function of  the temperature  and  voltage  in  the  localized  regime  below  1  K  have  been  performed.  Three  universal  laws  are  reported.  They    relate  the  coefficient  α (1)  to  the 

most organics and polymers. This large refractive index enhances the confinment in light waveguiding. Further, the tubular aspect of nanotube give a new degree of modulation in photonics. In literature, Zhao et al. 3 have shown that the tubular geometry of nanotube can uniquely confine part of the energy density in the lowest refractive index area, the core of the nanotube. This original confinment allows tuning of light propagation and light-matter interaction. In this present work, we detail keys for: low-cost production of SU8 polymer nanowires and nanotubes by wetting template method, experimental protocol to directly inject light into such nanostructures, numerical analysis of sub-wavelength propagation, and finally characterisation of optical losses for SU8 nanotubes.

For instance, technological progresses have permitted a reduction of system dimensions so that quantum effects may be observed. In fact, a non-negligible part of both theoretical and technological breakthroughs came from the miniaturization of materials. To summa- rize, when magnetism goes down to nano, quantum effects become visible and unexpected magnetic behaviors can be discovered. As a matter of fact, these novel nanoscaled ma- terials exhibit a characteristic size which is of the same order as the interaction lengths [Blü2005]. For instance, new phenomena have appeared in confined systems (2D: thin films, 1D: nanowires, 0D: nanoclusters) or at the interfaces of magnetic and non-magnetic layered materials. Concerning confined systems, the reduction of one or several spatial dimensions of a system brings many changes such as variations on the thermodynamic properties, on the values of the spin and orbital moments, on the size of magnetic domains or even on the dynamical properties [Blü2005]. It must however be reminded that prob- ing low-dimensional systems does not only consist in investigating geometrical nanosized systems but also larger systems which exhibit special properties on a 2D or 1D region (for example Kohn anomaly in low-dimensional chains [Hoe2009], or conduction of electrons through the Cu-O 2D-planes of cuprates superconductors). The dimensional reduction of the exchange interaction (2D: J 1 = J 2  J 3 or 1D: J 1  J 2 = J 3 ) leads to collective prop-

cating a high magnetic stability. As pointed out above, our SAED and HRTEM results show that we have preferential orientations in the bundles, and that the ensemble of bundles is randomly dis- tributed in all directions. The ferromagnetic behavior is consistent with the observed fact that after the sample is magnetized, an important fraction of the nanowires conserves its magnetization, and this leads to the observed remanence and coercive field. It is also clear from the Mössbauer result that at room temperature no trace of superparamagnetism is present in those samples, even though this is a nanostructured a-Fe system. There must be at least two reasons to maintain the magnetization of these nanowires in one direction: the shape anisotropy and the magnetic surface ef- fects [7,17] .

The magnetic properties of nanowires have been studied in different systems through several works 11–15 . However, few studies have focused on the magnetic states of isolated nanowires 16-18 . The observation of magnetization in individual nanowires requires techniques combining high sensitivity and spatial resolution. Transmission Electron Microscopy (TEM) is the appropriate tool: its broad sensitivity ranges from atomic structure to electromagnetic fields and includes atomic-scale analysis of valence states and chemistry. Its ability to probe individual nano-objects instead of assemblies of nano-objects provides a remarkable potential for discoveries. Among the different TEM methods, off-axis electron holography (EH) 19 is a powerful interferometric method, giving access to the in-plane induction inside and outside of the nano-object. 20,21 A recent study has for instance presented the 3D vectorial magnetic analysis using EH on two Fe platelets separated by a Cr layer. They demonstrated the appearance of coupled vortices between Fe disks. 22

DOI: 10.1103/PhysRevLett.123.217201 It is well known that specific properties in condensed- matter and nanosized systems can be obtained by either acting on the electronic structure by selecting an appropriate material composition and crystalline structure, or by making use of finite-size and interfacial effects, strain, gating with an electric field, etc. [1] . These approaches have proven suitable for tailoring charge transport, optical properties, electric or magnetic polarization, etc., however, there are limits regarding what can be achieved with materials, or realized with device fabrication. An alternative strategy entails considering a specific topology in order to develop the desired properties of a system, yielding diverse appli- cations such as the design of wide-band-gap photonic crystals [2] and the control of flow of macromolecules [3] , or novel theoretical methods such as for the description of defects [4] , or intringuing 3D vector-field textures such as hopfions and torons [5] . As regards magnetism, unusual properties resulting from topological features have been predicted, such as the existence of a domain wall (DW) in the ground state of a Moebius ring [6] , or the nonreciprocity of spin waves induced by curvature and boundary conditions in nanotubes [7] .

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We used a modified CVD reactor for this study; Figure 4-6 shows the schematics of the modified CVD system. A hot tungsten filament was placed 150 mm upstream of the sample holder, in order to pre- decompose the gas before it reaches the substrate. The filament was thermally isolated by a boron nitride cylinder, and it was operated at ~ 1800 °C. The temperature was measured with an optical pyrometer. Since the substrate heating (including the possible effect of the tungsten filament) was automatically adjusted by a thermocouple situated at the same position as the substrate, the temperature was correctly controlled at the desired value during the growth. The samples were first placed at the end of the quartz tube holder, which is almost at room temperature, and rapidly pushed in the center region of the quartz tube using a stainless steel rod, once the temperature has been stabilized at the desired value and once the H· had been activated by switching-on the corresponding tungsten filament (this was done only for the SiNW growth). But for the CNT growth, the hot filament was always in the off mode. The gas (SiH 4 or

Simulations micromagnétiques Pour analyser plus profondément les résultats expérimentaux, les états magnétiques résiduels 3D et des champs parasites de 20 bicouches de Co/Cu ont été calc[r]

Simulations micromagnétiques Pour analyser plus profondément les résultats expérimentaux, les états magnétiques résiduels 3D et des champs parasites de 20 bicouches de Co/Cu ont été calc[r]

Fig. 2 b is reasonably reproduced by the simulation of MFM contrast of a Bloch-point wall (Fig. 2 c), although not taking into account the above-mentioned tilts. Note also that it is not granted that experimentally a Bloch- point wall may be distinguished from a transverse wall, due to the finite spatial resolution. Anyway, it would be wrong to interpret Fig. 2 a as a signature for a trans- verse wall; it is an instrumental feature, which has the strongest signature for large-diameter wires as in Fig. 1 c compared to Fig. 1 e, and more generally for small thick- ness of tip coating, small oscillation amplitude and small lift height (Fig. 2 f).

Basically, we can observe four different states which are color coded in Fig. 4. We will not focus on transition states (orange in Fig. 4(c)) which appear in particular cases we did not experimentally observe. The blue and turquoise colors correspond to uniform magnetic configurations, with the magnetization pointing either in the Co layer plane (In-plane “IP” state) or along the NC axis, i.e. z direction (Out-of-Plane “OP” state) respectively. The green and red colors represent vortex states with the core pointing either perpendicular (VOP state) to the layer plane or roughly in the layer plane (VIP state, the angle between the direction of the core and n being at least > 45 °). It is worth noting that micromagnetic simulations show the appearance of vortices for thicknesses down to 25 nm. For perfect layers, i.e. with surfaces perfectly perpendicular to the z axis (flat discs) the limit thickness to get vortices is at least 30 nm. The fact that vortex states are favoured in our particular geometry demonstrates the importance of being as close as possible of the real geometry of the sample in micromagnetic simulations at very small scales.

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University of Technology, 612 00 Brno, Czech Republic ‡ Electronic address: olivier.fruchart@cea.fr domain wall (BPW, also called vortex wall by some), host- ing a BP at its center even at rest. The BPW was predicted to reach a steady-state motion with high axial velocity even at high magnetic field[ 19 – 21 ]. Second is the transverse-vortex wall (TVW, also called transverse wall by some), with fast az- imuthal precession and axial mobility much lower than that of the BPW[ 19 , 22 ]. Both DWs have been predicted to retain their topology during motion. This makes a sharp contrast with thin strips, prone to DW transformations under both field and spin-polarized current[ 19 , 23 , 24 ]. The latter can be under- stood as all DWs share a single topology in strips[ 25 ], making transformations easier. As the existence of the BPW and TVW has been confirmed experimentally recently at rest[ 26 , 27 ], the question arises whether the different topology indeed prevents DW transformation in reality.

Theoretically  for  a  wire  of  hexagonal  Co  with  an  aspect  ratio  of  20  and  its  crystallographic  c-­‐axis  along  its  length,  a   value  of  H K =2.3  T  is  expected,  of  which  1.56  T  due  to  shape  anisotropy  and  the  rest  0.74  T  due  to  the  magneto-­‐ crystalline  anisotropy.  This  theoretical  value  is  closer  to  the  ones  obtained  by  the  perpendicular  measurement.  This   is   due   to   the   fact   that   in   the   field   range   close   to   saturation,   homogeneous   reversible   rotation   mechanisms   are   dominant  whereas  in  the  range  near  the  magnetization  switching  point  more  complex  mechanisms  and  irreversible   jumps   may   occur.   We   have   used   the   value   that   best   describes   the   former   range   for   the   hard   axis   loop   fit.   Furthermore  in  the  curves  measured  with  the  applied  field  perpendicular  to  the  easy  direction  (hard  axis),  there  is   an  apparent  contradiction  between  the  anisotropy  value  which  must  be  used  to  account  for  the  high  field  range   (approach   to   saturation)   as   opposed   to   the   value   that   predicts   the   correct   loop   shape.   In   conclusion   the   real   anisotropy  field  is  best  represented  by  H K (per)  while  the  deviation  of  the  ratio  H K (par)/  H K (per)  from  unity  gives  a  

the HF Hamiltonian except in some very special cases and that, even in the latter, the proportionality factor is not B as usually assumed but 3B/2. Very recently Nicolas et al.[6] have discussed the effect of orbital polar- ization, using either a Stoner-like TB Hamiltonian with the OPA or an HF Hamiltonian in which the one and two orbital matrix elements of the Coulomb interaction are treated exactly in the spherical harmonics (SH) basis but three and four orbital terms are neglected. These latter terms depend both on B and C in the SH basis which results in a symmetry breaking that they claim to overcome by averaging over different orbital basis. On the opposite, a recent work by Xiangang Wan et al.[7] is based on a complete HF decoupling. However their ef- fective intra-atomic potential (see Eq.4 of their work) is the same as in LSDA+U while the TB part of their total Hamiltonian is not spin polarized. As a result when the approximations leading to the Stoner model are carried out in their Eq.4, it does not lead to the correct Stoner parameter.

VIII. DISCUSSION AND CONCLUSIONS We have shown in a detailed case study how one can combine ab initio electronic structure calculations with nu- merical renormalization group to get quantitative estimates of the Kondo temperature and zero-bias anomaly in the transport across atomically, structurally and electronically controlled nanowires. The specific examples we have chosen to apply this strategy to are Co and Fe magnetic impurities adsorbed on single-wall carbon nanotubes and a carbon vacancy in a pristine nanotube. While there are no experimental data for these systems, their extreme simplicity and reproducibility recommend them as ideal test cases for future study. Even in the absence of experimental data, the effect of various approxi- mations and DFT errors can be tested here rather instructively. Our main results can be summarized very shortly. A Co atom (or a C vacancy) behaves effectively as a S = 1/2

13 geometric parameters to the critical magnetic field needed to unpin the domain wall. Numerical simulations and analytical predictions based on an approximation of uniformly magnetized magnetic spheres agreed best in the case of gently sloping modulations. To provide a more accurate analytical description of domain wall behavior close to the modulation may require some correction of the model or the inclusion of additional energy terms. These terms may concern such phenomena as, for example, magnetocrystalline anisotropy and its fluctuations in the polycrystalline structure [42], any defects, domain wall structure modification close to an abrupt modulation, or spin-polarized-current-induced effects. Nevertheless, the analytical model developed here is a simple scaling law which may be useful in resolving experimental and nanofabrication issues.