R.P. del Real

Structural Dependence of Magnetic Properties in Co-Based Nanowires: Experiments and Micromagnetic Simulations

The magnetic properties of several series of cylindrical Co and CoFe nanowires with tailored hexagonal or cubic crystalline symmetry are reviewed. Nanowires are prepared by electroplating filling the self-assembled nanopores of anodic alumina membranes. Their structure is tailored through the electroplating conditions to present cubic (fcc Co and bcc CoFe) or hcp hexagonal ([101], [110] and [002] textures) symmetry. Hysteresis parameters (i.e., coercivity and remanence) are measured in parallel and perpendicular to nanowires magnetic field configurations. Micromagnetic simulations have been performed taking into account the different crystalline anisotropy of nanowires. They show that the magnetization reversal process takes place in hcp symmetry crystal phase wires by vortex-like domain wall and quasi-curling mechanisms, while for fcc and bcc symmetries crystal phases only a vortex domain wall is involved resulting in high-squareness hysteresis loops.

Spin configuration of cylindrical bamboo-like magnetic nanowires

The surface and the internal magnetic structure of bamboo-like cylindrical nanowires with tailored diameter modulations have been determined exploiting the direct photoemission and transmission contrasts using photoemission electron microscopy combined with X-ray magnetic circular dichroism, as well as complementary magnetic force microscopy and micromagnetic simulations. Bamboo-like cylindrical nanowires with diameters of 130 and 140 nm, and a modulation periodicity of 400 nm were electrochemically grown into the pores of alumina templates. FeCoCu and Co nanowires were selected to offer parallel and perpendicular magnetization easy axis, respectively. For FeCoCu nanowires, a main longitudinal magnetization configuration is found consistent with the predominant shape anisotropy. In addition, a weaker modulated contrast along the wires’ axis is observed that matches the position of each diameter modulation: vortex-like structures are observed at the ends of the wires and at the surface around the modulations. In Co nanowires, a multi-segmented vortex-like structure with alternating opposite chirality is found not matching the periodicity of the diameter modulations. Such a spin configuration is interpreted considering that Co nanowires exhibit hexagonal symmetry with c axis nearly perpendicular to the nanowires defining strong uniaxial transverse magnetocrystalline anisotropy.

Comparative performance of static-mode ferrous MEMS gradiometers fabricated by a three-step DRIE process

Two MEMS structures—a cantilever beam and a quad-beam—have been designed and fabricated through a three-step deep reactive ion etching (DRIE) process. Devices feature target patterns to align with an external optical detection system and a micromachined cavity to embed an NdFeB hard mini-magnet, thus releasing the stress of structures. Structures are intended for magnetostatic gradient measurements. Induced magnetic fields generate an attracting force on the magnet that deflects the sensor. Deflection is optically detected through nanometer-resolution confocal microscopy. The static-mode sensitivity of up to 1.86 × 10−4 T m−1 demonstrates that MEMS gradiometers are able to perform in situ gradiometry with a single sensor and miniaturized size. Suitable techniques for integrated detection are discussed.

Dependence of magnetization process on thickness of Permalloy antidot arrays

Nanohole films or antidot arrays of Permalloy have been prepared by the sputtering of Ni80Fe20 onto anodic alumina membrane templates. The film thickness varies from 5 to 47 nm and the antidot diameters go from 42 to 61 nm, for a hexagonal lattice parameter of 105 nm. For the thinner antidot films (5 and 10 nm thick), magnetic moments locally distribute in a complex manner to reduce the magnetostatic energy, and their mostly reversible magnetization process is ascribed to spin rotations. In the case of the thicker (20 and 47 nm) antidot films, pseudodomain walls appear and the magnetization process is mostly irreversible where hysteresis denotes the effect of nanoholes pinning to wall motion.

Double large Barkhausen jump in soft/soft composite microwires

The magnetic properties of double layer microwires consisting of a soft FeSiBP amorphous core, an intermediate non-magnetic glass spacer and a softer FeNi outer shell have been investigated. As in the case of other magnetostatically coupled two-phase systems, the hysteresis loops are characterized by two well-defined Barkhausen jumps corresponding each to the magnetization reversal of the individual phases, separated by a plateau. The strong dipolar interaction that leads to the appearance of the plateau is investigated in terms of the microwire geometry. It is shown that this source of coupling is capable of increasing up to one order of magnitude the switching field of the Fe-rich core. Thus, magnetic bistability can be effectively controlled in these kinds of composite wires.

Direct observation of transverse and vortex metastable magnetic domains in cylindrical nanowires

The study has been performed under the framework of the Projects No. MAT2013-48054-C2-1-R, MAT2016-76824-C3-1-R, MAT-2015-68772-P, MAT2015-64110-C2-2-P supported by the MINECO from Spain and DURSI 2014SGR220 supported by the Catalan Government. J.A.F.-R. acknowledges financial support from MINECO and the ESF through the “Ayudas para contratos predoctorales para la formacion de doctores 2014.”

Micromagnetism of permalloy antidot arrays prepared from alumina templates

Magnetic hysteresis processes of hexagonal arrays of permalloy antidots have been studied by means of micromagnetic simulations as a function of geometrical parameters. The ideal system shows a maximum of the coercive field as a function of the antidot diameter. The simulated magnetic behavior has been compared with experimental values for antidot arrays of permalloy prepared from alumina templates with thicknesses between 2 and 60 nm, showing a monotonic increase of the coercive field as a function of the antidot diameter. We show that the introduction into simulations of the combination of variable antidot diameters from bottom to top due to the fabrication process and, more importantly, large geometrical domains, which break the sample symmetry, solves the discrepancy between the simulations and the experiment.

Nanomagnets with high shape anisotropy and strong crystalline anisotropy: perspectives on magnetic force microscopy

We report on a new approach for magnetic imaging, highly sensitive even in the presence of external, strong magnetic fields. Based on FIB-assisted fabricated high-aspect-ratio rare-earth nanomagnets, we produce groundbreaking magnetic force tips with hard magnetic character where we combine a high aspect ratio (shape anisotropy) together with strong crystalline anisotropy (rare-earth-based alloys). Rare-earth hard nanomagnets are then FIB-integrated to silicon microcantilevers as highly sharpened tips for high-field magnetic imaging applications. Force resolution and domain reversing and recovery capabilities are at least one order of magnitude better than for conventional magnetic tips. This work opens new, pioneering research fields on the surface magnetization process of nanostructures based either on relatively hard magnetic materials-used in magnetic storage media-or on materials like superparamagnetic particles, ferro/antiferromagnetic structures or paramagnetic materials.

Focused-Ion-Beam-Assisted Magnet Fabrication and Manipulation for Magnetic Field Detection Applications

A focused-ion-beam-assisted technique intended for ultrasmall, hard-magnet fabrication has been developed. By means of ion-beam-induced milling and deposition, reduced-size NdFeB magnets were extracted from a macroscopic quarry and bonded to the surface of a thin-film bulk acoustic resonator (FBAR). Electrical characterization of the FBAR before and after bonding of the magnet was carried out, thus observing both a downshifting of the resonance frequency and a reduction of the quality factor of the resonator. The magnetic behavior of the nanomagnet has been confirmed by means of magnetometry measurements based on atomic force microscopy.

Inhomogeneous nanocrystallization of Joule-heated amorphous Vitroperm alloy

Crystallization of Vitroperm® alloy has been realized by means of Joule heating. The influence of the gas flow around the sample during the heat treatment is dramatic and it produces an inhomogeneous crystallization. For large enough gas flow the temperature of the sample surroundings can be considered constant and a temperature gradient appears over the sample width, which means a gradient in size and concentration of Fe–Si and Fe–B nanocrystals. For adequate gas flow values the temperature of the sample edges is below the Fe–B crystallization value whereas the centre is above that value, showing therefore completely different magnetic behaviour.