Enric Menéndez

Direct Magnetic Patterning due to the Generation of Ferromagnetism by Selective Ion Irradiation of Paramagnetic FeAl Alloys

Sub-100-nm magnetic dots embedded in a non-magnetic matrix are controllably generated by selective ion irradiation of paramagnetic Fe(60)Al(40) (atomic %) alloys, taking advantage of the disorder-induced magnetism in this material. The process is demonstrated by sequential focused ion beam irradiation and by in-parallel broad-beam ion irradiation through lithographed masks. Due to the low fluences used, this method results in practically no alteration of the surface roughness. The dots exhibit a range of magnetic properties depending on the size and shape of the structures, with the smallest dots (<100 nm) having square hysteresis loops with coercivities in excess of micro(0)H(C) = 50 mT. Importantly, the patterning can be fully removed by annealing. The combination of properties induced by the direct magnetic patterning is appealing for a wide range of applications, such as patterned media, magnetic separators, or sensors.

Selective generation of local ferromagnetism in austenitic stainless steel using nanoindentation

Periodic arrays of magnetic structures with micrometrer and submicrometer lateral sizes have been prepared at the surface of an austenitic stainless steel by means of local deformation using a nanoindenter. This method takes advantage of the phase transformation (from nonmagnetic fcc austenite to ferromagnetic bct martensite) which occurs in this material upon plastic deformation. The local character of the induced ferromagnetism is confirmed by magneto-optical Kerr effect measurements together with magnetic force microscopy imaging. The generated ferromagnetism can be subsequently erased by subjecting the deformed steel to annealing processes at temperatures above the reverse, martensite-to-austenite, phase transformation temperature.

Two-fold origin of the deformation-induced ferromagnetism in bulk Fe60Al40 (at.%) alloys

The transition from the atomically ordered B2-phase to the chemically disordered A2-phase and the concomitant deformation-induced ferromagnetism have been investigated in bulk polycrystalline Fe60Al40 (at.%) alloys subjected to compression processes. A detailed correlation between structural, magnetic and mechanical properties reveals that the generated ferromagnetism depends on the stress level but is virtually independent of the loading rate. The mechanisms governing the induced ferromagnetism also vary as the stress level is increased. Namely, in the low-stress regime both lattice cell expansion and atomic intermixing play a role in the induced ferromagnetic behavior. Conversely, lattice expansion seems to become the main mechanism contributing to the generated ferromagnetism in the high-stress regime. Furthermore, a correlation is also observed between the order–disorder transition and the mechanical hardness. Hence, a combination of magnetic and mechanical measurements can be used, in synergetic manner, to investigate this deformation-induced phase transition.

Out-of-plane magnetic patterning on austenitic stainless steels using plasma nitriding

A correlation between the grain orientation and the out-of-plane magnetic properties of nitrogen-enriched polycrystalline austenitic stainless steel surface is performed. Due to the competition between the magnetocrystalline anisotropy, the exchange and dipolar interactions, and the residual stresses induced by nitriding, the resulting effective magnetic easy-axis can lay along unusual directions. It is also demonstrated that, by choosing an appropriate stainless steel texturing, arrays of ferromagnetic structures with out-of-plane magnetization, embedded in a paramagnetic matrix, can be produced by local plasma nitriding through shadow masks.

Microstructural evolution during solid-state sintering of ball-milled nanocomposite WC–10 mass% Co powders

The microstructural evolution during solid-state sintering of nanocrystalline WC–10 mass% Co powders, prepared by different ball-milling processes to give rise to diverse mechanical activation states, is investigated. During sintering, two ternary carbides, Co6W6C and Co3W3C, form at the binder phase which surrounds WC particle aggregates. The overall microstructure of the sintered material (including the formation of these carbides) is found to be very sensitive to the initial state of the powders, even more than to the sintering temperature and/or sintering time. In addition, the nanocrystalline nature of the composites is preserved during the solid-state sintering, in part assisted by the formation of these complex carbides and decarburization. This nanostructure yields moderate microhardness and fracture toughness values, in spite of the remaining levels of porosity.

Improving the magnetic properties of Co-CoO systems by designed oxygen implantation profiles

Oxygen implantation in ferromagnetic Co thin films is shown to be an advantageous route to improving the magnetic properties of Co-CoO systems by forming multiple nanoscaled ferromagnetic/antiferromagnetic interfaces homogeneously distributed throughout the layer. By properly designing the implantation conditions (energy and fluence) and the structure of the films (capping, buffer, and Co layer thickness), relatively uniform O profiles across the Co layer can be achieved using a single-energy ion implantation approach. This optimized configuration results in enhanced exchange bias loop shifts, improved loop homogeneity, increased blocking temperature, reduced relative training effects and increased retained remanence in the trained state with respect to both Co/CoO bilayers and O-implanted Co films with a Gaussian-like O depth profile. This underlines the great potential of ion implantation to tailor the magnetic properties by controllably modifying the local microstructure through tailored implantation profiles.

Fluence dependence of ion implantation-induced exchange bias in face centered cubic Co thin films

The fluence dependence of exchange bias induced by oxygen ion implantation has been studied in highly textured face centered cubic Co films. These films exhibit a strong magnetocrystalline anisotropy prior to implantation. Upon implantation, the crystalline order is strongly reduced, even for the lowest implantation fluence, as shown by an isotropic magnetic behavior. Detailed analysis of the structural changes shows that the crystallite size remains basically unaltered upon implantation, suggesting that CoxOy is formed at the Co grain boundaries. A large suppression of the magnetocrystalline anisotropy is observed after implantation. This anisotropy has no influence on the unidirectional anisotropy associated to the exchange bias effect. Our study identifies a narrow implantation fluence window in which exchange bias by oxygen ion implantation is established. With increasing oxygen fluence, an increase in the magnitude of the exchange bias effect for higher fluences and, finally, a saturation of the exch...

Controlled generation of ferromagnetic martensite from paramagnetic austenite in AISI 316L austenitic stainless steel

The strain-induced austenite (γ) to martensite (α′) transformation in AISI 316L austenitic stainless steel, either in powders or bulk specimens, has been investigated. The phase transformation is accomplished using either ball-milling processes (in powders)—dynamic approach—or by uniaxial compression procedures (in bulk specimens)—quasi-static approach. Remarkably, an increase in the loading rate causes opposite effects in each case: (i) it increases the amount of transformed α′ in ball-milling procedures, but (ii) it decreases the amount of α′ in pressed samples. Both the microstructural changes (e.g., crystallite size refinement, microstrains, or type of stacking faults) in the parent γ phase and the role of the concomitant temperature rise during deformation seem to be responsible for these opposite trends. Furthermore, the results show the correlation between the γ → α′ phase transformation and the development of magnetism and enhanced hardness.

Tailoring the magnetization reversal of elliptical dots using exchange bias (invited)

Exchange bias effects have been studied in elliptical dots composed of ferromagnetic Ni80Fe20–antiferromagnetic Ir20Mn80 bilayers. The magnetization reversal mechanisms and magnetic configurations have been investigated by magneto-optic Kerr effect and magnetic force microscopy. Although the obtained bias fields in these dots are relatively small, the magnetization reversal is found to be influenced by the ferromagnetic–antiferromagnetic coupling. Namely, for some off-axis angles of measurement, the magnetization reversal mechanism of the Ni80Fe20–Ir20Mn80 ellipses depends on whether exchange bias is induced along the minor or major axis of the ellipses. Hence, exchange bias is shown to be an effective means for tailoring the magnetization reversal of elliptical dots after sample fabrication.

Magnetic Properties of Single Crystalline Expanded Austenite Obtained by Plasma Nitriding of Austenitic Stainless Steel Single Crystals

Ferromagnetic single crystalline [100], [110], and [111]-oriented expanded austenite is obtained by plasma nitriding of paramagnetic 316L austenitic stainless steel single crystals at either 300 or 400 °C. After nitriding at 400 °C, the [100] direction appears to constitute the magnetic easy axis due to the interplay between a large lattice expansion and the expected decomposition of the expanded austenite, which results in Fe- and Ni-enriched areas. However, a complex combination of uniaxial (i.e., twofold) and biaxial (i.e., fourfold) in-plane magnetic anisotropies is encountered. It is suggested that the former is related to residual stress-induced effects while the latter is associated to the in-plane projections of the cubic lattice symmetry. Increasing the processing temperature strengthens the biaxial in-plane anisotropy in detriment of the uniaxial contribution, in agreement with a more homogeneous structure of expanded austenite with lower residual stresses. In contrast to polycrystalline expanded austenite, single crystalline expanded austenite exhibits its magnetic easy axes along basic directions.