Jordi Sort

Large anomalous enhancement of perpendicular exchange bias by introduction of a nonmagnetic spacer between the ferromagnetic and antiferromagnetic layers

In (Pt/Co)n/FeMn multilayers, the magnitude of exchange bias, HE, can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer (ML) and the antiferromagnetic (AFM) layer. The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased. This bias enhancement is accompanied by an increase of coercivity, HC. This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic (FM)/AFM spin projection, thus leading to the HE and HC enhancements. The decrease of HE and HC for thicker spacer layers is due to the limited range of the FM–AFM proximity effect.

Modern Trends in Tungsten Alloys Electrodeposition with Iron Group Metals 1

Theoretical and applied studies of tungsten alloys with iron group metals (Me-W) are being carried out worldwide, in the light of their versatile applications. The aim of this paper is to provide an overview of the works on electrodeposition of tungsten alloys with iron group metals, their properties and applications. There are 221 papers reviewing on the following theoretical and practical topics: chemistry of electrolytes used for electrodeposition, codeposition mechanisms, and properties of electrodeposited tungsten alloys. In addition, the formation of W(VI) and iron group metal (Me) complexes (polytungstates and complexes of Me(II) and W(VI)) with citrates and OH− is analysed based on the published data and the calculated distribution of species as a function of pH (ranged from 1 to 10) is provided for solutions with/without citrates. The adduced data are correlated with the compositions of electrodeposited alloys. Various codeposition models of tungsten with iron group metals described in the literature are critically discussed as well. The peculiarities of the structure of tungsten alloys and their thermal stability, mechanical, tribological, and magnetic properties, corrosion performance, their applications in hydrogen electrocatalysis, template-assisted deposition into recesses (aimed to obtain micro- and nanostructures) are also reviewed and mapped.

Size-dependent passivation shell and magnetic properties in antiferromagnetic/ferrimagnetic core/shell MnO nanoparticles.

The magnetic properties of bimagnetic core/shell nanoparticles consisting of an antiferromagnetic MnO core and a ferrimagnetic passivation shell have been investigated. It is found that the phase of the passivation shell (gamma-Mn(2)O(3) or Mn(3)O(4)) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly gamma-Mn(2)O(3) shell, larger ones have increasing amounts of Mn(3)O(4). A considerable enhancement of the Néel temperature, T(N), and the magnetic anisotropy of the MnO core for decreasing core sizes has been observed. The size reduction also leads to other phenomena such as persistent magnetic moment in MnO up to high temperatures and an unusual temperature behavior of the magnetic domains.

Room-temperature coercivity enhancement in mechanically alloyed antiferromagnetic-ferromagnetic powders

The coercivity, HC, and squareness of Co powders have been enhanced at room temperature by mechanically alloying them with antiferromagnetic powders with Neel temperature, TN, above room temperature. The enhancement is maximum after field annealing above TN. The existence of loop shifts and the dependence of HC on the annealing and measuring temperatures indicate that exchange bias effects are responsible for this behavior.

Coercivity and squareness enhancement in ball-milled hard magnetic–antiferromagnetic composites

The room-temperature coercivity, HC , and squareness, MR / MS ~remanence/saturation magnetizations!, of permanent magnet, SmCo 5 powders have been enhanced by ball milling with antiferromagnetic NiO ~with Neel temperature, T N 5590 K!. This enhancement is observed in the as-milled state. However, when the milling of SmCo 5 is carried out with an antiferromagnet with T N below room temperature ~e.g., for CoO, TN5290 K!, the coercivity enhancement is only observed at low temperatures after field cooling throughTN . The ferromagnetic-antiferromagnetic exchange coupling induced either by local heating during milling (SmCo51NiO) or field cooling (SmCo51CoO) is shown to be the origin of the HC increase.

Hard and Transparent Films Formed by Nanocellulose-TiO2 Nanoparticle Hybrids

The formation of hybrids of nanofibrillated cellulose and titania nanoparticles in aqueous media has been studied. Their transparency and mechanical behavior have been assessed by spectrophotometry and nanoindentation. The results show that limiting the titania nanoparticle concentration below 16 vol% yields homogeneous hybrids with a very high Young’s modulus and hardness, of up to 44 GPa and 3.4 GPa, respectively, and an optical transmittance above 80%. Electron microscopy shows that higher nanoparticle contents result in agglomeration and an inhomogeneous hybrid nanostructure with a concomitant reduction of hardness and optical transmittance. Infrared spectroscopy suggests that the nanostructure of the hybrids is controlled by electrostatic adsorption of the titania nanoparticles on the negatively charged nanocellulose surfaces.

Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

The fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic (AFM) matrix are discussed. The preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation. The magnetic properties of the FM/AFM composites are shown to depend strongly on the morphology of the system (e.g., nanoparticle size), the AFM anisotropy and the AFM-FM coupling. For example, all the studied systems exhibit coercivity enhancement below the Neel temperature of the AFM. However, while Co nanoparticles embedded in CoO exhibit loop shifts of thousands of Oe, Fe nanoparticles in Cr2O3 only show a few Oe shifts. An interesting effect evidenced in all systems is the increase of remanence (MR) which, in the case of Co-CoO, ultimately leads to an improvement of the superparamagnetic blocking temperature of the nanoparticles.

Silica aerogel–iron oxide nanocomposites: structural and magnetic properties

Abstract Magnetic nanocomposites formed by iron oxide particles hosted in silica aerogels pores have been synthesized by sol–gel processes and supercritical evacuation of the solvent. Two iron-containing salts have been essayed: (A) Fe(NO 3 )·9H 2 O and (B) FeNa(EDTA)·2H 2 O. The synthetic routes made use of the gel pores as nanoreactors. Structural and magnetic properties have been studied by combining X-ray diffraction (XRD), N 2 adsorption isotherms, transmission electron microscopy (TEM), 57 Fe Mossbauer spectroscopy and vibrating sample magnetometry (VSM). The nanocomposites properties, i.e., phase of the iron oxide, monolithic integrity, particle size distribution and magnetic phase dilution vary with the chosen synthetic path. The use of EDTA complex as a nanoparticle precursor increases the average pore diameter of the matrix. The nanocomposites are good candidates for applications in the field of magneto-optical sensors and magnetic devices due to their attractive properties, including soft magnetic behavior, low-density and electric resistivity.

A transparent hybrid of nanocrystalline cellulose and amorphous calcium carbonate nanoparticles

Nanocellulose hybrids are promising candidates for biodegradable multifunctional materials. Hybrids of nanocrystalline cellulose (NCC) and amorphous calcium carbonate (ACC) nanoparticles were obtained through a facile chemical approach over a wide range of compositions. Controlling the interactions between NCC and ACC results in hard, transparent structures with tunable composition, homogeneity and anisotropy.

Perpendicular exchange bias in antiferromagnetic-ferromagnetic nanostructures

Exchange bias effects have been induced along the perpendicular-to-film direction in nanostructures prepared by electron beam lithography, consisting of a ferromagnetic [Pt/Co] multilayer exchange coupled to an antiferromagnet (FeMn). As a general trend, the exchange bias field and the blocking temperature decrease, whereas the coercivity increases, as the size of the nanostructures is reduced.