We study the magnetoimpedance effect, using a ${\mathrm{Co}}_{67}{\mathrm{Fe}}_4{\mathrm{Mo}}_{\text{1.5}}{\mathrm{Si}}_{\text{16.5}}{\mathrm{B}}_{11}$ amorphous, ribbon-based sensitive element, in the presence of a commercial Ferrofluid^® liquid thin layer covering the ribbon surface. The magnetoimpedance response is clearly dependent on the presence of the magnetic ferroliquid, the value of the applied magnetic field, and the parameters of the driving current. The magnetoimpedance-based prototype is proposed as a biosensor with high sensitivity to the fringe field produced by magnetic nanoparticles. A special advantage of this sensor is its high stability to chemical aggressive media; hence, it can be used for in situ measurements during fabrication of biomaterials with a high level of affinity and specificity with nanoparticles employed as bimolecular labels.

In this work an enhancement of the Curie temperature of the intergranular amorphous region in nanocrystalline alloys with respect to amorphous ribbons of the same composition is shown. The Curie temperature reaches a value of 125 °C. The experimental results are discussed in terms of both an inhomogeneous distribution of Nb atoms in the interphase and the magnetic interactions between the Fe ferromagnetic grains and the matrix. Finally, it is proposed that the existence of a molecular field of about 80 T originated by the penetration of the exchange field of α-Fe nanocrystals into the amorphous paramagnetic intergranular region is the main cause of this Curie temperature enhancement. Moreover, the compositional dependence of the Curie temperature in Fe-B-Nb-Cu amorphous ribbons has been reported.

The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened up the path for extensive research activity in the nanoscale world. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. We combined X-ray diffraction, high-resolution transmission electron microscopy, extended X-ray absorption fine structure and magnetic measurements in order to describe the influence of the microstructure and morphology on the magnetic behaviour of NiO nanoparticles (NPs) with sizes ranging from 2.5 to 9 nm. The present findings reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order, typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni²⁺ magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, whereas larger NPs are formed by an uncompensated AFM core with a net magnetic moment surrounded by a SG shell. The coupling at the core–shell interface leads to an exchange bias effect manifested at low temperature as horizontal shifts of the hysteresis loop (∼1 kOe) and a coercivity enhancement (∼0.2 kOe).

The temperature dependence of the isothermal magnetic entropy change, ΔS_M, and the magnetic field dependence of the refrigerant capacity, RC, have been investigated in a composite system xA + (1 − x)B, based on Fe₈₇Zr₆B₆Cu₁ (A) and Fe₉₀Zr₈B₂ (B) amorphous ribbons. Under a magnetic field change of 2 T, the maximum improvement of the full-width at half maximum of ΔS_M(T) curve (47% and 29%) and the RC (18% and 23%), in comparison with those of the individual alloys (A and B), is observed for x ≈ 0.5. Moreover, a flattening over 80 K in the ΔS_M(T) curve around room temperature range is observed, which is a key feature for an Ericsson magnetic refrigeration cycle.