[en] We study the magnetic effects of the nano-oxide layers of specular spin valves, based on zero-field-cooling and field-cooling (under H=H₀) experiments in the 320-15 K temperature range. One finds a small increase (decrease) of ΔR/R at 15 K after H₀>0 (<0) cooling, but a sharp ΔR/R minimum occurs when H₀=-500 Oe. Also ΔR/R(T,H₀) departs from the ZFC curve at T∼175 K. These features are related to magnetic ordering in the oxide layers

[en] Magnetic refrigeration, based on the magnetocaloric effect, is an attractive alternative to the conventional vapor-compression technology. In a solid state magnetic refrigerator the traditional alternated fluid flow is replaced by two solid thermal switches (TSs) that control the heat flux. These TSs are materials, or devices, whose thermal conductivity changes with an external stimulus. Here, we numerically investigate how the performance of a solid state magnetic refrigerator depends on the TS thermal conductivities (k) and corresponding k-variations ($\mathrm{\Delta }k$) with the magnetic field. Two different scenarios were considered: a single TS and a double TS magnetic refrigerator. We have numerically varied the thermal conductivity of the thermal switches, operating frequency and working temperature. The use of only one thermal switch with near-ideal $\mathrm{\Delta }k$ is enough if the required performance does not overcome half of the adiabatic temperature change ($\mathrm{\Delta }{T}_{\mathit{ad}}$) of the magnetocaloric material. To overcome this value the device must use two effective thermal switches. Using an additional thermal switch is only beneficial when $\mathrm{\Delta }{T}_{\mathit{ad}}$ needs to be overcome, or when the first thermal switch shows a modest $\mathrm{\Delta }k$. These results simplify the operability control, which will increase the interest for magnetically activated thermal switches operating at acceptable magnetic fields.

[en] Magnetic-tunnel junctions MTJs with ultra-thin (∼6-9 A) oxide barriers, having the resistance-area product as low as 1-10 Ω μm², and magnetoresistance as high as ∼20%, were studied. In the temperature range from 300 to 25 K, they display resistance combined between metallic and activated laws. Under applied currents from 0.1 to 10 mA, the dynamic conductance varies from chaotic to periodic regimes, and the noise power spectrum over times from 10 ms to 10⁵ s shows a crossover from 1/f ² to 1/f behavior, all the features being different for parallel (P) and antiparallel (AP) magnetization of electrodes. A theoretical model is proposed, combining electron tunneling through localized defect states in the barrier and migration of respective defect atoms. A qualitative agreement with the experimental noise spectrum is achieved

[en] Highlights: • Irradiation of an Fe–V alloy by femtosecond laser triggers diffusion decomposition. • The decomposition occurs with strongly enhanced (∼4 orders) atomic diffusivity. • This anomaly is associated with the metallic glassy state achievable under laser quenching. • The ultrafast diffusion decomposition is responsible for laser-induced ferromagnetism. - Abstract: We investigate the origin of ferromagnetism induced in thin-film (∼20 nm) Fe–V alloys by their irradiation with subpicosecond laser pulses. We find with Rutherford backscattering that the magnetic modifications follow a thermally stimulated process of diffusion decomposition, with formation of a-few-nm-thick Fe enriched layer inside the film. Surprisingly, similar transformations in the samples were also found after their long-time (∼10"3 s) thermal annealing. However, the laser action provides much higher diffusion coefficients (∼4 orders of magnitude) than those obtained under standard heat treatments. We get a hint that this ultrafast diffusion decomposition occurs in the metallic glassy state achievable in laser-quenched samples. This vitrification is thought to be a prerequisite for the laser-induced onset of ferromagnetism that we observe