Laurent Carbone

Tuning macro-twinned domain sizes and the b-variants content of the adaptive 14-modulated martensite in epitaxial Ni–Mn–Ga films by co-sputtering

Abstract In order to obtain modulated martensite in our epitaxial Ni–Mn–Ga films, we tuned the composition by using a co-sputtering process. Here we present how the composition affects the variant distribution of the 14-modulated martensite at room temperature. The nature of such modulated martensites is still strongly debated for magnetic shape memory alloys. It has been very recently demonstrated that the modulated martensites in Ni–Mn–Ga are adaptive phases. The results presented here corroborate this theory for the first time, for three different compositions. Moreover, we demonstrate with the help of the adaptive modulations theory that b -variants of the 14-modulated martensite form close to the free surface of the film to release the stress induced by branching of macro-twinned domains during the martensitic transformation on a rigid substrate. At room temperature, the content of such b -variants is found to strongly decrease when the macro-twinned domain sizes increase.

Fabrication and characterization of a Ni-Mn-Ga uniaxially textured freestanding film deposited by DC magnetron sputtering

Homogeneous freestanding films have been obtained by the direct current (DC) magnetron sputtering technique using a sacrificial layer. After annealing, the films are crystallized with a strong out-of-plane texture along the (022) direction. The stoichiometry of the annealed films is close to the target composition and leads to a martensitic transformation around 255K. The annealed films demonstrate ferromagnetic behavior with a Curie temperature of about 362K. The magnetization process has been studied on the both states and during the martensitic transition. The saturation magnetizations have been determined by fitting the experimental data with a saturation approach law in the range 1-5T. Results show the saturation magnetization of the martensite is around 10% higher than that of the austenite. A model based on intrinsic magnetic properties of each state allowing the description of the magnetization M=f (H, T) of such polycrystalline films during the martensitic transformation is presented. The mass fraction of martensite inside the austenite phase can be determined using this model. The shape memory effect is analyzed both by scanning electron microscopy and by optical microscopy with in-situ measurement of the resistance temperature dependence.

Martensite structures and twinning in substrate-constrained epitaxial Ni–Mn–Ga films deposited by a magnetron co-sputtering process

Abstract In order to obtain Ni–Mn–Ga epitaxial films crystallized in martensite structures showing Magnetic-Induced Rearrangement (MIR) of martensite variants, a fine control of the composition is required. Here we present how the co-sputtering process might be helpful in the development of Ni–Mn–Ga epitaxial films. A batch of epitaxial Ni–Mn–Ga films deposited by co-sputtering of a Ni-Mn-Ga ternary target and a pure manganese target has been studied. The co-sputtering process allows a precise control of the film compositions and enables keeping the epitaxial growth of Ni-Mn-Ga austenite during deposition at high temperature. It gives rise to tune the content of the MIR-active 14-modulated martensite in the film at room temperature, as well as micro and macro-twinned domains sizes.

Thermal effects in YBCO/metal bilayers at high current densities

Abstract We report on the thermal aspects associated with the switching properties at high current densities of YBCO thin films shunted by a metallic layer. The role of the shunt is first detailed in a current ramp mode experiment under various conditions of temperature and magnetic field. We discuss specifically a regime associated with an almost total current diversion by the metallic layer, observed when the critical current density is reduced at temperatures close to the critical temperature T c or under magnetic field. This regime is associated to a thermal anomaly, as seen in the temperature signal measured on the rear side of the substrate, which can explain the partial recovery of the superconductive state under magnetic field. Under current limitation conditions at 50 Hz, 77 K and self-field, the temperature reaches 260 K at the rear side of the sapphire substrate after 0.4 s without any damage to the superconducting device.

Current limitation capability of bulk Bi2223 material

Abstract Bulk sintered and textured Bi2223 materials have been prepared in view of current limiting applications. Pellets of large diameter (100 mm) have been compacted at 1 GPa before sintering and cutting in meander shape. The total length of the sintered conductor is 70 cm with a section of 0.25 cm2. A homogeneous transport critical current (Ic=160 A) has been obtained for such a length. Pulsed measurements above the critical current have shown that the total (70 cm) length of the conductor goes to the normal state. A total voltage of 370 V has been obtained for a pulsed current of 3.3 kA in one sintered element of 100 mm of diameter. The parameters governing the transition to the normal state have been studied on sintered and textured 20 mm long samples with different Jc values. In Bi2223 material, the transition in the normal state is governed by the ρ flux flow J c 2 value. A textured sample with a Jc of 4500 A/cm2 has shown a transition in the normal state for a current lower than five times the critical current. This last result is very encouraging for the use of textured Bi2223 material as current limiting element operating at 77 K.

Switching of YBCO thin films into the dissipative state at high current densities

We report on the switching properties at high current densities of YBCO thin films shunted by an Ag layer. These experiments are performed either in liquid nitrogen or liquid argon (87.3K). Different transition regimes associated to specific electric field thresholds are observed. The nature of the thermal exchange with the bath and the role of the silver shunt and substrate are discussed. While only two regimes in the current-voltage characteristics are observed in liquid nitrogen, successively reversible and hysteretic, an intermediate behaviour appears at 87.3K associated with a total current diversion by the silver layer.

Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial growth

Ni-Mn-Ga films in the austenite and the martensite structures at room temperature have been obtained using the DC magnetron sputtering technique. Two elaboration processes were studied. A first batch of samples was deposited using a resist sacrificial layer in order to release the film from the substrate before vacuum annealing. This process leads to polycrystalline films with a strong (022) fiber texture. The martensitic phase transformation of such polycrystalline freestanding films has been studied by optical and scanning electron microscopy. A second batch of samples was grown epitaxially on (100)MgO substrates using different deposition temperatures. The texture has been analyzed with four-circle X-ray diffraction. Epitaxial films crystallized both in the austenite and the martensite structures at room temperature have been studied.