O. Heczko

Adaptive modulations of martensites.

Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape-memory alloys. To understand the origin of these phases, we employ and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase, and tetragonal martensite in Ni-Mn-Ga magnetic shape-memory alloy epitaxial films. We show that the modulated martensite can be constructed from nanotwinned variants of the tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes metastability, the sequence of 6M-10M-14M-NM intermartensitic transitions, and the magnetocrystalline anisotropy.

Magnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO(001)

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45 relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing microstructural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.

Epitaxial Ni–Mn–Ga films deposited on SrTiO3 and evidence of magnetically induced reorientation of martensitic variants at room temperature

Epitaxial Ni–Mn–Ga films were grown on SrTiO3 by sputter deposition. The films deposited at 673K are ferromagnetic and martensitic at room temperature. Pole figure measurements indicate that the twinned orthorhombic martensite microstructure of the film has a lower symmetry compared to bulk. Magnetically induced reorientation or magnetic shape memory effect is indicated by magnetization curve measurements. Though the overall extension of the film is constrained by a rigid substrate, the reorientation is possible due to the additional degree of freedom in the orthorhombic phase.

Metamagnetic transitions and magnetocaloric effect in epitaxial Ni–Co–Mn–In films

Due to their large strains and multifunctionality, magnetic shape memory alloys are of particular interest for microsystems. Here epitaxially grown metamagnetic Ni–Co–Mn–In films on MgO (001) are analyzed which exhibit a magnetically induced austenite transition. This opens the way to use Ni–Co–Mn–In films in microactuators combining high stroke with high forces. Additionally these films exhibit an inverse magnetocaloric effect with an entropy change of 8.8 J kg−1 K−1 in 9 T at 353 K. The high surface-to-volume fraction of films promises a fast heat exchange, which is beneficial for efficient magnetic cooling.

Stress induced martensite in epitaxial Ni–Mn–Ga films deposited on MgO(001)

Biaxial tensile stress in epitaxial Ni52Mn23Ga25 films on MgO(001) was measured at different temperatures using an adapted x-ray stress analysis. A stress of up to 105MPa originates from different thermal expansions of substrate and film and partially from the substrate-film misfit. The film transforms to twinned orthorhombic martensite at 319K. The stress increases the martensitic transformation temperature of about 63K. This is in good agreement with respect to the transformation temperature expected from the composition of the film. The presence of biaxial tensile stress leads to twin boundary selection, hence, there are no twin planes perpendicular to the substrate.

Magnetically induced martensite transition in freestanding epitaxial Ni–Mn–Ga films

The martensitic transformation in freestanding Ni–Mn–Ga films obtained by epitaxial growth on NaCl (001) is analyzed. A temperature-magnetic field phase diagram reveals that the martensitic phase, exhibiting a higher magnetization compared to austenite, is favored by an external field. A shift of martensite temperature of dT/dH=0.36 K/T is observed, in good agreement with the value expected from a Clausius–Clapeyron equation. The practicality and energy input for actuation using magnetically induced martensitic transition is compared with a magnetically induced reorientation of martensitic variants.

On the electronic origin of the inverse magnetocaloric effect in Ni?Co?Mn?In Heusler alloys

In order to understand the electronic origin of the inverse magnetocaloric effect observed in a Ni–Co–Mn–In system we used a combination of indirect experimental probes as magnetization, resistivity and specific heat. The findings are compared with the band structure of isostructural Heusler alloys such as Ni–Mn–Ga. We suggest that the inverse magnetocaloric effect in Ni–Co–Mn–In originates from the high density of states close to Fermi energy. Within the austenite state this causes ferromagnetic band splitting. The structural change to the martensite allows an alternative way to reduce the high density of states at lower temperatures, which does not require band splitting and thus does not support ferromagnetic order.

Magnetic properties of epitaxial Fe–Pd films measured at elevated temperatures

Magnetic properties of disordered Fe–Pd films with 28 and 33at.% Pd epitaxially grown at room temperature were measured at room and elevated temperatures up to 900 and 600K, respectively. Thermomagnetic measurements of the film with 28at.% Pd having a body-centered-tetragonal structure in the as-deposited state show that, due to their metastability, disordered Fe–Pd phases can decompose at elevated temperatures. For a film with 33at.% Pd having a face-centered-tetragonal (fct) structure in the as-deposited state, no indication of decomposition or first order transformation is observed up to 600K, suggesting that the fct phase required for the magnetic shape memory effect is quite stable in epitaxial films.