A.A. Likhachev

Effect of crystal structure on magnetic-field-induced strain in Ni-Mn-Ga

Magnetic shape memory materials are expected to have a high potential in practical applications. Several ferromagnetic materials exhibiting the large magnetic-field-induced strain have been found in recent years. The largest field-induced strain is observed in Ni-Mn-Ga system. The most important experimental results on crystal structure, magnetic anisotropy and twinning stress of martensitic phases in Ni-Mn-Ga having tetragonal five-layered, orthorhombic seven-layered and tetragonal non-layered crystal structures are reported. Depending on the martensite crystal structure Ni-Mn-Ga alloys are able to show a really giant strain response (approximately 6% in tetragonal five-layered or 10% in orthorhombic seven-layered martensitic phase) in a magnetic field less than 1 T. Contrary to these two phases, a detectable field-induced strain is not observed in non-layered tetragonal martensitic phase in Ni-Mn-Ga system. Effect of crystal structure is in a good agreement with calculation of the magnetic-field-induced strain based on the model developed by authors. The effect of composition on appearance of undesirable non-layered tetragonal martensitic phase in Ni-Mn-Ga system is discussed based on the new experimental results.

Influence of external stress on the reversibility of magnetic-field-controlled shape memory effect in Ni-Mn-Ga

Present publication gives a detailed report about the experimental results obtained concerning the effect of external constant stress on the magnetic field controlled strain response during the cyclic change of the magnetic field. Simultaneously we represent a brief overview of the most important structural and magneto-mechanical properties of Ni48Mn30Ga22 - family magnetic shape memory alloys. We also discuss the physical mechanism of this effect using our last model developments.

Optimizing work output in Ni-Mn-Ga and other ferromagnetic shape memory alloys

Present article briefly summarizes the mechanism of magnetic shape memory, main modeling principles and most important information about the main structural, magnetic and mechanical properties related to a family of non- stoichiometric Ni-Mn-Ga alloys. We also first consider in details the problems of energy balance, energy losses, optimization of work output and estimation of thermodynamic efficiency for Ni-Mn-Ga based MSMAs.

The model development and experimental investigation of giant magneto-mechanical effects in Ni–Mn–Ga

This paper represents some new experimental results and the quantitative model describing large magneto-strain effect and main mechanical and magnetic properties observed in NiMnGa and other ferromagnetic shape-memory alloys. The model application to giant magneto-strain effect recently found in some non-stoichiometric NiMnGa alloys is discussed.

Crystal structure, magnetic anisotropy, and mechanical properties of seven-layered martensite in Ni-Mn-Ga

Magnetic-field-induced strain of about 10 percent is reported in Ni48.8Mn29.7Ga21.5 alloy at ambient temperature in a magnetic field order of 1 T. It was confirmed by different experimental methods that the strain is contributed by twin boundary motion. The crystal structure of thermally-induced martensitic phase in this alloy was found to be close to orthorhombic one in temperature range from 245 K to 333 K with lattice parameters a equals 0.619 nm, b equals 0.580 nm, c equals 0.553 nm (relating to the cubic parent phase coordinates) at ambient temperature. More detailed x-ray studies revealed seven-layer shuffling-type modulation along and directions. High magnetic anisotropy properties were found for this phase. The magnetic measurements revealed that the shortest axis (c-axis) is the axis of easiest magnetization, the longest (a-axis) is the axis of hard magnetization, and b-axis is the intermediate one. The orthorhombic phase has low twinning stresses. The compressive stress applied along a-axis of single-variant sample at most 2 MPa is enough to produce approximately 10 percent strain realized by twin boundary motion. The necessary conditions for observation a giant magnetic-field-induced strain in non-stoichiometric Ni2MnGa alloys based on the new experimental data are discussed.

Magnetic forces controlling magnetic shape memory in Ni-Mn-Ga and their practical measurement from the mechanical testing experiments in constant magnetic fields

Performing mechanical testing experiments in all the different martensitic phases of Ni-Mn-Ga under the constant magnetic field applied perpendicular to the load direction we show that such a magnetic field can dramatically modify standard zero field strain stress relationships of MSMAs like Ni-Mn-Ga. In a particular case of 5M and 7M martensites we observe a so-called pseudo-elastic or rubber-like behavior during the standard compression-decompression cycling under the field at about 1T. This effect is finally discussed from the point of the general thermodynamic background and some particular modeling concepts.

Effect of Magnetostatic Energy on the Magnetic Driving Forces and Field Induced Superelastic Behavior in Ni-Mn-Ga

Abstract. Present publication gives a general theoretical concept and also presents the relevant experimental results concerning the effect of the magnetostatic coupling between the twin layers on the magnetic-field-controlled superelastic behavior during the mechanical cycling in magnetic field in Ni-Mn-Ga.