Oleg Heczko

Effect of intermartensite transformation on twinning stress in Ni-Mn-Ga 10?M martensite

Intermartensite transformations (IMTs) and temperature dependence of very low twinning stress associated with Type II twin boundaries were studied experimentally in four magnetic shape memory alloys with composition Ni50Mn25u2009+u2009xGa25u2009–u2009x (at. %), where x was between 3.2 and 3.9. The slightly different x or Mn content resulted in slightly different martensite transformation temperatures between 309 and 328u2009K, but in significantly different intermartensite transformation temperatures between 10u2009K and 251u2009K, with the transformation temperature being approximately proportional to x. In all four alloys, the initial twinning stress, about 0.1u2009MPa at room temperature, increases sharply when approaching the IMT at lower temperatures, up to 1u2009MPa and more. The effect is explained by the instability of 10M martensite near the IMT from which it follows that the twinning stress dependence shall be flat for x≤2.7. That is experimentally confirmed by an additional measurement on Ni50Mn27.7Ga22.3 alloy (xu2009=u20092.7), which e...

Room Temperature Synthesis of Magnetite (Fe3-δO4) Nanoparticles by a Simple Reverse Co-Precipitation Method

Magnetite (Fe3-δO4) nanoparticles with the size less than 30 nm have been synthesized by using a simple reverse co-precipitation method at room temperature. During the process, ferrous sulfate (FeSO4•7H2O) powder was used as an iron precursor, and ammonium hydroxide (NH4OH) as a precipitating agent. The experiment was carried out in ambient atmosphere without any surfactant added. In this method, the base solution for the precipitation process was adjusted to have a pH value suitable for the formation of the magnetite phase. The iron salt precursor was added into the solution during the synthesis by two different synthesis protocols. The phase, morphology and magnetic characteristic of differently synthesized magnetite particles were characterized by using an X-ray diffraction (XRD), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM). The morphologies of the particles were spherical or irregular in shape depending on the synthesis protocol used. Magnetic measurement shows that the particles are ferromagnetic at room temperature with relatively high saturation magnetization and low hysteresis. The saturation magnetization and magnetic hysteresis of the particles varied with preparation reaction conditions and the resulting oxidation state of the particles.

Magnetic and magnetoelastic properties of Ni-Mn-Ga – Do they need a revision?

The magnetic anisotropy, magnetostriction and behaviour of elastic constants in magnetic field are important for fundamental understanding of martensitic transformation and magnetic shape memory effects. Here we present the measurement of the properties of cubic austenite and tetragonal martensite as a function of temperature and compare it with previously published results. In austenite, while the magnetostriction broadly agrees with published data, measured anisotropy was about ten times smaller. Using single variant tetragonal martensite the magnetic anisotropy constants were revisited. The magnetic anisotropy of modulated (10M and 14M) and tetragonal martensites can be related using tetragonal distortion within the concept of adaptive phase consisting of nanotwinned tetragonal building blocks. The elastic constants measured by pulse echo methods vary only slightly with magnetic field with apparent exception of shear constant c, which is field dependent. However, due to very low value of c, the pulse echo method is not suitable and other method as resonant ultrasound spectroscopy has to be used.

Effect of Magnetic Ordering on the Stability of Ni–Mn–Ga(–Co–Cu) Alloys Along the Tetragonal Deformation Path

The influence of magnetic ordering on the stability of Ni–Mn–Ga(–Co–Cu) Heusler alloys is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. The paramagnetic (PM) state is described by disordered local moment approach. In stoichiometric Ni₂MnGa alloy, the total energy profile along the tetragonal deformation path differs between ferromagnetic (FM) ground state and PM state with high energy, where cubic structure of austenite exhibits lower total energy than tetragonally distorted structure of martensite. Martensitic structure is stabilized in ground state by FM interaction. In PM state it can be stabilized by partial substitution of Ni by Co or by partial substitution of Mn/Ga by Cu. Energy difference between PM and FM state <inline-formula> <tex-math notation=LaTeX>

Low temperature a / b nanotwins in Ni 50 Mn 25+x Ga 25−x Heusler alloys

Delta E_{mathrm {PM{-}FM}}

Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni 2 MnGa Compound

</tex-math></inline-formula> can be used for qualitative estimation of Curie temperature <inline-formula> <tex-math notation=LaTeX>

Magneto-optical Kerr effect of a Ni2.00Mn1.16Ga0.84 single crystal across austenite and intermartensite transitions

T_{C}

Novel iron oxide–silica coreshell powders compacted by using pulsed electric current sintering: Optical and magnetic properties

</tex-math></inline-formula>. Since Co doping to Ni sublattice slightly increases <inline-formula> <tex-math notation=LaTeX>

Magnetic properties of Ni-Mn-Ga-Co-Cu tetragonal martensites exhibiting magnetic shape memory effect

Delta E_{mathrm {PM{-}FM}}

Orthorhombic intermediate phase originating from {110} nanotwinning in Ni50.0Mn28.7Ga21.3 modulated martensite

</tex-math></inline-formula>, the <inline-formula> <tex-math notation=LaTeX>