Modeling of the magnetic shape memory effect in Ni<Subscript>2</Subscript>MnGa films

The magnetic shape memory effect in Ni₂MnGa films is discussed and simulated in terms of the theory of diffuse martensitic transformations. Theoretical temperature and field dependences of the film deformation are obtained. Changes in the film shape under the action of the magnetic field and temperature are calculated.     

Magnetostructural transformations in Ni<Subscript>51</Subscript>Mn<Subscript>36</Subscript>Sn<Subscript>13</Subscript> Heusler alloy thin films

In this paper we report structural, magnetic and transport properties of strongly textured Ni₅₁Mn₃₆Sn₁₃ thin films. The off-stoichiometric Heusler alloy films with 200 nm thickness were sputter-deposited on a MgO(100) substrate at 500 K and after annealed at 1000 K in UHV conditions. The textured growth was confirmed by x-ray diffraction in Bragg-Brentano geometry. The temperature dependence of the magnetic properties was measured by VSM and FMR methods. The electron transport measurements were carried out in function of temperature in 0 Oe and 50 kOe fields. All measurements corroborate the existence of the martensitic transformation in the film. Furthermore, transport measurements reveal an influence of the magnetic field on the transition temperature.     

Influence of magnetism on the structural stability of cubic L2<Subscript>1</Subscript>Ni<Subscript>2</Subscript>MnGa

We present calculations of magnetic field-dependent properties of magnetic shape memory (MSM) Heusler alloys by means of density functional theory calculations. The effects of an external magnetic field on structural properties are simulated by fixing the magnetic moments within the framework of the fixed spin moment (FSM) method. We calculate the binding surface as a function of the magnetic moment and the tetragonal distortion. For magnetizations of 10% below the equilibrium value, the energy of the martensitic L1₀ phase steeply increases leading to a relative stabilization of the L2₁ phase in a confined magnetization range. Calculations of the phonon dispersion in the direction [ξξ0]2π/a suggest that the instability at ξ≈1/3 disappears with decreasing magnetization, allowing a nearly stable spectrum in a small magnetization interval.     

Effect of severe plastic deformation and ultrarapid quenching on the properties of magnetic shape memory alloys near the Ni<Subscript>2</Subscript>MnGa composition

The effect of ultrarapid quenching from melt and subsequent severe plastic deformation by torsion in Bridgman anvils on the temperatures of martensitic (T _M) and magnetic (T _C ) phase transitions in magnetic shape memory alloys has been investigated. The features of the crystal structure and magnetic, electric, and optical properties of Ni₂MnGa-based alloys with different degrees of structural ordering are discussed. The effect of doping with copper and cobalt on the properties of nonstoichiometric Ni₅₀Mn_28.5Ga_21.5 alloy is considered.     

Ordering,martensitic transformation,and shape memory effect in submicron samples of rapidly quenched Ni<Subscript>50</Subscript>Ti<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy

The experiments aimed at developing micro- and nanomechanical devices based on shape memory alloys are described. A novel scheme and a model of microtweezers for manipulating nanoobjects with extremely small sizes (12 × 3 × 1 μm³) are proposed. The control of microtweezers deformation by local heating is demonstrated. The controlled cantilever bending was found to be 1000 nm; this value is in good agreement with the theoretical data.     

First-principles study of elastic and vibrational properties of Ni<Subscript>2</Subscript>MnIn magnetic shape memory alloys

We present the results of ab initio calculations of lattice dynamics and the second order elastic stiffness constants of nickel-based magnetic shape memory alloy Ni₂MnIn in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (σ-GGA) scheme of the density functional theory (DFT) is applied. Elastic constants are calculated by tetragonal and monoclinic isochoric strains on cubic L2₁ structure. The calculated elastic constants agree very well with the recent ultrasonic experimental data. Phonon dispersion spectra are investigated within linear response technique of the density functional perturbation theory (DFPT). A vibrational anomaly is observed in phonon spectra at the transverse acoustic mode (TA₂) in [ζ ζ0] direction at wavevector ζ = 0.3 as an indication of the structural instability of the system to shear deformation. This anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. Phonon dispersion curves are in excellent agreement with the results of recent neutron diffraction experiments.     

Reflection of elastic waves in a crystal of Heusler alloy Ni<Subscript>2</Subscript>MnGa in the phase transition range

The reflection of longitudinal and transverse acoustic waves from the free surface of the ferromagnetic shape memory alloy Ni₂MnGa that is located in the ranges of the premartensite and martensite phase transformations is considered. The propagation directions and amplitudes of the waves reflected in the (001) plane of the crystal are determined. They acquire the character of substantially quasi-longitudinal and quasi-transverse vibrations rather than being pure modes. The angles of wave reflection and conversion are shown to be effectively controlled by temperature and a magnetic field due to the colossal acoustic anisotropy of the crystal over the wide range of its phase transitions. Beginning from a certain critical angle of incidence of a quasi-transverse wave, the quasi-longitudinal wave having appeared upon reflection becomes an accompanying surface vibration, and it can be emitted into the bulk of the crystal when the phase transition point is approached. Two angles of full conversion of an incident quasi-longitudinal wave into a quasi-transverse wave are established, and their temperature dependences are found. Trivisonno’s experimental data for the ultrasound velocity and absorption in an Ni₂MnGa crystal are used to numerically estimate these acoustic effects.     

Electron acoustic imaging of Mn<Subscript>50</Subscript>Ni<Subscript>28</Subscript>Ga<Subscript>22</Subscript> ferromagnetic shape memory alloy

The novel ferromagnetic shape memory alloy Mn₅₀Ni₂₈Ga₂₂ exhibits a single martensite phase with tetragonal structure at room temperature. Its martensite structure was investigated by scanning electron acoustic microscopy. Stripe twin variants exist in every grain and exhibit the configurations of the typical self-accommodation arrangement. The interfaces between twin variants are straight and clear. However, the magnetic domain walls obtained by the Bitter method coincide well with the twin variant boundaries. The first direct evidence of the coincidence between micron-scale magnetic domains and martensite variants is presented. These results will be beneficial for the development of ferromagnetic shape memory alloys and for the understanding of the correlation between magnetic domains and crystallographic twin variant domains as well. PACS  68.37.Hk; 75.70.Kw; 75.60.Ch; 73.50.Rb; 43.58.+z     

Magnetostructural phase separation and giant isotope effect in R<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

Results of neutron diffraction studies of R_0.5Sr_0.5MnO₃ manganites (R = Sm, Nd_0.772Tb_0.228, and Nd_0.544Tb_0.456) performed to reveal the microscopic origins of the giant oxygen isotope effect recently discovered in Sm_0.5Sr_0.5MnO₃ are presented. It is shown that two crystalline phases differing in the type of Jahn-Teller distortions of oxygen octahedra and in the type of magnetic ordering coexist at low temperatures in all the studied compositions. A scenario for the observed phase transitions is suggested based on the diffraction data. It is found that the percolation transition from the metallic to insulating state in compositions with Sm upon substitution of ¹⁸O for ¹⁶O is associated with a sharp (from 65 to 13%) decrease in the volume of the ferromagnetic metallic phase.     

Crystal structure of martensite and intermediate phases in Ni<Subscript>2</Subscript>MnGa studied by neutron diffraction

We have investigated crystal structures of martensite and intermediate phases in stoichiometric Ni₂MnGa. The neutron diffraction profile of the martensite phase measured at T = 4.2 K exhibits four satellites between [2 0 0]_P^* and [0 2 0]_P^* reflections (P stands for the parent phase) at incommensurate positions of [h 2-h 0]_P^* with h = 0.428, 0.863, 1.136 and 1.572. The profile of the intermediate phase measured at T = 210K exhibits two satellites between [2 0 0]_P^* and [0 2 0]_P^* reflections at incommensurate positions of [h 2-h 0]_P^* with h = 0.343 and 1.657. Although each satellite of the martensite phase moves toward its nearest fundamental reflection as temperature increases, that of the intermediate phase does not move significantly. On the contrary, the intensity of each satellite decreases significantly in the intermediate phase as temperature increases while not in the martensite phase. A synchrotron X-ray diffraction and a Rietveld analysis of the result reveal that, for both the phases, the displacement of atoms from the parent phase are represented by a sine wave whose propagation vector is parallel to [1 1 0]_P.     

Recent topics in microstructure control by magnetic field in Ni<Subscript>2</Subscript>MnGa and CoPt and new transformation behavior in Ti-Ni-Fe alloys

Three topics related to solid-solid phase transformations are presented. The first topic is related to ferromagnetic shape memory alloys. The general condition for rearrangement of martensite variants by magnetic field is discussed quantitatively. The second topic is related to microstructure control of CoPt (tetragonal L1₀-type structure) during ordering heat-treatment under a magnetic field. We show that a single variant state is realized by magnetic field, and magnetic field is especially effective at the early stage of ordering. The third topic is related to the so-called precursor phenomena in Ti-Ni-Fe shape memory alloys. In the topic we will show the existence of a commensurate phase, which inherits the microstructure of the incommensurate phase and is probably different from the R-phase.     

Effect of plastic deformation on physical properties and structure of the shape memory alloy Ti<Subscript>49.5</Subscript>Ni<Subscript>50.5</Subscript>

The effect of severe plastic deformation by torsion (SPDT) in Bridgman anvils at a high pressure (6 GPa) on the physical properties and crystal structure of the shape memory alloy Ti_49.5Ni_50.5 has been studied. The behavior of the thermal expansion, electrical resistivity, absolute differential thermopower, Hall coefficient, magnetic properties, and optical characteristics of the amorphous/nanocrystalline and submicrocrystalline alloys obtained by the SPDT with subsequent heat treatment at 800 K has been discussed.     

Mechanical alloying of Ni<Subscript>80</Subscript>Ta<Subscript>20</Subscript> and Ni<Subscript>80</Subscript>Nb<Subscript>20</Subscript>. Crystallization of the amorphous phase

The sequence and crystallization kinetics of the amorphous phase have been analyzed by differential scanning calorimetry and X-ray diffraction for mechanochemically activated Ni₈₀Ta₂₀ and Ni₈₀Nb₂₀ samples. The formation of equilibrium products from the amorphous phase occurs through the formation of metastable products of the A3 type. The kinetic parameters (activation energy and reaction order) and thermodynamic characteristics of this process are determined. Crystallization of the amorphous phase for the Ni₈₀Ta₂₀ and Ni₈₀Nb₂₀ systems occurs due to the growth of existing nuclei through the polymorphic and eutectic mechanisms, respectively. The thermal effects of the synthesis of equilibrium products from a mechanochemically activated mixture of components are identified.     

Influence of deformation on the structural transformation of the Pd<Subscript>40</Subscript>Ni<Subscript>40</Subscript>P<Subscript>20</Subscript> amorphous phase

The influence of plastic deformation on the structure of the Pd₄₀Ni₄₀P₂₀ amorphous alloy has been investigated using X-ray diffraction and measurements of the velocity of sound. It has been revealed that the rolling of the sample leads to a change in the structure of the amorphous phase (distortion of the first coordination sphere) and that the structural transformations are more pronounced in the near-surface region of the sample. The rolling also results in a decrease in the transverse velocity of sound. The observed effects decrease with time. It has been demonstrated that the revealed effects are associated with the inelastic deformation of the amorphous alloy.     

Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>)<Subscript>2</Subscript>Zn<Subscript>0.8</Subscript>Ni<Subscript>0.2</Subscript>Cl<Subscript>4</Subscript> solid solutions

We have studied the signs of phase transitions and spatial modulation of the structure in the absorption spectra of an (NCH₃)₄)₂Zn_0.8Ni_0.2Cl₄ crystal. We have observed the existence of phase transitions in the given solid solution at temperatures of 155 K, 168 K, 275 K, 280 K, and 296 K. We have established that the thermooptic memory effect observed in the absorption spectra is completely consistent with a model of defect ordering in the sample in the field of the modulated structure. According to this model, stabilization of the sample in an incommensurable phase leads to fixing of a certain symmetry in the crystal (usually a lower symmetry than the average symmetry of the incommensurable phase) and a metal-halogen complex corresponding to the defect wave. As a result, we observe an appreciable shift of the intra-ionic absorption bands and an increase in their intensity. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 717–723, September–October, 2008.     

Effect of alloying additions and atomic disordering on the physical properties of magnetic Ni<Subscript>2</Subscript>MnGa-based shape memory alloys

The effect of atomic disordering induced by melt quenching or severe plastic deformation via high-pressure torsion on the physical properties (thermal expansion coefficient, electrical resistivity, thermoelectric power, magnetization) of a stoichiometric Ni₅₀Mn₂₅Ga₂₅ alloy and nonstoichiometric Ni₅₀Mn_28.5Ga_21.5 alloys with 2 at % Cu or Co is studied in the temperature range 2 K ≤ T ≤ 900 K and the magnetic field range H ≤ 7 MA/m.     

Experimental Study and Analysis of Absorption Spectra of Ni<Superscript>2+</Superscript> Ions in Nickel Orthoborate Ni<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

The results of studies of the absorption spectra of nickel orthoborate Ni₃(BO₃)₂ in the range of electronic d–d-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni²⁺ ions are located in two crystallographically nonequivalent positions 2a and 4f with point symmetry groups C_2h and C₂, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion ³A_2g (³F) to the sublevels of the ³T_2g (³F), ³T_1g (³F) and ³T_1g (³P) triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2a- and 4f-positions, as well as the parameters of electrostatic interaction between the 3d electrons and spin-orbit interaction constants.     

The evolution of electronic configuration and magnetic characterization of Fe<Subscript>9</Subscript>Ni<Subscript>1</Subscript>, Fe<Subscript>8</Subscript>Ni<Subscript>2</Subscript> alloy in theoretical calculation

To analyze the origin of the magnetic enhancement of Fe-Ni alloy, the electronicconfigurations and magnetic properties were investigated using density functional theorybased on the first-principle. The supercell (5 × 1 × 1) of Fe,Fe₉Ni₁ and Fe₈Ni₂ were constructed. Thedefect formation energy, band structure, density of states and electron density differencewere calculated. The results showed that Ni doping changed the electronic configuration ofFe atoms, resulting in the enhancement of spin polarization of Fe and the larger Bohrmagnetic moment in Fe-Ni alloys (Fe₉Ni₁). The results showed thatthe charge transfer and the atomic spacing between Fe atoms and the dopant Ni atoms playedan important role in determination of magnetic moment. The value of Fe supercell(5 × 1 × 1), Fe₉Ni₁ and Fe₈Ni₂ were 23.14,23.34 and 22.61μ _B, respectively.     

Optical and electronic properties of heusler Cu<Subscript>2</Subscript>MnAl alloy with shape memory effect

A ferromagnetic Heusler Cu₂MnAl alloy with a shape memory effect has been studied in the wide spectral region of 0.25–2.80 μm (0.44–5.35 eV) at room temperature using spectral ellipsometry methods. Dispersion dependences for optical constants n(hν) and κ(hν), dielectric permeability ε(hν), and optical conductivity σ(hν), which is proportional to the interband density of electronic states, have been analyzed. Optical conductivity spectra have been calculated by mathematical modeling based on data for the electronic state densities in both subbands of electrons with spins oriented along and against the magnetization direction. A model of indirect transitions based on the Berglund–Spicer formula has been shown to describe the experimental spectrum only in general terms. The main contribution to the conductivity in the high-frequency spectral region is due to direct transitions of electrons in the vicinities of L, X, and Γ points of the Brillouin zone coupled with Cu atoms whereas low-energy anomalies in the absorption spectrum σ(hν) are caused by electrons coupled with Mn and Al atoms.