Ladislav Straka

Magnetic anisotropy in Ni–Mn–Ga martensites

We study the temperature dependence of the magnetic anisotropy of three different martensites known to exist in the Ni–Mn–Ga alloys. The anisotropy constants were determined from magnetization curves measured at different temperatures. The anisotropy of five-layered modulated tetragonal martensite is uniaxial with easy magnetization direction along short crystallographic axis. At room temperature K1(rt)=1.65×105 J m−3 and K2 is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. K1(rt)=1.7×105 J m−3 and K2(rt)=0.9×105 J m−3 referring to hard and mid-hard magnetization axes. Nonmodulated tetragonal martensite possesses a uniaxial anisotropy with easy plane and hard magnetization direction along the long crystallographic axis with K1(rt)=−2.3×105 J m−3 and K2(rt)=0.55×105 J m−3. The temperature dependence of K1(T) of five-layered martensite follows magnetization power law with exponent n=3 suggesting a single ion origin ...

Temperature dependence and temperature limits of magnetic shape memory effect

We study the temperature dependence and low and high temperature limits of the magnetic shape memory effect (MSME) in five-layered tetragonal Ni–Mn–Ga martensite. Using a simple model we show that, additionally to the limits posed by transformation to austenite or an intermartensitic transformation, the temperature dependence of the magnetic anisotropy, tetragonality of the lattice, and twinning stress play important role when considering the temperature limits of the MSME. With decreasing temperature, the lattice distortion and magnetic anisotropy increase, but saturate in a low temperature region. The twinning stress does not saturate and its temperature dependence has exponential-like character that increases rapidly in the low temperature region. The model predicts that the low-temperature limit of the MSME is 165 K for Ni49.7Mn29.1Ga21.2 composition. This agrees very well with the value of 173 K determined from direct measurements. The high temperature limit is transformation to austenite at 315 K. T...

Superelastic response of Ni-Mn-Ga martensite in magnetic fields and a simple model

In this paper focused to a possibility of using magnetic shape memory Ni-Mn-Ga alloys cut along {100} faces as a magnetically controlled superelastic element and proposed a simple model of the phenomena. Stress-strain curves and magnetization curves are obtained.

Temperature dependence of magnetic anisotropy in Ni–Mn–Ga alloys exhibiting giant field-induced strain

Temperature dependence of structure and magnetic anisotropy of single crystalline Ni48.8Mn28.6Ga22.6 alloy exhibiting giant field-induced strain or magnetic shape memory (MSM) effect was studied in the temperature range 80–420 K. Upon cooling the alloy transforms from cubic austenite at 307 K to the martensite which exhibits five-layered (modulated) tetragonal structure (5M) with a=0.595 nm and c=0.559 nm. Reverse transformation occurs at 317 K. An additional intermartensitic transition takes place at about 95 K. The basic mechanism of the MSM effect was corroborated by direct simultaneous measurements of strain and magnetization as a function of magnetic field. The magnetic anisotropy of the martensite exhibiting the giant strain was determined from the magnetization curves measured by a vibrating sample magnetometer at different temperatures. The anisotropy of the single variant 5M martensite is uniaxial with easy axis along the tetragonal c axis. The first magnetic anisotropy constant is Ku1=2.0×105 J/...

Temperature dependence of single twin boundary motion in Ni–Mn–Ga martensite

Magnetic-field-induced reorientation in Ni–Mn–Ga five-layered martensite (10 M) mediated by the motion of single twin boundary was evaluated from magnetization measurements between 20 and 300 K. At 300 K, the single twin boundary moved in an exceptionally small field of 25 kA/m. Twinning stress, as a measure of the twin boundary mobility, was determined from the magnetization curves using a magnetic-energy-based model; it increased from ≈0.1 MPa at 300 K to ≈0.8 MPa at 20 K. The dependence is discussed in terms of thermal activation and the effect of intermartensitic transformation is considered.

Magnetic properties of various martensitic phases in Ni-Mn-Ga alloy

Single crystalline Ni/sub 48.7/Mn/sub 29.1/Ga/sub 22.2/ sample was studied in the temperature range 110 K-425 K. Magnetic properties of four different structure phases were measured at different temperatures. The magnetic anisotropies of the different phases were determined from magnetization curves. At room temperature, the structure is five-layered modulated tetragonal martensite with c/a 1. A single variant of low-temperature martensite was obtained by cooling in magnetic field 1.4 T. At 173 K, the sum of magnetic anisotropy constants is K/sub 1/+K/sub 2/=1.5/spl times/10/sup 5/ J.m/sup -3/ assuming an easy-plane magnetic anisotropy.

Twin interaction and large magnetoelasticity in Ni-Mn-Ga single crystals

We demonstrate experimentally the existence of triple twins in Ni-Mn-Ga magnetic shape memory single crystals with a modulated five-layered martensite structure using optical observations andx-ray diffraction. Subsequently, we investigate the response of the crystals with triple-twin segments to compressive loading up to several MPa. Such loading typically resulted in an abrupt rearrangement of the twin microstructure to a configuration with many fine twins (1–10 µm in size) ending at a twin boundary. This type of twin microstructure exhibited recoverable deformation with up to 0.3% macroscopic strain and an estimated 2.5% local strain, while the recoverable strain was much smaller for other studied microstructure configurations. The results indicate that by the creation of a suitable twin microstructure, the originally pseudoplastic or magnetoplastic material can be made rubberlike elastic or magnetoelastic with the macroscopic recoverable strain comparable to 2.5%.

Photocatalytic Activity of Atomic Layer Deposited TiO2 Coatings on Austenitic Stainless Steels and Copper Alloys

Photocatalytic activity of TiO 2 -coated austenitic stainless steel (AISI 304) and copper alloys [deoxidized high phosphorus (DHP) copper and Nordic Gold] was studied by means of decomposition of methylene blue model waste water and open-circuit electrochemical potential measurements, and the photoinduced hydrophilicity was studied by means of contact angle measurements of water under ultraviolet irradiation. The TiO 2 coatings were prepared by an atomic layer deposition technique from TiCl 4 and H 2 O. The thicknesses of the prepared coatings were 5, 10, 50, 100, 150, and 200 nm. Morphology and crystal structure of the TiO 2 coatings were studied using scanning electron microscope and X-ray diffraction techniques. Photocatalytic activity of the studied coatings was low with a coating thickness of 5 and 10 nm. When the coating thickness was 50 nm or higher for AISI 304 stainless steel, and 100 nm or higher for DHP and Nordic Gold copper alloys, the photoactivity was good, but no saturation or systematic effect of coating thickness or surface finish was observed. The photoinduced hydrophilicity was good with all studied coating thicknesses (50, 100, 150, and 200 nm), with some exceptions.

Magnetic shape memory effect at 1.7 K

Magnetic shape memory effect or magnetically induced structure reorientation (MIR) occurred down to 1.7 K in 10 M martensite with composition of Ni50.0Mn27.5Ga22.5 exhibiting no intermartensite transformation. The reorientation of the martensite microstructure was mediated by the motion of single Type II twin boundary. In contrast with weak thermal dependence of Type II boundary, MIR with Type I boundary in the same alloy showed strong thermal dependence resembling normal thermal activation process and the effect disappeared below 220 K. Thus the type of the boundary is decisive for MIR at low temperatures.

Magnetic anisotropy of nonmodulated Ni–Mn–Ga martensite revisited

Magnetic anisotropy of single crystal Ni50.5Mn30.4Ga19.1 having nonmodulated (NM) martensite structure (c=0.660 nm and b=a=0.547 nm at room temperature) was determined in the range of 10–300 K. The single variant microstructure needed for proper anisotropy determination was prepared by 40 MPa tensile stress in comparison with previous measurements where detwinned microstructure was obtained by compression. The tensile stress process yield the single variant microstructure with easy plane magnetic anisotropy and negligible second anisotropy constant in contrast of two anisotropy constants of the same order for compressed sample. The absolute value of anisotropy constant increases from 2.6×105 J/m3 at 300 K to 5×105 J/m3 at 10 K.