Critical exponents and intrinsic broadening of the field-induced transition in NiCl2_2⋅\cdot4SC(NH2_2)2_2

The field-induced ordering transition in the quantum spin system NiCl$_2$$\cdot$4SC(NH$_2$)$_2$ is studied by means of neutron diffraction, AC magnetometry and relaxation calorimetry. The interpretation of the data is strongly influenced by a finite distribution of transition fields in the samples, which was present but disregarded in previous studies. Taking this effect into account, we find that the order-parameter critical exponent is inconsistent with the BEC universality class even at temperatures below 100 mK. All results are discussed in comparison with previous measurements and with recent similar studies of disordered Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$

Giant elasticity in the Ni Mn Ga single crystalline FSMA martensites

The phenomenon of the giant elastic completely reversible linear strain is found at constant temperatures in Ni Mn Ga martensites. Single crystals of some off stoichiometric Ni Mn Ga compositions possess rubber like behavior in the martensite phase with completely elastic strain reaching over 10 at some 90MPa compressive stress at room temperature. The observed phenomenon is different from the conventional two phase superelasticity, since it is found completely in the martensite phase and no intermartensitic reaction is observed during the load unload cycling. The main features of the observed rubber like behavior are 1 complete elasticity and giant values of the strains achieved; 2 the linear character of stress strain dependence; 3 excellent stability to the mechanical cycling, stress rate and temperature variation, in contrast to the conventional two phase superelasticity due to the stress induced martensite transformation. Current work is directed to study the structural mechanisms of the found phenomenon by means of the neutron diffraction in situ under compressive stress cyclin

Giant elasticity in the Ni-Mn-Ga single crystalline FSMA martensites

The phenomenon of the giant elastic completely reversible linear strain is found at constant temperatures in Ni-Mn-Ga martensites. Single crystals of some off-stoichiometric Ni-Mn-Ga compositions possess rubber-like behavior in the martensite phase with completely elastic strain reaching over 10% at some 90MPa compressive stress at room temperature. The observed phenomenon is different from the conventional (two-phase) superelasticity, since it is found completely in the martensite phase and no intermartensitic reaction is observed during the load-unload cycling. The main features of the observed rubber-like behavior are: 1) complete elasticity and giant values of the strains achieved; 2) the linear character of stress-strain dependence; 3) excellent stability to the mechanical cycling, stress rate and temperature variation, in contrast to the conventional two-phase superelasticity due to the stress induced martensite transformation. Current work is directed to study the structural mechanisms of the found phenomenon by means of the neutron diffraction in-situ under compressive stress cycling

Crystal and magnetic structure temperature evolution in Ni Mn Ga magnetic shape memory martensite

Proceedings of the 7th European Symposium on Martensitic Transformations, ESOMAT 2006The current work presents the original results concerning the temperature evolution of the magnetic and crystal structure of Ni−Mn−Ga magnetic shape memory alloys within the entire temperature interval of the martensite phase. The atomic positions and the correct space group of the 5M martensite unit cell has been derived, which is crucial for the electronic structure calculations. Neutron diffraction studies of the Ni−Mn−Ga alloy, possessing a large magnetic shape memory effect (>5% shear strain), revealed that there are several magnetic transitions in the martensite phase due to the smooth strongly anisotropic temperature dependence of the martensite lattice parameters

A New Diffraction Approach To Crystal Structure Determination of Nano-twined Martensites

The intensity diffracted by crystals containing twin boundaries of the system {110}, $\langle 1 \bar{1} 0 \rangle$ were calculated in the reciprocal space assuming a short-range order in the twin-boundary distribution. The calculated intensity distributions have been analyzed to find the effect on the diffraction pattern of such parameters as (i) ratio b/a (c/a) between the crystal lattice constants, (ii) density of the twin boundaries, (iii) ratio between thicknesses of the twin-related lamellae, and (iv) crystallographic direction along which the intensity distribution is simulated. It is found that each of these parameters affects the diffraction-peak profiles and positions including the diffraction peaks of the basic structure. Therefore, the problem of the diffraction pattern identification for a twin-modulated crystal should be approached only by a combined consideration of the diffraction features. Note to the reader: On page 02025-p2 several mistakes have been corrected on October 19, 2009