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Register a new userNeutron diffraction evidence for kinetic arrest of first-order magneto-structural phase transitions in some functional magnetic materials
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Vasudeva Siruguri Dr
Publication date
2013
fields
Physics
and research's language is
English
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Neutron diffraction measurements, performed in presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first-order phase transition from (i) the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5, (ii) the higher temperature ferromagnetic phase to the lower temperature antiferromagnetic phase in the half-doped charge ordered compound La0.5Ca0.5MnO3 and (iii) the formation of a glass-like arrested state (GLAS). The CHUF (cooling and heating under unequal fields) protocol has been used to establish phase coexistence of metastable and equilibrium states of GLAS and also to demonstrate the devitrification of the arrested metastable states in the neutron diffraction patterns. We also explore the field-temperature (H,T) phase diagram for the two compounds, which depicts the kinetic arrest line TK(H). TK is seen to increase as H increases.
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Neutron diffraction measurements, performed in presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first-order phase transition from the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5 and the formation of a glass-like arrested state (GLAS). The CHUF (cooling and heating under unequal fields) protocol has been used to establish phase coexistence of metastable and equilibrium states of GLAS in the neutron diffraction patterns. We also explore the field-temperature (H,T) phase diagram for this composition which depicts the kinetic arrest line TK(H). TK is seen to increase as H increases.
Substitutions at the Mn-site of the charge-ordered Pr0.5Ca0.5MnO3 manganite is an effective way to induce abrupt jumps on the magnetic field driven magnetization curve. In order to get new insights into the origin of this remarkable feature, the Pr0.5Ca0.5Mn0.97Ga0.03O3 perovskite manganite has been studied by neutron diffraction, versus temperature and at 2.5K in an applied magnetic field up to 6 Tesla. A weak and complex antiferromagnetic order is found for the low temperature ground-state. Magnetic transitions, associated with structural ones, are evidenced for certain strengths of magnetic field, which gives rise to the step-like behavior corresponding to the magnetization curve. Small angle neutron scattering provides evidence for a nucleation process of micron size ferromagnetic domains which follows the magnetization behavior.
We combine spin polarised density functional theory and thermodynamic mean field theory to describe the phase transitions of antiperovskite manganese nitrides. We find that the inclusion of the localized spin contribution to the entropy, evaluated through mean field theory, lowers the transition temperatures. Furthermore, we show that the electronic entropy leads to first order phase transitions in agreement with experiments whereas the localized spin contribution adds second order character to the transition. We compare our predictions to available experimental data to assess the validity of the assumptions underpinning our multilevel modelling.
We investigate the structural and magnetic phase transitions in EuTi1-xNbxO3 with synchrotron powder X-ray diffraction (XRD), resonant ultrasound spectroscopy (RUS), and magnetization measurements. Upon Nb-doping, the Pm-3m to I4/mcm structural transition shifts to higher temperatures and the room temperature lattice parameter increases while the magnitude of the octahedral tilting decreases. In addition, Nb substitution for Ti destabilizes the antiferromagnetic ground state of the parent compound and long range ferromagnetic order is observed in the samples containing more than 10% Nb. The structural transition in pure and doped compounds is marked by a step-like softening of the elastic moduli in a narrow temperature interval near TS, which resembles that of SrTiO3 and can be adequately modeled using the Landau free energy model employing the same coupling between strain and octahedral tilting order parameter as previously used to model SrTiO3.
Gen Shirane began studying ferroelectrics while he was still based in Japan in the early 1950s. It was therefore natural that when he arrived at Brookhaven and began specialising in neutron scattering that he would devote much of his energy and expertise studying structural phase transitions. We review the ground breaking experiments that showed the behaviour of antiferroelectrics and ferroelectrics were reasonably described in terms of the soft mode concept of structural phase transitions. This work lead directly to Gen being awarded the Buckley prize and, jointly with John Axe, awarded the Warren prize. We then describe his work on incommensurate phase transitions and in particular how the giant Kohn anomalies are responsible for the structural instabilities in one-dimensional metals. Finally Gen carefully investigated the central peak and the two-length scale phenomena that occur at most if not all transitions. Due to Gens elegant experimental work we know a great deal about both of these effects but in neither case is theory able to explain all of his results
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