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Register a new userHigh-resolution structure studies and magnetoelectric coupling of relaxor multiferroic Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$
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Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$ (PFN), one of the few relaxor multiferroic systems, has a $G$-type antiferromagnetic transition at $T_N$ = 143 K and a ferroelectric transition at $T_C$ = 385 K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above $T_C$, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below $T_N$. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between Fe$^{3+}$ and Nb$^{5+}$ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.
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We report the structural, static, and dynamic properties of Cr$_{0.5}$Fe$_{0.5}$Ga by means of powder x-ray diffraction, magnetization, heat capacity, magnetic relaxation, and magnetic memory effect measurements. DC magnetization and AC susceptibility studies reveal a spin-glass transition at around $T_{rm f} simeq 22$~K. An intermediate value of the relative shift in freezing temperature $delta T_{rm f} simeq 0.017$, obtained from the AC susceptibility data reflects the formation of cluster spin-glass states. The frequency dependence of $T_{rm f}$ is also analyzed within the framework of dynamic scaling laws. The analysis using power law yields a time constant for a single spin flip $tau* simeq 1.1times10^{-10}$~s and critical exponent $z u^{prime}=4.2pm0.2$. On the other hand, the Vogel-Fulcher (VF) law yields the time constant for a single spin flip $tau_0 simeq 6.6times10^{-9}$~s, VF temperature $T_{rm 0}=21.1pm0.1$~K, and an activation energy $E_{rm a}/k_{rm B} simeq 16$~K. The value of $tau*$ and $tau_0$ along with a non-zero value of $T_{rm 0}$ provide further evidence for the cluster spin-glass behaviour. The magnetic field dependent $T_{rm f}$ follows the de Almeida-Thouless line with a non-mean-field type instability, reflecting either a different universality class or strong anisotropy in the spin system. A detailed non-equilibrium dynamics study via relaxation and memory effect experiments demonstrates striking memory effects. All the above observations render a cluster spin-glass behaviour which is triggered by magnetic frustration due to competing antiferromagnetic and ferromagnetic interactions and magnetic site disorder. Moreover, the asymmetric response of magnetic relaxation with respect to the change in temperature, below $T_{rm f}$ can be explained by the hierarchical model.
La$_{1.5}$Sr$_{0.5}$CoMn$_{0.5}$Fe$_{0.5}$O$_{6}$ (LSCMFO) compound was prepared by solid state reaction and its structural, electronic and magnetic properties were investigated. The material forms in rhombohedral $Rbar{3}c$ structure, and the presence of distinct magnetic interactions leads to the formation of a Griffiths phase above its FM transition temperature (150 K), possibly related to the nucleation of small short-ranged ferromagnetic clusters. At low temperatures, a spin glass-like phase emerges and the system exhibits both the conventional and the spontaneous exchange bias (EB) effects. These results resemble those reported for La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ but are discrepant to those found when Fe partially substitutes Co in La$_{1.5}$Sr$_{0.5}$(Co$_{1-x}$Fe$_{x}$)MnO$_{6}$, for which the EB effect is observed in a much broader temperature range. The unidirectional anisotropy observed for LSCMFO is discussed and compared with those of resembling double-perovskite compounds, being plausibly explained in terms of its structural and electronic properties.
In view of the recent experimental predictions of a weak structural transition in CoV$_{2}$O$_{4}$ we explore the possible orbital order states in its low temperature tetragonal phases from first principles density functional theory calculations. We observe that the tetragonal phase with I4$_1/amd$ symmetry is associated with an orbital order involving complex orbitals with a reasonably large orbital moment at Vanadium sites while in the phase with I4$_1/a$ symmetry, the real orbitals with quenched orbital moment constitute the orbital order. Further, to study the competition between orbital order and electron itinerancy we considered Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$ as one of the parent compounds, CoV$_{2}$O$_{4}$, lies near itinerant limit while the other, MnV$_{2}$O$_{4}$, lies deep inside the orbitally ordered insulating regime. Orbital order and electron transport have been investigated using first principles density functional theory and Boltzmann transport theory in CoV$_{2}$O$_{4}$, MnV$_{2}$O$_{4}$ and Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$. Our results show that as we go from MnV$_{2}$O$_{4}$ to CoV$_{2}$O$_{4}$ there is enhancement in the electrons itinerancy while the nature of orbital order remains unchanged.
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