Magnetization process and specific heat properties of geometrically frustrated pyrochlores R2FeSbO7 (R3+ = Dy, Y) and spin-ice magnetic phase in Dy2FeSbO7

摘要

We report the spin-ice magnetic phase in geometrically frustrated Dy2FeSbO7 from the systematic studies of dc and ac magnetization, and specific heat properties of two polycrystalline pyrochlore compounds, Dy2FeSbO7 and Y2FeSbO7. A non-magnetic Y2GaSbO7 powder is also synthesized and X-ray crystallographic structure has also been reported. DC magnetic susceptibility chi(dc)(T) shows that Dy2FeSbO7 remains in Curie-type paramagnetic phase down to T-fm approximate to 11 K, below which a small FC-ZFC irreversibility and magnetic hysteresis loop provide evidence of weak ferromagnetic ordering of Fe3+-ions. The positive value of Curie-Weiss temperature is indicative of net ferromagnetic interactions among magnetic ions at low temperatures. The magnetization of Dy2FeSbO7 at 3 K saturates to the value of 5.67 mu(B)/DyFe0.5, half of the effective moment and can be expressed by a local Ising anisotropy along the <111> D-3d trigonal axes of the tetrahedral unit for polycrystalline samples. The isothermal magnetization, M(H), have been demonstrated by the Arrott plot analysis, and zero-field inverse susceptibilities are compared to the measured chi(dc)(T). AC magnetic susceptibility chi(ac)(T) of Dy2FeSbO7 shows two distinct anomalies in its imaginary (chi '') data. One of them occurs at higher temperature, T-f similar to 13 K which corresponds to the ferromagnetic ordering of Fe3+-ions; another at lower temperature, T-s similar to 3 K, signalling spinice-like freezing of Dy3+-spins in Dy2FeSbO7. The magnetic susceptibility, chi(Dy)(T), related to the Dysublattice and the measured magnetization, M(T), of Dy2FeSbO7 are reproduced satisfactorily within the framework of crystal-field theory and Weiss mean-field approximation, employing the dipolar and anisotropic exchange interactions among the Dy3+-ions. Specific heat data of Dy2FeSbO7 and Y2FeSbO7 versus T are collected at different magnetic fields. The zero-field specific heat of Dy2FeSbO7 shows a broad peak at T-peak approximate to 3.1 K similar to T-s, which gradually broadens and shifts to the higher temperature with increasing the applied field. To extract the magnetic specific heat data of these two compounds, specific heat of non-magnetic insulating Y2GaSbO7 pellet is measured and utilized to calculate the Debye temperatures of these compounds. Magnetic entropy of Dy2FeSbO7 at the zero-field equals to the spin-ice value of Delta S approximate to 4.1 JK(-1) mol(-1) around 11 K. Application of the magnetic field releases the zero-point entropy, intrinsic to the macroscopic degeneracy of the spin-ice like configurations of Dy3+-spins in Dy2FeSbO7, and hence increases the total integrated entropy. (C) 2017 Elsevier B.V. All rights reserved.