Infrared study of lattice and magnetic dynamics in a spin-chain compoundGd2BaNiO5

Abstract

We present infrared spectra of Gd 2 BaNiO 5 , which is isostructural to a prototype S = 1 Haldane compound Y 2 BaNiO 5 containing Ni 2+ chains, in the spectral range 2 meV-0.55 eV. Unlike Y 2 BaNiO 5 , the studied compound contains magnetic rare-earth sublattices and orders antiferromagnetically at T N = 58 K. Detailed information on optical phonons is given. Temperature dependences of frequencies and half widths for the two lowest-frequency phonons polarized along the Ni-chain direction evidence the interaction of these lattice vibrations with magnetic excitations. With the help of lattice-dynamics calculations, we find relative displacement vectors of ions for all the phonon modes and use them to discuss the mechanism of phonon-magnon interaction. The optical spectra exhibit a broad absorption continuum for radiation polarized along the chains, probably of magnetic origin, gradually decreasing with lowering temperature. A new mode at about 30 cm −1 polarized along the chains ͑a axis͒ emerges below ϳ150 K. A midinfrared absorption peak at 1306 cm −1 ͑0.16 eV͒ is observed and found to sharpen and shift significantly at T N . We argue that it can be attributed to a phonon-assisted magnetic absorption and discuss its nature in the framework of the Lorenzana-Sawatzky-Eder model.

Multiferroicity and magneto-electric effect in Gd2BaNiO5

“…There is a sharp anomaly in (T) around ~24 K attributable to spin reorientation transition (SRT) which is followed by an increase on further lowering temperature for both 100 Oe and 5 kOe applied field [17,18]. Earlier reports based on the microscopic studies such as optical [8] and Mössbauer spectroscopy The plot of C/T as a function of temperature is shown in Fig. 2(b).…”

“…However, as the energy scale for controlling the magnetic and electric properties are different, the coupling between dielectric and magnetic properties is possible in a material in which a low frequency infrared-allowed optical phonon mode exists [7]. A recent study of the lattice and magnetic dynamics in Gd 2 BaNiO 5 through infrared spectroscopy has shown the existence of low frequency continuum and anomalies in optical conductivity near the anti-ferromagnetic (AFM) ordering temperature (T N ~55 K) suggesting the presence of spin-phonon coupling [8]. Therefore strong ME coupling is expected in this compound.…”

Extraordinarily large intrinsic magnetodielectric coupling of the Tb member within the Haldane spin-chain family R2BaNiO5

“…These compounds crystallizing in Immm-type orthorhombic structure has been of special interest in the field of magnetism for over a quarter century for anomalies associated with Haldane spin-chain gap and magnetic ordering of Ni and (magnetic-moment containing) R ions at the same temperature (see, for instance, Refs. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Though this family has been considered to be prototype for Haldane spin-chain behavior, there was very little focus to understand magnetoelectric behavior of these insulators till recently.…”

“…It may be noted that very high dielectric permittivity at room temperature for non-magnetic Y2BaNiO5 [17] and linear magnetoelectric coupling for magnetically ordering Ho analogue [18] were reported. This family of materials is characterized [6,7] by infra-red allowed optical phonon mode with low frequency, i.e., the existence of a soft phonon mode which is a clue [25] for exploring magnetoelectric coupling. In light of this, in recent years, we have subjected many members of this family to intense studies to explore the cross-coupling phenomenon and reported [7, 19 -23] a variety of magnetoelectric anomalies.…”

Effect of electron-phonon interactions on Raman line at ferromagnetic ordering

“…The interaction of phonons with magnons in antiferromagnets was discussed in the review article 10 and more recently in the analysis of the thermal conductivity 11 , the spin Seebeck effect 12,13 , hightemperature superconductivity 14 , and optical spectra 15 . The magnon-phonon interaction results in the magnon damping 16 , however, no effect for phonons was observed.…”