Magnetization And ESR Study Of Spin-chain Compounds Gd_2-xNd_xBaNiO₅

The spin-chain compounds R2BaNiO5(R = rare earths and Y) were recently recognized as promising model systems to study quasi-one-dimensional magnetism. The linear-chain nickelates R2BaNiO5, due to peculiarities of their structure, appeared to be excellent model materials for studying the crossover from 1D quantum to 3D classical behavior. However, the magnetism of R2BaNiO5 is still controversial. In this graduation thesis, we conduct a systematic study on the mixing effects of Gd and Nd in spin-chain compounds Gd2-xNdx BaNiO5(x=0~2.0) by magnetism and ESR measurements.Polycrystalline samples of Gd2-x Ndx BaNiO5(x=0~2.0) were synthesized using the solid-state reaction method. X-ray powder diffraction(XRD) spectra and the lattice constants show that all the samples are single phase with orthorhombic crystal structure(space group Immm). The spin-flop transition and spin reorientation in the M(H) and 1/?(T) curves are observed in Gd2-xNdx BaNiO5(x=0~2.0). Our magnetization measurements showed a possible classical to quantum transition by the mixing effects of Gd and Nd in spin-chain compounds Gd2-xNdxBaNiO5. First, the M(T) curves for the Gd-rich compounds exhibit a monotonous increase with decreasing temperature whereas those for the Nd-rich compounds consist of a maximum-minimum structure due to the Gd/Nd and Ni excitations. Second, the 1/?(T) curves for the Gd-rich compounds demonstrate Curie-Weiss-like behavior, which is absent for the Nd-rich side. It is therefore concluded that the Gd-rich compounds behave totally like classical magnets while the Nd-rich compounds quantum magnets. The enhanced magnetic anisotropy due to the Nd doping is proposed to be responsible for this classical-quantum transition.X-band and high-field electron spin resonance(ESR) has been employed to study the antiferromagnetic ordering state(T<TN=55 K) of spin-chain multiferroic Gd2BaNiO5. The spin reorientation at Tsr=24 K is well characterized by the temperature-dependent ESR spectra. Although the magnetization data evidence a field-induced spin-flop transition at 2 K, the frequency-field relationship can be explained by conventional resonance theory with uniaxial anisotropy, in good agreement with magnetization data.