Neutron scattering investigation of complex magnetic transitions in multiferroic Co3TeO6 and core@shell Prussian blue analogue Rb-Co-Fe@K-Ni-Cr nano-cubes

• Main Authors: Chi-Hung Lee, 李其紘
• Other Authors: Wen-Hsien Li
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/78647108134040562226

博士 === 國立中央大學 === 物理學系 === 103 === There are two parts in my thesis. The first part focuses on the complex magnetic couplings in Co3TeO6. The second part focuses on the magnetic phases in core/shell Prussian blue analogue Rb-Co-Fe@K-Ni-Cr nano-cubes. In the first part, neutron diffraction, magnetic susceptibility, specific heat, and dielectric constant of single crystal cobalt tellurate Co3TeO6 have all been measured to study the interplay between the ferroelectricity, commensurate and incommensurate magnetic ordering developed in the compound. Four critical temperatures are identified. A non-collinear arrangement of the Co spins is found. A negative thermal expansion of the crystalline unit cell is identified when electric polarization develops. Both applied magnetic field and electric field significantly affect the magnetic and electric order parameters as well. These behaviors characterize Co3TeO6 to be a type-II multiferroics. Four magnetic phases have been identified in nano-sized core/shell Prussian blue analogue cubes, with a 250 nm Rb-Co-Fe phase in the core coated by a 45 nm K-Ni-Cr phase on the shell. The stress preserved in the core results in separated CoN6 and FeC6 octahedra, which weaken the magnetism and photo-sensitivity of the core. Three separated magnetic phase transitions at 86, 69 and 67 K are found in the K-Ni-Cr phase on the shell. Two magnetic exchange paths are identified. One propagates along the three crystallographic axis directions. The other propagates along the [110] crystallographic direction for the Ni-Ni ions, but not for the Cr-Cr ions. The severe Cr-deficiency and the appearance of direct Ni-Ni exchange are used to understand the appearance of multiple magnetic phases. The formation of compact CoN6 and FeC6 octahedra weakens the charge transfer between the Fe and Co ions, which results in the development of weak magnetic moments in the core.