Structural and magnetic properties of YAl3(BO3)4 and EuAl3(BO3)4 single crystals doped with Co2+

Structural and magnetic properties of the cobalt-doped YAl3(BO3)(4) and EuAl3(BO3)(4) crystals were studied using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), X-ray diffraction analysis, and specific heat methods. It was found that cobalt, when doped to the studied crystals, is in divalent state, substitutes the aluminum ions and forms a system of three magnetically equivalent magnetically nonequivalent centers whose magnetic axes are rotated at 120 degrees relative to one another. We have determined parameters of the spin Hamiltonian of orthorhombic symmetry, describing the anisotropic angular dependence EPR spectra for both types of crystals. Strong broadening of EPR lines is caused by the spin-lattice relaxation. Specific heat of the EuAl3(BO3)(4) and YAl3(BO3)(4) crystals doped with 0.2% of Co2+ was measured over the temperature range from 3 to 395 K, in zero magnetic field. The Debye temperature was determined to be equal to 355 K and 358 K for YAl3(BO3)(4) and EuAl3(BO3)(4), respectively. Crystallographic structure at ambient conditions as well as variations of unit-cell parameters and thermal expansion coefficients with temperature were studied in detail in the range 302 K < T < 1073 K. NMR study on Al-27 and B-11 nuclei at room temperature was conducted. It is shown that on changing the rare-earth ion from yttrium to europium, the contribution of hyperfine interactions to the resonance characteristics of the samples increases. It is also shown that there are two nonequivalent environments of boron atoms with an attitude close to 1.1 and 1.2 for the samples containing 0.1 and 0.2% of cobalt ions, respectively. (C) 2018 Elsevier B.V. All rights reserved.