Structure and magnetic state of hydrothermally prepared Mn-Zn ferrite nanoparticles.

• Mn Zn ferrite particles with a size of ≈ 12 nm are prepared by hydrothermal synthesis. • Analysis by Mössbauer spectroscopy and neutron diffraction reveals a metastable state. • Mn 0.6 Zn 0.4 Fe 2 O 4 particles show a non-equilibrium distribution of Zn and Mn. • Superparamagnetism is studied on time scales of Mössbauer and magnetic measurements. • Transition to superparamagnetic state is easily controlled by Zn content. Magnetic behaviour of nanoparticles deviates from their bulk equivalents not only due to finite-size and surface effects but it may be also affected by occurrence of metastable states, such as non-equilibrium cation distribution. The chemical composition along with the specific cation distribution are decisive for magnetic properties of spinel ferrites, the nanoparticles of which can be easily tailored to exhibit peculiar properties suitable for various applications. The present study is devoted to hydrothermally prepared Mn Zn ferrite nanoparticles; namely to five samples with approximate composition Mn 1− x Zn x Fe 2 O 4 , where x = 0.21–0.63, that were prepared by a surfactant-free hydrothermal procedure at a rather low temperature of 180 °C. XRD measurements evidenced the cubic spinel structure and a gradual decrease of the mean crystallite size from 14 to 10 nm with increasing zinc content, which was further confirmed by TEM analysis. 57Fe Mössbauer spectroscopy and temperature-variable magnetic measurements provided valuable insight into the blocking behaviour of particles on two dramatically different time scales and demonstrated that Néel relaxation can be enhanced by increasing the zinc content. Nanoparticles of the selected composition Mn 0.62 Zn 0.41 Fe 1.97 O 4 were subjected to neutron diffraction study at 2 K to determine the ferrimagnetic order and in combination with Mössbauer spectroscopy to analyse the cation distribution. The non-zero occupancy of Zn2+ in octahedral sites evidenced a metastable distribution, and the supplemental DFT study revealed that the distribution of Mn2+ is non-equilibrium as well. [ABSTRACT FROM AUTHOR]