ИСТИНА

Single molecule and single ion magnets (SMMs and SIMs, respectively) are atomic scale objects that show slow magnetization relaxation at low temperatures. Most modern SIMs are represented by metalorganic complexes of transition element ions and lanthanide ions with low coordination numbers providing strong axial anisotropy. Such molecular complexes are often unstable, so transition metal ions may be introduced into positions with crystal field anisotropy in crystal structure of inorganic solid forming magnetic centers with bistable ground states (inorganic SIMs or ISIMs). High-spin Fe3+ free ion possesses a half-filled 3d-level without orbital momentum in its ground state. However slow relaxation of magnetization was observed in several compounds of iron(III) exhibiting low Ueff. In addition, the reduced electron-phonon interaction of such electronic configuration may provide a longer spin coherence time required to create qubits for quantum computers. In the present work imbedding of iron in an inorganic solid and possibility of forming the first Fe3+ based ISIM are considered. Mayenite Ca12Al14O33 was chosen as a crystalline host. It represents an anti-zeolite structure with positively charged cages partially filled with oxygen-based anions. Al atoms are found in two types of distorted tetrahedra of oxygen atoms (Al1 and Al2). Fe3+ can partially substitute Al3+ at the trigonally distorted tetrahedral site Al1 keeping its high-spin configuration. Such distortion may provide a crystal field anisotropy required to form a SIM unit. Ceramic samples with nominal compositions Ca12Al14 xFexO33, x = 0, 0.05, and 0.25 (1, 2, and 3, respectively) were prepared via annealing of pellets in oxygen flow at 1400 °C. According to X-ray powder diffraction data samples represent cubic mayenite phase. The crystal structure was refined by Rietveld method. From 1 to 3 slight increasing of unit cell parameters is represented suggesting the replacement of a smaller Al3+ by a large Fe3+. According to dc magnetization measurements 2 and 3 show paramagnetic behavior down to 2 K. 1 also reveals weak paramagnetic response detectable at low temperatures. For the further analysis, magnetic response of 1 was subtracted from that of 2 and 3. It’s worth noting, that paramagnetic behavior of 1 corresponds to that of a high-spin Fe3+ with its content x=0.009. Chi-1(T) is fitted well with the Curie-Weiss equation, demonstrating meff of 5.64 and 5.73 mB and a very small  of 0.0 and -0.1 K for 2 and 3, respectively. These parameters also correspond to high-spin Fe3+ ions diluted in a diamagnetic matrix with very small inter-ion interactions. For 2 and 3 temperature dependence of susceptibility-temperature product at 4 kOe and field dependence of magnetization at 2 K are in great agreement with model suggesting zero-field splitting axial parameter D = -0.67 cm-1. Ac susceptibility measurements (ac) under a dc magnetic field of 4 kOe reveal slow magnetization relaxation (SR) in all the samples. The undoped sample 1 also demonstrates similar SR. At low temperatures for 2 and 3, two maxima on Chi’‘(f) were observed corresponding to two relaxation processes. The lower frequency relaxation process (LSR) represents a minor fraction of slowly relaxing magnetic centers. The LSR fraction decreases with increasing temperature and disappears at 10 K. For the major SR process,  decreases fast with increasing Fe3+ content. In 3 at temperatures above 5 K,  becomes too low to be estimated. For 1 and 2, relaxation time temperature dependences are obtained. For these samples practically the same values for Ueff are calculated. The refined Ueff is in good agreement with the energy gap 6D = 4.2 cm-1, derived from the dc-magnetization data.