Search

Single-domain Sr1−x/12Cax/12Fe12−xAlxO19 (x = 4–6) particles are synthesized by a simple citrate auto-combustion method. The room temperature coercivity of the materials rises with aluminum content from 21.3 kOe (x = 4) to a maximum of 36 kOe (x = 5.5). This value is the highest among ferrite materials to date. Moreover, the magnetic alignment of the particles leads to further coercivity improvement up to 40 kOe. Due to large magnetic anisotropy the samples demonstrate sub-terahertz electromagnetic wave absorption by natural (zero-field) ferromagnetic resonance (NFMR). The absorption lines shift with aluminum substitution from 160 GHz (x = 4) to 250 GHz (x = 5.5), which is the record NFMR frequency known for a magnetic material. This research paves the way for development low-cost materials with extremely high coercivity and sub-terahertz NFMR.

Uniform quasi-one-dimensional integer spin compounds are of interest as a potential realization of the Haldane conjecture of a gapped spin liquid. This phase, however, has to compete with magnetic anisotropy and long-range ordered phases, the implementation of which depends on the ratio of interchain J′ and intrachain J exchange interactions and both uniaxial D and rhombic E single-ion anisotropies. Strontium nickel selenite chloride, Sr2Ni(SeO3)2Cl2, is a spin-1 chain system which passes through a correlations regime at Tmax ~ 12 K to long-range order at TN = 6 K. under external magnetic field it experiences the sequence of spin-flop at Bc1 = 9.0 T and spin-flip transitions Bc2 = 23.7 T prior to full saturation at Bsat = 31.0 T. Density functional theory provides values of the main exchange interactions and uniaxial anisotropy which corroborate the experimental findings. The values of J′/J = 0.083 and D/J = 0.357 place this compound into a hitherto unoccupied sector of the Sakai-Takahashi phase diagram. © 2021, The Author(s). Support by the P220 program of Government of Russia through the project 075-15-2021-604 is acknowledged. ANV acknowledges support by the RFBR Grant 19-02-00015. Work at Heidelberg was supported by BMBF via the project SpinFun (13XP5088) and by Deutsche Forschungsgemeinschaft (DFG) under Germany’s Excellence Strategy EXC2181/1-390900948 (the Heidelberg STRUCTURES Excellence Cluster) and through project KL 1824/13-1. We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL). Theoretical calculations using density functional theory were supported by the Russian Science Foundation via project 20-62-46047. Experimental research was supported by the Russian Science Foundation via project 19-42-02010.

Magnetically hard ferrites attract considerable interest due to their ability to maintain a high coercivity of nanosized particles and therefore show promising applications as nanomagnets ranging from magnetic recording to biomedicine. Herein, we report an approach to prepare nonsintered single-domain nanoparticles of chromium-substituted hexaferrite via crystallization of glass in the system SrO–Fe2O3–Cr2O3–B2O3. We have observed a formation of plate-like hexaferrite nanoparticles with diameters changing from 20 to 190 nm depending on the annealing temperature. We demonstrated that chromium substitution led to a significant improvement of the coercivity, which varied from 334 to 732 kA m−1 for the smallest and the largest particles, respectively. The results provide a new strategy for producing high-coercivity ferrite nanomagnets.

Chloromenite, Cu9O2(SeO3)4Cl6, is the product of volcanic activity. To conduct various physical studies both natural and synthetic samples of chloromenite were used which resulted in a long-standing controversy regarding its crystal structure. We carried out the temperature dependent single crystal X-ray diffraction study in the range 100 – 370 K and established that the reflection conditions are consistent with a primitive monoclinic cell with the only possible P21/n space group. Raman scattering evidences a complex interplay of crystal lattice and spin degrees of freedom. The observation of a sequence of short range fluctuation effects points to the existence of competing interactions or nearly degenerate energy scales. To gain microscopic understanding of the magnetic properties of Cu9O2(SeO3)4Cl6 the first principles density functional theory calculations have been performed. Basing on the new data on magnetization and the hierarchy of exchange interaction parameters a model of Cu9O2(SeO3)4Cl6 magnetic structure is suggested.

Our study of FeSe 1 − x Te x crystals with x d − d exchange in the iron plane in the framework of the basic phenomenological description such as the Bethe–Slater curve. With this approach, when the distance between iron atoms is close to the value at which the sign of the magnetic exchange for some d orbitals changes, structural and electronic instability can occur. Anomalies in the crystal field near the point of charge neutrality can also be a significant component of this instability.

${\mathrm{Fe}}^{3+} S=5/2$ ions form saw-tooth-like chains along the $a$ axis of the oxoselenite ${\mathrm{Fe}}_{2}\mathrm{O}{({\mathrm{SeO}}_{3})}_{2}$ and an onset of long-range magnetic order is observed for temperatures below ${T}_{C}=105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T=150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

The iron oxoselenite Fe2O(SeO3)2 hosts unique tilted saw-tooth chains of vertex-sharing iron triangles. It experiences long-range magnetic order below TN ≈ 105 K characterized by a sharp dichotomy in zero-field-cooled and field-cooled behavior (T ≈ 0.31), which results from a spin-wave treatment. The mean-field analysis of the temperature evolution of the fields Bhf,Fei(T) allowed evaluation of exchange interactions between different iron sublattices. A spin relaxation behavior of Fe2O(SeO3)2 in the temperature range T*

We demonstrate that the simultaneous substitution of calcium and aluminum for strontium and iron in strontium hexaferrite results in a significant increase of coercivity up to a record high of 21.3 kOe. We propose that the effect is originated from a crystal structure distortion causing an increase of the magnetocrystalline anisotropy.