Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

Invited review “sol-gel” preparation of high temperature superconducting oxides

Heterostructured ABA thin films consisting of two different Prussian blue analogues, where A is a ferromagnet and B is a photoinducible ferrimagnet, have been fabricated for the first time. This novel arrangement allows the magnetization to be decreased by irradiation with white light and significantly increases the ordering temperature of the photoinduced magnetism from 18 to 75 K.

Persistent photoinduced magnetism in heterostructures of prussian blue analogues.

Multiferroics materials, which exhibit coupled magnetic and ferroelectric properties, have attracted tremendous research interest because of their potential in constructing next-generation multifunctional devices. The application of single-phase multiferroics is currently limited by their usually small magnetoelectric effects. Here, we report the realization of giant magnetoelectric effects in a Y-type hexaferrite Ba0.4Sr1.6Mg2Fe12O22 single crystal, which exhibits record-breaking direct and converse magnetoelectric coefficients and a large electric-field-reversed magnetization. We have uncovered the origin of the giant magnetoelectric effects by a systematic study in the Ba2-x
 Sr
 x
 Mg2Fe12O22 family with magnetization, ferroelectricity and neutron diffraction measurements. With the transverse spin cone symmetry restricted to be two-fold, the one-step sharp magnetization reversal is realized and giant magnetoelectric coefficients are achieved. Our study reveals that tuning magnetic symmetry is an effective route to enhance the magnetoelectric effects also in multiferroic hexaferrites. Control of the electrical properties of materials by means of magnetic fields or vice versa may facilitate next-generation spintronic devices, but is still limited by their intrinsically weak magnetoelectric effect. Here, the authors report the existence of an enhanced magnetoelectric effect in a Y-type hexaferrite, and reveal its underlining mechanism.

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Giant magnetoelectric effects achieved by tuning spin cone symmetry in Y-type hexaferrites.

Magnetoelectric (ME) properties were investigated for BaSrCo2−xZnxFe11AlO22 ceramics with the Y-type hexaferrite structure. This system exhibits a magnetic ordering above room temperature (RT) and high electrical resistivity exceeding 500 MΩ·cm by annealing under 10 atm O2. It is found that the samples with x ≤ 0.4 exhibit the robust ME effect with a reversal of electrical polarization induced by a low magnetic field at RT. Moreover, the electric-field-induced magnetization switching is also observed at RT. These results clearly demonstrate the mutual control of magnetization and electrical polarization at RT in the Y-type hexaferrite system.

Mutual control of magnetization and electrical polarization by electric and magnetic fields at room temperature in Y-type BaSrCo2−xZnxFe11AlO22 ceramics

We investigate the phase diagram of ${\mathrm{TmB}}_{4}$, an Ising magnet on a frustrated Shastry-Sutherland lattice, by neutron diffraction and magnetization experiments. At low temperature we find N\'eel order at low field, ferrimagnetic order at high field, and an intermediate phase with magnetization plateaus at fractional values $M/{M}_{\mathrm{sat}}=1/7,1/8,1/9,\dots{}$ and spatial stripe structures. Using an effective $S=1/2$ model and its equivalent two-dimensional fermion gas we suggest that the magnetic properties of ${\mathrm{TmB}}_{4}$ are related to the fractional quantum Hall effect of a 2D electron gas.

Fractional magnetization plateaus and magnetic order in the Shastry-Sutherland magnet TmB4.

Field-induced incommensurate-to-commensurate phase transition in the magnetoelectric hexaferrite Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1 − x Al x ) 12 O 22

We have investigated the magnetic ordering of the frustrated fcc – antiferromagnet HoB12. Below TN= 7.4 K antiferromagnetic order and a complex phase diagram is observed. Above TN neutron scattering experiments show strong diffuse scattering. The diffuse signal indicates strong correlations between rare earth moments along the [111] direction well above TN. The behavior of this component resembles low dimensional magnets which are known to show long range order only at T = 0. Close to TN correlations perpendicular to the [111] direction get relevant, they diverge toward TN. Thus we observe a complex ordering process where the frustration is lifted in steps. The experimental data and their interpretation are presented, some of the possible microscopic origins are discussed.

Magnetic Properties of the Frustrated fcc – Antiferromagnet HoB12 Above and Below TN

We have investigated the magnetic structure of the fcc antiferromagnet $\mathrm{Ho}{\mathrm{B}}_{12}$ by magnetization and specific heat measurements on small single crystals prepared from natural elements and by neutron diffraction on isotopically enriched powder samples. Magnetization measurements up to $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ show up to three magnetic phases in the $B$ vs $T$ phase diagram, depending on the orientation of the applied field. The specific heat in zero field exhibits a very steep increase at ${T}_{N}=7.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but its maximum is reached only at a lower temperature. In applied magnetic field up to $8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ additional $\ensuremath{\lambda}$-like anomalies are observed which confirm the phase boundaries from the magnetization measurements. Powder neutron diffraction in zero magnetic field reveals an antiferromagnetic structure below ${T}_{N}$. The basic reflections can be indexed with $(1∕2\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}\phantom{\rule{0.3em}{0ex}}1∕2\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}\phantom{\rule{0.3em}{0ex}}1∕2\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta})$, where $\ensuremath{\delta}=0.035$, pointing to an incommensurate magnetic structure. In a field below $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ (in the lowest-field magnetic phase) the principal reflections remain; in a higher magnetic field they become suppressed. Moreover, the magnetic background strongly decreases with applied field. The analysis of results shows that an amplitude-modulated, incommensurate structure likely represents the magnetic order of $\mathrm{Ho}{\mathrm{B}}_{12}$. The very complex phase diagram of this compound can arise from the interplay between the RKKY and dipole-dipole interaction and/or from frustration effects in the fcc-symmetry lattice.

Phase diagram and magnetic structure investigation of the fcc antiferromagnet Ho B 12

We present a simple model for a description of magnetization processes in rare-earth tetraborides. The model is based on the coexistence of two subsystems, and, namely, the spin subsystem described by the Ising model and the electronic subsystem described by the Falicov-Kimball model on the Shastry-Sutherland lattice (SSL). Moreover, both subsystems are coupled by the anisotropic spin-dependent interaction of the Ising type. We have found, that the switching on of the spin-dependent interaction $({J}_{z})$ between the electron and spin subsystems and taking into account the electron hopping on the nearest $(t)$ and next-nearest $({t}^{\ensuremath{'}})$ lattice sites of the SSL leads to a stabilization of magnetization plateaus. In addition, to the Ising magnetization plateau at ${m}^{sp}/{m}_{s}^{sp}=1/3$ we have found three relevant magnetization plateaus located at ${m}^{sp}/{m}_{s}^{sp}=1/2$, 1/5, and 1/7 of the saturated spin magnetization ${m}_{s}^{sp}$. The ground states corresponding to magnetization plateaus have the same spin structure consisting of parallel antiferromagnetic bands separated by ferromagnetic stripes.

S. Matas

Papers

Fractional magnetization plateaus and magnetic order in the Shastry-Sutherland magnet TmB4.

Field-induced incommensurate-to-commensurate phase transition in the magnetoelectric hexaferrite Ba 0 . 5 Sr 1 . 5 Zn 2 (Fe 1 − x Al x ) 12 O 22

Magnetic Properties of the Frustrated fcc – Antiferromagnet HoB12 Above and Below TN

Phase diagram and magnetic structure investigation of the fcc antiferromagnet Ho B 12

Numerical study of magnetization processes in rare-earth tetraborides