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Register a new userTerahertz-infrared dielectric properties of lead-aluminum double-cation substituted single-crystalline barium hexaferrite
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Hexaferrite materials are highly demanded to develop and manufacture electronic devices operating at radio- and microwave frequencies. In the light of the prospects for their use in the forthcoming terahertz electronics, here, we present our results on the terahertz and infrared dielectric response of a typical representative of hexaferrites family, lead-substituted M-type barium hexaferrite doped with aluminum, Ba0.2Pb0.8AlxFe12-xO19, x(Al)=0.0, 3.0, and 3.3. We studied uniquely large and high-quality single crystals of the. Systematic and detailed investigations of the dependences of terahertz-infrared (frequencies 8 - 8000 cm-1) spectra of complex dielectric permittivity on the temperature, 4 - 300 K, and on the chemical composition, x(Al)=0.0, 1.2, 3.0, 3.3, were performed for polarizations of the electric field E-vector of the probing radiation normal and parallel to the crystallographic c-axis. A number of resonance absorption bands are discovered at infrared-terahertz frequencies and assigned to polar phonons and transitions between energy levels of the fine-structured ground state of Fe2+ (5E) ions. In contrast to undoped BaFe12O19, no softening of the lowest frequency A2u phonon is observed, indicating suppression of a displacive phase transition in substituted compounds. Basing on dielectric data and detailed X-ray experiments, we find that for all concentrations of Al3+ ions, x(Al)=0.0, 1.2, 3.0, and 3.3, they mainly occupy the 2a and 12k octahedral site positions and that the degree of substitution of iron in tetrahedral positions is not substantial. Along with fundamental findings, the obtained data on broad-band dielectric properties of Ba0.2Pb0.8AlxFe12-xO19 crystals provides the information that can be used for development and manufacture of electronic devices with operating frequencies lying in the terahertz spectral band.
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We use first-principles total-energy calculations based on density functional theory to study the site occupancy and magnetic properties of Al-substituted $M$-type strontium hexaferrite SrFe$_{12-x}$Al$_{x}$O$_{19}$ with $x=0.5$ and $x=1.0$. We find that the non-magnetic Al$^{3+}$ ions preferentially replace Fe$^{3+}$ ions at two of the majority spin sites, $2a$ and $12k$, eliminating their positive contribution to the total magnetization causing the saturation magnetization $M_s$ to be reduced as Al concentration $x$ is increased. Our formation probability analysis further provides the explanation for increased magnetic anisotropy field when the fraction of Al is increased. Although Al$^{3+}$ ions preferentially occupy the $2a$ sites at a low temperature, the occupation probability of the $12k$ site increases with the rise of the temperature. At a typical annealing temperature ($> 700,^{circ}{rm C}$) Al$^{3+}$ ions are much more likely to occupy the $12k$ site than the $2a$ site. Although this causes the magnetocrystalline anisotropy $K_1$ to be reduced slightly, the reduction in $M_s$ is much more significant. Their combined effect causes the anisotropy field $H_a$ to increase as the fraction of Al is increased, consistent with recent experimental measurements.
The site preference and magnetic properties of Zn, Sn and Zn-Sn substituted M-type strontium hexaferrite (SrFe$_{12}$O$_{19}$) have been investigated using first-principles total energy calculations based on density functional theory. The site occupancy of substituted atoms were estimated by calculating the substitution energies of different configurations. The distribution of different configurations during the annealing process at high temperature was determined using the formation probabilities of configurations to calculate magnetic properties of substituted strontium hexaferrite. We found that the magnetization and magnetocrystalline anisotropy are closely related to the distributions of Zn-Sn ions on the five Fe sites. Our calculation show that in SrFe$_{11.5}$Zn$_{0.5}$O$_{19}$, Zn atoms prefer to occupy $4f_1$, $12k$, and $2a$ sites with occupation probability of 78%, 19% and 3%, respectively, while in SrFe$_{11.5}$SnO$_{19}$, Sn atoms occupy the $12k$ and $4f_2$ sites with occupation probability of 54% and 46%, respectively. We also found that in SrFe$_{11}$Zn$_{0.5}$Sn$_{0.5}$O$_{19}$, (Zn,Sn) atom pairs prefer to occupy the ($4f_1$, $4f_2$), ($4f_1$, $12k$) and ($12k$, $12k$) sites with occupation probability of 82%, 8% and 6%, respectively. Our calculation shows that the increase of magnetization and the reduction of magnetic anisotropy in Zn-Sn substituted M-type strontium hexaferrite as observed experimentally is due to the occupation of (Zn,Sn) pairs at the ($4f_1$, $4f_2$) sites.
Superconductivity in group IV semiconductors is desired for hybrid devices combining both semiconducting and superconducting properties. Following boron doped diamond and Si, superconductivity has been observed in gallium doped Ge, however the obtained specimen is in polycrystalline form [Herrmannsdorfer et al., Phys. Rev. Lett. 102, 217003 (2009)]. Here, we present superconducting single-crystalline Ge hyperdoped with gallium or aluminium by ion implantation and rear-side flash lamp annealing. The maximum concentration of Al and Ga incorporated into substitutional positions in Ge is eight times higher than the equilibrium solid solubility. This corresponds to a hole concentration above 10^21 cm-3. Using density functional theory in the local density approximation and pseudopotential plane-wave approach, we show that the superconductivity in p-type Ge is phonon-mediated. According to the ab initio calculations the critical superconducting temperature for Al- and Ga-doped Ge is in the range of 0.45 K for 6.25 at.% of dopant concentration being in a qualitative agreement with experimentally obtained values.
We report the evolution of structural, magnetic and dielectric properties due to partial substitution of Ba by Sr in the high temperature multiferroic YBaCuFeO5. This compound exhibits ferroelectric and antiferromagnetic transitions around 200 K and these two phenomena are presumed to be coupled with each other. Our studies on magnetic and dielectric properties of the YBa1-xSrxCuFeO5 (x = 0.0, 0.25 and 0.5) show that substitution of Sr shifts magnetic transition towards higher temperature whereas dielectric transition to lower temperature. These results points to the fact that magnetic and dielectric transitions get decoupled as a result of chemical pressure in form of Sr substitution. The nature of magnetodielectric coupling changes across the series with the presence of higher order coupling terms. Additionally in these compounds glassy dynamics of electric dipoles is observed at low temperatures.
In the present work the authors report results of broadband dielectric spectroscopy on various samples of CaCu3Ti4O12, including so far only rarely investigated single crystalline material. The measurements extend up to 1.3 GHz, covering more than nine frequency decades. We address the question of the origin of the colossal dielectric constants and of the relaxational behavior in this material, including the second relaxation reported in several recent works. For this purpose, the dependence of the temperature- and frequency-dependent dielectric properties on different tempering and surface treatments of the samples and on ac-field amplitude are investigated. Broadband spectra of a single crystal are analyzed by an equivalent circuit description, assuming two highly resistive layers in series to the bulk. Good fits could be achieved, including the second relaxation, which also shows up in single crystals. The temperature- and frequency-dependent intrinsic conductivity of CCTO is consistent with the Variable Range Hopping model. The second relaxation is sensitive to surface treatment and, in contrast to the main relaxation, also is strongly affected by the applied ac voltage. Concerning the origin of the two insulating layers, we discuss a completely surface-related mechanism assuming the formation of a metal-insulator diode and a combination of surface and internal barriers.
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