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The electrical conductivity σ and magnetization M of KBiFe2O5$\left(\text{KBiFe}\right)_{2} \left(\text{O}\right)_{5}$ (KBFO) nanoparticles (NPs) are systematically examined using a microscopic model and Green's function theory. KBFO is characterized by a narrow bandgap and relatively weak electrical conductivity σ, which presents a problem in carrier transportation and collection. Therefore, ways to enhance σ are found. The first is reducing the NP size. The second one is doping at the Fe and Bi sites with different ions which cause a compressive strain, that is, their ionic radius is smaller than that of the host ion. It is shown that doping with Al at the Fe site as well as with Ru or La ions at the Bi site leads to enhancing the electrical conductivity σ. The magnetization M increases with increasing concentration of all dopants. [ABSTRACT FROM AUTHOR]

Investigating the multiferroic properties of Cu2OCl2$\left(\text{Cu}\right)_{2} \left(\text{OCl}\right)_{2}$, their temperature and magnetic‐field dependencies around the magnetic and ferroelectric phase transition temperatures TN$T_{N}$ and TE$T_{E}$ are analyzed using a microscopic model and Green's function technique. Near the Neel temperature TN$T_{N}$, a kink in the specific heat Cp(T)$C_{p} \left(\right. T \left.\right)$ and also in the phonon energy ω(T)$\omega \left(\right. T \left.\right)$ and damping γ(T)$\gamma \left(\right. T \left.\right)$ due to strong anharmonic spin–phonon interaction is observed. The kink in Cp(T)$C_{p} \left(\right. T \left.\right)$ disappears with increasing magnetic field h. The polarization P(T)$P \left(\right. T \left.\right)$ shows also a small anomaly at TN$T_{N}$ and decreases with increasing magnetic field h. This behavior strongly suggests that Cu2OCl2$\left(\text{Cu}\right)_{2} \left(\text{OCl}\right)_{2}$ exhibits multiferroic characteristics. A kink at TN$T_{N}$ can be seen in the real part of the dielectric constant ε′(T)$\left(\epsilon\right)^{&aposx;} \left(\right. T \left.\right)$, too. It should be noted that ε′$\left(\epsilon\right)^{&aposx;}$ demonstrates a significant increase with increasing field strength h below TN$T_{N}$, which is a pronounced magneto‐dielectric effect. These findings qualitatively agree with the experimental data. [ABSTRACT FROM AUTHOR]

Based on a microscopic model and the Green's function theory, the temperature, size, magnetic field, and ion doping dependence of the magnetic, electric, and dielectric properties in Fe‐and Nd‐doped ferroelectric Bi4Ti3O12 bulk and nanoparticles is investigated. The multiferroism can be achieved in two ways. At once the magnetism appears by substitution of the nonmagnetic Ti4+$^{4 +}$ ion with Fe3+$^{3 +}$ or other transition metal ions in bulk Bi4Ti3O12. Otherwise, the simultaneous existence of Fe3+$^{3 +}$ and Fe4+$^{4 +}$ gives rise to a superexchange mechanism responsible for ferromagnetic interaction. Moreover, multiferroism is possible also in pure Bi4Ti3O12 nanoparticles. Due to surface effects and oxygen vacancies leading to different valence states on the surface of Ti3+$^{3 +}$ or Ti2+$^{2 +}$ with spin value S≠0$S \neq 0$, there appears weak magnetism, which can be improved by Fe ion doping. It is demonstrated that doping with rare earth ions, for example Nd3+$^{3 +}$, at the Bi3+$^{3 +}$ site is able to improve the ferroelectric, multiferroic, and dielectric properties. [ABSTRACT FROM AUTHOR]

Using a microscopic model, the magnetic and electric properties of pure and ion‐doped (on the Lu or Fe sites) hexagonal LuFeO3 (LuFO) are studied and compared with the properties of ion‐doped LaFeO3 (LaFO). The magnetization increases by Sr and Sc doping which is caused by the strain and the changes of the exchange interaction constants at the doped states. It is observed that by Sm doping of LuFO the polarization decreases, whereas by Sm doping of LaFO, the polarization increases with increasing Sm concentration. By Ir or Co ion doping on the Fe site, the magnetization and the phonon energy increase whereas the bandgap energy decreases. By doping on the Fe sites with the same ion, for example, Sc, there are no differences between LuFO and LaFO. A good agreement with the experimental data is observed. The doping effects can be used for different applications. [ABSTRACT FROM AUTHOR]

The multiferroic properties of BiCrO3 (BCO) bulk and thin films are investigated for the first time using a microscopic model. The influence of ion doping on the multiferroic properties of BCO is also studied because the doping process changes the physical properties and can be used for potential applications in magnetoelectronic devices. The magnetization M increases by Mn and decreases by Ga‐ion doping whereas the Néel temperature TN decreases in both cases. There are some discrepancies by polarization whether it is ferroelectric or antiferroelectric. It is shown that bulk pure BCO is antiferroelectric because the electric field dependence of polarization P shows a double hysteresis loop and the dielectric constant has no anomalies at TN. In Ga‐, Mn‐ or Ti‐doped BCO a ferroelectric polarization appears. The specific heat shows a peak at TN for BCO. By Ga doping, this peak vanishes with increasing x. The multiferroic behavior of BCO is demonstrated in the magnetic field dependence of the dielectric constant. In BCO thin films, ferroelectric and ferromagnetic behavior is observed. [ABSTRACT FROM AUTHOR]

Herein, doped yttrium garnet (YIG) nanoparticles (NPs) Y3[Fe2−yMy]a(Fe3−zMz)dO12 (M= Al, Ga, Sc are nonmagnetic ions) are studied, appropriate for self‐controlled magnetic hyperthermia (SCMH) for in vivo and in vitro applications. A microscopic model (modified Heisenberg Hamiltonian) and the Green's function technique are used to investigate the temperature, doping, and size dependence of magnetic phase transition, saturation magnetization, the coercivity, and the specific absorption rate coefficient to fulfill the conditions for magnetic hyperthermia. A set of mixed yttrium garnet magnetic NPs is found, which are suitable for applications in medicine for cancer therapy. There are two Sc‐doped YIG NPs which are the best candidates for SCMH with x = 1.08, d = 27.5 nm, and SARmax = 13.52 W g−1, as well as x = 1.10, d = 29.7 nm, and SARmax = 15.44 W g−1 for which the conditions for biocompatibility and maximization of SAR are fulfilled simultaneously. Moreover, also Al‐ and Ga‐doped YIG NPs are found that can be applied for magnetic hyperthermia but with smaller SAR effectivity. [ABSTRACT FROM AUTHOR]

The dielectric properties of CuCrO2 bulk and thin films are studied by evaluating the complex dielectric function ε. In addition to the small peak near the Néel temperature TN, a secondary broad peak is found at high temperatures around Tm=450 K≫TN. As a feature of relaxor ferroelectrics, the maximum temperature Tm increases with increasing frequency. The real part of ε decreases with increasing magnetic field h and the peak at TN vanishes. The secondary peak becomes smaller and is broadened with increasing magnetic field. ε increases with increasing film thickness. Near Tm, the dielectric function offers a critical behavior expressed by an exponent γ=1.74. Using scaling arguments, critical exponents β=0.13 and δ=13.38 are deduced. The exponents depend on the film thickness. While β increases, the exponent γ is reduced with increasing film thickness. [ABSTRACT FROM AUTHOR]

The size, doping concentration, and magnetic field dependences of the magnetization M and the real part of the dielectric function ε' in doped LaFe1−xMxO3 nanoparticles (NPs) are studied using a microscopic model. Although M increases, ε′ decreases with decreasing NPs size in pure LaFeO3. Doping with different ions causes different strain which modifies the exchange interaction. As a consequence, M and ε′ grow with increasing Mn or Zn ion doping, whereas both quantities decrease by doping with Ti or Al ions. The magneto‐dielectric coefficient increases with an applied external magnetic field and Mn doping concentration in pure and Mn‐doped LaFeO3 indicating that pure and doped LaFeO3 NPs exhibit a multiferroic behavior. A microscopic model is proposed to study multiferroic Bi2NiMnO6 thin films. [ABSTRACT FROM AUTHOR]

Investigating the multiferroic properties of Cu2OCl2$\left(\text{Cu}\right)_{2} \left(\text{OCl}\right)_{2}$, their temperature and magnetic‐field dependencies around the magnetic and ferroelectric phase transition temperatures TN$T_{N}$ and TE$T_{E}$ are analyzed using a microscopic model and Green's function technique. Near the Neel temperature TN$T_{N}$, a kink in the specific heat Cp(T)$C_{p} \left(\right. T \left.\right)$ and also in the phonon energy ω(T)$\omega \left(\right. T \left.\right)$ and damping γ(T)$\gamma \left(\right. T \left.\right)$ due to strong anharmonic spin–phonon interaction is observed. The kink in Cp(T)$C_{p} \left(\right. T \left.\right)$ disappears with increasing magnetic field h. The polarization P(T)$P \left(\right. T \left.\right)$ shows also a small anomaly at TN$T_{N}$ and decreases with increasing magnetic field h. This behavior strongly suggests that Cu2OCl2$\left(\text{Cu}\right)_{2} \left(\text{OCl}\right)_{2}$ exhibits multiferroic characteristics. A kink at TN$T_{N}$ can be seen in the real part of the dielectric constant ε′(T)$\left(\epsilon\right)^{&aposx;} \left(\right. T \left.\right)$, too. It should be noted that ε′$\left(\epsilon\right)^{&aposx;}$ demonstrates a significant increase with increasing field strength h below TN$T_{N}$, which is a pronounced magneto‐dielectric effect. These findings qualitatively agree with the experimental data. [ABSTRACT FROM AUTHOR]

Using a microscopic model, the magnetic and electric properties of pure and ion‐doped (on the Lu or Fe sites) hexagonal LuFeO3 (LuFO) are studied and compared with the properties of ion‐doped LaFeO3 (LaFO). The magnetization increases by Sr and Sc doping which is caused by the strain and the changes of the exchange interaction constants at the doped states. It is observed that by Sm doping of LuFO the polarization decreases, whereas by Sm doping of LaFO, the polarization increases with increasing Sm concentration. By Ir or Co ion doping on the Fe site, the magnetization and the phonon energy increase whereas the bandgap energy decreases. By doping on the Fe sites with the same ion, for example, Sc, there are no differences between LuFO and LaFO. A good agreement with the experimental data is observed. The doping effects can be used for different applications. [ABSTRACT FROM AUTHOR]