Multiferroic BiFe0 Research Articles

Impacts of substitution of BaTiO3 on structure, dielectric, transport, and magnetic properties of multiferroic BiFe0.5Mn0.5O3

Impacts of substitution of BaTiO3 on structure, dielectric, transport, and magnetic properties of multiferroic BiFe0.5Mn0.5O3

Out-of-plane polarization reversal and changes in in-plane ferroelectric and ferromagnetic domains of multiferroic BiFe0.9Co0.1O3 thin films by water printing

BiFe0.9Co0.1O3 is a promising material for an ultra-low-power-consumption nonvolatile magnetic memory device because local magnetization reversal is possible through application of an electric field. Here, changes in ferroelectric and ferromagnetic domain structures in a multiferroic BiFe0.9Co0.1O3 thin film induced by “water printing”, which is a polarization reversal method involving chemical bonding and charge accumulation at the interface between the liquid and the film, was investigated. Water printing using pure water with pH = 6.2 resulted in an out-of-plane polarization reversal from upward to downward. The in-plane domain structure remained unchanged after the water printing process, indicating that 71° switching was achieved in 88.4% of the observation area. However, magnetization reversal was observed in only 50.1% of the area, indicating a loss of correlation between the ferroelectric and magnetic domains because of the slow polarization reversal due to nucleation growth.

Effect of cobalt substitution on the structural, ferroelectric, and magnetic properties of bismuth ferrite thin films

Epitaxial films of multiferroic BiFe1−xCoxO3 (BFCO) with 0≤x≤0.35 are grown on (001)-oriented SrTiO3 and SrRuO3 buffered SrTiO3 substrates using the pulsed laser deposition technique. The effect of structural transformation from rhombohedral to tetragonal phase with increasing cobalt substitution on the magnetic, electrical, and piezo-/ferroelectric properties is investigated. Piezoresponse force microscopy is used to quantify the coercive voltage from the phase hysteresis loops for different thickness films to investigate the semi-empirical Kay–Dunn scaling law with varying cobalt concentrations. For the rhombohedral structure, a reduction of the coercive voltage is observed with increasing substitution of Fe by Co. The coercive voltage of a 10 nm BFCO (x=0.35) film is found to be 0.63 V, which is 67% lower than that of a pure BiFeO3 (BFO) (1.9 V) film of the same thickness. Cobalt substitution also leads to changes in the magnetic and electrical properties due to modification of spin ordering and reduction of the bandgap, respectively. Further, to validate the experimental results, we have performed theoretical calculations using density functional theory. The theoretical results indicate a reduction in unit cell volume and enhancement in net magnetization can be achieved with cobalt substitution, in agreement with experimental results. Partial Co substitution can, thus, provide a pathway to realize BFO-based nonvolatile magnetoelectric devices with reduced operating voltage.

Engineering Photoresponse in Epitaxial BiFe0.5Cr0.5O3 Thin Films through Structural Distortion

Multiferroic BiFe0.5Cr0.5O3 (BFCO) thin films are promising candidates for emerging optoelectronics and all-oxide solar absorbers. Yet, a thorough understanding of the structural evolution and associated changes in the functional properties of BFCO is lacking. Here, we explore thickness-dependent structural phase transitions in the epitaxial BFCO films and ascertain the impact of the accompanying crystallographic distortions on their photoresponse. The results show that the strain imposed by the substrate changes the crystal symmetry, inducing a transition from a tetragonal-like phase to a rhombohedral-like phase through a rather complex strain relaxation mechanism upon increasing film thickness. This change in crystallographic distortion also induces a shift of ∼150 meV in the bandgap. Moreover, wavelength-resolved photocurrent measurements reveal that the absorption onset is red-shifted for the tetragonal-like structure, implying light absorption up to wavelengths of 800 nm. First-principles calculations shed further light on the symmetry-induced changes in the electronic structure of the BFCO films. The crystallographic symmetry is shown to be a decisive factor in modifying the valence band maximum and conduction band minimum characteristics in the perovskite oxides, revealing an emerging type of Mott multiferroic in the BFCO system. This work provides a practical strategy to further engineer the optoelectronic properties of the multiferroic oxide films through thickness-induced phase transitions.

The effects of N2 atmosphere annealing on the physical properties of BiFe0.5Mn0.5O3 ceramic

Multiferroic BiFe0.5Mn0.5O3 (BFMO) was elaborated by the conventional solid-state method under ordinary sintering conditions (air) and N2 atmosphere at 1193 K, and the effects of the two different atmospheres on the structural properties were studied. Indeed, it was found that BFMO prepared in the atmospheric air exhibited an impurity phase, BFMO annealed under an N2 atmosphere, displayed an intensity of secondary phase that decreased significantly (As for the Rietveld analysis), indicating that BFMO annealed under N2 flow crystallized in rhombohedral structure with R3c space group. The phase crystallinity was confirmed by Raman spectroscopy. Moreover, a dielectric study revealed the enhancement of dielectric permittivity ε’r and a low dielectric loss. Concerning the electrical hysteresis loop at room temperature, it indicated that BFMO sample exhibited ferroelectric characteristics. Besides, the impedance spectroscopy and conductivity were investigated in the frequency range of 102-106 Hz. Nyquist plots indicated the presence of only grain contributions. Furthermore, the conduction mechanism was evaluated by Ac conductivity using Jonsher low. From the thermal variation of the exponent S, it was concluded that the model of Correlated Barrier Hopping (CBH) and the Non-overlapping Small Polaron Tunneling (NSPT) were the dominant conduction mechanisms present in our sample. From the magnetic measurement, FC and ZFC were reported to show the weak ferromagnetism character of our sample. The obtained physical properties of BFNO at room temperature suggest that this material can be used in technological applications.

Impedance spectroscopy and conduction mechanism of a BiFe0.95Mn0.05O3 thin film

Dielectric response and conduction mechanism were investigated for a multiferroic BiFe0.95Mn0.05O3 epitaxial thin film. A contribution from a thermally activated interface (0.37 eV) and the bulk of the film on the dielectric response were observed through the comparison between experimental results and equivalent circuit model. The low frequency interface relaxation signatures strongly suggest a Maxwell-Wagner space charge origin. The alternative current conductivity deduced from the model follows a power law frequency dependence suggesting a polaronic hopping mechanism while the low frequency limit is in perfect agreement with the direct current conduction mechanism. The current-voltage characteristics were indeed correlated with Schottky-Simmons interface limited transport with activation energy of 0.36 eV, close to the one extracted from the impedance analysis. Such analysis of the electrostatic landscape and dielectric behaviour may help to further understanding the anomalous photo-induced properties in the BiFeO3 system.

Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe_{1-x}Co_{x}O_{3} Solid Solutions near Room Temperature.

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe_{1-x}Co_{x}O_{3} solid solutions with 0.25≤x≤0.50 (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1-1.0GPa, depending on the composition, the electric polarization of bulk BFCO increases by ∼150%. The origin of such an exceptional behavior is a phase transformation involving a low-T rhombohedral (R) phase and a high-T supertetragonal (T) phase. Both R and T phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 μ_{B} per formula unit. Contrary to what occurs in either bulk BiFeO_{3} or BiCoO_{3}, the T phase is stabilized over the R by increasing temperature due to its higher vibrational entropy. This extraordinary T-induced R→T phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.

Stable electric polarization switching accompanied by magnetization reversal in B-site-substituted multiferroic BiFe0.9Co0.1O3 thin films

We report the substitution of manganese or scandium as a third element into the B-site of multiferroic BiFe0.9Co0.1O3 thin film in the ABO3 perovskite structure. Stable electric polarization switching via a biased cantilever was achieved multiple times in the BiFe0.892Co0.1Mn0.008O3 and BiFe0.87Co0.1Sc0.03O3 on SrTiO3 (001) while the piezoresponse force microscope image after the poling was inhomogeneous and was not reversible in BiFe0.9Co0.1O3 thin film on SrTiO3 (001). Although the electric domains are fragmented by Mn substitution, magnetization reversal by electric field was observed in the BiFe0.892Co0.1Mn0.008O3 thin film on SrTiO3 (001) as well as in the BiFe0.9Co0.1O3 on GdScO3 (110) studied previously.

Exchange bias effect in bulk multiferroic BiFe0.5Sc0.5O3

Below the Néel temperature, TN ∼ 220 K, at least two nano-scale antiferromagnetic (AFM) phases coexist in the polar polymorph of the BiFe0.5Sc0.5O3 perovskite; one of these phases is a weak ferromagnetic. Non-uniform structure distortions induced by high-pressure synthesis lead to competing AFM orders and a nano-scale spontaneous magnetic phase separated state of the compound. Interface exchange coupling between the AFM domains and the weak ferromagnetic domains causes unidirectional anisotropy of magnetization, resulting in the exchange bias (EB) effect. The EB field, HEB, and the coercive field strongly depend on temperature and the strength of the cooling magnetic field. HEB increases with an increase in the cooling magnetic field and reaches a maximum value of about 1 kOe at 5 K. The exchange field vanishes above TN with the disappearance of long-range magnetic ordering. The effect is promising for applications in electronics as it is large enough and as it is tunable by temperature and the magnetic field applied during cooling.

Impedance spectrum and magnetic properties of BiFe0.95Ti0.05O3 ceramics

Multiferroic BiFe0.95Ti0.05O3 (BFTO) powders were synthesized by sol–gel method, and then compacted by spark plasmas sintering (SPS) at 650 °C. Analysis of structure and morphology suggested the formation of a distorted rhombohedral in BFTO ceramics with a homogeneous distribution in grain sizes. BFTO ceramics revealed electric homogenous properties verified from the plots of complex impedance versus frequency. The contribution of oxygen vacancies in the behaviors of dielectric and ac conductivity was analyzed. At high temperature (473 K), the dielectric relaxation and ac conductivity were mainly attributed to the motion of oxygen vacancies. Doping of Ti4+ in BiFeO3 ceramics showed a high remanent magnetization due to the structural evolution and the broken of the spiral magnetic ordering in BiFeO3 system.

Effect of the Synthetic Method on the Formation of BiFe0.93Mn0.07O3 Multiferroic Agglomerates

Multiferroic BiFe0.93Mn0.07O3 was prepared by ultrasonic spray pyrolysis (USP), high-temperature self-propagating synthesis (SPS), and thermohydrolysis (TH). The synthetic method was shown to impact the sizes, shapes, and mutual arrangement of particles in agglomerates. Magnetic properties were found to depend on the morphology of agglomerates. Ultrasonic spray pyrolysis produced porous spherical particles that had record-breaking values of coercive force.

Changes in the Magnetic Structure of Multiferroic BiFe0.80Cr0.20O3 with Temperature

The results of a Mossbauer study of the magnetic structure of multiferroic BiFe0.80Cr0.20O3 in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the G type is present in BiFe0.80Cr0.20O3 at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at T ~ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Neel temperature, TN = 505 ± 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T ~ 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe0.80Cr0.20O3 multiferroic.

Температурные изменения магнитной структуры мультиферроика BiFe-=SUB=-0.80-=/SUB=-Cr-=SUB=-0.20-=/SUB=-O-=SUB=-3-=/SUB=-

AbstractThe results of a Mössbauer study of the magnetic structure of multiferroic BiFe_0.80Cr_0.20O_3 in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the G type is present in BiFe_0.80Cr_0.20O_3 at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at T ~ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Néel temperature, T _N = 505 ± 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T ~ 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe_0.80Cr_0.20O_3 multiferroic.

Oxygen vacancies induced structural distortions, valence fluctuations and enhanced optical properties in BiFe0.83Ni0.17O3

The present work reports the impact of the oxygen vacancies (OVs) created in multiferroic BiFe0.83Ni0.17O3 in both a qualitative and quantitative manner by employing first principles density functional theory calculations. We have investigated the role of OVs in modulating the structural and optical properties of BiFe0.83Ni0.17O3. Our results have shown that the higher concentration of OVs increases the electron hopping process which ultimately results in the presence of Fe2+ and Fe3+ and the increasing trend of lattice parameters is due to the existence of larger ionic radius Fe2+ in addition to Fe3+ ions. More distortion was realized for the NiO6 octahedron when the OVs were created farther from the Ni ion resulting in changes in the electronic state by decreasing band gaps and providing conductivity. Considerably less distortion was observed when OVs were created nearer to the Ni ion, increasing the band gap values in both up and down spin channels. With respect to optical properties, we conclude that OVs, when created nearer to the Ni ion, increase the absorption behavior in a significant manner. The observed refractive index values are known to be one of the highest values known in the UV-visible range of the spectrum and thus the existence of OVs can drive this material towards applications in optical communication devices. The maximum dielectric constant values of 9–10.5 are observed in all the configurations in the UV and visible region of the electromagnetic spectrum.

Unusual magnetic properties of the polar orthorhombic BiFe0.5Sc0.5O3 perovskite

Magnetic properties of new multiferroic BiFe0.5Sc0.5O3 perovskite which exists in two different orthorhombic polymorphs, the antipolar Pnma and the polar Ima2, were studied using a SQUID-magnetometer technique in magnetic fields up to 50 kOe in the temperature range of 5–300 K. Both polymorphs show a weak ferromagnetic component below the Néel temperature, TN∼220 K. A wide cusp-like anomaly of the temperature dependent magnetic susceptibility was revealed below 200 K. The anomaly is dependent on the magnetic field applied and can be associated with the glassy magnetic behavior of the compound. The best fit of the initial magnetization curves taken at each temperature can be obtained as a sum of two contributions, namely the nonlinear in field and the linear one. The nonlinear contribution appeared to be temperature and magnetic field dependent, which is in a good agreement with the magnetization experimentally obtained as a function of temperature in different magnetic fields applied. Increase of external magnetic field leads to a decrease of the nonlinear contribution to the total magnetic moment.

Occurrence of spintronics behaviour (half-metallicity, spin gapless semiconductor and bipolar magnetic semiconductor) depending on the location of oxygen vacancies in BiFe0.83Ni0.17O3.

The current communication signifies the effect of oxygen vacancies (OVs) both qualitatively and quantitatively in multiferroic BiFe0.83Ni0.17O3 by an in-depth atomic-level investigation of its electronic structure and magnetization properties, and these materials have a variety of applications in spintronics, optoelectronics, sensors and solar energy devices. Depending on the precise location of OVs, all the three types of spintronic material namely half-metallic, spin gapless semiconductor and bipolar magnetic conductor have been established in a single material for the first time and both super-exchange and double-exchange interactions are possible in accordance with the precise location of OVs. We have also calculated the vacancy formation energies to predict their thermodynamic stabilities. These results can highlight the impact and importance of OVs that can alter the multiferroic properties of materials.

Local Piezoresponse and Electrical Behavior of Multiferroic BiFe0.95Mn0.05O3 Epitaxial Thin Films Deposited Under Different Oxygen Pressure

Multiferroic BiFe0.95Mn0.05O3 (BFMO) epitaxial thin films were grown on SrRuO3, SrTiO3 and TiN-buffered (001)-oriented Si substrates under different oxygen pressure by pulsed laser deposition. The influence of oxygen pressure on the microstructures, domain configurations and their local piezoresponse and electrical behaviors were investigated by the piezoresponse force microcopy and conductive atomic force microscopy. BFMO thin film at low oxygen pressure was found to demonstrate pure phase state, inhomogeneous piezoresponse and low leakage behavior with diode-like behavior, while high oxygen pressure leads to impurity Bi2O3 phase-enclosed BFMO films with higher leakage current above 2 nA, revealing the importance of the oxygen pressure in governing the physical properties of BFMO films.

Optimization of the multiferroic BiFeO3 thin films by divalent ion (Mn, Ni) co-doping at B-sites

Multiferroic BiFe0.96−xMn0.04NixO3 (BFMNO) thin films were prepared on fluorine doped tin oxide (FTO) substrates by the chemical solution deposition (CSD) method. X-ray diffraction, Rietveld refinement and Raman spectra reveal the structural transition from rhombohedral (R3c:H) to the biphasic structure (R3c:H+R3m:R) with Mn/Ni co-doping. The X-ray photoelectron spectroscopy analysis confirms the coexistence of Fe3+ and Fe2+ ions in the thin films. The lower leakage current density (8.1×10−6A/cm2 at 300kV/cm) via co-doping is obtained. The two-phase coexistence of BFMN3O gives rise to the superior ferroelectric (2Pr=226.4μC/cm2) and the enhanced ferromagnetic properties (Ms=15.9emu/cm3).

High ferromagnetic transition temperature in multiferroic BiFe0.95Ni0.05O3 compound

We have prepared BiFe0.95Ni0.05O3 by the standard solid-state reaction and rapid sintering. The structural analysis by x-ray diffraction for BiFe0.95Ni0.05O3 shows rhombohedral perovskite structure (R3c), and lattice constants a and c of BiFe0.95Ni0.05O3 are slightly increased as compared to those of BiFeO3. The Raman studies present the increase of oxygen vacancy by the Ni2+ substitution. The magnetic properties of BiFe0.95Ni0.05O3 are greatly improved, owing to the enhanced ferromagnetic interaction caused by the Fe/Ni coupling. BiFe0.95Ni0.05O3 sample is ferromagnetic with a cluster-glass state unlike the antiferromagnetic BiFeO3-based compounds.

High-temperature phases of multiferroic BiFe0.7Mn0.3O3

We report the results of a high-resolution powder neutron-diffraction study of multiferroic BiFe${}_{0.7}$Mn${}_{0.3}$O${}_{3}$. We have confirmed the previous assignment of the $\ensuremath{\alpha}$ phase and $\ensuremath{\beta}$ phase as having $R3c$ and $Pbnm$ symmetry, respectively. The $\ensuremath{\gamma}$ phase, however, has been shown unequivocally not to be cubic, as previously reported, but rather to retain octahedral tilting distortions leading to a lower symmetry, most likely rhombohedral with space group $R\overline{3}c$. The $\ensuremath{\gamma}$ phase of BiFe${}_{0.7}$Mn${}_{0.3}$O${}_{3}$ is therefore different than that of the parent compound BiFeO${}_{3}$, which retains an orthorhombic structure up to its decomposition temperature. The lattice parameters of the $\ensuremath{\gamma}$ phase of BiFe${}_{0.7}$Mn${}_{0.3}$O${}_{3}$ are pseudocubic throughout its entire stability range, despite the fact that there are still significant distortions from the cubic aristotype structure even at the onset of decomposition.