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Register a new userMagnetic properties of BiFeO3 micro-cubes synthesized by microwave agitation
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Added by
Bartlomiej Andrzejewski
Publication date
2012
fields
Physics
and research's language is
English
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In this report we present results of magnetization measurements and investigation of aging and memory effect in bismuth ferrite multiferroic micro-cubes obtained by means of simple microwave synthesis procedure. It is found that difference between FC and ZFC magnetizations appears at the temperature of freezing of ferromagnetic domain walls. The decay of the magnetic moment vs. time described by power-law relation and the absence of memory effect indicate domain growth mechanism rather than the spin-glass phase.
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The process of magnetic relaxation was studied in bismuth ferrite BiFeO3 multiferroic micro-cubes obtained by means of microwave assisted Pechini process. Two different mechanisms of relaxation were found. The first one is a rapid magnetic relaxation driven by the domain reorientations and/or pinning and motion of domain walls. This mechanism is also responsible for the irreversible properties at low temperatures. The power-law decay of the magnetic moment confirms that this relaxation takes place in the system of weakly interacting ferromagnetic or superferromagnetic domains. The second mechanism is a longterm weak magnetic relaxation due to spin glass-phase.
Magnetoelectric multiferroic composite of two types of multiferroic (Type I and II) consisting BiFeO3 and TbMnO3 is studied for enhanced magnetic and transport properties. A narrower band gap is estimated from the UV-visible absorption spectrum from that of BiFeO3 and TbMnO3. With known value of band gap, the band structure was estimated from the valence band x-ray photoemission spectra (XPS) and ultra violet photoemission spectra (UPS). The valence and conduction band was found at 1.0 eV and 0.45 eV above and below the Fermi level respectively. Thus the insulating behavior of the system is understood from the reconstruction of the energy bands at the interface which happens due to lattice mismatch of the two materials. The large coercivity and the increase on the magnetization value are understood to be due to superexchange interaction between different Mn ions (Mn2+, Mn3+ and Mn4+). From the composition study of EDXA and core level x-ray photoemission spectra oxygen vacancy was found which in turn creates the mixed valence state of Mn to maintain the charge neutrality.
The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO$_3$ were measured for temperatures $200 leq T leq 750,$K, using inelastic neutron scattering (INS). Our results indicate that the magnetic spectrum of BiFeO$_3$ closely resembles that of similar Fe perovskites, such as LaFeO$_3$, despite the cycloid modulation in BiFeO$_3$. We do not find any evidence for a spin gap. A strong $T$-dependence of the phonon DOS was found, with a marked broadening of the whole spectrum, providing evidence of strong anharmonicity. This anharmonicity is corroborated by large-amplitude motions of Bi and O ions observed with neutron diffraction. These results highlight the importance of spin-phonon coupling in this material.
First principles electronic structure calculations have been carried out on ordered double perovskites Sr_2BBO_6 (for B = Cr or Fe and B 4d and 5d transition metal elements) with increasing number of valence electrons at the B-sites, and on Ba_2MnReO_6 as well as Ba_2FeMoO_6. The Curie temperatures are estimated ab initio from the electronic structures obtained with the local spin-density functional approximation, full-potential generalized gradient approximation and/or the LDA+U method (U - Hubbard parameter). Frozen spin-spirals are used to model the excited states needed to evaluate the spherical approximation for the Curie temperatures. In cases, where the induced moments on the oxygen was found to be large, the determination of the Curie temperature is improved by additional exchange functions between the oxygen atoms and between oxygen and B and B atoms. A pronounced systematics can be found among the experimental and/or calculated Curie temperatures and the total valence electrons of the transition metal elements.
The ZFC and FC magnetization dependence on temperature was measured for BiFeO3 ceramics at the applied magnetic field up to H=10T in 2K-1000K range. The antiferromagnetic order was detected from the hysteresis loops below the Neel temperature TN=646K. In the low magnetic field range there is an anomaly in M(H), probably due to the field-induced transition from circular cycloid to the anharmonic cycloid. At high field limit we observe the field-induced transition to the homogeneous spin order. From the M(H) dependence we deduce that above the field Ha the spin cycloid becomes anharmonic which causes nonlinear magnetization, and above the field Hc the cycloid vanishes and the system again exhibits linear magnetization M(H). The anomalies in the electric properties, which are manifested within the 640K-680K range, coincide to the anomaly in the magnetization M(T) dependence, which occurs in the vicinity of TN. We propose to ascribe this coincidence to the critical behaviour of the chemical potential, related to the magnetic phase transition.
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