Showing papers on "Magnetocapacitance published in 2006"

Multiferroic and magnetoelectric materials

Abstract: A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Magnetocapacitance without magnetoelectric coupling

Abstract: The existence of a magnetodielectric (magnetocapacitance) effect is often used as a test for multiferroic behavior in new material systems. However, strong magnetodielectric effects can also be achieved through a combination of magnetoresistance and the Maxwell-Wagner effect, unrelated to true magnetoelectric coupling. The fact that this resistive magnetocapacitance does not require multiferroic materials may be advantageous for practical applications. Conversely, however, it also implies that magnetocapacitance per se is not sufficient to establish that a material is multiferroic.

Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr2S4

Abstract: We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel ${\mathrm{HgCr}}_{2}{\mathrm{S}}_{4}$. Similar to the findings in ferromagnetic ${\mathrm{CdCr}}_{2}{\mathrm{S}}_{4}$, the dielectric constant of ${\mathrm{HgCr}}_{2}{\mathrm{S}}_{4}$ becomes strongly enhanced in the region below 60--80 K, which can be ascribed to polar relaxational dynamics triggered by the onset of ferromagnetic correlations. In addition, the observation of polarization hysteresis curves indicates the development of ferroelectric order below about 70 K. Moreover, our investigations in external magnetic fields up to 5 T reveal the simultaneous occurrence of magnetocapacitance and magnetoresistance of truly colossal magnitudes in this material.

Effect of ferroelectric layers on the magnetocapacitance properties of superlattices-based oxide multiferroics

Abstract: A series of superlattices composed of ferromagnetic La0.7Ca0.3MnO3 (LCMO) and ferroelectric/paraelectric Ba1−xSrxTiO3 (0⩽x⩽1) were deposited on SrTiO3 substrates using pulsed laser deposition. Magnetotransport properties of the films reveal a ferromagnetic Curie temperature in the range of 145–158K, and negative magnetoresistance as high as 30%, depending on the type of ferroelectric layers employed for their growth (i.e., “x” value). Ferroelectricity at temperatures ranging from 55Kto105K is also observed, depending on the barium content. More importantly, the multiferroic nature of the film is determined by the appearance of negative magnetocapacitance, which is maximum around the ferroelectric transition temperature (3% per tesla). These results are understood based on the role of the ferroelectric/paraelectric layers and strains in inducing the multiferroism.

Enhancement and inverse behaviors of magnetoimpedance in a magnetotunneling junction by driving frequency

Abstract: The magnetoimpedance effect was employed to study magnetotunneling junction (MTJ) with the structure of Ru(5nm)∕Cu(10nm)∕Ru(5nm)∕IrMn(10nm)∕CoFeB(4nm)∕Al(1.2nm)-oxide∕CoFeB(4nm)∕Ru(5nm). A huge change of more than ±17000% was observed in the imaginary part of the impedance between the magnetically parallel and antiparallel states of the MTJ. The inverse behavior of the magnetoimpedance (MI) loop occurs beyond 21.1MHz; however, the normal MI at low frequency and the inverse MI at high frequency exhibit the same magnetization reversal as checked by the Kerr effect. The reversal in MI was due to the dominance of magnetocapacitance at high frequency.

Positive and negative magnetocapacitance in magnetic nanoparticle systems

Abstract: The dielectric properties of MnFe2O4 and γ-Fe2O3 magnetic nanoparticles embedded in insulating matrices were investigated The samples showed frequency dependent dielectric anomalies coincident with the magnetic blocking temperature and significant magnetocapacitance above this blocking temperature, as large as 04% at H=10kOe For both samples the magnetic field induced change in dielectric constant at high temperatures was proportional to the square of the sample magnetization These measurements suggest that the dielectric properties of magnetic nanoparticles are closely related to the disposition of magnetic moments in the system This magnetodielectric coupling is believed to arise from extrinsic effects, which are discussed in light of recent work relating magnetoresistive and magnetocapacitive behaviors

Positive and negative magnetocapacitance in magnetic nanoparticle systems

Abstract: The dielectric properties of MnFe$_2$O$_4$ and $\gamma$-Fe$_2$O$_3$ magnetic nanoparticles embedded in insulating matrices were investigated. The samples showed frequency dependent dielectric anomalies coincident with the magnetic blocking temperature and significant magnetocapacitance above this blocking temperature, as large as 0.4% at H = 10kOe. For both samples the magnetic field induced change in dielectric constant was proportional to the square of the sample magnetization. These measurements suggest that the dielectric properties of magnetic nanoparticles are closely related to the disposition of magnetic moments in the system. As neither bulk gamma-Fe2O3 nor MnFe2O3 are magnetoelectric materials, this magnetodielectric coupling is believed to arise from extrinsic effects which are discussed in light of recent work relating magnetoresistive and magnetocapacitive behavior.

Magnetodielectric effect of Bi6Fe2Ti3O18 film under an ultra-low magnetic field

Abstract: Good-quality and fine-grain Bi6Fe2Ti3O18 magnetic ferroelectric films with single-phase layered perovskite structure have been prepared successfully via the metal organic decomposition (MOD) method. The results of low-temperature magnetocapacitance measurements reveal that an ultra-low magnetic field of 10 Oe can produce a non-trivial magnetodielectric response in zero-field cooling conditions, and the relative variation of dielectric constants in a magnetic field is positive, i.e. [er(H)− er(0)]/er(0) = 0.05, when T<55 K, but negative with a maximum of [er(H)− er(0)]/er(0) = −0.14 when 55 K

Role of the spin-pair correlation in magnetic relaxor ferroelectrics

Abstract: In magnetic relaxor ferroelectrics, for the coupling interaction between the relaxor ferroelectricity and magnetic order, the dielectric susceptibility exhibits a sharp increase around the magnetic phase transition temperature. By adding an appropriate coupling term between the electrical and magnetic subsystems, which is related to the interaction of the Heisenberg spin and the order parameter field, the replica theory based on the spherical random-bond-random-field model and the mean-field theory based on the Heisenberg model are successfully applied to the magnetic relaxor ferroelectrics. We find that the fluctuation of the spin-pair correlation plays an important role in the change of dielectric susceptibility around the magnetic phase transition temperature. The obtained static dielectric susceptibility and the magnetocapacitance are in good agreement with the experimental results.

Comments on "Giant Dielectric Response in the One-Dimensional Charge-Ordered Semiconductor (NbSe_{4})_{3}I" and "Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr_{2}S_{4}"

Abstract: Comments on "Giant Dielectric Response in the One-Dimensional Charge-Ordered Semiconductor (NbSe_{4})_{3}I" (D. Staresinic et al., Phys. Rev. Lett. 96, 046402 (2006)) and "Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr_{2}S_{4}" (S. Weber et al., Phys. Rev. Lett. 96, 157202 (2006))

Giant Dielectric Permittivity and Colossal Magnetocapacitance Effect in Complex Manganites with High Conductivity

Abstract: The temperature dependences of the dielectric properties and conductivity of the La 1 − x Sr x MnO3 single crystals (x = 0.1, 0.11, 0.125) are investigated in a wide temperature range. It is found that the giant dielectric permittivity arises due to the pre-percolation regime of the charge separation. These single crystals display also the colossal magnetocapacitance effect. The features of the temperature dependence of these dielectric properties are explained by the nanoscale dynamic inhomogeneties appearing due to the charge and spin ordering.