[en] The magneto-resistivity and electric field-current density (E-J) curves were investigated up to a magnetic field 9 T in the optimally doped BaFe_1.8Co_0.2As₂ single crystal with a superconducting temperature T_c of 24.6 K. The E-J Scaling behaviors below and above vortex glass transition temperature T_g were found, confirming the existence of the vortex glass phase transition. The critical exponents for the diverging spatial and time correlations at T_g, were obtained as v = 1.1±0.1 and z = 4.5±0.3, respectively. The obtained critical exponents are in good agreement with the predicted values of v ∼ 1 - 2 and z > 4 within the 3D vortex glass theory.

[en] In order to investigate whether magnetism and superconductivity coexist in Co-doped SrFe₂As₂, we have prepared single crystals of SrFe_2-xCo_xAs₂, x = 0 and 0.4, and characterized them via x-ray diffraction, electrical resistivity in zero and applied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility. At x = 0.4, there is both magnetic and resistive evidence for a spin density wave transition at 120 K, while T_c = 19.5 K-indicating coexistent magnetism and superconductivity. A discussion of how these results compare with reported results, both in SrFe_2-xCo_xAs₂ and in other doped 122 compounds, is given. (fast track communication)

[en] We report the observation of multiferroicity in a clinopyroxene NaFeGe₂O₆ polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T_N1 = 13 K, a second magnetic transition appears at T_N2 = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T_N2 rather than T_N1 and its magnitude increases up to ∼13 μC m^-2 at 5 K, supporting the idea that the ferroelectric state in NaFeGe₂O₆ stems from a helical spin order stabilized below T_N2. When a magnetic field of 90 kOe is applied, the electric polarization decreases to 9 μC m^-2 and T_N2 slightly increases by 0.5 K. At intermediate magnetic fields, around 28 and 78 kOe, anomalies in the magnetoelectric current, magnetoelectric susceptibility, and field derivative of magnetization curves are found, indicating field-induced spin-state transitions. Based on these electrical and magnetic properties, we provide a detailed low temperature phase diagram up to 90 kOe, and discuss the nature of each phase of NaFeGe₂O₆. (paper)

[en] Electrical transport across lateral geometrical nanoconstrictions realized in 100 nm thick GaMnAs epifilms is studied. The constrictions are patterned with the aid of chemical etching techniques, as opposed to plasma-assisted methods. Transport behavior across the constrictions, where domain walls can be formed and pinned, changes from Ohmic to non-Ohmic below temperatures corresponding to epifilm T_C for junctions with high resistances. Magnetoresistance measurements across such junctions qualitatively show similar behavior to unpatterned epifilms attributable to anisotropic magnetoresistance. The experimental IV curves are in good agreement with theoretical models accounting for spin flop across a region of high resistance

[en] BiMnFe_2O_6 exhibits a spiral antiferromagnetic ordering below 212 K and a reentrant spin glass transition at 34 K. Further, magnetic and dielectric anomalies occur at the same temperature (T = 170 K) with a significant magnetodielectric effect. Upon substitution of Sm"3"+ for Bi"3"+ ions in Bi_1_−_xSm_xMnFe_2O_6 (x = 0.1 and 0.2), the dielectric anomaly shifts to low temperatures (T = 135 and 72 K, respectively), whereas the magnetic anomaly develops into a weak ferromagnetism. For x = 0.2, the weak ferromagnetism occurs in a wide temperature range (90–201 K). Below 90 K, it undergoes a transition to an antiferromagnetic state. In contrast to the parent compound (x = 0), the magnetodielectric effect is observed both in the antiferromagnetic region (T < 90 K) with a maximum at the dielectric anomaly (72 K) and also in the weak ferromagnetic region. It has been shown that the magnetodielectric effect in the antiferromagnetic region has an intrinsic capacitive origin while that observed at the weak ferromagnetic region originates from magnetoresistance

[en] The magnetoelectric properties of hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂ are significantly improved by Al substitution and thermal annealing. Measuring the enhancement factor of ⁵⁷Fe NMR, we found direct microscopic evidence that the magnetic moments of the L and S blocks are rotated by a magnetic field in such a way as to increase the net magnetic moment of a magnetic unit, even after the field is removed. Al substitution makes magnetoelectric property arise easily by suppressing the easy-plane anisotropy. The effect of thermal annealing is to stabilize the multiferroic state by reducing the number of pinning sites and the electron spin fluctuation. The transverse conic structure gradually changes to the alternating longitudinal conic structure where spins fluctuate more severely. (paper)

[en] The magnetocaloric effect (MCE) associated with the spin transitions of alternating longitudinal conical (ALC)–mixed conical (MC) and MC–ferrimagnetic (FIM) states in a Ba_0.3Sr_1.7Co₂Fe₁₂O₂₂ single crystal has been investigated. For magnetic field directions applied along either the [120] or [001] directions, the crystal is found to exhibit the conventional and inverse MCE near the ALC–MC (T_N1 = 235 K) and MC–FIM (T_N2 = 348 K) states, respectively. The dependence of the magnetic entropy on the magnetic field also exhibits such sign change behaviors in the MCE, which is attributed to the magnetic field induced gradual collapse of heliconical magnetic order. (paper)

[en] Using single crystal neutron diffraction, we confirmed the existence of the commensurate transverse helical ordering in ${\mathrm{B}\mathrm{a}}_{0.52}{\text{Sr}}_{2.48}{\mathrm{C}\mathrm{o}}_2{\mathrm{F}\mathrm{e}}_{24}{\mathrm{O}}_{41}$ upon zero-field cooling, in which room-temperature magnetoelectric coupling has been previously observed. Its magnetic structure remained commensurate to the lattice under the external field perpendicular to the $c$ axis. We also found the commensurate ordering to be directly responsible for the electric polarization, which is the common feature of many types of hexaferrites independently of their zero-field orderings.

[en] Cobalt (Co) and tantalum (Ta) co-substituted BiFeO₃ polycrystalline ceramics were prepared by a solid-state reaction and their magnetic and dielectric properties were investigated. Magnetic hysteresis loops were clearly observed in co-substituted specimens and magnetization was greatly improved. The co-substitution decreased the electrical conductivity by six orders of magnitude along with the reduction of grain size. The magnetoelectric coupling was estimated in co-substituted BiFeO₃ by determining the changes of the dielectric constant with an external magnetic field.

[en] Whether or not a large ferroelectric polarization P exists in the orthorhombic HoMnO₃ with E-type antiferromagnetic spin ordering remains one of the unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty in determining the P of polycrystalline specimens, namely that conventional pyroelectric current measurements performed after a poling procedure under high dc electric fields are subject to large errors due to the problems caused by leakage currents or space charges. To overcome the difficulty, we employed the positive-up negative-down (PUND) method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO₃ specimens below their Neel temperatures. We found that all the HoMnO₃ samples had similar remnant polarization P_r values at each temperature, regardless of their variation in resistivity, dielectric constant and pyroelectric current levels. Moreover, the P_r value of ∼0.07 μC cm^-2 at 6 K is consistent with the P value obtained from the pyroelectric current measurement performed after a short pulse poling. Our findings suggest that the intrinsic P of polycrystalline HoMnO₃ can be determined through the PUND method and P at 0 K may reach ∼0.24 μC cm^-2 in a single crystalline specimen. This P value is still much smaller than the theoretically predicted one but is one of the largest observed in magnetism induced ferroelectrics.