Electric Polarization Induced by a Proper Helical Magnetic Ordering in a Delafossite Multiferroic CuFe1-xAlxO2

Multiferroic CuFe1-xAlxO2 (x=0.02) exhibits a ferroelectric ordering accompanied by a proper helical magnetic ordering below T=7K under zero magnetic field. By polarized neutron diffraction and pyroelectric measurements, we have revealed a one-to-one correspondence between the spin helicity and the direction of the spontaneous electric polarization. This result indicates that the spin helicity of the proper helical magnetic ordering is essential for the ferroelectricity in CuFe1-xAlxO2. The induction of the electric polarization by the proper helical magnetic ordering is, however, cannot be explained by the Katsura-Nagaosa-Balatsky model, which successfully explains the ferroelectricity in the recently explored ferroelectric helimagnets, such as TbMnO3. We thus conclude that CuFe1-xAlxO2 is a new class of magnetic ferroelectrics.Comment: 4 pages, 4 figure

Anisotropic magnetic field responses of ferroelectric polarization in a trigonal multiferroic CuFe1-xAlxO2 (x=0.015)

We have investigated magnetic field dependences of a ferroelectric incommensurate-helimagnetic order in a trigonal magneto-electric (ME) multiferroic CuFe1-xAlxO2 with x=0.015, which exhibits the ferroelectric phase as a ground state, by means of neutron diffraction, magnetization and dielectric polarization measurements under magnetic fields applied along various directions. From the present results, we have established the H-T magnetic phase diagrams for the three principal directions of magnetic fields; (i) parallel to the c axis, (ii) parallel to the helical axis, and (iii) perpendicular to the c and the helical axes. While the previous dielectric polarization (P) measurements on CuFe1-xGaxO2 with x=0.035 have demonstrated that the magnetic field dependence of the `magnetic domain structure' results in distinct magnetic field responses of P [S. Seki et al., Phys. Rev. Lett., 103 237601 (2009)], the present study have revealed that the anisotropic magnetic field dependence of the ferroelectric helimagnetic order `in each magnetic domain' can be also a source of a variety of magnetic field responses of P in CuFe1-xAxO2 systems (A=Al, Ga).Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.

The complex multiferroic phase diagram of Mn1−x_{1-x}Cox_xWO4_4

The complete magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_{x}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, and polarization experiments. We show that the ferroelectric polarization $\overrightarrow{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x$_{c1}$=0.075, $\overrightarrow{P}$ rotates from the $b-$axis into the $a-c$ plane and at x$_{c2}$=0.15 it flips back to the $b-$axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the ferroelectric polarization is derived by taking into account the intrachain ($c-$axis) as well as the interchain ($a-$axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1$\leq$x$\leq$0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO$_4$.Comment: 18 pages, 6 figure

Field induced ferroelectric state in frustrated magnet CuFe1 xAlxO2

By magnetization and neutron diffraction measurements in an applied magnetic field up to 15 T, we obtained the field H versus temperature T magnetic phase diagram of a geometrically frustrated triangular lattice antiferromagnet CuFe1 amp; 8722;xAlxO2 with x 0.012, and found that the first field induced incommensurate noncollinear magnetic phase of CuFeO2 accompanying spontaneous electric polarization continuously connects, in the H T phase diagram for x 0.012, with the impurity induced low temperature LT phase, in which the spontaneous polarization has recently been observed even in zero field. By pyroelectric measurements on the x 0.012 sample in zero magnetic field, we also demonstrated that the thermal evolution of ferroelectric polarization corresponds to that of LT magnetic orderin