Commensurate Magnetism Research Articles

Metastable and localized Ising magnetism in α−CoV2O6 magnetization plateaus

$\alpha$-CoV$_{2}$O$_{6}$ consists of $j_{\mathrm{eff}}={1 \over 2}$ Ising spins located on an anisotropic triangular motif with magnetization plateaus in an applied field. We combine neutron diffraction with low temperature magnetization to investigate the magnetic periodicity in the vicinity of these plateaus. We find these steps to be characterized by metastable and spatially short-range ($\xi\sim$ 10 $\r{A}$) magnetic correlations with antiphase boundaries defining a local periodicity of $\langle \hat{T}^{2} \rangle =\ \uparrow \downarrow$ to $\langle \hat{T}^{3} \rangle =\ \uparrow \uparrow \downarrow$, and $\langle \hat{T}^{4} \rangle=\ \uparrow \uparrow \downarrow \downarrow$ or $\uparrow \uparrow \uparrow \downarrow$ spin arrangements. This shows the presence of spatially short range and metastable/hysteretic, commensurate magnetism in Ising magnetization steps.

Structural dimerization in the commensurate magnetic phases ofNaFe(WO4)2andMnWO4

The structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related MnWO$_4$ and NaFe(WO$_4$)$_2$. Both systems exhibit a competition of incommensurate spiral and commensurate spin up-up-down-down ordering along the magnetic chains. In the latter commensurate phases, the alternatingly parallel and antiparallel arrangement of Fe$^{3+}$ respectively Mn$^{2+}$ moments leads to sizeable bond-angle modulation and thus to magnetic dimerization. For NaFe(WO$_4$)$_2$ this structural distortion has been determined to be strongest for the low-field up-up-down-down arrangement, and the structural refinement yields a bond-angle modulation of $\pm 1.15(16)$ degrees. In the commensurate phase of MnWO$_4$, superstructure reflections signal a comparable structural dimerization and thus strong magneto-elastic coupling different to that driving the multiferroic order. Pronounced anharmonic second- and third-order reflections in the incommensurate and multiferroic phase of MnWO$_4$ result from tiny commensurate fractions that can depin multiferroic domains.

Classification and Interpretation of the Polarization of MultiferroicRMn2O5

We have collected and compared temperature-dependent polarization data of multiferroic R Mn 2 O 5 for 11 kinds of rare earths ( R ) from the literature and our own measurements. Although the reported data are inconsistent, it was found that the maximum polarization values for R =Y, Dy, Ho, Er, and Tm are almost equal above 30 K except for the phase transition temperature where the polarization disappears. The findings suggest a standard polarization value for R Mn 2 O 5 of 100–120 nC/cm 2 at 30 K in a commensurate magnetic (CM) phase. We discuss the nature and issues of the measured polarization of R Mn 2 O 5 from the data and our detailed measurement results for YMn 2 O 5 . Temperature-dependent ferroelectric hysteresis loops of YMn 2 O 5 by the double-wave method reveal that an antiferroelectric-like high-temperature incommensurate magnetic (ICM) phase and a ferroelectric low-temperature ICM phase, while the intermediate CM phase is ferrielectric.

Magnetically Induced Ferroelectricity in Multiferroic Compounds of RMn2O5

A review is given on the studies of microscopic magnetic properties and macroscopic dielectric properties as functions of temperature, magnetic field, and the variety of R 3 + (R = rare-earth, Bi, and Y) ions in the multiferroic RMn 2 O 5 compounds by neutron diffraction and dielectric measurements. We have observed successive dielectric and magnetic phase transitions, which concomitantly takes place and a so-called “multiferroics” is realized. In this system, the ferroelectricity is magnetically induced and the commensurate magnetism is indispensable to produce a spontaneous electric polarization. The microscopic origin of the magnetically induced ferroelectricity and the role of rare-earth magnetic moment for a colossal magnetoelectric effect in RMn 2 O 5 will be discussed in the viewpoint of the magnetic structure.

Rapid Suppression of the Commensurate Magnetic Ordered Phase of PrB6 by La Doping

We have studied the magnetization, electrical resistivity and magnetic phase diagram of Pr x La 1- x B 6 . The x dependence of the incommensurate (IC) magnetic ordering temperature, T IC1 , is simi...

Neutron Diffraction Study of Successive Magnetic Phase Transitions in Ferroelectric TbMn2O5

Neutron diffraction measurements have been performed on a single crystal of TbMn 2 O 5 to elucidate the interplay between magnetic ordering and ferroelectricity. On cooling below T N1 = 42 K, the system enters a lock-in commensurate magnetic (CM) state with the magnetic propagation vector q = (1/2,0,1/4) from a two-dimensionally modulated (2D) incommensurate magnetic (ICM) one with the temperature-dependent q = ( q x ,0, q z ). Unlike isomorphous YMn 2 O 5 and ErMn 2 O 5 where a 1D-ICM phase with q = ( q x ,0,1/4) appears and separates both 2D-ICM and CM phases, the CM phase coexists with the 2D-ICM one at temperatures between T = 36.5 and 37.8 K, associated with a kink and a sharp peak in the dielectric constant along the b axis. On further cooling, the system reenters an ICM state at T = 22.4 K whose q is pinned at (20/41,0,5/16) at low temperatures. The comparison between the temperatures of dielectric anomalies and magnetic phase transitions indicates that the formation of the CM ordering is related t...

Reinvestigation of Simultaneous Magnetic and Ferroelectric Phase Transitions in YMn2O5

Magnetic ordering around the ferroelectric transition temperature T C1 of weakly ferroelectric YMn 2 O 5 has been reinvestigated by single-crystal neutron diffraction and dielectric measurements. Besides a two-dimensionally modulated incommensurate magnetic (2D-ICM) phase with the propagation vector q = ( q x ,0, q z ) and a lock-in commensurate magnetic (CM) one with q = (1/2,0,1/4) reported previously, we observed a 1D-ICM phase characterized by q = ( q x ,0,1/4) with q x ∼0.492 that appears in a narrow temperature interval of ∼1 K and separates the 2D-ICM and CM phases. Corresponding to the 2D-ICM-to-1D-ICM and 1D-ICM-to-CM phase transitions at T = 40.8 K and 40.0 K, a sharp peak and a shoulder in the dielectric constant along the b -axis were detected. These results suggest that the ferroelectricity appears simultaneously with the magnetic phase transition from the 2D-ICM to 1D-ICM phase in YMn 2 O 5 .

2D-to-1D Modulated-to-Lock in Successive Magnetic Phase Transitions Associated with Ferroelectricity in ErMn2O5

We report results of dielectric and neutron diffraction measurements in zero field on magnetic ordering of weakly ferroelectric ErMn 2 O 5 . Around the ferroelectric transition temperature of T C1 ∼38 K below T N1 = 44 K, we observed successive magnetic phase transitions from a two-dimensionally-modulated (2D) incommensurate magnetic (ICM) phase with the propagation vector q = ( q x ,0, q z ) to another 1D-ICM one with \(\mathbf{q} = (q_x,0,\frac{1}{4})\) at T = 39.1 K, and then to a commensurate magnetic (CM) phase with \(\mathbf{q} = (\frac{1}{2},0,\frac{1}{4})\) at T = 37.7 K. These two magnetic phase transitions (39.1 K and 37.7 K) were found to be associated with an anomaly in the dielectric constant along the b axis. In addition, we found that in the case of a rapid-cooling procedure a low-temperature ICM state characterized by \(\mathbf{q} = (\frac{1}{2},0,0.265)\) shows up and coexists with the CM phase below T N2 = 11.5 K where evolution of magnetic order of the Er ions is inferred from measureme...