arXiv subjects

R. D. Johnson

Publications and source records attributed to R. D. Johnson.

60 records · Page 4Linked to original sources

arXiv2011

Giant improper ferroelectricity in the ferroaxial magnet CaMn7O12

In rhombohedral CaMn7O12, an improper ferroelectric polarization of magnitude 2870 μC m-2 is induced by an incommensurate helical magnetic structure that evolves below TN1 = 90 K. The electric polarization was found to be constrained to the high symmetry three-fold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in CaMn7O12 gives rise to the largest magnetically induced, electric polarization measured to date.

cond-mat.str-el↗

arXiv2011

Cu3Nb2O8: a multiferroic with chiral coupling to the crystal structure

By combining bulk properties, neutron diffraction and non-resonant X-ray diffraction measurements, we demonstrate that the new multiferroic Cu$_3$Nb$_2$O$_8$ becomes polar simultaneously with the appearance of generalised helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins --- an observation that cannot be reconciled with the "conventional" theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically-induced structural chirality.

cond-mat.str-el↗

arXiv2011

The magnetic structures of the anisotropic inter-metallic compounds Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$

Two members of the iso-structural R$_2$CoGa$_8$ inter-metallic series, Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed Er$_2$CoGa$_8$ to adopt a 3D universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium and thulium based compounds. The corresponding magnetic space groups were determined to be $P_{2a}mmm'$ and $P_Cmmm$. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

cond-mat.str-el↗

arXiv2011

Magnetically induced electric polarization reversal in multiferroic TbMn$_2$O$_5$: Terbium spin reorientation studied by resonant x-ray diffraction

In multiferroic TbMn$_2$O$_5$, the behavior of the terbium ions forms a crucial part of the magneto-electric coupling. The result is a magnetically induced reversal of the electric polarization at 2 T. In this article we present the first direct measurement of the terbium magnetic structure under applied magnetic fields. Contrary to the current interpretation of the magnetic properties of \tmo, we show that upon the electric polarization reversal the terbium sub-lattice adopts a canted antiferromagnetic structure with a large component of magnetic moment parallel to the \textit{a}-axis. Furthermore, we provide evidence for a coupling between the manganese 3d magnetic structure and the terbium $4f$ magnetism, which is of great significance in the elusive magneto-electric mechanisms at play.

cond-mat.str-el↗

arXiv2010

Antiferromagnetically spin polarized oxygen observed in magneto-electric TbMn2O5

We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wavevector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization.

cond-mat.str-el↗

arXiv2008

Determination of magnetic order of the rare-earth ions in multiferroic TbMn2O5

We have employed resonant x-ray magnetic scattering to specifically probe the magnetic order of the rare-earth ions in multiferroic $\mathrm{TbMn_2O_5}$. Two energy resonances were observed, one originated from the E1-E1 dipolar transition and the other from the E2-E2 quadrupolar transition. These resonances directly probe the valence 5d band and the partially occupied 4f band, respectively. First, full polarization analysis, which is a measurement of the scattered polarization as a function of incident polarization, confirmed a spin polarization of the terbium valence states (probed by the E1-E1 transition) by the $\mathrm{Mn^{4+}}$ spin density in the commensurate phase. Second, full polarization analysis data were collected in the low-temperature incommensurate and commensurate phases when tuned to the E2-E2 resonance. By employing a least-squares fitting procedure, the spin orientations of the terbium ion sublattice were refined.

cond-mat.str-el↗