X-ray resonant magnetic scattering investigations of hexagonal multiferroics RMnO3 (R = Dy, Ho, Er)

Nandi, Shibabrata
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This dissertation is concerned with the magnetic structure of hexagonal multiferroic compounds RMnO3 (R = Ho, Dy, Er) in both zero and applied electric field. Microscopic magnetic structures in zero field were studied using x-ray resonant magnetic scattering (XRMS). Magnetic structure in applied electric field was studied using bulk magnetization, x-ray resonant magnetic scattering (XRMS), and x-ray magnetic circular dichroism (XMCD).

The magnetic structures of Ho, Dy, and Er members have been determined using high-quality single-crystal samples grown by optical floating zone technique. We have determined that the magnetic structure of Ho3+ in HoMnO3 to be Γ3 in the intermediate temperature magnetic phase ITP (between 40 K and 4.5 K). The magnetic Ho3+ moments are aligned along the c axis and, at 12 K, the ratio between the magnetic moments of the Ho(2a) and Ho(4b) Wyckoff site is ~ -2. The moments at the Ho(2a) site are antiferromagnetically aligned to the moments at the Ho(4b) site in the a-b plane. We also conclude that there is a change of the magnetic structure of Ho3+ at 4.5 K. Below 4.5 K, the magnetic phase can be well described by the co-existence of the ITP (Γ3) with a decreasing `ordered moment' and a new magnetic phase LTP with magnetic representation Γ1 with a rapidly increasing Ho (4b) moment for decreasing temperatures. We failed to observe resonant magnetic scattering from Mn K-edge due to the presence of non-magnetic anisotropic tensor scattering at the magnetic Bragg peaks. Therefore, existence of a c component of the Mn3+ moments, predicted by symmetry analysis, can not be tested.

We have also determined the magnetic structures of Dy3+ and Er3+ moments in DyMnO3 and ErMnO3, respectively. Dy3+ moments order according to the magnetic representation Γ3 in the intermediate temperature magnetic phase, ITP (between 68 K and 8 K). In the low temperature phase, LTP (below 8 K), XRMS together with magnetization measurements indicate that Γ2 is the magnetic representation and the Dy3+ moments are ferrimagnetically aligned in the hexagonal c direction. For ErMnO3, we conclude that no ITP exists and the Er3+ moments order ferrimagnetically below 3 K according to magnetic representation Γ2.

We note that the magnetic structure in DyMnO3 is the same as in HoMnO3 in the ITP, however, in the LTP the magnetic order is different: the Ho3+ moments are antiferromagnetically aligned according to Γ1 in contrast to the ferromagnetic alignment of the Dy3+ moments in DyMnO3. For both the Ho3+ and Dy3+, magnetism in the ITP can be explained assuming an exchange interaction between R3+ and Mn3+ and a crystal electric field splitting of the R3+ ground state quasidoublet/doublet. The crystal electric field splitting for Dy is ~6 meV and that of Ho is ~1.3 meV.

From the extensive single crystal SQUID magnetization, XRMS and XMCD as well as XMCD on powder samples for two different hexagonal multiferroics, HoMnO3 and DyMnO3, we conclude that electric field up to 1x107 V/m does not change the magnetic structure of Ho3+ moments.

Antiferromagnetism, Hexagonal multiferroics, Induced magnetic order, Magnetic order in an applied electric field, Rare earth magnetic order, x-ray resonant magnetic scattering and x-ray magnetic circular dichroism