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

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\u27 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

Charge density wave and Weyl Semimetal phase in Y2_2Ir2_2O7_7

The subtle interplay of band topology and symmetry broken phase, induced by electron correlations, has immense contemporary relevance and potentially offers novel physical insights. Here, we demonstrate charge density wave (CDW) in bulk Y$_2$Ir$_2$O$_7$ for T < 10 K, and its transition to the Weyl semimetal (WSM) phase at higher temperatures. The CDW phase is evidenced by a) current induced nonlinear conductivity with negative differential resistance at low temperature, b) low frequency Debye like dielectric relaxation at low temperature with a large dielectric constant, and c) an anomaly in the temperature dependence of the thermal expansion coefficient. The WSM phase at higher temperature is confirmed by the DC and AC transport measurements which show an inductive response at low frequencies. More interestingly, we show that by reducing the crystallite size, the low temperature CDW phase can be eliminated leading to the restoration of the WSM phase.Comment: 5 pages, 4 figures; minor correction

Short-range magnetic correlations in Tb5Ge4

We present a single crystal neutron diffraction study of the magnetic short-range correlations in Tb$_5$Ge$_4$ which orders antiferromagnetically below the Neel temperature $T_N$ $\approx$ 92 K. Strong diffuse scattering arising from magnetic short-range correlations was observed in wide temperature ranges both below and above $T_N$. The antiferromagnetic ordering in Tb$_5$Ge$_4$ can be described as strongly coupled ferromagnetic block layers in the $ac$-plane that stack along the b-axis with weak antiferromagnetic inter-layer coupling. Diffuse scattering was observed along both $a^*$ and $b^*$ directions indicating three-dimensional short-range correlations. Moreover, the $q$-dependence of the diffuse scattering is Squared-Lorentzian in form suggesting a strongly clustered magnetic state that may be related to the proposed Griffiths-like phase in Gd$_5$Ge$_4$.Comment: 6 pages, 5 figure

Unconventional pairing in the iron arsenide superconductors

We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.Comment: 3 figures and 4 pages; final version as published

Multiferroic quantum material Ba2Cu1−xMnxGe2O7 (0 ≤ x ≤ 1) as a potential candidate for frustrated Heisenberg antiferromagnet

Multiferroic Ba2CuGe2O7 was anticipated as a potential member of the exciting group of materials hosting a skyrmion or vortex lattice because of its profound Dzyaloshinskii–Moriya interaction (DMI) and the absence of single ion anisotropy (SIA). This phase, however, could not be evidenced and instead, it exhibits a complex incommensurate antiferromagnetic (AFM) cycloidal structure. Its sister compound Ba2MnGe2O7, in contrast, is characterized by a relatively strong in-plane exchange interaction that competes with a non-vanishing SIA and the weak DMI, resulting in a quasi-two-dimensional commensurate AFM structure. Considering this versatility in the magnetic interactions, a mixed solid solution of Cu and Mn in Ba2Cu1−xMnxGe2O7 can hold an interesting playground for its interactive DMI and SIA depending on the mixed spin states of the transition metal ions towards the skyrmion physics. Here, we present a detailed study of the micro- and macroscopic spin structure of the Ba2Cu1−xMnxGe2O7 solid solution series using high-resolution neutron powder diffraction techniques. We have developed a remarkably rich magnetic phase diagram as a function of the applied magnetic field and x, which consists of two end-line phases separated by a potentially quantum-critical phase at x = 0.57. An AFM conical structure at zero magnetic field is demonstrated to persist up to x = 0.50. Our results provide crucial information on the spin structure and magnetic properties, which are necessary for the general understanding and theoretical developments on multiferroicity in the frame of skyrmion type or frustrated AFM lattice where DMI and SIA play an important role

Magnetic structure of Dy3+ in hexagonal multiferroic DyMnO3

Element specific x-ray resonant magnetic scattering (XRMS) investigations were undertaken to determine the magnetic structure of the multiferroic compound, hexagonal DyMnO3. In the temperature range from 68 K down to 8 K the Dy3+ moments are aligned and antiferromagnetically correlated in the c direction according to the magnetic representation Γ3. The temperature dependence of the observed intensity can be modeled assuming the splitting of ground-state doublet crystal-field levels of Dy3+ by the exchange field of Mn3+. XRMS together with magnetization measurements indicate that the magnetic representation is Γ2 below 8 K

Structural transition and anisotropic properties of single-crystalline SrFe2As2

Platelike single crystals of SrFe2As2 as large as 3×3×0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at To=198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 to 160 K, single-crystal x-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases

Flux growth at ambient pressure of millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO

Millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3 * 4 * 0.05-0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the plate-like single crystals. Some crystals manifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal scattering confirmed that LaFeAsO crystals exhibit a structural phase transition at Ts ~ 154 K and a magnetic phase transition at TSDW ~ 140 K. The transition temperatures agree with those determined by anisotropic magnetization, in-plane electrical resistivity and specific heat measurements and are consistent with previous reports on polycrystalline samples. Co and F were successfully introduced into the lattice leading to superconducting LaFe1-xCoxAsO and LaFeAsO1-xFx single crystals, respectively. This growth protocol has been successfully employed to grow single crystals of NdFeAsO. Thus it is expected to be broadly applicable to grow other RMAsO (R = rare earth, M = transition metal) compounds. These large crystals will facilitate the efforts of unraveling the underlying physics of iron pniticide superconductors

Spin-flop transition in Gd5Ge4 observed by x-ray resonant magnetic scattering and first-principles calculations of magnetic anisotropy

X-ray resonant magnetic scattering was employed to study a fully reversible spin-flop transition in orthorhombic Gd5Ge4 and to elucidate details of the magnetic structure in the spin-flop phase. The orientation of the moments at the three Gd sites flop 90° from the c axis to the a axis when a magnetic field, Hsf=9 kOe, is applied along the c axis at T=9 K. The magnetic space group changes from Pnm′a to Pn′m′a′ for all three Gd sublattices. The magnetic anisotropy energy determined from experimental measurements is in good agreement with the calculations of the magnetic anisotropy based on the spin-orbit coupling of the conduction electrons and an estimation of the dipolar interactions anisotropy. No significant magnetostriction effects were observed at the spin-flop transition