Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F32: Focus Session: Manganites |
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Sponsoring Units: GMAG DMP Chair: Hyowon Park, University of Illinois at Chicago Room: 207B |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F32.00001: Domain walls in improper ferroelectrics as functional oxide interfaces Invited Speaker: Manfred Fiebig The coexistence of magnetic and electric order in multiferroics and the resulting magnetoelectric coupling have triggered an immense research interest. The most prominent mechanisms promoting magnetic and ferroelectric order, however, tend to be mutually exclusive. As a result, multiferroics are an inherent source of ``unusual'' ferroelectricity. In many cases the ferroelectric state is improper, i.e., induced by the ordering of a different parameter like magnetism or strain. This secondary nature can lead to properties not normally found in ferroelectrics. In my talk I will discuss consequences for the ferroelectric domain walls of various multiferroics. For example, in magnetically induced ferroelectrics like MnWO$_4$ or TbMnO$_3$ the electric polarization within the wall is expected to rotate instead of passing through zero, as in conventional displacive ferroelectrics. This affects the distribution and propagation of the ferroelectric domains. In addition, a magnetic-field-induced rotation can reversible charge and discharge the domain walls. In strain-induced ferroelectrics like SrMnO$_3$ the interplay of strain and oxygen vacancies leads to a polar state in which domain walls act as insulating boundaries to the conducting domains which therefore acts as nano-capacitors. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F32.00002: Neutron Scattering Studies of Magnetic Structure and Excitations in Na$_{5/8}$MnO$_2$ Robin Chisnell, Xin Li, Xiaohua Ma, Dong Su, Lei Liu, Shyue Ping Ong, Hailong Chen, Alexandra Toumar, Juan-Carlos Idrobo, Yuechuan Lei, Jianming Bai, Feng Wang, Jeffrey Lynn, Young Lee, Gerbrand Ceder Na$_x$TMO$_2$ (TM=transition metal) materials consist of alternating layers of Na and TM ions with the TM ions arranged on a geometrically frustrated triangular lattice. Na can be easily and reversibly removed from these materials, making them of interest for application in rechargeable batteries and allowing for exploration of their rich phase diagrams as a function of Na concentration. Na ordering is an important factor in ground state selection, and is driven by electrostatic interactions in many Na$_x$TMO$_2$ systems. The series Na$_x$MnO$_2$ differs in that Na ordering is driven by a cooperative Jahn-Teller effect, due to the coexistence of Jahn-Teller active Mn$^{3+}$ and non-active Mn$^{4+}$ ions. We have recently shown the existence of a charge stripe ordering a in the material Na$_{5/8}$MnO$_2$ [1]. At low temperatures a magnetic stripe order also develops. We present neutron diffraction and inelastic scattering measurements and examine the details of the magnetic structure and excitations in the magnetic stripe ordered phase. [1] X. Li \textit{et al.} Nature Mater. \textbf{13}, 586 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F32.00003: Scanning Tunneling Microscopy of Bilayer La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ Single Crystals Leujen Chen, Xinzhou Tan, Seong Heon Kim, Jeehoon Kim, J.-S. Zhou, J.B. Goodenough, Alex de Lozanne We employed a spin-polarized scanning tunneling microscope to image the (001) surface topography and spectroscopy in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x$=$0.32, 0.40 {\&} 0.52) single crystals below the Curie temperature. As the doping is increased from x$=$0.3 to x$=$0.5 the spins arrange themselves in antiferromagnetic, ferromagnetic, in-plane and out-of-plane configurations. From local density of states (LDOS) maps, we observed a charge density wave with a wavelength of about 1.6 nm along the tetragonal a or b axes in the x$=$0.32 sample, which is known to be ferromagnetic with spins perpendicular to the surface. On the other hand, the x$=$0.52 crystal is expected to be type A antiferromagnetic with spins parallel to the surface, which is confirmed by our LDOS maps. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F32.00004: Inhomogeneity driven giant magneto-resistance in compressed LaMnO3 Maria Baldini, Takaki Muramatsu, Mohammad Sherafati, Ho-kwang Mao, Lorenzo Malavasi, Paolo Postorino, Sashi Satpathy, Viktor Struzhkin CMR in rare-earth manganites has been intensively studied over the past decades. However, the mechanism underlying the CMR is still not completely clarified. Up to now, CMR was only observed in doped manganites suggesting that the presence of mixed valence Mn ions is an essential ingredient of the CMR phenomenon. Since its discovery, phase separation has been strongly linked to CMR, although the exact mechanism was not established yet [1-5]. We performed high pressure transport measurement varying temperature and magnetic field in a pure compound LaMnO3, and observe CMR at around 32 GPa. This result leaves aside many ambiguities inherent to compounds with complex chemical composition. We used pressure to modify the material's property and to clarify in a clean way the role played by phase separation. We found that pressure induces the formation of a mixed phase which consists of two components: an insulating one with Jahn Teller distortion and a metallic one without distortion. The volume fraction of the metallic phase grows with pressure and the CMR is observed just below the percolation threshold. The experimental results are well reproduced by theoretical calculations and percolation theory. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F32.00005: Polaron-Mediated Spin Correlations in Metallic and Insulating La$_{1-x}A_{x}$MnO$_{3}$ Joel Helton, Daniel Pajerowski, Yiming Qiu, Yang Zhao, Dmitry Shulyatev, Yakov Mukovskii, Georgii Bychkov, Sergei Barilo, Jeffrey Lynn Neutron spectroscopy measurements reveal short-range spin correlations near and above the ferromagnetic-paramagnetic phase transition in manganite materials of the form La$_{1-x}A_{x}$MnO$_{3}$ ($A$=Ca, Sr, or Ba), including samples with an insulating ground state as well as colossal magnetoresistive samples with a metallic ground state. Quasielastic magnetic scattering is revealed that forms clear ridges running along the [100]-type directions in momentum space. A simple model consisting of a conduction electron hopping between spin polarized Mn ions that becomes self-trapped after a few hops captures the essential aspects of this magnetic component of the scattering. We associate this scattering component with the magnetic part of diffuse polarons, as we observe a temperature dependence similar to that of the diffuse nuclear scattering arising from individual polarons. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F32.00006: Unexpected nanoscale metamagnetic transition in a colossal magnetoresistance manganite Lingjia Shen, Elizabeth Blackburn, Alexander. T. Holmes, Edward. M. Forgan, Sebastian Muehlbauer, Andre Heinemann Intrinsic inhomogeneities, both on the nanometer and micrometer scale, are crucial to the phase separation seen in the colossal magnetoresistance (CMR) perovskite manganites. The origin of these inhomogeneities has been an open question for the past decade [1, 2], but they clearly play an important role. To investigate these inhomogeneities, we have studied Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.97}$Ga$_{0.03}$O$_3$ using small angle neutron scattering in fields up to 16 T. In zero field at $\sim$ 220 K, there is a transition to a charge/orbital ordered (CO/OO) paramagnet, as in Pr$_{1-x}$Ca$_{x}$MnO$_3$ [3]. At 150 K, the CO/OO phase is destroyed by applying a field of 6 T, and we find that above this field, nanoscale inhomogeneities undergo a separate, uncoupled, metamagnetic transition. We have also explored metamagnetic avalanches at low temperature [4] in this material, and a possible link between the avalanches and the magnetic nanoscale phases will be illustrated. [1] E. Dagotto et al. Physics Reports 344, 1-153 (2001). [2] K. H. Ahn et al. Nature, 428, 401 (2004). [3] Y. Tomioka et al. Phys. Rev. B 53, R1689 (1996). [4] C. Yaicle et al. Physical Review B, 68, 224412 (2003). [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F32.00007: Tunable metamagnetic transitions in double-perovskite Eu$_{2}$CoMnO$_{6}$ single crystals Hwan Young Choi, Nara Lee, M.S. Seo, S.Y. Park, Y.J. Jo, Y.J. Choi Double perovskite single crystals of Eu$_{2}$CoMnO$_{6}$ were first synthesized using flux method and their magnetic properties were investigated. Magnetic field dependence of magnetization reveals a metamagnetic transition in as-grown crystals. Controlling valences of magnetic ions in different gas annealing conditions leads to the complete change of shapes and locations of metamagnetic transitions in the isothermal magnetization. This remarkable variation originates from the formation of magnetic clusters with different valences of magnetic ions. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F32.00008: Spin excitations used to uncover the nature of the magnetically ordered ground state of Pr0.5Ca0.5MnO3 Russell Ewings, Toby Perring, Olga Sikora We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in its magnetic and orbitally ordered phase. Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the excitations, with several different models shows that only the CE-type ordered state provides an adequate description of the magnetic ground state. We are able to rule out the Zener polaron and magnetic dimer models as magnetic ground states of the system, the former on the basis of gross features of the observed spin wave spectrum and the latter due to subtle discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F32.00009: A Combined Density Functional Theory and Monte Carlo Study of Manganites for Magnetic Refrigeration Romi Korotana, Giuseppe Mallia, Zsolt Gercsi, Nicholas Harrison Perovskite oxides are considered to be strong candidates for applications in magnetic refrigeration technology, due to their remarkable properties, in addition
to low processing costs.
Manganites with the general formula R$_{1-x}$A$_x$MnO$_3$, particularly for A=Ca and $0.2 |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F32.00010: Giant orthorhombic distortions by Cu$^{2+}$ in ferrimagnetic spinel Mn$_3$O$_4$ Jae-Ho Chung, Kee Hwan Lee, Hun Chang, In Yong Hwang, Hyun Wook Kang, Su Jae Kim, Seongsu Lee Mn$_3$O$_4$ is a tetragonal ($c > a$) spinel that exhibits noncollinear Yafet-Kittel ferrimagnetic ordering at low temperatures. We report large orthorhombic distortions in its ferrimagnetic phase stabilized by a few percent of Cu doping. The orthorhombic strains of the ferrimagnetic phases increased linearly to the doping and reached up to $\epsilon \approx 8.2 \times 10^{-3}$ for $x$ = 0.19, which is three times larger than the saturated value under external magnetic fields. For high doping ($x \agt 0.17$), the distortions first appeared in the paramagnetic phases and underwent further enhancement simultaneously with the onset of the noncollinear ferrimagnetic ordering. We present the rich magnetostructural phase diagram of Cu$_x$Mn$_{3-x}$O$_4$, and argue that the diluted $t_2$ orbital degeneracy of Cu$^{2+}$ under tetrahedral crystal field breaks the global symmetry and triggers the orthorhombic instability inherent in Mn$_3$O$_4$. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F32.00011: Magnetic Order and Frustration in Doped Sr$_{2}$Mn$_{3}$As$_{2}$O$_{2}$ Christopher Georgen, Chih-Wei Chen, Jiakui Wang, Emilia Morosan Crystal structure is often key in dictating a material's magnetic and physical properties. Two components of crystal structure that can be particularly influential are reduced dimensionality (1D or 2D) or geometric frustration. The layered crystal structure of Sr$_{2}$Mn$_{3}$As$_{2}$O$_{2}$ which crystallizes in the tetragonal I4 /mmm space group, exhibits both these features. This compound is of particular interest because it consists of Fe pnictide-\textit{like} tetrahedral planes that alternate with Cu-O-\textit{like} octahedral layers. This structure is a promising avenue to study geometric effects in relation to unconventional superconductivity. Here we report a magnetic study of the effects of 3d and 4d transition metal substitutions on the Mn site, with emphasis on the resulting long range \textit{and} short range order. AC and DC magnetization data provide evidence for spin glass to cluster glass crossover with 4d metal doping, while 3d metal doping suppresses both the magnetic order and glassy state. This can be attributed to half-filled Mn 3d shells in a 2D magnetic structure as suggested by neutron diffraction data and band structure calculations. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F32.00012: Spin reorientation and Ce-Mn coupling in antiferromagnetic oxypnictide CeMnAsO David Vaknin, Qiang Zhang, Spencer Peterson, Kevin Dennis, Wei Tian Structure and complex magnetic properties of CeMnAsO, a parent compound of the ``1111"-type oxypnictides, have been investigated using neutron powder diffraction and magnetization measurements. Whereas there is no structural transition from the P4/nmm tetragonal phase below 420 K, CeMnAsO undergoes a C-type antiferromagnetic order with Mn$^{2+}$ ($S=5/2$) moments pointing along the \textit{c}-axis below a relatively high N\'{e}el temperature of $T_{\rm N} = 345$ K. Below $T_{\rm SR} = 35$ K, two instantaneous transitions occur where the Mn moments reorient to the \textit{ab}-plane preserving the C-type magnetic order, and Ce moments undergo long-range AFM ordering with moments in the \textit{ab}-plane. Another transition to a noncollinear magnetic structure occurs below 7 K. We find that CeMnAsO primarily falls into the category of a local-moment antiferromagnetic insulator in which the nearest-neighbor interaction ($J_{1}$) is dominant. The spin reorientation transition driven by the coupling between rare earth Ce and transition metal seems to be common to Mn, Fe and Cr ion,but not to Co and Ni ions in the iso-structural oxypnictides. [Preview Abstract] |
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