arXiv subjects

D. Meier

Publications and source records attributed to D. Meier.

50 records · Page 3Linked to original sources

arXiv2013

Symmetry-Tunable Environment in Functional Ferroic Heterostructures

We demonstrate a concept for post-growth symmetry control in oxide heterostructures. A functional oxide is sandwiched between two ferroelectric layers and inversion symmetry at the position of the oxide constituent is reversibly switched on or off by layer-selective electric-field poling. The functionality of this process is monitored by optical second harmonic generation. The generalization of our approach to other materials and symmetries is considered. We thus establish ferroic trilayer structures as device components in which symmetry-driven charge-, spin-, and strain-related functionalities can be activated and deactivated at will.

cond-mat.mtrl-sci↗

arXiv2012

Mutual induction of magnetic 3d and 4f order in multiferroic hexagonal ErMnO3

The complex interplay between the 3d and 4f moments in hexagonal ErMnO3 is investigated by magnetization, optical second harmonic generation, and neutron-diffraction measurements. We revise the phase diagram and provide a microscopic model for the emergent spin structures with a special focus on the intermediary phase transitions. Our measurements reveal that the 3d exchange between Mn^{3+} ions dominates the magnetic symmetry at 10 K < T < T_N with Mn^3+ order according to the Gamma_4 representation triggering 4f ordering according to the same representation on the Er^{3+}(4b) site. Below 10 K the magnetic order is governed by 4f exchange interactions of Er^{3+} ions on the 2a site. The magnetic Er^{3+}(2a) order according to the representation Gamma_2 induces a magnetic reorientation (Gamma_4 --> Gamma_2) at the Er^{3+}(4b) and the Mn^{3+} sites. Our findings highlight the fundamentally different roles the Mn^{3+}, R^{3+}(2a), and R^{3+}(4b) magnetism play in establishing the magnetic phase diagram of the hexagonal RMnO3 system.

cond-mat.str-el↗

arXiv2012

Observation of Coherent Helimagnons and Gilbert damping in an Itinerant Magnet

We study the magnetic excitations of itinerant helimagnets by applying time-resolved optical spectroscopy to Fe0.8Co0.2Si. Optically excited oscillations of the magnetization in the helical state are found to disperse to lower frequency as the applied magnetic field is increased; the fingerprint of collective modes unique to helimagnets, known as helimagnons. The use of time-resolved spectroscopy allows us to address the fundamental magnetic relaxation processes by directly measuring the Gilbert damping, revealing the versatility of spin dynamics in chiral magnets. (*These authors contributed equally to this work)

cond-mat.str-el↗

arXiv2011

Independent ferroelectric contributions and rare-earth-induced polarization reversal in multiferroic TbMn2O5

Three independent contributions to the magnetically induced spontaneous polarization of multiferroic TbMn2O5 are uniquely separated by optical second harmonic generation and an analysis in terms of Landau theory. Two of them are related to the magnetic Mn3+/4+ order and are independent of applied fields of up to 7 T. The third contribution is related to the long-range antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application of a magnetic field and mediates the change of sign of the spontaneous electric polarization in TbMn2O5. The close relationship between the rare-earth long-range order and the non-linear optical properties points to isotropic Tb-Tb exchange and oxygen spin polarization as mechanism for this rare-earth induced ferroelectricity.

cond-mat.str-el↗

arXiv2010

Translation domains in multiferroics

Translation domains differing in the phase but not in the orientation of the corresponding order parameter are resolved in two types of multiferroics. Hexagonal (h-) YMnO$_3$ is a split-order-parameter multiferroic in which commensurate ferroelectric translation domains are resolved by piezoresponse force microscopy whereas MnWO$_4$ is a joint-order-parameter multiferroic in which incommensurate magnetic translation domains are observed by optical second harmonic generation. The pronounced manifestation of the generally rather "hidden" translation domains in these multiferroics and the associated drastic reduction of symmetry emphasize that the presence of translation domains must not be neglected when discussing the physical properties and functionalities of multiferroics.

cond-mat.str-el↗

arXiv2010

Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4

A comprehensive analysis of optical second harmonic generation (SHG) on an incommensurate (IC) magnetically ordered state is presented using multiferroic MnWO4 as model compound. Two fundamentally different SHG contributions coupling to the primary IC magnetic order or to secondary commensurate projections of the IC state, respectively, are distinguished. Whereas the latter can be described within the formalism of the 122 commensurate magnetic point groups the former involves a breakdown of the conventional macroscopic symmetry analysis because of its sensitivity to the lower symmetry of the local environment in a crystal lattice. Our analysis thus foreshadows the fusion of the hitherto disjunct fields of nonlinear optics and IC order in condensed-matter systems.

cond-mat.str-el↗

arXiv2010

Incompatible Magnetic Order in Multiferroic Hexagonal DyMnO3

Magnetic order of the manganese and rare-earth lattices according to different symmetry representations is observed in multiferroic hexagonal (h-) DyMnO$_3$ by optical second harmonic generation and neutron diffraction. The incompatibility reveals that the 3d-4f coupling in the h-$R$MnO$_3$ system ($R$ = Sc, Y, In, Dy - Lu) is substantially less developed than commonly expected. As a consequence, magnetoelectric coupling effects in this type of split-order parameter multiferroic that were previously assigned to a pronounced 3d-4f coupling have now to be scrutinized with respect to their origin.

cond-mat.str-el↗

arXiv2009

Topology and manipulation of multiferroic hybrid domains in MnWO$_4$

An investigation of the spatially resolved distribution of domains in the multiferroic phase of MnWO$_4$ reveals that characteristic features of magnetic and ferroelectric domains are inseparably entangled. Consequently, the concept of "multiferroic hybrid domains" is introduced for compounds in which ferroelectricity is induced by magnetic order. The three-dimensional structure of the domains is resolved. Annealing cycles reveal a topological memory effect that goes beyond previously reported memory effects and allows one to reconstruct the entire multiferroic multidomain structure subsequent to quenching it.

cond-mat.mtrl-sci↗

arXiv2007

Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO$_3$ and TbMnO$_3$ due to 4f crystal-field excitations

We present measurements of the thermal conductivity $κ$ and the thermal expansion $α$ of NdMnO$_3$ and TbMnO$_3$. In both compounds a splitting of the $4f$ multiplet of the $R^{3+}$ ion causes Schottky contributions to $α$. In TbMnO$_3$ this contribution arises from a crystal-field splitting, while in NdMnO$_3$ it is due to the Nd-Mn exchange coupling. Another consequence of this coupling is a strongly enhanced canting of the Mn moments. The thermal conductivity is greatly suppressed in both compounds. The main scattering process at low temperatures is resonant scattering of phonons between different energy levels of the $4f$ multiplets, whereas the complex 3d magnetism of the Mn ions is of minor importance.

cond-mat.str-el↗

arXiv2007

New features in the phase diagram of TbMnO$_3$

The (H,T)-phase diagram of the multiferroic perovskite TbMnO$_3$ was studied by high-resolution thermal expansion $α(T)$ and magnetostriction $ΔL(H)/L$ measurements. Below $T_{N}\simeq 42$ K, TbMnO$_3$ shows antiferromagnetic order, which changes at $T_{FE}\simeq 28$ K where simultaneously a spontaneous polarization $P||c$ develops. Sufficiently high magnetic fields applied along $a$ or $b$ induce a polarization flop to $P||a$. We find that all of these transitions are strongly coupled to the lattice parameters. Thus, our data allow for a precise determination of the phase boundaries and also yield information about their uniaxial pressure dependencies. The strongly hysteretic phase boundary to the ferroelectric phase with $P||a$ is derived in detail. Contrary to previous reports, we find that even in high magnetic fields there are no direct transitions from this phase to the paraelectric phase. We also determine the various phase boundaries in the low-temperature region related to complex reordering transitions of the Tb moments.

cond-mat.str-el↗

arXiv2006

Spitzer Observations of 3C Quasars and Radio Galaxies: Mid-Infrared Properties of Powerful Radio Sources

We have measured mid-infrared radiation from an orientation-unbiased sample of 3CRR galaxies and quasars at redshifts 0.4 < z < 1.2 with the IRS and MIPS instruments on the Spitzer Space Telescope. Powerful emission (L_24micron > 10^22.4 W/Hz/sr) was detected from all but one of the sources. We fit the Spitzer data as well as other measurements from the literature with synchrotron and dust components. The IRS data provide powerful constraints on the fits. At 15 microns, quasars are typically four times brighter than radio galaxies with the same isotropic radio power. Based on our fits, half of this difference can be attributed to the presence of non-thermal emission in the quasars but not the radio galaxies. The other half is consistent with dust absorption in the radio galaxies but not the quasars. Fitted optical depths are anti-correlated with core dominance, from which we infer an equatorial distribution of dust around the central engine. The median optical depth at 9.7 microns for objects with core-dominance factor R > 10^-2 is approximately 0.4; for objects with R < 10^-2, it is 1.1. We have thus addressed a long-standing question in the unification of FR II quasars and galaxies: quasars are more luminous in the mid-infrared than galaxies because of a combination of Doppler-boosted synchrotron emission in quasars and extinction in galaxies, both orientation-dependent effects.

astro-ph↗

arXiv2006

Uniaxial pressure dependencies of the phase transitions in GdMnO$_3$

GdMnO$_3$ shows an incommensurate antiferromagnetic order below $\simeq 42$ K, transforms into a canted A-type antiferromagnet below $\simeq 20$ K, and for finite magnetic fields along the b axis ferroelectric order occurs below $\simeq 12$ K. From high-resolution thermal expansion measurements along all three principal axes, we determine the uniaxial pressure dependencies of the various transition temperatures and discuss their correlation to changes of the magnetic exchange couplings in $R$MnO$_3$ ($R = {\rm La, ... Dy}$).

cond-mat.str-el↗

arXiv2006

Hysteresis effects in the phase diagram of multiferroic GdMnO$_3$

We present high-resolution thermal expansion $α(T)$ and magnetostriction $ΔL(H)/L$ measurements of GdMnO$_3$, which develops an incommensurate antiferromagnetic order (ICAFM) below $T_{\rm N}\simeq$ 42 K and transforms into a canted A-type antiferromagnet (cAFM) below $T_{\rm c}\simeq 20 $K. In addition, a ferroelectric polarization ${\bf P}||a$ is observed below $T_{\rm FE} $ for finite magnetic fields applied along the $b$ direction. In zero magnetic field we find a strongly anisotropic thermal expansion with certain, rather broad anomalous features. In finite magnetic fields, however, very strong anomalies arise at $T_{\rm c}$ for fields applied along each of the orthorhombic axes and at $T_{\rm FE}$ for fields along the b axis. Both phase transitions are of first-order type and strongly hysteretic. We observe a down-bending of the ICAFM-to-cAFM phase boundary $T_{\rm c}(H)$ for low magnetic fields and our data give evidence for coexisting phases in the low-field low-temperature range.

cond-mat.str-el↗

arXiv2002

A sample of southern Compact Steep Spectrum radio sources: The VLBI observations

A small sample of 7 southern Compact Steep Spectrum (CSS) radio sources has been selected as part of the study of a larger flux-limited complete sample of radio sources. High resolution images, using the VLBI network in the southern hemisphere and the high resolution MERLIN array, are presented for all sources in the CSS sample. The overall morphology of each source consists of well-defined double lobes but with substantial diffuse and extended components present. In the majority of cases only a fraction of the total flux density is detected on the VLBI baselines, indicating the presence of larger extended radio structures. However, all sources are unresolved at arcsecond scales and are of sub-galactic size, with linear size in the range 0.1-2 kpc. The radio properties of the sources agree well with CSS sources in other samples.

astro-ph↗