Neue Magnetoelektrika und magnetoelektrische Multiferroika

Der Schwerpunkt der vorliegenden Arbeit liegt in der Suche nach neuen multiferroischen Materialien mit starker magnetoelektrischer Kopplung. Zunächst wird die Etablierung und Optimierung der für die dielektrischen Untersuchungen notwendigen Tieftemperaturmesstechnik vorgestellt. In der ersten untersuchten Substanzklasse, der Gruppe der Wolframate MWO4, bzw. MIMII(WO4)2 wird die Verbindung MnWO4 als neues Multiferroikum identifiziert, dessen magnetoelektrischer Kopplungsmechanismus auf einer spiralartigen Spinanordnung beruht. Für die Verbindung NaFe(WO4)2 wird ebenfalls multiferroisches Verhalten beobachtet. Da diese Substanz eine kollineare Spinanordnung aufweist, deutet dieses Ergebnis auf das Vorliegen eines andersartigen, möglicherweise bisher unbekannten magnetoelektrischen Kopplungsmechanismus hin. In der zweiten untersuchten Substanzklasse, der Gruppe der Pyroxene AMSi2O6, wird zunächst das Mineral Ägirin mit der durch Mikrosondenanalyse bestimmten Zusammensetzung Na1.04Fe0.83Ca0.04Mn0.02Al0.01Ti0.08Si2O6 als neues Multiferroikum identifiziert: In dieser Verbindung wird unterhalb 6K das Auftreten einer spontanen elektrischen Polarisation von bis zu 13 muC/m2 in einer antiferromagnetisch geordneten Phase beobachtet. Eine starke magnetoelektrische Wechselwirkung tritt in Form der Schaltbarkeit der elektrischen Polarisation von der kristallographischen b- in die c- Richtung mittels eines senkrecht zur b-Richtung applizierten Magnetfeldes auf. Ein entsprechendens magnetoelektrisches Phasendiagramm wird vorgestellt. Als zweites Pyroxen wird die synthetisch hergestellte Verbindung LiFeSi2O6 untersucht. Diese Substanz wird als neues lineares Magnetoelektrikum identifiziert, dessen magnetoelektrische, antiferromagnetische Phase unterhalb einer magnetfeldabhängigen Übergangstemperatur kleiner-gleich 18.5 Kelvin auftritt. Bei Applizierung von Magnetfeldern größer etwa 5 Tesla tritt ein Spinflop-Übergang auf, der das magnetoelektrische Verhalten zerstört. Aus einer kompletten Vermessung der dielektrischen Anisotropie wird der vollständige magnetoelektrische Tensor bestimmt. Eine Symmetrieanalyse führt zur Ableitung der exakten mikroskopischen magnetischen Struktur dieser Substanz allein auf der Basis makroskopischer dielektrischer und magnetischer Messungen: In der magnetoelektrischen Phase wird die magnetische Symmetrie von LiFeSi2O6 durch die magnetische Raumgruppe P21/c' mit parallel c angeordneten magnetischen Momenten beschrieben. In LiFeSi2O6 tritt, signalisiert durch einen starken nicht-symmetrischen Anteil im magnetoelektrischen Tensor, ein ferrotoroidaler Effekt auf. Ein Ausdruck für das entsprechende Toroidisierungsgitter wird aus der ermittelten magnetischen Struktur abgeleitet. Als dritte Verbindung aus der Klasse der Pyroxene wird synthetisch hergestelltes LiCrSi2O6 untersucht. Diese Substanz zeigt ebenfalls linear magnetoelektrisches Verhalten. Der Übergang von der ungeordneten in eine antiferromagnetisch geordnete linear magnetoelektrische Phase erfolgt in LiCrSi2O6 bei einer magnetfeldunabhängigen Phasenübergangstemperatur von etwa 11K

Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)

We report measurements of the thermal expansion for two Eu$^{+2}$- and two Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal expansion coefficient $\alpha$ which become smeared and/or displaced in an external magnetic field. Together with specific heat data we determine the initial pressure dependences of the transition temperatures at ambient pressure using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure dependences indicating strong spin-phonon coupling, in particular for GdAgMg and EuAuMg where a quantum phase transition might be reached at moderate pressures of a few GPa.Comment: 6 pages, 3 figure

Spin-State Transition and Metal-Insulator Transition in La1−x_{1-x}Eux_xCoO3_3}

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La$_{1-x}$Eu$_x$CoO$_3$. LaCoO$_3$ shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La$^{3+}$ by the smaller Eu$^{3+}$ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO$_3$ to about 2000 K in EuCoO$_3$, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state with orbital order for $x<0.5$ and without orbital order for larger $x$. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the Co$^{3+}$ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all $x$ and approaches a doping-independent value around 1000 K indicating that well above the metal-insulator transition the same spin state is approached for all $x$.Comment: 10 pages, 6 figure

CdV2O4: A rare example of a collinear multiferroic spinel

By studying the dielectric properties of the geometrically frustrated spinel CdV2O4, we observe ferroelectricity developing at the transition into the collinear antiferromagnetic ground state. In this multiferroic spinel, ferroelectricity is driven by local magnetostriction and not by the more common scenario of spiral magnetism. The experimental findings are corroborated by ab-initio calculations of the electric polarization and the underlying spin and orbital order. The results point towards a charge rearrangement due to dimerization, where electronic correlations and the proximity to the insulator-metal transition play an important role.Comment: 4+ pages, 3 figure

Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds

We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic $RET$Mg intermetallics with $RE = {\rm La}$, Eu, Gd, Yb and $T = {\rm Ag}$, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic ($RE = {\rm La}$, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K. All of them are metallic, but the resistivity varies over 3 orders of magnitude. The magnetic order causes a strong decrease of the resistivity and around the ordering temperature we find pronounced magnetoresistance effects. The magnetic ordering also leads to well-defined anomalies in the specific heat. An analysis of the entropy change leads to the conclusions that generally the magnetic transition can be described by an ordering of localized $S=7/2$ moments arising from the half-filled $4f^7$ shells of Eu$^{2+}$ or Gd$^{3+}$. However, for GdAgMg we find clear evidence for two phase transitions indicating that the magnetic ordering sets in partially below about 125 K and is completed via an almost first-order transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.Comment: 12 pages, 7 figures include

Magnetoelectric effect due to local noncentrosymmetry

Magnetoelectrics often possess ions located in noncentrosymmetric surroundings. Based on this fact we suggest a microscopic model of magnetoelectric interaction and show that the spin-orbit coupling leads to spin-dependent electric dipole moments of the electron orbitals of these ions, which results in non-vanishing polarization for certain spin configurations. The approach accounts for the macroscopic symmetry of the unit cell and is valid both for commensurate and complex incommensurate magnetic structures. The model is illustrated by the examples of MnWO4, MnPS3 and LiNiPO4. Application to other magnetoelectrics is discussed.Comment: 11 pages, 2 figures, 2 table

Phase diagram and isotope effect in (PrEu)_0.7Ca_0.3CoO_3 cobaltites exhibiting spin-state transitions

We present the study of magnetization, thermal expansion, specific heat, resistivity, and a.c. susceptibility of (Pr$_{1-y}$Eu$_y$)$_{0.7}$Ca$_{0.3}$CoO$_3$ cobaltites. The measurements were performed on ceramic samples with $y = 0.12 - 0.26$ and $y = 1$. Based on these results, we construct the phase diagram, including magnetic and spin-state transitions. The transition from the low- to intermediate-spin state is observed for the samples with $y > 0.18$, whereas for a lower Eu-doping level, there are no spin-state transitions, but a crossover between the ferromagnetic and paramagnetic states occurs. The effect of oxygen isotope substitution along with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope substitution ($^{16}$O by $^{18}$O) is found to shift both the magnetic and spin-state phase boundaries to lower Eu concentrations. The isotope effect on the spin-state transition temperature ($y > 0.18$) is rather strong, but it is much weaker for the transition to a ferromagnetic state ($y < 0.18$). The ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by the Co$^{4+}$ ions, which favor the formation of the intermediate-spin state of neighboring Co$^{3+}$ ions.Comment: 13 pages, including 11 figures, to be published in Phys. Rev.

Correlation between dielectric properties and chemical composition of the tourmaline single crystals

Dielectric responses were studied on piezoelectric tourmaline single crystals of widely varying chemical composition from different geological origins. The dielectric constants at constants stress, and dissipation factor were measured as a function of frequency (100-1000 kHz) using method of substitution. A correlation between two independent dielectric constants (along and perpendicular to crystallographic c-axis) is observed, and dependence of dielectric constants on chemical composition is presented. (C) 2011 American Institute of Physics. [doi:10.1063/1.3646912