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Abstract

This thesis mainly deals on single-layered manganites Re

1−x

A

1+x

MnO

4

with Re = La, Pr, Nd and A = Sr, Ca and with doping levels 0.25 ≤ x ≤ 0.67. The com- plex interplay between charge, orbital, and magnetic degrees of freedom has been studied by neutron diffraction techniques.

Magnetic excitations on half-doped La

0.5

Sr

1.5

MnO

4

, Pr

0.5

Ca

1.5

MnO

4

, and Nd

0.5

Sr

0.5

MnO

3

yield qualitatively a comparable spin-wave dispersion. In La

0.5

Sr

1.5

MnO

4

it has been possible to study magnetic excitations of the acoustical and those of all optical branches whereas in Pr

0.5

Ca

1.5

MnO

4

and in the perovskite Nd

0.5

Sr

0.5

MnO

3

excitations up to about 12 meV were recorded. The perfect agree- ment between experimental data and a theoretical description by linear spin-wave theory based on AFM CE-type ordering is evident and a qualitative validation of the Goodenough model could be confirmed.

The ground state of charge-, orbital-, and antiferromagnetic ordering in over- doped 214-manganites Re

1−x

A

1+x

MnO

4

with doping 0.5 ≤ x ≤ 0.67 has been stud- ied by an area detector. In contrast to half-doped manganites incommensurate superstructure reflections associated with charge and orbital ordering were found.

Satellites referring to magnetic ordering of Mn

3+

moments are incommensurate as well while magnetic superstructure reflections of Mn

4+

spins have been recorded at half-indexed i.e. at commensurate position for the doping level 0.5 < x ≤ 0.60.

This finding appears inverse for the doping x =

2

/

3

as superstructure reflections as- sociated with magnetic ordering of Mn

4+

spins were detected at incommensurate and of Mn

3+

spins at commensurate positions. For the first time, a full consistent model based on stripe ordering of extra Mn

4+

ions could be developed to describe the studied ordering parameters.

Magnetic excitations in Pr

0.33

Ca

1.67

MnO

4

and Nd

0.33

Sr

1.67

MnO

4

exhibit a qual- itatively comparable spin-wave dispersion as the highly discussed hourglass dis- persion in high-T

c

copper-oxides. The tight connection between the magnetic cor- relation lengths and the magnetic excitation spectrum at low energies could be depicted.

The ground state of electron-doped La

1−x

Sr

1+x

MnO

4

with x = 0.4, 0.33, 0.25 as well as that of Pr

0.6

Ca

1.4

MnO

4

has been characterized. Former experimen- tal studies were not able to clearly connect slightly electron-doped (x ≈ 0.40) and highly electron-doped (x ≈ 0.125) 214-manganites. The experimental data clearly show incommensurate superstructure reflections associated with orbital ordering in agreement to hole-doped manganites. However, the incommensurabil-

1

(2)

Abstract

ity is smaller compared to the overdoped systems. The experimental observation can be explained by an orbital flop of the e

g

orbital on Mn

3+

site out-of the MnO

2

plane.

2

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