Raman and IR spectroscopy study of the spin-lattice coupling in (LaMn3)Mn4O12 

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High-pressure/high-temperature syntheses

As explained in the previous chapter, in nature, electric and magnetic orders tend to avoid each other for several fundamental reasons. One effective solution to stabilize new crystal structures that allow simultaneously electric and magnetic order is to use high-pressure. Indeed, high-pressure enables a large increase in density, changes in orbital hybridization and coordination number, with deep modification of the electronic structure and transport properties. Exemple include insulator-metal transitions [50] and induction of superconductive properties [51]. In the case of quadruple perovskite structure [42], subject of the present thesis, high-pressure helps to stabilize the Mn3+ ions on the A0-sites in the square planar coordination by driving the very large tilt of the corner-sharing BO6 octaedra which results in a structure 20% densier than the simple perovskite. The technical difficulty is to apply a very high isotropic pressure during the synthesis and obtain homogeneous phase products. In order to reach this objective we have used a 1000 tons multianvil Walker-type press available at the IMEM-CNR in Parma represented in Fig 2.1.

Difference between Infrared and Raman spectroscopy

A plan-wave is composed of electric and magnetic field, in phase and perpendicular to each other and to the direction of propagation. When an electromagnetic wave goes through a solid, it undergoes 2 types of scattering:
• Elastic scattering: No energy transfer, only the direction of the incident photon of energy h is modified. After the sample, the light is diffused and this diffusion can be coherente (Diffraction) or incoherente (Diffusion Rayleigh).
• Inelastic scattering: The incident photon of energy h exchanges a quantity of energy with the sample. This quantity is characteristic of one specific excitation of the crystal. If the energy transfer is total, the incident photon is totally anihilated (or emmitted) and it is the case of infrared absorption phenomenon (1 photon processus, see Fig 2.6 a). If the energy transfer is partial, the incident photon loses or gains a quantity of energy and this is the Raman diffusion effect (two photons processus which combines absorption and emmission, see Fig 2.6 b).

Sample preparation for spectroscopy

The preparation of samples is a critical phase for spectroscopy measurements. For example, Raman spectroscopy is a surface sensitive technique as the penetration depth of the incident radiation is of few hundred of nanometers. In order to obtain a smooth surface, we tried to polish the sample with a mechanical polisher using a very thin grain-paper of 0.1 μm but this was not successful as we obtained poor Raman signal and thus we conclude that the polishing probably degradates the first layers probed during the Raman spectroscopy experiment. We obtain the best Raman signal by cliving the samples just before putting it under high-vacuum in the cryostat. With this preparation, we were able to recover a good signal at each temperature even if we notice a change in the intensity of modes depending on the position of the few micrometers spot on the surface.
On the other hand, IR spectroscopy is a bulk probe as the penetration depth in the reflectivity mode is of few micrometers, but the reflected intensity is very dependent on the quality of the crystal surface. The diameter of the incident light spot is of the same order of magnitude than the sample size ( 700 μm). In this case, we obtained a good reflective surface by polishing the sample with a mechanical Leica polisher using diamond grain-paper of 0.1 μm.

Fourier Transform InfraRed (FTIR) Spectrometer

The FTIR is based on a Michelson interferometer (see Fig. 2.7) after which the recombined beam (interferogram) proceeds through the sample and then the detector. The advantage of an FTIR spectrometer is that it exposes the sample to all available infrared light frequencies simultaneously. Therefore, each portion of the interferogram contains encoded information of the sample that was derived from every infrared frequency it was exposed to. In parallel of the infrared interferogram, the spectrometer also acquires a laser interferogram for every scan, used to sample the infrared interferogram.
The detector sends the signal toward a computer and with the help of a Fourier transformation, we obtain the plot of the intensity as a function of frequency. The FTIR spectrometer used at ESPCI is a Bruker IFS66/s, composed of several chambers. The source is in the same chamber than the Michelson interferometer. Because our sample absorbs all frequencies in the IR range, we performed the measurement in the reflectivity configuration. The sample is mounted out of the spectrometer in a cryostat sitting just in front of the sample chamber. The detector is also moved out of the detector chamber and mounted beside the sample cryostat. We performed two series of measurements as a function of temperature.
• In the far-infrared range [20-700 cm−1], using a Globar SiC source, a Mylar- Germanium beamsplitter and a Bolometer as detector and a resolution of 2 cm−1.
• In the mid-infrared region, we used the Globar SiC source combined with a KBr: Germanium beamsplitter, a 4.2 K Photoconductor and a resolution of 4 cm−1 (for a reasonable time of acquisition 10 min).

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Article IV: Evidence of centrosymmetry breaking in the magnetic ferroelectric (YMn3)Mn4O12

We report on the unusual properties of magnetic ferroelectricity of the quadruple perovskite (YMn3)Mn4O12, where the comparatively small Y3+ ion exerts a large chemical pressure on the crystal structure. Pyrocurrent, dielectric constant and transport measurements performed on polycristalline samples give evidence of spontaneous electric polarization reaching the remarkable saturation value Psat = 0.54 μC cm−2, two times higher than the value previously reported in the related compound (CaMn3)Mn4O12.
Surprisingly, ferrolectricity shows up at a characteristic temperature, T= 70 K, where magnetic susceptibility shows a large anormal peak not compatiblewith a long range magnetic order, as reported previously [108]. The observation of ferroelectricity is consistent with complementary single-crystal synchrotron x-ray diffraction data showing the appearance of a commensurate non-centrosymmetric structural modulation at the structural phase transition Ts = 200 K. To the best of our knowledge, this is the first unambigous observation of non-centrosymmetric distortion in a magnetic ferroelectric.
The P-values expected in single-crystals bulk or thin films samples being significantly larger than those reported here suggest that (YMn3)Mn4O12 may be a suitable material for practical multiferroic applications.

Table of contents :

Abstract
Résumé
Riassunto
1 Introduction 
1.1 Generalities about magnetoelectric multiferroics
1.2 Magnetically induced ferroelectricity
1.2.1 Classification of multiferroics
1.2.2 Phenomenological theory
1.2.3 Microscopic theories
1.3 (AMn3)Mn4O12 quadruple perovskites
1.3.1 Open questions and strategy for the Thesis
2 Experimental methods 
2.1 High-pressure/high-temperature syntheses
2.2 Diffraction techniques
2.3 Thermodynamics measurements
2.3.1 Transport properties
2.3.2 Magnetization
2.3.3 Heat capacity
2.4 Spectroscopy techniques
2.4.1 Difference between Infrared and Raman spectroscopy
2.4.2 Sample preparation for spectroscopy
2.4.3 Fourier Transform InfraRed (FTIR) Spectrometer
2.4.4 Triple-stage Raman spectrometer
2.5 Ferroelectric characterizations
2.5.1 Pyroelectric currents
2.5.2 Dielectric constant
3 Multiferroic properties of (LaMn3)Mn4O12 
3.1 Introduction
3.2 Article I: Large ferroelectricity induced by collinear magnetism in the quadruple perovskite (LaMn3)Mn4O12
Fig. 3.1 Cell-parameters vs temperature
Fig. 3.2 Pyrocurrents vs Temperature
Fig. 3.3 Remnant polarization vs T
Fig. 3.4 Magnetoelectric coupling measurements
4 Raman and IR spectroscopy study of the spin-lattice coupling in (LaMn3)Mn4O12 
4.1 Introduction
4.2 Article II: Evidence of strong magnetoelastic coupling driving magnetic ferroelectricity
Fig 4.1 Infrared spectra
Fig 4.2 Raman spectrum
Fig 4.3 Infrared frequencies vs temperature
Fig 4.4 Raman frequencies vs temperature
Fig 4.5 Dielectric constant vs frequency
Fig 4.6 Specific heat vs magnetic field
5 Effect of the chemical pressure (La3+/Y3+ substitution) 
5.1 Introduction
5.2 Article III: Effect of chemical pressure induced by the La3+/Y3+ substitution on the magnetic ordering of (AMn3)Mn4O12 quadruple perovskites
5.3 Supplementary information on article III
6 Multiferroic properties of the new quadruple perovskite (YMn3)
6.1 Motivations
6.2 Article IV: Evidence of centrosymmetry breaking in the magnetic ferroelectric (YMn3)Mn4O12
Fig. 6.1 Pyroelectric currents vs temperature
Fig. 6.2 Remnant polarization vs temperature
Fig. 6.3 Remnant polarization vs poling electric field
Fig. 6.4 Capacitance vs temperature
Fig. 6.5 Electrical resistivity vs temperature
Fig. 6.6 Single-crystal diffraction patterns
Fig. 6.7 Scheme of the super-cell
Conclusions and perspectives
Bibliography 

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