1. Introduction of hexagonal manganites as multiferroic materials

2. Brief overview of the synthesis method

DOI: http://dx.doi.org/10.5772/intechopen.81753

following starting materials were used:

both desired samples are shown below:

• 0.5 Ho2O3 + 0.5 Mn2O3 ! HoMnO3

• 0.5 Y2O3 + 0.5 Mn2O3 ! YMnO3

• Holmium oxide (Ho2O3), 99.99%, Strem Chemicals

• Manganese (III) oxide (Mn2O3), 99%, Strem Chemicals

pact resulting in more uniform shrinkage during sintering.

101

Sintering is a heating process in which the atomic mobility of the compact is sufficient to permit the decrease of the free energy associated with the grain boundaries [13]. It is the most critical and expensive process step as it yields the required crystal structure, oxidation state, microstructure, and physical condition

• Yttrium oxide (Y2O3), 99.99%, Alfa Aesar

Polycrystalline holmium and yttrium manganite samples were synthesized by a solid state reaction via high-energy ball milling (HEBM) with 12 h milling time. This was carried out by mixing and milling together according to the stoichiometric ratios of the required metal oxide powders, followed by pelletizing and furnace heating. The most important measurements involved in this project are ferroelectric parameters with the evolution of their microstructure. These are, ferroelectric hysteresis P-E loop along with microstructure analysis by FESEM measurement. For the purpose of the multiferroic material preparation in this project, the

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline…

The chemical equation via mechanical alloying (high-energy ball milling) for

The requirement for the sample preparation is to have the powder materials in the form of nanosized starting particles. With the use of HEBM, the duration of milling time has been chosen to be 12 h, which is optimum in order to obtain nanosized particles. The mixed material was crushed by using high-energy ball milling (HEBM) in order to facilitate the solid state reaction. The mechanical alloying (MA) process starts with mixing the powders in the right proportion and loading them into the mill along with the grinding medium (usually steel balls). The mixture is then milled for the desired length of time until a steady state is reached when the composition of every powder sample contains the same proportion of the elements in the starting powder mix. The milled powder is then transformed into a bulk shape and heat-treated to obtain the desired microstructure and properties. The key of the process involved is the raw materials, the mill, and the process variables [12]. The selected optimum parameters for the milling and ball-to-powder ratio (BPR) were 12 h and 10:1, respectively, and were used for the preparation of all the multiferroic nanoparticles used in this project. For pellet preparation, �1.0 g is required from the as-milled powder for each sintering temperature. The mechanically alloyed nanoparticle materials were weighed according to the calculated formula using an analytical microbalance (A&D, model GR-200) and granulated by using 2% PVA. The samples were then lubricated with zinc stearate in order to reduce the density gradient caused by friction of the powder along the wall of the mold. The transformation of the previously granulated powder into a pellet shape was carried out by pressing the mold with a force of 200 MPa by using a pressing machine. Suitable pressure is important in order to obtain a uniform density com-

Multiferroic materials consist of more than one ferroic polarization, and the term multiferroic was first coined by Schmid in 1994 to indicate a material that has either two or three different kinds of ferroic orders like ferromagnetism, ferroelectricity, ferroelasticity, and ferrotoroidicity in the same phase [1]. The definition is often extended to antiferroic orderings. Ferroic materials are defined as possessing a spontaneous order, the direction of which can be switched by using an external field. A ferromagnet has a spontaneous magnetization M and shows hysteresis under an applied magnetic field H. A ferroelectric has a spontaneous polarization P and shows hysteresis under an applied electric field E. As for ferroelastic, it has a spontaneous strain and shows hysteresis under an applied stress. Such spontaneous order typically occurs as a result of a phase transition when the material is cooled below a particular temperature. Following Schmid in 1994, these kinds of materials are recognized as multiferroics (Figure 1). Nowadays, what most people mean by multiferroic predominantly applies to the coexistence of magnetism and ferroelectricity.

In the past 5 years, research into multiferroics has branched into several different areas. The use of multiferroics for technology is currently a widely researched field, and the fundamental physics governing the strong magnetoelectric coupling seen in multiferroics is still not fully understood. Now that the causes of multiferroic behavior are better understood, it has been possible for us to observe the evolution of their microstructure with respect to their ferroelectric behavior acting in the nanoscale-to-micron-sized grain regimes. As for the chosen materials, the hexagonal rare-earth manganites, RMnO3, were first discovered in 1963 [2–4]. Hexagonal RMnO3 compounds show a strong ferroelectric ordering with saturated polarization larger than 5.6 μCcm<sup>2</sup> [5–7]. Thus, hexagonal rare-earth manganites are classified as an interesting family with multiferroic properties, which is also the driving force for them to be the focus of this study.

Bertaut et al. in 1963 discovered the ferroelectricity in hexagonal REMnO3 [3, 4]. Precise structural investigations have been employed to find the origin of ferroelectricity in hexagonal rare-earth manganites since its discovery by Bertaut et al. [8–11]. Consequently, the origin of ferroelectricity is geometrically driven by the displacement between RE3+ and O<sup>2</sup> resulting from a structural phase transition. A structural phase transition from a high-temperature paraelectric phase (space group P63/mmc) to a low-temperature ferroelectric phase (space group P63 cm) has been observed in hexagonal REMnO3. YMnO3 may serve as a representative of all hexagonal rare-earth manganite systems, especially for HoMnO3 with its similar size of the rare-earth ion and lattice parameters.

Figure 1. Multiferroics combine the properties of ferroelectrics and magnets.

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline… DOI: http://dx.doi.org/10.5772/intechopen.81753

## 2. Brief overview of the synthesis method

1. Introduction of hexagonal manganites as multiferroic materials

ferroelectricity.

Functional Materials

Figure 1.

100

Multiferroic materials consist of more than one ferroic polarization, and the term multiferroic was first coined by Schmid in 1994 to indicate a material that has either two or three different kinds of ferroic orders like ferromagnetism, ferroelectricity, ferroelasticity, and ferrotoroidicity in the same phase [1]. The definition is often extended to antiferroic orderings. Ferroic materials are defined as possessing a spontaneous order, the direction of which can be switched by using an external field. A ferromagnet has a spontaneous magnetization M and shows hysteresis under an applied magnetic field H. A ferroelectric has a spontaneous polarization P and shows hysteresis under an applied electric field E. As for ferroelastic, it has a spontaneous strain and shows hysteresis under an applied stress. Such spontaneous order typically occurs as a result of a phase transition when the material is cooled below a particular temperature. Following Schmid in 1994, these kinds of materials are recognized as multiferroics (Figure 1). Nowadays, what most people mean by multiferroic predominantly applies to the coexistence of magnetism and

In the past 5 years, research into multiferroics has branched into several different areas. The use of multiferroics for technology is currently a widely researched field, and the fundamental physics governing the strong magnetoelectric coupling

multiferroic behavior are better understood, it has been possible for us to observe the evolution of their microstructure with respect to their ferroelectric behavior acting in the nanoscale-to-micron-sized grain regimes. As for the chosen materials, the hexagonal rare-earth manganites, RMnO3, were first discovered in 1963 [2–4]. Hexagonal RMnO3 compounds show a strong ferroelectric ordering with saturated polarization larger than 5.6 μCcm<sup>2</sup> [5–7]. Thus, hexagonal rare-earth manganites are classified as an interesting family with multiferroic properties, which is also the

Bertaut et al. in 1963 discovered the ferroelectricity in hexagonal REMnO3 [3, 4]. Precise structural investigations have been employed to find the origin of ferroelectricity in hexagonal rare-earth manganites since its discovery by Bertaut et al. [8–11]. Consequently, the origin of ferroelectricity is geometrically driven by the displacement between RE3+ and O<sup>2</sup> resulting from a structural phase transition. A structural phase transition from a high-temperature paraelectric phase (space group P63/mmc) to a low-temperature ferroelectric phase (space group P63 cm) has been observed in hexagonal REMnO3. YMnO3 may serve as a representative of all hexagonal rare-earth manganite systems, especially for HoMnO3 with its similar size of

seen in multiferroics is still not fully understood. Now that the causes of

driving force for them to be the focus of this study.

the rare-earth ion and lattice parameters.

Multiferroics combine the properties of ferroelectrics and magnets.

Polycrystalline holmium and yttrium manganite samples were synthesized by a solid state reaction via high-energy ball milling (HEBM) with 12 h milling time. This was carried out by mixing and milling together according to the stoichiometric ratios of the required metal oxide powders, followed by pelletizing and furnace heating. The most important measurements involved in this project are ferroelectric parameters with the evolution of their microstructure. These are, ferroelectric hysteresis P-E loop along with microstructure analysis by FESEM measurement. For the purpose of the multiferroic material preparation in this project, the following starting materials were used:


The chemical equation via mechanical alloying (high-energy ball milling) for both desired samples are shown below:


The requirement for the sample preparation is to have the powder materials in the form of nanosized starting particles. With the use of HEBM, the duration of milling time has been chosen to be 12 h, which is optimum in order to obtain nanosized particles. The mixed material was crushed by using high-energy ball milling (HEBM) in order to facilitate the solid state reaction. The mechanical alloying (MA) process starts with mixing the powders in the right proportion and loading them into the mill along with the grinding medium (usually steel balls). The mixture is then milled for the desired length of time until a steady state is reached when the composition of every powder sample contains the same proportion of the elements in the starting powder mix. The milled powder is then transformed into a bulk shape and heat-treated to obtain the desired microstructure and properties. The key of the process involved is the raw materials, the mill, and the process variables [12]. The selected optimum parameters for the milling and ball-to-powder ratio (BPR) were 12 h and 10:1, respectively, and were used for the preparation of all the multiferroic nanoparticles used in this project. For pellet preparation, �1.0 g is required from the as-milled powder for each sintering temperature. The mechanically alloyed nanoparticle materials were weighed according to the calculated formula using an analytical microbalance (A&D, model GR-200) and granulated by using 2% PVA. The samples were then lubricated with zinc stearate in order to reduce the density gradient caused by friction of the powder along the wall of the mold. The transformation of the previously granulated powder into a pellet shape was carried out by pressing the mold with a force of 200 MPa by using a pressing machine. Suitable pressure is important in order to obtain a uniform density compact resulting in more uniform shrinkage during sintering.

Sintering is a heating process in which the atomic mobility of the compact is sufficient to permit the decrease of the free energy associated with the grain boundaries [13]. It is the most critical and expensive process step as it yields the required crystal structure, oxidation state, microstructure, and physical condition of a material. The purpose of the sintering process is to complete the interdiffusion of the component metal ions into the desired crystal lattices and to develop the polycrystalline microstructure at as low a temperature as possible, first to ensure a high grade of microstructural homogeneity, second to avoid cannibal grain growth, and third to save energy cost. At this stage, atoms are closely in contact with each other, hence reducing the space between them. Therefore, shrinkage occurs as a consequence of aggregation of the entire substance. As required by commercial production practice, the desired shrinkage is between 10 and 20%.

reactivity in the starting powder is required in order to prepare a series of samples in which the observation of nanosized starting particles followed by nano-to-micron

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline…

A control of the crystal orientation is significantly important, because the ferroelectric polarization of HoMnO3 and YMnO3 appears along the hexagonal c-axis. Figure 3(a) and (b) reveals the XRD spectra of HMO and YMO samples, respectively. At 12 h high-energy ball milling, broad diffraction peaks belonging to

size grains is required in this study.

DOI: http://dx.doi.org/10.5772/intechopen.81753

Figure 3.

103

(a) XRD patterns of for as milled continued with sintering from 600 to 1250<sup>o</sup>

C for YMO.

for as milled continued with sintering from 600 to 1250o

C for HMO. (b) XRD patterns of

Since the aim of this research is to study in detail the evolution of the microstructure and the subsequent effect of the ferroelectric processes, sintering was used as an agent to control such desired changes that would be observed in the microstructure. The sintering temperature used for the purpose of detecting the sintering effects on the P-E hysteresis loops was from 600 to 1250°C with the increase of 50°C giving a total of 14 samples for each batch. They were sintered separately for 10 h in an electric furnace. The average dimensions of the final product with the pellet shape were about 10 mm diameter and 1–2 mm thickness.
