Chapter 2
Multiferroics
2.1 Introduction to Multiferroics:
H. Schmid used the term multiferroic for the first time in 1994. Those materials which combine multiple ferroic properties such as ferromagnetism, ferroelectricity and ferroelasticity are known as multiferroics. Simultaneous coexistence of at least two ferroic properties takes place in the same phase in multiferroics. It has the feasibility of exhibiting coupling between ferroelectricity and magnetism which is known as the magnetoelectric effect (ME). This ME enables the external electric field to change magnetization [1].
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Each multiferroic property is closely connected to symmetry. The principal ferroic properties can be characterized by their behavior under time and space inversion. For example the direction of polarization P is reversed by Space inversion while leaving the magnetization M invariant. In turn, time reversal will change the sign of M, while the sign of P remains invariant.
Space invariant
Space variant
Time invariant
Ferroelastic
Ferroelectric
Time variant
Ferromagnetic
Ferrotoroidic
A simultaneous violation of space and time inversion symmetry is required by Magnetoelectric multiferroics [2].
There are also various potential applications of multiferroic such as information storage, spintronics, sensors and microelectronics devices in the field of material science due to the presence of strong coupling of electric, magnetic and structural order parameters. These parameters gave rise to simultaneous occurrence of ferroelectricity, ferroelasticity and ferromagnetism [3].
Application of magnetic field can induce intrinsic polarization and application of external electric field can induce magnetization in any magnetoelectric compound. Apart of industrial application, these coupling of properties in magnetoelectric compounds makes them important from physics point of view because of their enriched physical properties. However very few materials shows both these ferroic properties at or above room temperature [4].
Two fundamental forces of nature are magnetism and electricity. Combination of these two properties in a single multiferroic material is applicable for many practical applications such as they can be used as magnetic sensors in which the sign of their electric polarization changes with a small magnetic field. These effects are important to understand as multiferroics are not only quite rare but their properties also helps to develop materials where these effects are suitably strong for applications.
A beam of x-rays are used to study the magnetic properties of multiferroics. The electronic states of the iron ions in the crystal are specifically probed by the x-rays which are related to its magnetic properties. This experiment reveals that these electronic states extend throughout the material in a periodic manner. It breaks the crystal symmetry and leads to a shift of the electrically charged atoms in the crystal which is responsible for multiferroic properties.
Each iron atoms is surrounded by a symmetric arrangement of oxygen atoms and the magnetic moments of the iron atoms are in disorder at room temperature whereas the magnetic moments are assumed to have the shape of a screw at low temperatures. The energy of the chemical bonds are slightly altered by each magnetic moment in the crystal which depends on the relative orientation between the chemical bond direction and the magnetic moment. The resulting force distorts the crystal structure which leads to an electric polarization [5].
Recently, multiferroics due to their potential properties such as comprising of ferroelectric and ferromagnetic ordering in elastically distorted systems have drawn a major attention of researchers for fabrication of magnetic and ferroelectric devices. High dielectric constant, low dielectric loss, high temperature phase transition and small structural distortion occurs mainly due to the electric, magnetic and stress field applied on the materials for multifunctional applications. In the process of development of new materials, many novel materials have been detected for different purposes due to their useful and interesting properties [6].
2.2 Types of Multiferroics:
Multiferroics can be divided into two groups:
Type-I Multiferroics2) Type-II Multiferroics
Type-I Multiferroics:
This type of multiferroics are older, more numerous and are good ferroelectrics. Above room temperature, the critical temperatures of the magnetic and ferroelectric transitions can be well. In these materials, the coupling between magnetism and ferroelectricity is unfortunately weak [14]. Different origin of ferroelectricity and magnetism in type-I multiferroic are mostly due to different active subsystems of a material. There is a certain coupling between breaking time reversal symmetry, breaking spatial inversion symmetry, ferroelectric order parameter, magnetic order parameter in such type-I multiferroics. In these materials, ferroelectricity can have a number of possible microscopic origins [7].
For example: BiFeO3 with the ferroelectric transition temperature Tc higher then the Neel transition temperature TN. [8]
Type-II Multiferroics:
Due to the recent discovery of a novel class of multiferroics, there is the biggest excitement as ferroelectricity exists only in a magnetically ordered state and is caused by a particular type of magnetism. A nonzero electric polarization occurs in the low temperature phase [14] . For example CuFeO2 with Tc = TN [15].
The magnetic and/or electric polarization of the barrier controls the current driven through a magnetic tunnel junction (MTJ) with a multiferroic tunnel barrier. Multiferroic tunnel junctions is referred to the junctions with a multiferroic tunnel barrier. The use of a multiferroic material as a tunnel barrier and ferromagnetic materials as leads in MFTJs would lead to 8 possible resistive states of such junctions [9].
Figure 1.3: Multiferroic tunneljunction (left) and eight resistive states that it provides (right). The ferroelectric polarization is depicted with the black arrow; the white arrow stands for the magnetization.
Cupric Ferrite (CFO): type-II multiferroic has attracted increasing attention due to the recent discovery of ferroelectricity in the first magnetic field induced phase. It is considered as a distinct class of magnetoelectric (ME) multiferroics [10]. Magnetic field induced generates a spontaneous electric polarization parallel to the helical axis in delafossite compound CuFeO2 [11]. Delafossite crystals have general formula ABO2, where A represents cations which are linearly co-ordinated with two oxygen ions and B represents cations, situated in distorted edge-sharing BO6 octahedra [12]. The materials possesse R m space group and have found very useful device applications because of different properties such as superconductivity, large magnetoresistance, thermoelectric effects and multiferroicity [13]. It has Hexagonal crystal structure.
Figure 1.Crystal structure of CuFeO2 with the hexagonal unit cell [14]
These effects make them potential candidates for device applications in such kind of multiferroic materials. CFO was first discovered by Friedel and Hebd12 in 1873 and it is considered to be one of the promising materials of this group [15].
CFO was broadly studied in past decade due to its pleasing antiferromagnetic properties at liquid helium temperatures. Numerous magnetic phase transitions and multiferroicity due to geometrical frustrations at low temperature are seen by this antiferromagnet on a triangular lattice [16, 17, 18, 19].
The delafossite structure of CFO consists of hexagonal layers of Cu, O and Fe accumulated with a Cu-O-Fe sequence along the c-axis in order to form a layered triangular lattice antiferromagnet. It is a p-type semiconductor with low conductivity σ=1.53 53 S/cm, high Seebeck coefficient S=544 V/K, and a small bandgap of 1.15 eV. The electrical and optoelectrical properties of CFO were explored by by Benko and Koffyberg [20, 21].
By combining two of the fundamental forces of nature i.e magnetism and electricity in a single multiferroic material in which one controls the other is not only of basic interest, but also significant for practical applications. Multiferroic materials can also be used as magnetic sensors in which the sign of their electric polarization is changed with a small magnetic field. A new mechanism has been verified after studying the properties of the multiferroic CuFeO2 by which magnetism and electricity can be coupled in a single material. Magnetism and ferroelectricity are coupled in different ways in multiferroics. Apart of multiferroics being quite rare, a better understanding of their properties is essential as it helps to develop materials where these effects are suitably strong for applications. the magnetic properties of CuFeO2 using a beam of x-rays were studied by the researchers. This study reveals that these electronic states extend throughout the material in a periodic manner which is directly responsible for the multiferroic properties as it breaks the crystal symmetry and leads to a shift of the electrically charged atoms in the crystal. Each of the iron atoms is surrounded by a symmetric arrangement of oxygen atoms and the magnetic moments of the iron atoms are in disorder at room temperature.
The energy of the chemical bonds is slightly altered by each magnetic moment in the crystal which depends on the relative orientation between the magnetic moment and the chemical bond direction. The crystal structure is then distorted by the resulting force which leads to an electric polarization [22].
Several routes for the synthesis of multiferroics are being applied such as solid state synthesis hydrothermal synthesis, sol-gel processing, Sol-Gel autocombustion, vacuum based deposition, floating zone [23]. In the present study, modified Sol-Gel autocombustion technique is used.
Processing techniques influence the physical properties and the ideal synthesis techniques provide superior control over the parameters such as crystallite size, distribution of particle sizes and interparticle spacing which have the greatest impact on the magnetic and other properties [24, 25]. In present work we have adopted sol-gel auto-combustion technique because of some advantages over other methods like the reagents are simple compounds, special equipments are not required, agglomeration of powders remains limited and dopant can be easily introduced into the final product. The properties of the final product such as particle size, surface area and porosity depend on the method of combustion [26, 27]
2.3 Plants:
According to the literature reviews, various microorganisms such as fungi, yeasts algae and bacteria are used for the biosynthesis of nanoparticles but presently a new trend has come to force the use of plants for the fabrication of nanoparticles because of its spontaneous, economical, eco-friendly protocol, suitable for large scale production and single step technique for the biosynthesis process [28].
The major mechanism examined for the synthesis of nanoparticles mediated by the plants is due to the presence of phytochemicals which are responsible for the spontaneous reduction of ions are flavonoids, terpenoids, carboxylic acids, quinones, aldehydes, ketones and amides [29].
The botanical details about the currently used flowers for the study of synthesis of Cupric Ferrite are as follows:
Delonix Regia [30]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Rosidae
Order
Fabales
Family
Fabaceae ⁄ Leguminosae – Pea family
Genus
Delonix Raf. – delonix
Species
Delonix regia (Bojer ex Hook.) Raf. – royal poinciana
Rosa indica: [31]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Rosidae
Order
Rosales
Family
Rosaceae – Rose family
Genus
Rosa L. – rose
Species
Rosa indica L. – cyme rose
Vinca [32]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Asteridae
Order
Gentianales
Family
Apocynaceae– Dogbane family
Genus
Vinca L. – periwinkle
Hibiscus [33]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Dilleniidae
Order
Malvales
Family
Malvaceae – Mallow family
Genus
Hibiscus L. – rosemallow
Species
Hibiscus rosa-sinensis L. – shoeblackplant
Jasmine [34]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Asteridae
Order
Scrophulariales
Family
Oleaceae – Olive family
Genus
Jasminum L. – jasmine
Euphobia milli [35]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Rosidae
Order
Euphorbiales
Family
Euphorbiaceae – Spurge family
Genus
Euphorbia L. – spurge
Alamanda [36]
Rank
Scientific Name and Common Name
Kingdom
Plantae – Plants
Subkingdom
Tracheobionta – Vascular plants
Superdivision
Spermatophyta – Seed plants
Division
Magnoliophyta – Flowering plants
Class
Magnoliopsida – Dicotyledons
Subclass
Asteridae
Order
Gentianales
Family
Apocynaceae – Dogbane family
Genus
Allamanda L. – allamanda
Species
Allamanda cathartica L. – golden trumpet
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[8] Randy Fishman, Oak Ridge National Laboratory, Materials Science and Technology Division, Monday, 22 September, 2014
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[23] D. Varshney et al., J. Alloys Compd. 509, 8421 (2011)
[24] Candac T S, Carpenter E E, O’Connor C J, John V T and Li S, IEEE Trans. Magn. 34, 1111 (1998).
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[30] http://plants.usda.gov/core/profile?symbol=DERE
[31] http://plants.usda.gov/core/profile?symbol=ROIN5
[32] http://plants.usda.gov/core/profile?symbol=VINCA
[33] http://plants.usda.gov/core/profile?symbol=HIRO3
[34]https://plants.usda.gov/java/ClassificationServlet?source=display&classid=JASMI
[35]https://plants.usda.gov/java/ClassificationServlet?source=display&classid=EUPHO
[36] http://plants.usda.gov/core/profile?symbol=ALCA7
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