QUALITATIVE UNDERSTANDING OF MULTIFERROIC BEHAVIOR OF MN-BASED SYSTEM AND SYNTHESIS OF ROD-LIKE NANOSTRUCTURE OF TbMnO3

GAIKWAD V.M.¹*, KHULE S.M.², ACHARYA S.A.^3
1Depatment of Physics, Rashtrasant Tukdoji Maharaj Nagpur University, Nagpur, MS, India.
²Wainganga College of Engineering and Management, Dongargaon, Nagpur, MS, India.
³Depatment of Physics, Rashtrasant Tukdoji Maharaj Nagpur University, Nagpur, MS, India.
* Corresponding Author : vish9823@gmail.com, saha275@yahoo.com

Received : 28-02-2012     Accepted : 06-03-2012     Published : 15-03-2012
Volume : 3     Issue : 1       Pages : 55 - 57
Int J Knowl Eng 3.1 (2012):55-57

Conflict of Interest : None declared

In the present project work, to understand the role of A and B atoms of ABO3 pervoskite in multiferroic behavior, forty multiferroics are systematically reviewed on the basis of parameters like electronegativity, ionic radii, bond covalence and tolerance factor. The correlation of tolerance factor with Neel temperature and Curie Temperature of Mn-based orthorhombic pervoskite multiferroics are studied. The nature of A-O chemical bonding and its influence on multiferroic behaviour is discussed. In the second part of this project the multiferroic TbMnO3 was successfully prepared and characterized by XRD and SEM. The XRD shows the amorphous nature of as-synthesized TbMnO3, after calcinations at 800oC the crystalline orthorhombic structure is formed. The SEM images confirm the rod-like morphology TbMnO3 having diameter in range of 40-60 nm and length is in range of µm.

Multiferroic, TbMnO3, Bond covalence

Multiferroics are materials that exhibit coexistence of ferromagnetic, ferroelectric or ferroelastic. They display the phenomenon of controlling electrical polarization by the application of an external magnetic field. They have great possibility for application in multifunctional devices. Magnetoelectrics multiferroics have recently attracted a lot of interest due to these exceptional properties. Unlike in conventional ferroelectrics in the manganite pervoskite such as TbMnO₃ and DyMnO₃, ferroelectricity arises from of peculiar coupling of the lattice to a spiral ordering of Mn spin. A strong anisotropic dependence of the spontaneous electric polarization on the direction of magnetic field is found which can be related to the spiral ordering of the magnetic moments. Orthorhombic TbMnO₃ is multiferroic material which upon cooling show successive phase transition related to antiferromagnetic ordering at T_N ~ 41K and ferroelectric ordering at T_C ~ 23K respectively, while DyMnO₃ have antiferromagnetic ordering at 30 K and ferroelectric ordering at T_C = 18 K [1-10] .
Generally, it is observed that changing of A or B element in ABO3 multiferroic, vary the ferromagnetic and ferroelectric phase transition temperature. To understand multiferroic behavior it is very essential to recognize the role of A and B. In the present project work, more than forty pervoskite (ABO₃) multiferroics are reviewed. The role of parameters like ionic radii, elecronegativity and nature of chemical bonding (A-O and B-O) of A and B elements are tried to correlate with Curie and Neel temperature of the system [10-17] .

The ionic radii and electronegativity is used to calculate the tolerance factor and bond covalence. The tolerance gives the knowledge of compactness of unit cell. Bond covalence helps us to understand the atomic interaction in the molecular level.
The bond covalence is calculated by using formula Covalence

(A-O) = exp {-1/4(XO-XA)}………… (i)

Where,
XO - Electronegativity of O-atom,
XA - Electronegativity of A-atom,
XB - Electronegativity of B-atom.
The tolerance factor is calculated by using formula,

Tolerance factor = 1/√2 {(RA + Ro) / (RB + RO)}…. (ii)

where,
RA - Ionic radius of A-atom,
RB - Ionic radius of B-atom,
Ro­ - ionic radius of o-atom.

Tolerance factor of almost forty Mn-base multiferroics are calculated and are found to be less than 1.
The tolerance factors versus Neel temperature and Curie temperature plot of some Mn-based pervoskite multiferroic is shown in [Fig-1] . It point out that tolerance factor is directly related with temperatures. It clearly indicates that ionic size of A (in present case B is constant) playing important role in magnetic and electric ordering, which are responsible for the magnetic and electric phase transition.
The covalence of A-O and B-O bonds of the perovskite system are calculated by using Pauli’s electronegativity equation (i). It is found that bonds covalence of A-O lies in between 0.55 to 0.7, while for B-O, it is in between 0.62 to 0.665 It clearly indicates that both the bonds are partial covalent and partial ionic.
Covalence for B-O bond is observed to be in average 65 % and A-O bond is in average 70%. It means in B-O bonds 60% of charge is shared between B and O atoms and 40% charge shifted towards O. Same can be explained for A-O bond also. Oxygen electron cloud is commonly share with A and B type of atoms in the structure. Change in A or B position due to temperature sensitive structural phase transition, changes sharing of electron cloud between A-O and B-O. Its gives rise to change of covalence of A-O and B-O bond.
The position of B is shifted slightly from the centre position, which is responsible for charge imbalance at B site in the molecular level as a consequence of phase transition. Variation of Neel temperature with bond covalence [Fig-2] exhibits that Neel temperature is inversely related with bond covalence. It can be interpreted as electron sharing between A and O affect on spiral ordering of Mn spin which is responsible for successive phase transition upon cooling in Mn-based orthorhombic pervoskite multiferroic. The systematic study to comprehensive understanding this behavior is under investigation.

TbMnO₃ particles are prepared by taking stoichiometric (1:1) amounts of 0.05 M solutions of TbCl₃.6H₂O (99.9%, Aldrich) and MnCl₂.4H₂O (AR, Fishers), in double distilled water, were mixed and stirred for 2 hour. For synthesis, we followed two different routes to study the effect of precipitation conditions on samples properties. In first route, 0.05 M solution of KOH (GR, Merck) was added to this solution of the mixed salts, under vigorous stirring, leading to a dark brown color solution. After 17 hours,
magnetic stirring at room temperature, the colloidal solutions was filtered and washed with water. After drying at room temperature, these samples were calcined (heat treated in air, at atmospheric pressure) at 800ËšC for 90 minutes. The powder is used for further characterization. XRD and SEM are shown.

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	Fig. 1- Tolerance factor versus Neel Temperature of Mn-based pervoskite multiferroic.
	Fig. 2- Bond covalence of B-O bond versus Neel Temperature of Mn-based pervoskite multiferroic.
	Fig. 3- XRD patterns of TbMnO3
	Fig. 4- SEM image of TbMnO3­ nanorods