**Theoretical Backgrounds and Principles**

**1** 

*India* 

**Analysis of Thermodynamic** 

**and Phononic Properties of** 

*Department of Physics, Barkatullah University, Bhopal* 

The perovskite manganites are of recognized importance, owing to the development of new materials designed for different potential scientific and technological applications, such as *magnetic sensors in the tracking and location of emergency personnel, ferroelectric thin films, microwaves, spintronics, semiconductor technologies and memory devices* etc. These materials have been the subject of significant research interest because of the intriguing underlying physics showing marked colossal magnetoresistance (CMR) effect and the anticipated multifunctional and advanced applications for the next generation electronics. The recent discovery of large magnetoelectric effects in the *R*1−*xAx*MnO3 (where *R* and *A* are trivalent rare earth and divalent alkaline earth ions, respectively) has kindled interest among investigators in understanding the complex relationships between lattice distortion, magnetism, dielectric and thermodynamic properties of undoped *R*MnO3 and are good candidates for certain sensor applications, bolometers, magnetic refrigeration, read head devices, magnetic storage in hard disk and floppy disks, and spin valve devices (such as the 21st Century electronics i.e. spintronics (Yamasaki et al., 2007). The huge magnetoresistance or colossal magnetoresistance effect (CMR) has been observed in the perovskite manganites (Jin et al., 1994). The *magnetoresistance* is defined as (Δ*R/RH* = [*R(T,H)* − *R(T,* 0*)*]*/R(T, H).*  Magnetoresistance can be negative or positive. In most nonmagnetic solids the magnetoresistance is positive. In non-magnetic pure metals and alloys MR is generally positive and MR shows a quadratic dependence on H. MR can be negative in magnetic materials because of the suppression of "Spin Disorder" by the magnetic field. The Giant magnetoresistive effect GMR was recently discovered by Sir Peter Grünsberg (Germany) and Sir Albert Flert (France) (see photographs 1 and 2) and both of them were awarded Nobel Prize in the year 2007 (Baibich et al., 1988; Binasch et al., 1989). It has been used extensively in read heads of modern hard drives. Another application of the GMR effect is non-volatile, magnetic random access memory. Read heads of modern hard drives are based on the GMR or TMR effect.The GMR effect is observed, mainly, in artificial multilayer systems with alternating magnetic structure and is used in many applications. The large magnetoelectric effects in the rare-earth manganites RMnO3 has reopened the field of the socalled *multiferroic materials.* Multiferroic means, that several ferro-type orders, such as

ferromagnetism, ferroelectricity or ferroelasticity coexist.

**1. Introduction** 

**Perovskite Manganites** 

Renu Choithrani
