Magnetic Ordering in Bulk and Nanoparticles of Certain Bismuth Based Manganites Bi1-xAxMnO3 (A = Ca, Sr) : Electron Paramagnetic Resonance and Magnetization Studies

Abstract

The study of bulk and nanoparticles of perovskite rare earth manganites has been an extensive area of research in the recent past due to their rich and interesting physics and potential applications [1-5]. Manganites have potential applications in the emerging field of spintronics because of their colossal magnetoresistance (CMR) [6] and half-metallic [7] properties. Nano sized materials exhibit enhanced and different electronic and magnetic properties and expected to behave quite differently compared to their bulk counterparts due to quantum confinement effects and high surface/volume ratio. Magnetic nanoparticles in particular have great potential for use in a wide range of applications including magnetic recording media, various sensors, catalysts, magnetic refrigeration, medicine etc. In this thesis we study changes in the magnetic ordering of certain bismuth based manganites Bi1-xAxMnO3 (A = Ca and Sr) using various experimental probes when we reduce their particle size to nano scale. The general formula for doped manganites is R1-xAxMnO3 where R is a trivalent rare-earth ion such as La, Nd, Pr, Sm and A is a divalent alkaline earth ion such as Ca, Sr, Ba, Pb. They became interesting due to their many intriguing properties like CMR (Colossal Magnetoresistance), phase separation, charge ordering (CO), orbital ordering (OO) and many more. These properties depend sensitively on many factors like temperature, magnetic field, pressure and doping concentration x. There is a strong coupling of spin, orbital and lattice to each other in manganites. The complex interplay of all these couplings make them strongly correlated systems. In the parent compound RMnO3 Mn ion is in Mn3+ state while it is present as Mn4+ in the compound AMnO3. The manganites with x = 0 and x = 1 are both antiferromagnetic insulators, magnetism in them being mediated by superexchange through oxygen. On doping with a divalent alkaline earth ion in RMnO3, there is a transition The properties of nanoparticles of manganites show strong surface effects. The magnetic behavior is strongly governed by the free surface spins in nanoparticles. And as the size reduces, there is suppression of charge ordering which can also disappear in very small particles [11]. Antiferromagnetism in bulk gives way to ferromagnetism in nanoparticles [12-14]. In the following we give a chapter wise summary of the results reported in the thesis. Chapter 1: This chapter of the thesis consists of a brief introduction to the physics of manganites. Further we have written a detailed overview of bismuth based manganites, properties of nano manganites and the technique of EPR. There is a section about different line shapes observed in EPR of manganites, their origin and how to fit them to appropriate lineshape function [15]. This chapter also includes a detailed account of experimental methodologies used in thesis which are: EPR spectrometer, SQUID magnetometer, X-ray diffractometer and TEM and the analysis procedure adopted in this work. Chapter 2: This chapter deals with the magnetic and EPR studies of nanoparticles (average diameter ~ 30 nm) of Bi0.25Ca0.75MnO3 (BCMO) and their comparison with the results on bulk BCMO. Bulk Bi0.25Ca0.75MnO3 (BCMOB) shows charge ordering at 230 K followed by a transition to an antiferromagnetic phase at 130 K [16]. The bulk and the nanoparticles (D ~ 30 nm) of Bi0.25Ca0.75MnO3 were prepared by solid-state reaction method and sol-gel method respectively. The two samples were investigated by using XRD, TEM, SQUID and EPR techniques. Our magnetization and EPR results show that the charge ordering disappears in nanoparticles of this composition and there emerges a ferromagnetic phase similarly to the rare earth manganites. The nanoparticles of the rare earth based manganites are found to consist of an antiferromagnetic core and a ferromagnetic shell/surface region [3, 17] and thus are expected to exhibit the ‘exchange bias (EB) effect’ [18-22] resulting in a shift of the magnetic hysteresis loop. Indeed many nanomanganites do show EB effect. However, contrary to this expectation, we find that in BCMON samples the EB effect is absent. Chapter 3: In this chapter, we report the results of temperature dependent magnetization and electron paramagnetic resonance studies on bulk and nanoparticles of electron (x = 0. 6, BCE) and hole (x = 0.4, BCH) doped Bi1-xCaxMnO3 (BCMO) and the effect of the size reduction on the electron-hole asymmetry observed in the bulk sample. Bulk sample of Bi0.4Ca0.6MnO3is a paramagnetic insulator at room temperature with Tco = 330 K and TN ~ 120 K while BiCaMnO3 undergoes a charge ordering transition at TCO = 315 K with TN ~ 150 K [16]. All the four samples were investigated by using XRD, TEM, SQUID and EPR techniques. It is shown that antiferromagnetism and charge order persist in the hole doped nano sample while ferromagnetism has emerged in the electron doped nano sample. Our magnetization and EPR results show that spin glass phase exists in bulk BCE, bulk BCH and nano BCE whereas no sign of either spin glass state or ferromagnetism is seen in nano BCH. We have shown that electron-hole asymmetry in terms of ‘g’ parameter has reduced in the nanoparticles but it has not completely disappeared in contrast with the results on Pr1-xCaxMnO3 [23]. We understand these interesting results in terms of the presence of the highly polarizable 6s2 lone pair electrons on bismuth which is known to cause many interesting departures from the behavior of rare earth manganites. We study the temperature dependence of the linewidth behavior by fitting it to the different models [24¬27] and find that Shengelaya’s model [25, 26] fits well to all the four samples describing the spin dynamics satisfactorily in the present samples. Chapter 4: In this chapter, we present the fabrication, characterization and the results obtained from the magnetization and EPR measurements carried out on bulk and nanoparticles of Bi0.1Ca0.9MnO3. We prepared the nanoparticles of BCMO by standard sol¬gel technique and bulk samples by solid state reaction method. We investigated magnetic ordering by doing temperature dependent magnetic and EPR studies on both the samples and compared the properties with each other. Bulk Bi0.1Ca0.9MnO3 (BCMB) shows mixed phase of antiferromagnetism and ferromagnetism without any charge ordered state. Our results show that the ferromagnetism exists in the bulk BCMO which is present in the nano sample as well but with somewhat weakened strength with the size reduction. The nanoparticles of the rare earth based manganites are found to consist of an antiferromagnetic core and a ferromagnetic shell/surface region and thus are expected to exhibit the more uncompensated spins on the surface which reduce the magnetization in the nanoparticles. We calculated activation energy for the two samples by fitting the intensity behavior to the Arrhenius equation [28]. Activation energy was found to decrease for nano BCMO which indicates the weaker intracluster double-exchange interaction in it. Chapter 5: This chapter deals with the comparative study of the temperature dependent magnetic properties and EPR parameters of nano and bulk samples of Bi0.2Sr0.8MnO3 (BSMO). Nanoparticles and bulk sample of BSMO were prepared by sol-gel technique and solid state reaction method respectively. Bulk BSMO has high antiferromagnetic transition temperature TN ~ 260 K and robust charge ordering (TCO ~ 360 K) [29]. We confirm that the bulk sample shows an antiferromagnetic transition around ~ 260 K and a spin-glass transition ~ 40 K. For nano sample we see a clear ferromagnetic transition at around ~ 120 K. We conclude that mixed magnetic state exists in the bulk sample whereas it is suppressed in the nano sample and strong ferromagnetism is induced instead. Chapter 6: This chapter summarizes the main conclusions of the thesis, also pointing out some future directions for research in the field.