Synthesis and ESR Study of Transition from Ferromagnetism to Superparamagnetism in La_0.8Sr_0.2MnO₃ Nanomanganite

Abstract

Electron spin resonance (ESR) spectroscopy was used to determine the magnetic state transitions of nanocrystalline La 0.8 Sr 0.2 MnO 3 at room temperature, as a function of crystallite size. Ferromagnetic nanoparticles having an average crystallite size ranging from 9 to 57 nm are prepared by adopting the autocombustion method with two-step synthesis process. Significant changes of the ESR spectra parameters, such as the line shape, resonance field (Hr), g-factor, linewidth (∆Hpp), and the low-field microwave absorption (LFMA) signal, are indicative of the change in magnetic domain structures from superparamagnetism to single-domain and multi-domain ferromagnetism by increase in the crystallite size. Samples with crystallite sizes less than 24.5 nm are in a superparamagnetic state. Between 24.5 and 32 nm, they are formed by a single-domain ferromagnetic. The multi-domain state arises for higher sizes. In superparamagnetic region, the value of g-factor is practically constant suggesting that the magnetic core size is invariant with decreasing crystallite size. This contradictory observation with the core-shell model was explained by the phenomenon of phase separation that leads to the formation of a new magnetic state that we called multicore superparamagnetic state.

Effects of Process Variables on Properties of CoFe2O4 Nanoparticles Prepared by Solvothermal Process

“…• C is believed to be coming from the increase of particle size [38][39][40]. As the particle size decreased below the critical size, magnetization can randomly flip the direction under the influence of temperature, causing the residual magnetization to be 0 [41]. These results were in good agreement with previously reported values of 6-10 nm for the critical particle size of CoFe 2 O 4 nanoparticles [5,18].…”

“…The magnetization value of the CoFe2O4 nanoparticles at 15,000 Oe of magnetic field increased from 7.03 emu/g to 14.85 emu/g, 27.98 emu/g, 49.35 emu/g, and, finally, 53.3 emu/g as the The transition in magnetic property from superparamagnetic to ferromagnetic behavior at 200 °C is believed to be coming from the increase of particle size [38][39][40]. As the particle size decreased below the critical size, magnetization can randomly flip the direction under the influence of temperature, causing the residual magnetization to be 0 [41]. These results were in good agreement with previously reported values of 6-10 nm for the critical particle size of CoFe2O4 nanoparticles [5,18].…”

Structural and magnetic analysis of Cd-Zn spinel ferrite nanoparticles

“…This increase depends on several factors such as grain size, lattice strain and magneto-crystalline anisotropy. In this work, coercivity was found to increase with the increase in grain size that suggests single-domain structure of prepared samples [60][61][62]. Another reason is the overall increase in lattice strain, table 1, that hinders the movement of domain walls by forming strong pinning sites [60,63].…”

Electron Spin Resonance Study of Magnetite Nanoparticles in iPP Matrix

“…Table 1 demonstrated the values of the g-factor, the width of the absorption bands, the intensity of the absorption bands and the area of the absorption region depending on the amount of Fe 3 O 4 nanoparticles in the polymer matrix. The value of the g-factor was calculated by the following equation: The shape and intensity of the ESR signal can provide information about the interaction between magnetic centers, ultrafine structures, defects in the material structure [7]. Such broad-band absorption lines are characteristic of magnetic particles and are related to the inhomogeneity of the material.…”