A remarkable transformation of magnetic resonance spectra as a result of a mutual influence of coexisting para- and ferromagnetic phases

Abstract

In this work, we carry out the analysis of the resonance absorption of electromagnetic radiation for the system in which para- and ferromagnetic phases coexist over a wide temperature region. It is found that taking account of the mutual influence of coexisting phases gives rise to the appearance of substantial changes in the curves of resonance absorption and values of resonance fields, as well as to making the geometry of a phase distribution dependent on an external magnetic field. Near the temperature boundaries of the phase coexistence region, the expressions for description of the curves of the dispersive absorption of electromagnetic radiation are obtained and the rules of the behaviour of the resonance fields for each of the phases are specified. As follows from the calculations, the resonance field for the paramagnetic phase becomes dependent on the shape of the sample, the saturation magnetization and the fraction of ferromagnetic phase. It is shown that the character of magnetic resonance spectra and the features of their temperature change agree well with the experimental data, obtained by various groups of researchers on the single crystalline and polycrystalline samples of doped perovskite manganites.

Peculiar features of electron spin resonance spectra in (Ca,Na)-doped lanthanum manganites

“…In a high-temperature region (T > 304 K), the spectrum consists of a single symmetric line L1 which keeps discernible down to 280 K. At T = 320 K, the integrated curve is well fitted by a Lorentzian with the parameters which correspond to the paramagnetic state of manganites (resonance field B L 0 1 ≈ 318 mT, linewidth w L1 ≈ 44 mT) [9,10,12]. With the temperature lowering, the resonance field of this line remains almost constant, but the linewidth gradually increases.…”

“…Electron spin resonance (ESR) is an advantageous tool to probe the manganite behavior within the regions of phase coexistence but the interpretation of the results is far from being unambiguous [6][7][8]. It is the mutual influence of coexisting phases that may strongly change the resonance conditions for each of the coexisting phases [6,9,10].The analysis of the behavior of the system in which paramagnetic (PM) and ferromagnetic (FM) phases coexist over a wide temperature range was carried out in Refs. 6 and 10.…”

“…The fact that the FM phase doesn't occupy the whole volume of a particle changes the resonance conditions for the FM phase, as they in the first place are governed by the shape of FM regions [11]. On the other hand, the magnetic fields created by the FM regions strongly transform the resonance conditions for the PM phase [6,10].To date, there have been only few works with the reasonable interpretation of experimental data within the temperature range where the FM and PM phases coexist [6,9,12,13]. To clarify the picture, one should carry out a comprehensive analysis of the ESR spectra on the carefully characterized samples with a broadened magnetic transition.…”

“…The fact that the FM phase doesn't occupy the whole volume of a particle changes the resonance conditions for the FM phase, as they in the first place are governed by the shape of FM regions [11]. On the other hand, the magnetic fields created by the FM regions strongly transform the resonance conditions for the PM phase [6,10].…”

“…10. It was shown that the mutual influence of the coexisting phases results in the following effects: the resonance field for the PM phase increases with the temperature lowering, while that for the FM phase first decreases and then approaches a value characteristic of the uniformly magnetized ferromagnetic sample.…”