Microstrain induced deviation from N\'eel's 1/d behaviour: Size-dependent magnetization in Bi1-xCaxFe1-yTiyO3-delta nanoparticles

Pavana S. V. Mocherla; M. B. Sahana; Ehab Abdelhamid; Debarati Hajra; B. Nadgorny; R. Naik; R. Gopalan; M. S. Ramachandra Rao; B. R. K. Nanda; C. Sudakar

arXiv · 1703.07190

Microstrain induced deviation from N\'eel's 1/d behaviour: Size-dependent magnetization in Bi1-xCaxFe1-yTiyO3-delta nanoparticles

Pavana S. V. Mocherla·M. B. Sahana·Ehab Abdelhamid·Debarati Hajra·B. Nadgorny·R. Naik·R. Gopalan·M. S. Ramachandra Rao·B. R. K. Nanda·C. Sudakar

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Abstract

Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to N\'eel's model. Here we report a deviation from this conventional linear 1/d dependence, altered significantly by the microstrain, in Ca and Ti substituted BiFeO3 nanoparticles. Magnetic properties of microstrain-controlled Bi1-xCaxFe1-yTiyO3-delta (y = 0 and x = y) nanoparticles are analyzed as a function of their size ranging from 18 nm to 200 nm. A complex interdependence of doping concentration (x or y), annealing temperature (T), microstrain (epsilon) and particle size (d) is established. X-ray diffraction studies reveal a linear variation of microstrain with inverse particle size, 1/d nm-1 (i.e. epsilon.d = 16.5 nm.%). A rapid increase in the saturation magnetization below a critical size dc ~ 35 nm, exhibiting a (1/d)^alpha (alpha ~ 2.6) dependence, is attributed to the influence of microstrain. We propose an empirical formula M is proportional to (1/d)epsilon^beta (beta ~ 1.6) to highlight the contributions from both the size and microstrain towards the total magnetization in the doped systems. The magnetization observed in nanoparticles is thus, a result of competing magnetic contribution from the terminated spin cycloid on the surface and counteracting microstrain present at a given size. Large magnetodielectric response of ~ 9.5 % is observed in spark plasma sintered pellets with optimal size and doping concentration, revealing a strong correlation between magnetic and ferroelectric order parameters.

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Pavana S. V. Mocherla, M. B. Sahana, Ehab Abdelhamid, Debarati Hajra, B. Nadgorny, R. Naik, R. Gopalan, M. S. Ramachandra Rao, B. R. K. Nanda, C. Sudakar. 2017-03-21. Microstrain induced deviation from N\'eel's 1/d behaviour: Size-dependent magnetization in Bi1-xCaxFe1-yTiyO3-delta nanoparticles. https://arxiv.org/abs/1703.07190

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