Observation of three-dimensional Ising to uniaxial dipolar crossover in a double exchange ferromagnet

The present study examines the physical properties of a double-exchange ferromagnet $L{a}_{0.7-y}{\square }_{y}S{r}_{0.3}Mn{O}_3$ (LSMO) with (y = 0.11) and without (y = 0) La-deficiency using X-ray diffraction, X-ray photoelectron spectroscopy, dc magnetization, and AC susceptibility measurement techniques. X-ray diffraction analysis confirms that both samples are single-phase and crystallize in rhombohedral crystal symmetry ($R\bar{3}c$). The unit cell volume, octahedral bond length (Mn-O), and Curie temperature ($T_C$) decrease while the Mn-O-Mn bond angle increases in the La-deficient ($y=0.11$) sample. Magnetic analysis discloses enhanced magnetic anisotropy and inhomogeneities in the La-deficient sample due to a partial increase in $M{n}^{4+}$ concentration. In the asymptotic critical region, the ac susceptibility and critical isotherm analyses unveiled mean-field behaviour in La-deficient ($y=0.11$) sample as opposed to 3D Ising behaviour in the parent ($y=0$) compound. This is in accordance with renormalization group (RG) theory prediction wherein dipolar interactions drastically modify the critical behaviour of a 3D Ising (d = 3 and n = 1) system so much so that the system behaves as a uniaxial dipolar ferromagnet in the asymptotic critical region characterized by mean-field critical exponents with logarithmic corrections to the single power laws. The dipole–dipole interactions operating between the localized $e_g$/$t_{2g}$ electrons get progressively enhanced with La-deficiency and consequently, the nature of ferromagnetic (FM)-paramagnetic (PM) phase transition is altered from 3D Ising to a 3D uniaxial dipolar ferromagnet in the asymptotic critical region. These observations, indeed, mark the importance of the interplay between magnetic anisotropy and dipolar interactions in governing the critical behaviour of hole-doped manganites.