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A detailed muon spin relaxation (μSR) and magnetization study is presented of the paramagnetic-toferromagnetic transition in the compound SrFe 0.80 Co 0.20 O 3 . The critical exponents derived from the static critical analysis are close to the theoretical predictions for the Heisenberg model in three dimensions. However, a small drift toward mean-field values is interpreted as arising from the presence of long-range dipolar interactions between the Fe(IV) centers. The evolution of spin dynamics across the transition determined from the μSR study is consistent with this interpretation. μSR and magnetization data also provide evidence of an inhomogeneous magnetic state both above and below T c , placing this system in line with other double-exchange materials such as manganites and cobaltites where spontaneous electronic and magnetic phase separation appears conspicuous and explains much of the encountered experimental phenomenology.

We present the results of the high-temperature neutron and x-ray diffraction experiments on the Ca 3-xSrxTi2O7 (x = 0.5, 0.8, 0.85, 0.9) compounds. The ferro-to paraelectric transition in these hybrid improper ferroelectric materials arises from the so-called trilinear coupling. Depending on the Strontium content, various structures and phase transitions, different from theoretical predictions, emerge. The in-situ x-ray powder diffraction indicates a direct ferro-to paraelectric transition between the orthorhombic A2 1am and tetragonal undistorted I4/mmm phase for x ≤ 0.6. We identified a reduction in the trilinear coupling robustness by increasing the Sr-doping level to lead to the emergence of the intermediate tetragonal P4 2/mnm phase and the gradual suppression of the orthorhombic phase. The observed character of the structure transitions and the Ca 3-xSrxTi2O7 phase diagram are discussed in the framework of theoretical models of other related hybrid improper ferroelectric systems.