Neutron powder diffraction (NPD) and magnetization measurements have been performed for the La0.7Sr0.3Mn0.7Ti0.3−xAlxO3 (0≤x≤0.15) stoichiometric compounds. Increase of the Al3+content enlarges the fraction of Mn4+ ions from 0% for the sample with x=0 up to around 20% for x=0.15 one. The compound without Al content undergoes a structural transition from rhombohedral phase to orbitally disordered orthorhombic one below room temperature whereas crystal structure of the compounds with x=0.1 and 0.15 remain to have rhombohedral one down to 2 K. The structural transition occurs well above the magnetic transition temperature. The substitution of Ti4+ by Al3+ ions is accompanied by a gradual increase in the bond angle Mn–O–Mn and Mn–O bond length thus strengthening the covalent component of the related chemical bonds. All these compounds are insulators and have ferromagnetic components. Magnetic moments calculated per manganese ion based on NPD data obtained at 2 K are found to change from 1.3 µB for x=0 compound up to 1.7 µB for x=0.1 and 0.15 ones. It is suggested that ferromagnetism is predominantly caused by superexchange interactions Mn3+–O–Mn3+ and Mn3+–O–Mn4+ whereas fluctuations in the bond angles and distances frustrate magnetic interactions. It is assumed that increase of covalency within the chemical bonds Mn–O slightly enforces ferromagnetic interactions.
Troyanchuk, I. O., et al. "Magnetic ordering in manganites doped by Ti and Al." Ceramics International 43.1 (2017): 187-191.