Competing magnetic interactions driven tuning of spin-flop field and anomalous thermal expansion in MnTi1−xMnxO3

Abstract

Multiferroic MnTiO3 (MTO) is made up of alternate Mn2+ (3d5) and Ti4+ (3d0) layers along the c-axis. It undergoes a magnetic spin-flop driven ferroelectric polarization flop beyond ∼6 T below TN∼64 K. MTO also exhibits a weak negative thermal expansion along the c-axis below ∼90 K. We demonstrate that doping Mn ions at non-magnetic Ti layers acts as a very effective microscopic control parameter to systematically tune both these functional responses. Tuning of c/a lattice parameters in MnTi1−xMnxO3 leads to significant reduction in the spin-flop critical magnetic field from ∼6 T to more easily accessible fields (<2 T for x=0.05). Simultaneously, the controlled tuning of magnetic frustration due to Mn-doping at Ti sites also helps in a systematic enhancement of anomalous thermal expansion in regard to increasing the onset temperature for anomalous expansion to higher values (by ∼40 K for x=0.10) and also strengthening its magnitude (by ∼30% at 15 K for x=0.10 in comparison to x=0.00). Increase in the c/a ratio with Mn doping in MnTiO3 likely causes reduction in the effective magnetocrystalline anisotropy, which leads to a decrease in the value of the spin-flop magnetic field. Detailed temperature-dependent structural analyses shed light on the critical role of exchange striction mechanism in tuning the negative thermal expansion in doped MnTiO3.