Recent computational work predicted orthorhombic antiferromagnetic NaMnF3 to undergo a ferroelectric transition if an a = c structural distortion is imposed. Such a distortion may be achieved by epitaxial growth of NaMnF3 on a cubic substrate in thin film form. It has been shown that the a = c distortion leads to the freezing of a soft polar mode resulting in a structural transformation from a non-polar Pnma to a polar Pna21 space group with a polarization of P = 6 μ C/cm2 along the long axis. A weak ferromagnetic phase and an amplification of magnetoelectric coupling are also expected in the strained films, with a magnetoelectric response comparable to Cr2O3.
In this work, I investigate multiferroicity in NaMnF3 thin films. Epitaxial NaMnF3 thin films were grown on cubic SrTiO3 (001) single crystal substrates via molecular beam epitaxy (MBE). Thin film structural quality was investigated as a function of the substrate temperature and film thickness using X-ray diffraction (XRD), in-situ reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The best films were smooth single phase grown at 250 − 300 ◦C substrate temperatures with four different twin domains. In-plane magnetization measure- ments using a superconducting quantum interference device (SQUID) magnetometer revealed antiferromagnetic ordering with weak ferromagnetism below the Neel tem- perature TN = 66 K. Reproducible room temperature ferroelectric switching by a
voltage-biased scanning probe was detected with preferred up (out-of-plane) polar- ization using piezoresponse force microscopy. An interesting 180◦ out-of-plane polar- ization flip by the application of an in-plane electric field was also observed.
For the exchange bias and dielectric studies, NaMnF3 films were grown on 30 nm SrRuO3 (001) layer, which was used as a ferromagnetic layer as well as a bottom electrode, and grown via pulsed laser deposition. After field cooling below the Neel temperature of NaMnF3, hysteresis loops were measured as the function of tempera- ture. A negative exchange bias was found with the blocking temperature (TB) ∼ 45 K. The complex permittivity as a function of frequency measurements indicated a strong Debye-like relaxation. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K.