We report on the observation of the spin Seebeck effect in antiferromagnetic MnF2. A device scale on-chip heater is deposited on a bilayer of MnF2 (110) (30 nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF2 (110) substrate. Using Pt as a spin detector layer it is possible to measure thermally generated spin current from MnF2 through the inverse spin Hall effect. The low temperature (2 -80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel the easy axis of the MnF2 thin film. When magnetic field is applied perpendicular to the easy axis, the spin flop transition is absent, as expected.

The NaMnF3 fluoride-perovskite has been found, theoretically, to be ferroelectric under epitaxial strain becoming a promising alternative to conventional oxides for multiferroic applications. Nevertheless, this fluoroperovskite has not been experimentally verified to be ferroelectric so far. Here we report signatures of room temperature ferroelectricity observed in perovskite NaMnF3 thin films grown on SrTiO3. Using piezoresponse force microscopy, we studied the evolution of ferroelectric polarization in response to external and built-in electric fields. Density functional theory calculations were also performed to help understand the strong competition between ferroelectric and paraelectric phases as well as the profound influences of strain. These results, together with the magnetic order previously reported in the same material, pave the way to future multiferroic and magnetoelectric investigations in fluoroperovskites.

Multiferroic materials have simultaneous magnetic and ferroelectric long-range orders and can be potentially useful for a wide range of applications. Conventional ferroelectricity in oxide perovskites favors non-magnetic electronic configurations of transition metal ions, thus limiting the number of intrinsic multiferroic materials. On the other hand, this is not necessarily true for multiferroic fluorides. Using molecular beam epitaxy, we demonstrate for the first time that the multiferroic orthorhombic fluoride BaCoF 4 can be synthesized in thin film form. Ferroelectric hysteresis measurements and piezoresponse force microscopy show that the films are indeed ferroelectric.From structural information, magnetic measurements, and first principles calculations, a modified magnetic ground state is identified which can be represented as a combination of bulk collinear antiferromagnetism with two additional canted spin orders oriented along orthogonal axes of the BaCoF 4 unit cell. The calculations indicate that an anisotropic epitaxial strain is responsible for this unusual magnetic ground state.

The orthorhombically distorted perovskite NaMnF 3 has been predicted to become ferroelectric if an a = c distortion of the bulk Pnma structure is imposed. In order to test this prediction, NaMnF 3 thin films were grown on SrTiO 3 (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature T N = 66 K. For the dielectric studies, NaMnF 3 films were grown on a 30 nm SrRuO 3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics.There is much interest in multiferroic (MF) materials that show coexistence of two or more long-range orders such as ferroelectricity, (anti-) ferromagnetism and/or ferroelasticity. Magnetoelectric (ME) coupling can allow for the control of magnetization M (electric polarization P) using an electric field E (a magnetic field H), which can be used for potential applications in data storage, sensors and spintronic devices.1-5 As a result, there has been much effort expended towards synthesizing MF insulating materials with strong ME coupling at room temperature. 6-9Many MF materials are oxides, but multiferroicity can also be found in fluorides. For example, in the orthorhombic BaM F 4 family the M = Co and Ni compounds in bulk form are ferroelectric (FE) and antiferromagnetic.10 While in oxide perovskites ABO 3 the simultaneous presence of FE and magnetic orderings is limited by conflicting requirements for the d n electronic configuration of the transition metal in the B-site, 11 ferroelectricity in BaMnF 4 is mainly due to geometric (topological) reasons.12 Recent experiments showed that BaCoF 4 thin films are weakly ferromagnetic at low temperatures due to strain. 13NaMnF 3 (NMF) is another fluoride compound that is possibly MF with ME coupling. The NMF crystal structure is described by the orthorhombic space group Pnma. At room temperature, the lattice constants are a o = 5.751Å, b o = 8.008Å, and c o = 5.548Å (Fig. 1a). 14In the pseudo-cubic unit cell, the corresponding lattice parameters are a pc = c pc = a 2 o + c 2 o /2 = 3.995Å and b pc = b o /2 = 4.004Å. NMF exhibits G-type antiferromagnetism of the Mn magnetic moments centered within tilted MnF 6 octahedra, with additional weak ferromagnetic canting.14-16 The Néel temperature has been reported to be T N = 66 K.15-20 Recent computational work predicted a ferroelectric instability in NMF originating from Na displacements (the A-site perovskite cati...