Weak ferromagnetism and short range polar order in NaMnF3 thin films

Abstract

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...

Room temperature ferroelectricity in fluoroperovskite thin films

“…Therefore, the confirmation of the ferroelectric state in NaMnF 3 is of high importance in the multiferroics field since it would be the first perovskite-like fluoride to exhibit a multiferroic behavior. We note that, in films grown on a conducting SrRuO 3 epi-layers pre-deposited on SrTiO 3 , temperature dependent dielectric measurements showed signs of an onset of low-temperature ferroelectric order-disorder transition, but long range ferroelectric order was not observed above 10 K 17 . For films grown without the SrRuO 3 back contact layer, similar measurements were not possible, and the ferroelectric properties of these films need to be evaluated by other methods.…”

“…X-ray diffraction (XRD) results reported in ref. 17 show that the MBE grown films are mostly relaxed and domains with b- axis in-plane and out-of-plane are both present. Therefore, the majority portions of the films are likely paraelectric.…”

“…NaMnF 3 thin films were grown on SrTiO 3 (001) substrates by molecular beam epitaxy (MBE) 17 . Bulk NaMnF 3 at room temperature has an orthorhombically distorted perovskite structure with lattice parameters a = 5.757 Å, b = 8.008 Å, c = 5.548 Å (Fig.…”

“…Slight variations of substrate or films thickness can produce very different material properties, as reported in ref. 17 . Out of the total fourteen samples grown on different substrates and characterized by PFM, only three with films directly grown on SrTiO 3 showed signatures of ferroelectricity.…”

“…The resultant weak ferromagnetic component could become useful to tune the polarization by an external field. Recently, quasi-epitaxial thin films of NaMnF 3 on SrTiO 3 substrates were successfully grown by molecular beam epitaxy (MBE), in which the low temperature antiferromagnetic order and spin-canting induced magnetization were verified experimentally 17 . Remarkably, only a few fluoroperovskites, such as CsPbF 3 18 , 19 and NaCaF 3 20 , have shown ferroelectricity so far.…”