Perovskite-like Metal Formates with Weak Ferromagnetism and as Precursors to Amorphous Materials

Abstract

Three isomorphous compounds M(CHOO)3[NH2(CH3)2] (M = Mn(1 x Mn), Co(2 x Co), Ni(3 x Ni)) have been synthesized in solvothermal conditions. Single-crystal X-ray diffraction shows that they are all crystallized in the trigonal space group R c with small differences in the lattice parameters. Bridged by the three-atom single-bridge CHOO-, M ions form a three-dimensional distorted perovskite-like structure with dimethylamine (DMA) cations located in the cages of the network. Based on the magnetic data, these three 3D compounds are weak ferromagnets with the critical temperature Tc = 8.5 K (1 x Mn), 14.9 K (2 x Co), and 35.6 K (3 x Ni), and for 2 x Co and 3 x Ni, spin reorientation might take place at 13.1 and 14.3 K, respectively. At 1.8 K, hysteresis loops can be observed for all three compounds with the coercivity field ca. 90 Oe (1 x Mn), 920 Oe (2 x Co), and 320 Oe (3 x Ni). The canting angles are estimated to be 0.08 degrees, 0.5 degrees, and 0.6 degrees for 1 x Mn, 2 x Co, and 3.Ni, respectively. The magnetic coupling between MnII ions in 1.Mn was estimated based on the model developed by Rushbrook and Wood for a Heisenberg antiferromagnet on a simple cubic lattice and the best fit gives J = -0.23 cm(-1). At the same time, according to molecular field theory of antiferromagnetism, the J values for compounds 1 x Mn, 2 x Co, and 3 x Ni were estimated to be -0.32 cm(-1), -2.3 cm(-1), and -4.85 cm(-1), respectively. The spin cant in these compounds may originate from the noncentrosymmetric character of the three-atom single-bridge CHOO-. Furthermore, amorphous materials 4 x Mn238, 5 x Mn450, 6 x Co320, and 7 x Ni300 were prepared from precursors 1-3 under an argon atmosphere at different temperatures according to the thermogravimetric analyses. As an interesting result, 5 x Mn450 was confirmed to be an amorphous form of Mn3O4 with a considerably large coercivity field HC = 4.1 kOe at 30 K compared to that value (250 Oe) for bulk Mn3O4.

Magnetic field-temperature phase diagram of multiferroic [(CH3)2NH2]Mn(HCOO)3<

“…19 Figure 1 displays the crystal structure of this compound and summarizes the aforementioned energy scales. The Mn 2+ centers are connected with formate super-exchange ligands with J=-0.64 K. 20 Dimethylammonium resides inside the anion pocket and is disordered (ordered) above (below) T C . Surprisingly and counter-intuitively, polarization is enhanced in magnetic field at temperatures well above T N .…”

First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3]

“…[15][16][17] Out of all [(CH 3 ) 2 NH 2 ][M(HCOO) 3 ] compounds the Mn-formate -which shows a magnetic transition at T N ≈ 8.5 K, [ 7 ,11,12] a dielectric transition at T t ≈ 190 K 7 ,10 and according to a very recent report a new magnetoelectric coupling in the paramagnetic state near the ferroelectric transition 17 -is so far the best characterized one, 11,12 even if a number of questions remain open, for example, if the structural transition is a first or second order transition.…”

Dimethylammonium copper formate [(CH3)2NH2]Cu(HCOO)Wang1, Jain2, Choi3 et al. 2013Phys. Rev. B

“…11 Recently, multiferroic properties were found for those dimethylammonium metal formate), where M = Mn, Co, Ni, and Fe. 8,[11][12][13] These studies present a new approach to synthesizing multiferroic materials.…”

“…Concerning the magnetic properties, Wang et al 11,12 have reported that DMAMnF, Dimethylammonium copper formate (DMACuF) was first reported by Sletten and Jensen 14 and has been studied by muon spin relaxation measurements. 15 It crystallizes in a monoclinic space group of C2/c and has a distorted ReO 3 -like structure as depicted in Fig.…”