Nanosized helical magnetic domains in strongly frustratedFe3PO4O3

Abstract

Fe3PO4O3 forms a non-centrosymmetric lattice structure (space group R3m) comprising triangular motifs of Fe 3+ coupled by strong antiferromagnetic interactions (|ΘCW | > 900 K). Neutron diffraction from polycrystalline samples shows that strong frustration eventually gives way to an ordered helical incommensurate structure below TN = 163 K, with the helical axis in the hexagonal ab plane and a modulation length of ∼ 100Å. The magnetic structure consists of an unusual needlelike correlation volume that extends past 900Å along the hexagonal c-axis but is limited to ∼ 70 A in the ab plane, despite the three-dimensional nature of the magnetic sublattice topology. The small in-plane correlation length, which persists to at least T = TN /40, indicates a robust blocking of long-range order of the helical magnetic structure, and therefore stable domain walls, or other defect spin textures, must be abundant in Fe3PO4O3. Temperature dependent neutron powder diffraction reveals small negative thermal expansion below TN . No change in lattice symmetry is observed on cooling through TN , as revealed by high resolution synchrotron X-ray diffraction. The previously reported reduced moment of the Fe 3+ ions (S=5/2), µ ∼ 4.2 µB, is confirmed here through magnetization studies of a magnetically diluted solid solution series of compounds, Fe (3−x) GaxPO4O3, and is consistent with the refined magnetic moment from neutron diffraction 4.14(2) µB. We attribute the reduced moment to a modified spin density distribution arising from ligand charge transfer in this insulating oxide.

Tuning the antiferromagnetic helical pitch length and nanoscale domain size in Fe3PO4O3 by magnetic dilution

“…A slight splitting between the FC and ZFC curves in the parent compound (x = 0, shown in Ref. 17 persists down to at least 2 K. Similar features are observed for x = 0.012 and x = 0.06 ( Fig. 3), although T N decreases with increasing x. T N is estimated at 160 K for x = 0.012 and 146 K for x = 0.06, based on the location of the the largest positive dM/dT in the ZFC susceptibility, a feature which is coincident with a peak in the specific heat for x = 0 as shown in Ref.…”

“…3), although T N decreases with increasing x. T N is estimated at 160 K for x = 0.012 and 146 K for x = 0.06, based on the location of the the largest positive dM/dT in the ZFC susceptibility, a feature which is coincident with a peak in the specific heat for x = 0 as shown in Ref. 17.…”

“…To within the constraints of our powder averaged neutron diffraction data, we cannot distinguish between coplanar (helical) versus some non-coplanar (conical) magnetic structures, as discussed in Ref. 17; we thus chose the former for simplicity. Furthermore, the crystal structure lacks any inversion center, implying the presence of Dzyaloshinskii-Moriya (DM) interactions for every pair of Fe atoms, which may favor non-coplanar conical structures.…”

“…Polycrystalline samples in the solid solution series Fe 3x Ga x PO 4 O 3 were prepared as previously described 17 . Fe 2 O 3 and Ga 2 O 3 were dried overnight at 600 • C and FePO 4 · nH 2 O was converted to anhydrous FePO 4 by heating in air at 900 • C overnight in alumina crucibles, and then stored in a desiccator.…”

“…domains) 17 , but the origin of the high density of these defects is presently unknown. Here, we shed some light on this issue by showing that the defect spacing is intimately tied to the helical pitch length in Fe 3 PO 4 O 3 , which we show can be varied by greater than a factor of two through dilution of the magnetic lattice.…”