Pressure dependence of spin canting in ammonium metal formate antiferromagnets

Abstract

High-pressure single-crystal X-ray diffraction at ambient temperature and high-pressure SQUID measurements down to 2 K were performed up to ∼2.5 GPa on ammonium metal formates, [NH][M(HCOO)] where M = Mn, Fe, and Ni, in order to correlate structural variations to magnetic behaviour. Similar structural distortions and phase transitions were observed for all compounds, although the transition pressures varied with the size of the metal cation. The antiferromagnetic ordering in [NH][M(HCOO)] compounds was maintained as a function of pressure, and the magnetic ordering transition temperature changed within a few kelvins depending on the structural distortion and the metal cation involved. These compounds, in particular [NH][Fe(HCOO)], showed greatest sensitivity to the degree of spin canting upon compression, clearly visible from the twenty-fold increase in the low-temperature magnetisation for [NH][Fe(HCOO)] at 1.4 GPa, and the change from purely antiferromagnetic to weakly ferromagnetic ordering in [NH][Mn(HCOO)] at 1 GPa. The variation in the exchange couplings and spin canting was checked with density-functional calculations that reproduce well the increase in canted moment within [NH][Fe(HCOO)] upon compression, and suggest that the Dzyaloshinskii-Moriya (DM) interaction is evolving as a function of pressure. The pressure dependence of spin canting is found to be highly dependent on the metal cation, as magnetisation magnitudes did not change significantly for when M = Ni or Mn. These results demonstrate that the overall magnetic behaviour of each phase upon compression was not only dependent on the structural distortions but also on the electronic configuration of the metal cation.

Effect of Alkali and Trivalent Metal Ions on the High-Pressure Phase Transition of [C2H5NH3]MI0.5MIII0.5(HCOO)3 (MI = Na, K and MIII = Cr, Al) Heterometallic Perovskites

“…The replacement of Na + ions by larger K + ions not only gives a more compressible material but also leads to a lowering of the phase transition pressure as well as an additional transition. The reduction in phase transition pressure due to changes in the metal cations with larger radii is also observed for DMANaCr and DMAKCr (by ∼2 GPa) as well as for DMAM II (M II = Mn, Fe) and ammonium metal formates (M II = Mn, Ni, Fe, Zn). , …”

“…The reduction in phase transition pressure due to changes in the metal cations with larger radii is also observed for DMANaCr and DMAKCr (by ∼2 GPa) 18 as well as for DMAM II (M II = Mn, Fe) 36 and ammonium metal formates (M II = Mn, Ni, Fe, Zn). 24 , 25 …”

“…The reduction in phase transition pressure due to changes in the metal cations with larger radii is also observed for DMANaCr and DMAKCr (by ∼2 GPa) 18 as well as for DMAM II (M II = Mn, Fe) 36 and ammonium metal formates (M II = Mn, Ni, Fe, Zn). 24,25 The refined crystal structures within the three phases (ambient phase I, phase II, and phase III) are given in Figure 3. Both high-pressure phase transitions did not show any evidence of a change in monoclinic symmetry, and the same centrosymmetric P2 1 /n space group as found at ambient conditions could be used to refine both high-pressure phases (Table S1).…”

“…We also perform high-pressure Raman measurements for EtANaAl, which has very similar temperature-dependent properties to EtANaCr, to investigate how the substitution of the M III cation influences the high-pressure behavior. Both techniques have previously been employed for the studies of many formate-based MOFs. ,,− Nevertheless, their combination allows the examination of long-range crystalline order together with very subtle local symmetry changes at the same time. DFT calculations are employed to obtain information about the energy of the hydrogen bonds (HBs) and to describe changes related to the HBs during the experiment.…”

Local Structure of Ferroic Iron Formates at Low Temperature and High Pressure Studied by Mössbauer Spectroscopy

“…In particular, the ordering of the DMA cations in DMAFeF could be induced at 7.3(2) GPa due to the volume reduction arising from the large iron formate distortion, although an antiferroelectric arrangement of the DMA cations is adopted. 20 AFeF has a weaker resilience to pressure and displays two transitions at p = 0.50 (7) and p = 1.95(14) GPa, 21,22 but the possible ordering of the ammonium cation could not be reliably determined.…”