Importance of interplane coupling on the magnetic phases of quasi-two-dimensional tantalites

Abstract

We propose a three-dimensional model to describe magnetic interactions in a class of tantalite compounds of compositions A(x)A'(1-x)Ta(2)O(6), with A,A' = Fe, Co or Ni. Due to the quasi-two-dimensional nature of the magnetism in these compounds, experimental data have been previously interpreted using two-dimensional models. These are anisotropic Heisenberg models or Ising models and include competing exchange interactions from different neighbors. Taking into account all the relevant exchange terms, which include interplane interactions, we show that the latter allows us to understand the various low-temperature magnetic phases observed by neutron diffraction in this family of compounds. This is done by studying the eigenvalues of the exchange-interaction matrix in wavevector space for different sets of coupling parameters, of which those relative to in-plane interactions have been obtained from high-temperature series analysis of the magnetic susceptibility. This approach is rather general and the model presented here is directly applicable to isostructural compounds like ASb(2)O(6).

Strongly correlated one-dimensional magnetic behavior ofNiTa2O6

“…[51] Applying this scenario to the EPR spectra of Ni 2+ in MgTa The measured values of D, E and the g-factor are in a range typically found for Ni 2+ in a slightly distorted octahedral environment and a lot more reasonable than what was concluded by Santos et al. [30,31] Highly resolved temperature dependent measurements of the low-field part of the spectrum reveal small linear changes of the resonance positions of the resonance lines A' and B' (see Figure 5), which indicates a marginal temperature dependence of the crystal field parameters D (≈ 3 %) and E (≈1.5 %). This can be understood as due to thermal contraction of the lattice.…”

“…[30,31] Highly resolved temperature dependent measurements of the low-field part of the spectrum reveal small linear changes of the resonance positions of the resonance lines A' and B' (see Figure 5), which indicates a marginal temperature dependence of the crystal field parameters D (≈ 3 %) and E (≈1.5 %). This can be understood as due to thermal contraction of the lattice.…”

“…The magnetic properties of NiTa 2 O 6 have been the subject of a number of studies, but a unanimous consensus especially on the appropriate spin-exchange model has not been reached until now. [21][22][23][24][25][26][27][28][29][30] Previous magnetization measurements have reported Curie-Weiss behavior with Curie-Weiss temperatures ranging from -19.9 K to -50 K indicating a predominant AFM SEI. [21,22,29] Long-range AFM ordering was consistently reported to appear below 11 K. [23,24,28,30] The magnetic structure has been previously determined by powder neutron diffraction revealing a rather large magnetic unit cell (propagation vector (1/4, -1/4, 1/2)) with the magnetic moments being collinearly aligned parallel to [1 1 0].…”

“…[21][22][23][24][25][26][27][28][29][30] Previous magnetization measurements have reported Curie-Weiss behavior with Curie-Weiss temperatures ranging from -19.9 K to -50 K indicating a predominant AFM SEI. [21,22,29] Long-range AFM ordering was consistently reported to appear below 11 K. [23,24,28,30] The magnetic structure has been previously determined by powder neutron diffraction revealing a rather large magnetic unit cell (propagation vector (1/4, -1/4, 1/2)) with the magnetic moments being collinearly aligned parallel to [1 1 0]. [25] The ordered moment was refined to 1.6 µ B and the difference to the expected moment of g × m S ∼ 2 µ B was attributed to incomplete ordering of the spins.…”

“…They found that both SEI were roughly equal and AFM with a value of 3.4 K, D was found to be 56.7 K and an average g-factor of 3.08 (where g = 3.51 and g ⊥ = 2.03). [30] Such a large single ion anisotropy implies that NiTa 2 O 6 is close to an Ising-like system. However, the g-factors deviating so greatly from the free-electron g-factor, g e , are unprecedented for Ni 2+ in an octahedral oxygen environment.…”