Control of the third dimension in copper-based square-lattice antiferromagnets

Abstract

Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu(HF2)(pyz)2]ClO4 [pyz = pyrazine], [CuL2(pyz)2](ClO4)2 [L = pyO = pyridine-N-oxide and 4-phpyO = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)2] 2+ nearly square layers, but exhibit interlayer spacings that vary from 6.5713Å to 16.777Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed-and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz)2(ClO4)2. We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the HF2 axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpyO) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz)2(ClO4)2, the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal S = 1/2 two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.

Spin-flop transition accompanied with changing the type of magnetic ordering

“…On the other hand, the value of the inter-plane dipolar coupling is of the order of 10 −2 K as it is seen, e.g., from Eqs. (8).…”

“…g-factor is anisotropic in Cu(pz) 2 (ClO 4 ) 2 : g = g bc = 2.04(3) for in-plane components of magnetic moments and g = g x = 2.25(5) for components perpendicular to square planes. 8,12 Then, ω 0 can possess three slightly different values in Eq. (2) depending on α and γ that is taken into account in calculations below.…”

“…[5][6][7][8][9][10][11] Exchange coupling constant between spins from neighboring planes does not exceed 9 mK and dipolar forces are expected to play a significant role in the interlayer coupling. 8 Recent antiferromagnetic resonance (AFR) experiment 12 reveals also an in-plane easy-axis anisotropy (that is an order of magnitude smaller than the easy-plane one) and a related spin-flop transition at magnetic field H = H sf ≈ 0.42 T having anomalous properties. According to the common wisdom, 13 one expects four possible scenarios summarized in Fig.…”

“…The unit cell containing four Cu 2+ magnetic atoms (two atoms from two neighboring bc planes) has parameters a = 13.9276(3)Å, b = 9.7438(2)Å, and c = 9.7871(2)Å. 8 There are right angles between b and c axes as well as between a and b ones. The angle β = 96.924 (1) • between a and c axes differs slightly from 90…”

“…8 We neglect below for simplicity the tiny difference between lattice parameters b and c. Thus, we assume that Cu 2+ ions are arranged in perfect square lattice in bc planes. On the other hand, we show below that the small departure of angle β from 90…”