Spin–Lattice Coupling Across the Magnetic Quantum-Phase Transition in Copper-Containing Coordination Polymers

Abstract

We measured the infrared vibrational properties of two copper-containing coordination polymers, [Cu­(pyz)2(2-HOpy)2]­(PF6)2 and [Cu­(pyz)1.5(4-HOpy)2]­(ClO4)2, under different external stimuli in order to explore the microscopic aspects of spin–lattice coupling. While the temperature and pressure control hydrogen bonding, an applied field drives these materials from the antiferromagnetic → fully saturated state. Analysis of the pyrazine (pyz)-related vibrational modes across the magnetic quantum-phase transition provides a superb local probe of magnetoelastic coupling because the pyz ligand functions as the primary exchange pathway and is present in both systems. Strikingly, the PF6 – compound employs several pyz-related distortions in support of the magnetically driven transition, whereas the ClO4 – system requires only a single out-of-plane pyz bending mode. Bringing these findings together with magnetoinfrared spectra from other copper complexes reveals spin–lattice coupling across the magnetic quantum-phase transition as a function of the structural and magnetic dimensionality. Coupling is maximized in [Cu­(pyz)1.5(4-HOpy)2]­(ClO4)2 because of its ladderlike character. Although spin–lattice interactions can also be explored under compression, differences in the local structure and dimensionality drive these materials to unique high-pressure phases. Symmetry analysis suggests that the high-pressure phase of the ClO4 – compound may be ferroelectric.

High-Field Magnetoelectric and Spin-Phonon Coupling in Multiferroic (NH4)2[FeCl5·(H2O)]

“…The coupling constants for the two Fe-containing modes are on the order of −0.2 and −0.3 cm –1 , and that of the ammonium libration is an order of magnitude lower. These values are significantly smaller than the coupling constants of other molecule-based materials such as the low-dimensional copper halides across their magnetic saturation transitions . We attribute the difference to (i) through-space hydrogen- and halogen-bond exchange pathways in (NH 4 ) 2 [FeCl 5 ·(H 2 O)] as compared with superexchange pathways created by metal···ligand···metal linkages in low-dimensional quantum magnets and (ii) the value of the spin ( S = 5/2 for Fe 3+ vs 1/2 for Cu 2+ ) which determines the size of the spin–spin correlation function.…”

“…We evaluate spin-phonon coupling across the magnetically driven phase transition at 30 T as ω = ω 0 + λ⟨ S i · S j ⟩ where ω and ω 0 are the perturbed and unperturbed phonon frequencies, λ is the mode-dependent spin-phonon coupling constant, and ⟨ S i · S j ⟩ is the nearest-neighbor spin–spin correlation function. The magnitude of λ quantifies how the magnetic exchange interaction J is modulated by the various phonon displacements. , Using the limiting low temperature value of ⟨ S i · S j ⟩ = S 2 = (5/2) 2 = 6.25, we calculate λ for several important distortions including the Fe–O stretch, the Fe–OH 2 rocking mode, and the NH 4 + twist near 60 cm –1 . These values are summarized in Table .…”

“…The magnitude of λ quantifies how the magnetic exchange interaction J is modulated by the various phonon displacements. 51,52 Using the limiting low temperature value of ⟨S i •S j ⟩ = S 2 = (5/2) 2 = 6.25, we calculate λ for several important distortions including the Fe−O stretch, the Fe−OH 2 rocking mode, and the NH 4 + twist near 60 cm −1 . These values are summarized in Table 1.…”

Magnetic properties of a quantum spin ladder in proximity to the isotropic limit

“…With increasing temperature (starting approximately at 400 mK) the phase transi- Both critical fields shift to higher values, when the applied magnetic field is tilted to the c direction (Fig. 8), so that for H ∥ c we obtained µ 0 H c1 = 4.80(5) T and µ 0 H c2 = 23.88(5) T. Interestingly, recent infrared spectroscopy studies of CPPP [41] showed pronounced changes in the pyrazine-related vibrational phonon spec- As mentioned, at H c1 the system undergoes a transition into the low-temperature TTL phase. Typically, upon decreasing temperature, 3D interactions become relevant, resulting in a field-induced magnetically ordered ground state.…”

Tight-Binding Approach to Pyrazine-Mediated Superexchange in Copper–Pyrazine Antiferromagnets

“…The exchange interactions in structurally homologous, chemically similar materials can often vary widely, , offering an opportunity to design and investigate materials by finely tuning their geometries. Although an abundance of experimental information is available for these materials, ,− the factors that control the magnetic interactions are still debated. To be able to design materials with specific sets of magnetic interactions, these factors need to be understood in great detail.…”