Magnetic properties of a buckled honeycomb lattice antiferromagnet

Abstract

The intriguing interplay between competing degrees of freedom in frustrated magnets can lead to non-trivial magnetic phenomena with exotic low-energy excitations that are highly relevant for addressing some of the fundamental questions in quantum condensed matter as well as potential technological applications. Herein, we report the synthesis and thermodynamic results on a frustrated magnet Co3ZnNb2O9. The Co2+ moments constitute buckled AB-type honeycomb layers in the ab-plane. The temperature-dependent magnetic susceptibility shows a sharp anomaly at 14 K, indicating the onset of long-range magnetic ordering. The Curie-Weiss fit of the magnetic susceptibility above 100 K, yields a Curie-Weiss temperature of -70 K, suggesting strong antiferromagnetic (AFM) interactions between the Co2+ spins and an effective magnetic moment of 5.54 muB, indicating the presence of unquenched orbital angular momentum. A field-induced spin-flop-like metamagnetic transition below the ordering temperature is characterized by a critical magnetic field of 1.2 T. The specific heat shows a lambda-type anomaly at 14 K, confirming the presence of long-range magnetic ordering, due to finite interlayer interaction. Interestingly, our study of the magnetocaloric effect near the transition temperature revealed an entropy change of 2.81 J/kg.K, which is ascribed to competing interactions, underlying anisotropy, and reduced net magnetization lead to relatively small isothermal entropy changes that suggest that frustrated honeycomb magnets are promising contenders for field-induced exotic phases and magnetocaloric response.

Figures (5)

• Figure 1: (a) Rietveld refinement of the powder X-ray diffraction data of Co$_3$ZnNb$_2$O$_9$ taken at room temperature. (b) Crystal structure of Co$_3$ZnNb$_2$O$_9$. (c) The buckled honeycomb lattice formed by the Co$^{2+}$ moments in the crystallographic $ab$ plane. (d) Splitting of energy levels for a high-spin $3d^7$ ion (Co$^{2+}$) in an edge-sharing octahedral environment.
• Figure 2: (a) A comparison between the temperature dependence of magnetic susceptibility $\chi(T)$, of Co$_3$ZnNb$_2$O$_9$ and Co$_4$Nb$_2$O$_9$, measured under an applied magnetic field of 0.1 T. The $\chi(T)$ of CNO reproduced with permission from ref. PhysRevB.102.174443. (b). The inverse magnetic susceptibility $\chi^{-1}(T)$ for CZNO is fitted to the Curie-Weiss law. (c) ZFC and FC magnetic susceptibilities measured under an applied field of 100 Oe; the inset shows linear behavior of polarization and magnetization at temperatures well below the antiferromagnetic transition. The polarization data is reproduced with permission from ref. martin2024compositional.
• Figure 3: (a) The schematic diagram of the magnetocaloric refrigeration cycle, where two steps are involved: an adiabatic step in which the entropy remains constant and temperature changes and an isothermal process that induces a change in entropy while maintaining a constant temperature nair2018magnetocaloric. (b) The schematic diagram of entropy as a function of temperature and magnetic field, the vertical line represents the isothermal entropy change ($\Delta S_m$), while the horizontal line denotes the adiabatic temperature change ($\Delta T_\mathrm{ad}$) pecharsky1999magnetocaloric. (c) Isothermal magnetization at various temperatures in the first quadrant up to a magnetic field of 7 T. The inset shows derivative of magnetization with respect to magnetic field at 5 K as a function of the magnetic field. (d) The entropy change as a function of temperature for various magnetic fields derived from magnetization isotherms recorded at different temperatures.
• Figure 4: (a) Temperature dependence of specific heat in zero magnetic field. The solid red line represents the lattice contribution arising from phonon excitations. (b) The field dependence of specific heat was measured up to 9 T and showing almost no significant shift in $T_\text{N}$. This result is compared with the electric polarization data adapted from ref. martin2024compositional. (c) The magnetic specific heat of CZNO at zero field, obtained after subtracting the lattice contribution using Debye-Einstein fitting, and the red line shows the power law behavior of $C_\text{m}$ with temperature below $T_\text{N}$. A sharp $\lambda-$type anomaly at 14 K, indicates the onset of long-range magnetic ordering. In the inset, we show the dielectric permittivity $\varepsilon'$ measured under various applied fields, reproduced with permission from ref. martin2024compositional. (d) Magnetic entropy obtained from the corresponding $C_\text{m}$ for zero field specific heat result.
• Figure 5: Correlated evolution of magnetic energy scales across the A$_4$B$_2\text{O}_9$ family ($A=\text{Mn, Fe, Co, Zn}$; $B=\text{Nb, Ta}$): [Top] Variation of the spin-flop transition field ($\mu_0 H_{sf}$) as a function of Néel temperature ($T_{\rm N}$), illustrating the relationship between magnetic anisotropy energy and thermal stability. [Bottom] The Curie-Weiss temperature ($\theta_{\rm CW}$) plotted against $T_{\rm N}$, highlighting the evolution of exchange coupling strength and magnetic frustration ($f = |\theta_{\rm CW}|/T_{\rm N}$) across the series PhysRevB.94.094427kolodiazhnyi2011spinPhysRevB.97.161106maignan2024enhancementmaignan2021fe. The figure highlights a feasible scaling between anisotropy and exchange energy scales, consistent with $H_\text{sf}\propto\sqrt{H_EH_A}$, where the exchange field $H_\text{E}$ and $H_\text{A}$ set the ordering temperature $T_\text{N}$.