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Register a new userMagnetization plateaux cascade in the frustrated quantum antiferromagnet Cs$_2$CoBr$_4$
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We have found an unusual competition of two frustration mechanisms in the 2D quantum antiferromagnet Cs$_2$CoBr$_4$. The key actors are the alternation of single-ion planar anisotropy direction of the individual magnetic Co$^{2+}$ ions, and their arrangement in a distorted triangular lattice structure. In particular, the uniquely oriented Ising-type anisotropy emerges from the competition of easy-plane ones, and for a magnetic field applied along this axis one finds a cascade of five ordered phases at low temperatures. Two of these phases feature magnetization plateaux. The low-field one is supposed to be a consequence of a collinear ground state stabilized by the anisotropy, while the other plateau bears characteristics of an up-up-down state exclusive for lattices with triangular exchange patterns. Such coexistence of the magnetization plateaux is a fingerprint of competition between the anisotropy and the geometric frustration in Cs$_2$CoBr$_4$.
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The field induced magnetic phase transitions of Cs$_2$CuBr$_4$ were investigated by means of magnetization process and neutron scattering experiments. This system undergoes magnetic phase transition at Ne{e}l temperature $T_mathrm{N}=1.4$ K at zero field, and exhibits the magnetization plateau at approximately one third of the saturation magnetization for the field directions $Hparallel b$ and $Hparallel c$. In the present study, additional symptom of the two-third magnetization plateau was found in the field derivative of the magnetization process. The magnetic structure was found to be incommensurate with the ordering vector $boldsymbol{Q}=(0, 0.575, 0)$ at zero field. With increasing magnetic field parallel to the c-axis, the ordering vector increases continuously and is locked at $boldsymbol{Q}=(0, 0.662, 0)$ in the plateau field range $13.1 mathrm{T} < H < 14.4 mathrm{T}$. This indicates that the collinear textit{up-up-down} spin structure is stabilized by quantum fluctuation at the magnetization plateau.
We report $^{133}$Cs nuclear magnetic resonance (NMR) measurements on the 2D frustrated Heisenberg antiferromagnet Cs$_2$CuCl$_4$ down to 2 K and up to 15 T. We show that $^{133}$Cs NMR is a good probe of the magnetic degrees of freedom in this material. Cu spin degrees of freedom are sensed through a strong anisotropic hyperfine coupling. The spin excitation gap opens above the critical saturation field. The gap value was determined from the activation energy of the nuclear spin-lattice relaxation rate in a magnetic field applied parallel to the Cu chains (b axis). The values of the g-factor and the saturation field are consistent with the neutron-scattering and magnetization results. The measurements of the spin-spin relaxation time are exploited to show that no structural changes occur down to the lowest temperatures investigated.
We argue that collinearly ordered states which exist in strongly frustrated spin systems for special rational values of the magnetization are stabilized by thermal as well as quantum fluctuations. These general predictions are tested by Monte Carlo simulations for the classical and Lanczos diagonalization for the S=1/2 frustrated square-lattice antiferromagnet.
The crystal structure of Cs$_2$CuBr$_4$ is the same as that of Cs$_2$CuCl$_4$, which has been characterized as a spin-1/2 quasi-two-dimensional frustrated system. The magnetic properties of Cs$_2$CuBr$_4$ were investigated by magnetization and specific heat measurements. The phase transition at zero magnetic field was detected at $T_{rm N}=1.4$ K. It was observed that the magnetization curve has a plateau at about one-third of the saturation magnetization for magnetic field $H$ parallel to the $b$- and $c$-axes, while no plateau was observed for $Hparallel a$. The field-induced phase transition to the plateau state appears to be of the first order. The mechanism leading to the magnetization plateau is discussed.
Quantum triangular-lattice antiferromagnets are important prototype systems to investigate phenomena of the geometrical frustration in condensed matter. Apart from highly unusual magnetic properties, they possess a rich phase diagram (ranging from an unfrustrated square lattice to a quantum spin liquid), yet to be confirmed experimentally. One major obstacle in this area of research is the lack of materials with appropriate (ideally tuned) magnetic parameters. Using Cs$_2$CuCl$_4$ as a model system, we demonstrate an alternative approach, where, instead of the chemical composition, the spin Hamiltonian is altered by hydrostatic pressure. The approach combines high-pressure electron spin resonance and magnetization measurements, allowing us not only to quasi-continuously tune the exchange parameters, but also to accurately monitor them. Our experiments indicate a substantial increase of the exchange coupling ratio from 0.3 to 0.42 at a pressure of 1.8 GPa, revealing a number of emergent field-induced phases.
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