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Register a new userCrossover from impurity-induced ordered phase to uniform antiferromagnetic phase under hydrostatic pressure in the doped spin-gap system TlCu$_{1-x}$Mg$_x$Cl$_3$
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Magnetic phase transition under hydrostatic pressure in TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$ was investigated by magnetization measurements. The parent compound TlCuCl$_3$ is a coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at $P_{rm c} = 0.42$ kbar due to the shrinkage of the gap. At ambient pressure, the present doped system exhibits impurity-induced magnetic ordering at $T_{rm N}=2.5$ K. With increasing pressure, $T_{rm N}$ increases. This is because the effective exchange interaction $J_{rm eff}$ between unpaired spins is enhanced by the shrinkage of the gap. With a further increase in pressure, the present system undergoes a phase transition to a uniform antiferromagnetic phase due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at $P simeq 1.3$ kbar.
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Magnetic phase transitions under hydrostatic pressures in spin gap systems TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$ and KCu$_{0.973}$Mg$_{0.027}$Cl$_3$ were investigated by magnetization measurements. The present doped systems exhibit impurity-induced magnetic orderings. With increasing pressure, ordering temperature $T_{rm N}$ increases. With a further increase in pressure, the present systems undergo phase transitions to uniform antiferromagnetic phases due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at $P simeq 1.3$ kbar for TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$.
The spin-1/2 stacked triangular antiferromagnet CsCu$_{1-x}$Co$_x$Cl$_3$ with $0.015<x<0.032$ undergoes two phase transitions at zero field. The low-temperature phase is produced by the small amount of Co$^{2+}$ doping. In order to investigate the magnetic structures of the two ordered phases, the neutron elastic scattering experiments have been carried out for the sample with $xapprox 0.03$. It is found that the intermediate phase is identical to the ordered phase of CsCuCl$_3$, and that the low-temperature phase is an oblique triangular antiferromagnetic phase in which the spins form a triangular structure in a plane tilted from the basal plane. The tilting angle which is 42$^{circ}$ at $T=1.6$ K decreases with increasing temperature, and becomes zero at $T_{rm N2} =7.2$ K. An off-diagonal exchange term is proposed as the origin of the oblique phase.
The relationship is established between the Berry phase and spin crossover in condensed matter physics induced by high pressure. It is shown that the geometric phase has topological origin and can be considered as the order parameter for such transition.
We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y$_2$BaNi$_{1-x}$Zn$_x$O$_5$. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility $chi(T) sim C / (Theta+T)$ as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the ``impurity susceptibility $chi_{imp}(T)$ by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by $T chi_{imp}(T) = C_{imp} (1 + T_{imp}/T )^{-gamma}$. In the temperature range [100 mK, 1 K] the experimental data are well fitted by $T chi_{imp}(T) = A ln{(T/T_c)}$, where $T_c$ increases with $x$. This fit suggests the existence of a finite Neel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility $chi(T)$ which suggests the existence of antiferromagnetic correlations at very low temperature.
Spin-lattice relaxation rate $T_1^{-1}$ of $^1$H-NMR has been measured in (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ with $x=0.88$, which has been reported to be gapped system with singlet ground state from the previous macroscopic magnetization and specific heat measurements, in order to investigate the bond randomness effect microscopically in the gapped composite Haldane system (CH$_3$)$_2$CHNH$_3$CuCl$_3$. It was found that the spin-lattice relaxation rate $T_1^{-1}$ in the present system includes both fast and slow relaxation parts indicative of the gapless magnetic ground state and the gapped singlet ground state, respectively. We discuss the obtained results with the previous macroscopic magnetization and specific heat measurements together with the microscopic $mu$SR experiments.
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