The influence of pressure on the structure and magnetic properties of the layered hybrid compounds Cu 2 (OH) 3 (n-C m H 2mϩ1 CO 2 )•zH 2 O is investigated for mϭ10 and 12. It is shown that the distance between magnetic copper͑II͒ layers, up to 40.7 Å, is not significantly modified and that the temperature of the ferromagnetic ordering decreases linearly with pressure increase. We present a new analysis of the susceptibility data, based on the scaling theory of phase transitions, which clearly shows up a crossover from a hightemperature two-dimensional ͑2D͒ behavior to a 3D regime at about 30 K, around 10 K above the long-range ordering temperature. A model of quantum ferromagnetic layers interacting through dipolar coupling, taking into account the temperature dependence of correlated spin domains in a mean-field approach, allows us to explain the stabilization of a 3D order at a T C value very close to that observed experimentally. The decrease of T C under pressure is shown to be mainly driven by the decrease of in-plane interactions, which can be caused by small variations of the Cu-O-Cu bond angles within the layer.
We present experimental data on the magnetization M(H) of a CuMn 1 at. % in the range Tc≤ T ≤ 4 Tc, 0 < H < 7 teslas. We took special precautions in order to eliminate systematic errors and improve the reliability of the data. It was then possible to study the temperature dependence of the first coefficients A 1, A3, A5 in the expansion of the low field magnetization data in terms of odd powers of [FORMULA], in the range 1.1 Tc ≤ T ≤ 4 Tc. From the divergence of A3 and A5 (which vary over 3 and 6 orders of magnitude respectively in this range), we derive two exponents (γ = 3.25, β = 0.75 ± 0.25) which allow the rescaling of all our data points onto a universal function. The success of the scaling argument is strong evidence in favour of the existence of a phase transition in three dimensions for RKKY spin glasses
The phase diagram of a model describing doped CuGeO 3 is derived. The model emphasizes the role of local moments released by the impurities and randomly distributed inside the gaped singlet background. The phase diagram is investigated by two methods: (i) in a mean field treatment of the interchain coupling and (ii) in a real space decimation procedure in a two dimensional model of randomly distributed moments. Both methods lead to similar results, in a qualitative agreement with experiments. In particular, a transition to an inhomogeneous Néel phase is obtained for arbitrary small doping. From the decimation procedure, we interpret this phase at very low doping as a Griffith antiferromagnet. Namely, it does not have a true long range order down to zero temperature. Nonetheless, large magnetically ordered clusters appear already at relatively high temperatures. This demonstrates the role of disorder in the theoretical description of doping in CuGeO 3 . A detailed comparison with other approaches is also given. * U.P.R. 5001 du CNRS, Laboratoire conventionné avec l'Université Joseph Fourier
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