The effect of disorder on the critical properties of the ferromagnetic phase transition in colossal magnetoresistive manganite ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ has been studied by substituting $\mathrm{Ga}$ for $\mathrm{Mn}$. It is found that, upon 10% $\mathrm{Ga}$ substitution, the peak in the specific heat at the Curie point ${T}_{C}$ changes drastically and appears as a small anomaly. Static magnetization data analyzed in the asymptotic critical region using modified Arrott plots and the Kouvel-Fisher method give values for the critical exponents $\ensuremath{\beta}=0.387(6)$, $\ensuremath{\gamma}=1.362(2)$, and $\ensuremath{\delta}=4.60(3)$. The results show that the first-order transition in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ becomes continuous by $\mathrm{Ga}$ substitution. The critical properties of the rounded transition in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{Mn}}_{0.9}{\mathrm{Ga}}_{0.1}{\mathrm{O}}_{3}$ suggest that the magnetic subsystem in this mixed-valent perovskite is close to that of a conventional isotropic ferromagnet belonging to the Heisenberg universality class with short-range interactions. It is concluded that the first-order magnetic transition in pure ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ is induced by fluctuations from a competing mode, which couples to the magnetic subsystem.

Rounding of a first-order magnetic phase transition in Ga -doped La 0.67 Ca 0.33 MnO 3

The effect of disorder on the critical properties of the ferromagnetic phase transition in colossal magnetoresistive manganite La0.67Ca0.33MnO3 has been studied by substituting Ga for Mn. It is found that, upon 10% Ga substitution, the peak in the specific heat at the Curie point T_C changes drastically and appears as a small anomaly. Static magnetization data analyzed in the asymptotic critical region using modified Arrott plots and the Kouvel-Fisher method give values for the critical exponents beta=0.387(6), gamma=1.362(2), and delta=4.60(3). The results show that the first-order transition in La0.67Ca0.33MnO3 becomes continuous by Ga substitution. The critical properties of the rounded transition in La0.67Ca0.33Mn0.9Ga0.1O3 suggest that the magnetic subsystem in this mixed-valent perovskite is close to that of a conventional isotropic ferromagnet belonging to the Heisenberg universality class with short-range interactions. It is concluded that the first-order magnetic transition in pure La0.67Ca0.33MnO3 is induced by fluctuations from a competing mode, which couples to the magnetic subsystem.

Rounding of a first-order magnetic phase transition in Ga doped La0.67Ca0.33MnO3

The temperature dependence of the magnetization and quadrupole order parameters of the Blume–Emery–Griffiths (BEG) model Hamiltonian with the nearest-neighbor ferromagnetic exchange interactions [both bilinear ( J ) and biquadratic ( K )] and crystal field interaction ( D ) is studied using the lowest approximation of the cluster variation method. Besides the stable solutions, metastable and unstable solutions of the order parameters are found for various values of the two different coupling parameters, α = J / K and γ = D / K . These solutions are classified using the free energy surfaces in the form of a contour map. The phase transitions of the stable, metastable and unstable branches of the order parameters are investigated extensively. The critical temperatures in the case of a second-order phase transition are obtained for different values of α and γ calculated by the Hessian determinant. The first-order phase transition temperatures are found using the free energy values while increasing and decreasing the temperature. The temperature where both the free energies equal each other is the first-order phase transition temperature. Finally, the results are also discussed for the Blume–Capel model which is the special case of the BEG model.

Stable, metastable and unstable solutions of the Blume–Emery–Griffiths model

Phase diagrams of the Blume–Emery–Griffiths model calculated by the mean-field approximation including the transverse fields effects

The three-dimensional BC model is simulated on a cellular automaton which improved from the Creutz cellular automaton for simple cubic lattice. The phase diagram characterizing phase transition of the model is obtained for a comparison with those obtained from other calculations. The simulations confirm the existence of a tricritical point at which the phase transition changes from second-order to first-order at D/J =2.82. For the determined of the tricritical point, the thermodynamics quantities are computed using two different procedures which is called as the standard and the cooling algorithm for the anisotropy parameter values in the interval 3≥D/J≥-8. The simulations indicates that the cooling algorithm is a suitable procedure for the calculations near the first-order phase transition region, and the cooling rate is an important parameter in the determining of the phase boundary. The estimated critical temperatures for D/J =0 and 2.82 are compatible with the series expansion results.

The tricritical behavior of the 3d blume–capel model on a cellular automaton

The effect of bond randomness on tricritical and critical end-point phenomena is studied by renormalization-group theory. In three dimensions, the pure-system tricritical point is replaced by a line segment of second-order transitions dominated by randomness and bounded by a multicritical point and a random-bond tricritical point, which reaches zero temperature at threshold randomness. This topology indicates a violation of the empirical universality principle. The random-bond tricritical point renormalizes onto the fixed distribution of random-field Ising criticality. {copyright} {ital 1996 The American Physical Society.}

A. Falicov

Papers

Tricritical and critical end-point phenomena under random bonds.