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Register a new userA theoretical study of the cluster glass-Kondo-magnetic disordered alloys
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The physics of disordered alloys, such as typically the well known case of CeNi1-xCux alloys, showing an interplay among the Kondo effect, the spin glass state and a magnetic order, has been studied firstly within an average description like in the Sherrington-Kirkpatrick model. Recently, a theoretical model (PRB 74, 014427 (2006)) involving a more local description of the intersite interaction has been proposed to describe the phase diagram of CeNi1-xCux. This alloy is an example of the complex interplay between Kondo effect and frustration in which there is in particular the onset of a cluster-glass state. Although the model given in Ref. PRB 74, 014427 (2006) has reproduced the different phases relatively well, it is not able to describe the cluster-glass state. We study here the competition between the Kondo effect and a cluster glass phase within a Kondo Lattice model with an inter-cluster random Gaussian interaction. The inter-cluster term is treated within the cluster mean-field theory for spin glasses, while, inside the cluster, an exact diagonalisation is performed including inter-site ferromagnetic and intra-site Kondo interactions. The cluster glass order parameters and the Kondo correlation function are obtained for different values of the cluster size, the intra-cluster ferromagnetic coupling and the Kondo intra-site coupling. We obtain, for instance, that the increase of the Kondo coupling tends to destroy the cluster glass phase.
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The competition between spin glass, ferromagnetism and Kondo effect is analysed here in a Kondo lattice model with an inter-site random coupling $J_{ij}$ between the localized magnetic moments given by a generalization of the Mattis model which represents an interpolation between ferromagnetism and a highly disordered spin glass. Functional integral techniques with Grassmann fields have been used to obtain the partition function. The static approximation and the replica symmetric ansatz have also been used. The solution of the problem is presented as a phase diagram giving $T/{J}$ {it versus} $J_K/J$ where $T$ is the temperature, $J_{K}$ and ${J}$ are the strengths of the intrasite Kondo and the intersite random couplings, respectively. If $J_K/{J}$ is small, when temperature is decreased, there is a second order transition from a paramagnetic to a spin glass phase. For lower $T/{J}$, a first order transition appears between the spin glass phase and a region where there are Mattis states which are thermodynamically equivalent to the ferromagnetism. For very low ${T/{J}}$, the Mattis states become stable. On the other hand, it is found as solution a Kondo state for large $J_{K}/{J}$ values. These results can improve the theoretical description of the well known experimental phase diagram of $CeNi_{1-x}Cu_{x}$.
The low-energy physics of a spin-1/2 Kondo impurity in a gapless host, where a density of band states $rho_0(epsilon)=|epsilon|^r/(|epsilon|^r+beta^r)$ vanishes at the Fermi level $epsilon=0$, is studied by the Bethe ansatz. The growth of the parameter $Gamma_r=beta{rm g}^{-1/r}$ (where ${rm g}$ is an exchange constant) is shown to drive the system ground state from the Kondo regime with the screened impurity spin to the Anderson regime, where the impurity spin is unscreened, however, in a weak magnetic field $H$, it exceeds its free value, $S_i(H)>{1/2}$, due to a strong coupling to a band. It is shown also that a sufficiently strong potential scattering at the impurity site destroys the Anderson regime.
We report magnetic properties in the random spinel magnet CoGa2O4. Rietveld analysis of the x-ray diffraction profile for CoGa2O4 reveals that the Co and Ga ions are distributed randomly in the tetrahedral A-sites and octahedral B-sites in the cubic spinel structure. CoGa2O4 exhibits a spin-glass transition at TSG = 8.2 K that is confirmed by measurements of the dc- and ac-susceptibilities and thermoremanent magnetization (TRM) that develops below TSG. From the frequency dependence of the freezing temperature Tf for CoGa2O4, it is indicated that the relaxation time follows a Vogel-Fulcher law. Magnetic entropy is considerably reduced, probably because magnetic cluster formation developed even at T > TSG. The relaxation rate of TRM is considerably enhanced at TSG and decays rapidly above and below TSG. The time course of TRM is reproduced by non-exponential relaxation forms, such as a stretched exponential (Kohlrausch) as well as Ogielski and Weron relaxation forms. This behavior is displayed universally in glass systems, and the characteristic parameters associated with these functions were reasonable.
The quantum critical behavior of the Ising glass in a magnetic field is investigated. We focus on the spin glass to paramagnet transition of the transverse degrees of freedom in the presence of finite longitudinal field. We use two complementary techniques, the Landau theory close to the T=0 transition and the exact diagonalization method for finite systems. This allows us to estimate the size of the critical region and characterize various crossover regimes. An unexpectedly small energy scale on the disordered side of the critical line is found, and its possible relevance to experiments on metallic glasses is briefly discussed.
An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means of an ab-initio based approach. First-principles calculations are employed to obtain effective chemical, strain-induced and magnetic exchange interactions, as well as static atomic displacements, and the interactions are subsequently used in thermodynamic simulations. It is shown that the calculated atomic and magnetic short-range order accurately reproduces the results of neutron-scattering experiments. In particular, it is confirmed that the alloy exhibits a tendency toward ordering and the corresponding ordered phase is revealed. The magnetic structure is represented by spin-spiral clusters accompanied by weaker ferromagnetic short-range correlations. The spin-glass transition temperature obtained in Monte Carlo simulations by a finite-size scaling technique, 57 K, is in reasonable agreement with experimental data, 78 K.
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