Numerical Study of Competing Spin-Glass and Ferromagnetic Order

Two and three dimensional random Ising models with a Gaussian distribution of couplings with variance $J$ and non-vanishing mean value $J_0$ are studied using the zero-temperature domain-wall renormalization group (DWRG). The DWRG trajectories in the ($J_0,J$) plane after rescaling can be collapsed on two curves: one for $J_0/J > r_c$ and other for $J_0/J < r_c$. In the first case the DWRG flows are toward the ferromagnetic fixed point both in two and three dimensions while in the second case flows are towards a paramagnetic fixed point and spin-glass fixed point in two and three dimensions respectively. No evidence for an extra phase is found.Comment: a bit more data is taken, 5 pages, 4 eps figures included, to appear in PR

Spin glasses without time-reversal symmetry and the absence of a genuine structural glass transition

We study the three-spin model and the Ising spin glass in a field using Migdal-Kadanoff approximation. The flows of the couplings and fields indicate no phase transition, but they show even for the three-spin model a slow crossover to the asymptotic high-temperature behaviour for strong values of the couplings. We also evaluated a quantity that is a measure of the degree of non-self-averaging, and we found that it can become large for certain ranges of the parameters and the system sizes. For the spin glass in a field the maximum of non-self-averaging follows for given system size a line that resembles the de Almeida-Thouless line. We conclude that non-self-averaging found in Monte-Carlo simulations cannot be taken as evidence for the existence of a low-temperature phase with replica-symmetry breaking. Models similar to the three-spin model have been extensively discussed in order to provide a description of structural glasses. Their theory at mean-field level resembles the mode-coupling theory of real glasses. At that level the one-step replica symmetry approach breaking predicts two transitions, the first transition being dynamical and the second thermodynamical. Our results suggest that in real finite dimensional glasses there will be no genuine transitions at all, but that some features of mean-field theory could still provide some useful insights.Comment: 11 pages, 11 figure

Ordering in the dilute weakly-anisotropic antiferromagnet Mn(0.35)Zn(0.65)F2

The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated by neutron scattering in zero field. The Bragg peaks observed below the Neel temperature TN (approximately 10.9 K) indicate stable antiferromagnetic long-range ordering at low temperature. The critical behavior is governed by random-exchange Ising model critical exponents (nu approximately 0.69 and gamma approximately 1.31), as reported for Mn(x)Zn(1-x)F2 with higher x and for the isostructural compound Fe(x)Zn(1-x)F2. However, in addition to the Bragg peaks, unusual scattering behavior appears for |q|>0 below a glassy temperature Tg approximately 7.0 K. The glassy region T<Tg corresponds to that of noticeable frequency dependence in earlier zero-field ac susceptibility measurements on this sample. These results indicate that long-range order coexists with short-range nonequilibrium clusters in this highly diluted magnet.Comment: 7 pages, 5 figure

Nonperturbative renormalization group approach to frustrated magnets

This article is devoted to the study of the critical properties of classical XY and Heisenberg frustrated magnets in three dimensions. We first analyze the experimental and numerical situations. We show that the unusual behaviors encountered in these systems, typically nonuniversal scaling, are hardly compatible with the hypothesis of a second order phase transition. We then review the various perturbative and early nonperturbative approaches used to investigate these systems. We argue that none of them provides a completely satisfactory description of the three-dimensional critical behavior. We then recall the principles of the nonperturbative approach - the effective average action method - that we have used to investigate the physics of frustrated magnets. First, we recall the treatment of the unfrustrated - O(N) - case with this method. This allows to introduce its technical aspects. Then, we show how this method unables to clarify most of the problems encountered in the previous theoretical descriptions of frustrated magnets. Firstly, we get an explanation of the long-standing mismatch between different perturbative approaches which consists in a nonperturbative mechanism of annihilation of fixed points between two and three dimensions. Secondly, we get a coherent picture of the physics of frustrated magnets in qualitative and (semi-) quantitative agreement with the numerical and experimental results. The central feature that emerges from our approach is the existence of scaling behaviors without fixed or pseudo-fixed point and that relies on a slowing-down of the renormalization group flow in a whole region in the coupling constants space. This phenomenon allows to explain the occurence of generic weak first order behaviors and to understand the absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure

Scaling and density of states of fractal lattices from a generating function point of view

It is shown that the analogy between the free energy in critical phenomena and the complex generating function allows one to exploit well known position-space renormalization group techniques to easily derive scaling properties as well as the exact density of states for electronic or vibrational problems on fractal lattices. Certain self-similar lattices whose spectral dimension d can be larger or less than 2 are also studied.Il est démontré que l'analogie entre l'énergie libre des phénomènes critiques et la fonction génératrice complexe rend possible l'utilisation de techniques bien connues du groupe de renormalisation dans l'espace direct, permettant ainsi de dériver facilement les propriétés d'échelle ainsi que la densité d'états exacte pour des problèmes électroniques ou de vibration sur des réseaux fractals. On discute aussi l'existence d'une famille de réseaux dont la dimension spectrale d peut être inférieure ou supérieure à la valeur 2

Search for universality in disordered 2d ferromagnets

Transfer matrix simulations of the Ising and the 3-state Potts models on the square lattice, with strong and weak interactions distributed randomly, are compatible with a universal ratio of / 2 independent of the amount of disorder. Our results suggest that this ratio differs from that of the pure system in the 3-state Potts model, but in the Ising case the data are consistent with the same universal value for both the disordered and the pure system.Des calculs de matrice de transfert pour les modèles d'Ising et de Potts à 3 états sur le réseau carré, avec des interactions aléatoires prenant deux valeurs, donnent des résultats compatibles avec une valeur universelle du rapport / 2 indépendante de l'amplitude du désordre. Nos résultats suggèrent que la valeur de ce rapport diffère de celle du système pur pour le modèle de Potts à 3 états. Pour le modèle d'Ising, nos résultats sont compatibles avec une valeur universelle unique pour le système pur et pour le système désordonné

Enhancement of the exchange-bias onset temperature in a columnar nanocrystalline Ni80Fe20/Co3O4 thin film

Typically, exchange coupling can only occur when ferromagnetic spins couple to antiferromagnetic spins below the Neel temperature. We present results on a Ni80Fe20/Co3O4 thin film that is composed of similar to 10 nm diameter nanocrystalline columns. Field cooling the film reveals low-temperature exchange-bias hysteresis loop shifts that are the same magnitude as those measured in a similar Ni80Fe20/CoO system, although the bulk magnetocrystalline anisotropy is a factor of 5 smaller. The coercivity and exchange-bias loop shift exhibit the same temperature dependence with individual &quot;blocking&quot; temperatures. Most surprising is that the onset temperature of the exchange-bias is nearly four times higher than the bulk Neel temperature. (C) 2006 Institute of Physics

Proximity effects in an exchange-biased Ni80Fe20/Co3O4 thin film

An unusual enhancement of the antiferromagnetic ordering (Neel) temperature is observed in a Ni80Fe20/Co3O4 thin film. This significant increase of the Neel temperature indicates an exchange energy of interface spins between the ferromagnet (FM) and antiferromagnet (AF) that is significantly greater than that of spins in the bulk of the film. Exchange bias is established even when the thin film is zero field cooled. Field cooling the film sets a significantly stronger exchange coupling between the FM and AF spins at the interface with a concomitant increase in the exchange-bias field

Exchange bias in a columnar nanocrystalline Ni80Fe20/CoO thin film

The effects of interfacial coupling at the boundary of ferromagnetic and antiferromagnetic components in a nanoscale columnar-structured thin film of Ni80Fe20/CoO have been examined. Field-cooling the film results in very different temperature dependences of the enhanced coercivity and exchange-bias shift of the hysteresis loop. The exchange-bias temperature dependence is well described by thermal fluctuations of the interfacial spins while the coercivity temperature dependence indicates that single-domain-like columns are being coherently rotated by the thermal fluctuations of the interface spins. Furthermore, only a portion of the spins in the antiferromagnetic layer seem to be associated with the spin coupling that results in exchange bias. X-ray magnetic resonant scattering measurements show clearly the presence of canted Co interfacial moments that provide a local field which enables exchange bias at a significantly higher temperature than the onset of an enhanced coercivity