J. R. L. de Almeida

Glassy behaviour in pyrochlores: a spin glass approach

Using a model for antiferromagnetism in disordered systems with many sublattices, it is suggested that the spin glass behaviour in pyrochlore systems is basically due to fluctuations in the spin interactions quenched on the time scale of measurements. A mean field calculation is carried out for antiferromagnetism in structures with many sublattices. By allowing quenched disorder (fluctuations) in the exchange interactions, our results clearly exhibit the interplay between the effects of lattice frustration and disorder on the systems properties. Even small fluctuations in the exchange parameters do induce sizeable glassy behaviour in structures with many sublattices. Spin glass behaviour in apparently non-disordered systems, such as certain pyrochlores, may thus be accounted for within the present context.

Spin-glass and random-field effects in exchange-biased NiFe film on a NiO single-crystal substrate

Magnetization (M) data on a ferromagnetic/antiferromagnetic (FM/AF) sample made of a permalloy thin film sputtered on a thick NiO single-crystal substrate reveal remarkable irreversibilities. The field, temperature (T), and time dependencies of M were measured after the sample was cooled in zero-field or in the presence of a field, keeping T below the Neel temperature, with the field applied in the plane, either perpendicular or parallel to the anisotropy direction. While the behavior observed in the perpendicular configuration is characteristic of spin glasses, in the parallel configuration it exhibits features of random-field systems. The observed metastabilities and thermal history dependencies are shown to result from frustrations and randomness due to the interface roughness.

Irreversibility line in the ferromagnet Co70.4Fe4.6Si15B10 alloy

We report magnetization measurements in the zero magnetostriction Co70.4Fe4.6Si15B10 amorphous alloy. The measurements were carried out from room temperature up to 430 °C and for applied magnetic fields up to 45 Oe. An irreversible behavior in the zero‐field and field‐cooled magnetization was observed. The irreversibility line was fitted to a de Almeida–Thouless line with a crossover critical exponent φ=4.66. The experimental results are analyzed in the framework of an extended random anisotropy model for amorphous ferromagnets.

Strongly frustrated random antiferromagnets

Abstract:A high dimensionality calculation (Weiss like) has been carried out for antiferromagnetism (AFM) in structures with many sublattices. By allowing quenched disorder in the exchange interactions our results clearly exhibit the interplay between the effects of lattice frustration and disorder on the systems properties. For given number of sublattices present, there are several possible phases (ordering of the spins) and as many metastable states in the ergodic phases. It is found that the glassy behavior, and metastability, for multi-sublattices systems is substantially enhanced as compared with simple structures, exhibiting structure dependent de Almeida-Thouless lines. Strongly disordered systems have the long-range AFM ordering, ergodic metastable states and glassy phases intermingled in a non-trivial way. Also, even small fluctuations in the exchange parameters do induce sizeable glassy behavior in structures with many sublattices. Spin glass behavior in apparently non-disordered systems as certain pyrochlores may be accounted for within the present context.

Multifractility of Ising models on hierarchical lattices: pure and spin glass cases

The multifractal properties of the order parameter of Ising spin models on hierarchical lattices are investigated by an exact recursion procedure. Both pure and spin glass cases exhibit an order parameter with multifractal structure at the critical point. The connection between the multifractal ƒ(α) function and the critical exponents governing the transition is established. A continuous infinite set of exponents is required to describe the critical behavior of the local order parameter. Scaling relations between these exponents are also obtained.

Field induced transition in the amorphous ferromagnet Co70.4Fe4.6Si15B10

Abstract Magnetization measurements were carried out in the Co 70.4 Fe 4.6 Si 15 B 10 alloy for both amorphous and polycrystalline phases from room temperature to 900° C and for applied magnetic fields up to 50 Oe. Besides an irreversible behavior of the zero-field-cooled and field-cooled magnetization which cas characterized by an irreversibility line in the field-temperature diagram, we found a second line characterizing a first order transition induced by the magnetic field.

Field dependence of the interface energy in AF/AFM bilayers

Abstract In the investigations of antiferromagnetic (AF)/ferromagnetic (FM) bilayer samples, often distinct experimental techniques yield different values for the measured exchange anisotropy field (HE). Using a simple microscopic model for representing the AF/FM interface, which incorporates the effect of interface roughness, we propose that the observed discrepancy may be accounted for by the dependence of the interface energy between the AF and FM layers with the value of the external applied field (H) as recently observed in anisotropic magnetoresistance measurements, lending support to our proposal.

Susceptibilities of the antiferromagnet REM model

Abstract:The linear and non-linear susceptibilities of the two sub-lattices Random Energy Model (REM) allowing antiferromagnetic order is studied as a function of the external field (h) and temperature (T). Due to the competition between external field and the internal exchange field acting on the spins there is a drastic change of the systems behavior as the parameters (h,T) are varied. The behavior of the susceptibilities in low and high fields is very different in that the latter may grow as the temperature decreases. Moreover, the critical region undergoes a substantial enlargement as the external field increases.

On the Entropy of the Viana-Bray Model

The entropy of the Viana-Bray model at zero temperature and external field is calculated within the solution which takes into account only delta functions for the global order parameter P(h). It is shown that such solution is unsatisfactory both from the viewpoint of stability analysis and for not reproducing the well known Sherrington-Kirkpatrick result in the large connectivity limit thus pointing out the relevance of considering solutions with continuous part in P(h) for such model and possibly related models.

On the three-sublattice random antiferromagnet

Abstract Some properties of a model for random antiferromagnets with a three-sublattice structure (Ising model) which is a generalization of the well-known Sherrington–Kirkpatrick model are studied. Using standard techniques, we obtain the replica symmetric and permutation broken replica symmetry (RSB) solutions (numerically) and the phase diagram in the field versus temperature plane. The solution indicates that metastability is greatly enhanced in many sublattice systems as compared with simple systems. It is found that there may appear two kinds of RSB solutions for some values of the temperature and external field one of them with a Parisis order-parameter function displaying non-monotonic behavior.