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The nanomagnetism of monodisperse 7 nm γ-Fe2O3 nanoparticles exhibits unique features due to a significant amount of surface spin disorder. To correctly characterize the superparamagnetism of a dilute dispersion requires including the effects of the magnetic anisotropy and a shell of disordered spins surrounding the ordered core. The nanoparticle shell’s disordered spin structure is exchange coupled to that of the ordered core. This enables an exchange bias loop shift, Hex, when the nanoparticle dispersion is field cooled. The surface spin disorder also leads to an unusual exponential-like decrease of the nanoparticle’s total saturation magnetization with increasing temperature.

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We report magnetic measurements on a macroscopic three-dimensional fcc array of iron-oxide nanoparticles. We observe typical nanomagnetism for the randomly packed configuration of nanoparticles, including dynamical freezing and superparamagnetism. By contrast, the nanoparticle "atoms" in the fcc configuration that form the crystal exhibit a low coercivity that is weakly temperature dependent with no superparamagnetism up to 400 K.

We report a Monte Carlo study of the classical antiferromagnetic Heisenberg model on the triangular lattice. The free-energy cost for the formation of free vortices is obtained from a vorticity modulus. Evidence of a Kosterlitz-Thouless type of defect-mediated phase transition at a finite temperature is found.

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The authors examine the nature of two-magnon excitations in the alternating bond ferromagnetic S=1/2 spin chain. Both a direct analytic approach as well as a method based on a scaling transformation are used to study the bound state branches and their relationship to the two-magnon continuum. Several features are expected to be observable in two-magnon Raman scattering experiments.

Monte Carlo methods are used to study a family of three-dimensional XY frustrated models interpolating continuously between the stacked triangular antiferromagnets and a variant of this model for which a local rigidity constraint is imposed. Our study leads us to conclude that generically weak first order behavior occurs in this family of models in agreement with a recent nonperturbative renormalization group description of frustrated magnets.In spite of intensive study during the last 25 years the critical behavior of XY or Heisenberg frustrated magnets with a noncollinear ground state is still a strongly debated topic (see Refs. 1 and 2 and references therein). This is, for instance, the case of the celebrated XY Stacked Triangular Antiferromagnet (STA) that we consider here, whose Hamiltonian iswhere the S i are two-component vectors and the sum runs over all pairs of nearest neighbor spins on a stacked triangular lattice. In Eq.(1), the in-plane interactions are antiferromagnetic ͑J Ͻ 0͒ and the interplane interactions are taken to be ferromagnetic ͑J Ͼ 0͒ with ͉J͉ = 1. The competition between the in-plane antiferromagnetic interactions produces a ground state where the three spins S 1 , S 2 , and S 3 on each elementary triangular plaquette are oriented at 120°one to another, that is,For such a system, the order parameter is given by two orthogonal vectors of the same norm, a fact that has led to the hypothesis that noncollinear magnets could undergo a second order phase transition associated with a new-chiraluniversality class. 3 However, it is now becoming more and more widely accepted that the physics of frustrated magnets is not given by such a simple picture. 1,4-6 The complexity of the critical behavior of STA systems is exemplified in the experimental results (for a review see Ref. 1, and references therein). Indeed, scaling behavior is generally found while universality is violated since materials which belong, a priori, to the same universality class display different critical exponents. It is thus difficult to interpret the experimental data within the usual picture of critical phenomena.On the theoretical side, the situation is controversial. 1,5 A nonperturbative renormalization group (NPRG) method 1,4 finds that the phase transitions in these systems are generically weakly first order with the possibility of standard strongly first order transitions. More precisely while there is no RG fixed point there exists a whole region where the RG flow is very slow, corresponding to large-but finitecorrelation lengths at the transition, which allows one to compute pseudocritical exponents. Hence the violation of universality observed experimentally has a natural explanation since, in the absence of a fixed point, the pseudocritical exponents found depend on the microscopic Hamiltonians. On the other hand, Pelissetto et al. have derived and resummed the six-loop ␤ functions in three dimensions. 7 They find a fixed point and therefore predict a second order phase transition. Calabrese et al. have claimed tha...