First-Order Phase Transition Induced by Quantum Fluctuations in Heisenberg Helimagnets

We calculate the ground-state energy of an isotropic quantum Heisenberg ferromagnet on an hexagonal lattice with ferromagnetic exchange interactions J1 and J’ between nearest neighbors in the same basal plane and adjacent basal planes and, respectively, competing interactions J2 and J3 between second- and third-nearest neighbors in the same basal plane, respectively. When the ground-state energy of a helical state with wave vector Q is expanded for small Q as EG(Q)=E0+E2Q2+E4Q4+...., then the coefficients E2 and E4 can be evaluated exactly at zero temperature, with the result that E2 is given by its classical (S→∞) value, whereas E4 has quantum corrections. At the ferromagnet-helix transition (at which E2=0) E4 is positive for S=∞ indicating that this transition is continuous, whereas as S−1 is nonzero, a region develops wherein the transition becomes discontinuous

First‐Order Phase Transition Induced by Quantum Fluctuations in Heisenberg Helimagnets

The ground‐state energy of a spin‐density wave of wave vector Q in a Heisenberg ferromagnet with competing interactions is calculated for small Q as E G (Q)=E G (0)+A 1 Q 2 +A 2 Q 4+ . . . . The coefficients A 1 and A 2 are calculated exactly at zero temperature in the limit as the ferromagnet to helix transition is approached. For some ranges of parameters we find that quantum zero point motion causes A 2 to become negative thus leading to a first‐order transition not present in the classical system

Phase Locking in Heisenberg Helimagnets

We consider a Heisenberg model with ferromagnetic nearest‐neighbor and competing further‐neighbor exchange interactions in a small applied magnetic field at low temperature T. As a function of the exchange constants, the modulation vector is shown to have devil’s staircase behavior. We consider the effects of nonzero temperature and quantum effects. We find a special modulation wave vector at which the incommensurability energy vanishes for the classical system at T=0

Locking of Commensurate Phases in the Planar Model in an External Magnetic Field

Commensurate configuration locking is known in models like the anisotropic next-nearest-neighbor Ising model and the Frenkel-Kontorova model. We find an analogous scenario in the planar model with competing interactions when an external magnetic field is applied in the plane in which the spins lie. This model falls in the same symmetry class of the Heisenberg model with planar anisotropy. We performed a low-field, low-temperature expansion for the free energy of the model and we find phase locking energy for states with wave vectors of the form G/p where p is an integer and G is a reciprocal-lattice vector. The helix characterized by p=3 is peculiar because the commensuration energy vanishes at zero temperature. The helix corresponding to p=4 is not stable against the switching of a magnetic field that forces the spins into an up-up-down-down configuration analogous to the spin-flop phase of an antiferromagnet. For a generic commensurate value of p\u3e4, we expect locking both at zero and finite temperature as we have verified for p=5 and 6. The consequences of our results are examined for the 3N model (a tetragonal spin lattice with in-plane competitive interactions up to third-nearest neighbors)

Metric Features of a Dipolar Model

The lattice spin model, with nearest neighbor ferromagnetic exchange and long range dipolar interaction, is studied by the method of time series for observables based on cluster configurations and associated partitions, such as Shannon entropy, Hamming and Rohlin distances. Previous results based on the two peaks shape of the specific heat, suggested the existence of two possible transitions. By the analysis of the Shannon entropy we are able to prove that the first one is a true phase transition corresponding to a particular melting process of oriented domains, where colored noise is present almost independently of true fractality. The second one is not a real transition and it may be ascribed to a smooth balancing between two geometrical effects: a progressive fragmentation of the big clusters (possibly creating fractals), and the slow onset of a small clusters chaotic phase. Comparison with the nearest neighbor Ising ferromagnetic system points out a substantial difference in the cluster geometrical properties of the two models and in their critical behavior.Comment: 20 pages, 15 figures, submitted to JPhys

The microbial safety of edible insects: a matter of processing

Abstrac

Statistical mechanics of magnetic excitations: from spin waves to stripes and checkerboards

The aim of this advanced textbook is to provide the reader with a comprehensive explanation of the ground state configurations, the spin wave excitations and the equilibrium properties of spin lattices described by the Ising-Heisenberg Hamiltonians in the presence of short (exchange) and long range (dipole) interactions.The arguments are presented in such detail so as to enable advanced undergraduate and graduate students to cross the threshold of active research in magnetism by using both analytic calculations and Monte Carlo simulations.Recent results about unorthodox spin configurations su

Relaxation time of the nanomagnet Fe4

The magnetic behaviour of the molecular nanomagnet Fe4 is very well simulated by a single spin model Hamiltonian in a crystal field with S=5. The crystal field parameters were determined from the inelastic neutron scattering (INS) spectra. Here we show that the quantum effects are crucial to understand the saturation of the relaxation time observed at very low temperature at variance with the standard master equation result that leads to an Arrhenius law at any temperature. Very deep downward spikes in correspondence of the anticrossing fields are found in the relaxation time versus field at low temperature. We compare our results with those obtained by previous approaches worked out to fit experimental data on Mn12