Monte Carlo study of first-order transition in Heisenberg fcc antiferromagnet

Nearest-neighbor Heisenberg antiferromagnet on a face-centered cubic lattice is studied by extensive Monte Carlo simulations in zero magnetic field. The parallel tempering algorithm is utilized, which allows to overcome a slow relaxation of the magnetic order parameter and fully equilibrate moderate size clusters with up to N ~ 7*10^3 spins. By collecting energy and order parameter histograms on clusters with up to N ~ 2*10^4 sites we accurately locate the first-order transition point at T_c=0.4459(1)J.Comment: 5 pages, 5 figure

Quantum order by disorder and accidental soft mode Er2Ti2O7

Motivated by recent neutron scattering experiments, we derive and study an effective "pseudo-dipolar" spin-1/2 model for the XY pyrochlore antiferromagnet Er2Ti2O7. While a bond-dependent in-plane exchange anisotropy removes any continuous symmetry, it does lead to a one-parameter `accidental' classical degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the non-coplanar spin structure observed experimentally -- a rare experimental instance of quantum order by disorder. A non-Goldstone low-energy mode is present in the excitation spectrum in accordance with inelastic neutron scattering data. Our theory also resolves the puzzle of the experimentally observed continuous ordering transition, absent from previous models.Comment: 5 pages, 4 figures, final versio

Topological solitons in highly anisotropic two dimensional ferromagnets

e study the solitons, stabilized by spin precession in a classical two--dimensional lattice model of Heisenberg ferromagnets with non-small easy--axis anisotropy. The properties of such solitons are treated both analytically using the continuous model including higher then second powers of magnetization gradients, and numerically for a discrete set of the spins on a square lattice. The dependence of the soliton energy $E$ on the number of spin deviations (bound magnons) $N$ is calculated. We have shown that the topological solitons are stable if the number $N$ exceeds some critical value $N_{\rm{cr}}$. For $N < N_{\rm{cr}}$ and the intermediate values of anisotropy constant $K_{\mathrm{eff}} <0.35J$ ($J$ is an exchange constant), the soliton properties are similar to those for continuous model; for example, soliton energy is increasing and the precession frequency $\omega (N)$ is decreasing monotonously with $N$ growth. For high enough anisotropy $K_{\mathrm{eff}} > 0.6 J$ we found some fundamentally new soliton features absent for continuous models incorporating even the higher powers of magnetization gradients. For high anisotropy, the dependence of soliton energy E(N) on the number of bound magnons become non-monotonic, with the minima at some "magic" numbers of bound magnons. Soliton frequency $\omega (N)$ have quite irregular behavior with step-like jumps and negative values of $\omega$ for some regions of $N$. Near these regions, stable static soliton states, stabilized by the lattice effects, exist.Comment: 17 page

Fractal structures in systems made of small magnetic particles

This article was published in the journal, Physical Review B [© American Physical Society]. It is also available at: http://link.aps.org/abstract/PRB/v72/e014433.We have found that in a system consisting of small magnetic particles a phenomenon related to the formation of fractal structures may arise. The fractal features may arise not only in the distribution of magnetic moments but also in their energy spectrum. The magnetization and the susceptibility of the system also display fractal characteristics. The multiple structures are associated with exponentially many locally stable minima in a highly complex energy landscape. The signature of these fractal structures can be experimentally detected by various methods

Order by disorder and spiral spin liquid in frustrated diamond lattice antiferromagnets

Frustration refers to competition between different interactions that cannot be simultaneously satisfied, a familiar feature in many magnetic solids. Strong frustration results in highly degenerate ground states, and a large suppression of ordering by fluctuations. Key challenges in frustrated magnetism are characterizing the fluctuating spin-liquid regime and determining the mechanism of eventual order at lower temperature. Here, we study a model of a diamond lattice antiferromagnet appropriate for numerous spinel materials. With sufficiently strong frustration a massive ground state degeneracy develops amongst spirals whose propagation wavevectors reside on a continuous two-dimensional ``spiral surface'' in momentum space. We argue that an important ordering mechanism is entropic splitting of the degenerate ground states, an elusive phenomena called order-by-disorder. A broad ``spiral spin-liquid'' regime emerges at higher temperatures, where the underlying spiral surface can be directly revealed via spin correlations. We discuss the agreement between these predictions and the well characterized spinel MnSc2S4

Dental status of patients with squamous cell carcinoma of the oropharyngeal region

Purpose of the study. To evaluate changes in the oral cavity in patients with squamous cell carcinoma of the oropharyngeal region after radiation therapy.Materials and methods. The study included 70 patients with oropharyngeal squamous cell carcinoma. All patients underwent external beam radiation therapy. We used dose fractionation mode: 5 fractions per week, single focal dose — 2G. The severity of oral mucositis was assessed using the RTOG evaluation scale.Results. Patients with carcinoma of the oropharyngeal region before the start of the main treatment had pathology of hard tooth tissues (CPU-14.62±1.08), gingival inflammation (PMA-41.71±4.98), periodontal inflammation (CPITN-2.54±0.62). After the end of radiation therapy, oral mucositis was detected in all patients (100%). We found changes in the negative side for all indicators of dental status indices compared to primary CPUs-15.26±0.90; CPITN-3.36±0.58; PMA-49.75±5.28 (p&lt;0.05). Clinical signs of oral mucositis begin to appear as early as 7 days after the start of radiation therapy, RTOG=1,5. After 3 weeks, patients develop severe mucosal lesions with fibrinous plaque and severe pain syndrome (RTOG=3.0), requiring intensive drug therapy (p&lt;0.05).Conclusion. In connection with the early appearance of post-radiation oral mucositis, preventive and therapeutic measures to improve the regeneration

The High field magnetization of FePS 3

International audienceHigh field magnetisation measurements in pulsed fields up to 65 Tesla have been performed on FePS3, which is nominally a good example of a two-dimensional Ising-like antiferromagnet on a honeycomb lattice. Measurements with the field parallel to the moment direction confirm the presence of two first-order transitions above 35 Tesla, to M/Msat = 1/2 and M/Msat = 1, respectively, at 4 K. The measurements are in contradiction with published estimates for the magnetic exchange parameters, but the contradiction can be resolved by allowing for anisotropic exchange parameters in the Hamiltonian. The magnetisation with the field perpendicular to the moment direction is anisotropic, with no transitions observed for fields along the a axis while a cascade of first-order transitions is observed for fields above 50 Tesla along the b axis, the latter case also showing a strong degradation of the sample after repeated pulses. The results indicate a strong magneto-lattice coupling in FePS3. Temperature-dependent measurements hint at a possible tricritical point