A new approach to (quasi) periodic boundary conditions in micromagnetics: the macrogeometry

We present a new method to simulate repetitive ferromagnetic structures. This macro geometry approach combines treatment of short-range interactions (i.e. the exchange field) as for periodic boundary conditions with a specification of the arrangement of copies of the primary simulation cell in order to correctly include effects of the demagnetizing field. This method (i) solves a consistency problem that prevents the naive application of 3d periodic boundary conditions in micromagnetism and (ii) is well suited for the efficient simulation of repetitive systems of any size

Spin-polarized currents in exchange spring systems

We present a computational study of the magnetization dynamics of a trilayer exchange spring system in the form of a cylindrical nanopillar in the presence of an electric current. A three-dimensional micromagnetic model is used, where the interaction between the current and the local magnetization is taken into account following a recent model by Zhang and Li [Phys. Rev. Lett. 93, 127204 (2004)] We obtain a stationary rotation of the magnetization of the system around its axis, accompanied by a compression of the artificial domain wall in the direction of the electron flow

Magnetic switching modes for exchange spring systems ErFe2/YFe2/DyFe2/YFe2 with competing anisotropies

The magnetization reversal processes of ½10nm ErFe2=nYFe2=4nm DyFe2=nYFe2" multilayer films with a (110) growth axis and a variable YFe2 layer thickness n are investigated. The magnetically soft YFe2 compound acts as a separator between the hard rare earth (RE) ErFe2 and DyFe2 compounds, each of them bearing different temperature dependent magnetic anisotropy properties. Magnetic measurements of a system with n ¼ 20nm reveal the existence of three switching modes: an independent switching mode at low temperatures, an ErFe2 spin flop switching mode at medium high temperatures, and an YFe2 dominated switching mode at high temperatures. The measurements are in qualitative agreement with the findings of micromagnetic simulations which are used to illustrate the switching modes. Further simulations for a varied YFe2 layer thickness n ranging from 2 to 40nm are carried out. Quantitative criteria are defined to classify the reversal behavior, and the resultant switching modes are laid out in a map with regard to n and the temperature T. A new coupled switching mode emerges above a threshold temperature for samples with thin YFe2 separation layers as a consequence of the exchange coupling between the magnetically hard ErFe2 and DyFe2 layers. It reflects the increasing competition of the two conflicting anisotropies to dominate the magnetic switching states of both RE compounds under decreasing n

A new approach to (quasi) periodic boundary conditions in micromagnetics: The macrogeometry

We present a new method to simulate repetitive ferromagnetic structures. This macro geometry approach combines treatment of short-range interactions (i.e. the exchange field) as for periodic boundary conditions with a specification of the arrangement of copies of the primary simulation cell in order to correctly include effects of the demagnetizing field. This method (i) solves a consistency problem that prevents the naive application of 3d periodic boundary conditions in micromagnetism and (ii) is well suited for the efficient simulation of repetitive systems of any size

Anastomosis configuration and technique following ileocaecal resection for Crohn's disease: a multicentre study

A limited ileocaecal resection is the most frequently performed procedure for ileocaecal CD and different anastomotic configurations and techniques have been described. This manuscript audited the different anastomotic techniques used in a national study and evaluated their influence on postoperative outcomes following ileocaecal resection for primary CD. This is a retrospective, multicentre, observational study promoted by the Italian Society of Colorectal Surgery (SICCR), including all adults undergoing elective ileocaecal resection for primary CD from June 2018 May 2019. Postoperative morbidity within 30 days of surgery was the primary endpoint. Postoperative length of hospital stay (LOS) and anastomotic leak rate were the secondary outcomes. 427 patients were included. The side to side anastomosis was the chosen configuration in 380 patients (89%). The stapled anastomotic (n = 286; 67%), techniques were preferred to hand-sewn (n = 141; 33%). Postoperative morbidity was 20.3% and anastomotic leak 3.7%. Anastomotic leak was independent of the type of anastomosis performed, while was associated with an ASA grade ≥ 3, presence of perianal disease and ileocolonic localization of disease. Four predictors of LOS were identified after multivariate analysis. The laparoscopic approach was the only associated with a reduced LOS (p = 0.017), while age, ASA grade ≥ 3 or administration of preoperative TPN were associated with increased LOS. The side to side was the most commonly used anastomotic configuration for ileocolic reconstruction following primary CD resection. There was no difference in postoperative morbidity according to anastomotic technique and configuration. Anastomotic leak was associated with ASA grade ≥ 3, a penetrating phenotype of disease and ileo-colonic distribution of CD

National variations in perioperative assessment and surgical management of Crohn's disease: a multicentre study

Aim: Crohn's disease (CD) requires a multidisciplinary approach and surgery should be undertaken by dedicated colorectal surgeons with audited outcomes. We present a national, multicentre study, with the aim to collect benchmark data on key performance indicators in CD surgery, to highlight areas where standards of CD surgery excel and to facilitate targeted quality improvement where indicated. Methods: All patients undergoing ileocaecal or redo ileocolic resection in the participating centres for primary and recurrent CD from June 2018 to May 2019 were included. The main objective was to collect national data on hospital volume and practice variations. Postoperative morbidity was the primary outcome. Laparoscopic surgery and stoma rate were the secondary outcomes. Results: In all, 715 patients were included: 457 primary CD and 258 recurrent CD with a postoperative morbidity of 21.6% and 34.7%, respectively. Laparoscopy was used in 83.8% of primary CD compared to 31% of recurrent CD. Twenty-five hospitals participated and the total number of patients per hospital ranged from 2 to 169. Hospitals performing more than 10 primary CD procedures per year showed a higher adoption of laparoscopy and bowel sparing surgery. Conclusions: There is significant heterogeneity in the number of CD surgeries performed per year nationally in Italy. Our data suggest that high-volume hospitals perform more complex procedures, with a higher adoption of bowel sparing surgery. The rate of laparoscopy in high-volume hospitals is higher for primary CD but not for recurrent CD compared with low-volume hospitals

Simulations of ferromagnetic nano structures

The magnetic properties of nanometre-scale structures are of fundamental scientific interest and have the potential to play a major role in future data storage technologies. In particular, arrays of small magnetic elements, also called bit-patterned media, are one of the most promising candidates for the future generation of data storage devices. In this thesis we study potential bit patterned element geometries which are below 1micrometre in size. Their magnetic behaviour is hard to predict using analytical methods and computer simulations are the principal tool for in-depth analysis. The relevant micromagnetic equations are solved using the combined Finite Element/Boundary Element method, and finite differences. Patterned media are (quasi) periodic arrangements of identical objects, with each object typically representing one bit. While one or some of these objects can be simulated with today’s simulation capabilities, the investigation of arrays with hundreds of objects requires novel simulation methods.To deal with such large arrays we introduce and evaluate the new “macro geometry” approach. In most real samples this is superior to using conventional periodic boundary conditions as it takes account of the macroscopic shape of the sample.The micromagnetic simulation package Nmag developed at Southampton has been extended to provide the macro geometry capabilities, and subsequently used to study demagnetising effects between the elements of triangular ring arrays. We find that in a square array of 50-nm size triangular elements these effects are governed by the first and second nearest neighbours and can be considered negligible when the spacing between the rings is larger than 30 nm. We also study the transport properties via the Anisotropic Magneto Resistance (AMR) signal of connected rings arrays using the multi-physics features of Nmag. The simulations use a self-consistent approach to determine the AMR values, a technique able to explain experimental AMR measurements of real structures. We also show how the spatially varying current distribution affects the computation of the AMR values and found that the uniform current model, sometimes used in the study of AMR effects, is a very inaccurate approximation and can easily lead to qualitatively wrong results

Parallel execution and scriptability in micromagnetic simulations

We demonstrate the feasibility of an ‘encapsulated parallelism’ approach towards micromagnetic simulations that combines offering a high degree of flexibility to the user with the efficient utilization of parallel computing resources.While parallelization is obviously desirable to address the high numerical effort required for realistic micromagnetic simulations through utilizing now widely available multiprocessor systems (including desktop multicore CPUs and computing clusters), conventional approaches towards parallelization impose strong restrictions on the structure of programs: numerical operations have to be executed across all processors in a synchronized fashion. This means that, from the user’s perspective, either the structure of the entire simulation is rigidly defined from the beginning and cannot be adjusted easily, or making modifications to the computation sequence requires advanced knowledge in parallel programming.We explain how this dilemma is resolved in the Nmag simulation package in such a way that the user can utilize without any additional effort on his side both the computational power of multiple CPUs and the flexibility to tailor execution sequences for specific problems: simulation scripts written for single processor machines can just as well be executed on parallel machines and behave in precisely the same way, up to increased speed. We provide a simple instructive magnetic resonance simulation example that demonstrates utilizing both custom execution sequences and parallelism at the same time. Furthermore, we show that this strategy of encapsulating parallelism even allows to benefit from speed gains through parallel execution in simulations controlled by interactive commands given at a command line interface