Calculation of coercivity of magnetic nanostructures at finite temperatures

We report a finite temperature micromagnetic method (FTM) that allows for the calculation of the coercive field of arbitrary shaped magnetic nanostructures at time scales of nanoseconds to years. Instead of directly solving the Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any free parameter by solving a non linear equation, which arises from the transition state theory. The method is applicable to magnetic structures where coercivity is determined by one thermally activated reversal or nucleation process. The method shows excellent agreement with experimentally obtained coercive fields of magnetic nanostructures and provides a deeper understanding of the mechanism of coercivity.Comment: submitted to Phys. Rev.

Fluxgate Principle Applied to a Magnetic Tunnel Junction for Weak Magnetic Field Sensing

Breth L, Dimopoulos T, Schotter J, Rott K, Brueckl H, Suess D. Fluxgate Principle Applied to a Magnetic Tunnel Junction for Weak Magnetic Field Sensing. IEEE Transactions on Magnetics. 2011;47(6):1549-1553.A method is presented for measuring weak magnetic fields by applying a measurement principle known from fluxgate magnetometers to a magnetic tunnel junction. The fluxgate measurement principle based on second harmonic detection is transferred to magnetic tunnel junctions embedded in current lines. Based on Fourier analysis, we developed an analytical model to describe the response of a magnetic tunnel junction fluxgate sensor. The analytical result is compared to MATLAB simulations using Fast Fourier Transformation. Experimental results are obtained with a sensor prototype using lock-in amplification to detect the second harmonic component of the signal generated by the resistance change of the junction. A linear sensor characteristic with a sensitivity in the order of mV/mT is detected in an unshielded setup. Design improvements of the sensor layout as well as a low RA product of the magnetic tunnel junctions should make it possible in the future to detect pT-fields. A cheap sensing technology using magnetic tunnel junctions with the ability of measuring fields in the order of pT could promote novel diagnostic methods such as magnetocardiography into daily clinical routine

Cryogenic Testing of the RF Input Waveguide for the CEBAF Upgrade Cryomodule

In order to support the planned CEBAF upgrade at the Jefferson Lab a new cryomodule has been designed. A key component of the new cryomodule is the upgraded RF input waveguide, which may couple as much as 10 kW of Radio Frequency (RF) power to Superconducting Radio Frequency (SRF) cavities contained in a bath of superfluid helium. The coupler consists of a straight copper plated stainless steel waveguide as a thermal transition between 2K and 300K and one ceramic window at the warm end of the waveguide. The waveguide interior shares a common vacuum with the cavity. Initial testing of the upgraded coupler has been conducted at Jefferson Lab in a representative cryomodule. During testing, data was obtained regarding waveguide temperature profiles as well as coupler arcing and multipacting.Predicted temperature profiles were used to determine the optimum location of the 50K heat intercept on the waveguide, and were found to be comparable to the actual measured profiles.The coupler was found to be free of multipacting up to 1.7 kW. No arcing occurred during multiple eight-hour runs in heavy field emission with typical radiation levels of 0.5 to 1.0 R/hr outside the cryostat

Reduced order model for hard magnetic films

In the pursuit of rare earth-lean permanent magnets for green technologies, microstructural optimisation offers a promising strategy to enhance coercivity while minimising critical element content. For this approach, the combination of experimental work on hard magnetic films and numerical investigations is necessary. However, computational limitations restrict micromagnetics to small systems, motivating the development of a reduced order model for investigating large multigrain systems. The model is based on the embedded Stoner-Wohlfarth method and is used to investigate the influence of a nonmagnetic grain boundary phase thickness and the aspect ratio of the magnetic grains on the overall coercivity. It is possible to simulate large NdFeB multigrain structures which can be compared to hard magnetic films. We derive design recommendations to increase coercivity by increasing the grain boundary phase thickness and the aspect ratio of the grains in hard magnetic materials

sj-pdf-1-aic-10.1177_0310057X231212211 - Supplemental material for Exploring anaesthetists’ views on the carbon footprint of anaesthesia and identifying opportunities and challenges for reducing its impact on the environment

Supplemental material, sj-pdf-1-aic-10.1177_0310057X231212211 for Exploring anaesthetists’ views on the carbon footprint of anaesthesia and identifying opportunities and challenges for reducing its impact on the environment by Matilde Breth-Petersen, Alexandra L Barratt, Forbes McGain, Justin J Skowno, George Zhong, Andrew D Weatherall, Katy JL Bell and Kristen M Pickles in Anaesthesia and Intensive Care</p