The development of a suitable technique for isolating unreprocessed nuclear reactor wastes from the biosphere has led to the Swedish multibarrier concept KBS 2 with two engineered ccmponents, a thick-walled copper canister and a clay body which confines the canister, Fig. 1. The clay consists of well fitting blocks of highly compacted Na bentonite made by “isostatic” compression of bentonite powder. They are not water saturated when placed in the deposition holes but take up water from the surrounding rock, swell and ultimately form a tight contact with the rock and the canisters. When the bentonite is in physical equilibrium with the surroundings it forms a medium with a number of valuable properties, such as self-healing and ion exchange capacities. The healing means that initial joints between blocks and voids, or local passages in the clay cased by slight rock or canister displacements, will be sealed by the swelling potential of the clay.
Magnetic exchange interactions determine the magnetic groundstate, as well as magnetic excitations of materials and are thus essential to the emerging and fast evolving fields of spintronics and magnonics. The magnetic force theorem has been used extensively for studying magnetic exchange interactions. However, short-ranged interactions in itinerant magnetic systems are poorly described by this method and numerous strategies have been developed over the years to overcome this deficiency. The present study supplies a fully self-consistent method for systematic investigations of exchange interactions beyond the standard Heisenberg model. In order to better describe finite deviations from the magnetic ground state, an extended Heisenberg model, including multi-spin interactions, is suggested. Using cross-validation analysis, we show that this extended Heisenberg model gives a superior description for non-collinear magnetic configurations. This parameterisation method allows us to describe many different itinerant magnetic systems and can be useful for high-throughput calculations.
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