A model for the modifications of the exchange bias (EB) field of antiferromagnet (AF)/ferromagnet (F) bilayers with a polycrystalline or multidomain AF layer induced by ion bombardment (IB) in an external magnetic field is proposed. The model is based on a known two-energy level model for an antiferromagnetic grain or domain in contact with a ferromagnet where two free energy minima are separated by an energy barrier. The model explains the as yet unexplained increase of upon IB on the basis of the grain/domain size and magnetic anisotropy constants distributions in the antiferromagnetic layer after its deposition and on the basis of a twofold effect of the IB on the antiferromagnetic grains/domains: (1) IB acts like local hyperthermal heating leading to an almost immediate increase of the sample's EB. (2) Defects induced by IB in the antiferromagnetic grains/domains lead to a decrease of the energy barrier between the two minima, resulting in a slow additional increase of with time (with temperature T as a parameter) after the bombardment. The model is tested by experiments on the time dependence of the EB and coercive fields after the IB of NiO/NiFe bilayers.

Although equal sharing problems appear to support the development of fractions as multiplicative structures, very little work has examined how children's informal solutions reflect this possibility. The primary goal of this study was to analyze children's coordination of two quantities (number of people sharing and number of things being shared) in their solutions to equal sharing problems and to see to what extent this coordination was multiplicative. A secondary goal was to document children's solutions for equal sharing problems in which the quantities had a common factor (other than 1). Data consisted of problem-solving interviews with students in 1st, 3rd, 4th, and 5th grades (n = 112). We found two major categories of strategies: (a) Parts Quantities strategies and (b) Ratio Quantities strategies. Parts strategies involved children's partitions of continuous units expressed in terms of the number of pieces that would be created. Ratio strategies involved children's creation of associated sets of discrete quantities. Within these strategies, children drew upon a range of relationships among fractions, ratio, multiplication, and division to mentally or physically manipulate quantities of sharers and things to produce exhaustive and equal partitions of the items. Additionally, we observed that problems that included number combinations with common factors elicited a wider range of whole-number knowledge and operations in children's strategies and therefore appeared to support richer interconnections than problems with relatively prime or more basic number combinations.

In the present paper we investigate whether the ion bombardment induced magnetic modifications in exchange biased bilayers are stable in time, whether the direction of the exchange bias can be set to any arbitrary (in-plane) direction by the ion bombardment and whether the exchange bias field can be changed in successive bombardment steps. These three fundamental characteristics are prerequisites for ion bombardment used for an efficient, practical, and stable magnetic patterning of exchange biased layer systems.

No abstract

Magnetic tunnel junctions (Mn83Ir17/Co70Fe30/AlOx/Ni80Fe20) were bombarded by 10–20 keV He+ ions in an applied magnetic field to manipulate the exchange bias of the magnetically hard electrode. The tunneling magnetoresistance of the bombarded junctions is up to 37.8% for ion doses high enough to ensure a well defined exchange bias. This should allow the preparation of high quality tunnel junctions with magnetic micropatterned hard electrodes. For very high ion doses the tunneling magnetoresistance starts to decrease, whereas the area resistance product increases. Possible explanations based on structural alterations of the tunneling barrier are discussed.