uncoupled display paramagnetism, th

uncoupled display paramagnetism, thus paramagneticmaterials have moments with no long-range order and there isa small positive magnetic susceptibility (χ ≈ 0); e.g., pyrite[29]. Materials that possess ferromagnetism have alignedatomic magnetic moments of equal magnitude and theircrystalline structures allows for direct coupling interactionsbetween the moments, which may strongly enhance the fluxdensity (e.g., Fe, Ni and Co). Furthermore, the alignedmoments in ferromagnetic materials can confer a spontaneousmagnetization in the absence of an applied magnetic field.Materials that retain permanent magnetization in the absenceof an applied field are known as hard magnets. Materialshaving atomic magnetic moments of equal magnitude that arearranged in an antiparallel fashion display antiferromagnetism(e.g., troilite FeS). The exchange interaction couples themoments in such a way that they are antiparallel thereforeleaving a zero net magnetization [30]. Above the Neeltemperature (TN) thermal energy is sufficient to cause theequal and oppositely aligned atomic moments to randomlyfluctuate leading to a disappearance of their long range order.In this state the materials exhibits paramagnetic behavior.Ferrimagnetism is a property exhibited by materials whoseatoms or ions tend to assume an ordered but non-parallelarrangement in zero applied field below a certain characteristictemperature known as the Neel temperature (e.g., Fe3O4 andFe3S4). In the usual case, within a magnetic domain, asubstantial net magnetization results from the antiparallelalignment of neighboring non-equivalent sublattices. Themacroscopic behavior is similar to ferromagnetism. Above theNeel temperature, the substance becomes paramagnetic.