. The Bell System technical journal . able, there is an excess offive spins. Only in incomplete shells such as this, shells that are beingfilled as new and heavier atoms are made, is there such excess completed shells are magnetically neutral because the spinsmutually compensate one another. The outermost electrons are those responsible for the ordinarychemical properties, and they are influenced by chemical do not contribute to ferromagnetism for reasons that will appearlater. Exchange Forces Only in certain parts of the periodic table are there found electronsbeing
. The Bell System technical journal . able, there is an excess offive spins. Only in incomplete shells such as this, shells that are beingfilled as new and heavier atoms are made, is there such excess completed shells are magnetically neutral because the spinsmutually compensate one another. The outermost electrons are those responsible for the ordinarychemical properties, and they are influenced by chemical do not contribute to ferromagnetism for reasons that will appearlater. Exchange Forces Only in certain parts of the periodic table are there found electronsbeing added to inner shells, and one of these places is in the irongroup; but since there are other parts, notably those occupied by thepalladium, platinum, and rare earth metals, where these inner groupsare being filled, there arises the further question: Why are not theseother elements also ferromagnetic? For an element to be ferromagnetic, it is necessary not only thatthere be uncompensated spin in the electron orbits, but also that the. 68 BELL SYSTEM TECHNICAL JOURNAL resultant spins in neighboring atoms be parallel. Calculation of theenergies of the electrons indicates that to align the spins in all theatoms in a small region, the diameter of an atom must bear the properratio to the diameter of the electron shell in which the electron spinsare uncompensated ^^ (Fig. 3). This proper ratio is required because I n ferromagnetic substances, D/d is greater than (Slater) Fig. 3—Incomplete shells in neighboring atoms. the electron spins and charges influence each other to an amountdepending upon the distance between them; and it is only when thisinfluence, which is known technically as the exchange, has the rightvalue that the spins all can be aligned in the same direction,^^ that is,that the material can become ferromagnetic.^^ The forces of exchange, the existence of which has been realizedonly in the last few years, act to keep the spins parallel, while thermalagitation tends
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