So where do we find magnetoresistance, and why is it important? It's seen in systems having sensitive, magnetically dependent transport. Novel structures comprising magnetic layers separated by non-magnetic metallic spacer layers can exhibit GMR behavior in which there is a 100% change in resistance upon the application of an appropriate magnetic field, resulting from the non-parallel spin alignment of the magnetic layers at zero field. In simple terms, GMR compounds are insulators which become conductors when there is an external magnetic field.

   The discovery of high-temperature superconductivity in hole-doped copper oxide compounds aroused interest in transition-metal oxides, and in the course of these studies the electronic properties of hole-doped manganese oxides with perovskite structure were revisited. Perovskites have the form R_1-xA_xMnO₃ where R is a trivalent rare-earth ion, A is a divalent alkali ion such as Ca or Sr, and 0 < x < 1. The three Hot Papers in this period dwell on the ferromagnet La_1-xSr_xMn0₃. GMR refers to the pronounced decrease in electrical resistance in magnetically inhomogeneous materials when an applied field brings the moments into alignment. In these compounds with values of 0.2 <x < 0.4 the resistance drops hugely when a magnetic field is applied.

   All three Hot Papers deal with the basic physics of GMR, aiming to decouple the many variables of temperature, hole concentration x, ion types, and sample structure, in order to determine precisely how GMR operates in bulk materials.

   Paper #2 is a "shooting star," having lurked under the Top Ten for almost a year gathering a creditable 114 citations along the way. Its 40 citations this period catapult it to prominence. But to understand the importance of #2 we must first look closely at #10, from a group at AT&T Bell Laboratories. This takes the traditional model of the manganite system which assumes that the only relevant physics is magnetic coupling as a result of exchange of electrons between Mn⁺³ and Mn⁺⁴ ions. The researchers solve this model and then show that it collapses in the face of the data. Instead, they argue that a strong electron-phonon interaction plays a crucial role, and thereby they introduce new physics into models of GMR.

   Experimental study #2 looks at lattice effects by substituting different rare-earth ions (Pr, Y) for La and then seeing how the magnetic properties differ when the hole concentration x is fixed. From this they confirm that the double-exchange model is flawed and additional factors are at work in accounting for GMR.

   Newcomer #9, from a group representing the University of Tokyo and institutions in Tsukuba, Japan, establishes the use of single crystals rather than films in experiments on the insulator-metal transition in doped manganites. For Science Watch, Yutaka Moritomo (JRCAT, Tsukuba) explains that #9 "is the first systematic investigation of the magnetic and transport properties of the dope manganites using high-quality single crystals grown with a floating-zone furnace. Before the results reported in #9 there were only a few papers based on single crystals. In 1995-96 researchers used thin films or polycrystalline (ceramic) samples in their investigation of the GMR compounds. However, mounting experimental data reveal that grain boundaries in the polycrystalline samples significantly alter the resistivity as well as the magnetoresistance. Many researchers are now using single crystals." This may account for the high level of citations. The Japanese team's results include a phase diagram as a function of the hole concentration x.