What Is Colossal Magnetoresistance?

Colossal magnetoresistance (CMR) is a phenomenon in which certain materials exhibit an extraordinarily large decrease in electrical resistance when placed in an external magnetic field. The term "colossal" distinguishes the effect from ordinary magnetoresistance, where resistance changes by a few percent, and from giant magnetoresistance (GMR), which occurs in nanoscale multilayer metallic structures. In CMR materials, the resistance can drop by several orders of magnitude near a characteristic transition temperature, making the effect one of the largest known magnetoresistive responses in condensed matter physics.

CMR was observed and characterized systematically in the early 1990s in thin films of perovskite-structured manganese oxides, commonly called manganites, with the general formula RE(1-x)AE(x)MnO3, where RE is a trivalent rare-earth ion such as La, Pr, or Nd, and AE is a divalent alkaline-earth ion such as Ca, Sr, or Ba. These materials had been studied as early as the 1950s, but the growth of epitaxial thin films enabled controlled experiments that revealed the full magnitude of the effect.

Double Exchange and Electronic Phases

The primary mechanism underlying CMR in manganites is the double-exchange interaction, proposed by Zener in 1951 and later elaborated by Anderson and de Gennes. In this model, the transfer of an eg electron between neighboring Mn3+ and Mn4+ ions is facilitated when the core spins of those ions are aligned, linking ferromagnetic order to metallic conductivity. As an applied magnetic field aligns the manganese spins across the paramagnetic-to-ferromagnetic transition, the hopping probability increases sharply and electrical resistance drops. However, double exchange alone cannot fully account for the magnitude of the CMR effect; lattice distortions arising from the Jahn-Teller activity of Mn3+ ions and electronic phase separation between competing ferromagnetic metallic and charge-ordered insulating regions are also considered essential contributors.

Research on the origin of colossal magnetoresistance in LaMnO3 manganite identifies the interplay among spin, charge, orbital, and lattice degrees of freedom as the distinguishing feature of the CMR phenomenon, setting manganites apart from simpler transition-metal oxides.

Material Classes and Tuning

Perovskite manganites at hole-doping levels between x = 0.2 and x = 0.5 show the largest CMR responses, with the transition temperature tunable by choice of the rare-earth and alkaline-earth combination and by epitaxial strain in thin-film form. Double-perovskite manganites, containing two distinct B-site cations in an ordered arrangement, have attracted interest because some compositions show CMR responses at or near room temperature and at lower applied fields. An arXiv preprint on room-temperature low-field colossal magnetoresistance in double-perovskite manganite demonstrates that structural engineering of the double-perovskite lattice can shift the transition to technologically accessible conditions. Beyond manganites, CMR has been reported in a small number of other strongly correlated systems including certain pyrochlores and, more recently, in non-oxide materials.

Characterization and Measurement

CMR is quantified as the fractional change in resistance normalized either to the zero-field resistance or to the high-field resistance. Measurements typically require precise temperature control near the ferromagnetic transition, where the resistance is most sensitive to field. Transport measurements are combined with neutron diffraction, synchrotron X-ray scattering, and scanning probe microscopy to correlate macroscopic resistance with the nanoscale phase distribution documented in studies such as those on low-field magnetotransport in manganites.

Applications

Colossal magnetoresistance has applications in a range of technology domains, including:

• Magnetic field sensors exploiting the large resistance-field response near the transition
• Spintronic memory and data-storage research, where CMR provides a large readout signal
• Infrared bolometers and uncooled thermal detectors using thin-film manganite as the sensing element
• Fundamental condensed-matter physics research on strongly correlated electron systems