A ferromagnet having no net magnetic moment

Coupling between the spins (magnetic moments) of assemblies of ions can lead to ordered magnetic system-classified as ferromagnetic, antiferromagnetic, ferrimagnetic and so forth, depending on the nature of the ordering(1). A simple picture of the coupling and ordering is usually adequate for describing properties of these systems, such as the magnitude and thermal variation of the magnetization. Here we describe a system whose magnetic behaviour appears counterintuitive on the basis of this picture. The materials in question are based on the ferromagnetic compound, SmAl2, in which some of the magnetic samarium ions, Sm3+, have been substituted with other rare-earth elements. The resulting system exhibits the seemingly incompatible properties of large spin polarization but no bulk magnetization: this state occurs at a specific temperature, when the two components of the magnetization (the electron's spin and its orbital motion) exactly compensate for one another. This property should be generic to ferromagnets containing trivalent samarium ions, and may find potential application in, for example, spin-resolving devices for charged particles.