1. NHMFL, Florida State University, Tallahassee, Florida 32306, USA
2. Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87454, USA
3. Laboratorium fr Festkperphysik, ETH-Hnggerberg, CH-8093 Zrich, Switzerland
4. Department of Physics, San Diego State University, San Diego, California 92182, USA
5. Department of Physics, Louisiana State University, Baton Rouge, Louisiana 70803, USA
6. Centro Atomico Bariloche, Bariloche RN 8400, Argentina

Correspondence to: Z. Fisk^1,2 Correspondence and requests for materials should be addressed to Z.F. (e-mail: Email: fisk@magnet.fsu.edu).

Abstract

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades¹. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB₆) which, we argue, show that—at a density of 7 10¹⁹ electrons cm^-3—the ground state is ferromagnetically polarized with a saturation moment of 0.07 _B per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.