High-Temperature Ferromagnetism in CaB2C2
J. Akimitsu,12*
K. Takenawa,1
K. Suzuki,3
H. Harima,4
Y. Kuramoto5
We report a high Curie-temperature ferromagnet,
CaB2C2. Although the compound has neither
transition metal nor rare earth ions, the ferromagnetic transition
temperature Tc is about 770 Kelvin. Despite this
high Tc, the magnitude of the ordered moment at
room temperatures is on the order of 10
4 Bohr magneton
per formula unit. These properties are rather similar to those of doped
divalent hexaborides, such as
Ca1xLaxB6. The calculated electronic states also show similarity near the Fermi
level between CaB2C2 and divalent hexaborides.
However, there is an important difference:
CaB2C2 crystallizes in a tetragonal structure,
and there are no equivalent pockets in the energy bands for electrons
and holes--in contrast with CaB6. Thus, the disputed threefold degeneracy, specific to the cubic structure, in the energy
bands of divalent hexaborides turns out not to be essential for
high-temperature ferromagnetism. It is the peculiar molecular orbitals
near the Fermi level that appear to be crucial to the high-Tc ferromagnetism.
1 Department of Physics, Aoyama-Gakuin
University, Tokyo 157-8572, Japan.
2 Core Research
for Evolutional Science and Technology (CREST) of the Japan
Science and Technology Corporation, Japan.
3 Faculty
of Environmental and Information Sciences, Yokohama National
University, Yokohama 240-8501, Japan.
4 Institute of Scientific and Industrial Research,
Osaka University, Osaka 567-0047, Japan.
5 Department of Physics, Tohoku University, Sendai
980-8578, Japan.
*
To whom correspondence should be addressed. E-mail:
jun{at}phys.aoyama.ac.jp
