Fig. 1-11 The temperature-pressure phase diagram of CeP

When macroscopic quantities such as the magnetization, the electric conductivity, or the specific heat are measured for a compound CeP of rare earth elements (Ce), at constant pressure but with decreasing temperature, we observe a peculiar phase transition phenomenon where the original paramagnetic phase changes to the beta phase at a temperature of Tc2 and then changes to the alpha phase at a temperature of Tc1. Using neutron scattering experiments we succeeded in explaining the peculiar macroscopic phenomena by the corresponding microscopic phenomena at the atomic level. Because a neutron has a finite magnetic moment, it can be scattered not only by a nucleus but also by the atoms constituting a magnetic material. The scattering can therefore provide very important information regarding the mechanism of the phase transition of a magnetic material. By utilizing this property of the neutron scattering experiment we can obtain the temperature-pressure phase diagram of the compound CeP, and we can obtain the arrangement of the microscopic atomic magnets in the corresponding phase of the phase diagram, as shown in Figs. 1-11 and 1-12. The Ce atom can be in one of two states: in the gamma8 state where the atom has a strong magnetic moment, or in the gamma7 state where the atom has a weak magnetic moment. In the alpha phase, under atmospheric pressure, the material takes on an antiferromagnetic structure composed of atoms in the gamma7 state, arranged with alternating moment directions. With increasing pressure, the structure changes regularly until an antiferromagnetic structure, composed of atoms in the gamma8 state, is realized at a pressure of about 20 GPa. In the beta phase, on the other hand, atoms in the gamma7 state have random moment characters and atoms in the gamma8 state form a ferromagnetic structure with increasing pressure, whereas the structure changes regularly in similar manner to the alpha phase. This change in the structure has not previously been observed in any magnetic material.

Reference T. Osakabe et al., Novel Magnetic Structures of the Low-Carrier System CeP Under High Pressure, Physica B, 230-232, 645 (1997).

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