Magnetic Susceptibility of Cerium Metal Under Pressure

Abstract

The demagnetization of the 4f state of metallic Ce as function of pressure was studied by magnetic susceptibility measurements. In contrast to the p and y phases, the collapsed a phase does not show a local moment but exhibits instead a strong local exchange enhancement of the susceptibility. The susceptibility decreases linearly with increasing pressure in our measuring range and is estimated to decrease by 80 % between the boundaries of the magnetic and the superconducting phases. The results are consistent with the Anderson model of local moment formation over the entire range of interest of the parameter E^E V .In the j3 and y phases cerium exhibits an effective moment very close to the value of the singly occupied 4/ shell, and the j3 phase undergoes an antiferromagnetic transition at 12.S^. 1 On the other hand, in the a f phase above -50 kbar, Wittig found superconductivity at 1.8°K. 2 Since cerium is the only metal known to have a magnetic and a superconducting phase, it is of paramount interest for studies of local moment formation and the interplay of magnetism and superconductivity, especially in the intermediate a phase.We have developed a technique to measure small paramagnetic susceptibilities under pressure allowing, for the first time, unambiguous magnetic studies of the a phase. Our results are consistent with a demagnetization of the local Af state on the Ce atom according to the Friedel-Anderson model 3 which has been applied 4 to Ce with the assumption that the energy of the local 4/ level E^ moves upwards with respect to the Fermi level E F when the pressure increases. It seems that Ce is suitable for quantitative experimental and theoretical studies of many aspects of the Friedel-Anderson model over a very wide range of the fundamental parameter E^-EY. The samples were arc melted from ingots (Johnson-Matthey batch No. 269, nominal purity 99.99%) in an argon atmosphere and machined into cylinders. They were annealed at 300°C for about 5 h, fitted into Teflon jackets, and pressurized in a small clamp (1.2 by 0.6 in. in diameter) made of very pure BeCu. The magnetic moment of this assembly was measured as a function of magnetic field (0 to 11 kG) and temperature (0.38 to 300°K) in a Faraday magnetometer described elsewhere. 5 The large diamagnetic moment of the fairly massive clamp was compensated by a strip of pure Ta sheet of suitable length which was wrapped around its diameter. The residual magnetic moment of the empty clamp assembly was measured separately as a function of field and temperature. The absolute accuracy of the susceptibility, after correction for the clamp, is about 1 %, whereas the relative accuracy (between changes of pressure) is 0.3%. Figure 1 shows the pressure dependence of the 28 24 20 16 h 12 h 0

Different routes to pressure-induced volume collapse transitions in gadolinium and terbium metals

“…8,9 That the 4f electrons play an important role in this transition is clear from the large (80%) and abrupt drop in the magnetic susceptibility at 0.7 GPa, 35 thus supporting the Kondo or Mott-Hubbard models. However, recent results also point to the importance of lattice dynamics in Ce's volume collapse.…”

Magnetism of europium under extreme pressures

“…Because of their strong 4f local magnetic moments, they are normally not superconducting. Only Ce [9,10] and Eu [11] are known to become superconducting under pressure. At ambient pressure both Eu and Ce possess local magnetic moments.…”

“…At ambient pressure both Eu and Ce possess local magnetic moments. With increasing pressure Ce undergoes an isostructural γ-α transition near 0.7 GPa with a 16% volume collapse and a strong suppression of Ce's paramagnetism from Curie-Weiss type to enhanced Pauli paramagnetism [12,13]. In the α-phase Ce is superconducting with a maximum T c of 1.7 K at 5 GPa [9,10]; however, the strongly localized character of the 4f-electron state was shown to remain intact [14].…”

On the existence of heavy fermion ytterbium compounds

“…Compounds of cerium are known covering the entire range from fully localized f without Kondo interactions, to heavy fermion, to mixed valent, to fully delocalized. Application of external pressure works in the direction of delocalizing the f electron of cerium, and this aspect has been investigated in many experiments [8][9][10].…”