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Heavy fermion cerium hexaboride

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Heavy fermion cerium hexaboride

11 May 2014

Inelastic neutron scattering on the IN5 time-of-flight instrument reveals an unexpected strong ferromagnetic ordering propensity of the magnetic moments in the heavy fermion cerium hexaboride.

The cerium hexaboride (CeB6) intermetallic compound is known technologically for its high electron emissivity at low voltage and is already used in technical applications such as cathods for electron microscopes, electron beam welder or even for X-ray free electron lasers.
Despite its high technical employability, its fundamental properties are still partially controversial and unexplored.
At low temperature, below 10 K, this archetypal “dense Kondo lattice” – “heavy fermion” system shows a rich variety of ordered phases formed by the complicated interplay between spin and orbital degree of freedom. From a paramagnetic (PM) high temperature phase (phase I) the cerium 4f electron moments form an antiferromagnetic (AFM) order at TN = 2.4K (phase III) but above, at TQ = 3.2 K, enter a hidden antiferroquadrupolar (AFQ) order  (phase II) that, in contrast to conventional magnetic order, are invisible (hidden) to standard neutron diffraction experiments.
The ground state (the low temperature - phase III) being characterized by a conventional AFM order, the low-temperature physics of this system was so far assumed to be driven solely by AFM interactions. Inelastic neutron scattering revealed a strong ferromagnetic (FM) low-energy collective excitation that dominates the magnetic spectrum in this compound.

The experiment performed on the IN5 time-of-flight spectrometer at the ILL allowed to map on a single crystal of CeB6 all the magnetic excitations through the entire reciprocal space with unprecedented details. A direct intensity comparison shows that the FM mode intensity substantially exceeds that of conventional AFM modes. This propensity for ferromagnetism may account for much of the unexplained behaviour of CeB6, and should lead to a re-examination of existing theories that have so far largely neglected the role of FM interactions.

As the authors said: “The complete high-resolution mapping of the energy-momentum space, which became possible in this work due to the recent advances in the cold neutron time-of-flight spectrometer instrumentation, offers a strict testing ground for such new theoretical models.”

Re.: Nature Materials, 11 May 2014 (AOP). Doi:10.1038/nmat3976

Contact: Dr Jacques Ollivier, ILL