The nature of magnetism in metals is one of the long-standing research topics in the Quantum Matter group. What happens, when a ferromagnetic metal is tuned - by applying pressure or by varying the composition - so that the magnetic ordering temperature is gradually reduced to zero? If the transition is continuous, or second order, then the point in the phase diagram where magnetism just vanishes would be a so-called quantum critical point, but experimentally this appears to be exceedingly rare. Instead, most clean metals escape either into first order, discontinuous phase transitions. Some form another ordered state, such as spin density wave order.
Three dimensional composition-field-temperature phase diagram of NbFe2, showing the ferromagnetic (blue) and spin density wave (red) ordered phases, as well as the newly reported quantum tricritical points.
The band magnet NbFe2 is a nice example of this phenomenon, and in this Nature Physics publication, a team led by QM investigator Malte Grosche, together with colleagues from Bristol (Cavendish alumnus Sven Friedemann), Royal Holloway, Dresden and Munich, was able to show that in NbFe2 the interplay between spin density wave order and ferromagnetism can be nicely modelled in a two-order- parameter Landau theory, that this allows the location of the ferromagnetic quantum critical point to be pinpointed as it is buried inside a spin-density-wave region in the phase diagram, and that moreover all of this causes tricritical points to arise, when the spin density wave order is tuned in a magnetic field. The tricritical points, again, can be suppressed to zero temperature by varying composition or pressure, giving rise to the above-mentioned 'quantum tricritical points', where both the uniform and the staggered magnetic susceptibility diverge down to low temperatures. This phenomenon may be of more general interest, as it could apply, for instance, to certain f-electron systems near the threshold of magnetism, such as the archetypal non-Fermi liquid material YbRh2Si2. More information can be found in press statements by colleagues at MPI-CPfS Dresdenand at the Dept. of Physics, Bristol.(20/9/2017)