“First and second order Ferromagnetic quantum phase transitions”

G. Lonzarich
Cavendish Laboratory
Cambridge, UK

Border of Itinerant Electron Ferromagnetism

Great diversity of new physics develops on the brink of ferromagnetic instability.
Experiments over the past few years have led to the discovery that : -

The strange metal formed in the approach to the FM quantum critical point is consistent with the marginal Fermi liquid model (Ni₃Al)

There is a low-temperature tricritical point and T^3/2 resistivity in "non Fermi liquid phase" of MnSi.

p-wave pairing develops within the in ferromagnetic state (UGe₂) and the paramagnetic state (Sr₂RuO₄)

There is an unstable quantum critical end point in Sr₃Ru₂O₇.

These new discoveries raise many fundamental questions about the strange metal
near a FM quantum critical point. In particular:

- Does the elementary-excitation concept still hold ?

- What is the physics of a dissipative interaction field and fermion field?

- Do magnetic droplets form near the tricritical point (instantons)?

- Is the Ginzburg-Landau model still applicable and what happens to non-analytic terms (m⁴ ln m and q² ln q terms in 3D?

- Are there new instabilities at a quantum critical end point? branching?

- Is there anisotropic particle-hole pairing?

There are many possible applications of these new phenomena. One exciting prospect currently under investigation is the development of magnetic cooling
(Cambridge Magnetic Refrigeration, CamFridge)?