The spin-polarized ferromagnetic state of a cold Fermi gas

TL;DR

This paper explores the conditions under which a cold Fermi gas can become ferromagnetically polarized, showing that significant baryon dipole moment increases are needed, resulting in extremely strong magnetic fields with isotropic pressure.

Contribution

It demonstrates that a ferromagnetic state in a cold Fermi gas can be achieved through enhanced baryon dipole moments, leading to strong internal magnetic fields without pressure anisotropy.

Findings

Achieving ferromagnetization requires large baryon dipole moment increases.

Induced magnetic fields can reach around 10^{17} gauss.

Ferromagnetized state exhibits isotropic pressure, unlike externally magnetized gases.

Abstract

The spin-polarized ferromagnetic state of a cold Fermi gas is investigated for interacting and non-interacting charge-neutral and $\beta$-equilibrated gases. The standard minimal couplings between the magnetic field and the fermions' charges and magnetic dipole moments define the fermions' interaction with the magnetic field. Assuming a variable coupling strength between the magnetic field and the fermion (baryon) dipole moments, it is shown that a ferromagnetized state can be achieved that corresponds to a lower energy spin-polarized state with a magnetic field entirely due to the gas's magnetic response. We find that, depending on the density, a very large increase in the baryon dipole moments is needed to achieve this ferromagnetized state. While the required increase seems unlikely, the induced magnetic field is of the order $\sim10^{17}$ gauss. Furthermore, while externally magnetized Fermi gases have an anisotropic pressure, the pressure of the ferromagnetized gas is completely isotropic and the thermodynamically preferred magnetized state.