High-field moment polarization in the ferromagnetic superconductor UCoGe

We report magnetization and magnetoresistivity measurements on the isostructural ferromagnetic superconductors UCoGe and URhGe in magnetic fields up to 60 T and temperatures from 1.5 to 80 K. At low temperature, a moment polarization in UCoGe in a field ${\mu }_0\mathbf{H}\parallel \mathbf{b}$ of around 50 T leads to well-defined anomalies in both magnetization and magnetoresistivity. These anomalies vanish in temperatures higher than 30–40 K, where maxima in the magnetic susceptibility and the field-induced variation of the magnetoresistivity are found. A comparison is made between UCoGe and URhGe, where a moment reorientation in a magnetic field ${\mu }_0\mathbf{H}\parallel \mathbf{b}$ of 12 T leads to field-induced reentrant superconductivity.

Figure 1

(a) Magnetic susceptibility vs temperature at $H=1$ kOe (in an extended scale in the inset) and (b) magnetization vs magnetic field at $T=1.5$ K of UCoGe for $\mathbf{H}\parallel \mathbf{a}$, $\mathbf{b}$, and $\mathbf{c}$.

Figure 2

(a) Magnetization divided by the magnetic field and (b) magnetoresistivity vs field, in a semi-log scale, of UCoGe for $\mathbf{H}\parallel \mathbf{b}$ and at temperatures from 1.5 to 84 K.

Figure 3

(a) Magnetization divided by the magnetic field and (b) magnetoresistivity vs field, in a semi-log scale, of URhGe for $\mathbf{H}\parallel \mathbf{b}$ and at temperatures from 1.5 to 50 K. The insets of (a) show, on the left, the magnetic susceptibility of URhGe vs temperature for ${\mu }_0\mathbf{H}\parallel \mathbf{b}$ of 0.5 T and, on the right, the magnetization of URhGe vs magnetic field for $\mathbf{H}\parallel \mathbf{a}$, $\mathbf{b}$, and $\mathbf{c}$ and at $T=1.8$ K.

Figure 4

Magnetic field vs temperature phase diagram of UCoGe and URhGe extracted here from magnetization and magnetoresistivity measurements with $\mathbf{H}\parallel \mathbf{b}$. For each compound, I and II indicate the regimes below and above the moment polarization field.