Magnetic monopole density and antiferromagnetic domain control in spin-ice iridates

Pyrochlore iridates RE$_2$Ir$_2$O$_7$ (with RE - rare earth) exhibit a number of interesting electronic and magnetic properties thanks to their geometrically frustrated lattice [1,2]. Their structure consists of corner-sharing tetrahedra of RE and Ir ions, respectively, and of oxygen octahedra that surround each Ir. As opposed to the spin-ice forming titanates, in RE$_2$Ir$_2$O$_7$ both the RE and the Ir are magnetic. This additional complexity leads, for example, to a metal-to-insulator (MIT) transition below 150 K in most RE$_2$Ir$_2$O$_7$ systems which is accompanied by all-in all-out order of the Ir spins [3], and to anisotropic behaviour in applied magnetic field.
We present magnetisation $M$ and electrical transport $R$ measurements on Ho$_2$Ir$_2$O$_7$ single crystals at low temperatures down to 0.5 K and in magnetic fields up to 15 T. These experiments are complemented by dipolar Monte Carlo simulations of the field-dependence of magnetisation and monopole density. [4]
A MIT was found to occur at 85 K and the field-dependent measurements $M(H)$ and $R(H)$ show strongly anisotropic and hysteretic behaviour, indicating the formation of magnetic monopoles for $H\parallel[111]$ and the occurrence of two types of domains with antiferromagnetic Ir order. The comparison to simulations allows us to draw conclusions about the relation between experimental $M$ and $R$ data, correlations of the Ir and RE moments, and the monopole density. Moreover, we find that the ratio between the two afm domains can be manipulated in a controlled manner by the application of an external magnetic field.

[1] C. Castelnovo, R. Moessner & S. L. Sondhi, Nature 451, 42 (2008)
[2] J. S. Gardner, M. J. P. Gingras & J. E. Greedan, Rev. Mod. Phys. 82, 53 (2010)
[3] K. Matsuhira, M. Wakeshima, Y. Hinatsu & S. Takagi, JPSJ 80, 094701 (2011)
[4] M. J. Pearce et al., https://arxiv.org/abs/2102.04483