Magnetic phase diagram of UIr2Si2 single crystal

UIr$_{2}$Si$_{2}$ crystallizes with a tetragonal CaBe$_{2}$Ge$_{2}$-type structure (space group $P4/nmm$) with lattice parameters $a$ = 4.0851 Å and $c$ = 9.8348 Å. In the unit cell, iridium sites are partly occupied by silicon atoms. The excess of silicon introduces disorder to the crystal structure [1,2]. UIr$_{2}$Si$_{2}$ was characterized as an antiferromagnet with the Néel temperature, $T\mathrm{_{N}}$ between 4.9 and 6 K [1-3]. Neutron diffraction measurements revealed that the magnetic moments of 0.1 $\mu\mathrm{_{B}}$ are localized on uranium atoms and aligned along $c$ axis of the unit cell [2]. The compound undergoes metamagnetic-like transition at low temperatures in magnetic fields, $B\mathrm{_{c}}$, of 1.6–2 T [1-3]. Remarkably, the magnetic moment measured in magnetic fields $B$ > $B\mathrm{_{c}}$ is distinctly larger than the moment determined by neutron diffraction and amounts to 0.37 $\mu\mathrm{_{B}}$ [4]. Polarized neutron diffraction experiments revealed that magnetic field causes significant enhancement of the U magnetic moment due to mixing with excited crystal field levels [4].
Up to date, only few reports have been published on UIr$_{2}$Si$_{2}$. There is no consensus about the nature of its magnetic behavior, nor the values of $T\mathrm{_{N}}$ and $B\mathrm{_{c}}$. Therefore, we have undertaken an exhaustive re-investigation of the compound, with the main aim to build its direction-dependent magnetic phase diagrams covering wide ranges of temperature and magnetic field. Single crystal of UIr$_{2}$Si$_{2}$ was grown from stoichiometric amounts of the elemental constituents using Czochralski pulling technique in a tetra-arc furnace. Its quality and crystallographic orientation were checked on a Laue diffractometer. Physical properties were studied by means of magnetization, resistivity and heat capacity measurements carried out at temperatures from 1.8 to 300 K and in magnetic fields up to 14 T. The experiments confirmed that UIr$_{2}$Si$_{2}$ orders antiferromagnetically at low temperatures, as evidenced by characteristic anomalies in the magnetic susceptibility, electrical resistivity and heat capacity. The observed value of $T\mathrm{_{N}}$ = 5.5 K falls in between those reported in the literature. Significant difference in the magnitude of magnetization along main crystallographic directions indicated the $c$ axis to be easy magnetic direction. For this orientation, the metamagnetic transition occurs in $B\mathrm{_{c}}$ = 1.5 T at $T$ = 2 K. In stronger fields, the magnetization does not saturate and reaches in 14 T an enhanced value that corresponds to the magnetic moment of 0.43 $\mu\mathrm{_{B}}$. The magnetization measured perpendicular to the $c$ axis is straight-linear up to the highest magnetic fields applied. The electrical resistivity, $\rho$($T$), is fairly large and hardly changes with temperature. The residual resistivity ratio, defined as RRR = $\rho$(300 K)/$\rho$(2 K), is close to 1 for both directions. This feature can be attributed to substantial intermixing of Ir and Si atoms at the transition metal site. At 2 K, the specific heat over temperature ratio attains a large value of 300 mJ mol$^{-1}$K$^{-2}$, that may result from both strong interaction between 5$f$ and conduction electrons and the crystallographic disorder. Based on the collected experimental data, a tentative magnetic phase diagram of UIr$_{2}$Si$_{2}$ has been constructed for magnetic field applied along the $c$ axis.

This work was supported by the National Science Centre of Poland (grant no. 2018/31/D/ST3/03295).

[1] A. Vernière, et al., $Physica B$, \textbf{206 \& 207}, 509 (1995).

[2] A. Vernière, et al., $J. Magn. Magn. Mater.$, \textbf{153}, 55 (1996).

[3] A. J. Dirkmaat, et al., $Phys. Rev. B$, \textbf{41}, 2589 (1990).

[4] A. Vernière, et al., $Physica B$, \textbf{267-268}, 41 (1999).