High Temperature Magnetic Properties of UIr Single Crystals

Abstract

The discovery of a superconducting phase under pressure in the ferromagnet UGe2 1) and even at ambient pressure in URhGe has put a renewed interest on ferromagnetic compounds with a relatively small Curie temperature. U–T binary compounds (T = Ru, Rh, Ir) were previously investigated on their large variety of magnetic properties. With a Curie temperature of 46K, UIr is a natural candidate for searching exotic properties such as non-Fermi liquid behavior or superconductivity. The magnetic behavior of UIr constitutes itself an interesting subject. First, low-temperature magnetic investigations and neutron diffraction studies on polycrystalline samples indicated that this compound is a ferromagnet with an easy axis oriented along the 1⁄2010 direction. Magnetic investigations performed on single crystals revealed the easy axis parallel to 1⁄210 1 . De Haas van Alphen experiments carried out on single crystals showed evidence of moderate heavy fermion properties at low temperatures, indicating cyclotron effective masses ranging from 10 to 30m0 (m0, rest mass of an electron), in agreement with an enhanced coefficient of the electronic specific heat of about 50mJ/molK. Such characteristics are better explained in an itinerant magnetism picture. On the other hand, the strong magnetic anisotropy and the Curie–Weiss behavior observed above 500K on polycrystalline samples are pointing to a localized picture. Furthermore, studies under pressure indicated a strong suppression of TC in UIr with increasing pressures. For a better understanding of the magnetic behavior of UIr, here we report magnetic investigations performed on UIr single crystals in the temperature range from 2K to 800K. UIr single crystals were grown by the Czochralski method in a tetra-arc furnace. Magnetic measurements were performed using a SQUID magnetometer for investigations at both low and high temperatures. Above 300K we used the commercial oven option. In this temperature region the samples were fixed using Cu wires (99.9%) previously annealed up to 800K. This procedure allowed a temperature independent background. The measurements were carried out on several pieces cut from a main single crystal. This was necessary since some of the samples were found to break apart around 600K, mostly along the ð010Þ plane. The main crystallographic directions used for measurements were determined by Laue diffraction patterns. As shown in Fig. 1, UIr crystallizes in the monoclinic structure (space group P21, a 1⁄4 5:62 A, b 1⁄4 10:59 A, c 1⁄4 5:60 A and 1⁄4 98:9 ). The arrows indicate an easy axis for the magnetic moments as determined from the low temperature magnetization curves plotted in Fig. 2. We can conclude only that the easy axis is close to 1⁄210 1 , supporting the conclusion of Yamamoto et al. and in contradiction to other results. Along this direction, a saturation moment is 0.5 B. We note that the magnetization along 1⁄2010 increases linearly with increasing fields, which is compared to the previous magnetization curve containing a small ferromagnetic component. This is mainly due to the sample alignement. The anisotropy is preserved even at high temperatures, as it can be seen from the temperature dependence of the magnetization plotted in Fig. 3. Even at 800K the ratio between magnetization along the easy axis and the hard axis remains double. A plot of the reciprocal susceptibility, as shown in Fig. 4, confirms this fact. Curie–Weiss fits in the high temperature region yield effective magnetic moment values of 3.40, 3.45 and 3.85 B a b