Physical properties and field-induced metamagnetic transitions in UAu0.8Sb2

We have successfully synthesized single crystals of UAu0.8Sb2 using a flux method and present a comprehensive study of its physical properties by measuring the magnetic susceptibility, electrical resistivity and specific heat. Evidence for at least three magnetic phases is observed in the field-temperature phase diagram of UAu0.8Sb2. In zero field, the system undergoes an antiferromagnetic transition at 71 K, and upon further cooling it passes through another antiferromagnetic phase with a ferromagnetic component, before reaching a ferromagnetic ground state. A complex magnetic field-temperature phase diagram is obtained for fields along the easy c-axis, where the antiferromagnetic order eventually becomes polarized upon applying a magnetic field.


Results
Crystal structure and physical properties. Figure 1 displays the crystal structure, x-ray diffraction (XRD) and neutron diffraction patterns of UAu 0.8 Sb 2 at room temperature. The XRD measurements in Fig. 1(b) were performed on a single crystal sample, and the peaks can all be accounted for by the (00 l) reflections, indicating that the c-axis is perpendicular to the large face of the crystal. The neutron diffraction pattern indicates that the sample is single phase, and can be refined with the tetragonal HfCuSi 2 -type structure 18 . Table 1 shows the results of the refinement, while the atomic coordinates and occupancy factors are provided in Table 2. The refined lattice parameters and unit cell volume are a = 0.436320 (6) nm, c = 0.978574(3) nm and V = 0.1862963(6) nm 3 , which are smaller than those reported for UAuSb 2 (a = 0.4375(1) nm, c = 0.9831(2) nm) 19 . Upon refining the site occupancies keeping those of the U site fixed to one, the Sb sites are very close to being fully occupied, but the occupancy of the Au site is 0.792 (2). Figure 2(a,b) display the dc magnetization M(T) measured in a magnetic field of 0.1 T applied parallel to the c-axis and ab plane, respectively. The magnetization for fields applied in the ab plane is much smaller than along the c-axis, indicating significant magnetic anisotropy in UAu 0.8 Sb 2 , where the c-axis is the easy magnetization direction. Above 140 K, M(T) can be fitted using a modified Curie-Weiss expression: , giving an effective moment of μ eff = 3.47 μ B /U and a Curie-Weiss temperature of θ p = 84.8 K for fields parallel to [001], and μ eff = 3.07 μ B /U and θ p = −9.7 K for fields in the ab-plane. The μ eff values are reduced compared to the effective magnetic moment of the free U 4+ ions and U 3+ ions (µ eff = 3.58μ B /U and 3.62 μ B /U, respectively). The value of θ p is positive along the c-axis and negative in the ab plane, indicating the presence of both ferromagnetic and antiferromagnetic exchange interactions. The magnetization increases with decreasing temperature before reaching a maximum and abruptly dropping, which likely corresponds to a magnetic transition at T N = 71 K. At lower temperatures, splitting of the zero-field cooled (ZFC) and field-cooled (FC) curves can be seen at around 38 K for fields parallel to [001], along with another small peak at 34 K. This suggests the onset of a ferromagnetic component and a transition to a different magnetic state. Figure 3(a) shows the temperature dependence of the resistivity. The resistivity increases with decreasing temperature, reaching maximum at about 78 K, which is a little higher than the T N value derived from M(T), before dropping at lower temperatures. As displayed in the main panel, below 40 K the temperature dependence of the resistivity can be fitted using the expression 11,16 .
where ρ 0 is the residual resistivity and the second term describes scattering of the conduction electrons by spin-wave excitations with an energy gap Δ. The fitted values are ρ 0 = 221.7 μΩ cm, A = 0.0257 μΩ cm/K 2 and Δ = 16 K. In the paramagnetic state, above 100 K, the resistivity follows logarithmic behavior which may be due to Kondo scattering following k where ρ′ is a temperature independent term. As shown in Fig. 3(b), this expression can describe the higher temperature data, indicating the presence of significant Kondo scattering. Figure 4(a) shows the temperature dependence of the specific heat, where a kink in C/T is observed at 78 K, which likely corresponds to the magnetic transition. As shown in Fig. 4(b), at low temperatures C/T ~ T 2 , and from extrapolating to zero temperature a value of the electronic specific heat coefficient γ of 76 mJ/mol•K 2 is estimated, indicating that UAu 0.8 Sb 2 has moderately enhanced correlations due to the Kondo effect.
Field dependent measurements. Figure 5 shows the field dependence of the magnetization for fields applied parallel to the c axis at different temperatures. At 50 K, the zero-field magnetization is nearly zero for both the up-sweep and down-sweep processes with little hysteresis, indicating that at this temperature the system orders antiferromagnetically. Two metamagnetic transitions are observed with increasing magnetic field, one at around 0.9 T and another at 1.9 T, above which the magnetization changes little suggesting that this corresponds to a spin-polarized phase with a saturated moment of around 1.35 μ B /U. The plateau inbetween 0.9 and 1.9 T has a magnetization of about one third of the saturated value, suggesting that this corresponds to a field-induced antiferromagnetic phase which also has an FM component (AFM2). Such a magnetization plateau could arise if the spins are arranged periodically in a collinear 'up-up-down' arrangement, which yields a net magnetization of one third of that of the ferromagnetic state. It can be seen that at 35 K, the transition to the AFM2 phase with the one third magnetization moves to lower fields. The hysteresis at both metamagnetic transitions significantly increases, and a small hysteresis develops around zero-field, which is consistent with the splitting of the ZFC and FC curves at a slightly higher temperature in Fig. 2(a). At 25 and 15 K, there is now a clear hysteresis loop about zero-field, where the remanent magnetization is nearly one-third of the saturated value, indicating that the AFM2 state with both AFM and FM components is present at zero-field in this temperature range. A number of additional steps in the magnetization can also be detected below the metamagnetic transition to the field-induced FM state. While those in the upsweep curves (from 0 to ±9 T) may arise from the alignment of magnetic domains, at least one step can be resolved in the down-sweep measurements. This may either signify an additional change of magnetic structure, or the coexistence of the AFM2 state with small regions of either the AFM or FM phases.
At lower temperatures still, as shown for example by the curve at 3 K, it can be seen that upon sweeping down from high fields to zero-field, the magnetization remains very near the high-field saturated value, giving strong evidence for a ferromagnetic ground state. The magnetization changes little as the field is reduced into the negative field range, before the sign of the magnetization switches. It can be seen that the magnetization undergoes a number of steps before reaching the saturated negative value, which is likely related to the domain reorientation processes. It is noted that if the field sweep-rate is increased these steps are found to disappear. Figure 6 shows the in-plane resistivity of UAu 0.8 Sb 2 as a function of applied magnetic field along the c axis at different temperatures. In agreement with the magnetization measurements, the data at 50 K display two jumps of the resistivity in field with little hysteresis. At both of these metamagnetic transitions there is a sharp drop of the resistivity with increasing field, which is consistent with the sudden transition to a magnetic phase with a larger ferromagnetic component. Meanwhile below 35 K, the hysteresis at the low-field metamagnetic transition again significantly increases. At 15 and 25 K where the magnetization indicates the AFM2 ground state, the in-field hysteresis in the magnetoresistance is observed over a broader field range, only disappearing when the moments align in the field-induced FM state. Meanwhile at lower temperatures, where the magnetization loops indicate Phase diagram. Figure 7 shows the field-temperature phase diagram derived from the magnetoresistance and magnetization data, when the external field is applied parallel to the c axis. For the field dependent quantities, the locations of the transitions were determined from the down-sweep measurements from high fields. It can be seen that upon cooling from high temperatures in zero field the system undergoes an antiferromagnetic transition to a phase labeled AFM1. Meanwhile upon increasing the field along the c-axis, the system changes to the  AFM2 phase, which has both antiferromagnetic and ferromagnetic components. Since the net magnetization is around one third of the saturated value, this suggests a magnetic structure along the lines of an 'up-up-down' spin configuration. At higher fields still, the spins align in a field induced ferromagnetic state. Upon cooling further, the AFM1 phase is not found and in zero field the system is in the AFM2 state, where there is a finite remanent magnetization in zero field. The position of the additional magnetization steps, are marked by the dashed line. Meanwhile at low temperatures the ground state becomes ferromagnetic, as evidenced by the ferromagnetic-like hysteresis loops in the magnetization. To understand the nature of the magnetic phases and determine the magnetic structure, low temperature neutron diffraction measurements are highly desirable.

Discussion
The UT 1-x X 2 materials present variety of magnetic properties including AFM and FM ground states [11][12][13][14][15][16] , which were suggested to arise due to the modification of the magnetic exchange interactions by hybridization between the f electrons and conduction electrons 11 . In most of these materials, only one magnetic ordering transition is reported, whereas others such as UCuBi 2 display two AFM transitions 12 . The partially occupied UAu 0.8 Sb 2 shares some similarities to both UAuSb 2 and USb 2 . UAuSb 2 likely undergoes two magnetic transitions, a ferromagnetic one at T C = 31 K and a possible antiferromagnetic transition at T N = 43 K 19 . Meanwhile USb 2 undergoes an antiferromagnetic transition at 203 K, with no other magnetic transitions at lower temperature 20 . This suggests that the effect of Au doping into USb 2 suppresses the antiferromagnetic interactions, reducing T N , but induces a ferromagnetic transition at lower temperature. On the other hand, since the partial occupancy leads to smaller lattice parameters, UAu 0.8 Sb 2 may correspond to UAuSb 2 at positive pressure, and therefore it is of interest to study the stoichiometric UAuSb 2 under pressure to examine if the antiferromagnetism is enhanced accordingly.
In conclusion, we have successfully synthesized UAu 0.8 Sb 2 single crystals and performed a detailed investigation of its crystal structure and physical properties. We find evidence for at least three magnetic phases, a high temperature antiferromagnetic phase, a phase with both ferromagnetic and antiferromagnetic components at intermediate temperatures, and a low temperature ferromagnetic phase. We also constructed a field-temperature phase diagram from measurements performed with a field applied along the easy direction.

Methods
UAu 0.8 Sb 2 single crystals were grown using a self-flux method 21 . U (99.9%), Au (99.999%) and Sb (99.9999%) were combined in an atomic ratio of 1:3.5:14 and placed in an alumina crucible. The crucible was sealed in an evacuated silica tube, heated up to 1150 °C and held at this temperature for 24 h before being cooled to 1050 °C over 1 h and slowly cooled down to 700 °C. The excess Sb flux was removed by centrifuging and plate like crystals were mechanically separated from the crucible. The typical dimensions of the crystals are about 4 × 4 × 2 mm 3 .
The x-ray diffraction measurements of the single crystal were performed using a PANalytical X′Pert Pro diffractometer (Cu K α -radiation). The neutron diffraction experiments for UAu 0.8 Sb 2 powder were carried out using the high resolution neutron powder diffractometer (HRND) (λ = 0.1884 nm) at the China Mianyang Research Reactor (CMRR). The structural refinements were performed using the Fullprof software 22 . The measurements of the resistivity, magnetization and specific heat were performed using a Physical Property Measurement System (PPMS-9).

Figure 7.
Field-temperature phase diagram derived from the resistivity and magnetization data, for an external field applied parallel to the c axis.