Ferromagnetism and heavy fermion semiconductor-like behavior in UFe0.6Sb2 single crystals

Journal of Alloys and Compounds 663 (2016) 672e676

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Ferromagnetism and heavy fermion semiconductor-like behavior in UFe0.6Sb2 single crystals Donghua Xie a, b, Wen Zhang b, Qin Liu b, Yi Liu b, Shiyong Tan b, Wei Feng b, Yun Zhang b, Yanzhi Zhang b, Xiegang Zhu b, Qiuyun Chen b, Lizhu Luo b, Bingkai Yuan b, Bo Wang a, Xinchun Lai b, * a b

Beijing Institute of Technology, Beijing, 100081, China Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang, Sichuan, 621700, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 November 2015 Received in revised form 18 December 2015 Accepted 22 December 2015 Available online 25 December 2015

Single crystals of UFe0.6Sb2 grown by Sb flux method have been investigated by means of X-ray diffraction, magnetic susceptibility, electrical resistivity and specific heat. UFe0.6Sb2 crystallizes in the tetragonal HfCuSi2-type structure (space group P4/nmm). UFe0.6Sb2 undergoes a ferromagnetic transition at TC ¼ 28 K with an easy axis along the c-axis. The resistivity resembles heavy fermion semiconductinglike behavior and implies three pseudogaps are formed in both directions. One pseudogap formed in the range of 9e30 K originates from states of impurity due to the partial occupancies of the Fe sites, and it can be closed by applying high magnetic field. The magnetic field, however, has little effects on the other two pseudogaps. A large electronic specific heat coefficient g ¼ 233 mJ/mol$K2 reveals this compound is a strong electron correlation material. © 2016 Elsevier B.V. All rights reserved.

Keywords: UFe0.6Sb2 Magnetic properties Electronic properties Specific heat

1. Introduction Recently, the “112” family of uranium and cerium compounds have attracted considerable attentions to shed light on the understanding of their intricate properties owing to complicated manyebody interaction [1e3]. Uranium diantimonides UTSb2 (T ¼ transition metal) crystallize in a simple tetragonal HfCuSi2type structure (space group P4/nmm, No. 129) [3,4], whose planar layers of Sb, T, and U are stacked along the c-axis (inset (a) in Fig. 1). The UTSb2 compounds exhibit different magnetic behaviors according to different transition metal ions at T site, such as ferromagnetic (FM) ordering for T ¼ Co, Cu, Ag, and Au, while antiferromagnetically when T ¼ Ni, Ru, and Pd [4]. Among various “112” family compounds, the medium correlated system UFeSb2 with the Sommerfeld coefficient g ¼ 51(1) mJ/ mol.K2 undergoes a ferromagnetic-type transition at 31(1) K [5]. However, the basic natures of the UFeSb2 compound are difficult to clarify due to small amounts of impurity phases (FeSb1þx and a-Fe) in the arc-melting polycrystalline samples [4,5]. Previous systematic studies on the ternary UeFeeSb compounds, which were

prepared by melting the elements in an arc furnace under a high purity argon atmosphere, found that two ternary phases, UFe1-xSb2 and U3Fe3-ySb4 were stable at 700  C and 750  C. Indeed, UFe1-xSb2 is a solid solution stable while 0 < x < 0.27 at 700  C and 0 < x < 0.42 at 750  C, which indicates that the UFeSb2, the end composition of the UFe1-xSb2 phase, is not a single steady solid solution [6]. The UTSb2 compounds of partial occupancies at the T sites, such as UNi0.5Sb2 [7], UCo0.5Sb2 [8], UCu0.9Sb2 [9], and UPd0.6Sb2 [10], show interesting properties which are different from those of the compounds with full occupancies at the T sites. Therefore, further investigations on the physical properties of one single phase in UFeSb2 compounds of partial occupancies at the T sites are necessary to promote our understanding of the structural and magnetic properties of UFe1-xSb2. Here, we present the study on UFe0.6Sb2 consisting of the determination of the crystal structure, measurements of the resistivity under magnetic fields up to 90 kOe, magnetic susceptibility and specific heat. All these reproducible studies were performed on the samples from the same batch.

2. Experimental details * Corresponding author. E-mail address: [email protected] (X. Lai). http://dx.doi.org/10.1016/j.jallcom.2015.12.188 0925-8388/© 2016 Elsevier B.V. All rights reserved.

The UFe0.6Sb2 single crystals were grown by the so-called self-

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673

Fig. 1. Observed (points) and calculated (line) x-ray diffraction patterns for UFe0.6Sb2 powder at room temperature. The difference plot is shown at the bottom and vertical green bars represent the Bragg peak positions for UFe0.6Sb2. The insets are the crystal structure of UFe0.6Sb2 (a) and X-ray diffraction patterns for UFe0.6Sb2 single crystal (b). Two colors of the iron atom in the inset (a) indicate partial occupancies.

flux method in a similar way to the preparation of the parent compound USb2 [11]. Uranium (purity 99.9%), iron (purity 99.99%) and antimony (purity 99.9999%) were weighed in the atomic ratio of 1:4:20 and placed in an alumina crucible. The crucible was sealed in an evacuated silica tube, heated up to 1180  C and held at this temperature for 24 h followed by slow cooling (3  C/h) down to 700  C. Then, the single crystals were separated from the flux by means of centrifuging. Several shining plate-like single crystals were obtained and had the typical dimensions of 4  4  2 mm3. Excess of Fe was necessary for the title single crystal growth, which hindered the growth of USb2 and FeSb2. In other atomic ratios, the regular plate-like single crystals hardly formed and the needle-type FeSb2 crystals were easily obtained. The chemical compositions were determined on the fresh cleavage plane with an energy dispersive X-ray (EDX) spectrometer. X-ray powder diffraction measurements were performed with a PANalytical X'Pert Pro diffractometer (Cu Ka-radiation) on powder prepared by grinding several single crystals in the argon protected gloves box inside an agate mortar. The scans were taken in the q/2q mode with the following parameters: 2q region 18e90 , step scan 0.02 , counting time per step 10 s. The X'Pert HighScore Plus program was used for Rietveld refinements. The measurements of the electronic, magnetic and thermal properties were performed using a commercial PPMS-9 system (Quantum Design), which served as a variable temperature and magnetic field platform. 3. Results and discussion The results of EDX analysis on the fresh cleavage plane of the selected crystals (U, 28%; Fe, 17%; Sb, 55%) indicate a significant deviation from the idea 1:1:2 stoichiometry and yield the chemical composition around UFe0.6Sb2. The X-ray diffraction pattern of single crystals (inset (b) in Fig. 1), reveals a group of peaks of the 00l-type reflections indicating the c-axis is perpendicular to the visible analysis plane of the crystals. All the observed diffraction lines of the powder prepared by grinding several single crystals could be indexed as the UFe0.6Sb2 single phase with no impurity phases (Fig. 1). The crystal structure of the UFe0.6Sb2 is successfully refined using an X'Pert HighScore Plus program. The starting atomic parameters are taken as those of the UTSb2 compounds (T: transition metals). The crystallographic and experimental details are summarized in Table 1. Atomic coordinates and occupancy

Table 1 Crystal data and structure refinements for the UFe0.6Sb2 compound. Compounds

UFe0.6Fe2

Crystal system, space group Cell parameters (nm) Volume (nm3) Wavelengths (Å) Data range (2q/ ) Counting step (2q/ ) Counting time per step (s) Profile function: Background: Profile parameters U: V: W: Rietveld reliability factors (%) R (expected)/%: R (profile)/%: R (weighted profile)/%: Goodness-of-fit:

tetragonal, P4/nmm a ¼ 0.43116(3), c ¼ 0.89815(7) 0.166966(3) 1.54060, 1.54443 (Cu Ka) 18e90 0.02 10 Pseudo Voigt Polynomial 0.4569 0.1989 0.1003 8.88043 9.89736 12.46685 1.97082

factors are given in Table 2. The refinement of the cell parameters and unit cell volume converges to a ¼ 0.43116(3) nm, c ¼ 0.89815(7) nm and V ¼ 0.166966(3) nm3. The occupancy factors present no significant deviation from the full occupancies for uranium and antimony, and a value of 0.61 for iron atoms represented in two colors in the inset (a) of Fig. 1. The occupancies for iron atoms are in agreement with the result of EDX. The lattice parameters of the UFe0.6Sb2 are smaller than that of the poly-crystalline UFeSb2 (a ¼ 0.43238(1) nm, c ¼ 0.90949(3) nm) [5]. This is consistent with the partial occupancies of the Fe sites in the unit cell of this uranium ternary intermetallic compound. The metallic radius is 0.153 nm, 0.126 nm, 0.161 nm for uranium, iron and antimony atom, respectively. As for uranium atom in

Table 2 Crystallographic parameters for the UFe0.6Sb2 compound. Atom

Wyckoff position

x

y

z

s.o.f.

U Fe Sb1 Sb2

2c 2b 2c 2a

0.25 0.75 0.25 0.75

0.25 0.25 0.25 0.25

0.2690(2) 0.50 0.6388(4) 0.00

1.00 0.61 (9) 1.00 1.00

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UFe0.6Sb2, the distances of the nearest neighbors are higher than the sum of metallic radii except the distance of UeSb1, which almost equals to the metallic radii sum (Table 3). These data are similar to that of UFeSb2. In addition, the FeeSb1 distances are much shorter than the metallic radii sum. The observed variety of magnetic ordering in these series has been attributed to both superexchange and RKKY exchange interactions due to f-p and fd hybridization, respectively. It is deemed that the hybridization in UFeSb2 is mainly contributed by the f-p (UeSb1), the very short FeeSb1 distances result in a probable collapse of Fe magnetic moments [5]. The distance of UeSb1 in UFe0.6Sb2 are slightly larger than that of UFeSb2 (0.3155 nm), and the distance of FeeSb1 in UFe0.6Sb2 are shorter than that of UFeSb2 (0.2538 nm). Those differences of atomic distances inevitably lead to the discrepancy of properties between UFe0.6Sb2 and UFeSb2. The dc magnetic susceptibilities c(T) of UFe0.6Sb2 single crystal measured in a magnetic field 500 Oe applied parallel to the crystallographic a- and c-axes are displayed in Fig. 2. Although the trend of the cc is similar to that of ca, ca is very small compared with cc, indicating that the c-axis is equivalent to an easy magnetization axis. The cc(T) increases with decreasing temperature and shows a ferromagnetic-type transition at 28 K (according to the minimum of temperature derivative of the cc(T), seen in inset of Fig. 2), which is smaller than the ferromagnetic transition temperature of the polycrystalline UFeSb2 [5]. A small kink observed at 70 K may be ascribed to the variation of local magnetic environment due to the iron partial occupations. The cc data in the paramagnetic region follow the modified CurieeWeiss law c ¼ c0þC/(TQCW), where C ¼ m0NAmeff/3kB. The linear fitting of the c1 c (T) data between 70 K and 300 K yields an effective moment meff ~ 3.61 mB

and a CurieeWeiss temperature QCW ~ 25 K (Fig. 2). The positive sign of QCW is consistent with a ferromagnetic phase transition. The meff is good agreement with the free ion values of U3þ (3.62 mB) and U4þ (3.58 mB). Isothermal magnetization and hysteresis curves of UFe0.6Sb2 are shown in the Fig. 3, whose fields are parallel to the easy magnetization axis. The magnetization at 15 K rapidly increases with increasing field at the low fields and almost saturates at the high fields, reaches a value about 1.45 mB at H ¼ 90 kOe (inset of Fig. 3). The linear behavior of magnetization curve above TC (45 K and 60 K) indicates no ferromagnetic impurities with Curie temperature higher than 45 K. A full hysteresis loop at 2 K is displayed in the Fig. 3. Magnetization measurements and hysteresis curves provide strong evidences for the ferromagnetic state in UFe0.6Sb2 below 28 K. Further measurements are required to determine the exact nature of the magnetic transition in this material. Fig. 4 displays the temperature variation of the electrical resistivity of UFe0.5Sb2. At room temperature rc(T) and ra(T) are about 330 and 368 mU cm, respectively, which are one tenth of the values of UFeSb2 reported previously [5]. Below 300 K, the rc(T) and ra(T) increase with decreasing temperature, except two kinks at around 50 K and 28 K. It is interesting to fit the resistivity of UFe0.6Sb2 to an activation behavior of the type r ¼ r0 exp(△/kBT), where D is a gap energy, kB is the Boltzmann's constant and r0 is a constant resistivity. To estimate the gap energy, we have plotted lnr vs l/T (the inset in Fig. 4). It is noted that there are three distinct regions with a linear behavior, indicating the formation of three gaps in the electronic density of states in both directions of UFe0.6Sb2 (Table 4). The gaps in both directions seem to be decreased with decreasing temperature. The value of the gap in UFe0.6Sb2 revealed by the resistivity in the range of 50e150 K are the same order of magnitude as Kondo insulators or heavy fermion semiconductors, such as CeRhSb and CeRhAs, in which insulating gaps are believed to originate from the Kondo coupling between conduction electrons and localized 4f electrons [12,13]. To investigate the effect of magnetic fields on the resistivities, the resistivities under the magnetic fields are measured shown in Fig. 5 a) and Fig. 5c). The ra(T) below 80 K is enhanced when applying a field  10 kOe. When the fields are larger than 20 kOe, ra(T) below 80 K is obviously suppressed. The ra(T) increases with decreasing temperature and exhibits a broad maximum peak, followed by a decrease and an upturn at 7 K. The peak broadens and shifts to higher temperature with increasing field. The abrupt rise in

Fig. 2. Left axis: Temperature dependence of the magnetic susceptibility measured with H ¼ 500 Oe applied parallel to the c-axis (cc) and a-axis (ca). Right axis: Inverse 1 magnetic susceptibility c1 c (T) for 500 Oe, the solid line is the best fit of cc (T) to the modified CurieeWeiss law. Inset: The temperature derivative of the cc.

Fig. 3. Hysteresis curves M(H) at 2 K, 15 K and 45 K, fields are parallel to the c-axis. Inset: Magnetizations as a function of the magnetic fields applied parallel to the c-axis at 15 K, 45 K and 60 K.

Table 3 UFe0.6Sb2 interatomic distances (d) and nearest neighbor (NN) numbers. NN U (2c) 4 4 4 1 Sb1 (2c) 4 4 1

Atoms

d(nm)

Fe (2b) Sb1 (2c) Sb2 (2a) Sb1 (2c)

0.2992 0.3159 0.3238 0.3321

Fe (2b) U (2c) U (2c)

0.2490 0.3159 0.3321

NN

Atoms

d(nm)

Fe (2b) 4 4 4

Sb1 (2c) U (2c) Fe (2b)

0.2490 0.2992 0.3049

Sb2 (2a) 4 4

Sb2 (2a) U (2c)

0.3049 0.3238

D. Xie et al. / Journal of Alloys and Compounds 663 (2016) 672e676

Fig. 4. Electrical resistivity vs temperature for UFe0.5Sb2 along a-axis and c-axis. The inset shows the lnr vs 1/T curve, the solid lines show the line fitting of activated behavior.

Table 4 Activation energy obtained by fitting the activation behavior. Temperature range (K)

△/kBjra (K)

△/kBjrc (K)

150e50 30e9 7e2

10 0.3 0.02

14 0.6 0.03

675

9 K is correlated with the partial occupations of the Fe sites, which are similar to the states of impurities. The present results also suggest the existence of residual states at the Fermi level, which is consistent with the large value of Sommerfeld coefficient g in the specific heat. Therefore, the gaps can be called pseudogaps as those appeared in other Kondo insulators. The resistivity behaviors in UFe0.6Sb2 under the field indicate that one pseudogap can be closed by the applied high magnetic field. Even the maximum available magnetic field of our instrument, however, cannot destroy the other two pseudogaps. On the other hand, rc(T) below 60 K is only slightly enhanced and not obviously changed in high temperature range by the magnetic field (Fig. 5c). The differences of resistivity between the two axes are attributed to the anisotropic nature of the gap. The strong anisotropy, a common feature in “112” family, has been ascribed to a pronounced f-p hybridization of U-ions with Sb. The ra(T) below 80 K, especially in the range of 9e30 K, is sensitive to the magnetic field, which is also revealed by the transverse magnetoresistivity (TMR), defined as Dr/r(0) ¼ r(H)r(0))/r(0). The TMR along a-axis below 80 K is significantly enhanced at low fields, and the negative TMR at high fields is also enhanced (Fig. 5b). The enhancement of the negative TMR with increasing field above 4 T can be well understood as a result of an increase of carrier number by the close of one pseudogap under the strong magnetic field, and was similar to the phenomenon observed in other Kondo insulators [14e16]. There exist some experimental evidences and theoretical speculations that the material properties of these many body insulators are particularly sensitive to impurities. If the gap is caused by the impurity states,

Fig. 5. a) Variation of the ra(T) with the temperature under the constant magnetic field, the fields are parallel to c-axis. b) Normalized plot of the transverse magnetoresistivity at the constant temperature under the varied magnetic field for I//a, H//c. c) Variation of the rc(T) with the temperature under zero field and 70 kOe, the fields are parallel to c-axis.

resistivity below 7 K is similar to that observed in the so-called Kondo insulators. Those behaviors under the magnetic fields are consistent with the previous results of UFeSb2 compounds, however our resistivity under zero field are different from the data of UFeSb2 reported by A.P. Gonçalves et al., whose tendency without field are similar to that of the resistivity under the field [5]. Therefore, it can be concluded that the gap in UFe0.6Sb2 from 30 to

the magnetic field will have a strong influence on it [17]. Thus, the magnetic field dependence of the resistivity below 80 K for UFe0.6Sb2 implies that this pseudogap formed in the range of 9e30 K is caused by states of the impurities, originating from the partial occupations of the Fe sites. The semiconductor-like behavior below 7 K and above 80 K under all conditions, and the enhancement of the positive TMR at low fields could be interpreted as the

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pseudogap in UFe0.6Sb2 ranging from 50 to 150 K is the same order of magnitude as those in heavy fermion semiconductors. The pseudogap in the range of 9e30 K, originating from states of impurities due to the partial occupations of the Fe sites, can be closed with the applied high magnetic field, which is similar to other heavy fermion semiconductors. The magnetic field, however, has little effects on the other two pseudogaps. The value of g estimated from the analysis of the specific heat is large, 233 mJ/mol$K2, indicating that UFe0.6Sb2 is a strong electron correlation material. Acknowledgements

Fig. 6. Temperature dependence of the specific heat of UFe0.6Sb2. Upper inset: temperature derivative of the specific heat dC/dT. The arrows mark the ferromagnetic phase transition. Lower inset: low-temperature data plotted as C/T versus T2. The solid line represents a least-squares fitting of experimental data to the formula C ¼ gTþbT3.

little effects of the limited magnetic field on the other two pseudogaps. The specific heat decreases continuously down to 2 K, only one anomaly is observed in the maximum of dC/dT at TC ¼ 28 K (inset of Fig. 6), which agrees with the existence of ferromagnetic ordering in UFe0.6Sb2, derived from the magnetization and electrical resistivity results. The specific heat between 2 and 10 K follows C ¼ gTþbT3, where gT and bT3 are the electronic and the lattice contributions, respectively. The C/T versus T2 is shown in the lower inset of Fig. 6. The solid line represents a fitting of the experimental data to the standard formula C ¼ gTþbT3, with the fitting parameters: g ¼ 233 mJ/mol$K2 and b ¼ 4.3 mJ/mol.K4. The Sommerfeld coefficient g is two orders of magnitude larger than those of simple metals and reveals a relatively high density of electronic states at the Fermi level, indicating that a strong electron correlation exists in UFe0.6Sb2. 4. Conclusions In summary, the crystal structure, magnetic properties, electrical properties, and thermal properties of UFe0.6Sb2 are comprehensively investigated by means of XRD and temperature dependent magnetic susceptibility, electrical resistivity, and heat capacity measurements. Our data reveals that UFe0.6Sb2 orders ferromagnetically at Tc ¼ 28 K, with the easy magnetization along caxis. The resistivities show three distinct regions following activation behavior, indicating the formation of three pseudogaps in the electronic density of states in both directions. The value of

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