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Ferromagnetism in the Kondo-lattice compound CePd2P2
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 255602 (http://iopscience.iop.org/0953-8984/26/25/255602) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 255602 (11pp)
doi:10.1088/0953-8984/26/25/255602
Ferromagnetism in the Kondo-lattice compound CePd2P2 Vinh Hung Tran and Zbigniew Bukowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, PO Box 1410, 50-422 Wrocław, Poland E-mail:
[email protected] Received 7 January 2014, revised 24 March 2014 Accepted for publication 27 March 2014 Published 3 June 2014 Abstract
We report physical properties of CePd2P2 crystallizing in the tetragonal ThCr2Si2-type structure (space group I4/mmm). Dc-magnetic susceptibility, magnetization, specific heat, electrical resistivity and magnetoresistance measurements establish a ferromagnetic ordering below the Curie temperature TC = 28.4 ± 0.2 K. Critical analysis of isothermal and isofield magnetization yields critical exponents of β = 0.405 ± 0.005, γ = 1.11 ± 0.05 and δ = 3.74 ± 0.04. The ordered state is characterized by saturation moment Ms ∼ 0.98μB and magnon energy gap Δ/kB ∼ 25–35 K. The studied properties reflect a competing influence of the Kondo and crystalline electric field (CEF) interactions. The strength of the Kondo effect is assigned by a lowtemperature Kondo scale TK ∼ 19 ± 10 K and a high-temperature Kondo scale TKH ~ 117 ± 10 K. A model of the inelastic scattering of the conduction electrons with an exchanged CEF energy ΔCEF was applied to the magnetic resistivity. An average value ΔCEF = 260 ± 30 K is consistent in the relationships with TK and TKH. We argue that the CePd2P2 compound appears to be a new ferromagnetic Kondo-lattice among the Ce-based intermetallics. Keywords: ferromagnetic Kondo lattices, strongly correlated electron systems, Ce-based intermetallics (Some figures may appear in colour only in the online journal)
1. Introduction
the different types of exchange interactions—Kondo effect and crystal field splitting—has given rise to various magnetic ground states [4–9]. The majority of these compounds, namely, Ce(Ni, Cu, Rh, Pd, Ir and Pt)2Sn2, Ce(Cu, Rh, Pd, Ag, Au and Pt)2Ge2 and Ce(Rh, Pd, Ag and Au)2Si2 order antiferromagnetically at low temperatures, except Ce(Fe, Co, Ni and Ir)2Ge2 and Ce(Fe, Ni, Cu, Ru, Os, Os, Ir and Pt)2Si2 which are paramagnetic down to low temperatures measured and surprisingly CeRu2Ge2 undergoes as much as two phase transitions; a ferromagnetic one at TC ∼ 7.4 K and an antiferromagnetic one at TN = 8.5 K. It turns out that, owing to an unstable 4f-electron shell and to a strong hybridization of the localized 4f electrons with the conduction electrons leading to a high density of states near the Fermi energy, some Ce-based ternary compounds exhibit exotic phenomena at low temperatures. Outstanding examples found in 122-type intermetallics are unconventional superconductivity in CeCu2Si2 [10], and heavy-fermion behavior in CePt2Sn2 [11, 12]. The heavy fermion antiferromagnet CePd2Si2 is an intriguing system which exhibits
In the late years of 20th century, Ce-based ternary intermetallic CeT2X2 compounds, where T is a transition metal and X either Si, Ge or Sn, were the focus of intensive investigations. The two polymorphic tetragonal crystal structures, either the ThCr2Si2(space group I4/mmm) or CaBe2Ge2-type structure (space group P4/nmm) are adopted by these intermetallics [1–3]. Both structures are built up from alternating layers of Ce, T and M atoms, but different from each other by the stacking of the layers along the c-axis. The sequence Ce-X-T-X-Ce-X-T-X-Ce occurs in the ThCr2Si2 - type, when the Ce-T-X-T-Ce-X-T-X-Ce stack in the CaBe2Ge2-type structure. Such an arrangement of atoms causes each atom in compounds with the ThCr2Si2-type structure to occupy a specific position (Ce at 2a, T at 4d and X at 4e positions), whereas in the CaBe2Ge2-type structure the Ce atom locates at 2c but the T and M atoms are distributed on two different positions, i.e. T at 2a and 2c and X at 2b and 2c-positions. The well-documented data indicate that the competition among 0953-8984/14/255602+11$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
Vinh Hung Tran and Zbigniew Bukowski
J. Phys.: Condens. Matter 26 (2014) 255602
Figure 1. Powder x-ray diffraction patterns of CePd2P2 measured at room temperature. The solid line through the experimental points is the Rietveld refinement profile calculated for the tetragonal ThCr2Si2-type structure (space group I4/mmm). The short vertical bars mark the Bragg peak positions. The bottom curve represents the difference between the experimental and calculated intensities. Inset: a schematic structure of CePd2P2 with space group I4/mmm.
Figure 2. Inverse susceptibility of CePd2P2 measured at 5 T as a function of temperature. The dashed line is a Curie–Weiss fit. The insets a and b show the low-temperature magnetic data measured respectively at a low field of 0.1 T and at a high field of 5 T. The arrows indicate the inflection point of the M(T) curves. The solid line in the inset b is a fit based on the spin-wave theory.
quantum critical behavior together with pressure-induced superconductivity [13, 14]. Recently, the magnetic properties of the Ce-based arsenide CeNi2As2 have been reported [15]. This fact was motivated by the discovery of the high-Tc superconductivity in AFe2As2 (A = Ca, Ba, Sr or Eu) compounds under pressure and also upon doping [16–19]. Contrary to the extensive studies of silicides, germanides, stanides and arsenides, no physical properties of Ce-based phosphorides CeT2P2 can be found in the literature. The existence of the CePd2P2 compound crystallizing in the tetragonal ThCr2Si2-type structure was reported for the first time in the year 1983 by Jeitschko and Hofmann [20]. Expecting CePd2P2 to be an interesting subject to explore the novel electronic ground state properties, especially in the context of searching for quantum phase transitions in ferromagnetic Ce-based systems, we undertook to synthesize CePd2P2 and its isostructural LaPd2P2 compound and study their dc-magnetization, specific heat and electron transport properties at low temperatures. The non-f-electron LaPd2P2 is used as a non-magnetic reference for CePd2P2. As we will show below, the intermetallic CePd2P2 undergoes ferromagnetic transition with a relatively high Curie temperature of TC = 28.4 K. The physical properties in the ordered state can be described with a magnon energy gap of about 25–50 K and competing Kondo and CEF interactions, characterized by the low-temperature Kondo scale TK ∼ 20 K, high-temperature Kondo scale TKH ~ 120 K and the CEF splitting energy ΔCEF = 290 K, respectively. It will be emphasized that the observed Kondo effect coexisting with ferromagnetism in CePd2P2 classifies the material to a relatively rare family of S =1/2 ferromagnetic Kondo lattices, consisting of CeNiSb [21], CePd2Ga3 [22], CeRu2Ge2 [23], CeAgSb2 [24] and CeRuPO [25].
and Pd2P were synthesized by heating pieces of Ce (La) or Pd with red phosphorus in evacuated and sealed silica ampoules. Then, CeP (LaP) and Pd2P were mixed in a stoichiometric ratio and pressed into pellets in an Ar filled glove box. The pellets were put into an alumina crucible, sealed in evacuated silica tubes and heated at 950° C for 10 d. The powder x-ray diffraction technique using an X′Pert PRO diffractometer with monochromatized CuKα radiation was used to check phase purity and to determine the lattice parameters. The obtained powder x-ray diffraction (XRD) pattern of CePd2P2 at room temperature is shown in figure 1. A calculation of a theoretical XRD pattern model based on the space group I4/mmm was performed using the Powder Cell 2.4 software [26]. Good agreement between measured and calculated profiles confirms the tetragonal ThCr2Si2-type structure of CePd2P2. We obtained the atomic coordinates: Ce in Wyckoff position 2a, Pd in 4d and P in 4e with zP = 0.3769 and the lattice parameters: a = 0.4157(6) and c = 0.9895(7) nm. The latter parameters agree very well with the previously reported values a = 0.4156 nm and c = 0.9887 nm [20]. The tetragonal structure of CePd2P2 is shown in the inset of figure 1. The unit-cell of CePd2P2 is built up by stacking alternating layers ...-Ce-P-Pd-P-... in the c direction. There is a clear two-dimensional nature of this structure, in which the conductive layers of the Ce and Pd atoms are separated by non-conductive phosphorus layers. We may recall that the main parameter determining the degree of localization of f electrons in cerium and light actinide intermetallics is the shortest distance between the f-electron atoms, df−f [27]. Remarkably, as pointed out by Sereni [28], the magnetic ordering in Ce binary compounds appears just at dCe−Ce > 0.32 nm. Moreover, the type of magnetic ordering is firmly dependent on dCe−Ce, i.e., an antiferromagnetic ground state is expected for dCe−Ce < 0.37 nm or dCe−Ce > 0.41 nm, whereas a ferromagnetic order for 0.37 nm < dCe−Ce
