Colossal Magnetoresistance in a Rare Earth Zintl Compound with a

Nov 9, 2005 - Single crystals of a new Zintl compound, EuIn2P2, were grown from indium metal as ... interaction between R and T.11 In the case of rare...
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Chem. Mater. 2006, 18, 435-441

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Colossal Magnetoresistance in a Rare Earth Zintl Compound with a New Structure Type: EuIn2P2 Jiong Jiang and Susan M. Kauzlarich* Department of Chemistry, UniVersity of California, One Shields AVenue, DaVis, California 95616 ReceiVed September 9, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005

Single crystals of a new Zintl compound, EuIn2P2, were grown from indium metal as a flux solvent. The compound crystallizes in the hexagonal P6(3)/mmc space group with a unit cell of a ) 4.0829(6) Å, c ) 17.595(4) Å, and Z ) 2. It contains alternating Eu2+ layers and [In2P2]2- layers. This compound is paramagnetic at high temperatures with a magnetic transition at 24 K. In the magnetically ordered state, it shows large magnetic anisotropy. The temperature-dependent resistivity of this compound suggests interaction between conduction electrons and local spins. Negative colossal magnetoresistance of up to -398% (MR ) {[F(H) - F(0)]/F(H)} × 100%) at 5 T is observed at 24 K.

Introduction The classical Zintl concept requires a complete charge transfer from an alkali or alkaline earth metal to a posttransition element from groups 13-15 to form a valenceprecise intermetallic compound.1 This concept has been broadly extended to rationalize more complicated intermetallic compounds discovered in the last several decades. These compounds have more varied compositions than the original Zintl concept and now include transition and rare earth metals as well as covalent frameworks and cluster units.2,3 This ongoing extension of the Zintl concept has supported many synthetic explorations of new Zintl phases and has led to the discovery of Zintl compounds with new structure types4,5 and novel physical properties such as thermoelectricity6-8 and superconductivity.9,10 Rare earth metal containing Zintl compounds are of particular interest because of the magnetism-related physical properties which derive from the unique localized magnetic moment of the f orbital electrons. In these compounds, indirect exchange interaction plays an important role in the magnetic coupling. In the case of a R-T-X system (R ) rare earth metals, T ) 3d transition metals, X ) 13-15 group elements), the magnetic interaction between R f and T d (1) Kauzlarich, S. M. Chemistry, Structure, and Bonding of Zintl Phases and Ions; VCH Publishers: New York, 1996; p 306. (2) Artmann, A.; Mewis, A.; Roepke, M.; Michels, G. Z. Anorg. Allg. Chem. 1996, 622 (4), 679-682. (3) Payne, A. C.; Olmstead, M. M.; Kauzlarich, S. M.; Webb, D. J. Chem. Mater. 2001, 13 (4), 1398-1406. (4) Kim, H.; Condron, C. L.; Holm, A. P.; Kauzlarich, S. M. J. Am. Chem. Soc. 2000, 122 (43), 10720-10721. (5) Holm, A. P.; Olmstead, M. M.; Kauzlarich, S. M. Inorg. Chem. 2003, 42 (6), 1973-1981. (6) Mahan, G.; Sales, B.; Sharp, J. Phys. Today 1997, 50, 42-47. (7) Kim, S.-J.; Hu, S.; Uher, C.; Kanatzidis, M. G. Chem. Mater. 1999, 11, 3154-3159. (8) Ferguson, M. J.; Ellenwood, R. E.; Mar, A. Inorg. Chem. 1999, 38, 4503-4509. (9) Mills, A. M.; Lam, R.; Ferguson, M. J.; Deakin, L.; Mar, A. Coord. Chem. ReV. 2002, 233-234, 207-222. (10) Deakin, L.; Lam, R.; Marsiglio, F.; Mar, A. J. Alloys Compd. 2002, 388, 69-72.

electrons is indirect. It involves an intra-atomic, ferromagnetic f-d interaction within the R and an interatomic d-d interaction between R and T.11 In the case of rare earth compound systems without a transition metal such as the R-X system, the magnetic interaction between the R sites is believed to involve conduction-valence hybridization (X p-R d) and intra-atomic f-d interaction on R.12 This mechanism is an interband analogue of the famous Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. Although these mechanisms can qualitatively explain the magnetic properties in many systems, the study of the indirect exchange interaction is still not complete, and more experiments and theoretical analysis are required. Another exciting property recently discovered in some rare earth Zintl phases is colossal magnetoresistance (CMR), a property originally seen in perovskite oxides. Previous studies on R14MnPn11 (R ) Eu, Yb; Pn ) P, As, Sb) has found CMR behavior for almost all compounds in this system.3,13,14 It was proposed that the s-d interactions based on the RKKY type exchange contribute to the CMR effect and both the magnetism and the resistivity originate from the dilute Mn spin. However, the importance of the rare earth atom to the CMR was not clear; thus, further research on rare earth metal Zintl systems without transition metals is necessary. For the purposes of discovering new structures, further understanding the indirect magnetic exchange between rare earth atoms, and explaining the origin of the CMR behavior in rare earth Zintl phases, we have turned our focus to the Eu-In-P system. Among all the R14MnPn11 compounds, Eu14MnP11 has by far the largest magnetoresistance. This is another reason we choose the Eu-In-P system for (11) de Boer, F. R.; Zhao, Z. G. Physica B (Amsterdam) 1995, 211 (1-4), 81-86. (12) Lee, V. C.; Liu, L. Phys. ReV. B: Condens. Matter 1984, 30 (4), 20262035. (13) Fisher, I. R.; Wiener, T. A.; Bud′ko, S. L.; Canfield, P. C.; Chan, J. Y.; Kauzlarich, S. M. Phys. ReV. B: Condens. Matter 1999, 59 (21), 13829-13834. (14) Chan, J. Y.; Kauzlarich, S. M.; Klavins, P.; Shelton, R. N.; Webb, D. J. Chem. Mater. 1997, 9 (12), 3132-3135.

10.1021/cm0520362 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/18/2005

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study. So far, we have studied Eu3InP315 and Eu3In2P416 in this system and have discovered unique magnetic phenomena. Eu3InP3 shows several magnetic orderings characteristic of antiferromagnetic interactions, and Eu3In2P4 shows magnetic features reminiscent of both antiferro- and ferromagnetic ordering, along with magnetoresistance. Both compounds are semiconductors. In this article, we are presenting a newly discovered compound, EuIn2P2. Experiment Synthesis. EuIn2P2 crystals were grown by reaction of pure elements in a cylindrical crucible with the ratio Eu:In:P ) 3:110:6, where In acts as both a reactant and a flux.17 All reactants were handled in a glovebox with a nitrogen atmosphere. Europium metal of 99.999% purity, obtained in ribbon form from the Ames Laboratory, was cut into small pieces, and red phosphorus, obtained from J. Matthey or Puratronic, was ground into small grains with an agate mortar and pestle. The europium and phosphorus were packed into an alumina crucible between layers of granular indium metal of 99.99% purity obtained from Aesar. Another cylindrical crucible stuffed with quartz wool was inverted and capped on the first crucible. These two crucibles were sealed in a quartz jacket under 1/5 atm argon air. The reaction contents were heated to 1100 °C, allowed to stand for 16 h, cooled to 600 °C at 2 °C/h, allowed to stand for 2 h, and centrifuged at 600 °C. The crystals were then removed by breaking the quartz jacket in air. Single-Crystal Diffraction. The crystal structure of EuIn2P2 was determined by single-crystal X-ray diffraction. A Bruker APEX diffractometer was used in this experiment. The crystal was cut to a suitable size (0.10 × 0.07 × 0.06 mm3) with a stainless steel blade, mounted on a glass fiber tip with Exxon Paratone-N, and positioned on a triaxis goniometer head. The temperature of the sample was controlled at 90 ( 2 K by a cold N2 stream from a CRYO Industries low-temperature apparatus. The data were collected using Mo KR radiation. Absorption correction was applied using the computer program SADABS, v2.10. The crystal structure was solved and refined using the SHELXTL 6.10 program package.18 Direct methods were used to solve the structure. The final result of the refinement gives R1 ) 0.0176 and wR2 ) 0.0345, with the largest difference peak in the Fourier map being 0.900 e Å-3. The detailed parameters for data collection and refinement are listed in Table 1. Powder X-ray Diffraction. EuIn2P2 crystal was ground into a fine powder with an agate mortar and pestle and placed between pieces of cellophane tape with about 10% of National Bureau of Standards silicon (standard 640b). The sample was mounted on a Guinier camera monochromated for Cu KR1 radiation. The film data were compared with the theoretical diffraction pattern generated from the single-crystal diffraction data. The diffraction pattern matched the generated pattern, and no impurity was observed. Magnetism Measurements. A Quantum Design MPMS with a superconducting quantum interference device (SQUID) detector was used to measure the magnetic properties of EuIn2P2. A 0.29 mg crystal was fixed into a drinking straw with Kapton tape. To obtain the anisotropic magnetic properties, we measured the crystal with (15) Jiang, J.; Payne, A. C.; Olmstead, M. M.; Lee, H.-O.; Klavins, P.; Fisk, Z.; Kauzlarich, S. M.; Hermann, R. P.; Grandjean, F.; Long, G. J. Inorg. Chem. 2005, 44 (7), 2189-2197. (16) Jiang, J.; Olmstead, M. M.; Lee, H.-O.; Klavins, P.; Fisk, Z.; Kauzlarich, S. M. Inorg. Chem. 2005, 44 (15), 5322-5327. (17) Canfield, P. C.; Fisk, Z. Philos. Mag. B 1992, 65 (6), 1117-1123. (18) Sheldrick, G. M. SHELXTL, 6.10 Edition; Bruker Analytical X-ray Instruments, Inc.: Madison, WI, 1997.

Jiang and Kauzlarich Table 1. Crystal Data and Structure Refinement for EuIn2P2 formula weight temperature wavelength crystal system space group unit cell dimensions volume Z density (calculated) absorption coefficient F(000) crystal size theta range for data collection index ranges reflections collected independent reflections completeness to theta ) 30.00° refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I >2σ(I)]a,b R indices (all data)a,b extinction coefficient largest diff. peak and hole a

443.54 90(2) K 0.710 73 Å hexagonal P6(3)/mmc a ) 4.0829(6) Å; R ) 90° b ) 4.0829(6) Å; β ) 90° c ) 17.595(4) Å; γ ) 120° 254.02(7) Å3 2 5.799 g/cm3 21.636 mm-1 382 0.10 × 0.07 × 0.06 mm3 2.31-33.14°. -6 e h e 6, -6 e k e 6, -27 e l e 26 4188 231 [R(int) ) 0.0265] 100.0% full-matrix least-squares on F2 231/0/10 1.589 R1 ) 0.0176, wR2 ) 0.0345 R1 ) 0.0176, wR2 ) 0.0345 0.0182(10) 0.900 and -1.075 e‚Å-3

R1 ) ∑||Fo| - |Fc||∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2.

the magnetic field along the a axis, c axis, and a-b direction (the direction 30° away from the a axis). Zero-field-cooling (ZFC) and field-cooling (FC) measurements were performed between 5 and 240 K with applied fields of 0.01 and 0.1 T. Magnetization curves were measured between -4 and 4 T at 5 K. The data were collected utilizing the MultiVue software provided by Quantum Design.19 Several crystals from separate reactions were measured, and reproducible results were obtained. Resistivity Measurements. A standard four-probe method was used to perform resistivity measurements. Four platinum leads were attached onto a plate-shaped crystal with dimensions of 1.00 × 0.64 × 0.10 mm3. A Keithley model 224 current source was used to apply a constant current of 1 mA to the sample along the a axis through the two outer leads. The voltage between the two inner leads was measured using a Keithley model 181 voltmeter. The resistivity with current along the c axis was not measured because the crystals of EuIn2P2 were too thin in that direction. Magnetoresistance was measured under fields of 0.01, 0.1, 1, 3, and 5 T. The magnetic field was applied along the a axis.

Results and Discussion Synthesis. EuIn2P2 is another new phase in the Eu-In-P system after Eu3InP315 and Eu3In2P4.16 The crystals of EuIn2P2 were also grown in indium flux as were the previous two compounds. The optimized ratio of reactants is 3:110:6 (Eu:In:P). This phase was first discovered as an impurity in a reaction to produce Eu3In2P4. Peak heating profile temperatures of 900, 1000, and 1100 °C were explored in an effort to optimize the reaction conditions. The final products obtained from the reaction heated to 900 °C, centrifuged at either 850 or 600 °C, were mostly chunks of material along with a few small crystals of Eu3In2P4 and EuIn2P2. These results are consistent with incomplete melting or diffusion of the (19) MPMS MultiVue Application, Revision 1.54 Build 061, Copyright 1998-2003.

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Figure 1. Comparison of the structures of (a) ThCr2Si2, (b) CaAl2Si2, (c) EuIn2P2, and (d) CaIn2. One unit cell of each compound is shown in the blue box.

elements at 900 °C. The final products obtained from heating to 1000 and 1100 °C and centrifuging at 850 °C were a few tiny black flake crystals on the crucible wall. These results implied that the crystallization process had just begun when the sample was taken out of furnace for centrifugation. Centrifugation at 600 °C provided crystals of suitable size for our property measurements. A slower cooling rate was found to be better, and we typically used 2 °C/h. The crystals of EuIn2P2 are black and air-stable. They mostly grow as hexagonal-shaped plates and occasionally grow as rhombus-shaped plates. Structure. ThCr2Si220 structure type and CaAl2Si221 structure type are two common structure types of Zintl compounds with the AB2X2 composition, where A is a rare earth or alkaline earth element, B is a transition metal or main group element, and X is a group 14 or 15 element. Perspective views of these two structure types are shown in Figure 1a,b. The ThCr2Si2 structure type is tetragonal with I4/mmm space group. It contains A layers separated by B2X2 layers. Within the B2X2 layers, the B atoms form a two-dimensional tetragonal mesh with X atoms alternating above or below the center of each tetragon, surrounding the B atoms in a tetrahedral shape. Each of these BX4 tetrahedra share four edges with others.22 EuCo2P2 and EuNi2P2 are two examples of europium phosphides with ThCr2Si2 structure type.23 The CaAl2Si2 structure type is trigonal and belongs to the P3hm1 space group. It also has the alternate A layers separated by B2X2 layers. In the B2X2 layers, the B atoms are again tetrahedrally surrounded by X atoms, as in the ThCr2Si2 type. However, the packing style of the tetrahedra is different: each of them only share three edges with others.22 EuMn2P2 and EuCd2P2 are examples of this type.2, 24 EuIn2P2 adopts a structure type different from the two above. It crystallizes in the hexagonal P6(3)/mmc space group. The refinement parameters and results are listed in Table 1. (20) Ban, Z.; Sikirica, M. Acta Crystallogr. 1965, 18 (4), 594-599. (21) Gladyshevskii, E. I.; Kripyakevich, P. I.; Bodak, O. I. Ukr. Fiz. Zh. (Ukrainian Edition) 1967, 12 (3), 447-52. (22) Zheng, C.; Hoffmann, R. J. Solid State Chem. 1988, 72 (1), 58-71. (23) Marchand, R.; Jeitschko, W. J. Solid State Chem. 1978, 24 (3-4), 351-357. (24) Payne, A. C.; Sprauve, A. E.; Olmstead, M. M.; Kauzlarich, S. M.; Chan, J. Y.; Reisner, B. A.; Lynn, J. W. J. Solid State Chem. 2002, 163 (2), 498-505.

Among the AB2X2 structure types, the CaAl2Si2 structure can be easily rationalized by the Zintl concept;25 A atoms donate electrons to B2X2 so that each atom has four electrons to form the network. In the simplest formulism, the oxidation states can be assigned to help account for the complex; for example, the ions in EuMn2P2 can be considered as Eu2+, Mn2+, and P3-.22 ThCr2Si2 type structure is electron deficient and is relatively hard to explain. It requires the multicenter bonding concept.26 However, the A element in this structure is still considered as simply an electron donor. In the case of EuIn2P2, a simple oxidation state assignment such as Eu2+(In3+)2(P3-)2 does not work. Typically, for the CaAl2Si2 structure type, there are four electrons per atom (16 electrons total). In the case of EuIn2P2, there are a total of 18 electrons for the (In2P2)2- slab. As a result, EuIn2P2 crystallizes in a structure different from the two common AB2X2 structure types. Figure 1c gives a perspective view of the structure of EuIn2P2, in comparison to the CaAl2Si2 and ThCr2Si2 structure types. EuIn2P2 contains Eu layers and In2P2 layers alternating along the c axis. This structure is closely related to the structure of CaIn2 (Figure 1d).27 EuIn2P2 can be derived from the CaIn2 structure by removing every alternate layer of Ca and breaking the In-In bonds in the remaining Ca layers. In this manner, both In and P have a filled octet. Each In is four-coordinated, and P is in a trigonal pyramidal arrangement. Local environments of Eu and In are shown in Figure 2. The In-P-In angle is 102.58(4)°, which is more similar to the expected angle of a tetrahedron (109.47°) than an octahedron (90°). The Eu-P distance is 3.0175(10) Å in this compound. Eu-P covalent interaction is considered significant in some Eu-P binaries with this distance.28 The Eu ions are between In2P2 layers and form a close packing type of feature with P atoms. The P-Eu-P angles are 85.15(4)° when the two Ps are from same layer and 94.85(4)° when the two Ps are from adjacent layers; these angles are close to ideal close packing. The slight difference of the angles may be due to the rigidity of the P positions because (25) Burdett, J. K.; Miller, G. J. Chem. Mater. 1990, 2 (1), 12-26. (26) Zheng, C.; Hoffmann, R. Z. Naturforsch., Teil B: Anorg. Chem., Org. Chem. 1986, 41B (3), 292-320. (27) Nuspl, G.; Polborn, K.; Evers, J.; Landrum, G. A.; Hoffmann, R. Inorg. Chem. 1996, 35 (24), 6922-6932. (28) Hulliger, F.; Rare Earth Pnictides. In Handbook on the Physics and Chemistry of Rare Earths; Gshneidner, K. A.; Eyring, L., Eds.; Elsevier North-Holland, Inc.: New York, 1979; Vol. 4, pp 153-236.

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Figure 3. Magnetic susceptibility as a function of temperature along different crystal directions with an applied field of 0.01 T. The inset is the data collected along the c axis for T ) 2-50 K.

Figure 2. Local environments of In (a) and Eu (b) atoms. Lengths are listed in angstroms.

they bond with indium. The Eu-Eu distance in this compound is 4.0829(6) Å, much longer than the shortest EuEu distances in Eu3InP3 (3.5954(7) Å) and Eu3In2P4 (3.7401(6) Å). As discussed in the next section, the oxidation state of Eu was determined to be +2 by magnetic susceptibility measurements. Consequently, each In2P2 unit has a charge of -2. The [In2P2]2- layer is shown in Figure 2a. We can assume that the In-In bonding is best described as a two-centertwo-electron bond. Within each [In2P2]2- layer, the distance between two indiums with the same x and y coordinates is 2.7608(10) Å. This is slightly shorter than the In-In distance in SrInGe (2.8857(9) Å)29 and is similar to the distances found in many coordination compounds. For example, R2In2(µ-O2CC6H5-O,O′)2 has an In-In bond of 2.6538(8) Å,30 and [(C6H5)4P]2[In2Cl6] has a bond of 2.727(1) Å.31 In these compounds the In-In distances are slightly shorter than in EuIn2P2; this may be due to the lower electron negativity of P compared to those of O and Cl, and the extra bridges in R2In2(µ-O2CC6H5-O,O′)2 make the distance even shorter. The In-P distance in EuIn2P2 is 2.6161(8) Å, which is in good agreement with other Eu-In-P Zintl phases.15,16 Magnetism. Magnetic susceptibilities (χ) as a function of temperature (T) were measured under 0.01 and 0.1 T applied fields and are shown in Figures 3 and 4, respectively. From the data, we can see an obvious magnetic transition at about 24 K. Above this temperature, the χ versus T curves are identical in all three directions and are typical of paramag(29) Mao, J.-G.; Goodey, J.; Guloy, A. M. Inorg. Chem. 2002, 41 (4), 931937. (30) Uhl, W.; El-Hamdan, A. Eur. J. Inorg. Chem. 2004, 2004 (5), 969(31) 972. Bubenheim, W.; Frenzen, G.; Mueller, U. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1995, 51 (6), 1120-1124.

Figure 4. Magnetic susceptibility as a function of temperature along different crystal directions with an applied field of 0.1 T. The inset is the data collected along the c axis for T ) 2-50 K.

netism. Generally, the diamagnetic component in this type of Zintl compound is negligible; however, because Kapton tape was used to fix the crystal in place during measurements, it is more suitable to fit the paramagnetic data with a modified Curie-Weiss law, χ)

C - χ0 T-θ

so that we can subtract the diamagnetic contribution from the tape. Also, in an effort to minimize possible effects from short-range magnetic ordering, we only used data above 50 K. The result gives an average Curie constant (C) of 8.13(3) emu K/mol. This C value corresponds to an effective moment µeff ) 8.06(3) µB per formula, which agrees with the theoretical µeff value (7.94 µB) calculated for a free Eu2+ ion. The Weiss constant (θ) is 27.40(8) K. This value is close to the observed transition temperature of 24 K and suggests ferromagnetic ordering of the europium cations. The low-temperature magnetic data for EuIn2P2 are anisotropic with respect to the crystal axis. When a 0.01 T field was applied, as shown in Figure 3, EnIn2P2 goes through a

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Figure 6. Resistivity of EuIn2P2 as a function of temperature. Figure 5. Magnetization curve measured along different crystal directions at 5 K.

ferromagnetic transition in both the a axis and the a-b direction at 24 K. The a axis has the largest susceptibility and is the easy magnetization direction. A small hysteresis can be observed from the slight difference between ZFC and FC. When the measurement direction is along the c axis, the transition is no longer ferromagnetic. Instead, the χ versus T curve shows a sharp maximum at around 24 K, which is suggestive of a typical antiferromagnetic transition. The anisotropy is observable even several kelvin above the 24 K transition temperature, indicating the existence of shortrange magnetic ordering. In the enlarged inset of Figure 3, a divergence of ZFC and FC data can be seen below 24 K along the c axis. This may be attributed to the imperfect alignment of the crystal during measurement so that the difference between ZFC and FC along the c axis derives from some addition of the ferromagnetic component perpendicular to the c axis. The 0.1 T data are similar in shape to data obtained at 0.01 T; however, there are differences in the magnitude of the susceptibility. The magnitude of the susceptibility obtained at the higher field (0.1 T) is suppressed. In addition, the susceptibilities obtained at 0.1 T along the a axis and the a-b direction overlap with each other even below 24 K. The difference between ZFC and FC also disappears in both directions. These are indications of magnetization saturation. The susceptibility value along the c axis does not change, which is reasonable for an unsaturated antiferromagnet. The divergence of ZFC and FC along the c axis also disappears when the a axis and the a-b direction saturate. Because no difference in ZFC-FC data was expected for the c axis, the fact that no divergence is observed when the perpendicular directions are saturated further supports the suggestion that the ZFC-FC divergence along the c axis observed at 0.01 T was due to an imperfect crystal alignment. The magnetization versus field measurements at 5 K are shown in Figure 5. A very small hysteresis can be seen at the low field along the a axis and the a-b direction. The magnetization starts to saturate at around 0.04 T along the a-b direction and at around 0.06 T along the a axis. The

saturation magnetization is about 7.04 µB per formula unit, which agrees well with the electron configuration of Eu2+. The magnetization along the c axis does not start to saturate until about 1.8 T. Below the saturation field, the magnetization increases linearly with the applied field, indicating antiferromagnetism along the c axis. The saturation magnetization along the c axis is slightly smaller than in the other two directions, perhaps caused by the demagnetization effect of the crystal shapesthe c axis is the short direction of the plate crystal. A simple and reasonable way to explain the magnetic properties above is a canted magnetic system.32 Below the magnetic transition temperature, magnetic moments align in the same direction within the ab plane with a certain amount of tilting off this plane. The tilting directions are opposite between every next Eu2+ layer. Because the Eu-Eu distance is relatively large, direct magnetic exchange should be weak. Indirect Eu-Eu interaction through phosphorus is possible between two europiums in the same layer but is not likely between europiums in the different layers. And as we will discuss later, this compound was found to have electrical resistivity in semimetal range, so RKKY interaction and Bloembergen-Rowland interaction12 are two good models to describe the magnetic interaction of this compound. However, more detailed magnetic and electronic measurements, along with theoretical calculations, are necessary to draw a conclusion. Resistivity. EuIn2P2 is a black crystal whose structure can be rationalized within the extended Zintl concept, suggesting that it should be a semiconductor. However, temperaturedependent resistivity measurements (Figure 6) show that the resistivity of EuIn2P2 is in the scale of 10-5 Ω m, which is in a semimetal range. The resistivity increases with temperature in the high-temperature range as expected for a metal, but the increase is not linear with temperature, instead, the slope decreases with decreasing temperature and eventually becomes negative at about 100 K. This suggests a sp(d)-f admixture interaction in the magnetic disordered states of this compound.33 (32) Nguyen, H. C.; Goodenough, J. B. Phys. ReV. B: Condens. Matter 1995, 52 (1), 324-34.

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Below 60 K the resistivity rises quickly as the temperature drops. It reaches a maximum at the magnetic transition temperature and forms a sharp peak. Below 24 K, the magnetic transition temperature, the resistivity drops dramatically. This phenomenon is similar to the insulator-metal transition observed in EuO34 and the semiconductorsemimetal transition in EuB6.35 Interestingly, when comparing the data between 29 and 60 K with the simple semiconductor model, ln F ) Eg/2kBT + F, where Eg is the band gap energy and F is a constant, we can get a pretty good fit resulting in a band gap of 0.0032 eV. This value is two to three magnitudes smaller than those of normal semiconductors. The metallic properties observed at high temperature may arise from this small band gap. At high temperatures, the electrons in the valence band can be excited across this small band gap easily and the band gap can basically be neglected. When temperatures decrease to around 60 K, the thermal excitation is small enough and the band gap becomes apparent. The coincidence of the magnetic and resistivity transition is common in perovskite CMR materials and some rare earth Zintl compounds.14,35-38 One common explanation of this phenomenon in a ferromagnet consists of a change of band shape due to magnetic ordering and thus a change of carrier density.34,35,38 This mechanism is also suitable for EuIn2P2. Because of the interaction between the spin of the electrons in the conduction band and the spin on Eu2+ cations, the conduction band splits when the canted ferromagnetic ordering forms at 24 K. The lowered band overlaps with the conduction band or a trap level39 (due to defects) and induces the metallic resistivity below 24 K. Magnetic ordering may also change the Hall mobility, which could be another reason for the resistivity transition. Hall effect measurements would be useful to probe this further. Figure 7 shows the F versus T measurements under different applied magnetic fields. We can see that the external magnetic field has a strong effect on the resistivity between 5 and 90 K. If we define MR ) [(F(H) - F (0))/F(H)] × 100%, we can plot MR as a function of temperature (Figure 8). The negative MR can be observed beginning from 90 K and increases to its maximum of -398% at around 25 K with 5 T field. After that, the MR drops again with decreasing temperature. The MR magnitude of this compound is much larger than most CMR Zintl compounds with 14-1-11 structure type, like Eu13CaMnSb11 (80% at 6 T)40 and Eu14MnAs11 (203% at 5 T),3 with the exception of Eu14MnP11 (33) Gratz, E.; Zuckermann, M. J. Transport Properties (Electrical Resistivity, Thermoelectric Power and Thermal Conductivity) of Rare Earth Intermetallic Compounds. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring, L., Eds.; NorthHolland: Amsterdam, 1982; Vol. 5, pp 117-216. (34) Shapira, Y.; Foner, S.; Reed, T. B. Phys. ReV. B: Solid State 1973, 8 (5), 2299-2315. (35) Guy, C. N.; Von Molnar, S.; Etourneau, J.; Fisk, Z. Solid State Commun. 1980, 33 (10), 1055-1058. (36) Ramirez, A. P. J. Phys.: Condens. Matter 1997, 9 (39), 8171-8199. (37) Chan, J. Y.; Kauzlarich, S. M.; Klavins, P.; Shelton, R. N.; Webb, D. J. Phys. ReV. B: Condens. Matter 1998, 57 (14), R8103-R8106. (38) Lehmann, H. W. Phys. ReV. 1967, 163 (2), 488-496. (39) Oliver, M. R.; Dimmock, J. O.; McWhorter, A. L.; Reed, T. B. Phys. ReV. B: Solid State 1972, 5 (3), 1078-1098. (40) Kim, H.; Olmstead, M. M.; Klavins, P.; Webb, D. J.; Kauzlarich, S. M. Chem. Mater. 2002, 14 (8), 3382-3390.

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Figure 7. Resistivity as a function of temperature (0-100 K) measured under different applied magnetic fields.

Figure 8. MR ({[F(H) - F(0)]/F(H)} × 100%) as a function of temperature with different applied magnetic fields.

(116 300% at 5 T).3 These CMR compounds of the Ca14AlSb11 structure type all contain Mn and have a higher magnetic and resistivity transition temperature compared to those of EuIn2P2. Previous solid solution studies confirmed that the primary source of conduction electrons is the Mn 4s band, and it was proposed that the CMR property in 14-1-11 compounds comes from the s-d interactions based on the RKKY type exchange.41 Another major class of CMR compounds is doped perovskites. In these compounds, the spin alignment of Mn 4d electrons enhanced by magnetic field favors the electron hopping between heterovalent Mn and is considered the source of CMR (double exchange).42 EuIn2P2 is different than either of the above two types of compounds by not containing a transition metal. The CMR in this compound should be related to the interaction between the f moments and the conduction electrons. External magnetic field boosts the ferromagnetic alignment of the f moments and thus decreases the scattering of the conduction electrons by disordered local moments. Another reasonable (41) Kim, H.; Chan, J. Y.; Olmstead, M. M.; Klavins, P.; Webb, D. J.; Kauzlarich, S. M. Chem. Mater. 2002, 14 (1), 206-216. (42) Zener, C. Phys. ReV. 1951, 82, 403-405.

Colossal Magnetoresistance in EuIn2P2

interpretation is from the band structure point of view. Increasing external magnetic field can enhance magnetic order. As a result, the conduction band goes through a strong splitting and crosses the valence band. This effect can cause an increase of density of state at the Fermi level and consequently a decrease in the resistivity. An external field can even split the conduction band above the magnetic transition temperature, Tc, and move the metallic transition to a higher temperature, as observed in Figure 7. Summary. In this article, we reported a new Zintl compound EuIn2P2. The compound adopts a special structure because of the valence electron count different from those of other 1-2-2 compounds. EuIn2P2 has a magnetic transition at around 24 K. The magnetic ordering is best described as antiferromagnetic along the unit cell c axis and ferromagnetic perpendicular to the c axis. A canted ferromagnetic structure is proposed to explain the anisotropic magnetic properties. Resistivity of EuIn2P2 increases with

Chem. Mater., Vol. 18, No. 2, 2006 441

temperature above 100 K, with a curvature being due to electrons scattering from disordered moments. The data between 29 and 60 K reveal a small gap consistent with semiconducting property. There is a semiconductor-metal transition along with the magnetic transition at 24 K. CMR was observed in the temperature range from 5 to 90 K with a maximum MR ratio of 398%. Acknowledgment. The authors thank Prof. Marilyn Olmstead and Han-Oh Lee for useful discussion. We acknowledge funding by the NSF (DMR-0120990). J.J. also acknowledges a Tyco Electronics Fellowship for Research in Functional Materials for financial support. Supporting Information Available: Crystallographic data is provided in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CM0520362