Antiferromagnetism in Semiconducting Srmn2sb2 and Bamn2sb2 Single Crystals N

Ames Laboratory Accepted Manuscripts Ames Laboratory

1-3-2018 Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single crystals N. S. Sangeetha Ames Laboratory

V. Smetana Stockholm University

David C. Johnston Iowa State University and Ames Laboratory, [email protected]

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Recommended Citation Sangeetha, N. S.; Smetana, V.; and Johnston, David C., "Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single crystals" (2018). Ames Laboratory Accepted Manuscripts. 93. http://lib.dr.iastate.edu/ameslab_manuscripts/93

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Abstract Crystals of SrMn 2 Sb 2 and BaMn 2 Sb 2 were grown using Sn flux and characterized by powder and single- crystal x-ray diffraction, respectively, and by single-crystal electrical resistivity ρ , heat capacity C p , and magnetic susceptibility χ measurements versus temperature T , and magnetization versus field M ( H ) isotherm measurements. SrMn 2 Sb 2 adopts the trigonal CaAl 2 Si 2 -type structure, whereas BaMn 2 Sb 2 crystallizes in the tetragonal ThCr 2 Si 2 -type structure. The ρ ( T ) ad ta indicate semiconducting behaviors for both compounds with activation energies of ≳ 0.35 eV for SrMn 2 Sb 2 and 0.16 eV for BaMn 2 Sb 2 . The χ ( T ) and C p ( T ) data reveal antiferromagnetic (AFM) ordering at T N = 110 K for SrMn 2 Sb 2 and 450 K for BaMn 2 Sb 2 . The nia sotropic χ ( T ≤ T N ) data also show that the ordered moments in SrMn 2 Sb 2 are aligned in the hexagonal a b plane, whereas the ordered moments in BaMn 2 Sb 2 are aligned collinearly along the tetragonal c axis. The a b -plane M ( H ) data for SrMn 2 Sb 2 exhibit a continuous metamagnetic transition at low fields 0 < H ≲ 1 T, whereas BaMn 2 Sb 2 exhibits no metamagnetic transitions up to 5.5 T. The χ ( T ) nda C p ( T ) data for both SrMn 2 Sb 2 and BaMn 2 Sb 2 indicate strong dynamic short-range AFM correlations above their respective T N up to at least 900 K within a local-moment picture, corresponding to quasi-two-dimensional magnetic behavior. The present results and a survey of the literature for Mn pnictides with the CaAl 2 Si 2 and ThCr 2 Si 2 crystal structures show that the T N values for the CaAl 2 Si 2 -type compounds are much smaller than those for the ThCr 2 Si 2 -type materials.

Disciplines Condensed Matter Physics

This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_manuscripts/93 PHYSICAL REVIEW B 97, 014402 (2018)

Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single crystals

N. S. Sangeetha,1 V. Smetana,2 A.-V. Mudring,2 and D. C. Johnston1,3 1Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA 2Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16 C, 106 91 Stockholm, Sweden 3Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA

(Received 16 October 2017; published 3 January 2018)

Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn ﬂux and characterized by powder and single- crystal x-ray diffraction, respectively, and by single-crystal electrical resistivity ρ, heat capacity Cp,and magnetic susceptibility χ measurements versus temperature T , and magnetization versus ﬁeld M(H ) isotherm

measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type structure, whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type structure. The ρ(T ) data indicate semiconducting behaviors for both compounds with activation energies of 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2.Theχ(T )andCp(T ) data reveal antiferromagnetic (AFM) ordering at TN = 110 K for SrMn2Sb2 and 450 K for BaMn2Sb2. The anisotropic χ(T TN) data also show that the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane, whereas the ordered moments in BaMn2Sb2 are aligned collinearly along the tetragonal c axis. The ab-plane M(H ) data for SrMn2Sb2 exhibit a continuous metamagnetic transition at low ﬁelds 0

exhibits no metamagnetic transitions up to 5.5 T. The χ(T )andCp (T ) data for both SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range AFM correlations above their respective TN up to at least 900 K within a local-moment picture, corresponding to quasi-two-dimensional magnetic behavior. The present results and a

survey of the literature for Mn pnictides with the CaAl2Si2 and ThCr2Si2 crystal structures show that the TN values for the CaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type materials.

DOI: 10.1103/PhysRevB.97.014402

I. INTRODUCTION pounds for superconductivity because they could form a bridge between the superconducting iron-arsenide and cuprate fam- The discoveries of high-T superconductivity in layered c ilies of high-T materials. The 1111-type LaMnPO and 122- iron pnictides and chalcogenides since 2008 revealed Fe- c type BaMn As have been most extensively studied. These based materials to be a rich source of unconventional super- 2 2 compounds exhibit insulator (at temperature T = 0) to metal conductors. In marked contrast to high-Tc layered cuprates, the undoped ferropnictides are metallic and show itinerant transitions either upon doping or application of pressure, but no antiferromagnetic (AFM) spin-density wave and coincident superconductivity in these compounds has been observed. The or nearly coincident structural transitions. The suppression AFM ordering (Néel) temperature TN, ordered moment μ at of these transitions by chemical doping resulted in supercon- low T , and activation energy have been reported for several ductivity in the “1111”-type parent compound LaFeAsO with semiconducting 1111-type Mn pnictides, including LaMnPO [31], LaMnAsO [32], and LaMnSbO [33], as summarized in Tc = 26 K in 2008 [1]. Since then, many compounds with related layered crystal structures and chemical compositions Table I for these compounds and also for those below. They were discovered which are classiﬁed as “11”-type (e.g., binary tend to have a simple checkerboard (G-type) AFM structure, iron chalcogenide FeSe), “111”-type (e.g., ternary LiFeAs or indicating stronger ab-plane nearest-neighbor (NN) exchange NaFeAs), 1111-type with the primitive-tetragonal ZrCuSiAs- interactions J1 compared to twice the next-nearest-neighbor type structure with space group P 4/nmm (e.g., RFeAsO, interactions J2, i.e., J1 > 2J2 [3]. A high-pressure measure- R = rare earth), “122”-type with the body-centered-tetragonal ment on LaMnPO showed a semiconductor to metal transition − ThCr2Si2 structure with space group I4/mmm (e.g., AFe2As2, at a pressure of 10 GPa [38]. Fluorine-doped LaMnPO1 x Fx A = Ca,Sr,Ba,Eu), and other related structures [2–5]. The also exhibits an insulator-to-metal transition on doping which − highest reported Tc for a bulk Fe-based superconductor is 56 K only weakly affects TN and μ [39], similar to La1 x Srx MnAsO for Gd0.8Th0.2FeAsO [6]. [40]. On the other hand, hydrogen- and deuterium-substituted − In attempts to further enhance the Tc of such supercon- LaMnAsO1 x (H/D)x exhibits insulator-to-metal transitions ductors and search for other exotic ground states, additional associated with the emergence of itinerant ferromagnetism isostructural compounds based on other transition metals in [41]. place of Fe including Cr, Mn, Co, and Ni have been studied. The 122-type BaMn2X2 compounds with the ThCr2Si2- These pnictides include metallic Co-based [7–15], itinerant type structure containing X = P, As, or Bi have also been AFM Cr-based [16–24], semiconducting AFM Mn-based [25– extensively studied. BaMn2P2 [25], BaMn2As2 [26,42], and 33], and superconducting Ni-based [34–37] compounds. BaMn2Bi2 [29,43] undergo G-type AFM ordering with the Semiconducting layered quasi-two-dimensional AFM Mn ordered moments aligned along the c axis, where the NN pnictides received special attention as potential parent com- exchange interactions J1 are again dominant. A high-pressure

TABLE I. Properties of 1111-type and 122-type Mn pnictides, The compounds in Table I also manifest strong dy- including results from the present work (PW). Included are the struc- namic short-range AFM correlations at T TN, similar to ture type, AFM ordering temperatures TN, low-temperature ordered BaMn2As2 [56]. Neutron diffraction studies of both CaMn2Sb2 moment μ, and semiconducting activation energy . The ZrCuSiAs and CaMn2Bi2 revealed a collinear AFM structure with the structure is primitive tetragonal with space group P 4/nmm,the ordered moments aligned in the ab plane [55,57]. How- ThCr Si structure is body-centered tetragonal with space group 2 2 ever, whereas one neutron structural study of CaMn2Sb2 I4/mmm, and the CaAl2Si2 structure is trigonal with space group concluded that collinear AFM moments are aligned in the ¯ P 3m1. Unless otherwise indicated, was determined from electrical ab plane [53], another suggested that the collinear ordered resistivity data with the current in the crystallographic ab plane of moments are canted by 25◦ with respect to the ab plane [52]. single-crystal samples. Single-crystal neutron diffraction measurements on trigonal SrMn2As2 showed that the Mn moments are ordered in the ab Compound Structure type TN (K) μ (μB/Mn) (eV) Ref. plane in a collinear Néel AFM structure below TN = 118(2) K 1111-type [51] and magnetic susceptibility data exhibited quasi-2D be- LaMnPO ZrCuSiAs 375 2.26 [31] havior at T TN [30], as does ThCr2Si2-type BaMn2As2 LaMnAsO ZrCuSiAs 360 3.33 [32] as noted above. SrMn2P2 exhibits a novel pressure-induced LaMnSbO ZrCuSiAs 255 3.45 [33] structural transition from the trigonal CaAl2Si2-type to the 122-type body-centered-tetragonal ThCr2Si2-type structure [58]. a To extend the above work on the Mn pnictides and attempt to BaMn2P2 ThCr2Si2 4.2 0.073 [25] BaMn2As2 ThCr2Si2 625 3.88 0.027 [26,42] discover new properties, herein we report the growth, crystal b BaMn2Sb2 ThCr2Si2 1.12 [28] structure, ρ(T ), magnetization versus applied magnetic field BaMn2Sb2 ThCr2Si2 450 0.16 PW M(H ) isotherms, χ(T ), and heat capacity Cp(T ) measure- BaMn2Bi2 ThCr2Si2 400 3.83 0.003 [29,43] ments of BaMn2Sb2, which crystallizes in the ThCr2Si2-type structure [59], and SrMn2Sb2 which has the CaAl2Si2-type CaMn2As2 CaAl2Si2 62 0.061 [30] structure [60]. Since the Mn sublattice in SrMn Sb is a CaMn Sb CaAl Si 85−88 3.46 [52–54] 2 2 2 2 2 2 corrugated honeycomb or triangular bilayer sublattice whereas CaMn Bi CaAl Si 150 3.85 0.031 [55] 2 2 2 2 the Mn sublattice in BaMn Sb is a stacked square lattice, SrMn P CaAl Si 53 0.013 [25]a 2 2 2 2 2 2 the difference in the geometry of the Mn network between SrMn2As2 CaAl2Si2 120 3.6 0.085 [30,51] BaMn2Sb2 and SrMn2Sb2 may be important to take into SrMn2Sb2 CaAl2Si2 110 0.35 PW account when comparing their properties. aPolycrystalline sample. Previous theoretical electronic structure studies on b Measured at 470–773 K. BaMn2Sb2 suggested a G-type AFM ordering [61,62], but TN and the AFM structure have not been experimentally determined. Transport measurements revealed that BaMn2Sb2 has a study of BaMn2As2 showed an insulator-to-metal transition room-temperature Seebeck coefficient of ∼225 μV/K, consis- ∼ at 5.8 GPa with a downturn in the resistivity below 17 K tent with semiconducting behavior. With regard to SrMn2Sb2,a and no change in the crystal structure [44]. Interestingly, polycrystalline sample showed “complex magnetic ordering of substitution of only 1.6% of K for Ba results in a metallic the Mn moments below ≈ 265 K” and ρ(T ) measurements on ground state in BaMn2As2 [45]. Furthermore, itinerant FM a crystal suggested a “metallic-like temperature dependence” occurs in Ba1−x Kx Mn2As2 with x = 0.19 and 0.26 [46] and [63]. However, in contradiction to both of these results, the half-metallic FM behavior is found below the Curie tempera- present study on SrMn2Sb2 crystals reveals AFM ordering at ≈ ture TC 100 K for 40% K-doped [47] and 60% Rb-doped TN = 110 K with an insulating ground state. BaMn2As2 [48]. This itinerant FM [49] is aligned in the The remainder of the paper is organized as follows. Fol- ab plane and coexists with the G-type local-moment AFM lowing the experimental details in Sec. II, our structural data of the Mn spins aligned along the c axis [50]. Similar to are given in Sec. III A.Theρ(T ) data for BaMn2Sb2 and BaMn2As2, the isostructural AFM semiconductor BaMn2Bi2 SrMn2Sb2 crystals are presented in Sec. III B. Our studies of exhibits metallic behavior with K doping, and the magnetic M(H ) and χ(T ) for these crystals are presented in Sec. III C. character of Ba1−x Kx Mn2Bi2 up to 36% K doping is inferred We suggest in Secs. III B and III C1 the probable origins of to be local-moment AFM [29]. the erroneous experimental ρ(T ) and χ(T ) results reported in Interestingly, unlike body-centered-tetragonal 122-type Ref. [63], respectively. The Cp(T ) measurements on SrMn2Sb2 BaMn2(P, As, Sb, Bi)2, the 122-type (Ca, Sr)Mn2(P, As, and BaMn2Sb2 are presented in Sec. III D, and a summary is Sb, Bi)2 compounds crystallize in the trigonal CaAl2Si2-type given in Sec. IV. structure with space group P 3¯m1. Empirically, the CaAl2Si2- type structure is found if the transition metal has a d0, d5,ord10 II. EXPERIMENTAL DETAILS electronic configuration, whereas the ThCr2Si2-type structure has no such preferences. The Mn sublattice can be viewed Single crystals of BaMn2Sb2 and SrMn2Sb2 were grown in either as a corrugated honeycomb layer or a triangular-lattice Sn flux. High-purity elements Sr (99.99%) and Ba (99.99%) bilayer, which suggests the possibility of geometric frustration. from Sigma Aldrich, and Mn (99.95%), Sb (99.9999%), and Some of these trigonal compounds are known to be AFM Sn (99.999%) from Alfa Aesar, were weighed out in the semiconductors: SrMn2P2 [25], CaMn2As2 [30], SrMn2As2 molar ratio (Ba, Sr):Mn:Sb:Sn = 1.05:2:2:20 and placed in [30,51], CaMn2Sb2 [52–54], and CaMn2Bi2 [55]. an alumina crucible that was sealed under an Ar pressure of

014402-2 ANTIFERROMAGNETISM IN SEMICONDUCTING SrMn … PHYSICAL REVIEW B 97, 014402 (2018)

≈1/4 atm in a silica tube. Excess Sr or Ba was used in the 5000 syntheses to avoid the formation of MnSb and thus to assure the formation of magnetically pure samples. After preheating Iobs ◦ 4000 the mixture at 600 C for 7 h, the assembly was heated to I ◦ ◦ cal 1150 C at a rate of 50 C/h and held at this temperature 3000 Iobs Ical for 15 h for homogenization. Then the furnace was slowly | Bragg positions SrMn Sb ◦ ◦ 2 2 cooled at a rate of 4 C/hto700C. Shiny platelike and 2000 | Bragg positions Sn hexagon-shaped single crystals of BaMn2Sb2 and SrMn2Sb2, respectively, were obtained after decanting the Sn flux using 1000 a centrifuge. Subsequent handling of the crystals was carried out in a dry box, and exposure of them to air during transfer to 0 measurement instruments was kept to a minimum. Intensity (counts/bin) Semiquantitative chemical analyses of the single crystals -1000 were performed using a JEOL scanning electron microscope (SEM) equipped with an EDX (energy-dispersive x-ray anal- -2000 ysis) detector, where a counting time of 120 s was used. 20 40 60 80 100 A room-temperature powder x-ray diffraction (XRD) pattern 2θ (degree) was recorded on crushed single crystals of SrMn2Sb2 using a Rigaku Geigerflex powder diffractometer with Cu Kα radia- ◦ ◦ FIG. 1. Powder x-ray diffraction patterns (open circles) of tion (λ = 1.5418 A)˚ at diffraction angles 2θ from 10 to 110 SrMn Sb at room temperature. Shown are the observed intensity ◦ 2 2 witha0.02 step width. The data were analyzed by Rietveld data (Iobs, open red circles), the Rietveld refinement fit calculated for refinement using FULLPROF software [64]. the CaAl2Si2-type trigonal structure with space group P 3¯m1 together Single-crystal x-ray structural analysis of BaMn2Sb2 was with small amounts of adventitious elemental Sn impurity on the performed at room temperature using a Bruker D8 Venture crystal surfaces from the Sn flux (Ical, solid black curve through the diffractometer equipped with a Photon 100 CMOS detector, a data), the difference Iobs − Ical (bottom blue plot), and the Bragg peak flat graphite monochromator, and a Mo Kα IμS microfocus x- positions for SrMn2Sb2 (vertical green tick marks) and Sn (vertical ray source (λ = 0.710 73 A)˚ operating at a voltage of 50 kV and orange tick marks). a current of 1 mA. The raw frame data were collected using the Bruker APEX3 program [65], while the frames were integrated with the Bruker SAINT software package [66] using a narrow- frame algorithm integration of the data and were corrected the XRD pattern is shown in Fig. 1, confirming that SrMn2Sb2 for absorption effects using the multiscan method (SADABS) has the trigonal CaAl2Si2-type structure. Weak peaks from [67]. The atomic thermal factors were refined anisotropically. adventitious Sn from the flux are also visible and were refined Initial models of the crystal structures were first obtained with together with the main phase. The crystallographic parameters the program SHELXT-2014 [68] and refined using the program of SrMn2Sb2 are listed in Table II. The hexagonal lattice SHELXT-2014 [69] within the APEX3 software package. parameters a and c obtained are in good agreement with Magnetization M versus temperature T measurements at previously reported values [60,63]. fixed field over the T range 1.8 T 350 K and M versus Single-crystal XRD measurements on BaMn2Sb2 con- applied field H isotherm measurements for H 5.5Twere firmed the single-phase nature of the compound and the carried out using a Quantum Design, Inc., Magnetic Properties ThCr2Si2-type crystal structure. The crystallographic parame- Measurement System (MPMS). The high-temperature M(T ) ters are listed in Table II. The lattice parameters a and c are in for 300 T 900 K was measured using the vibrating- good agreement with the previous values [59]. sample magnetometer (VSM) option of a Quantum Design, Figures 2(a) and 2(c) show perspective views of the unit Inc., Physical Properties Measurement System (PPMS). Four- cells of BaMn2Sb2 (tetragonal ThCr2Si2-type) and SrMn2Sb2 probe dc ρ(T ) and Cp(T ) measurements were carried out on (trigonal CaAl2Si2-type), respectively. Both structures look the PPMS, where electrical contacts to a crystal for the ρ(T ) similar in the way they are composed of Mn-Sb tetrahedra measurements were made using annealed 0.05 mm diameter separated by Sr/Ba layers. The primary difference between Pt wires and silver epoxy. them is the geometry of the Mn layers. In BaMn2Sb2,theMn network is square planar where each Mn atom is coordinated by four Mn atoms at a 90◦ angle between them, as shown III. EXPERIMENTAL RESULTS in Fig. 2(b).InSrMn2Sb2, the planar Mn network can be considered to be either a corrugated Mn honeycomb layer A. Crystal structures or a triangular-lattice bilayer, as shown in Fig. 2(d).The SEM images of the crystal surfaces indicated single-phase difference between the structures of the Mn layers in the two crystals. EDX analyses of the chemical compositions were compounds may be expected to have a significant influence on in agreement with the expected 1:2:2 stoichiometry of the their physical properties. compounds, and the amount of Sn incorporated into the crystal The three smallest Mn-Mn intralayer distances d1, d2, and structure from the Sn flux is zero to within experimental error. d3 and the smallest Mn-Mn interlayer distances dz1 and dz2 Powder XRD data on crushed SrMn2Sb2 single crystals con- are shown by arrows in Figs. 2(a) and 2(c) for BaMn2Sb2 and firmed their single-phase nature. The Rietveld refinement of SrMn2Sb2, respectively. These distances are listed in Table II.

014402-3 SANGEETHA, SMETANA, MUDRING, AND JOHNSTON PHYSICAL REVIEW B 97, 014402 (2018)

TABLE II. Reﬁned crystallographic parameters obtained from XRD of SrMn2Sb2 and BaMn2Sb2 crystals. The atomic coordinates in SrMn2Sb2 in hexagonal notation are Sr: 1a (0, 0, 0); Mn: 2d (1/3, 2/3, zMn); and Sb: 2d (1/3, 2/3, zSb). The atomic coordinates in BaMn2Sb2 dz2 are Ba: 1a (0, 0, 0); Mn: 2d (0, 1/2, 1/4); and Sb: 2d (0, 0, zSb). The Ba shortest Mn-Mn interatomic distances in SrMn2Sb2 and BaMn2Sb2 (see Fig. 2) are also listed. Mn

dz1 SrMn2Sb2 BaMn2Sb2 d Structure CaAl2Si2-type ThCr2Si2-type 3 Sb trigonal tetragonal Space group P 3¯m1 I4/mmm b c d d Lattice parameters c 1 2 a a (A)˚ 4.5888(2) 4.397(4) b a c (A)˚ 7.7529(4) 14.33(2) (a) (b) c/a 1.6895(2) 3.259(8) 3 Vcell (A˚ ) 141.38(1) 277.15(6) Atomic coordinates zMn 0.62123(12) d z2 zSb 0.26133(20) 0.3642(1) ˚ Sr Shortest Mn-Mn distances (A) d z1 d d1 3.2471(7) 3.2140(3) 2 d2 4.5965(3) 4.4180(6) Mn d d3 5.6277(5) 6.2480(6) 1 dz1 6.4753(12) 7.2010(2) d 3 dz2 7.778(2) 14.4020(3) Sb b c c a (d) b a (c) B. In-plane electrical resistivity

FIG. 2. Outlines of the unit cells of (a) BaMn2Sb2 (I4/mmm, Figures 3(a) and 3(b) show the ab-plane ρ(T )forBaMn2Sb2 ThCr2Si2-type) and (c) SrMn2Sb2 (P 3¯m1, CaAl2Si2-type). The and SrMn2Sb2, respectively. For both compounds, ρ first increases slowly with decreasing T below 400 K, but then smallest Mn-Mn interatomic distances within the Mn layer (d1, d2, and d ) and between layers (d and d ) are indicated by arrows. The ρ increases rapidly with decreasing T below 300 K. The 3 z1 z2 projections of the manganese networks onto the ab plane with a slight data clearly indicate that both BaMn2Sb2 and SrMn2Sb2 have c-axis tilt are shown in (b) BaMn2Sb2 (square net) and (d) SrMn2Sb2 insulating ground states. We fitted the ρ(T ) data over restricted (corrugated honeycomb net or triangular bilayer). temperature intervals by Our results for SrMn2Sb2 contradict a previous report [63] log ρ(T ) = A + 0.4343 , (1) where it was claimed that their Sn-flux-grown SrMn Sb crys- 10 k T 2 2 B tals exhibit a “metallic-like” in-plane ρ(T ). In particular, the ≈ where A is a constant, kB is Boltzmann’s constant, and is ρ(T )inFig.6ofRef.[63] increases almost linearly from 30 the activation energy. m cm at low T to ≈ 100 m cm at T = 300 K. These values Plots of log10 ρ vs 1/T are shown in the insets of Figs. 3(a) are too large for a metallic layered pnictide [3]. Furthermore, and 3(b) for BaMn2Sb2 and SrMn2Sb2, respectively. For the intrinsic ρ(T ) values in Fig. 3(b) over the same T range BaMn2Sb2, the data between (i) 240 and 400 K and between (ii) are orders of magnitude larger than in Ref. [63]. We therefore 80 and 140 K are nearly linear in T and were separately fitted by suggest that the origin of these discrepancies is the presence Eq. (1), yielding the intrinsic and extrinsic activation energies of metallic Sn inclusions in the crystal measured in Ref. [63], 1 = 0.16 eV and 2 = 0.018 eV,respectively, corroborating which would short-circuit the much higher intrinsic resistance previous results [28]. The fits are shown as the solid straight of the crystal and lead to their observed T dependence of ρ. lines in the inset of Fig. 3(a), and the extrapolations as dashed We note that the lowest temperature of the ρ(T ) measurement lines. The fitted activation energies are listed in the inset and in the inset to Fig. 6 of Ref. [63] was 4.5 K, just above the Table I and are of the same order as found previously for superconducting Tc = 3.7 K of Sn metal, which would likely isostructural BaMn2P2 [25] and BaMn2As2 [26]. have been detected by extending their ρ(T ) measurement to In the case of SrMn2Sb2, there was no extended region of somewhat lower T . the ln ρ versus 1/T plot that showed linear behavior, so the data between 340 and 400 K were fitted by Eq. (1), yielding C. Magnetization and magnetic susceptibility the lower limit to the intrinsic activation energy as = 0.35 eV as shown in the inset and Table I. The fit is shown as the 1. SrMn2Sb2 solid straight line in the inset of Fig. 3(b) and the extrapolations Figure 4(a) shows the zero-field-cooled (ZFC) magnetic by dashed lines. susceptibility χ(T ) ≡ M(T )/H of SrMn2Sb2 in a small mag-

014402-4 ANTIFERROMAGNETISM IN SEMICONDUCTING SrMn … PHYSICAL REVIEW B 97, 014402 (2018)

300 6 2.5 SrMn Sb (a) ] (a) 2 2 250 ) 2.0

( 1.5 Δ = 0.018(2) eV 2

ρ 200 [ 4

g 1.0 3 /mol)

l 3 /mol K) cm) 3 150 0.5 Δ = 0.16(1) eV 2 1 cm cm -5 −3 ρ (Ω 0.0 1 100 0 4 8 12 H = 0.1 T BaMn2Sb2 3 -1 2 χ (10 10 /T (K ) /dT (10

χ ab 0 ab χ 50 100 150 200 50 χ d c T (K) 0 0 100 200 300 400 0 0 50 100 150 200 250 300 350 T (K) T (K) 6 6 (b) SrMn2Sb2 5 (b) 5 SrMn2Sb2

]

)

4 m

c 4 cm)

( 4

[ /mol) Ω 3 3 4 10 H = 3 T g 3

l cm Δ = 0.353 eV −3 ρ (10 2 χ 2 ab 2 3 4 5 2 χ

3 -1 χ (10 c 1 10 /T (K )

0 200 250 300 350 400 0 T (K) 0 200 400 600 800 FIG. 3. Temperature T dependence of the electrical resistivity, T (K) ρ(T ), in the ab plane for (a) BaMn2Sb2 and (b) SrMn2Sb2. The insets show plots of log ρ vs 1/T. The solid straight lines through the FIG. 4. (a) Zero-field-cooled magnetic susceptibility χ of 10 = inset data are fits over restricted temperature intervals by Eq. (1)as SrMn2Sb2 vs temperature T in a magnetic field H 0.1 T applied discussed in the text, and the dashed lines are extrapolations. in the ab plane (χab) and along the c axis (χc). Inset: derivative dχ(T )/dT vs T for H || ab to identify TN.(b)χab and χc vs T for 1.8 T 900 K measured in H = 3T. netic field H = 0.1 T applied in the ab plane (χab) and along the c axis (χc). These data exhibit a clear AFM transition at TN = 110(5) K. The value of TN is confirmed from a plot of magnetocrystalline anisotropy in SrMn2Sb2 is rather small, +2 5 dχab(T )/dT versus T in the inset of Fig. 4(a). The anisotropy as expected for Mn with a d electronic configuration of χ below TN indicates that the c axis is the hard axis and hence and spin S = 5/2. These results also show that contrary to that the ordered moments lie in the ab plane. The substantial a three-dimensional antiferromagnet for which χ decreases value of χab for T → 0 indicates that the AFM structure of above TN, the data for SrMn2Sb2 increase above TN, reach SrMn2Sb2 could be either a collinear antiferromagnet with a broad maximum at ∼300 K, and then slowly decrease. A multiple domains aligned within the ab plane or an intrinsically Curie-Weiss behavior is not attained up to 900 K, i.e., positive noncollinear structure; this behavior is similar to that of the curvature in χ(T ) is not clearly evident, indicating that strong isostructural compound SrMn2As2 [30] that was ultimately dynamic AFM fluctuations occur up to at least 900 K. Within found to have a collinear AFM structure with the ordered a local-moment picture, these features in χ(T )atT>TN are moments aligned in the ab plane, but with three approximately characteristic of a quasi-two-dimensional antiferromagnet. ◦ equally populated domains at an angle of 60 to each other M(H ) isotherms for a single crystal of SrMn2Sb2 with H which led to χab(T → 0) ≈ χ(TN)/2[51], as also observed in ab and H c are shown in Figs. 5(a) and 5(b), respectively. Fig. 4(a). The data for H c are proportional to H at all temperatures, Figure 4(b) shows that the anisotropy in χ(T )almost indicating lack of significant ferromagnetic or saturable para- disappears above TN in a field of 3 T, which indicates that the magnetic impurities. However, for H ab, the magnetization

014402-5 SANGEETHA, SMETANA, MUDRING, AND JOHNSTON PHYSICAL REVIEW B 97, 014402 (2018)

500 60 (a) SrMn2Sb2 1.8 K (a) SrMn2Sb2 400 50 K 50 100 100 K /mol) 3 300 K 40 350 K /mol)

3 300 50 /mol T) 3 M (G cm 30 1.8 K 200 0 H || ab 0 1 2 3 50 K M (G cm H (T) 20 100 K 200 K M/H (G cm 100 H || ab 300 K 10 350 K

0 0 0 1 2 3 4 5 1 2 3 4 5 H (T) H (T) 250 60

(b) SrMn2Sb2 (b) SrMn Sb 50 2 2 200 H || c 1.8 K 40 100 K

150 /mol T) /mol) 3 3 200 K 300 K 30 1.8 K H || c 100 350 K 50 K 20 100 K M (G cm 200 K M/H (G cm 50 300 K 10 350 K

0 0 0 1 2 3 4 5 1 2 3 4 5 H (T) H (T)

FIG. 5. Magnetization M of SrMn2Sb2 as a function of magnetic FIG. 6. M/H vs H of SrMn2Sb2 at various listed temperatures T field H at various temperatures T with (a) H in the ab plane (H || ab) with (a) H in the ab plane (H || ab)and(b)H along the c axis (H || c). and (b) H along c axis (H || c). The inset in (a) shows expanded plots The data in (a) for T = 1.8 and 50 K show a smooth metamagnetic of M(H )forT = 1.8, 50, and 100 K. transition that is spread out between H = 0and1T. shows positive curvature for H 1 T at low temperatures. in χ(T ) measured in 0.1 T which bears no resemblance to Expanded plots of the data with H 3TforT = 1.8, 50, and the powder average of the χ(T ) data in H = 0.1 T in our 100 K are shown in the inset of Fig. 5(a), where the positive Fig. 4(a). Second, the average of their data from T = 5K 3 curvature is somewhat more apparent. One can greatly amplify to 300 K is ≈ 0.05 cm /mole-SrMn2Sb2, whereas the value the low-field M(H ) behaviors of SrMn2Sb2 by plotting M/H of the powder-averaged χ up to 300 K in our Fig. 4(a) −3 3 versus H as shown for H ab and H c in Figs. 6(a) and 6(b), is ≈ 4 × 10 cm /mole-SrMn2Sb2, more than an order of respectively. If Mab ∝ H at low fields, the result would just be magnitude smaller. We infer that their results were strongly a horizontal line. Instead, one sees in Fig. 6(a) a strong increase affected by FM MnSb impurities, which have a strongly in Mab/H with increasing H up to 1 T, where it levels out. This composition-dependent Curie temperature TC that can vary indicates that a continuous ab-plane metamagnetic transition between 100 K and 320 K [70,71] and with a large ordered for H ab with 0

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150 BaMn Sb 2 2 TN = 450(10) K (a) BaMn Sb 3 2 2 H = 3 T

100 H || ab 8 /mol) /mol) 3 3 2 χ T = 450(5) K ab N /mol K) cm χ 3 4 -3 c χ

1.8 K cm ab -5

50 100 K (10 M (G cm 0 200 K χ 1

300 K /dT (10 χ -4 d 400 600 800 T (K) 0 0 1 2 3 4 5 0 0 200 400 600 800 H (T) T (K) 100 FIG. 8. ZFC magnetic susceptibility χ(T )ofBaMnSb as a BaMn Sb 2 2 (b) 2 2 function of temperature T for 1.8–550 K in H = 3 T applied along the c axis and χ(T )for1.8 T 900 K in H = 3Tappliedinthe 80 H || c ab plane. The TN obtained from these data is 450(10) K. Inset: the 1.8 K derivative dχab/dT vs T , which was used to obtain another estimate = 60 50 K TN 450(5) K from the temperature of the peak. /mol)

3 100 K 200 K 300 K = 40 applied along the c axis (H c, χc), and χ(T )forT 1.8to

900 K with H = 3 T applied in the ab plane (H ab, χab), M (G cm are shown in Fig. 8. The anisotropy between the c-axis and 20 ab-plane data below TN = 450(10) K is a textbook example of collinear long-range AFM ordering with the ordered moments oriented along the c axis [72,73]. A more precise estimate of 0 TN is obtained from a plot of the derivative dχab/dT versus T 0 1 2 3 4 5 in the inset of Fig. 8. A clear peak in these data gives the same H (T) Néel temperature but with a smaller error bar, TN = 450(5) K. Figure 8 further shows that χab(T ) does not decrease above FIG. 7. Isothermal magnetization M of BaMn Sb as a function 2 2 TN as would be expected for a three-dimensional antifer- of magnetic field H at the temperatures listed with (a) H in the ab romagnet, but instead increases and appears to approach a || || plane (H ab)and(b)H along the c axis (H c). Note the different maximum at ∼600 K, followed by a slow decrease. This scales on the ordinates in (a) and (b). paramagnetic behavior is similar to that of SrMn2Sb2 discussed above and is characteristic of strong short-range dynamic an M(H ) isotherm that was measured at each temperature for two-dimensional AFM fluctuations above TN [56]asseen each field direction [26]. The small amount of MnAs impurities previously in, e.g., tetragonal BaMn2As2 [56] and trigonal present in those crystals did not contribute significantly to the CaMn2As2 and SrMn2As2 [30]. magnetization when the measurement temperature was above Our overall χ(T ) and M(H ) results for BaMn2Sb2 are similar to those for other isostructural Mn compounds such TC of MnAs. as BaMn2As2 [26], BaMn2P2 [25], (Ca,Ba)Mn2Ge2 [74], and BaMn2Bi2 [29]. Electronic structure studies of a G-type 2. BaMn2Sb2 collinear AFM structure in BaMn2Sb2 were carried out previ- = Plots of the isothermal magnetization M of a BaMn2Sb2 ously, and the ordered Mn moment at T 0 was calculated to ≈ single crystal versus H for H || ab and H || c at various be 3.8 μB/Mn [61]or3.55 μB/Mn [62]. temperatures are shown in Figs. 7(a) and 7(b), respectively. Anisotropic behavior between H ab and H c is evident at low temperatures. One sees from these plots that the contri- D. Heat capacity bution of ferromagnetic or saturable paramagnetic impurities The Cp(T ) data for BaMn2Sb2 and SrMn2Sb2 in the tem- to the magnetization data is insignificant at all measured perature range between 1.8 and 300 K are shown in Figs. 9(a) temperatures. and 9(b), respectively. The temperature of the sharp λ-type The zero-field-cooled (ZFC) magnetic susceptibilities χ ≡ anomaly in Cp(T ) for SrMn2Sb2 at 110 K is in good agreement M/H of BaMn2Sb2 versus T from 1.8 to 550 K with H = 3T with the TN found from the above χ(T ) data for this material.

014402-7 SANGEETHA, SMETANA, MUDRING, AND JOHNSTON PHYSICAL REVIEW B 97, 014402 (2018)

150 and for SrMn2Sb2 we obtain

2 (a) BaMn2Sb2 γ = 0.02(4) mJ/mol K , (4a) β = 0.680(3) mJ/mol K4. (4b) 30 100 The null values of γ are consistent with the insulating

) 25

2 ground states found from the respective ρ(T ) measurements Data 20 in Sec. III B. Assuming no contribution from 3D AFM spin Fit 15 waves, we estimate the Debye temperatures ( D)ofthetwo (J/mol K)

P compounds from the β values using the expression

C 10 /T (mJ/mol K 50 P 4 1/3 C 12π Rn data 5 = , (5) C D 5β latt 0 0 5 10 15 20 25 Θ 2 2 where R is the molar gas constant and n is the number of D = 222(1) K T (K ) atoms per formula unit (n = 5forBaMn Sb and SrMn Sb ). 0 2 2 2 2 0 50 100 150 200 250 300 We thus obtain

T (K) D = 202(1) K (BaMn2Sb2), (6a)

150 = 242.5(3) K (SrMn2Sb2). (6b) (b) TN = 110 K The lattice contributions Clatt(T )toCp(T )forBaMn2Sb2 SrMn2Sb2 and SrMn2Sb2 at higher temperatures were obtained by ﬁtting 20 the respective Cp(T ) data over speciﬁed temperature ranges (where the magnetic contribution Cmag(T ) was expected to be 100 ) 2 15 small) by the molar heat capacity expression Data 10 Fit Clatt = nCV Debye, (7a) (J/mol K) P

/T (mJ/mol K 5 where n is deﬁned above. Here C is the Debye lattice

C V Debye P

50 C data C heat capacity per mole of atoms given by 0 C 0 5 10 15 20 25 3 D/T 4 x latt 2 2 T x e T (K ) C (T ) = 9R dx, (7b) Θ = 222(3) K V Debye x 2 D D 0 (e − 1)

0 where D is the Debye temperature. An accurate analytic Padé 0 50 100 150 200 250 300 approximant was used for the Debye function in Eq. (7b)for T (K) fitting the Cp(T ) data [75]. The fit of Cp(T )forBaMn2Sb2 by Eqs. (7) over the temperature range from 50 to 100 K and its FIG. 9. Heat capacity C vs temperature T and fits of C (T )in extrapolation to higher temperatures is shown by the solid blue p latt = Eqs. (7) to the data over restricted temperature intervals (see text) curve in Fig. 9(a), yielding D 222(1) K. This value of D for (a) BaMn2Sb2 and (b) SrMn2Sb2. The fits and extrapolations are is comparable with the value of 202 K in Eq. (6a) determined shown as blue curves, and the fitted Debye temperatures D are shown from the fit of the Cp for BaMn2Sb2 at low T by Eq. (2). 2 in each panel. The respective insets show Cp(T )/T vs T for T<5K, For SrMn2Sb2,theCp (T ) data for 1.8 T 10 K and where the straight lines through the respective data are fits by Eq. (2), 50 T 70 K were fitted simultaneously by Eqs. (7). The fit where the fitted parameters are given in Eqs. (3)and(4), respectively. and its interpolation and extrapolation are shown by the blue curve in Fig. 9(b) obtained using the fitted Debye temperature D = 222(3) K. Again, the value of D for SrMn2Sb2 is The insets in Figs. 9(a) and 9(b) show a linear Cp/T versus T 2 behavior for 1.8 T 5 K. Hence we fit these data by the comparable with the value of 242.5 K in Eq. (6b) determined from the fit of the C (T ) data at low T by Eq. (2). expression p The fitted constant-volume Clatt(T )inFig.9(b) is used to Cp obtain an estimate of C (T ) to the measured heat capacity of = γ + βT 2, (2) mag T SrMn2Sb2. The molar magnetic heat capacity is obtained from where the coefficient γ in metals is due to the conduction elec- Cmag(T ) = Cp(T ) − Clatt(T ). (8) tron contribution (Sommerfeld coefficient) if nonzero, and β reflects the low-T Debye T 3 lattice contribution together with The Cmag(T ) for SrMn2Sb2 obtained is shown in Fig. 10(a), = a three-dimensional AFM spin-wave contribution if present. where a sharp peak at TN 110 K is seen. The small broad bump below TN is an artifact, and the nonzero Cmag above TN For BaMn2Sb2, the fit yields indicates strong dynamic short-range AFM order above TN. γ = 0.01(1) mJ/mol K2, (3a) It is well known that the heat capacity of a material at constant pressure Cp is larger than at constant volume CV, = 4 β 1.17(2) mJ/mol K , (3b) and hence the Cmag(T )inFig.10(a) obtained using Eqs. (7)

014402-8 ANTIFERROMAGNETISM IN SEMICONDUCTING SrMn … PHYSICAL REVIEW B 97, 014402 (2018)

30 The magnetic entropy Smag(T ) for SrMn2Sb2 is calculated from Cmag(T )inFig.10(a) using TN = 110 K (a) T SrMn2Sb2 Cmag(T ) Smag(T ) = dT, (10) 20 0 T and the result is shown in Fig. 10(b). The entropy of completely disordered spins S per mole of SrMn2Sb2 is S(T →∞) = 2R ln(2S + 1), which gives (J/mol K) 10 mag

C J S (T →∞) = 23.1 (S = 3/2), (11a) mag mol K J = 29.8 (S = 5/2), (11b) 0 mol K 0 50 100 150 200 as shown by the horizontal dashed black lines in Fig. 10(b). The Smag(300 K) ≈ 15 J/mol K for SrMn2Sb2 in Fig. 10(b) is T (K) only ≈50% of the value for S = 5/2inEqs.(11) and is still only ≈65% of the value for S = 3/2. The remaining entropy is evidently not released until much higher temperatures. Thus (b) SrMn2Sb2 30 the strong dynamic short-range AFM order revealed in the 2R ln(6) Cmag(T ) and Smag(T ) data above TN is consistent with the above conclusion from the χ(T ) data that dynamic short-range AFM order survives from TN up to at least 900 K. 20 2R ln(4) IV. SUMMARY (J/mol K) T = 110 K

mag N Reﬁnements of our x-ray diffraction data conﬁrm that

S 10 SrMn2Sb2 crystallizes in the trigonal CaAl2Si2-type structure with space group P 3¯m1, whereas BaMn2Sb2 adopts the body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. Heat capacity Cp(T ) and electrical resistivity ρ(T ) 0 measurements indicate that both compounds are insulators at 0 50 100 150 200 250 300 low temperatures. At higher temperatures, the ρ(T ) data show that SrMn Sb and BaMn Sb are semiconductors with activa- T (K) 2 2 2 2 tion energies estimated to be 0.35 and 0.16 eV, respectively. Long-range antiferromagnetic (AFM) order arising from FIG. 10. (a) Molar magnetic heat capacity Cmag vs temperature +2 5 T for SrMn Sb obtained using Eq. (8). The λ anomaly at T = 110 the nominal Mn spins with d electronic configuration and 2 2 N = K arises from AFM order with the moments aligned in the ab plane. spin S 5/2 is reported for single crystals of SrMn2Sb2 and BaMn Sb at T = 110 and 450 K, respectively, as determined (b) Molar magnetic entropy Smag(T )ofSrMn2Sb2 obtained using the 2 2 N data in (a) and Eq. (10). The dashed lines are Smag(T →∞)forS = from magnetic susceptibility χ(T ) and Cp(T ) measurements. 5/2and3/2inEqs.(11), as indicated. We infer from the anisotropy in the χ(T ) data below TN that the ordered moments in SrMn2Sb2 lie in the ab plane and exhibit either intrinsic planar noncollinear AFM order or extrinsic would then be less than shown in the figure. This difference is noncollinear AFM order arising from multiple collinear AFM given by the conventional expression ab-plane domains. From magnetization versus field isotherms, we discovered that SrMn2Sb2 exhibits a continuous metamag- 2 netic transition at low ab-plane fields between 0 and 1 T. The α nature of this transition remains to be determined. On the other Cp − CV = TVM , (9) κT hand, the classic anisotropy of the χ(T ) data for BaMn2Sb2 at temperatures below TN indicates that this compound exhibits where α is the volume thermal expansion coefficient, κT is collinear AFM order with the moments aligned along the c the isothermal compressibility, and since C is normalized axis. −5 3 here per mole of compound, VM ≈ 4.26 × 10 m /mol is The χ(T ) above the respective TN is isotropic for both the molar volume for SrMn2Sb2. For the similar compound SrMn2Sb2 and BaMn2Sb2, indicating weak magnetocrys- −5 SrCo2As2 at T ∼ 200 K, one has α = 2.4 × 10 /K[13] and talline anisotropy. Both compounds exhibit broad maxima in −1 κT ≈ 0.0078 GPa [76]. Using these values, Eq. (9)gives χ(T ) above TN as shown in Figs. 4(b) and 8, respectively, Cp − CV ≈ 0.6J/mol K at 200 K, which is only about 6% of and do not settle into a Curie-Weiss behavior up to 900 K. Cmag(200 K) in Fig. 10(a). Hence we ignore this difference in Furthermore, the magnetic entropy of SrMn2Sb2 obtained at the subsequent discussion. 300 K from the heat capacity measurements is only half that

014402-9 SANGEETHA, SMETANA, MUDRING, AND JOHNSTON PHYSICAL REVIEW B 97, 014402 (2018)

+2 expected for disordered Mn spins S = 5/2. These results the Mn layers in SrMn2Sb2 and BaMn2Sb2 also appear to be indicate that within a local-moment picture, SrMn2Sb2 and important. From Table I, these differences may contribute to the BaMn2Sb2 exhibit strong dynamic AFM fluctuations up to large differences in Néel temperatures of layered Mn pnictides at least 900 K and hence are quasi-two-dimensional (2D) between the CaAl2Si2 and ThCr2Si2 crystal structures. Theo- local-moment antiferromagnets [56]. retical studies of the origins of these behaviors would be most The differences between the smaller intraplanar and larger interesting. interplanar interatomic distances between Mn atoms in Table II and the resultant differences between their interactions in ACKNOWLEDGMENTS SrMn2Sb2 and BaMn2Sb2 are expected to strongly influence We thank Yogesh Singh for helpful correspondence. The their magnetic properties. In both compounds, the smallest research in Ames was supported by the U.S. Department of intraplanar distances between the Mn atoms are much smaller Energy, Office of Basic Energy Sciences, Division of Materials than the smallest interlayer distances. This apparently leads Sciences and Engineering. Ames Laboratory is operated for the to the 2D dynamic short-range correlations in SrMn2Sb2 and U.S. Department of Energy by Iowa State University under BaMn2Sb2 above TN. The differences in the structures of Contract No. DE-AC02-07CH11358.

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