Semimetallic Antiferromagnetism in the Half-Heusler Compound Cumnsb

PHYSICAL REVIEW B 71, 184103 ͑2005͒ Semimetallic antiferromagnetism in the half-Heusler compound CuMnSb T. Jeong,1 Ruben Weht,2 and W. E. Pickett1 1Department of Physics, University of California, Davis, California 95616, USA 2Departamento de Física, CNEA, Avda. General Paz y Constituyentes, 1650-San Martín, Argentina ͑Received 17 September 2004; revised manuscript received 5 January 2005; published 20 May 2005͒ The half-Heusler compound CuMnSb, the first antiferromagnet ͑AFM͒ in the Mn-based class of Heuslers and half-Heuslers that contains several conventional and half metallic ferromagnets, shows a peculiar stability of its magnetic order in high magnetic fields. Density functional based studies reveal an unusual nature of its unstable ͑and therefore unseen͒ paramagnetic state, which for one electron less ͑CuMnSn, for example͒ would be a zero gap semiconductor ͑accidentally so͒ between two sets of very narrow, topologically separate bands of Mn 3d character. The extremely flat Mn 3d bands result from the environment: Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below the Fermi level, and the other four tetrahedrally coordinated sites are empty, leaving chemically isolated Mn 3d states. The AFM phase can be pictured heuristically as a self-doped Cu1+Mn2+Sb3− compensated semimetal with heavy mass electrons and light mass holes, with magnetic coupling proceeding through Kondo and/or anti-Kondo coupling separately through the two carrier types. The ratio of the linear specific heat coefficient and the calculated Fermi level density of states indicates a large mass enhancement m* /mϳ5, or larger if a correlated band structure is taken as the reference. DOI: 10.1103/PhysRevB.71.184103 PACS number͑s͒: 71.28.ϩd, 71.20.Be, 71.18.ϩy I. INTRODUCTION states and/or Fermi velocity, and an interband absorption 8 ⍀2 peak was reported at 0.8 eV. For a metal, the square p is ͑ ͒ 2 ͑ The study of unusual magnetic materials has increased proportional to N EF vF often interpreted as an effective ͑ ͒ ͒ dramatically in the past decade, due to new phenomena such carrier density to mass ratio n/m eff , and this square is a as colossal magnetoresistance, half-metallic ferromagnetism, factor or 3–15 lower than its FM counterparts. spin gaps and novel heavy fermion behavior, to name a few. The resistivity of CuMnSb shows a fairly rapid drop be- ͑ ͒ ͑ 9 The Heusler AT2X type and half-Heusler ATX type, T low TN, with the samples of Schreiner and Brandão drop- ␮⍀ ␮⍀ =transition metal, X=metal or metalloid͒ structures have ping from around 170 cm above TN to 50 cm at low produced a number of unusually interesting compounds. For temperature. More recent work4 has lowered these numbers 1 ␮⍀ ␮⍀ example, ferromagnetic ͑FM͒ PtMnSb was discovered to somewhat: 120 cm above TN to 45 cm at low tem- display the largest known magneto-optic Kerr effect at room perature. Such resistivity drops at magnetic ordering transi- temperature ͑at photon energy ប␻=1.75 eV͒. A number of tions are common and reflect spin scattering that gets frozen them have been proposed as half metallic ferromagnets, and out in the ordered phase. In contradiction to this inference, NiMnSb ͑with one more electron than CuMnSb͒ is one of and quite unusual for magnetic compounds, CuMnSb shows the most widely accepted examples of half metallic ferro- little magnetoresistance.4,6 The still large residual resistivity magnetism. ͑Ϸ45 ␮⍀ cm͒10 in the best samples suggests some intersite We address here the half-Heusler compound CuMnSb, disorder or nonstoichiometry ͑the empty site makes half which has its own peculiarities. In a class of intermetallic Heuslers susceptible to such defects͒. Our work indicates this magnets where ferromagnetism ͑FM͒ can occur as high at compound to be a semimetal, however, so this residual resis- 1000 K, CuMnSb develops antiferromagnetism ͑AFM͒ at tivity alternatively may reflect primarily a low carrier density the relatively low temperature, with reports of TN =55 K for a metal. In both AFM CuMnSb and several FM half- from Endo,2 62 K from Helmholdt et al.,3 and 50 K from Heusler compounds, it has reported that the resistivity is not Boeuf.4 A large value of the ordered Mn moment m T2 at low temperature,10 however Boeuf4 obtains essentially ␮ 2 =3.9–4.0 B has been obtained from neutron scattering aT behavior. measurements.3,5 This moment suggests that a “monovalent Possibly related to some of these features is the fact that 5 1 Mn” may provide a good picture, with d", d# high spin con- CuMnSb displays a curious stability of its magnetic order figuration. The Curie–Weiss moment is large, reported vari- under applied magnetic fields. The Néel temperature is in- ␮ 2 ␮ 4 ␮ 6 ously as 5.4 B, 6.3 B and 7.2 B, the latter two of variant for fields up to 14 T. Whereas many AFMs display 5 0 which would be more consistent with a “divalent Mn” d", d# metamagnetic transitions in a field, the induced moment at ␪ ␮ configuration. The Curie–Weiss CW=−160 K suggests large 5 K and H=12 T is only 0.25 B, again reflecting its imper- AFM interactions between the Mn moments,2 consistent with viousness to applied fields. In a more recent work Doerr et the AFM ordering. al.11 have seen no changes of the antiferromagnetic charac- CuMnSb is metallic, but experimentally much less so than teristics up to 50 T. the FM compounds in its class. Its Drude plasma energy was The remaining evidence about the electronic state of 7 ប⍀ ͑ ͒ 4,6 reported as p =1.6 eV compared to 2.6–6 eV for the FM CuMnSb is from heat capacity CV data. The linear spe- Heusler compounds, suggesting a much smaller density of cific heat coefficient ␥=17 mJ/mol K2 corresponds to a qua- 1098-0121/2005/71͑18͒/184103͑7͒/$23.00184103-1 ©2005 The American Physical Society JEONG, WEHT, AND PICKETT PHYSICAL REVIEW B 71, 184103 ͑2005͒ *͑ ͒ siparticle density of states N EF =7.2 states/eV per for- The half-Heusler structure has space group F4¯3m ͑216͒ 3 mula unit. The T coefficient of CV was reported to be two with the tetrahedral point group. The crystal structure5 is fcc, orders of magnitude larger than the anticipated lattice contri- lattice constant a=6.088 Å, with Mn ͑A͒ at ͑0,0,0͒,Cu͑T͒ at bution, which by itself would suggest it is dominated by soft ͑ 1 , 1 , 1 ͒ and Sb ͑X͒ at ͑ 1 , 1 , 1 ͒. The symmetry of all the sites magnetic fluctuations. Half-Heusler antimonides have re- 4 4 4 2 2 2 is¯ 43m. It will become significant that in this structure the cently attracted attention due to their large thermopower and magnetic atom Mn is coordinated tetrahedrally by four Cu other promising thermoelectric properties,12 but the Seebeck atoms at ͑ͱ3/4͒a=2.64 Å, with the other four nearest sites coefficient of CuMnSb has not been reported. being vacant. Mn is second neighbored by six Sb atoms at a Although there has been almost no electronic structure distance of a/2=3.04 Å. The Mn Cu distance is almost study of CuMnSb, many other Heusler and half-Heusler u identical to the sum of their atomic radii ͑1.35 and 1.28 Å, compounds have been studied rather thoroughly. A recent respectively͒, while the Mn Sb distance is 3% greater than analysis of trends in the electronic structure and magnetiza- u the sum of their radii ͑1.35 and 1.59 Å, respectively͒. The tion of Heusler compounds was been presented by AFM structure consists of alternating ͑111͒ planes of Mn Galanakis, Dederichs, and Papanikolaou,13 which references atoms with aligned spins. The Mn Mn nearest neighbor much of the earlier work. A recent review concentrating on u distance of 4.31 Å assures that direct Mn Mn exchange is spintronics applications has been provided by Palmstrom.14 u not a dominant factor in the magnetic ordering. An extensive set of calculations for many half-Heusler com- Two types of band structure methods have been used. The pounds has been presented by Nanda and Dasgupta.15 None full-potential nonorthogonal local-orbital minimum-basis of these address Cu-containing compounds, however. scheme ͑FPLO͒16,17 was used for scalar relativistic calcula- In this paper we lay the foundations for an understanding tions in the local density approximation ͑LDA͒ for the of the electronic properties and magnetism of CuMnSb by exchange-correlation energy18.Cu3s,3p,4s,4p,3d,Mn3s, reporting local spin density results for the electronic and 3p,4s,4p,3d, and Sb 4s,4p,5s,5p,4d states were included magnetic structures, and the equation of state, for unpolar- as valence states. All lower states were treated as core states. ized, ferromagnetic, and antiferromagnetic alignment of the We included the relatively extended semicore 3s,3p states of Mn spins. The results, such as the AFM ordering and the low Cu and Mn, and 4s,4p semicores of Sb as band states be- density of states at the Fermi level N͑E ͒, are in reasonable F cause of the considerable overlap of these states on nearest agreement with experimental data, so it appears that local neighbors. This overlap would be otherwise neglected in the spin density approximation ͑LSDA͒ is accurate for CuMnSb FPLO scheme. The spatial extension of the basis orbitals, as it is in most other intermetallic 3d compounds. The result- controlled by a confining potential ͑r/r ͒4, was optimized to ing band structure in the AFM phase is rather unusual, being 0 minimize the total energy.

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