JournalofMagnetismandMagneticMaterials272–276(2004)312–314 Effectsofalloyingonelectronicstructureandmagnetismof YðCo 1x M x Þ 2 (M=Al,Si,Fe,Cu)compounds PeterMohn 1 ,IljaTurek,SergiiKhmelevskyi* Center of Computational Materials Science, Vienna University of Technology, Getreidemarkt 9/134, Vienna A-1060, Austria Abstract TheeffectsofvarioussubstitutionsontheelectronicstructureofCoinYðCo 1x M x Þ 2 compounds(M=Al,Si,Fe, Cu)arestudiedemployingthecoherentpotentialapproximationasembodiedinanall-electronTB-LMTOmethod. TwoprincipalmechanismsaffectthemagneticpropertiesofCoastheimpurityconcentrationincreases:(i)achangeof the3d-electronconcentration;(ii)effectsofbandsmootheningduetodisorder.Thelattermechanismisdominantfor AlandSisubstitutions.Substituting3d-elementtheformerapplies,butalsodisordereffectsarepresent. r 2003ElsevierB.V.Allrightsreserved. PACS: 71.20.Lp;75.10.Lp;75.50.Cc Keywords: Lavesphase;Disorder;Metamagnetism The Laves phase compounds RCo 2 (R=Y or rare- earth) and RðCo 1x M x Þ 2 have been intensely studied during the recent decades [1,2]. The main reason was that the Co 3d-electron subsystem in the parent compound YCo 2 isveryclosetoamagneticinstability andthusreactssensitivelytochangesoftheexternaland internal parameters such like pressure, magnetic field, chemical concentration x and to the molecular field exerted by magnetic rare-earth ions. This all together makesthesematerialsveryattractivefortestingvarious approachesinthetheoryofitinerantelectronmagnetism [1,2]. YCo 2 is a strongly exchange-enhanced Pauli para- magnet exhibiting an itinerant electron metamagnetic (IEM) transition at a critical field H c B70T [3]. SubstitutionofCobyvariouselementsleadstoacouple of interesting phenomena as documented in the litera- ture [1].Perhapsthemostintriguingoneistheonsetof themagneticorderinYðCo 1x Al x Þ 2 at x > 0:12 [4] anda huge reduction of H c with increasing concentration of non-magneticAl [5] orSi [6].Theferromagneticstateis alsostabilizedinYðCo 1x Fe x Þ 2 for xB0:12 [7],whereas smallconcentrationsofNileadtoadecreaseof H c [8]. The interpretation of these experiments quite naturally wasbasedonthewell-knownpictureoftheYCo 2 DOS [9,10] whosecharacteristicfeatureisthepositionofthe Fermilevel E F onasharplyfallingportionoftheDOS just above a narrow peak (Fig. 1). Small changes in thestructureoftheDOSand/orinthepositionofthe Fermi level due to chemical substitutions can thus drive the system towards or off of the magnetic instability. From the observation that the calculated DOS of YCo 2 ; YFe 2 and YNi 2 [9] differs mainly by the position of E F ; a simple rigid band picture for the explanationofthemagneticpropertiesofYðCo 1x M x Þ 2 ; including also M=Al or Si was widely adopted [2]. This concept implies that two basic mechanisms drive the evolution of magnetism in YðCo 1x M x Þ 2 with increasing concentration x: These are (i) a change of the d-electron concentration, which leads to a rigid shift of the Fermi level [11] and (ii) a narrowing of thebandsasthelatticeexpandsduetothesubstitution [12]. However, by performing ab initio calculations of theelectronicstructure,ithasbeenshownrecently [10] ARTICLE IN PRESS *Correspondingauthor. E-mail address: phm@cms.tuwien.ac.at(P.Mohn). 1 Alsoforcorrespondence. 0304-8853/$-seefrontmatter r 2003ElsevierB.V.Allrightsreserved. doi:10.1016/j.jmmm.2003.11.111