Incommensurate magnetic structure in TmCuAl at low temperatures

AbstractWepresentaneutronpowderdiffractionstudyonTmCuAl,crystallizingintheZrNiAl-typehexagonalstructure.OurdataconfirmmagneticorderingbelowT c ¼ 1:9K.Themagneticstructureisalongitudinalspinwavedescribedbyapropagationvectork ¼ð 12 0qÞ withTmmagneticmomentsalongthec-axis.Inadditiontothat,acommensurateantiferromagneticcomponentwithinthebasalplanewithk ¼ð 12 02Þ developsbelowthemagneticphasetransitionatT 1 D1:2K. It leads to a rather complex magnetic structure below T 1 : One-third of the thulium moments has aconsiderablyreducedmagnitudecomparedtotheremainingones.Nofurtherphasetransitionswereobserveddownto50mK.r 2002ElsevierScienceB.V.Allrightsreserved. PACS: 75.25.+z;75.30. m;61.12.LdKeywords: Magneticstructures;Neutrondiffraction;Rare-earthintermetallics 1. IntroductionTheternaryRCuAl(R=rareearth)compoundscrystallize,exceptforLaCuAl,intheZrNiAl-typehexagonal structure (space group P-62m). Pre-viousstudiesrevealedaferromagneticorderingforthe compounds containing the heavy rare-earth(Gd–Er)[1,2].Onthecontrary,anantiferromag-neticbehaviorwasfoundinCeCuAl[3],PrCuAland NdCuAl [4]. The magnetic properties ofTmCuAlremainunclear.Nomagneticorderdownto 1.8K was found in Ref. [1], while magneticordering below 1.9K and a possible additionalphase transition at lower temperatures are re-ported in Refs. [5,6]. The negative Curie–Weissconstantandamaximuminthemagneticsuscept-ibilitysuggesttheimportanceofantiferromagneticinteractions in this compound. The