Studies of the magnetic ordering of some mixed spinel oxides by neutron diffraction

Abstract

Magnetlc ordering In two series of mixed spinel oxide systems have been investigated by neutron diffraction studies as iUlicliom; of composition and temperature. The ~y~tem, .,ladied arc (i) Zn,Mgll, .,Ni",FcI04 with composition, x = 00, 02, [) 4, 0.6 and (ii) CO,MllhAh,Fc).7,O, wilh composition, x = 0.2, 0.4, 06, 08. The samples were prepared in the ceramic Sintering method. The chemical structures of the systems have been determined by X-ray and neutron diffraction and the two re'lilts are compared. Bulk magnetization for each of the compllSlllOns urlhe (wo systems were measured <it room temperature using a Vibrating Sample Magnelometer (VSM), in order to have a preliminary idea of the overall magnetic properties of the system The detail magnetic behavior of the system is found out from the ncutron diffraction measurement> on them at a serics of temperatures buth above and below the respective Curie temperatures. For system (i) neutron diffraction measurements were done in the temperature range 15K,,; T ,,; 823K. For ,y,tem (ii) the neutron mea,urements were also performed at a number of temperatures '-anging from 773K down to 15K For system (i) the observed sublattlCe moment, do not follow the ~rnooth trend as expected in collinear ferrimagnetic :;tmcture, A ,Iuggi,h and ~lep like behavior in the temperature dependence of the magnetic moment ha:; been ob:.erved below Tc which manifests the pre~ence of spin clusters superimpo,ed on the inJinite magnctic netwurk. For the ~amplc \vith x = 0 6 the ordered moment in the 13sublattice reduces dra:;tically indicating the presence of strong lloncollmeanty in the B ~ite. But the absence of (200) rellection in thc neutron patterns at all temperatures suggests that there is no long range spatial ordering of the transver,e spin components However, for this sample thcre is con~iderahle broadening of the (111) dlffractron peak at 15K due to the diffuse ,ignal The presence of this dlffuse scattering and the decrease in '" • the B site moment at temperatures below SOKindicate that a re-entrant or semi-spingla:; s like :;tructure 1\ more probable for the composition with x = 0.6 The results are described qualitatively in the light of the eXisting tbcorie~ and experimental observation:; FOI system (ii). it appear:; that it ]Jo:;:;esse:;a long range ferrimagnetic ordering at x '" 0 6 along with some randomly canted :;pin components At x ~ 0 8, a 8eml-;pin gla:;s beiJaVlllr 'WaSobserved The laller composition e"hibits three tran:;itions' at temperatures higher than lOOK,there is formation of magnelic ,pin c1u,ters showing ,uper-paramagnetic type behavior before the true pawm;lgnetlC region is reaehed at about 200K, a ferrimagnetLc long-range ordering is observed in the temperature range lOOKto SOK; and ]jnally below SOKthere is freezIng of the magnetic 8pin cluster8, The competitIOn between the long range ferrimagnetie order and the fluctuating :;mall spin clusters has been probed through developing a computer program and fitting the (Ill) Blagg dim-action peaks in the diffraction patterns al different temperatures lor the composition with x = 0.8