Abstract:
Jl,.lagnetore,istivc propcrhcs of La,_"SrH"Mn,O, (x = 0.3-0.5) bulk polycrystallinc samples
prepared for doping levels X = 0.3 .(l,S and sinte,eJ at lcmpcralun.: 1IOO"C for 24 hour, in air
have heen invc,I;!;alc<1 [rom room tcmperatulG down lu hquld nitrogen temperature u,ing
standard four-probe !CCh'"qUC The tempe, alorc dcpcn<1cill'c of normal i7.cd re,ist;,.;ty for various
poly~rystall,nesamples in zero m"gn"(l~ flcld and ,n a magnetic field of 0.86 Tesla were
ltlvcstigated. The coITc'pondmg behavLor wa, observed upon Ihe partial substitution of small
amount of Gd in place of La amI then al,o in place of Sr of La,_"S',."MIl,O, (x = 0.3-0.5) bulk
samples, Most of the "ample, sho", a mclul-in,ulalor (11.1-1t)ran,ilion with a peak in (he ~lcclneal
re,i,tLvity, P,', at a temperature Tp. Th" sort of I\1-l transItion may be explained within the
fmmc"ork of Jnteraetion mechani,m, (double exchange interactions) bel ween the manganese
ions that occur via oxygen Ions, l'I'om the p,e,ent inve,tigation it IS observed that the substitution
of a ,mall amount of Gd in place of La relllmIllng the SI' unchanged I'esults in lowering of mctal
ill,ubwr tranSition temperature by" fcw KclvlIl, As the atomic size ofGd i, less than that of La,
the ,ubshtuted Od lOn., loweI' the Mn-M" exchange interaetion sub,tantially by bendlllg the
Mn"O-Mn bond angle The higher perecntage of SI' in the," Gornp(}unds enhances paramagnetic
insulating phase. Btlt when non-magnetic SI' is replaced wilh magneUe Od atom the tla",itioll
temperatufC IS found to increase dramatically favoring meTallic phase The magnetic proP~Tty of
(jd i, thoughl tu be responsibk 1'01'higher transition temperamrc. Tlie M-l transinon temperatul'e
IS also increased in pre.,ellce ofO.So T magnetIc field and this is may be due to the ,uPPl'e"ion of
the spin lluchlations with the applJcJ fieltl in the paramagnetic region. M~gncturGsistanGe
mcasurements wel'e carried oul for these polyerystalline bulk .,amples in ~ lllagllellc field of
around 0.% T, R[)(}mtcmperature MR is found to be vcry low, almosl 1.5% - 2% and is almost
linear wilh fidd, The exhibited lal'ge MR effects in thcse compound. at low tempcmtUl'c (78K)
and very low fieltl may be associated with magnetic-domain based scattering ()f spin-polal'ized
tunneling belwecn nusaligned grains In p{TY p(RT) _TL plots for the pl'e,ent inve,tigated
samples suggest thai C[)nJuctlOn occurred through a thel1nally activaled process,
