ACTA PHYSICA POLONICA B

Vol. 34 (2003)

No 2A

EFFECT OF-SITE CATION SIZE MISMATCH
ON FIRST-ORDER-LIKE FERROMAGNETIC
TRANSITION IN PEROVSKITE MANGANITES
M. Ikebe, H. Fujishiro, and H. Ozawa

Fa ulty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan(Re eived July 10, 2002)
The ee t of the A-site ation radius variation  2 on the ferromagneti (FM) and metalinsulator (MI) transition in R3+ 0:75 M2+ 0:25 MnO3
mixed rystals has been studied by measuring the magnetization M(T),ele tri al resistivity (T) and thermal ondu tivity (T). With in reasing 2 , the rst-order-like hara ter of FM and MI transition observed inLa0:75 Ca0:25 MnO3 is promptly suppressed in mixed rystals in spite of thesame toleran e fa tor value kept onstant to that of La0:75 Ca0:25 MnO3 .
PACS numbers: 65.40.b, 75.30.Kz, 72.15.Eb
1. Introdu tion

A series of arrier-doped perovskite manganites with the formulae RE1 X
AEX MnO3 (RE=lanthanide: AE=alkaline earth) exhibit a variety of novel
physi al properties and have been the subje t of re ent revived resear hes[1, 2℄. The novel properties originate from ompetitions and ooperations ofvarious intera tions of nearly equal importan e, i.e., the double ex hange,
superex hange, harge and orbital order, JahnTeller (JT) ee t due to
Mn3+ ions, A- and B -site ation radii mismat h in the ABO3 perovskite
latti e, et . The hara terisit metalinsulator (MI) transition on omitant with the ferromagneti (FM) order annot be explained by the double ex hange me hanism only, and the importan e of the latti e ee t iswidely admitted. The latti e ee t is enhan ed through the JT ee tand AB ation radius mismat h. In a previous paper [3℄, we reported
an anomalous rst-order-like ferromagneti MI transition in mixed rystals, (La1 Y PrY )0:75 (Ca1 Z SrZ )0:25 MnO3 . The anomalous transition did
not involve hysteresis and took pla e for the average A- ation radius,

Presented at the International Conferen e on Strongly Correlated Ele tron Systems,(SCES 02), Cra ow, Poland, July 1013, 2002.

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M. Ikebe, H. Fujishiro, H. Ozawa

1.195Å<rA <1.212Å(rA in 9-hold oodination). The most lear and typi al rst-order-like behavior was exhibited by La0:75 Ca0:25 MnO3 (LCMO)
with rA =1.207Å. The rA value determines the latti e pdistortion from theubi form through the toleran e fa tor (=(rA +rO )/( 2(rB +rO )), rB andrO being the ioni radii of Mn and oxygen). The single ele tron energy
bandwidth W be omes wider as the value approa hes 1 and, resultantly,the FM metal (FMM) phase is more strongly stabilized in a nearly ubi
latti e. Rodoriguez et al. [4℄ and Sundaresan et al. [5℄ pointed out that not
only the rA value but also its varian e  2 signi antly ae t the FM andMI transition in the manganite systems. In this paper, we investigate theee t of  2 on the FMM phase transition, with spe ial emphasis on therst-order-like behavior, by monitoring the magnetization M(T), ele tri al
resistivity (T) and the thermal ondu tivity (T).

2. ExperimentalRE1 X AEX MnO3 samples (RE: La, Pr, Nd, Sm and La0:50 Sr0:50 , AE:Ca, Sr, Ba and thier mixtures) were prepared by a solid-state rea tion
method. The mixtures of raw powders were al ined at 1000Æ C for 24h
in air, pressed into pellets and sintered at 1500Æ C for 8h in air. The mixing
ratios of AE ions were sele ted so as to keep the average A-site ation radius
onstant (rA =1.207Å, =0.917) and the hole on entration at X=0.25. The
mixing ratios, the sample symbols used in the text and the values of  2 aresummarized in Table 1. The radius varian e is given by  2 =yi ri2 -rA2 , whereyi is the fra tional o upan y of the A-site spe ies and ri is its ioni radius.
For al ulation, tabulated radii ri with 9-hold oordination [6℄ were used.
The magnetization M(T) was measured using a SQUID magnetometer. Thethermal ondu tivity (T) was measured by a steady-state heat ow method
in a Giord-M Mahon (GM) y le helium refrigerator.
TABLE I

Sample ompositions and  2 values
SampleSymbol2La0:75 Ca0:25 MnO3
(La0:50 Pr0:50 )0:75 (Ca0:573 Sr0:427 )0:25 MnO3
Pr0:75 (Ca0:146 Sr0:854 )0:25 MnO3
Nd0:75 (Sr0:819 Ba0:181 )0:25 MnO3
Sm0:75 (Sr0:237 Ba0:763 )0:25 MnO3LCMO
(LaPr)(CaSr)
Pr(CaSr)
Nd(SrBa)
Sm(SrBa)

0.00060.00150.00300.00680.0180Ee t of A-Site Cation Size Mismat h on : : :829

3. Results and dis ussion

Figure 1(a) shows M(T) of respe tive samples under the eld of 0.5 T.
With de reasing temperature T, M(T) of LCMO ( 2 = 0:0006) jumps up
step-like just below T , whi h is a hara teristi feature of the rst-order-liketransition. The step-up-like M(T) is somewhat obs ured for (LaPr)(CaSr)( 2 = 0:0015) and M(T) be omes ompletely se ond-order-like for larger  2values. For Sm(SrBa) with the largest  2 value (=0.0180), the FM saturation
moment in the low T region is remarkably redu ed. Figure 1(b) presents (T)for the same samples as in Fig. 1(a). A very sharp drop of (T) is noti ed
for LCMO and (LaPr)(CaSr), while it exhibits gentle variations at T forthe larger  2 . The (T) behaviors are all metalli below the FM transition
temperature T ex ept for Sm(SrBa).

Fig. 1. (a) M(T) and (b) (T) for the samples given in Table I.

Figure 2(a) shows (T), whi h is ontributed overwhelmingly by phonons( ' ph ) in the present samples. For LCMO, (T) shows a step-like rise
just below T with de reasing temperature and then is gradually enhan edwith further de rease of T. In (LaPr)(CaSr), the (T) jump just near T
and the gradual enhan ement be ome pretty moderate. In ontrast, (T) ofPr(CaSr), whi h shows the se ond-order-like MI transition, does not showsu h anomalous behaviors. Figure 2(b) presents T vs  2 . T de reases
with in reasing  2 . It is worthwhile to noti e that the perovskite manganite
system with the hole on entration X = 0:25 and the toleran e fa tor =0:917 is transformed to a FM-insulator by the large 2 values (Sm(SrBa)).
In summary, the A-site ation radius varian e  2 vitally ae ts the ferromagneti and the metal-insulator transition in the perovskite manganite
system. In the mixed rystals, the rst-order-like FM and MI transition

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M. Ikebe, H. Fujishiro, H. Ozawa

promptly disappeared with in reasing  2 in spite of the onstant toleran e
fa tor (= 0:917) and the onstant hole on entration X = 0:25. The behavior of the thermal ondu tivity (T) was also heavily inuen ed by the2 value.

Fig. 2. (a) (T) for the LCMO, (LaPr)(CaSr) and Pr(CaSr) samples and (b) T vs 2 for the samples given in Table I. The spe imens denoted by open symbols showthe rst-order-like transitions.

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