Physica B 281&282 (2000) 491}493

First-order-like ferromagnetic transition in(LaPr )(CaSr ) MnO (x&0.25)
1~y : 1~x
1~z ; x3
H. Fujishiro*, M. Ikebe, T. Kikuchi, H. Ozawa
Department of Materials Science, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan

Abstract
The magnetization M(¹), electrical resistivity o(¹) and thermal conductivity i(¹) have been measured for themanganese oxides, RA
MnO (R"LaPr : A"CaSr ) (0)y)1.0, 0)z)1.0). Two types of the "rst0.75 0.2531~y y1~z zorder-like ferromagnetic transition have been veri"ed; the transition shows no hysteresis for 1.345)r )1.362 AsRA(r "average ionic radius of R}A site), while it accompanies hysteresis for 1.320(r (1.345 As . For r '1.362 AsRARARAand r (1.320 As , the ferromagnetic transition is of the second order. The order of the transition may be closelyRA
correlated with the strength of the electron}phonon coupling as Millis et al. have predicted. ( 2000 Elsevier ScienceB.V. All rights reserved.
Keywords: Manganese oxides; Ferromagnetic order; First-order}like transition; Electron}phonon coupling

Perovskite-based manganese oxides, (RA )MnO
1~x x3(R"trivalent rare-earth ions such as La, Pr: A"
divalent alkaline-earth ions such as Sr, Ca) display a variety of physical properties such as the paramagnetic (PM)to ferromagnetic (FM) transition synchronized with theinsulator-to-metal (I}M) transition and the colossal
magnetoresistance, etc. [1,2]. The novel physical properties of this system are greatly in#uenced by the holeconcentration x and the average ionic radius r of (R}A)
RAsite ions. The e!ect of the di!erence in the ionic radius
can be systematically interpreted in terms of the tolerance factor f [3]. The La
Ca MnO system exhibits a1~x x
3"rst-order-like PM to FM transition at the Curie temperature ¹ for 0.25)x)0.30, showing an almost dis#continuous increase of the magnetization [4,5], thoughno trace of hysteresis is seen. The electrical resistivity
also shows a sharp drop at ¹ . Generally, the FM#transition is of the second order, but a "rst-order
FM transition may be possible in case of a strong
magneto}elastic or electron-phonon coupling [4,6].
In this note, we investigate the magnetization M(¹),* Corresponding author. Fax: #81-19-621-6373.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro)

electrical resistivity o(¹) and thermal conductivity i(¹)
of (LaPr )(CaSr )
MnO samples. The
1~y y 0.75
1~z ; 0.25
3hole concentration x is "xed at 0.25, because
LaCa
MnO shows the most typical "rst-order0.75 0.25
3like transition. r is changed by varying y and z values.
RAThe ranges of y and z, where the "rst-order-like ferromagnetic transition is observable, have been determined.
La(CaSr )MnOand(LaPr )0.75
1~z ; 0.25
3
1~y y 0.75
CaMnO samples were prepared by a solid-state
0.25
3
reaction method. The mixtures of raw powders werecalcined at 10003C for 24 h in air, pressed into pellets andsintered at 15003C for 8 h in air. o(¹) was measured bya standard four-point probe method and M(T) was measured using a SQUID magnetometer under a magnetic"eld of 0.5 T after zero-"eld cooling. i(¹) was automatically measured by a continuous heat-#ow method.
Fig. 1 shows the temperature dependence of the magnetization M(¹) of (LaPr )(CaSr )MnO
1~y y 0.75
1~z z 0.25
3samples. ¹ increases with increasing average ionic#radius r (r (r and r (r ). The step-like M(¹)
RA P3
L!
C!
S3
anomaly around ¹ can be observed for the samples#of (LaPr )CaMnO(0)y)0.2) and
1~y y 0.75 0.25
3
La(CaSr )MnO (0)z)0.3).
0.75
1~z z 0.25
3
Fig. 2 shows the temperature dependence of the resistivity o(¹). o(¹) decreases below ¹ and shows the#0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 8 8 9 - 3

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H. Fujishiro et al. / Physica B 281&282 (2000) 491}493

Fig. 1. The temperature dependence of the magnetization M(¹)
of (LaPr )(Ca Sr )MnO samples.
1~y y 0.75
1~z z 0.25
3Fig. 3. The temperature dependence of the thermal conductivityi(¹) for typical samples.

Fig. 2. The temperature dependence of the resistivity o(¹) of thesamples.

Fig. 4. The average ionic radius of (R}A) site ions r versus theRAferromagnetic transition temperature ¹ determined using the#results in Figs. 1}3.

metallic behavior at low temperatures except forPrCa
MnO . A step-like decrease of o(¹) without
0.75 0.25
3hysteresis is observed just below ¹ for the samples in#which the step-like M(¹) anomaly is observable. o(¹) of(LaPr )CaMnO (y"0.3 and 0.4) shows1~y y 0.75 0.25
3a hysteresis around ¹ which suggests the "rst-order#nature of the transition. The observation of a similar
hysteresis in o(¹) was also reported by other authors [7].
The absolute value of o decreases with increasing average
ionic radius r . For La
SrMnO , however, theRA0.75 0.25
3o value increases again, which is somewhat contradictory
to our anticipation.
Fig. 3 shows the temperature dependence of the thermal conductivity i(¹) for several samples. For thesample of (LaPr )(Ca Sr )MnO (y"0.4,
1~y y 0.75
1~z ; 0.253z"0) with smaller r , i(¹) shows a local minimum
RAaround ¹ . A step-like anomaly (without hysteresis) of#i(¹) at ¹ can be seen for the (y"0.2, z"0), (y"0,#z"0) and (y"0, z"0.2) samples. For the (y"0,z"0.4) sample with larger r , the step-like anomaly ofRAi(¹) disappears again.
Fig. 4 shows r versus the ferromagnetic transitionRA
temperature ¹ for the present samples using the results#in Figs. 1}3. r is calculated from the tabulated radii ofRA(R}A) site cations with 12-fold coordination (La3`"
1.36 As , Pr3`"1.30 As , Sr2`"1.44 As , Ca2`"1.34 As )[8]. ¹ is scaled by r as reported for the other "xed#RAvalues of the hole concentration [7]. We classify the"rst-order-like FM transition into two groups, i.e., withand without hysteresis. The range of r , where theRA"rst-order-like FM transition shows no hysteresis, is1.345)r )1.362 As and that where it shows hysteresis
RAis 1.320(r (1.345 As . The second-order-like FMRAtransition occurs beyond both ends of these r regions.
RAIt is not unreasonable to anticipate that the strength ofthe e!ective electron}phonon coupling is correlated withr and because of increasing internal stress acting onRAMn}O}Mn bonds, the coupling may become stronger
for the smaller r . Millis et al. theoretically predicted
RAthat a "rst-order transition may be possible between PM
insulator and FM metal phases for an intermediately
strong electron}phonon coupling [6]. The experimental
results of this study may exemplify a typical case of the"rst-order-like FM transitions which is in accord withthe prediction.

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