J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

Journal of the Physical Society of Japan
Vol. 69, No. 6, June, 2000, pp. 1865-1871

Canted Antiferromagnetic Order and Large Magnetoresistance Eﬀect
in La1−X CaX MnO3 , Pr1−X CaX MnO3 and other RE1−X AEX MnO3
Manganese Oxides (X ∼ 0.9)
Hiroyuki Fujishiro, Manabu Ikebe, Shuichi Ohshiden and Koshichi Noto
Faculty of Engineering, Iwate University, Morioka 020-8551(Received January 17, 2000)

The magnetization M(T) and the electrical resistivity ρ(T) of lightly electron-doped
La1−X CaX MnO3 , Pr1−X CaX MnO3 and other RE1−X AEX MnO3 (RE; rare-earth, AE; alkalineearth) have been measured and the canted antiferromagnetic (CAF) phase which competes withthe charge ordered (CO) phase has been extensively studied. The CAF phase was found to
be more strongly stabilized for the smaller A-site ion radius of the perovskite structure. Bysystematically changing the concentration and the average A-site ion radius, a detailed phase
diagram in the electron-doped region was established. The main origin of the CAF phase wassuggested to be the antisymmetric exchange interaction associated with the G-type antiferromagnetic structure of these systems.
KEYWORDS: RE1−X CaX MnO3 , La1−X (Ca1−y Sry )X MnO3 , electron doping, canted antiferromagnetic order,
charge order, magnetoresistance, ionic radius, antisymmetric exchange interaction§1.

Introduction

Perovskite-based manganese oxides RE1−X AEX MnO3
(RE = trivalent rare-earth ions such as La, Pr: AE =divalent alkaline-earth ions such as Sr, Ca) show a variety of dramatic phenomena such as a metal-insulator(MI) transition, a colossal magnetoresistance (CMR) anda ﬁeld-induced structural transformation.1, 2) By replacing RE3+ ions by AE2+ ions, Mn4+ ions are created andmobile holes are introduced in a low X region, which
results in a paramagnetic insulating (PM-I) phase athigh temperatures and a ferromagnetic metallic (FMM) phase at low temperatures. These dramatic properties have been interpreted on the basis of the doubleexchange mechanism and the dynamic Jahn-Teller effect etc.3) For the perovskite-based manganese system,
the average radius of A site ions (RE1−X AEX ) is also
known to play an important role in determining the single particle band-width. For small A site ions which result in a smaller Goldschmidt’s tolerance factor (Γ < 1)
the band-width becomes narrow favoring the insulatingphase, while for large A site ions (Γ ≈ 1), the metallic properties are enhanced. In the high X region near
X = 1, mobile electrons are to be doped changing apart of Mn4+ ions to Mn3+ . In contrast to the holedoped low X region (X ≤ 0.5), the investigations forelectron-doped high X region are scarce. Several Cadoped manganese oxides RE1−X CaX MnO3 have recently
been investigated resulting the existence of a large negative magnetoresistance (MR) eﬀect as in the hole-doped
samples. Bi1−X CaX MnO3 ,4, 5) Eu1−X CaX MnO3 6) andSm1−X CaX MnO3 7, 8) systems all exhibit a canted magnetic order accompanied with the negative MR eﬀect forhigher X region (X > 0.8). In these Ca-based manganese oxides, there are three common characteristic fea-tures; First, the saturation magnetic moment Ms at low
temperature shows 1/3 ∼ 1/4 of the fully aligned value.
Second, Ms is most enhanced at the Ca concentration of
X ∼ 0.90. Third, the magnetic transition temperatureis almost independent of the species of the RE-ions. We
denote this ferromagnetic-like order as the canted antiferromagnetic (CAF) order9) in this paper.
As for La1−X CaXMnO3 (LCMO) and Pr1−X CaXMnO3(PCMO) systems, both of which are very popular CMR
materials, the detailed investigations on the CAF phaseand the negative MR eﬀect in the high X region have notbeen reported. There are several investigations about themagnetic structure in the high X region by means of thepowder neutron diﬀraction technique for these systems.
Wollan et al. reported that the magnetic structures of
the LCMO samples for X = 0.8 and X = 1.0 are theC-type and G-type antiferromagnetic (AFM) order, respectively.10) The charge ordering accompanied by theC-type AFM order occurs for 0.5 < X < 0.9.9, 11) Jiraket al. reported on the PCMO system the C-type AFM
arrangement below TN ∼ 170 K for X = 0.80 and G-typealignment below TN ∼ 110 K for X = 1.0.12)
In this paper, we study systematically the eﬀect of
the radius of the A-site ions on the CAF phase andon the negative MR eﬀect of the lightly electron doped
RE1−X CaX MnO3 (RE = La, Pr) systems. Especially,
we investigate the CAF order and the related negative
MR eﬀect of these systems. La0.10 (Ca1−y Sry )0.90 MnO3
(0 ≤ y ≤ 0.40) and La1−X (Ca0.80 Sr0.20 )X MnO3
(0.875 ≤ X ≤ 0.95) systems are also investigated in order
to conﬁrm the correlation between the CAF phase andthe average ionic radius of (RE, AE) site. The origin of
the CAF phase is also discussed.

1865

J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

1866

§2.

Hiroyuki Fujishiro, Manabu Ikebe, Shuichi Ohshiden and Koshichi Noto

(Vol. 69,

Experimental

The La1−X CaX MnO3 (LCMO) and Pr1−X CaX MnO3
(PCMO) samples (0.75 ≤ X ≤ 1.00) were prepared fromstoichiometric mixtures of La2 O3 (Pr6 O11 ), CaCO3 and
Mn3 O4 raw powders. The mixtures were calcined at
1000◦ C for 24 h in air, pressed into pellets and then sintered at 1500◦ C for 8 h in air. RE0.10 Ca0.90 MnO3 (RE =Nd, Sm, Gd, Dy), La0.10 (Ca1−y Sry )0.90 MnO3 (0 ≤ y ≤0.40) and La1−X (Ca0.80 Sr0.20 )X MnO3 (0.875 ≤ X ≤0.95) samples were also prepared through the same procedures. The samples of the LCMO system were heattreated at 1500◦ C for 24 h in ﬂowing O2 gas to ensure theoxygen stoichiometry. The measured densities of eachsample are high, about 80–90% of the ideal one. X-ray
diﬀraction analyses at room temperature conﬁrmed thatthe samples were in a single phase. The electrical resistivity ρ(T) was measured by a standard four-pointprobe method under a magnetic ﬁeld of up to 5 T using a cryocooler cooled superconducting magnet. The
magnetization M(T) was measured from 10 to 400 K by a
SQUID magnetometer. The thermal dilatation dL(T)/L
was measured by a strain gauge method.
§3.

Results and Discussion

3.1 La1−X CaX MnO3 and Pr1−X CaX MnO3
Figures 1(a) and 1(b) show the temperature dependence of the magnetization M(T) for La1−X CaX MnO3
and Pr1−X CaX MnO3 under an applied ﬁeld of 0.5 T.
With decreasing temperature, M(T) of LCMO in
Fig. 1(a) sharply decreases at TCO for X ≤ 0.875, whichsuggests the onset of the charge ordering (CO) of Mn3+
and Mn4+ ions11) concomitant with the C-type AFM order.10) For X ≥ 0.90, M(T) is highly enhanced below
TCAF (≈110 K) suggesting the onset of the CAF order,
and the depression of M(T) related to CO is wiped out.
The spontaneous moment Ms is the largest for X = 0.90
and takes about 1/4 of the fully aligned value. For
X = 0.875, M(T) shows both the depression corresponding to CO and the enhancement corresponding to CAF.
LCMO (X = 0.875) may not be in a single phase but
may consist of a mixture of the CO and CAF phases.
For 0.875 ≤ X ≤ 1.0, TCAF , which is deﬁned as theonset of M(T) upturn, slightly increases with increasing
X; TCAF ≈ 110 K for X = 0.875 and TCAF ≈ 130 K for
X = 1.0.
In Fig. 1(b), overall X dependence of M(T) of PCMO
is similar to that of LCMO, but there is an important
quantitative diﬀerence between the two systems; LCMO(X = 0.875) shows the M(T) depression correlated toCO, while M(T) of PCMO (X = 0.875) shows only M(T)enhancement correlated to the CAF order. The CO transition is noticed only for X ≤ 0.85 in PCMO. Figure 2
shows the zero ﬁeld cooled (ZFC) spontaneous moment
Ms vs. X of LCMO and PCMO at 10 K under an appliedﬁeld of 0.5 T. In both systems, Ms is most enhanced ataround X = 0.90. The Ms value of PCMO (X = 0.90) is
about 50% larger than that of LCMO (X = 0.90). These
results imply that the CAF phase is more dominant for
PCMO than for LCMO. The main diﬀerence between
PCMO and LCMO is the diﬀerence in the RE ion radiusFig. 1. The temperature dependence of the magnetization M(T)of (a) the La1−X CaX MnO3 and (b) the Pr1−X CaX MnO3 samples under an applied ﬁeld of 0.5 T after zero ﬁeld cooling. The
arrows show the charge ordering temperature TCO (↓) and thecanted antiferromagnetic ordering temperature TCAF (↑).

Fig. 2. The spontaneous moment Ms at 10 K as a function of theconcentration X for the LCMO and PCMO samples applying
0.5 T after ZFC. Ms vs. X for the La1−X (Ca0.80 Sr0.20 )X MnO3
is also shown.

ri (Pr: ri = 1.30 Å, La: ri = 1.36 Å).13) The M(T) results
demonstrate that the smaller ri of the RE ions favors the
CAF phase in the Ca-based manganese systems. The behavior of M(T) as a function of X is analogous to that of

J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

2000)

Canted Antiferromagnetic Order and Large Magnetoresistance Eﬀect in. . . .

Fig. 3. The temperature dependence of the electrical resistivity
ρ(T) of (a) the La1−X CaX MnO3 and (b) the Pr1−X CaX MnO3
samples. The charge ordering temperature TCO is shown byarrows (↓).

Bi1−X CaX MnO3 4, 5) and Sm1−X CaX MnO3 7, 8) systems.
Figures 3(a) and 3(b) show the temperature dependence of the electrical resistivity ρ(T) of LCMO and
PCMO, respectively. At low temperatures, ρ(T) of
all the samples increases with decreasing temperature,
showing semiconductive or insulating behavior. Only
for X = 0.95, both LCMO and PCMO samples slightly
show a metallic ρ(T) at high temperatures (T ≥ 200K̇).
Electron doping into CaMnO3 results in a drastic reduction of ρ(T) in the CAF region and ρ(T) values at
low temperatures are the smallest for X = 0.90 in both
systems. With further electron doping, ρ(T) of the COphase specimens shows a clear increase below TCO . Comparing X = 0.875 samples of LCMO and PCMO, ρ(T)of LCMO is about two orders larger in magnitude thanthat of PCMO at low temperatures. The ρ value of
PCMO (X = 0.875) at 10 K is only about an order larger
than that of PCMO (X = 0.90), while the ρ value of
LCMO (X = 0.875) is more than two orders larger thanof LCMO (X = 0.90). These results suggest that, in
accord with the M(T) data, the main phase of LCMO(X = 0.875) is the CO phase and that the main phaseof PCMO (X = 0.875) is the CAF phase.
Figures 4(a) and 4(b) show ρ(T) of the typical LCMO
and PCMO samples in an applied magnetic ﬁeld of 5 T.

1867

Fig. 4. The temperature dependence of the electrical resistivity
ρ(T) of the (a) La1−X CaX MnO3 (X = 0.85, 0.875 and 0.90) and(b) the Pr1−X CaX MnO3 (X = 0.85, 0.875 and 0.90) samples
under the magnetic ﬁeld of 5 T. ρ(T) in zero ﬁeld is also shownfor comparison.

ρ(T) in zero ﬁeld is also shown for comparison. Large
negative MR is observed for the CAF phase samples below TCAF , making a marked contrast to the CO phasesamples which show almost no MR. Figure 5 presents theelectrical conductivity enhancement (positive magnetoconductance) at 10 K caused by the applied ﬁeld, i.e.,
Δσ = σ(5 T) − σ(0 T), for LCMO and PCMO. The conductivity measurements were performed under a ﬁeldcooled (FC) condition. Δσ is the largest for X = 0.90 inboth systems. Δσ of LCMO is larger than that of PCMO
for X = 0.90. Since Pr has a smaller ion radius than Ca,
the Pr substitution reduces the tolerance factor Γ andenhances the lattice distortion of the perovskite structure. The lattice distortion from cubic is known to reduce
the single electron bandwidth ΔW. Then the electrons
in LCMO are expected to be more easy to transfer thanin PCMO. The enhanced magnetoconductivity Δσ in
LCMO may be related to the diﬀerence of the ΔW values between LCMO and PCMO. For X = 0.875, Δσ of
LCMO is very small, while Δσ of PCMO (X = 0.875)
is nearly as large as that of PCMO (X = 0.90). These
results clearly demonstrate again that the main phaseof PCMO (X = 0.875) is the CAF phase and that of
LCMO (X = 0.875) is the CO phase.

J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

1868

Hiroyuki Fujishiro, Manabu Ikebe, Shuichi Ohshiden and Koshichi Noto

(Vol. 69,

Fig. 5. The electrical conductivity enhancement at 10 K,
Δσ(H) = σ(5 T) − σ(0 T), caused by the applied ﬁeld of 5 T after
the ﬁeld cooling for the LCMO and PCMO samples. An open
triangle represents the Δσ(H) on the zero ﬁeld cooling condition
for the La0.125 Ca0.875 MnO3 sample (see text).

3.2

On a possible ﬁeld induced lattice transformation in
Pr0.125 Ca0.875 MnO3
For the PCMO (X = 0.875) sample, we have founda signiﬁcant diﬀerence between FC and ZFC ρ(T) andM(T) data as Figs. 6(a) and 6(b) demonstrate. The
diﬀerence between the FC and ZFC data is very small
for X = 0.90. In contrast, we see a large diﬀerence
between FC and ZFC results for X = 0.875. On the
ZFC condition, the positive magnetoconductivity Δσ isreduced to about 2/5 of the FC data as shown by aopen triangle in Fig. 5. In Fig. 6(b), the M(T) valueof ZFC PCMO (X = 0.875) is reduced to about 2/3
of FC PCMO (X = 0.875) at 10 K. Figures 7(a) and7(b) show the ﬁeld dependence of Δσ(H) and M(H) at
10 K for ZFC PCMO (X = 0.875). In Fig. 7(a), Δσ(H)(= σ(H) − σ(0 T)) shows a pretty large hysteresis eﬀect
between up and down ﬁeld scans; after the ﬁeld scan up
to 5 T, the conductivity in zero-ﬁeld increases by about
10%. The M(H) curve in Fig. 7(b) also shows a hysteresis behavior, though it is very small in comparison to thecorresponding hysteresis of Δσ(H).
Now let us discuss a possible origin of the observed
hysteresis behavior of PCMO (X = 0.875). Figure 8
presents the thermal dilatation dL/L vs. T of PCMO
(0.80 ≤ X ≤ 1.0). With decreasing T , dL/L shows ananomalous shrink at TCAF for X ≥ 0.875 and at TCO for
X ≥ 0.850. The contraction of the sample at TCAF and
TCO suggests that a kind of structural transformation
may take place accompanied with these phase transitionsof PCMO. Jirak et al.12) observed by the low temperature neutron diﬀraction an elongated pseudotetragonal(T ) structure for 0.75 ≤ X ≤ 0.90 and pseudocubic (C)structure for 0.90 ≤ X ≤ 1.0. These results suggestthat the lattice structure of PCMO in the CAF phase(0.875 ≤ X ≤ 1.0) is pseudocubic and that of PCMOin the CO phase (0.80 ≤ X ≤ 0.85) is pseudotetragonal. Jirak et al. also observed a two-phase structure
for PCMO (X = 0.90). It is possible that the present
PCMO (X = 0.875) sample has the two-phase structure
because its Pr concentration is near the boundary (XC )Fig. 6. (a) The temperature dependence of the electrical resistivity ρ(T) of Pr1−X CaX MnO3 (X = 0.875 and 0.90) samples
under an magnetic ﬁeld of 5 T (ZFC and FC scans). (b) The
magnetization M(T) of the same samples as a function of T under an magnetic ﬁeld of 5 T (ZFC and FC scans).

Fig. 7. (a) The electrical conductivity enhancement at 10 K,
Δσ(H) = σ(H) − σ(0 T) caused by the applied ﬁeld after thezero ﬁeld co oling for the PCMO (X = 0.875) sample. (b) Magnetization curve at 10 K for the same sample.

between the CAF and CO phases. On FC condition in aﬁeld of 5 T, a larger part of the PCMO (X = 0.875) may
transform to the pseudocubic structure with the spontaneous moment Ms and Ms (T) in Fig. 6(b) is enhanced inthis case. Since only the CAF phase exhibits the large

J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

2000)

Canted Antiferromagnetic Order and Large Magnetoresistance Eﬀect in. . . .

Fig. 8. Temperature dependence of the thermal dilatationdL(T)/L for the Pr1−X CaX MnO3 samples. Downward arrows(↓) indicate the charge ordering temperature TCO and upward
arrows (↑) indicate the canted antiferromagnetic temperature
TCAF .

1869

Fig. 9. The temperature dependence of the magnetization M(T)of RE0.10 Ca0.90 MnO3 samples (RE = La, Pr, Nd, Sm, Gd, Dy).
The inset shows the saturation moment Ms at 10 K as a functionof the ionic radius of RE ions.

negative MR eﬀect, the FC ρ(T) of PCMO (X = 0.875)
in Fig. 6(a) shows larger negative MR eﬀect in comparison to ZFC ρ(T).The hysteresis behaviors of Δσ(H) in
Fig. 7(a) and M(H) in Fig. 7(b) suggest that a small partof PCMO (X = 0.875) makes the lattice transformationeven at 10 K in an applied ﬁeld of 5 T. The characteristic hysteresis behavior of ρ and Ms between FC and ZFC
conditions was not observed LCMO (X = 0.875) probably because the CO phase is overwhelmingly dominant
over the CAF phase in this sample.
3.3 RE0.10 Ca0.90 MnO3 (RE=Pr, Nd, Sm, Dy)
The CAF phase and the related large negative MR effect are peculiar and common characters for the electrondoped Ca-base manganese (X ∼ 0.9) and the smaller
ionic radius of RE ions seems to enforce the CAF state.
In order to conﬁrm the eﬀect of the ionic radius of
RE ions on the CAF phase, the magnetization M(T) of
RE0.10 Ca0.90 MnO3 samples (RE = Nd, Sm, Gd, Dy)
with a ﬁxed concentration X = 0.90 was measured. Figure 9 shows the M(T) curves of these samples under themagnetic ﬁeld of 0.5 T together with those of the LCMO
and PCMO. The inset shows Ms as a function of ri of RE
ions.13) Although the CAF phase was conﬁrmed below
nearly the same temperature TCAF (∼110 K) irrespectiveof the ionic radius of RE ions, the saturation moment Ms
increases with decreasing ionic radius of RE ions for La,
Pr, Nd, showing no dependence for smaller RE ions (Nd,Sm, Gd, Dy). The negative MR eﬀect similar to the
LCMO and PCMO systems was also observed for thesematerials.
and
La1−X
La0.10 (Ca1−y Sry )0.90 MnO3
(Ca0.80 Sr0.20 )X MnO3
Our experimental results on electron doped
RE1−X CaX Mn O3 have clearly indicated the importance of A site ion radius for the CAF phase stabilization. A more direct way to change the A site ion radius is to substitute Ca by Sr with a larger ionic radius. Figures 10(a) and 10(b) show M(T) and ρ(T) of
3.4

Fig. 10. (a) The magnetization M(T) and (b) the electrical resistivity ρ(T) of the La0.10 (Ca1−y Sry )0.90 MnO3 (0 ≤ y ≤ 0.4)
system with a ﬁxed AE ion concentration X = 0.90. Arrows
indicate the charge ordering temperature TCO .

La0.10 (Ca1−y Sry )0.90 MnO3 (0 ≤ y ≤ 0.40). By substituting only 5% Ca by Sr, the saturation moment Ms
drastically decreases and for y = 0.10, Ms reduces tojust 1/10 of La0.10 Ca0.90 MnO3 . With increasing y, theJ. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

1870

Hiroyuki Fujishiro, Manabu Ikebe, Shuichi Ohshiden and Koshichi Noto

(Vol. 69,

Fig. 11. The magnetization M(T) of the La1−X (Ca0.80 Sr0.20 )X
MnO3 system (0.875 ≤ X ≤ 0.95) with a ﬁxed Sr ion content
y = 0.20. Arrows indicate the charge ordering temperature TCO .

depression of M(T) which corresponds to the CO onset appears and for y = 0.10, the CO transition can
be clearly conﬁrmed in Fig. 10(a). In Fig. 10(b), ρ(T)drastically increases with increasing y. The negative MReﬀect can be seen up to y = 0.10, but the positive magnetoconductivity Δσ(H) is very small for y = 0.10. The
y = 0.10 sample may not be in a single phase and its
main phase may be the CO phase. The properties of
La0.10 (Ca0.90 Sr0.10 )0.90 MnO3 are very similar to thoseof La0.125 Ca0.875 MnO3 .
Figure 11 shows M(T) of the La1−X (Ca0.80 Sr0.20 )X
MnO3 samples. In this (Ca0.80 Sr0.20 )-based system withlarge RE and AE ionic radius, the CAF phase is very
rapidly quenched with decreasing X. For X = 0.95,
M(T) of this system shows a pretty large Ms at low temperatures characteristic of the CAF phase. But Ms is remarkably reduced for the X = 0.925 sample and the CAF
is already greatly damaged at this concentration. As canbe seen in this ﬁgure, a distinct depression of M(T) withdecreasing T characteristic of the CO phase is noticeable
for X = 0.90 and La0.10 (Ca0.80 Sr0.20 )0.90 MnO3 clearly
belongs to the CO phase. Thus in this system, the CAF
phase is dominant in a very narrow region of X ≥ 0.925.
This fact makes a marked contrast to Ca-based compounds because all the RE0.10 Ca0.90 MnO3 studied in thispaper shows the largest Ms for X = 0.90. Ms vs. X of
La1−X (Ca0.80 Sr0.20 )X MnO3 is also contained in Fig. 2.
The CAF state vs. the CO state in RE1−X AEX
MnO3 in the electron doping region
As we have seen in the preceding subsections, thelarger A-site ion radius results in a drastic deterioration of the CAF phase. Thus the experimental results for both RE ion substituted and AE ion substituted RE1−X AEX MnO3 systems indicate the important role of the A site ionic radius. Figure 12 shows
the phase diagram of the CO and CAF phases as afunction of the AE ion concentration X and the average ionic radius of A site. In this ﬁgure, open circles represent the occurence of the CO phase and theclosed circles represent the realization of the CAF phase.
3.5

Fig. 12. The phase diagram of the relation between the CO
and the CAF phases as a function of the AE ion concentration X and the average ionic radius of (RE,AE) site in the
RE1−X AEX MnO3 system. Open circles represent the CO phaseand closed circles represent the CAF phase. The ionic radius
was calculated from the tabulated radii with ninth-fold coodination.13)

Fig. 13. The CO transition temperature TCO determined in thiswork as a function of the electron doping concentration 1 − X.

The critical concentration XC which separates the twophases increases with increasing average radius of A-siteions. Figure 13 represents the CO transition temperature TCO studied in this work as a function of the electron doping concentration 1 − X. The average A siteionic radius becomes smaller in the consecutive orderof La0.10 (Ca0.60 Sr0.40 )0.90 , La0.10 (Ca0.80 Sr0.20 )0.90 and
La0.10 (Ca0.90 Sr0.10 )0.90 , La0.10 Ca0.90 and Pr0.10 Ca0.90 .
Figure 13 clearly demonstrates that the larger A siteionic radius always results in a higher TCO at a ﬁxed
value of electron-doping 1 − X. It is worthwhile to notethat the larger A site ionic radius, which should resultin a larger tolerance factor Γ and a wider single particle band-width, seems to favor the CO phase in thiselectron-doping region.

J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

2000)

Canted Antiferromagnetic Order and Large Magnetoresistance Eﬀect in. . . .

3.6 On the origin of the CAF state
The crystal structure of CaMnO3 is actually orthorhombic at low temperatures with space grouppbmn,14) which has no inversion symmetry at the midIn this lattice,point of the nearest Mn4+ spins.
Dzyaloshinsky-Moriya (DM) antisymmetric exchange interaction15) is possible. The DM interaction results inthe weak ferromagnetism (WF) with a canted spin conﬁguration in an antiferromagnet. The antiferromagnetic
structure in the region of X where the CAF phase occurs
were reported to be the G-type. The G-type structure
is suitable for WF because all the nearest neighbor Mn
spins are oriented antiparallel; parallel spin pairs cannot
contribute to WF. In contrast, in the region of X where
the CO phase dominates, the reported C-type structure
consists of ferromagnetic chains mutually oriented antiparallel.
As is shown in Fig. 2, the WF spontaneous moment
Ms is drastically enhanced by a small amount Ca substitution by La or Pr while the magnetic ordering temperature TCAF remains almost constant. The nearly constant
TCAF suggests that the antiferromagnetic superexchange
which mainly determines TCAF is not so much aﬀectedby initial Ca substitution. The drastic Ms enhancement
suggests the drastic enhancement of the DM interaction.
Qualitatively, the reduction in the local symmetry between Mn spins is expected to enhance the DM interaction. The Ca substitution by RE ions reduces the local symmetry between near Mn spins and the symmetry
reduction eﬀect should be stronger for the smaller RE
ion because of the stronger mismatch of the ionic radiusagainst Ca ions. It is very probable that the DM interaction is more strongly enhanced by the smaller RE
ion substitution and the DM interaction seems to bea key factor for the canted AF state in electron-doped
RE1−X CaX MnO3 .
In contrast to the hole-doped ferromagnetic region,16)
where the double exchange mechanism is dominant, thecanted antiferromagnetic transition does not accompany
any reduction of ρ(T) at or below TCAF . However, thedouble exchange-like mechanism,17, 18) through which
the doped electrons hop between parallel Mn4+ spin sites
enhancing the ferromagnetic component, may simultaneously be at work because the absolute values of ρ(T) aresigniﬁcantly reduced as a whole and the negative MReﬀect is conspicuous in the CAF phase. The DM antisymmetric exchange interaction and the double exchangemechanism may cooperatively stabilize the CAF phasein the RE1−X AEX MnO3 systems near X = 1.0.
§4.

Summary

The magnetization M(T) and the electrical resistivity ρ(T) of lightly electron-doped La1−X CaX MnO3 ,Pr1−X CaX MnO3 and other RE1−X AEX MnO3 (RE;rare-earth, AE; alkaline-earth) have been measured. Important experimental results and the noticeable points
obtained in this study are summarized as follows;(1) The canted antiferromagnetic (CAF) phase and therelated large negative magnetoresistance were observedfor RE1−X CaX MnO3 (RE = La, Pr, Nd, Sm, Gd, Dy)
for X > 0.90 and the charge ordered (CO) phase was1871

found for X < 0.90. The boundary concentration XC
between the CAF and CO phase depends on the RE
ion radius. The CAF phase became dominant over the
CO phase for smaller RE ion radius. The spontaneous
magnetic moment Ms in the CAF phase also increased
for the smaller RE ions.
(2) The substitution of Ca by Sr in theLa0.10 (Ca1−y Sry )0.90 MnO3 system promptly depressed
the CAF order, enforcing the CO phase. The result implies that the CAF phase become dominant also for thesmaller average AE ion radius.
(3) The phase boundary between the CAF and the COphase was determined as a function of the AE concentration X and the A site ion radius.
(4) Distinct diﬀerences were observed for ﬁeldcooled and zero-ﬁeld-cooled ρ(T) and M(T) of
Pr0.125 Ca0.875 MnO3 . A possibility of ﬁeld-induced structural transformation was suggested in this PCMO (X ≈XC ) sample.
(5) As a origin of the CAF phase, the importance of
Dzyaloshinsky-Morita antisymmetric exchange interaction was pointed out.
Acknowledgment
The authors wish to thank Prof. T. Fukase of the Institute for Materials Research, Tohoku University, forvaluable discussions. The authors also wish to thank T.
Kikuchi and T. Fujiwara of Iwate University for theirassistance in the sample preparation. This work is partially supported by the REIMEI Research Resources ofJapan Atomic Energy Research Institute.
1) H. Yoshizawa, H. Kawano, Y. Tomioka and Y. Tokura: J.
Phys. Soc. Jpn. 65 (1996) 1043.
2) Y. Tomioka, A. Asamitsu, Y. Morimoto, H. Kuwahara and Y.
Tokura: Phys. Rev. Lett. 74 (1995) 5108.
3) A. J. Millis, P. B. Littlewood and B. I. Shaiman: Phys. Rev.
Lett. 74 (1995) 5144.
4) H. Chiba, M. Kikuchi, K. Kusaba, Y. Muraoka and Y. Shono:
Solid State Commun. 99 (1996) 499.
5) Wei Bao, J. D. Axe, C. H. Chen and S.-W. Cheong: Phys.
Rev. Lett. 78 (1997) 543.
6) I. O. Troyanchuk, N. V. Samsonnenko, H. Szymczak and A.
Nabialek: J. Solid State Chem. 131 (1997) 144.
7) C. Martin, A. Maignan, F. Damay and B. Raveau: J. Solid
State Chem. 134 (1997) 198.
8) A. Maignan, C. Martin, F. Damay, B. Reveau and J.
Hejtmanek: Phys. Rev. B 58 (1998) 2758.
9) A. J. Millis: Nature 392 (1998) 147.
10) E. O. Wollan and W. C. Koehler: Phys. Rev. 100 (1955) 545.
11) A. P. Ramirez, P. Schiﬀer, S.-W. Cheong, C. H. Chen, W.
Bao, T. T. M. Palstra, P. L. Gammel, D. J. Bishop and B.
Zegarski: Phys. Rev. Lett. 76 (1996) 3188.
12) Z. Jirak, S. Krupicka, Z. Simsa, M. Dlouha and S. Vratislav:
J. Mag. Mag. Mater. 53 (1985) 153.
13) R. D. Shannon: Acta. Crystallegr. A 32 (1976) 751.
14) K. R. Poeppelmeiner, M. E. Leonowicz, J. C. Scanlon, J. M.
Longo and W. B. Yelon: J. Solid State. Chem. 43 (1982) 1137.
15) E. A. Turov: Physical Properties of Magnetically Ordered
Crystals, (Academic Press, New York and London 1965).
16) H. Fujishiro, T. Fukase, M. Ikebe and T. Kikuchi: J. Phys.
Soc. Jpn. 68 (1999) 1469.
17) C. Zener: Phys. Rev. 82 (1951) 403.
18) P. G. de Gennes: Phys. Rev. 118 (1960) 141.