Journal of Magnetism and Magnetic Materials 378 (2015) 451–456

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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm

Magnetic and thermodynamic properties of the lightly electron-doped
manganite compound (Ca,Sr)Mn0.95Sb0.05O3
Takahiro Fujiwara a, Michiaki Matsukawa a,n, Takahiro Aoyagi a, Satoru Kobayashi a,
Haruka Taniguchi a, Shigeki Nimori b, Ramanathan Suryanarayanan caDepartment of Materials Science and Engineering, Iwate University, Morioka 020-8551, Japan
National Institute for Materials Science, Tsukuba 305-0047,Japanc
Laboratoire de Physico-Chimie de L'état Solide, CNRS, UMR8182, Université Paris-Sud, 91405 Orsay, Franceb

art ic l e i nf o

a b s t r a c t

Article history:
Received 9 December 2013
Received in revised form
14 September 2014
Accepted 21 November 2014
Available online 26 November 2014

We report on the dc magnetization and ac magnetic susceptibility of the lightly electron-doped man4+
3++
ganite compound (Ca12+− xSr 2x+)Mn0.9
Mn0.05
Sb50.05
O23− (x ¼0.0, 0.05, 0.10, 0.15, 0.16, 0.17, and 0.2) with a ﬁxed
carrier content, to examine the effect of chemical pressure on magnetization reversal of this system. In a
weak magnetic-ﬁeld-cooled measurement, diamagnetic magnetization is observed for x ≤ 15% , whichchanges to positive values for x > 15% . We present a magnetic phase diagram as a function of Sr concentration in several magnetic ﬁelds. The ac magnetic susceptibility measurement indicates the existenceof magnetic frustration for the Sr substituted samples exhibiting diamagnetic behavior. To better understand the thermodynamic properties of this system, we have measured the speciﬁc heat as a functionof temperature over a ﬁeld cooling of 100 Oe. Our data show no anomalies associated with the temperature-dependent magnetization reversal, indicating the absence of long-range magnetic ordering.
& 2014 Elsevier B.V. All rights reserved.

Keywords:
Magnetization reversal
Electron doped manganites
Chemical pressure effect

1. Introduction
Extensive studies on magnetization reversal and negative
magnetization in magnetic materials demonstrate the potential
uses of these mechanisms in magnetic memory and related applications [1]. CaMnO3, the end member of the Ca1  xLaxMnO3
system, undergoes a G-type antiferromagnetic transition aroundTN ∼ 120 K , accompanied by a weak ferromagnetic component [2],in which each Mn ion spin is antiparallel to those of its nearest Mn
ion neighbors. In recent years, the electron-doped manganite
system (x < 0.5) [3] has attracted much attention because of thepossibility of observing a negative magnetoresistance effect in it
similar to that observed in its counterparts, the so-called holedoped manganites, with x > 0.5. The negative magnetization
phenomena in manganites were originally reported in compoundswith two sublattices of Mn ions and rare-earth ions (Nd, Gd, andDy) [4–6]. Some of these studies were discussed on the basis of aferrimagnetic scenario causing negative magnetization at temperatures below a particular compensation temperature, where
Mn and some rare-earth sub-lattices are antiferromagneticallycoupled. Previously, negative magnetization in CaMnO3 withnCorresponding author.
E-mail address: matsukawa@iwate-u.ac.jp (M. Matsukawa).

http://dx.doi.org/10.1016/j.jmmm.2014.11.068
0304-8853/& 2014 Elsevier B.V. All rights reserved.

B-site substitution had been reported [7]. Recently, we demonstrated the inﬂuence of hydrostatic pressure on magnetic andtransport properties in the lightly electron-doped manganite
CaMn1  xSbxO3 [8]. Anomalous magnetization reversals wereclearly observed for x ¼0.05 and 0.08 under ﬁeld-cooled magnetization while the application of external pressure suppressed thenegative magnetization. A theoretical work on lightly electrondoped manganite CaMnO3 predicted that spin canting in the
G-type antiferromagnetic structure is realized by electron doping
through the double exchange mechanism [9,10]. The weak FM
component observed in Ce-substituted CaMnO3 is explained wellby the spin-canting G-type AFM state with the double-exchange
hopping of electrons. For the lightly electron-doped samples ofþ3þ
5þCaMn41 
2xMnx Sbx O3, we believe that a weak ferromagnetic
trend in their MT curves is compatible with the present canted
AFM scenario. The larger Sb5 þ substitution at Mn4 þ sites introduces local lattice distortion of Mn3 þ O6 associated witheg-electron doping and then changes the orbital state ofeg-electron through the local Jahn–Teller (JT) effect. As a result, it
gives rise to considerable variation in the local spin conﬁguration
at the nearest neighbor of its Sb ion, leading to the formation of
diamagnetic clusters within its matrix. The canted AFM matrix
stabilized by light electron doping contributes to a weak ferromagnetic component. Furthermore, the diamagnetic clusters are452

T. Fujiwara et al. / Journal of Magnetism and Magnetic Materials 378 (2015) 451–456

abFig. 1. Lattice parameters and unit-cell volume as a function of Sr estimated from
X-ray diffraction patterns of (Ca1  xSrx)Mn0.95Sb0.05O3. The unit-cell volume isnearly proportional to the Sr content.

Table 1EPMA analysis of (Ca1  xSrx)Mn0.95Sb0.05O3 (x¼ 0.0, 0.15, 0.16, and 0.17).
Element
nominal composition

Ca

Sr

Mn

Sb

O

CaMn0.95Sb0.05O3(Ca0.85Sr0.15)Mn0.95Sb0.05O3(Ca0.84Sr0.16)Mn0.95Sb0.05O3(Ca0.83Sr0.17)Mn0.95Sb0.05O3

1.010
0.845
0.836
0.830

0.00
0.164
0.177
0.187

0.943
0.927
0.925
0.918

0.048
0.048
0.048
0.047

3.02
3.03
3.02
3.04

simultaneously formed by local lattice deformation because of thesubstitution of Sb with its larger ionic radius.
In this paper, we report on the dc magnetization and ac magnetic susceptibility of the lightly electron-doped manganite compound (Ca1  xSrx)Mn0.95Sb0.05O3 with a ﬁxed carrier content. Furthermore, to better understand the thermodynamic properties ofthis system, we conduct speciﬁc heat measurements for thesecompounds over a wide range of temperatures.

2. Experiment
Polycrystalline samples of (Ca1  xSrx)Mn0.95Sb0.05O3 (x¼ 0.0,
0.05, 0.10, 0.15, 0.16, 0.17, and 0.2) were prepared using a solidstate reaction method. Here, Sr substitution at Ca sites causes aﬁxed carrier content according to the chemical formula4+
3++(Ca12+− xSr 2x+)Mn0.9
Mn0.05
Sb50.05
O23−. The stoichiometric mixtures ofhigh purity CaCO3, Mn3O4, Sb2O3 and SrCO3 powders were calcined in air at 1000 °C for 24 h. The products were then groundand pressed into cylindrical pellets. The pellets were ﬁnally sintered at 1400–1450 °C for 12 h. X-ray diffraction data obtainedusing Rigaku Ultima IV revealed that all the samples had an almost
single phase with an orthorhombic structure (Pnma) [11]. The
lattice parameters and unit-cell volume estimated from X-ray
diffraction patterns using the RIETAN-FP program are shown as afunction of Sr content in Fig. 1. We note the linear relationshipbetween the volume and Sr substitution at Ca sites in the present
system, as previously reported in (Gd0.08Ca1  xSrx)MnO3 [12]. X-ray
photoemission spectroscopy on the CaMn0.8Sb0.2O3 sample performed at room temperature strongly supports that the valence ofFig. 2. Temperature dependence of the ﬁeld-cooled magnetic susceptibility M/H for(Ca1  xSrx)Mn0.95Sb0.05O3 recorded at 100 Oe and 200 Oe, (a) and (b), respectively.

the Sb ion is 5 þ [13]. The values obtained after electron probe
micro analyzer (EPMA) analysis on the nominal samples withx¼ 0.0, 0.15, 0.16, and 0.17 are listed in Table 1. We note that oursamples prepared by the solid-state reaction method are close to
the nominal composition and there is quite a small difference in
the Mn content between the samples. The dc magnetization
measurement was conducted using the commercial superconducting quantum interference device magnetometers at Iwate
University and the National Institute for Materials Science (NIMS).
The sample holder is attached with a nonmagnetic stainless steel
rod provided by quantum design.
The dimensions of the sample used to measure the magnetic
properties were typically 4.0  2.0  1.0 mm3. The ac magnetic
susceptibility measurement was obtained as a function of temperature for frequencies ranging from 1 Hz to 1000 Hz under an acmagnetic ﬁeld of 5 Oe at NIMS. The speciﬁc heat measurement inthe ﬁeld-cooled mode was conducted at temperatures from 120 K
down to 2 K using the physical property measurement system(quantum design PPMS).

3. Results and discussion
In Fig. 2(a), we demonstrate the effect of Sr substitution at Ca
sites on the temperature-dependent magnetization for(Ca1 xSrx)Mn0.95Sb0.05O3(x¼0.0, 0.05, 0.1, 0.15, 0.16, 0.17, and 0.20).
For the parent sample, on lowering the temperature, the magnetization shows a large enhancement near 80 K and diamagnetic

T. Fujiwara et al. / Journal of Magnetism and Magnetic Materials 378 (2015) 451–456

453

Fig. 3. (a) Temperature dependence of the ﬁeld-cooled magnetic susceptibility M/H for (Ca1  xSrx)Mn0.95Sb0.05O3 recorded at 500 Oe. (b) Magnetic phase diagram of the Srsubstituted (Ca1  xSrx)Mn0.95Sb0.05O3 (x¼ 0.0, 0.05, 0.1, 0.15, 0.16, 0.17, and 0.20), where PM, c-AFM, and DM denote paramagnetic, canted antiferromagnetic, and diamagnetic
phases, respectively. TDM and TN represent the diamagnetic and canted antiferromagnetic transition temperatures, respectively.

a

d

b

e

c

f

Fig. 4. Real part of the ac magnetic susceptibility for (Ca1  xSrx)Mn0.95Sb0.05O3 as a function of temperature collected at zero dc magnetic ﬁeld for frequencies ranging from
1 Hz to 1000 Hz. (a) x ¼0.0, (b) x ¼0.05, (c) x¼ 0.10, (d) x ¼0.15, (e) x ¼0.17, and (f) x¼ 0.20. The amplitude of the ac magnetic ﬁeld Hac was set to 5 Oe. The arrows point in the
direction of increasing frequency.

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T. Fujiwara et al. / Journal of Magnetism and Magnetic Materials 378 (2015) 451–456

a

d

b

e

c

f

Fig. 5. Imaginary part of the ac magnetic susceptibility of (Ca1  xSrx)Mn0.95Sb0.05O3 as a function of temperature collected at zero dc magnetic ﬁeld for frequencies ranging
from 1 Hz to 1000 Hz. (a) x ¼0.0, (b) x ¼0.05, (c) x ¼0.10, (d) x¼ 0.15, (e) x ¼ 0.17, and (f) x ¼0.20. The amplitude of the ac magnetic ﬁeld Hac was set to 5 Oe. The arrows point in
the direction of increasing frequency.

behavior is observed at lower temperatures. The Sr substitution
at Ca sites causes a strong suppression in the magnetic peaklocated around 80 K from 8 × 10−3 emu/g at x¼0.0 through5 × 10−3 emu/g at x¼0.05 down to 2 × 10−3 emu/g at x¼0.10. On
the other hand, a negative value of the magnetization at 20 K is not
signiﬁcantly changed in the case of Sr content up to x¼0.10. The
effect of Sr substitution on the crystal structures of perovskite
manganites (Ca1  xSrx)MnO3 and (Gd0.08Ca1 xSrx)MnO3 causes a
suppression of the Mn–O–Mn buckling [14,12]. It is revealed that inlightly electron doped manganites [12], the unit-cell volume increases linearly with increasing Sr substitution at Ca sites, which is
caused not by the expansion of the Mn–O bond length but by the
suppression of the tilting of the MnO6 octahedra. Furthermore, ourlattice data exhibit a linear relationship between volume and Srcontent as shown in Fig. 1(a). In the case of low Sr content, the Srions are distributed within the canted AFM matrix, contributing to a
weak ferromagnetic component. If the Sr substitution at Ca sites
gives rise to a suppression of the buckling of Mn–O–Mn bonds, it
results in the suppression of the magnetic peak. A further increasein Sr ion contents above 10% affects diamagnetic clusters occupying
small regions around the Sb sites, and ﬁnally, the addition of Sr
beyond 15% suppresses the diamagnetic magnetization. As mentioned previously in the Introduction, for the lightly electron doped

4+3++
sample of CaMn0.95
Mn0.05
Sb50.05
O3, the larger Sb5 þ substitution at
Mn4 þ sites introduces eg-electron and causes local lattice distortionof Mn3 þ O6 at the nearest neighbor of its Sb ion. The resultant local
lattice deformation changes the orbital state of eg-electron throughthe local JT effect. As a result, it gives rise to considerable variationin the local spin conﬁguration at the nearest neighbor of its Sb ion,
leading to the formation of diamagnetic clusters within its matrix.
The diamagnetic clusters are formed by local lattice deformation
because of the substitution of Sb with its larger ionic radius.
We suspect that the tilting of the Mn3 þ O6 octahedron near the
Sb ion is relaxed by the higher Sr substitution, resulting in a decrease in the negative component. One interesting feature is thatthe disappearance of negative magnetization accompanies a collapse of the clear peak in the magnetization curve. In a similar way
to the case of 100 Oe, the magnetization curves measured at
200 Oe for Sr¼ 0%, 5% and 10% show magnetization reversal andtheir lower temperature values reach similar negative values although their peaks are quite different between the samples. For
comparison, in Fig. 3(a), we present the magnetization data for(Ca1  xSrx)Mn0.95Sb0.05O3 measured at 500 Oe. For a relatively
higher ﬁeld, a diamagnetic state is suppressed but the phenomenon of magnetization reversal survives for Sr¼ 5% and 10%. At arelatively high ﬁeld of 500 Oe, the low-temperature magnetization
curve of the Sr10% sample retains a more negative value than the

T. Fujiwara et al. / Journal of Magnetism and Magnetic Materials 378 (2015) 451–456

a

b

c

Fig. 6. C/T vs. T for (Ca1  xSr x)Mn0.95Sb0.05O3 measured under a ﬁeld cooling of
100 Oe. (a) x ¼ 0, (b) x ¼0.1 and (c) x¼ 0.15 (inset of (c) x ¼0.17). The solid curves
denote the lattice contribution estimated from a polynomial approximation. The
insets of (a) and (b) represent the magnetic heat jump ΔC and the magnetic entropy Smag associated with the magnetic transition, respectively.

data of the lower Sr samples. This ﬁnding indicates that the Sr ion
acts as the pinning center of diamagnetic clusters, preventing therearrangement of their spins in the direction of applied ﬁeld. The
higher Sr substitution at Ca sites suppresses Mn–O–Mn buckling,deleting magnetization reversal, while the lower Sr substitution
maintains a diamagnetic state at low temperatures. Furthermore,magnetization reversal of all the samples studied is completely
suppressed when the applied ﬁeld exceeds a ﬁeld of 1000 Oe (notshown here). In terms of the total magnetic energy, we expectthat the system is stable to align its magnetic moment
along the applied magnetic ﬁeld when the applied ﬁeld reaches amoderately higher value. The magnetic phase diagram of(Ca1  xSrx)Mn0.95Sb0.05O3 established is summarized in Fig. 3(b).
Next, we conducted ac magnetic susceptibility measurementsfor (Ca1  xSrx)Mn0.95Sb0.05O3 (x¼ 0.0, 0.05, 0.1, 0.15, 0.17, and 0.20),
to clarify the dynamical effect on the diamagnetic state. The
temperature dependence of the real and imaginary parts, χ′ andχ″, is registered at zero dc magnetic ﬁeld with increasing frequency f ranging from 1 Hz to 1000 Hz, as shown in Figs. 4 and 5.
For the parent Sr0% sample, a sharp peak in the ac magnetic
susceptibility is observed around 100 K, which is in good agreement with the dc magnetic measurement [8]. Upon increasing f, asecond peak of χ′ and χ″ located at 90 K shifts slightly towards

455

lower temperatures. For lower values of Sr doping up to 10%, theamplitude of χ″ shows a strong decay with increasing frequency.
When the Sr content exceeds 15%, the frequency dependence of
the ac magnetic susceptibility almost disappears. The frequency
dependence of the ac magnetization shows the signature of a spinglass-like character; however, a substantial decrease in the peak inχ″ with increasing f is in contrast to the behavior of a conventional
spin-glass system [16], as previously indicated in the phase-separated Pr0.7Ca0.3MnO3 [17]. The ac magnetic susceptibility measurement of the lower Sr substituted samples suggests the existence of magnetic frustration between diamagnetic clusters andthe canted antiferromagnetic matrix. On the other hand, for the
Sr17% and Sr20% samples, the lack of frequency dependence of theac magnetization is related to their weak ferromagnetic behavior
under dc magnetization.
To further understand the thermodynamic properties of this system, we performed speciﬁc heat measurements over a wide range oftemperatures under a ﬁeld cooling of 100 Oe. Fig. 6 shows the speciﬁcheat as a function of temperature from 2 K to 120 K for(Ca1 xSrx)Mn0.95Sb0.05O3 (x¼ 0, 0.1, 0.15, and 0.17). Here, the latticecontribution Cl is estimated roughly from a polynomial approximation
as previously reported in manganites [15]. ΔC represents the maximum value of the magnetic contribution C − Cl and the magnetic
entropy is estimated by integrating (C − Cl )/T with respect to T. We
observe clear anomalies associated with a magnetic transition near TN
but no visible changes in the speciﬁc heat data at temperatures where
the diamagnetic behaviors appear. Our thermodynamic data strongly
support that the temperature-dependent magnetization reversal accompanies no long-range magnetic ordering. It is expected from theseﬁndings that short-range magnetic clusters around Sb ions have aclose relationship with the appearance of the observed weak diamagnetism. For the Sr-substituted samples, both the magnetic heatjump and the magnetic entropy do not change signiﬁcantly. In particular, we do not ﬁnd distinct differences in thermodynamic properties
between the diamagnetic Sr15% sample and the weakly ferromagnetic
Sr17% sample.

4. Summary
We have demonstrated the magnetic and thermodynamic
properties of the lightly electron-doped manganite compound(Ca1  xSrx)Mn0.95Sb0.05O3 (x¼ 0.0, 0.05, 0.1, 0.15, 0.16, 0.17, and0.20) with a ﬁxed carrier content, to examine the effect of chemical pressure on magnetization reversal of this system. In theinvestigated system, there exits a linear relationship between thevolume and the fraction of Sr substitution at Ca sites. In a weakmagnetic-ﬁeld-cooled measurement, the temperature-dependent
negative magnetization appears for x ≤ 15% , changing to positive
values for x > 15% . We checked using EPMA analysis that oursamples are close to the nominal composition and there is quite asmall difference in the Mn content between the samples. We established a magnetic phase diagram as a function of Sr concentration under a ﬁeld cooling of 100 Oe, 200 Oe and, 500 Oe. The
lower Sr substitution maintains a diamagnetic state at low temperatures but the higher Sr substitution at Ca sites suppresses Mn–
O–Mn buckling, deleting magnetization reversal.
Next, we conducted ac magnetic susceptibility measurements
for this system, to clarify the dynamical effect on the diamagnetic
state. The ac magnetic data indicate the existence of magnetic
frustration accompanied by the progressive formation of diamagnetic clusters within the AFM matrix under decreasing temperature. For the Sr17% and Sr20% samples, the lack of frequency
dependence of the ac magnetization is related to their weakly
ferromagnetic properties under dc magnetization. To further
understand the thermodynamic properties of this system, we456

T. Fujiwara et al. / Journal of Magnetism and Magnetic Materials 378 (2015) 451–456

performed speciﬁc heat measurements over a wide range oftemperatures under a ﬁeld cooling of 100 Oe. However, the speciﬁc heat data do not reveal any magnetic phase transition associated with the temperature-dependent magnetization reversal.

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