ARTICLE IN PRESS

Physica B 359–361 (2005) 1294–1296
www.elsevier.com/locate/physb

Magic d-electron number of doped ions for occurrence offerromagnetic metal phase in Pr0.65Ca0.35Mn0.96A0.04O3
Manabu Ikebea,, Hiroyuki Fujishiroa, Tatsuya Kumagaia,
Atsushi Fujiwaraa, Yoshihisa Obiba

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japanb
Institute of Material Research, Tohoku University, Sendai 980-8577, Japan

Abstract
The electrical resistivity r; Seebeck coefﬁcient S and magnetization M have been measured for polycrystallinePr0.65Ca0.35Mn0.96A0.04O3-doped with a vast variety of elements A. The ferromagnetic metal (FM) phase has beencaused by doping only by Cr, Co, Ni, Ru and Ir. A necessary condition for the electronic conﬁguration of the dopants
has been clariﬁed to materialize the FM phase in this system.
r 2005 Elsevier B.V. All rights reserved.
PACS: 75.30.Hx; 72.15.Eb; 72.10.Di
Keywords: Manganites; Ferromagnetic metal; Mn-site substitution; d-electron number

1. Introduction
The perovskite-type manganites, RE1XAEXMnO3 (RE ¼ La, Pr, etc., AE ¼ Ba, Sr, Ca, etc.),
exhibit intriguing physical properties, which originate from the competition between superexchange(SE), double exchange (DE) and charge order(CO)–orbital order (OO). In Pr1XCaXMnO3 withrather small A-site ion radii of Pr3+ and Ca2+, theCorresponding

author. Tel.: +81 19 621 6362;+81 19 621 6363.
E-mail address: ikebe@iwate-u.ac.jp (M. Ikebe).

fax:

ferromagnetic-metal (FM) phase does not actualize owing to the narrow one electron bandwidth.
The phase boundary between ferromagnetic-insulator (FI) and CO–OO phases is located at X 0:30; the CO–OO phase being stable for X X0:35:The hidden FM phase shows up for X X0:35 under
an applied magnetic ﬁeld or with the Mn-site
doping by suitable cations. In order to realize theFM phase, it is necessary for the dopants to
efﬁciently destroy CO–OO without seriously damaging the DE mechanism. We have studied the4%-doping effect of various cation dopants inPr0.65Ca0.35MnO3 (PCMO). This composition was0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.01.361

ARTICLE IN PRESS
M. Ikebe et al. / Physica B 359– 361 (2005) 1294–1296

1295

selected because the free energies of the CO-, FMand FI-phase are nearly degenerate.
Pr0.65Ca0.35Mn0.96A0.04O3 (A-PCMO) polycrystals were prepared by a solid-state reaction method(A ¼ Mg, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Zr,Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Ta, Ir). We
performed detailed Rietveld analyses for thepowder X-ray diffraction to determine the lattice
parameter change due to the doping.

2. Results and discussion
Fig. 1(a) shows the electrical resistivity r(T) ofthe Cr-, Co-, Ni-, Ru- and Ir-PCMO as a functionof temperature T. The spontaneous CO–FM phasetransition is exhibited for these dopants. Otherdopants have proved to cause no insulator–metal(I–M) transition.
Fig. 1(b) presents the Seebeck coefﬁcient S vs. T
for the respective samples. S(T) curves rapidly
drop toward S ¼ 0 level around the I–M transition
temperature TIM. The rapid reduction of S(T)conﬁrms the bulk nature of the I–M transition.
For all the other cation doping, S(T) diverges withdecreasing T.
Fig. 1(c) shows the magnetization M(T) underthe ﬁeld of 0.5 T. The M(T) values of Cr-, Co-, Niand Ru-PCMO reach about 3mB/Mn at low T,
while M(T) of Ir-PCMO remains somewhat
smaller (2.4mB/Mn).
Let us discuss the mechanism of the appearanceof the FM phase in the present PCMO system.
Hebert et al. [1] pointed out the importance ofimpurity eg level at the impurity site: hopping
electrons must take advantage of the extended
empty eg orbital of the dopant to preserve the DE
mechanism. In this paper, we propose a necessary
condition, for the FM phase occurrence: theavailable eg orbit must be energetically lowest.
Based on the Rietveld analyses, we havedetermined the lattice parameter variation on thedoping amount [2]. This process enabled us to ﬁx
the valence state of the majority of the cation
dopants. The key result is that Co and Ni are in
3+ valency and are in low spin (LS) conﬁguration(see Fig. 2). In Fig. 2, we can see that among the 3dtransition element, the Co3+(LS), Ni3+(LS) andFig. 1. (a) r(T), (b) S(T) and (c) M(T) as functions oftemperature T. TCO denotes the CO transition temperature inPr0.65Ca0.35MnO3.

high-spin Cr3+(HS) satisfy the necessary condition. The valencies of other 3d transition elements
are Ti4+, V3+ and Fe3+(HS), which are notcompatible with the condition. The 5d transition
element Ir3+ is in isomorphous d electron conﬁguration as Co3+(LS) and exhibits the I–M transition.
As is well known, Ru is a strong dopant to stagethe FM phase in manganites [3]. The Rietveld

ARTICLE IN PRESS
M. Ikebe et al. / Physica B 359– 361 (2005) 1294–1296

1296

@eg

()

(())

t 2g
Cr (LS)Co (HS)Ni (HS)

@eg

t 2g

( )
Cr (LS)()Co (HS)( )Ni (HS)

Fig. 2. Spin conﬁguration of Cr3+, Co3+ and Ni3+. The spinsin the parenthesis indicate the level to be occupied by thetransient hopping electron. The upper row is compatible withthe ferromagnetic metal phase formation.

analyses have supported the Ru4+ valency with
4d4 electron conﬁguration, which is isomorphouswith the parent Mn3+ conﬁguration. It is generally
believed that the 4d and 5d transition metalelements tend to take the LS conﬁguration incrystals. Our necessary condition demands thatRh4+ should be in the HS conﬁguration or, atleast, the HS conﬁguration should be almost

degenerate energetically with the LS conﬁgurationin Rh-PCMO.
The lattice parameter analyses suggest Cu3+
valency, which is compatible with the condition,
but the I–M transition does not take place. In Fig.
1(c), M(T) curve of Cu–PCMO is quite small andalmost falls on the pristine PCMO. The CO–OO ishardly damaged by the Cu3+ dopants, which may
be the origin of the absence of the FM phase in
Cu–PCMO.
In summary, we determined the spin conﬁgurations of Co and Ni dopant as the low spin t2g andt62ge1g, respectively, from the analyses on the lattice
parameter change upon doping in Pr0.65Ca0.35MnO3. The result leads us to a rigorous necessary
condition for the occurrence of the spontaneous
ferromagnetic-metal phase, i.e., the dopant musthave empty eg-orbital energetically lowest.
References[1] S. Hebert, et al., Solid State Commun. 121 (2002) 229.
[2] To be reported elsewhere.
[3] C. Martin, et al., Phys. Rev. B 63 (2001) 174402.