1. IntroductionThe electrical resis

1. Introduction
The electrical resistivity of La3+Mn3+O3
2−–Me2+Mn4+O3
2−
changes by orders of magnitude depending on the ratio of Mn3+
toMn4+ [1]. Both LaMnO3 andMeMnO3 are antiferromagnetic.
However, the 3d electrons in (La,Me)MnO3 are ferromagnetically
ordered and the compound shows low electrical resistivity.
This magnetic interaction betweenMn3+ andMn4+ is called double
exchange interaction. More recently, this effect has attracted
strong interest due to its magnetoresistance properties and a
strong correlation between magnetization and resistivity [2,3].
There have been two approaches for this study. One method is
to dope divalent elements into site A ofMe3+Mn3+O3
2− and the
other is to dope trivalent elements into siteAofMe2+Mn4+O3
2−.
For the latter case in particular, a number of elements have been
used as dopants. Y, La, Ce, Sm, Eu, In, Sn, Sb, Pb and Bi have
been used as dopants [4–9]. The complex changes in magnetic
and crystalline structures depending on the amount of the dopant
that has also been reported [10–14]. These manganese oxide
materials with high magnetoresistance have been extended to
(La,Sr)n+1MnnO3n+1 [15]. In these studies, the valence of Mn
has been maintained with in the range of 3+ and 4+.
In the study by Yamashita et al. on (Ca1−xAx)2MnO4 (A=La
and Na), it was discovered that Ca2MnO4 doped with La and
∗ Current address: Microwave and Materials Designs Pty. Ltd., BTP Technology
and Conference Centre, Clunies Ross Court, Eight Mile Plains, Qld 4113,
Australia.
E-mail address: yamashitatoru@hotmail.com.
Na has spontaneous magnetization and that the magnetic susceptibility
of (Ca0.925Na0.075)2MnO4 became negative below
20K [16]. It may be assumed that the interaction between Mn4+
and Mn6+ in (Ca0.925Na0.075)2MnO4 causes this spontaneous
magnetization and the negative magnetic susceptibility. Since
Ca2MnO4 has two-dimensional Mn–O planes, if the distance
between the planes and/or number of the Mn–O planes are
changed, the magnetic interactions between Mn4+ and Mn6+
may be influenced. If the temperature for transition from positive
to negative magnetic susceptibility increases it may be possible
to make new devices, such as current limiters or switching
devices which operate at close to room temperature.
In the present study, the magnetic and transport properties of
(Ca,Na)MnO3, which has infinite Mn–O layers [15], are evaluated
under a range of conditions. The distance between nearest
Mn–Mn in the nearest Mn–O layers of CaMnO3 is about 4 °A for
CaMnO3. This distance is one third that for Ca2MnO4 (12A° ).