Journal of Magnetism and Magnetic Materials 272–276 (2004) e1407–e1409

ARTICLE IN PRESS

*Corresp

91-372-062

E-mail

(J. S!anchez

0304-8853/

doi:10.1016

Improving room temperature magnetoresistance in derivativesof ferrimagnetic CaCu3Mn4O12 perovskite

J. S!anchez-Ben!ıtez*, J.A. Alonso, A. de Andr!es, M.J. Mart!ınez-Lope, M.T. Casais,J.L. Mart!ınez

Instituto de Ciencia de Materiales de Madrid, CSIC. Cantoblanco, E-28049 Madrid, Spain

Abstract

Two series of derivatives of the complex CaCu3Mn4O12 perovskite have been prepared under moderate pressure

conditions (20 kbar), and characterized by neutron powder diffraction, magnetic and magnetotransport measurements.

In a susbtituted series with formula LnCu3Mn4O12 (Ln=rare earths) we observe a significant increment of TC and a

level of low field magnetoresistance at RT that compete with the best values described for the state-of-the art perovskite

oxide systems.

r 2004 Elsevier B.V. All rights reserved.

PACS: 75.47.�m

Keywords: Magnetoresistance; Ferrimagnetic oxide; Neutron diffraction; Magnetic structure

Half metallic ferromagnets have focused a lot of

attention as a source of magnetoresistive materials.

Recently, the complex perovskite CaCu3Mn4O12 [1] has

attracted the attention of the CMR community: it is

semiconducting, orders ferromagnetically at 355K, and

shows, at 300K, low-field magnetoresistance (MR)

values higher than hole-doped manganese perovskites

ones. The crystal structure of CaCu3Mn4O12 [2] has the

rare feature of containing Cu2+ (or other Jahn–Teller

transition metal cations, such as Mn3+) at the A

positions of the ABO3 perovskite. This Jahn–Teller

cation and Ca2+ are 1:3 ordered in a 2a0� 2a0� 2a0cubic cell of Im%3 symmetry (a0: unit cell of the

aristotype). This material, and other compounds of the

A0A3B4O12 family, have been prepared under high

pressure (70 kbar), necessary to stabilize the small A

cations in the perovskite.

onding author. Tel.: +34-91-334-9000; fax: +34-

3.

address: javier.sanchez@icmm.csic.es

-Ben!ıtez).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

/j.jmmm.2003.12.704

Recently, we have been able to synthesize some new

derivatives of CaCu3Mn4O12 at moderate pressures of

20 kbar, starting from very reactive precursors obtained

by wet-chemistry procedures, in the presence of KClO4

as oxidizing agent. These materials have been fully

characterized by neutron powder diffraction (NPD),

magnetic and magnetotransport measurements.

With the aim of improving the magnetotransport

properties of this promising system, we have prepared

and studied two series of compounds: (i) Cu2+ has been

replaced by Mn3+, in the series CaCu3�xMn4+xO12

(x ¼ 0:5; 1, 2, 3) and (ii) Ca2+ has been replaced by a

rare earth ion in the series LnCu3Mn4O12 (Ln= La3+,

Nd3+, Tb4+, Ce4+ and Th4+).

Both series present Mn valence mixing at the B

positions of the perovskite, induced by the introduction

of Mn3+ or Ln3+/4+ replacing divalent cations at the A

sublattice. The crucial difference of both series is that in

the last one the Cu sublattice is undisturbed.

Fig. 1 shows the magnetization vs. temperature of

some selected samples. As Mn3+ is introduced in the

first series, the Curie temperature (TC) shows a steep

decrease. Most interestingly, TC increases remarkably in

d.

ARTICLE IN PRESS

0 50 100 150 200 250 300 350 400

0

1

2

3

4

5

0 1 2 3 4 50

3

6

9

12

10.2 µB/f.u.

T = 5 K

CaCu2.5Mn4.5O12

M(µ

B/f.

u.)

Magnetic Field (T)

CaCu2Mn5O12

CaCuMn6O12

CaCu2.5Mn4.5O12

LaCu3Mn4O12

TbCu3Mn4O12

M(µ

B/f.

u.)

Temperature (K)

Fig. 1. Temperature dependence of the magnetization for some

selected samples. The inset shows the magnetization vs.

magnetic field plot at 5K of CaCu2.5Mn4.5O12.

0.0 0.5 1.00.0

0.5

1.0

1.5

2.0

2.5

3.0

~ 1 0 0 º

B site A site

M n 4 + M n 3 +

M n 4 + M n 3 + C u 2 + M n 3 +

C u 2 +

CeCu 3Mn 4

O 12NdCu3M

n4O 12

Ca 0.5La 0.5

Cu 3Mn 4

O 12

CaCu2Mn5O12

CaCu 2.5Mn 4.5

O 12

LaCu3Mn4O

12

- M

R %

Magnetic Field (T)

Fig. 2. The left panel shows the arrangement of magnetic

moments in the CaCu3�xMn4+xO12 (up) and LnCu3Mn4O12

(down) series. The right panel shows the magnetoresistance

curves at RT for some selected samples of both series.

J. S !anchez-Ben!ıtez et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1407–e1409e1408

the second series up to 395K for TbCu3Mn4O12. All the

samples, excepting CaMn7O12 (AF), show a magnetiza-

tion characteristic of a ferromagnet (illustrated at the

inset of Fig. 1). In the first series, the saturation

magnetizations are significantly higher than those

expected in a pure ferrimagnetic system. We have

developed a magnetic model for the complex magnetic

couplings of these compounds from the analysis of NPD

and magnetization data.

NPD patterns were collected at room temperature and

2K at the high-resolution D2B neutron diffractometer

of ILL-Grenoble. A wavelength of 1.594 (A was selected

from a Ge monochromator. Both crystallographic and

magnetic structures were refined from NPD by the

Rietveld method.

The left panel of Fig. 2 depicts a schematic micro-

scopic model on the relative arrangement of the

magnetic moments in the Mn and Cu sublattices of

both series. This model implies that magnetic moments

of Mn3+ in B position are oriented parallel to those of

Mn4+ of the same site. However, magnetic moments of

Mn3+ at Cu positions are not collinear to those of Cu,

but almost perpendicular.

As a result of this configuration, in the Ca-

Cu3�xMn4+xO12 series, the interactions between Mn

an Cu planes are disturbed because of the introduced

magnetic disorder. By contrast, in LnCu3Mn4O12, where

the Cu sublattice is not disturbed, the magnetic

configuration is purely ferrimagnetic and a remarkable

increase of TC is observed.

The CaCu3Mn4O12 pure sample displays a semicon-

ducting behavior in all the temperature range (0–400K)

(rRT ¼ 1800O cm), while a metallic behavior is observed

for most of the substituted samples at low temperature,

with very low resistivity values (0.05–0.5O cm). For

NdCu3Mn4O12 and TbCu3Mn4O12 we find a metallic

behavior up to 400K. This fact can be explained

through the band structure diagram proposed by Weth

and Picket [3], where Mn eg levels are involved in the

conduction band. When Mn3+ is introduced, the

compound is electron doped and the conduction band

is progressively occupied leading to a metallic ground

state. This also causes the strong decrease of several

orders of magnitude in the resistivity. In both series, a

transition in the electrical behavior is induced (metal–

insulator transition) around 180–300K that can be due

to the competition of different electronic transport

mechanisms not determined yet.

The right panel of Fig. 2 shows the magnetoresistance

(MR ¼ 100� ðrH � r0Þ=r0) curves at RT for some

selected samples. The most interesting feature of these

curves is the strong component of low-field MR

(LFMR) at room temperature.

MR values of LnCu3Mn4O12 series are higher than

those of CaCu3�xMn4+xO12 (right panel of Fig. 2).

LaCu3Mn4O12 reaches a MR value higher than 1% at

0.1 T and RT. The improved LFMR in the lanthanide

series is due to its direct relationship with the carriers

polarization degree. In the first series, the magnetic

moments of Mn3+ at Cu positions are set in an almost

perpendicular direction, thus the carriers polarization is

reduced, while this is not the case for the lanthanide-

substituted series. Moreover, the increase of TC in Ln

compounds favors the degree of polarization of the

carriers at RT. On the other hand, we observe the MR at

H ¼ 9T increases monotonically with decreasing tem-

perature and no anomaly is found around TC; indicatingthat the valence mixing does not seem to couple the

ARTICLE IN PRESSJ. S !anchez-Ben!ıtez et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1407–e1409 e1409

electronic transport to the magnetic order, as it occurs in

manganites.

We conclude that in the lanthanide-substituted series

we obtain both a substantial increment of TC and a

better low field MR, via an electron doping effect that

maintains undisturbed the Cu sublattice. The obtained

TC’s and the levels of LFMR compete with the best

values described for the state-of-the art perovskite oxide

systems.

We thank the financial support of CICyT to the

projects MAT 2003-01880, MAT 2001-0539 and CAM

07N/0080/2002 and we are grateful to ILL for making

all facilities available.

References

[1] Z. Zeng, M. Greenblatt, M.A. Subramanian, M. Croft,

Phys. Rev. Lett. 82 (1999) 3164.

[2] J. Chenavas, J.C. Joubert, M. Marezio, B. Bochu, J. Sol.

Stat. Chem. 14 (1975) 25.

[3] R. Weht, W.E. Pickett, Phys. Rev. B 65 (2001) 14415.