Charge localization, magnetic order, structural behavior, and spin dynamics
of „La-Tb…
2/3
Ca
1/3
MnO
3
manganese perovskites probed by neutron diffraction
and muon spin relaxation
J. M. De Teresa
Departamento de Fı ´sica de la Materia Condensada and Instituto de Ciencia de Materiales de Arago ´n, Universidad de Zaragoza
and Consejo Superior de Investigaciones Cientı ´ficas, 50009, Zaragoza, Spain
C. Ritter
Institut Laue-Langevin, Boı ˆte Postale 156, 38042 Grenoble Ce ´dex 9, France
M. R. Ibarra and P. A. Algarabel
Departamento de Fı ´sica de la Materia Condensada and Instituto de Ciencia de Materiales de Arago ´n, Universidad de Zaragoza
and Consejo Superior de Investigaciones Cientı ´ficas, 50009, Zaragoza, Spain
J. L. Garcı
´
a-Mun
˜
oz
Institut de Cie `ncia de Materials de Barcelona, CSIC, Campus U.A.B., ES-08193, Bellaterra, Spain
J. Blasco, J. Garcı
´
a, and C. Marquina
Departamento de Fı ´sica de la Materia Condensada and Instituto de Ciencia de Materiales de Arago ´n, Universidad de Zaragoza
and Consejo Superior de Investigaciones Cientı ´ficas, 50009, Zaragoza, Spain
Received 10 February 1997
Two microscopic techniques, muon spin relaxation and neutron diffraction, including small-angle neutron
scattering SANS, have been used to probe into the relevant mechanisms that determine the macroscopic
behavior in the series (La
1-x
Tb
x
)
2/3
Ca
1/3
MnO
3
. The magnetic ground state at low temperatures evolves from
ferromagnetic for x 0.25 to spin glass for 0.33x 0.75 and finally antiferromagnetic for x =1. Spin-glass
regions were observed for x 0.25. We propose the existence of two different volume states associated with
the metalliclike ferromagnetic state and the semiconductorlike paramagnetic, spin-glass, or antiferromagnetic
states, respectively. SANS experiments reveal the existence of magnetic clusters for x 0.33. The magnetic
correlation length diverges at T
c
for x 0.25 while magnetic clusters of around 18 Å stabilize in the x
=0.33 compound at low temperatures. Muon spin relaxation experiments confirm the absence of microscopic
local magnetic order for the x =0.33 compound and give evidence for the existence of static local fields
randomly oriented below 44 K, bringing about a glassy magnetic state below that temperature. The remark-
able electrical behavior in this series has been correlated with the microscopic properties.
S0163-18299700730-3
I. INTRODUCTION
Manganese perovskites are currently compounds of great
interest for two reasons. From a technological point of view,
their application in devices based on compounds with giant
magnetoresistive GMR properties offers remarkable
possibilities.
1
From a basic-research point of view, there are
many intriguing phenomena which still lack a quantitative
theoretical explanation. In this paper we investigate
the magnetic and structural properties of the series
(La
1 -x
Tb
x
)
2/3
Ca
1/3
MnO
3
using two microscopic techniques,
neutron-scattering diffraction and muon spin relaxation. As
we will see, a detailed study of this series provides informa-
tion on most of the relevant mechanisms in manganese per-
ovskites. We will correlate these results with the macro-
scopic measurements, giving a comprehensive picture of
these systems.
LaMnO
3
and CaMnO
3
are antiferromagnetic compounds
due to the superexchange interaction. However, the
(La
1 -x
Ca
x
)MnO
3
series shows a complex magnetic and
transport phase diagram.
2–4
Around the composition
La
2/3
Ca
1/3
MnO
3
the magnetoresistance shows a maximum
5
and consequently this compound has been thoroughly
investigated.
6–10
Due to the presence of La
3+
and Ca
2+
ions,
there is coexistence of Mn
3+
and Mn
4+
ions in the same ratio
in order to preserve the charge equilibrium. It brings about
the appearance of a ferromagnetic interaction: the double-
exchange mechanism.
11
It occurs because Mn
4+
ions intro-
duce holes in the external electronic levels of the Mn ions
and the external electrons with symmetry e
g
can hop from
Mn to Mn through the oxygen orbitals. In this process the
e
g
electrons retain the spin direction tending thereby to align
the spins of neighboring Mn ions due to a strong Hund’s
coupling.
Together with these two competing magnetic
interactions,
12
there are additional mechanisms that can play
an important role in determining the ground state of the sys-
tem. In some compounds with Mn
4+
/Mn
3+
concentrations
around 1:1, below a charge-ordering temperature, the e
g
electrons become attached at fixed lattice sites resulting in a
PHYSICAL REVIEW B 1 AUGUST 1997-II VOLUME 56, NUMBER 6
56 0163-1829/97/566/33178/$10.00 3317 © 1997 The American Physical Society