Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 89-92, 2007

Local Magnetic Field of a Perovskite Manganite La0.7Ca0.3MnO3W. Sato,*,a N. Ochi,a A. Taniguchi,b T. Terai,c T. Kakeshita,c A. Shinohara,a and Y. Ohkubob
aGraduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
cGraduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japanb

Received: January 13, 2007; In Final Form: January 13, 2007
The time-differential perturbed angular correlation method with the 140 Ce probe was applied to a microscopic
investigation on a perovskite manganite, La0.7Ca0.3MnO3. A drastic change was observed in the time evolution of
the directional anisotropies around the Curie temperature (Tc ~ 245 K), reflecting the paramagnetic-ferromagnetic
phase transition. The internal magnetic field at the probe nucleus stabilized at the A site was estimated to be Bint =6.9 ± 0.3 T at 240 K, whereas no appreciable magnetic field was observed at 260 K. From a comparative study
using two different probe-introduction methods, in addition, we found that ascendant ions of the probe, 140Ba and140La, can be located at the A site by heat treatment.

1. Introduction
Perovskite manganese oxides (AMnO3) are known to exhibit
the effect of colossal magnetoresistance (CMR), a phenomenon
that electrical resistivity undergoes a drastic change as large asfive to six orders of magnitude by an applied magnetic field.1
Because of this unique physical property expected for wide
application, much attention has been given to these oxides since
the first discovery of this phenomenon.2 In general, the CMR
mechanism is associated with the notion of double exchangeinteraction arising from the Mn-Mn electronic hopping. 3
However, discussion on the mechanism is, in most cases, basedon macroscopic quantities such as resistivity and magnetization, and there still remain discrepancies between those experimental results and theory.4 For better understanding of themechanism, it is necessary to provide insight into the intriguing property by different approaches as well.
From this point of view, our interest was directed to theatomic-level observation of what is happening at lattice sites ofmanganite compounds by means of the time-differential perturbed angular correlation (TDPAC) technique. The TDPAC
method is a nuclear spectroscopy that provides microscopic
information on local fields in matter through hyperfine interactions between probe nuclei and the surrounding spins andcharge distribution.5 By observing the time-evolving directional anisotropy of cascade γ rays, we can obtain direct information on the extranuclear field. With recent development of
the radiation detection technique, this method has been widely
applied to researches of condensed matter physics as well as
nuclear physics.6 Because of high sensitivity of this spectroscopy, we can expect to obtain unique information that is notavailable from the bulk measurements.
In the present work, we chose a perovskite La0.7Ca0.3MnO3 as
the sample, which exhibits the CMR effect by the paramagnetic-ferromagnetic phase transition (Tc ~ 245 K). It is reported
that the CMR effect is strongly related with the ordering of theA-site ions in some type of perovskite manganites;7,8 in thatsense, it is important to examine the hyperfine field acting at
the A-site ions. We thus employed the 140 Ce as the probe forthe present TDPAC measurements because their ascendants
140
La and 140Ba, which are initially introduced in the sample,*Corresponding author. E-mail: wsato@chem.sci.osaka-u.ac.jp.
Fax: +81-6-6850-5417.

are expected to substitute for the intrinsic A-site ions, La3+ andCa 2+. In the present work, two different probe introduction
methods were tested for the evaluation of the influences of
those methods on the lattice structure. Preliminary results arepresented here on the emergence of a hyperfine magnetic fieldat their residence sites by showing the drastic change of thetime evolutions of the directional anisotropies at temperatures
above and below Tc.
2. Experiments
2.1. Sample preparation and characterization. Powder
La0.7Ca0.3MnO3 was synthesized by a conventional solid-state
reaction in air.9,10 Stoichiometric amounts of starting powdermaterials (MnO2, La2O3, and CaCO3) were thoroughly mixed
in a mortar. The powder mixture was calcined on a platinum
plate at 1273 K for 12 h. After cooling down to room temperature, the sample was again ground to uniformity, and it wasthen pressed into a disc. Finally, the disc was sintered on theplatinum plate at 1473 K for 96 h. A powder X-ray diffraction
and magnetization measurements showed that the sample wasof a single phase, reproducing reports in literature on analogous perovskites.9,112.2. Probe introduction and TDPAC measurements. We
adopted neutron irradiation as a method of the production of
the 140 Ce probe. Powder La0.7Ca0.3MnO3 of 20 mg was sealed
in a quartz tube evacuated. Neutron irradiation was performed
for the sealed sample in a pneumatic tube at Kyoto UniversityReactor (KUR) with a thermal neutron flux of 2.75 × 1013 cm–2s –1 for 1 min. The parent of the probe was produced by the139
La(n,γ)140La reaction.
An ion-implantation method was tried as another way for theproduction and introduction of the 140Ce probe, a descendant ofa fission product 140Cs. A disk of polycrystalline La0.7Ca0.3MnO3with a size of approximately 6 mmφ × 1 mm was attached tothe ion beam collector equipped at the isotope separator on-line
installed at KUR (KUR-ISOL). The ion beam of 140 Cs waskept implanted in the La0.7Ca0.3MnO3 sample at the total acceleration voltage of 100 kV. The number of the implanted 140Cs
atoms was estimated to be approximately 1.6 × 1011 in total.
Schematic diagrams of the system of KUR-ISOL appear elsewhere,12,13 and the irradiation condition is described in detail in
our previous paper.14 It was confirmed by a γ -ray spectrum of
the implanted sample that the projectile was well isolated in the© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

90SatoJ. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
140Cs63.7 s-0.2

β−-0.15140Ba−12.8 d β(a) 260 K(c) 260 K(b) 240 K(d) 240 K-0.1

140La-0.05

2412 keV

γ1 : 329 keV
t1/2 = 3.45 ns 4+2083 keV

0-0.2-0.15-0.1

γ2 : 487 keV
2+A22G22(t)−40.3 h β3+-0.05

1596 keV

00.050+140Ce

0510152025 0510152025Time / nsstableFigure 1. Simplified decay scheme of 140Ce formed in the disintegration from 140Cs.

Figure 2. TDPAC spectra of 140Ce formed in a perovskite manganite
La0.7Ca0.3MnO3 by neutron irradiation. The measurements were performed at temperatures indicated. In (a) and (b) are shown those forthe as-irradiated sample, and in (c) and (d) for the annealed one.

mass-separation process.15
After the probe introduction by each method, TDPAC measurements were performed for the “as-irradiated” and “asimplanted” samples prepared by the neutron irradiation and the140Cs implantation, respectively, at temperatures around Tc forthe probe 140 Ce on the 329–487 keV cascade γ rays with theintermediate state of I π = 4 + having a half-life of 3.45 ns. A
simplified decay scheme of 140Ce formed in the disintegrationfrom 140Cs is shown in Figure 1.16 (Note that the disintegration
starts from the parent nucleus 140La for the probe production by
the neutron irradiation.) In the present work, the directional
anisotropy of the cascade γ rays was observed at π /2- and π radian directions with a conventional four-detector system.
BaF2 scintillation detectors were adopted due to their excellent
time resolution. The as-irradiated and as-implanted samples
underwent heat treatment at 1473 K in air, and TDPAC measurements were again carried out on the same condition for theinvestigation of the annealing effect.

damaged by radiation. The spectral change evidently showsthat the lattice structure was, to some extent, recovered by theannealing. Because the spectra for the annealed sample shouldreflect the original structure of La0.7Ca0.3MnO3 better, we discuss them below rather than those for the as-irradiated sample.
There are obvious distinctions in the time evolution between
the spectra at temperatures above and below Tc, which reflects
t h e p a r a m a g n e t i c -fe r r o m a g n e t i c p h a s e t r a n s it io n .
Reproducibility of the spectra was confirmed by repeated measurements changing temperature up and down. In the paramagnetic state, it is generally considered that an electric field
gradient (EFG) arising from the ambient charge distribution is
the only factor perturbing the probe nucleus. Accordingly, theslow attenuation of the directional anisotropy observed in thespectra at 260 K should be ascribed to a weak electric quadrupole interaction with the outer lattice. In the case of an axiallysymmetric EFG, the perturbation factor is expressed for thepresent case as

3. Results and Discussion

EFG

3.1. Electromagnetic field at the La sites. Figure 2 shows
the TDPAC spectra obtained at around Tc for the sample prepared by the neutron irradiation: on the left are those for theas-irradiated sample and on the right for the annealed one. The
directional anisotropy on the ordinate, A22 G 22 (t), was deducedwith the following simple operation:
A22 G 22 (t) =2[N(π,t) – N(π /2,t)].
N(π,t) + 2N(π /2,t)

G 22 (t) =1 [331 + 10cos(3ω t) + 81cos(9ω t)
Q
Q1155+ 180cos(12ωQt) + 175cos(15ωQt) + 196cos(21ωQt)+ 126cos(24ωQt) + 56cos(36ωQt)],(2)

where ω Q represents the electric quadrupole frequency
described by(1)Here, A22 denotes the angular correlation coefficient, G 22 (t) thetime-differential perturbation factor as a function of the timeinterval, t, between the relevant cascade γ -ray emissions, andN(θ,t) the number of the coincident events observed at an angle,θ.
One can see the effect of the heat treatment for the spectra
observed at 240 K. The neutron irradiation in a reactor is considered to cause damages to the crystal structure by the socalled radiation effect and/or hot atom effect. The slightly
damped structure of the 240 K spectra for the as-irradiatedsample suggests that the surroundings of the probe nuclei wereωQ =eQVzz
.
4I(2I – 1) h(3)Here, Q (= 0.35 (7) b for the relevant case16) denotes the nuclear
quadrupole moment of the intermediate state, Vzz the principal
axis of the EFG, and I (= 4) the nuclear spin at the intermediate
state of the cascade. However, a least-squares fit with theabove perturbation function for an assumed single component
does not reproduce well the 260 K spectrum as indicated by abroken line in Figure 2(c). Initial transition to the ferromagnetic phase may have been observed at this temperature, or thespectra may consist of ensemble of multiple components withA22G22(t)Local Magnetic Field of a Perovskite Manganite La0.7Ca0.3MnO3J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

-0.2

-0.2

-0.15

-0.15

-0.1

91(a)

-0.1

-0.05

-0.05

0.05

05

1015

2025

Time / nsFigure 3. TDPAC spectra of 140Ce formed in the disintegration from140Cs implanted in a perovskite manganite La0.7Ca0.3MnO3. The measurement was performed for the as-implanted sample at 240 K.

A22G22(t)00-0.2(b)

-0.15
-0.1

widely ranged EFGs due to insufficient heat treatment. As forthe 240 K spectrum, on the other hand, an oscillatory structureappears in the time evolution of the directional anisotropy,
which can be assigned to the Larmor precession of the probe
nucleus caused by a hyperfine magnetic interaction with theinternal field in the ferromagnetic phase. When the probe
nucleus is perturbed by an internal magnetic field, the timedifferential perturbation factor is described as-0.05
00.05

G 22 (t) = 1 [1 + 2cos(ωLt) + 2cos(2ωLt)],
5IMF(4)

where ω L is the Larmor frequency caused by the relevant
hyperfine interaction. Regarding the oscillatory structure in
Figure 2(d) as a perturbation by the magnetic interaction, aleast-squares fitting was carried out with eq 4 for the principal
component. Here, Gaussian distributions were assumed for theLarmor frequency in eq 4 asIMF

G 22 (t) =Σ a exp(– 12 δ ω t )cos(ω t).
ii2Li

2 2Li(5)The relative width δ in eq 5 is defined by δ = σ/ωLi, where σ isthe width of the distribution of the Larmor frequency in eachterm. From the fit, the internal magnetic field acting at the siteof the 140 Ce nucleus was estimated to be B int = 6.9 ± 0.3 T at
240 K. It is to be noted that this is a preliminary result because
perturbation by the EFG, which would be negligibly small
compared with that by the magnetic field, was not taken into
consideration for the present fitting. The evaluation of themagnitude is now underway in comparison with reports forother manganite in the ferromagnetic region.8,17
3.2. Residence sites of 140 Cs. In Figure 3 is shown theTDPAC spectrum measured at 240 K for the sample preparedby the 140Cs implantation (as-implanted). For the samples without the heat treatment, one can see contrasting time evolutions
between Figures 2(b) and 3 in spite of the identical sample
probed by the same nuclei. It is obvious from these figures thatthe implanted Ce nucleus does not feel a distinct magnetic field
below Tc as opposed to the one produced by the neutron irradiation. A conceivable cause for the little, if any, magnetic field atthe probe site is the formation of lattice defects. Since the 140Cs
ions were kinetically introduced into the host La0.7Ca0.3MnO3,0510152025

Time / ns
Figure 4. TDPAC spectra of 140Ce formed in the disintegration from140
Cs implanted in a perovskite manganite La0.7Ca0.3MnO3. The measurements were performed for the annealed sample (a) at 260 K and(b) at 240 K.

the implantation process may have caused defects in the crystal. There are two possible types of defects in the present case:one is the irradiation damage in the crystal structure and theother the occupation of some interstitial sites by the incident
140
Cs ions. Because the Cs ions are introduced as univalent
impurities in the system, they possibly occupy some interstitialsites rather than substitutional lattice. As a result, for the asimplanted sample, the descendant 140 Ce probes would remain
at the initial sites, where total magnetic field transferred from
Mn ions is not operative. The same is true of the case for theformation of the irradiation damage; the Cs ions could not bestabilized at the A sites in the as-implanted sample, meaningthat the 140Ce probe does not feel magnetic perturbation.
TDPAC spectra for the 140Cs-implanted sample with the heat
treatment are shown in Figure 4. We can see a drastic change
in the directional anisotropy at 240 K, and the oscillating patterns seen in Figures 2(d) and 4 are similar to each other.
Because the half-life of 140Cs is as short as 63.7 s, almost all theimplanted ions should have been disintegrated to 140Ba (T1/2 =12.8 d) or 140La (T1/2 = 40.3 h) during the heat treatment. Fromthe similarity of the spectra for the annealed samples, it would
be natural to consider that the implants thermally migrates tothe A sites, where they are well stabilized. This observation
clearly suggests that 140 Ba substitutes for the A-site atoms,
likely Ca, as the same alkaline-earth homologue exhibiting asimilar chemical behavior.

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

4. Summary
We have applied the TDPAC technique with the 140Ce probe
for the investigation of a transferred magnetic field at the A
sites of a perovskite manganite La0.7Ca0.3MnO3. Two different
methods were employed for the probe introduction to the sample: one is neutron irradiation in a nuclear reactor for the139
La(n,γ)140La reaction and the other is the ion-implantation of
140
Cs, an ascendant of the probe 140Ce.
A drastic change was observed in the time evolution of thedirectional anisotropies between temperatures above and below
Tc. The change can plausibly be attributed to the paramagneticferromagnetic phase transition. A least-squares fit was performed, accordingly, for the 240 K spectrum by assuming thatthe oscillatory structure reflects a hyperfine interaction
between the probe nucleus and the magnetic field transferredfrom ambient Mn ions, resulting in Bint = 6.9 ± 0.3 T. For further information on the A site, temperature dependence of theinternal magnetic field needs to be investigated.
No distinct magnetic field was observed for the as-implanted
sample even at temperature below Tc. Because the incident ion
is Cs +, it is considered that their residence sites are not necessarily the substitutional lattice sites. It was ascertained by a
TDPAC measurement after annealing the sample that β–-disintegrated implants, 140Ba and 140La, migrates to the A site during
the heat treatment. In order to examine the effect of theimplantation, it would be interesting to employ 140 Ba and/or140
La ions as an incident beam.
Acknowledgment. We are grateful to Prof. S. Kawata for
helpful instruction in the sample evaluation. We express ourgratitude to Prof. T. Saito and Dr. Y. Yamaguchi for providing
experimental facilities for the TDPAC measurements. The
present work was accomplished as part of the Visiting
Researcher’s Program of the Kyoto University Research
Reactor Institute (KURRI), and was supported in part by agrant from the Ministry of Education, Culture, Sports, andScience of Japan.
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