Yua a Department of Physics, Chungbuk National University, 361-763 Cheongju, Korea b Center for Materials Science, University of Science, 334 Nguyen Trai, Hanoi, Vietnam c Department of
Trang 1Physica B 371 (2006) 317–322
Spin dynamics, electrical and magnetic properties of
T.L Phana, , N.D Thob, M.H Phanc, N.D Had, N Chaub, S.C Yua
a Department of Physics, Chungbuk National University, 361-763 Cheongju, Korea
b Center for Materials Science, University of Science, 334 Nguyen Trai, Hanoi, Vietnam
c
Department of Aerospace Engineering, Bristol University, BS8 1TR, UK
d
Department of Materials Engineering, Chungnam National University, 305-764, Korea Received 21 July 2005; received in revised form 25 September 2005; accepted 21 October 2005
Abstract
Magnetization, resistivity, and electron spin resonance (ESR) measurements were carried out to investigate spin dynamics, electrical and magnetic properties of (La0.5Pr0.5)0.7Pb0.3Mn1xCuxO3 (x ¼ 0, 0.02) perovskites It was found that with Cu addition the Curie temperature decreased from 326 K for x ¼ 0 to 300 K for x ¼ 0:02 composition The x ¼ 0 sample underwent a metallic–insulator transition at 130 K, whereas the x ¼ 0:02 sample with higher resistivity exhibited an insulating behavior as a whole It was furthermore found that asymmetrical ESR signals at low temperatures became Lorentzian at high temperatures above Tmin The temperature dependence of the linewidth, DHðT Þ, at T 4Tmin fits well to the one-phonon model, DHðT Þ ¼ AT þ B The activation energy Ea, determined from the ESR intensity with respect to temperature, are 0.16 and 0.14 eV for x ¼ 0 and 0.02 compositions, respectively In terms of our experimental results, it is reasonable to conclude that the Cu addition led to a suppression of the ferromagnetism and conductivity of the parent compound (x ¼ 0)
r2005 Elsevier B.V All rights reserved
PACS: 71.30.+h; 72.15.Gd; 75.30.Kz
Keywords: Perovskite manganites; Magnetic and electrical properties; ESR
1 Introduction
Parent lanthanum-manganese oxides (the so-called
manganites), LaMnO3 and CaMnO3, with a perovskite
structure were first studied by Jonker and Santen[1] Both
these precursor compounds are an antiferromagnetic
(AFM) insulator, where the orientation of
nearest-neigh-bor spins is opposite [1–3] Recent progresses in
manu-facturing thin film materials have led to the discovery of
the colossal magnetoresistance (CMR) effect, observed
around the Curie temperature, TC, in several hole-doped
manganites of R1xA0
xMnO3 (where R ¼ La, Pr, Nd, and A0¼Ca, Sr, Ba, Pb, etc.) It has been found that
the TC value of the manganese perovskites strongly depends on the doping concentration and the effective-average radius of doped ions (A0) In order to make such a manganese perovskite possible for practical uses, its TC is often controlled to a value near room temperature
In view of existing CMR materials, a family of Ca-doped
La1xCaxMnO3 (0.2pxp0.5) manganites has attracted the widest interest in the aspects of experimental and theoretical researches [4–11], because they exhibited the largest CMR effect However, the TCvalue of this material family is about 270 K, still far away from room tempera-ture[10,11] Fortunately, two other systems with higher TC
were found to be Sr- and Pb-doped manganites of
La1x(Sr,Pb)xMnO3[11–15] In particular, La1xPbxMnO3
(0.1pxp0.5) compounds with TCaround room tempera-ture are promising candidates for room temperatempera-ture
www.elsevier.com/locate/physb
0921-4526/$ - see front matter r 2005 Elsevier B.V All rights reserved.
doi:10.1016/j.physb.2005.10.134
Corresponding author Department of Physics, University of Bristol,
Bristol BS8 1TL, UK Tel.: +44(0) 117 928 8750; fax: +82 43 274 7811.
E-mail address: ptlong2512@yahoo.com (T.L Phan).
Trang 2applications such as magnetic sensors, magnetoresistive
read heads, and magnetic refrigerators[13,16]
In accessing the physical mechanism of the CMR effect
in the doped manganites, a double-exchange (DE)
interac-tion model was proposed by Zener [17] This model
considers electronic-exchange processes between Mn3+
and Mn4+ ions via the Mn–O bond and the Mn–O–Mn
bond angle[17,18] However, Millis et al [19,20]recently
indicated that the only DE model is not sufficient enough
to elucidate the entire physical picture of the CMR effect in
the manganites and that addition into the DE model the
Jahn–Teller effect, which arises from a strong
electron-phonon coupling, is extremely important This has been
experimentally verified by several studies of Raman
scattering, neutron powder diffraction, and electron spin
resonance (ESR) [5,18] Despite a number of previous
studies [18–20], the understanding of several physical
phenomena such as phase-separation and
magneto-trans-port mechanism in the manganites still remains
controver-sial in part due to the complex nature of the problem[21]
With the hope of gaining some more insight into the
nature of the electrical transport and magnetic properties
as well as internal dynamics of such a doped manganite, in
the present work, a thorough study of the electrical and
magnetic properties of polycrystalline (La0.5Pr0.5)0.7Pb0.3
Mn1xCuxO3 (x ¼ 0, 0.02) perovskites were carried out
by means of magnetization, resistivity, and electron spin
resonance (ESR) measurements The experimental results
reveal that the partial substitution of Mn by Cu resulted
in the weakening of double-exchange ferromagnetic
interactions in the Cu-doped sample No DE process
took place between Mn3+ and Cu2+ ions The internal
dynamic properties of the samples were exposed by ESR
spectra
2 Experimental details
(La0.5Pr0.5)0.7Pb0.3Mn1xCuxO3 (x ¼ 0, 0.02)
polycrys-talline samples were prepared by conventional solid-state
reaction Powder precursors of La2O3, Pr2O3, PbO, CuO
and MnCO3with a high purity were well-mixed
stoichio-metrically and then pressed into two pellets The pellets
were pre-sintered in turn at 850 and 900 1C for 15 h, after
several intermediate grinding and pressing Finally, they
were annealed at 1000 1C for 15 h, and then slowly cooled
down to room temperature The whole processes were
carried out in the normal air condition The quality of the
final samples was checked by powder X-ray diffraction
patterns (D5005-Brucker) The temperature dependence of
the DC resistivity was measured by the four-probe
technique using a closed cycle-helium refrigerator
Zero-field-cooled (ZFC) and Zero-field-cooled (FC)
magnetiza-tions were performed on a vibrating sample
magneto-meter (VSM), DMS-880, with a maximum field value
of 1.5 T ESR measurements were carried out with a
JEOL-JES-TE300 ESR spectrometer operating at 9.2 GHz
(X-band)
3 Results and discussion
Fig 1 shows the X-ray diffraction patterns of (La0.5Pr0.5)0.7Pb0.3Mn1xCuxO3 (x ¼ 0, 0.02) samples The samples are single-phase with an orthorhombic structure; the lattice parameters (a, b, and c) are summarized in Table 1 As one can see clearly from the table, with addition of Cu, the lattice constants of the Cu-doped sample (x ¼ 0:02) slightly decreased comparing to the Cu-free sample (x ¼ 0) This is understandable because
of the fact that the radius of the Cu2+ion is smaller than that of the Mn3+ion Similar trend was reported by other authors[21]
MZFC(T) and MFC(T) curves measured at 20 Oe are displayed inFig 2 The ferromagnetic (FM)-to-paramag-netic (PM) phase transition temperature, TC, determined from these magnetization curves, is about 326 and 300 K for x ¼ 0 and 0.02 compositions, respectively This indicates a considerable reduction of TC in samples with
Cu substitution for Mn However, the TC of the x ¼ 0:02 sample is still of 300 K, which may be of interest in the development of magnetic refrigerants for room-tempera-ture magnetic refrigeration applications [16,22] A reduc-tion in magnetizareduc-tion of the x ¼ 0:02 sample compared to the x ¼ 0 sample could be ascribed to the decrease in FM interactions due to the Cu doping effect [21] It is worth noting that, at temperatures below TC, a separation of
30
25
20
15
10
5
0
2 Θ (°)
x = 0.02
x = 0
Fig 1 The X-ray diffraction patterns of (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3
(x ¼ 0, 0.02) samples.
Table 1 The lattice parameters of (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3 (x ¼ 0, 0.02) samples
Trang 3MFC(T) from MZFC(T) for both compositions was
observed This is typically symptomatic of ferromagnets
with a strong anisotropy field arising from FM clusters that
are usually observed in unconventional ferromagnets With
the presence of the FM clusters, magnetic moments of
spins inside the cluster could be frozen in directions
energetically favored by their local anisotropy or an
external magnetic field as the system is cooled down from
a high temperature in zero or non-zero magnetic field,
respectively[23] This feature seems to be very widespread
in manganese perovskites as FC magnetization is
per-formed at low applied fields When the applied magnetic
field, Hex, is high enough, such feature will disappear and,
MZFC(T) and MFC(T) curves become coincide with each
other because of the fact that the sufficiently high field
could suppress entirely anisotropy fields in the system (see
the inset of Fig 2, magnetization of the two samples
measured at 1 kOe)
Fig 3 shows the temperature dependence of the DC
resistivity of the x ¼ 0 and 0.02 samples As one can see
fromFig 3, with respect to the increase of temperature the
x ¼ 0 sample exhibited a metallic-to-insulator transition at
130 K while only insulating behavior was observed in the
x ¼ 0:02 sample in the whole temperature range
investi-gated In the present case, we did not measure resistivity
values at lower temperatures, maybe, there was the metallic
behavior [24] As shown earlier in Refs [7,24] on a
(La0.5Pr0.5)0.7Ca0.3MnO3 expitaxy film and Pr0.65Ca0.35x
SrxMnO3 compounds, a metallic-to-insulator transition
followed by a broadening band around the phase transition
temperature was observed This coincides with what we
observed here on (La0.5Pr0.5)0.7Pb0.3MnO3(x ¼ 0), and can
be explained due to the presence of charge-ordering states
[24] It should be noted that the magnitude of the resistivity
of the x ¼ 0:02 sample at a given temperature is larger than
that of the x ¼ 0 sample In connection with the magnetization data, it is reasonable to conclude that the
Cu addition leads to a decrease of the ferromagnetic interaction and conductivity of the parent compound (x ¼ 0) This might be related to changes in the exchange mechanism occurring among eg electrons of Mn and Cu ions[21] Here, an emerging question is why the resistivity
of the (La0.5Pr0.5)0.7Pb0.3MnO3 (x ¼ 0) sample becomes higher with Cu addition?
In order to address this question, the model of Mott’s variable-range-hopping (VHR) insulating behavior r(T) ¼
r0exp[(T0/T)1/4] (where r0is a pre-exponential factor and
T0 is a constant) has been used to analyze the resistivity data of the presently investigated samples Determined values of T0are 4.55 107and 16.12 107(K) for x ¼ 0 and 0.02 compositions, respectively In the present work,
we have taken into account the density of states of the system at the Fermi level N(EF)-which is obtained using the equation of T0¼18a3/kBN(EF), where kBis the Boltzmann factor and a is the electron wave-function decay constant Using a ¼ 2:22 nm1 [24,25], N(EF) is deduced to be 1.04 1025 and 0.63 1020eV1cm1 for x ¼ 0 and 0.02 compositions, respectively This enables us to state that the decrease of the density of states on the Fermi level in the Cu-doped sample is the origin leading to the decrease
of the electrical conductivity of the sample, as compared to the Cu-free sample As can be seen in Fig 3(b), the VHR model can describe the r(T) data for the x ¼ 0:02 sample in
a large temperature range, rather than for the x ¼ 0 sample This is in good agreement with what was reported
in Ref [21]with respect to an increase of Cu content
To understand internal spin dynamics of the presently investigated samples, we recorded ESR spectra at different temperatures above from TC, as shown inFig 4 It can be seen that asymmetrical ESR signals at low temperatures
100
10
1
60 120 180 240 300 0.24 0.27 0.30 0.33
T (K) T1/4 (K-1/4)
x = 0
x = 0.02
5
4 3 2 1
0
Fig 3 (a) The temperature dependence of resistivity, r(T), for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3 (x ¼ 0, 0.02) samples; (b) the logarithm
of resistivity Ln(r) versus T 1/4 , solid lines are fitting curves according to Mott’s variable-range-hopping model.
2.0
1.5
1.0
0.5
0.0
T (K)
FC
ZFC
x = 0
x = 0.02
75 60 45 30 15
T (K)
x = 0
x = 0.02
Fig 2 Temperature dependence of field-cooled (FC) and
zero-field-cooled (ZFC) magnetizations taken at 20 Oe for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn
1x-Cu x O 3 (x ¼ 0, 0.02) samples The inset shows the temperature dependence
of FC and ZFC magnetization for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3 (x ¼ 0)
measured at 1000 Oe.
Trang 4became symmetrical-Lorentzian at temperatures T4Tmin
(Tmin is, inTable 2, the temperature corresponding to the
narrowest ESR linewidth) With decreasing temperature in
the region TCoToTmin, the ESR signals were splitted into
two lines, in which the resonant line at a lower field
changed in its position with temperature[21,26], seeFig 5
In particular, these two lines strongly competed in
amplitude at temperatures around Tmin This may be
considered a signature of coexistence of two competing
phases, namely, FM and PM phases At temperatures
TpTC, the FM phase is dominant and it strongly
suppresses ESR signals of the PM phase As a matter of
fact, it was found in the ferromagnetic-low-temperature
region appearing several resonance lines[27–29]due to the
presence of FM correlations and spins in FM
micro-regions (or FM clusters) These spins were strongly
influenced by an anisotropy field (arising from itself FM
clusters) added to the external magnetic field On the other
hand, recent reports also pointed out experimental
evidences on the presence of phase separation, this led to
the additional appearance of resonance lines of mixed
phases [27,28], introducing to the broadening of the ESR
linewidth at low temperatures However, at temperatures
above Tmin, single ESR signals in the Lorentzian shape for
the samples were observed; where the samples are
completely in the PM state This differs from the results
of Ref.[21], where for the Cu-doped samples the two lines
arising from the coexistence of double-exchange FM and
super-exchange antiferromagnetic (AFM) interactions were observed even above Tmin This differential can be understood, because of the fact that the Cu-doping level in our system is so small that formation of AFM clusters due
to Cu2+–Mn3+and/or Cu2+–Mn4+interactions is trivial only It is furthermore suggested that the origin of the observed ESR signals comes from the participation of electron spins of Mn3+, Mn4+, and Cu2+ ions in correlation to crystal fields caused by the neighboring ions
[4–6,9,26]
Fig 6 shows the temperature dependence of the ESR linewidth, DHðT Þ, for the x ¼ 0 and 0.02 samples DHðTÞ reached a minimum value DH at T (Table 2), and its
423 428
DC field (mT)
Fig 4 X-band ESR spectra for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3 (x ¼ 0,
0.02) samples at selected temperatures around T min
Table 2
Experimental parameters obtained for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3
(x ¼ 0, 0.02) samples
x T C (K) T 0 107
(K)
T min
(K)
Y (K) B (Oe/K) DH min
(Oe)
E a
(eV)
0.02 300 16.12 351 327 6.64 716 0.14
340 320 300 280 260 340 320 300 280 260
300 320 340 360 380 400 420 440 460 480
T (K)
x = 0.02
x = 0
Hr
Fig 5 The plot of the resonance position of high and low field lines for
x ¼ 0 and 0.02 compositions with respect to temperature Solid lines are guides to the eye Two lines at ToT min due to FM and PM correlations become a sole line at T 4T min
x = 0
x = 0.02
1.4
1.2
1.0
0.8
0.6
T (K)
Fig 6 The temperature dependence of the ESR linewidth, DHðT Þ for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3 (x ¼ 0, 0.02) samples; the solid lines fit to
a function of DHðT Þ ¼ AT þ B.
Trang 5appearance is probably related to the exchange narrowing
and decay values of the correlation function around the
phase transition [9] In the temperature range studied,
DHðT Þ of the x ¼ 0:02 sample was higher than that of the
x ¼ 0 sample; this could be related to difference in the
concentration of Mn3+ and Mn4+ ions in the samples
In general, the ESR linewidth for manganites
con-taining either Mn3+or Mn4+ions individually is usually
larger than that for manganites containing both these
ions[4,9] Accordingly, in our system the Mn substitution
by Cu resulted in an increase in the concentration of Mn4+
ions
Paying attention to the variation of DHðT Þ at
tempera-tures (lower Tmin), it increases because of the development
of FM correlations and forming FM clusters [27–31]
However, the increase of DHðT Þ in the region T 4Tmin is
different, it relates to correlation of FM clusters existing on
a large range of temperature[14,26]; this is concerned via
the activation energy values as being presented later Based
upon the relation between the resistivity rðTÞ and DHðT Þ
data at temperatures above Tmin, on the other hand, it is
stated that the hopping rate of charge carriers could limit
the lifetime of the spin state, thereby resulting in a
broadening of EPR spectra with increasing temperature
[31,32]
Regarding the interaction mechanism in the PM region,
one see that the Lande factor g of the samples is close to
2.00, and it is temperature independent This means that
the spin–spin interaction is dominant in this temperature
range And, the obtained value of g is appropriate to the
one-phonon process [12,33], i.e the variation of the ESR
linewidth above Tminobeys to a linear function As can be
seen inFig 6, a function DHðT Þ ¼ A þ BT fits well to the
ESR linewidth data above Tmin, where A is a constant and
B is a parameter being related to exchange, dipolar, lattice
distortions, and/or Jahn–Teller fluctuations [33] The B
value is determined to be 7.02 and 6.64 Oe/K for the x ¼ 0
and 0.02 samples, respectively According to earlier studies
[12,32–36], we realize that La1xA0xMnO3 manganites were
often found to have the low B value (3 Oe/K) [34–36]
compared to praseodymium manganites Pr1xA0xMnO3
(5–7 Oe/K) [33] This is perhaps due to influences of
crystal fields, which are caused by La3+and Pr3+ions, on
Mn ions In our case, the estimated value of B is 7.0 Oe/K
and this is acceptable because of the presence of both La3+
and Pr3+ions in the samples
The temperature dependence of the ESR intensity I(T) at
T 4Tmin, determined by taking double integration of the
experimental curve, for the samples is shown in Fig 7
With increasing temperature, I(T) exponentially decreased
According to the one-phonon process, I(T)pwDC(T),
where wDC is the DC susceptibility, [33,36,37] and the
relation between DHðT Þ and r(T) [6,32], I(T) can be
expressed by the function I ðTÞ ¼ I0exp ðEa=kBT Þ, where
Eais the thermal activation energy for the dissociation of
the FM spin clusters [26,32,35] This expression describes
well the experimental I(T) data as shown inFig 6 The E
value is determined to be 0.16 and 0.14 eV for the x ¼ 0 and 0.02 samples, respectively The 1=I ðT Þ vs T plot and the extrapolation of the linear-high-temperature part of this curve to zero value, according to the Curie-Weiss law, allowed us to determine the Curie–Weiss temperatures of the samples, Y, which are summarized inTable 2 [4,34–36]
A reasonable agreement of this law was found at high temperatures (see the inset ofFig 7) implying an existence
of FM clusters as exposed earlier by the magnetization data and the activation energy Ea
Finally, to interpret the magneto-transport mechanism
of the samples, the electronic configuration of ions present
in the compounds has been taken into account In the case
of the x ¼ 0 sample, both Mn3+and Mn4+ions (Mn3+is 3d4, t32ge1g, with S ¼ 2, whereas Mn4+ is 3d3, t32ge0g, with
S ¼ 3=2) coexist and follow the strong Hund’s rule coupling The spins of these ions orient parallel to each other thus leading to the fact that the interaction governing them is FM [17] Meanwhile, the exchange couplings of
Mn4+–Mn4+ and/or Mn3+–Mn3+ ions are AFM super-exchange interactions [38,39] When substituting a small amount of Cu for Mn, the Mn4+ concentration in the sample increases, and the AFM interaction is intensified due to the additional appearance of the super-exchange couplings of Cu2+–O–Mn3+,4+and Cu2+–O–Cu2+ As a result, the FM interaction is declined That is why both the
TCand the activation energy value Eadecreased in the Cu-doped sample It should be furthermore noted that the x ¼ 0:02 sample contains Cu2+ ions with 3d9 configuration,
t82ge3g and S ¼ 1=2, one free-electron spin on the eg level which couples weakly with the t2g-core[38–40] Therefore, the interaction between Cu2+and Mn3+/4+ions is actually AFM
15
10
5
0
330 360 390 420 450 480
T (K)
T (K)
x = 0
x = 0
x = 0.02 Fitting curve
θ
Fig 7 The temperature dependence of the ESR intensity for (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1x Cu x O 3 (x ¼ 0, 0.02) samples; the symbols shows the experimental data and the solid lines are fitting curves with the function IðT Þ ¼ I 0 exp ðE a =k B T Þ The inset shows the 1=IðTÞ vs T curve, according to the Curie–Weiss law.
Trang 64 Conclusions
The electrical and magnetic properties of (La0.5Pr0.5)0.7
Pb0.3Mn1xCuxO3 (x ¼ 0, 0.02) perovskites have been
thoroughly studied It was found that with Cu addition
the Curie temperature decreased from 326 K for x ¼ 0
composition to 300 K for x ¼ 0:02 composition The
resistivity of the Cu-doped sample was higher than that
of the Cu-free sample It is interesting to note that the
x ¼ 0 sample underwent a metallic–insulator transition at
130 K, whereas the x ¼ 0:02 sample exhibited an
insulat-ing behavior in the entire temperature investigated The
results obtained from ESR measurements show that
asymmetrical ESR signals at low temperatures became
Lorentzian at high temperatures above Tmin Temperature
dependence of the linewidth, DHðT Þ, at T 4Tminfitted well
to the one-phonon model, DHðTÞ ¼ AT þ B The
activa-tion energy Ea are estimated to be 0.16 and 0.14 eV for
x ¼ 0 and 0.02 compositions, respectively In terms of our
experimental results, it is reasonable to conclude that the
Cu addition led to a suppression of the ferromagnetism and
conductivity of the parent compound (x ¼ 0)
Acknowledgements
This work in Vietnam was supported by the National
Fundamental Research Program (Project 421004), and in
Korea was supported by the Korea Research Foundation
Grant (KRF-2003-005-C00018)
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