Magnetic measurements for various the magnetic fields and temperatures were performed on a superconducting quantum interference device SQUID and a vibrating sample magnetometer VSM.. In
Trang 1(Received 2 December 2011)
CaMnO3 is an antiferromagnet, in which the super-exchange interaction taking place between
Mn4+ ions plays an important role The doping of a small amount of 15% Pr into the Ca site,
Ca0.85Pr0.15MnO3, leads to the appearance of Mn3+ ions, and introduces the ferromagnetic (FM)
double-exchange interaction between Mn3+and Mn4+ions, which is dominant in a narrow
temper-ature range of 90 ∼ 115 K The FM interaction becomes strong for Ca0.85Pr0.15MnO3 doped with
4 and 8% Ru into the Mn site (i.e., Ca0.85Pr0.15Mn1−xRuxO3 with x = 0.04 and 0.08) The Curie
temperature obtained for x = 0.04 and 0.08 are about 135 and 180 K, respectively While the FM
interaction in the former is dominant due to Mn3+-Mn4+exchange pairs, the latter has the
contri-bution of Ru ions This results in remarkable differences in the features of their FM-paramagnetic
phase transitions and their coercive fields Hc
PACS numbers: 75.30.Kz, 75.47.Lx, 75.60.Nt
Keywords: Ru-doped perovskite manganites, Magnetic properties
DOI: 10.3938/jkps.61.1568
I INTRODUCTION
Recently, CaMnO3-based perovskite manganites have
received much interest from the solid-state physics
com-munity because of their showing interesting electrical and
magnetic properties around their phase-transition
tem-peratures [1–5] In the parent compound of CaMnO3, the
formal valence of Mn is 4+, and the magnetic
interac-tion between Mn4+ ions is antiferromagnetic (AFM)
su-perexchange; its N´eel temperature is 125 K [5] The
pres-ence of intrinsic defects due to insufficient oxygen (i.e.,
CaMnO3−δ) leads to the appearance of Mn3+, which
stimulates a ferromagnetic (FM) double-exchange
inter-action between Mn3+and Mn4+ ions Random
compet-ing AFM and FM interactions can cause magnetic
frus-tration as in spin-glass or mictomagnetic systems [5] For
the case of sufficient oxygen content, Mn3+ ions can be
generated by doping trivalent lanthanide ions into the Ca
site in the chemical formulae of Ca1−yReyMnO3 (Re =
La, Pr, Nd, Lu, Sb, and so forth) [1,6] These materials
are suitable for high-temperature thermoelectric energy
conversion in power generators and refrigerators (known
as thermoelectric coolers or Peltier coolers) For Me =
∗ E-mail: scyu@chungbuk.ac.kr; Tel: 43-261-2269; Fax:
+82-43-275-6415
La, Pr, and Nd, Me-doping at high concentrations with y
= 0.5 – 0.8 enriches remarkably their electrical and netotransport properties, particularly the colossal mag-netoresistive effect (CMR) [7,8] This further widens their application range in electronic and spintronic devices, including reading/writing heads, sensitive sensors, and magnetoresistive random access memories [9]
Concerning Ca1−yPryMnO3, the coexistence of char-ge-ordering (CO) and antiferromagnetic states has been found as y = 0.5 [8,10] Babushkina and co-workers dis-covered that the CO state was suppressed strongly when
Ca0.5Pr0.5MnO3 was doped with Ru, where Ru sub-stitutes for Mn [4, 11] and acts as an electron dopant Also, the incorporation of Ru ions in Mn sites favors the formation of the FM phase [10] Such a feature was found in Ru-doped Sr0.5Pr0.5MnO3[11] Clearly, the Mn-site doping by Ru not only changes the carrier density, the bond angle O-Mni and the bond length
hMn-Oi but also leads to interesting magnetic interactions taking place between Mn and Ru ions To get more insight into this problem, we chose antiferromagnetic
Ca0.85Pr0.15MnO3 (a Mn4+-rich compound) as a host lattice and doped it with Ru in the chemical formula
Ca0.85Pr0.15Mn1−xRuxO3(x = 0.04 and 0.08) The syn-thesized samples were then studied to obtain detailed structural and magnetic properties
Trang 2
-1568-Fig 1 (Color online) XRD patterns of polycrystalline
Ca0.85Pr0.15Mn1−xRuxO3compounds prepared by using
con-ventional solid-state reaction
II EXPERIMENTAL DETAILS
Three polycrystalline samples of Ca0.85Pr0.15Mn1−x
-RuxO3 (x = 0, 0.04, and 0.08) were prepared by
us-ing conventional solid-state reaction High-purity
pow-ders of CaCO3, Pr2O3, MnO2, and RuO2were combined
in nominally stoichiometric quantities and were ground
well by using an agate mortar and pestle These
mix-tures calcined at 1000 ◦C for 24 hrs were then pressed
into three pellets under a pressure of about 5000 psi by
using a hydraulic press The pellets were then annealed
in air at 1180◦C for 24 hrs The final products obtained
were checked for crystal structure by using a Brucker
D5005 diffractometer working with an X-ray Cu-Kα1
ra-diation source (λ = 1.5406 ˚A) Magnetic measurements
for various the magnetic fields and temperatures were
performed on a superconducting quantum interference
device (SQUID) and a vibrating sample magnetometer
(VSM)
III RESULTS AND DISCUSSION
Figure 1 shows X-ray diffraction (XRD) patterns for
Ca0.85Pr0.15Mn1−xRuxO3 (x = 0, 0.04, and 0.08)
pre-pared by using the solid-state reaction method Detailed
analyses based on the XRD profiles reveal that the
sam-ples had a single phase with an orthorhombic
struc-ture and with no additional XRD peaks from the initial
powders The lattice constants determined for Ca0.85
-Pr0.15MnO3 (i.e., x = 0) are a = 5.311 ˚A, b = 5.321 ˚A,
and c = 7.540 ˚A They slightly increase with increasing
Ru doping from x = 0.04 to 0.08 (a = 5.319 ˚A, b =
5.321 ˚A, and c = 7.545 ˚A for x = 0.08) This is due to
the fact that the Ru ions that substituted for Mn ions
in Ca0.85Pr0.15Mn1−xRuxO3 had oxidation states of 4+
and 5+ [3, 4, 10, 11] The ionic radii of Ru4+ and Ru5+
are 0.62 ˚A and 0.565 ˚A while those of Mn3+ and Mn4+
Fig 2 (Color online) Temperature dependences of the magnetization for Ca0.85Pr0.15Mn1−xRuxO3 under an ap-plied field of 150 Oe measured after zero-field cooling
are 0.645 ˚A and 0.53 ˚A, respectively Under such circum-stances, two possibilities exist:
(i) Ru4+ tends to substitute for Mn3+ and Ru5+ sub-stitutes for Mn4+because of the agreement in their ionic radii For a small amount of Ru doping with x
= 0.04 - 0.08, the Mn4+concentration is decreased, and more Mn3+ions are introduced The substitu-tion of Ru5+for Mn4+is thus decreased, promoting the substitution of Ru4+ for Mn3+
(ii) The charge transfer Ru4++ Mn4+→ Ru5++ Mn3+ occurs because of doping Ru The average ionic ra-dius of Ru5+ and Mn3+ (0.605 ˚A) is greater than that of Ru4+ and Mn4+(0.575 ˚A)
Both the above possibilities lead to the lattice constants
of Ca0.85Pr0.15Mn1−xRuxO3 (x = 0.04 and 0.08) being greater than those of Ca0.85Pr0.15MnO3 However, X-ray absorption and neutron diffraction analyses revealed the second case is usually observed in Ru-doped perovskite manganites [4,11,12]
In an attempt to understand the influences of the coexistence of Mn3+, Mn4+, Ru4+, and Ru5+ ions on the magnetic properties of Ca0.85Pr0.15Mn1−xRuxO3,
we have measured the magnetization variation with re-spect to the temperature and magnetic field Figure 2, it plots the temperature dependence of the magnetization,
M (T ), for the samples under an applied field of 150 Oe after zero-field cooling One can see that the M (T ) curves exhibit maxima at a temperature of Tm≈ 102 K for x =
0 and 122 K for x = 0.04 and 0.08, which are associated with the AFM-FM phase transition [12] The Curie tem-peratures (TC) obtained from the minima of the dM /dT curves for x = 0, 0.04, and 0.08 at temperatures T >
Tm are 107, 135, and 180 K, respectively At tempera-tures higher than TC, the samples with x = 0.04 and 0.08 enter the paramagnetic (PM) region Meanwhile,
x = 0 is still in the AFM regime until T ≈ 145 K (see the inset in Fig 2) The increase in T with
Trang 3increas-Fig 3 (Color online) Hysteresis loops for x = 0 measured
at various temperatures above 90 K The inset shows the
coercivity Hcversus temperature
ing Ru content proves that the development of the FM
phase is due to additional contributions of ferromagnetic
interactions related to the Ru ions, in good agreement
with previous reports on Ru-doped Sr0.5Ca0.5MnO3[10],
Sr0.5Pr0.5MnO3 [11], and SrMnO3 [12] Comparing the
magnetic features of our samples with each other, we
found that the phase-transition region of x = 0 is very
sharp that of x = 0.04 is quite smooth, and that of x =
0.08 has a hump at ∼150 K It is assigned the above
phe-nomena to be due to the magnetic inhomogeneity inside
the samples caused by the Ru dopants
The magnetic natures of the AFM, FM and PM
phases existing in Ca0.85Pr0.15Mn1−xRuxO3 can be
fur-ther characterized by considering their hysteresis loops,
M (H) curves, recorded at various temperatures starting
from 90 K in the magnetic field range of 0 – 10 kOe
For Ca0.85Pr0.15MnO3 (the x = 0 sample without Ru
dopants), three factors contribute to the M (H) curves
at temperatures T < 110 K (see Fig 3): (i) The first
one associated with FM interactions causes the
hystere-sis loops in the field range from −4 to 4 kOe; (ii) the
second is due to AFM interactions, leading to a
split-ting of two lines at fields beyond (4 kOe (as measured
at a given temperature); and (iii) the other is PM
con-tributions due to paramagnetic centers, such as isolated
ions and/or lattice defects At temperatures T > 110
K, there is a narrow loop (nearly parallel) in the M (H)
curves, indicating a remarkable decline of the FM
inter-action Only AFM and PM contributions persist until T
≈ 150 K Such features agree well with those recorded
from M (T ) for x = 0, a shown in the inset of Fig 2
Notably, the coercive field Hc obtained from the M (H)
curves also decreases rapidly for magnetic fields from 670
Oe to ∼170 Oe for T > TC = 107 K, (see the inset of
Fig 3) We believe that in the x = 0 sample, AFM
interactions associated with the superexchange pairs of
Mn4+-Mn4+ and Mn3+-Mn3+ are dominant There is
also a random distribution of FM clusters due to Mn3+
-Fig 4 (Color online) Hysteresis loop for x = 0.04 mea-sured at various temperatures above 90 K The inset shows
Hcas a function of temperature
Mn4+pairs The magnetic moments of the Mn ions may
be governed by a locally anisotropic field generated from the FM clusters at temperatures below 102 K However,
at temperatures above 102 K, as the thermal energy is high enough to suppress the FM coupling, the magnetic moments of Mn ions would be governed by the external applied field The impact change of magnetic moments from a local field to an external one leads to the maxi-mum in the zero-field-cooled magnetization of the x = 0 sample Furthermore, because FM clusters are included
in the AFM host lattice, the competition between AFM and FM interactions, which take place around cluster boundaries, can lead to magnetic frustration
Concerning the x = 0.04 sample (TC = 135 K), Fig 4 shows some representative M (H) curves at various tem-peratures Different from the previous sample, the main contribution to these curves is the FM phase At tem-peratures T > 122 K, the saturation magnetization Ms gradually decreases, and the system enters the PM re-gion for T > TC The decrease in Msat temperatures T
< 122 K is due to AFM interactions The variation of Ms
is thus similar to that of M (T ) shown in Fig 2 Based on the M (H) curves, we also determined the temperature dependence of Hc, see the inset of Fig 4 Hc (= 335 Oe
at 95 K) appears to decrease with increasing temperature from 95 to ∼130 K This is due to the suppression of the
FM order under the thermal energy The slight fluctua-tion of Hcat temperatures above 130 K is assigned to an appearance of Ru-related magnetic phases, as discussed below
Assessing in detail the magnetic properties of x = 0.04, we considered the isothermal magnetization curves shown in Fig 5(a), which were recorded around the
FM-PM phase transition TC in temperature increments of
5 K Arrott plots of M2 versus H/M [13] will be sep-arated into two characteristic regions with T < 135 K being associated with the FM region and T > 140 K be-ing associated with the PM region, see Fig 5(b) The
Trang 4Fig 5 (Color online) (a) Isothermal-magnetization curves
of x = 0.04 recorded around the phase transition temperature
(b) The performance of Arrott plots, M2-H/M (c) Ms(T )
and χ−10 (T ) data are fitted to Eqs (1) and (2), respectively;
the critical parameters are indicated
FM-PM phase transition temperature thus lies between
135 and 140 K We also find that the slopes of the H/M
versus M2curves (not shown) in the vicinity of the phase
transition are positive This indicates that the x = 0.04
sample exhibits a second-order magnetic phase transition
according to the Banejeer criterion [14] It means that
the spontaneous magnetization (Ms) and the inverse
ini-tial susceptibility (χ−10 ) obey the asymptotic relations
[15]
Ms(T ) = M0(−ε)β, ε < 0, (1)
χ−10 (T ) = (h0/M0)εγ, ε > 0, (2)
where M0 and h0 are the critical amplitudes, and ε =
(T – TC)/TC is the reduced temperature The critical
exponents β and γ are associated with the exponents
of the Ms(T ) and the χ−10 (T ) curves, respectively The
features of the Arrott plots reflect that the values of β
and γ are close to those expected from the mean-field
(MF) theory (where β = 0.5, and γ = 1.0) [15] We
used the Arrott-Noakes method [16] to determine Ms(T )
and χ−10 (T ) from the data in Fig 5(b) The fittings of
Ms(T ) and χ−10 (T ) to Eqs (1) and (2), respectively,
in-troduce β = 0.478 ± 0.009, γ = 1.252 ± 0.025, and TC =
138.3 K, as shown in Fig 5(c) Obviously, the TC value
obtained from this technique is in good agreement with
that obtained from M (T ) While our β value is close
Fig 6 (Color online) Hysteresis loops for x = 0.08 mea-sured at various temperatures above 90 K The inset shows
Hcversus temperature
to that of the MF theory (with β = 0.5), γ is close to that of the tricritical MF theory (with γ = 0.25) [17] The deviation in β value compared to the MF theory demonstrates the presence of short-range ferromagnetic interactions Here, the short-range ferromagnetism is as-signed to AFM-interaction pairs, such as Mn4+-Mn4+ and Mn3+-Mn3+, besides the dominant FM interaction pair of Mn3+-Mn4+ Though the system goes to the PM region, FM clusters still persist at temperatures above
TC, making our γ = 0.25 value different from the γ = 1.0 of the MF theory This can explain why there is the fluctuation of Hc at temperatures T > 130 K, as shown
in the inset of Fig 4
The hysteresis loops of the last sample with x = 0.08 recorded at several temperatures are shown in Fig 6 Similar to the tendency observed in the x = 0.04 sample, the FM phase continuously develops with increasing Ru-doping content in Ca0.85Pr0.15Mn1−xRuxO3 Observing carefully, one can see clearly a remarkable difference in the shapes of the hysteresis loops at temperatures T >
140 K This is more visible in the variation of Hcgraphed
in the inset of Fig 6 At ∼150 K, Hc reaches to a maxi-mum value, coinciding with the temperature of the hump observed in M (T ) for x = 0.08 in Fig 2 A lowering of the temperature below 120 K leads to an enhancement
of Hc Because complicated variations of Hc, we believe that magnetic inhomogeneities and multiphases exist in the compound
An explanation for the variation in the behavior of the Hc data for the samples with x = 0.04 and 0.08 may be based on the coexistence of Mn3+, Mn4+, Ru4+ and Ru5+ ions, and the charge transfer of Ru4++ Mn4+
→ Ru5+ + Mn3+ as increasing Ru-doping concentra-tion Among these, Mn4+-Mn4+, Mn3+-Mn3+and Ru4+
-Ru5+ pairs are AFM while Ru5+-Ru5+and Mn3+-Mn4+ pairs are FM [2, 18] Other pairs related to Mn-Ru in-teractions are suggested to be AFM [2] For the case of
x = 0.04, the additional presence of Mn3+ and Ru5+ ions in Ca Pr MnO (i.e., x = 0 where the Mn4+
Trang 5at x = 0.04 in Ca0.85Pr0.15Mn1−xRuxO3 However, an
interesting situation occurs in the case of x = 0.08
Be-sides the continuous development of the main FM phase
associated with Mn3+-Mn4+, a secondary FM phase
as-sociated with Ru5+-Ru5+ becomes significant as shown
by the appearance of the hump at ∼150 K in M (T ) We
predict that further increasing the Ru-doping content to
x > 0.08 will broaden the FM phase towards higher
tem-peratures The change in interaction is impact related to
the FM phases due to Mn3+-Mn4+and Ru5+-Ru5+pairs
causes an interesting variation in the Hcversus
tempera-ture curve, as can be seen in the inset of Fig 6 As
men-tioned above for x = 0.04, the impact of the Ru5+-Ru5+
FM phase starts from T ≈ 130 K, and that of the Mn3+
-Mn4+ FM phase starts at lower temperatures However,
the change in the impact for the x = 0.08 sample starts
from ∼120 K, where Hc reaches a minimum value The
ascendancy of the Ru5+-Ru5+ FM phase increases with
increasing temperature, indicating an interaction
com-petition of Ru5+-Ru5+ with other Mn- and Ru-related
pairs This Ru5+-Ru5+ FM phase becomes strongest as
T = 150 K and will be suppressed at higher temperatures
by the thermal activation energy The above results and
explanations show that Ru-doping causes the electrical,
magnetic and/or magneto-transport properties of
per-ovskite manganites to become more interesting
IV CONCLUSION
We prepared three polycrystalline ceramic samples of
Ca0.85Pr0.15Mn1−xRuxO3 (x = 0, 0.04, and 0.08) and
then studied in detail their structures and magnetic
properties The XRD data revealed that the samples
had a single phase with an orthorhombic structure The
slight increases in the lattice constants with increasing
Ru content indicated the incorporation of Ru4+ and
Ru5+ in the Mn sites This leads to the development
of a FM phase associated with Mn3+-Mn4+ and Ru5+
-Ru5+pairs, and with TC= 135 K for x = 0.04 and TC=
180 K for x = 0.08 The change in the impact related to
the Mn3+-Mn4+ and the Ru5+-Ru5+ FM phases caused
interesting variations in the Hc versus temperature and
Ru-doping concentration, leading to the differences in
features of the magnetic phase transitions for the
sam-QG-11-02
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