Yu1 1Department of Physic, Chungbuk National University, Cheongju 361-763, Korea 2Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay,
Trang 1Inhomogeneous Ferromagnetism and Spin-Glass-Like Behavior
T L Phan1, V D Nguyen2, T A Ho1, N V Khiem3, T D Thanh2,
N X Phuc2, P D Thang4, and S C Yu1
1Department of Physic, Chungbuk National University, Cheongju 361-763, Korea
2Institute of Materials Science, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
3Department of Natural Science, Hongduc University, Thanhhoa, Vietnam
4Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology,
Vietnam National University, Hanoi, Vietnam
The magnetic properties of polycrystalline ceramic samples (Nd 1−xYx )0.7Sr 0.3MnO 3 with x = 0.21 − 0.35 were studied by
means of dc magnetization and ac susceptibility measurements Experimental results reveal a strong decrease of the ferromagnetic
(FM)-paramagnetic phase-transition temperature (T C ) from 97 to 65 K as increasing x from 0.21 to 0.35, respectively There is
magnetic inhomogeneity associated with short-range FM order Particularly, the samples undergo a spin-glass (SG) phase transition
at the so-called blocking temperature (T B ) below T C, which shifts toward lower temperatures with increasing the applied field,
Hex; T B → T g (the SG phase-transition temperature) as Hex → 0 The existence of the SG behavior in these samples was also
confirmed by frequency ( f ) dependences of the ac susceptibility For the in-phase/real component, χ(T), it shows a
frequency-dependent peak at the SG freezing temperature (T f ); T f → T g as f → 0 Dynamics of this process were analyzed by means of the
slowing down scaling law,τ/τ0∝ (T f /T g − 1) −zv, whereτ0 and zv are the characteristic time and critical exponent, respectively Fitting the experimental T f (f ) data to the scaling law gave the results of zv = 10.1–12.3 and τ0 = 10 −21 –10 −15 s These values
are different from those expected for canonical SG systems with zv = 10 and τ0 = 10 −13 s, revealing the cluster-SG behavior of (Nd 1−xYx) 0.7Sr 0.3MnO 3 samples Notably, the increase in Y content leads to the shift ofτ0and zv values toward those of canonical
SG systems, which is ascribed to an expansion of SG clusters.
Index Terms— Magnetic inhomogeneity, perovskite manganites, spin-glass (SG) behavior.
I INTRODUCTION
IT IS known that NdMnO3 is an antiferromagnetic (AFM)
insulator, and crystallized into the orthorhombic
struc-ture (space group Pbnm) At low temperastruc-tures, due to
the canting and reorientation of Mn spins, the
coexis-tence of ferromagnetic (FM) and AFM couplings leads
to a non-collinear magnetic structure [1] In
mangan-ites, AFM interactions are generated from super-exchange
(SE) pairs of Mn3+-Mn3 + and Mn4 +-Mn4 + while the FM
interaction is from a double-exchange (DE) pair Mn3+
-Mn4+ [2] The strength of these interactions strongly
depends on Mn3+and Mn4 + concentrations, the bond length
Mn-O, and the bond angle Mn–O–Mn Due to the
coex-istence of FM and AFM interactions, it has been suggested
that intrinsic defects related to oxygen content are present in
NdMnO3 [1] A dominancy of SE interactions leads to its
AFM nature
To widen the application range of NdMnO3 in electronic
devices, a divalent alkaline-earth metal A can be doped into
the Nd site to fabricate hole-doped manganites Nd1−x A
xMnO3
with A = Ca, Ba, or Sr This creates more Mn4 +
concen-tration, and results in noticeable physical effects (typically,
colossal magnetoresistance and magnetocaloric effects) at
phase-transition temperatures, depending on added A-dopant
content (x ) Various Nd1−x A
xMnO3compounds have different
Manuscript received November 13, 2013; revised January 8, 2014; accepted
January 14, 2014 Date of current version June 6, 2014 Corresponding author:
S C Yu (e-mail: scyu@chungbuk.ac.kr).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2014.2300852
Jahn–Teller distortions (caused by a strong electron-phonon coupling), which change the lattice symmetry, and the struc-tural parameters of Mn-O and Mn-O-Mn To characterize
the distortion level, it is proposed to use the tolerance factor
t = (R A + R O )/√2(R B + R O ), where R A and R B
are the average radii of cations located at the A and B sites
in the perovskite structure ABO3, respectively, and R O is the radius of oxygen anion [3], [4] Comparing with La-based
manganites, it has been found that the e g-electron bandwidth
of Nd1−x A
xMnO3is more sensitive to changes related to the
parameters t, Mn-O, and Mn-O-Mn Electrical, magnetic,
and magnetotransport properties of Nd-based manganites are thus more interesting and complicated than those of La-based manganites [5]–[9]
In general, FM interactions of Mn3+-Mn4 + DE pairs in
Nd1−x A
xMnO3become strongest at the doping concentration
x = 0.3, corresponding to Mn3 +/Mn4 + = 7/3 [2] Among
Nd0.7 A
0.3MnO3 compounds, Nd0.7Sr0.3MnO3 has attracted much more particular interest There is the contribution of the Nd–Mn exchange interaction [9], [10] though no Nd-related magnetic ordering has been found in NdMnO3at temperatures down to 1.8 K [1] Depending on investigated magnetic-field and temperature ranges, the NdMnO3compound exhibits other noticeable phenomena, such as charge-orbital ordering state [11], and magnetic frustration, and spin-glassy (SG)-like behavior [8] Though Nd0.7Sr0.3MnO3is known as a magnetic-frustrated and disordered system [8], the FM interaction between Mn3+and Mn4 + ions is still dominant The SG-like
behavior thus appears in cluster, namely SG clusters, causing the magnetic inhomogeneity It is possible to enlarge the size
of SG clusters by decreasing the parameter R A or t upon
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Trang 2doping a rare earth or transition-metal element into the site
La/Sr or Mn, depending on its ionic radius For the substitution
into the La/Sr site, the magnetic frustration (caused by the
competition of FM and AFM interactions) and SG behavior
become significant asR A < 1.2 Å [12].
In an attempt to obtain more insight into this problem, we
prepared (Nd1−xYx )0.7Sr0.3MnO3 with x = 0.21–0.35, and
then have studied in detail their magnetic properties based
on dc magnetization and ac susceptibility measurements Our
study points out that increasing Y-doping content results in the
shift of the characteristic time (τ0) and critical exponent (zv)
value toward those of canonical SG systems This is ascribed
to an expansion of SG clusters
II EXPERIMENTALDETAILS
Three polycrystalline samples (Nd1−xYx )0.7Sr0.3MnO3
with x = 0.21, 0.28, and 0.35 were prepared by
solid-state reaction High-purity powdered precursors of Nd2O3,
SrCO3, Y2O3, and MnO2 (99.9%) were combined in
nomi-nally stoichiometrical quantities, well mixed, and pressed into
pellets These pellets were then preannealed at 1150 °C for
24 h After several times of intermediate grinding, pressing
and preannealing, the calcined pellets were sintered in air at
1350 °C for 5 h The single phase in an orthorhombic structure
of the final products was confirmed by an X-ray diffractometer
as using Siemens D5000 Measurements of dc magnetization
and ac susceptibility were performed on a physical property
measurement system in the temperature range 5–300 K For
the ac susceptibility, measurements were performed in an ac
field Hac = 5 Oe, and zero dc field Hdc= 0 after the
zero-field cooling Frequency ( f ) can be changed in the range
12.7–9100 Hz
III RESULTS ANDDISCUSSION
Fig 1 shows temperature dependences of zero-field-cooled
(ZFC) and field-cooled (FC) magnetizations, MZFC/FC(T )
curves, of the samples (Nd1−xYx )0.7Sr0.3MnO3in an external
magnetic field Hex= 100 Oe For the MZFC(T ) curves, there
are cusps peaked at the so-called blocking temperature T B≈
82, 68, and 53 K, corresponding to the samples with x= 0.21,
28, and 35, respectively Below T B, there is a gradual decrease
of magnetization However, the decrease in magnetization at
temperatures below T B does not occur for the case of the
MZFC(T ) curves at the reversibility temperature T r (= 98, 80,
and 69 K for x = 0.21, 28, and 35, respectively) below the
FM-paramagnetic phase transition temperature T C = 97, 77,
and 65 K for x = 0.21, 28, and 35, respectively Here, the
TC values are obtained from the minima of the dMZFC/dT
(or dMFC/dT ) versus T curves, inset of Fig 1 Experimental
evidences revealed that both T r and T B values are shifted
according to a power function toward lower temperatures
with increasing Hex[13], [14] Notably, T Cand magnetization
values gradually decrease with increasing Y-doping content
It is suggested that the features of the MFC(T ) and MZFC(T )
curves shown in Fig 1 are related to a local anisotropic field
Ha generated from FM/AFM clusters (which can persist even
at temperatures above the T C [15]) due to the magnetic
inho-mogeneity and mixed phases The energy of the anisotropic
field acting on magnetic moments m of Mn ions is thus defined
by E a= Ha·m Magnetic moments can be frozen in the
direc-tions favored energetically by their local anisotropy Ha or by
Fig 1 Temperature dependences of ZFC and FC magnetizations,
MZFC/FC (T ) curves, for (Nd1−xYx )0.7Sr 0.3MnO 3with x= 0.21, 0.28, and
0.35 in the field 100 Oe Inset: dM/dT versus T curves, and their maximum shows the T Cvalues of the samples.
an external magnetic field Hex(with energy defined as Eex=
Hex·m) when the system is cooled from high temperatures in a
zero or nonzero field This leads to the separation between the
MZFC(T ) and MFC(T ) curves at temperatures below T r The
deviation of MFC(T ) from MZFC(T ) depends on the magnetic
homogeneity, and on the Hexmagnitude A large deviation is usually observed in magnetic samples having a coexistence of
FM and AFM phases, and exhibiting the magnetic frustration (i.e., a strong competition between FM and AFM interactions)
[8], [14], [16] At the temperature T B , E a is equal to Eex, and
thus MZFC reaches the maximum Meanwhile, at temperatures
T < T B , E a is higher than Eex These features are similar
to those observed in some manganites and cobaltites, and an indication of coexisting short-range FM order and SG-like
behavior below T B[8], [13], [14], [17] For a SG system, there
is the SG-phase-transition temperature T g (where T B → T gas
Hex→ 0) Magnetic moments experience random interactions
with other ones, leading to a state that is irreversible and metastable
To learn about the SG behavior and spin dynamics in (Nd1−xYx )0.7Sr0.3MnO3 with x = 0.21–0.35, we have
mea-sured frequency ( f ) and temperature (T ) dependences of the
ac magnetic susceptibilityχ(T, f ), which consists of the
in-phase/real, χ(T , f ), and out-of-phase/imaginary, χ (T , f ),
components Among these,χis the slope of the M (H ) curve
while χ indicates dissipative processes For ferromagnets,
non-zero χ values indicate irreversible domain-wall
move-ments or absorption due to a permanent moment Because both χ and χ are very sensitive to thermodynamic phase
changes (or spin dynamics), more information associated to transition temperatures is expected to be obtained from ac susceptibility measurements In Fig 2, it shows temperature dependences of the χ and χ components for the samples
(Nd1−xYx )0.7Sr0.3MnO3recorded at various f values ranging
from 12.7 to 9100 Hz For both components, one can see that the curves exhibit cusps at the freezing temperature (denoted
as T f and T
f for the χ(T ) and χ (T ) data, respectively).
Similar to the cusps peaked at T B in the MZFC(T ) curves, the
positions of the cusps at T f [or T] in theχ(T ) [or χ(T )]
Trang 3PHAN et al.: INHOMOGENEOUS FERROMAGNETISM AND SG-LIKE BEHAVIOR IN (Nd1−xYx )0.7Sr 0.3MnO 3 2502204
Fig 2 Temperature dependences of ac susceptibility components χ
and χ recorded at various frequencies in the range 12.7–9100 Hz for
(Nd 1−xYx )0.7Sr 0.3MnO 3 with (a) and (b) x = 0.21, (c) and (d) x = 0.28,
and (e) and (f) x= 0.35 The humps marked with asterisks, besides the cusps
at T f and T
f, reveal magnetic inhomogeneity (meaning an coexistence of
FM and SG behaviors) in the samples.
curves are also shifted toward lower temperatures with
increas-ing Y-dopincreas-ing concentration in (Nd1−xYx )0.7Sr0.3MnO3, which
is related to the weakening of FM interactions With f =
12.7 Hz, both T f and T
f values are ∼84, 73, and 53 K
for the samples with x = 0.21, 0.28, and 0.35, respectively
Particularly, these temperatures shift toward high
tempera-tures when f is increased, Fig 3 with an enlarged view
around T f The frequency dependence of the ac susceptibility
demonstrates an existence of the SG behavior, where there
is a strong competition of FM and AFM interactions [8],
[12]–[14], [17] As mentioned, the samples go into the SG
state at temperatures below the SG-phase-transition
tempera-ture T g that T f → T g as f → 0 Above T g, they usually
exhibit the PM behavior However, it comes to our attention
that besides the main cusps, small humps are also observed
(indicated as asterisks in Fig 2), particularly for the samples
with x = 0.28 and 0.35, even at temperatures above T f
(or T
inhomogeneity associated with FM/AFM clusters [8], [14],
[15] Magnetic moments of Mn ions can be aligned to different
anisotropic fields generated from FM/AFM clusters, leading to
the appearance of humps For theχ(T ) curves, the existence
of the humps and cusp is directly related to different magnetic
energy dissipations These results prove the coexistence of
FM and SG behaviors in the samples (Nd1−xYx )0.7Sr0.3MnO3
with x = 0.21–0.35 We also measured the ac susceptibility
of an additional sample x = 0.14 (not shown) However, no
frequency dependence of T f and T was observed.
Fig 3 Enlarged view of theχ(T ) data around T f at different frequencies
for the samples (a) x = 0.21, (b) x = 0.28, and (c) x = 0.35 exhibiting
SG-like behavior Insets: log10(τ) versus log10[(T f -T g )/T g] data fitted to the critical-slowing down law.
Dynamics of the SG behavior can be deeper understood
upon analyzing frequency dependences of T f (or T
f ) based
on conventional critical-slowing down [14], [17], τ/τ0 ∝
(T f /T g − 1) −zv, where τ0 and zv are the characteristic time and critical exponent, respectively In this equation, T f is understood as the frequency-dependent freezing temperature
at which the maximum relaxation time τ of the system
responds to the measured frequency By fitting the above
equation to the T f data in the range 12.7–9100 Hz, with
the T g values selected from the T f values at the lowest
frequency f = 12.7 Hz, we obtained zv = 10.1–12.3 and
τ0 = 10−21− 10−15 s, insets and their labels of Fig 3 It
appears that the obtained zv values are close to the value zv=
10 while theτ0 values are much different fromτ0 = 10−13 s
of canonical SG systems [17] Similar results were observed for the cases of La0.7Ba0/3Mn0.7Ti0.3O3(with τ0≈ 10−16 s)
[13] and Nd0.4Gd0.3Sr0.3MnO3 (with τ0 ≈10−17 s) [12] It
has been suggested that τ0 strongly depends on the size of
SG clusters, which is usually observed in unconventional
SG systems or inhomogeneous DE ferromagnets [12], [13]
In other words, SG clusters coexist with the FM phase in the samples (Nd1−xYx )0.7Sr0.3MnO3 with x = 0.21–0.35
Increasing the Y-doping concentration makesτ0and zv values
shifted to those of canonical SG systems due to an expansion
of SG clusters
To clarify the origin of the decrease in the T C and mag-netization, and the SG-like behavior when Y concentration
Trang 4in (Nd1−xYx )0.7Sr0.3MnO3 is increased, let us consider the
magnetic property of Nd0.7Sr0.3MnO3 withR A = 1.207 Å
In this parent compound, the Mn3+/Mn4 +ratio is 7/3, and the
FM interaction between Mn3+and Mn4 + ions plays a
domi-nant role With doping and increasing Y3+ concentration into
the Nd3+ site of (Nd
1−xYx )0.7Sr0.3MnO3, the Mn3+/Mn4 +
ratio is unchanged while R A gradually decreases because
the ionic radius of Y3+ (1.04 Å) is shorter than that of Nd3 +
(1.123 Å) The strength of magnetic interactions thus only
depends on the variation of R A; definitely, this variation
influences directly the bond length Mn–O and angle Mn–
O–Mn It has been found that with decreasing R A, the
FM DE interaction is weakened and resultantly the AFM SE
interaction is developed [18] The decreases in T C and
mag-netization are thus understandable Particularly, whenR A is
smaller than 1.2 Å, the FM and AFM interactions in samples
are comparable with each other, resulting in the magnetic
frustration and SG-like behaviors These phenomena happen
for the cases of Nd0.4Gd0.3Sr0.3MnO3 withR A = 1.198 Å
[12], and our samples (Nd1−xYx )0.7Sr0.3MnO3 withR A =
1.194, 1.190, and 1.186 Å for x = 0.21, 0.28, and 0.35,
respectively For manganites doped by a transition metal (M,
such as Fe, Cr, Ti, and so forth) into the Mn site (i.e., the B site
of the perovskite structure ABO3) [4], [13], [17], their SG-like
behavior is related to the suppression of FM DE Mn3+-Mn4 +
interactions due to the additional presence of AFM interactions
caused by M-dopant ions In addition, doping M ions modifies
the structural parameters of Mn–O and Mn–O–Mn, and
changes the concentration of Mn3+ and Mn4 + ions These
factors influence directly the FM phase of manganites
IV CONCLUSION
We prepared polycrystalline samples (Nd1−xYx )0.7Sr0.3
MnO3 with x = 0.21–0.35 by solid-state reaction Their
magnetic properties were then studied by measurements of
ZFC/FC magnetizations and ac susceptibility (at various
fre-quencies) versus temperature With increasing the Y-doping
concentration (x ), the T C is decreased from 97 K (for x =
0.21) to 65 K (for x = 0.35) We also found the magnetic
inhomogeneity associated with the coexistence of the FM
phase and SG-like behavior The samples undergo SG phase
transition at temperatures below T B < T C This phenomenon
was evidenced via frequency dependences of the freezing
temperature T f (or T ’ f ), which were recorded from the
ac susceptibility in the frequency range 12.7–9100 Hz The
SG dynamics were further analyzed by conventional
critical-slowing down law for the T f ( f ) data, τ/τ0 ∝ (T f /T g-1)−zv.
The obtained results reveal that the values zv= 10.1–12.3 and
τ0 = 10−21–10−15 s are quite different from those expected
for canonical SG systems with zv = 10 and τ0= 10−13s We
believe that this phenomenon is related to unconventional SG
behaviors usually observed in inhomogeneous DE
ferromag-nets In other words, there exists the cluster-SG behavior in
our (Nd1−xYx )0.7Sr0.3MnO3samples Increasing the Y-doping
concentration leads to the expansion in size of SG clusters, and
increases the τ0 and zv values, making these values shifted
toward those of canonical SG systems Here, the decreases of
TCand magnetization, and the appearance of the SG behavior
are directly related to the decrease of R A, which weakens
the FM DE interaction between Mn3+ and Mn4 +.
ACKNOWLEDGMENT
This work was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea, under Grant 2013K000405, and the National Foundation for Science and Technology Development under Grant 103.02-2012.57 in Vietnam
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