Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films
The amorphous, nanoclustered, and self-organizing bilayered Nd₀.₅Sr₀.₅MnO₃ films have been prepared by a rf-magnetron sputtering. The amorphous film turn out to be a typical paramagnet with a freely moving of the individual Mn spins, the magnetic properties of which are well described by the Curi...
Gespeichert in:
| Datum: | 2007 |
|---|---|
| Hauptverfasser: | , , , , , , , , , , |
| Format: | Artikel |
| Sprache: | English |
| Veröffentlicht: |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2007
|
| Schriftenreihe: | Физика низких температур |
| Schlagworte: | |
| Online Zugang: | http://dspace.nbuv.gov.ua/handle/123456789/120920 |
| Tags: |
Tag hinzufügen
Keine Tags, Fügen Sie den ersten Tag hinzu!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Zitieren: | Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films / V.G. Prokhorov, G.G. Kaminsky, V.A. Komashko, Y.P. Lee, S.Y. Park, Y.H. Hyun, J.B. Kim, J.S. Park, V.L. Svetchnikov, V.P. Pashchenko, V.A. Khokhlov // Физика низких температур. — 2007. — Т. 33, № 08. — С. 889–896. — Бібліогр.: 25 назв. — англ. |
Institution
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-120920 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1209202017-06-14T03:04:42Z Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films Prokhorov, V.G. Kaminsky, G.G. Komashko, V.A. Lee, Y.P. Park, S.Y. Hyun, Y.H. Kim, J.B. Park, J.S. Svetchnikov, V.L. Pashchenko, V.P. Khokhlov, V.A. Низкотемпеpатуpный магнетизм The amorphous, nanoclustered, and self-organizing bilayered Nd₀.₅Sr₀.₅MnO₃ films have been prepared by a rf-magnetron sputtering. The amorphous film turn out to be a typical paramagnet with a freely moving of the individual Mn spins, the magnetic properties of which are well described by the Curie–Weiss approximation. The nanoclustered film manifests the magnetic properties mimic to the superparamagnetic particles with a nonclassical magnetic dynamics. Taking into account the unique shape of the hysteresis loops, which have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, we suggest that each particle (nanocluster) is the closure magnetic domain (or magnetic vortex) rather than the single one. At the same time, the blocked to unblocked transition was observed with increasing temperature similar to the usual superparamagnet. The self-organizing bilayered film demonstrates a negative exchange bias, which is typical for the ferromagnet/antiferromagnet hybrid system in spite of that both layers in our case have a ferromagnetic origin. The magnetic properties of the films are discussed in detail on the base of modern theoretical models. 2007 Article Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films / V.G. Prokhorov, G.G. Kaminsky, V.A. Komashko, Y.P. Lee, S.Y. Park, Y.H. Hyun, J.B. Kim, J.S. Park, V.L. Svetchnikov, V.P. Pashchenko, V.A. Khokhlov // Физика низких температур. — 2007. — Т. 33, № 08. — С. 889–896. — Бібліогр.: 25 назв. — англ. 0132-6414 PACS: 73.63.Bd, 75.47.Gk, 75.47.Lx, 75.75.+a http://dspace.nbuv.gov.ua/handle/123456789/120920 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| topic |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм |
| spellingShingle |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Prokhorov, V.G. Kaminsky, G.G. Komashko, V.A. Lee, Y.P. Park, S.Y. Hyun, Y.H. Kim, J.B. Park, J.S. Svetchnikov, V.L. Pashchenko, V.P. Khokhlov, V.A. Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films Физика низких температур |
| description |
The amorphous, nanoclustered, and self-organizing bilayered Nd₀.₅Sr₀.₅MnO₃ films have been prepared
by a rf-magnetron sputtering. The amorphous film turn out to be a typical paramagnet with a freely moving
of the individual Mn spins, the magnetic properties of which are well described by the Curie–Weiss approximation.
The nanoclustered film manifests the magnetic properties mimic to the superparamagnetic particles
with a nonclassical magnetic dynamics. Taking into account the unique shape of the hysteresis loops, which
have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, we suggest that
each particle (nanocluster) is the closure magnetic domain (or magnetic vortex) rather than the single one.
At the same time, the blocked to unblocked transition was observed with increasing temperature similar to
the usual superparamagnet. The self-organizing bilayered film demonstrates a negative exchange bias,
which is typical for the ferromagnet/antiferromagnet hybrid system in spite of that both layers in our case
have a ferromagnetic origin. The magnetic properties of the films are discussed in detail on the base of modern
theoretical models. |
| format |
Article |
| author |
Prokhorov, V.G. Kaminsky, G.G. Komashko, V.A. Lee, Y.P. Park, S.Y. Hyun, Y.H. Kim, J.B. Park, J.S. Svetchnikov, V.L. Pashchenko, V.P. Khokhlov, V.A. |
| author_facet |
Prokhorov, V.G. Kaminsky, G.G. Komashko, V.A. Lee, Y.P. Park, S.Y. Hyun, Y.H. Kim, J.B. Park, J.S. Svetchnikov, V.L. Pashchenko, V.P. Khokhlov, V.A. |
| author_sort |
Prokhorov, V.G. |
| title |
Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films |
| title_short |
Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films |
| title_full |
Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films |
| title_fullStr |
Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films |
| title_full_unstemmed |
Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films |
| title_sort |
nonclassical magnetic dynamics and negative exchange bias in nd₀.₅sr₀.₅mno₃ films |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2007 |
| topic_facet |
Низкотемпеpатуpный магнетизм |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120920 |
| citation_txt |
Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films / V.G. Prokhorov, G.G. Kaminsky, V.A. Komashko, Y.P. Lee, S.Y. Park, Y.H. Hyun, J.B. Kim, J.S. Park, V.L. Svetchnikov, V.P. Pashchenko, V.A. Khokhlov // Физика низких температур. — 2007. — Т. 33, № 08. — С. 889–896. — Бібліогр.: 25 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
AT prokhorovvg nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT kaminskygg nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT komashkova nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT leeyp nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT parksy nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT hyunyh nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT kimjb nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT parkjs nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT svetchnikovvl nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT pashchenkovp nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films AT khokhlovva nonclassicalmagneticdynamicsandnegativeexchangebiasinnd05sr05mno3films |
| first_indexed |
2025-07-08T18:52:13Z |
| last_indexed |
2025-07-08T18:52:13Z |
| _version_ |
1837105923739353088 |
| fulltext |
Fizika Nizkikh Temperatur, 2007, v. 33, No. 8, p. 889–896
Nonclassical magnetic dynamics and negative exchange
bias in Nd0.5Sr0.5MnO3 films
V.G. Prokhorov, G.G. Kaminsky, and V.A. Komashko
Institute of Metal Physics of the National Academy of Sciences of Ukraine, Kiev 03142, Ukraine
E-mail: pvg@imp.kiev.ua
Y.P. Lee, S.Y. Park, Y.H. Hyun, J.B. Kim, and J.S. Park
q-Psi and Department of Physics, Hanyang University, Seoul 133-791, Korea
V.L. Svetchnikov
National Center for HREM, TU Delft, 2628AL, The Netherlands
V.P. Pashchenko and V.A. Khokhlov
Donetsk Physical and Technical Institute of the National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine
Received November 13, 2006, revised January 10, 2007
The amorphous, nanoclustered, and self-organizing bilayered Nd Sr MnO0.5 0.5 3 films have been prepared
by a rf-magnetron sputtering. The amorphous film turn out to be a typical paramagnet with a freely moving
of the individual Mn spins, the magnetic properties of which are well described by the Curie–Weiss approxi-
mation. The nanoclustered film manifests the magnetic properties mimic to the superparamagnetic particles
with a nonclassical magnetic dynamics. Taking into account the unique shape of the hysteresis loops, which
have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, we suggest that
each particle (nanocluster) is the closure magnetic domain (or magnetic vortex) rather than the single one.
At the same time, the blocked to unblocked transition was observed with increasing temperature similar to
the usual superparamagnet. The self-organizing bilayered film demonstrates a negative exchange bias,
which is typical for the ferromagnet/antiferromagnet hybrid system in spite of that both layers in our case
have a ferromagnetic origin. The magnetic properties of the films are discussed in detail on the base of mo-
dern theoretical models.
PACS: 73.63.Bd Nanocrystalline materials;
75.47.Gk Colossal magnetoresistance;
75.47.Lx Manganites;
75.75.+a Magnetic properties of nanostructures.
Keywords: manganites, microstructure, magnetization.
1. Introduction
The hole-doped manganites L1�xA xMnO 3, where L
and A are a trivalent lanthanide and a divalent alka-
line-earth ions, respectively, have attracted considerable
attention due to their interesting fundamental science and
potential for applications [1]. Their peculiar physical be-
havior originates from the strong coupling between mag-
netic, structural, electronic, and orbital degrees of free-
dom and is manifested more greatly in the half-doped
perovskite compounds. Nd Sr MnO1 3�x x is a typical sys-
tem whose ground state varies from a ferromagnetic (FM)
metal to an A-type antiferromagnetic (AFM) metal (or in-
sulator at x � 0.6) when the Sr-doping crosses 0.5 [2]. In
addition, the charge-ordered (CO) phase with the AFM
CE-type structural is formed in the very narrow doping
range 0.48 � �x 0.51. The band structure calculations re-
veal that the energy difference between the FM and AFM
phases is very small (around 0.01 eV/atom) [3]. As conse-
quence, the ground state of Nd Sr MnO0.5 0.5 3 is drastically
© V.G. Prokhorov, G.G. Kaminsky, V.A. Komashko, Y.P. Lee, S.Y. Park, Y.H. Hyun, J.B. Kim, J.S. Park, V.L. Svetchnikov,
V.P. Pashchenko, and V.A. Khokhlov, 2007
dependent on the degree of crystallinity, the lattice strain,
the chemical homogeneity, and the clustering of
microstructure, which are controlled by a making tech-
nique. Thus, the CO state, for example, observes in the
Nd Sr MnO0.5 0.5 3 film with a tensile in-plane strain [4],
while does not find in that with a compressive one [5].
The CO AFM CE-type state can be suppressed by the in-
creasing film thickness [6], the high hydrostatic pressure
[7,8], the decreasing grain size in polycrystalline samples
[9], and the applied electrical field [10]. On the other
hand, the influence of the structural quench disorder and
the fine-crystalline clustering on the magnetic and charge
ordering in this compound is still poorly understood.
In this paper, we report the experimental results for the
Nd Sr MnO0.5 0.5 3 films with the different fine-crystalline
microstructure. The observed evidence for the magnetic
vortex state and the negative exchange bias effect are dis-
cussed in detail.
2. Experimental techniques
The Nd Sr MnO0.5 0.5 3 films were prepared by the
rf-magnetron sputtering using a so-called «soft» (or pow-
der) target [11]. The substrate was a LaAlO 3 (001) single
crystal (LAO) with an out-of-plane lattice parameter
c � 0.379 nm for the pseudocubic symmetry. The sub-
strate temperature (Tsub ) during deposition was varied
from 20 to 800 °C, and only the as-deposited films were
used for a study. The thickness of all the films was
� 100 nm. The �–2� x-ray diffraction (XRD) patterns
were obtained using a Rigaku diffractometer with Cu K �
radiation. The lattice parameters evaluated directly from
the XRD data were plotted against cos / sin2 � �. From
the intercept of the extrapolated straight line to
cos sin2 � �� � 0, a more precise lattice parameter was ob-
tained. The high-resolution electron-microscopy
(HREM) and electron-diffraction (ED) studies were car-
ried out using a Philips CM300UT-FEG microscope with
a field emission gun operated at 300 kV. The point resolu-
tion of the microscope was in the order of 0.12 nm. The
cross-sectional specimens were prepared by the standard
techniques using mechanical polishing followed by
ion-beam milling at a grazing incidence. The magnetic
measurements were performed by using a Quantum De-
sign SQUID magnetometer.
3. Microstructure of films
Figure 1 presents the �–2� XRD scans for the films de-
posited at Tsub � 20 (a), 300 (b) and 750 (c) °C, respec-
tively. The film deposited at the lowest substrate tempera-
ture manifests the very fuzzy Bragg peaks near (00l)-like
reflections of LAO-substrate only that is typical for fully
disordered or amorphous crystal structure [12]. Inset in
Fig. 1,a displays the fast Fourier transform (FFT) of the
cross-sectional HREM image across the film/substrate in-
terface. It is seen that FFT produces a bright uniform halo
instead of a rectangular pattern of circular spots. Only the
slightly luminescent spots corresponding to a crystal lat-
tice of substrate are just-noticeable. Therefore, one can
conclude that the film deposited at Tsub � 20 °C reveals
an amorphous structure.
The film deposited at middle-Tsub manifests the (0kl)-
and (hkl)-like Bragg peaks in addition to the broad (00l)
XRD reflections. The FFT pattern, represented by the in-
set in Fig. 1,b, shows a wide ring, which is typical for a
nano-scale disordered crystal structure [12]. Additional
ED analysis (no shown) reveals that the average size
of the randomly-oriented crystallites turn out to be about
4 nm. This film hereinafter will be referred as a nano-
clustered one.
The more unexpected microstructure was found for the
film grown at Tsub � 750 °C. XRD scan displays the (00l)
Bragg peaks of high intensity only, indicating that the de-
position results in the highly c-oriented microstructure.
The FFT of cross-sectional HREM image produces
mainly a rectangular pattern of spots that is typical for a
perfect crystal lattice. It is confirmed by the analysis of
the ED pattern prepared at room temperature (see inset in
Fig. 2,a), which reveals the well-known orthorhombic
crystal structure with the lattice parameters for a simple
pseudocubic symmetry about a b� � 0.3852 nm and
c � 0.3856 nm, respectively, that are almost coincident
with the bulk [2,8,13]. Figure 2,b shows that the ED pat-
tern prepared at 87 K is the same. On the other hand, a
890 Fizika Nizkikh Temperatur, 2007, v. 33, No. 8
V.G. Prokhorov et al.
Fig. 1. �–2� XRD scans of amorphous (a), nanoclustered (b),
and bilayered (c) films. Insets are FFT of the corresponding
HREM images.
slight splitting of the Bragg reflections along c� direction
(indicated by white arrows) is observed for both tempera-
tures. Therefore, the high-Tsub film has an additional
crystal phase with the out-of-plane lattice parameter,
which is larger than that for the bulk, c � 0.3896 nm. The
existence of two phases with different crystal structure is
well-defined on the cross-sectional low-magnification
HREM image, taken at a room temperature and repre-
sented by the Fig. 2,a. It should be noted that the HREM
image prepared at 87 K remains without changing. Analy-
sis of the transmission contrast of the HREM image re-
veals that the film has a column-like microstructure (with
the average diameter of the columns about 10 nm) and the
sufficiently thick ( 20 nm) intermediate layer on the film
side (indicated by the white arrows). An additional FFT
analysis (not shown) manifests that particularly the inter-
mediate layer has a crystal lattice with the tetragonal dis-
tortion (c a/ � 1.015), while the rest of the film with a
column-like microstructure has lattice parameters coinci-
dent to the bulk. Since the intermediate layer has a
tetragonal distortion, one can assume that it is the slightly
strained crystalline phase due to the large mismatch be-
tween film and substrate lattice parameters. On the other
hand, as the thickness of grown layer exceeds a certain
critical value during the film’s deposition, the film tend to
stress relaxation by the formation of the misfit disloca-
tions or a columnlike microstructure directed normal to
the film plane [14]. This film hereinafter will be referred
as a self-organizing bilayered one.
The ED patterns of the high-Tsub NSMO film, pre-
pared at 300 K (inset in Fig. 2,a) and 87 K (Fig. 2,b), dis-
play only fundamental Bragg spots without traces of the
CO phase, which is observed at TCO � 150 K in bulk as a
rule. It is well-known that the CO phase is characterized
by the appearance of superlattice spots on the ED pat-
terns, with a wave vector q � a� /2, where a� is the recip-
rocal lattice vector along the a axis for the simple
pseudocubic symmetry. For comparison Fig. 2,c shows
the same ED pattern for the half-doped La Ca MnO0 5 0 5 3. .
(LCMO) film, prepared at the same conditions, in which
the CO state is observed [15]. The superlattice reflections
are indicated by a white arrow. The nonappearance of the
CO phase in the high-Tsub NSMO film even at low tem-
perature (87 K) can be explained by two following cir-
cumstances. First, the major part of the film consists the
small-size columnlike clusters that provide the complete
suppression of the CO phase [9]. Second, the intermediate
layer has an in-plane compressive strained crystal lattice
that also leads to a degradation of the CO state [5].
Summarizing the microstructural measurements one
can conclude that three kinds of the Nd Sr MnO0.5 0.5 3
films with different microstructure were prepared.
There are the amorphous (Tsub � 20 °C), the nanoclus-
tered (Tsub � 300 °C), and the self-organizing bilay-
ered (Tsub � 750 °C) films. All films have a nano-crys-
talline microstructure and do not manifest the traces of the
CO state with the decreasing temperature up to 87 K.
4. Experimental results
Figure 3 shows both FC (solid symbols) and ZFC
(open symbols) temperature-dependent magnetization
curves, M(T ), at different magnetic field for the amor-
phous film. The M(T ) behavior is typical for paramag-
netic (PM) materials [16] and could be written for the
whole temperature range as
M T H C T HPM
CW( , ) [ / ( )]�
� �0 ,
where � 0 is a temperature-independent susceptibility, and
the second term is the Curie–Weiss (CW)-type suscepti-
bility with a constant CCW and a characteristic tempera-
ture �. Inset shows that the experimental data are excel-
lently described by the CW expression with the following
Nonclassical magnetic dynamics and negative exchange bias in Nd0.5Sr0.5MnO3 films
Fizika Nizkikh Temperatur, 2007, v. 33, No. 8 891
NSMO
Intermediate layer
LAO
(001)
300 K
a
b
(001)
c
(001)
NSMO LCMO
87 K 87 K
20 nm
Fig. 2. a — Low-magnification cross-sectional HREM image
for the bilayered film across the interface with substrate mea-
sured at 300 K. Inset shows the ED pattern taken at room tem-
perature. b — ED pattern of the same film taken at 87 K.
c — ED pattern taken at 87 K for the La0 5. Ca0 5. MnO3 film.
White arrow indicates the superlattice reflections, connected
with an appearance of the CO state.
fitting parameters: CCW � 0.002 emu·K/T and � = 5 K.
The estimated from CCW effective magnetic moment was
�eff � 4.4� B /Mn, that is almost coincident with the
theoretical value, �
eff
theor
� 4.42� B /Mn, which is obtained
from following expression:
�
eff
theor �
�
g xS S x S S1 1 2 21 1 1( ) ( ) ( ).
Here x is the Sr concentration, S1 = 3/2 and S 2 = 2 are the
spin values of Mn 4
and Mn 3
ions, respectively, and
g = 2 is the Land� factor. Therefore, one can conclude that
the amorphous film is a typical paramagnet with a freely
moving of the individual Mn spins.
Figure 4 presents the M T( ) curves for the nanocluster-
ed film measured under the different applied magnetic
field. The magnetization value monotonically increases
with decreasing temperature for all the applied fields,
demonstrating the superparamagnetic (SPM) behavior up
to about 40 K. Moreover, the ZFC and FC M T( ) curves
are nonsplitted in this temperature range. At the same
time, the significant difference between ZFC and FC
M T( ) is occurred below of 40 K, indicating appearance of
an additional magnetic transition. Inset shows the
in-plane magnetization loops for this film at different
temperature. It is seen that the M(H) dependences mani-
fest a nonhysteretic behavior in the low-field range but
have the hysteretic lobes (indicated by arrows) at higher
magnetic field.
Figure 5 shows the temperature-dependent magnetiza-
tion of the self-organizing bilayered film. The main fea-
ture of this film lies in the unique M T( ) behavior with in-
creasing applied magnetic field. At low applied magnetic
field (100 Oe) M T( ) demonstrates a shape, which is typi-
cal for the FM state (the magnetization tends to a satu-
ration at low temperature), while at higher one the
SPM-like behavior is added (the magnetization perma-
nently grows with the decreasing temperature). There-
fore, the M T( ) dependences testify for a presence of two
different magnetic phases, FM and SPM, which are lo-
cated, probably, in the different regions of the film. Ta-
king into account the HREM analysis (Fig. 2,a), one can
conclude that FM belongs to the intermediate layer while
SPM occupies the film’s part with the nanocrystalline co-
lumn-like microstructure. Inset displays the in-plane mag-
netization loops for this film at different temperature. In
contrast to the nanoclustered film the M(H) dependence
reveals the well-defined symmetric hysteretic loop at T �
� 100 K, which confirms a presence of the FM phase. At
892 Fizika Nizkikh Temperatur, 2007, v. 33, No. 8
V.G. Prokhorov et al.
100 200
0
5
10
0.02 0.04
0
2
4
6
8
3000 Oe
500 Oe
TC
T, K
500 Oe
3000 Oe
M
,
em
u
/c
m
3 M
,
em
u
/c
m
3
Fig. 3. Temperature dependence of the in-plane FC (solid sym-
bols) and ZFC (open symbols) of a magnetization for the
amorphous film measured under an applied magnetic field of
500 and 3000 Oe. Lines are guides to the eyes. Arrow indi-
cates the location of the Curie temperature for this compound.
Inset is the M T( ) versus 1 � �( )T � plot for the same film.
100 200
0
5
10
15
–5 0 5
–10
0
10
TB
4
3
2
1
T, K
2 K
100 K
in–plane
H, kOe
M
,
em
u
/c
m
3 M
,
em
u
/c
m
3
Fig. 4. Temperature dependence of the in-plane FC (solid sym-
bols) and ZFC (open symbols) of a magnetization for the
nanoclustered film measured under an applied magnetic field
of 500 (1), 1500 (2), 3000 (3), and 10000 (4) Oe. Lines are
guides to the eyes. Arrow indicates location of the blocking
temperature. Inset shows the in-plane hysteresis loops at 2 and
100 K. Arrows indicate the hysteretic lobes in a high-field
range.
the same time, with decreasing temperature up to 5 K the
magnetization loops become greatly asymmetric. Similar
to the nanoclustered film, the additional magnetic transi-
tion is observed below 50 K with decreasing temperature
as well.
5. Discussion
Let us consider the peculiarity of the magnetic state in
the Nd Sr MnO0.5 0.5 3 film controlled by a different kind of
the microstructure disorder.
Figure 3 reveals that the amorphous film is a typical
paramagnet with a freely moving of the individual Mn
spins, the M T( ) dependence of which is well described in
framework of the Curie–Weiss approximation. Therefore,
the quench-disordered microstructure, which is formed
during the deposition at Tsub � 20 °C, results in the pa-
ramagnetic state of the film. Such effect have already
observed by us in the La Ca MnO0 7 0 3 3. . films [12] and can
be explained by the anomalously small size of the crystal-
lites.
In contrast to that the temperature dependence of a
magnetization for the nanoclustered film, deposited at
Tsub � 300 °C, cannot be described by the CW approxi-
mation. Figure 6 shows the M T( ) versus 1/ T plot for this
film, measured at different applied magnetic fields, which
does not demonstrate a linear behavior in the whole tem-
perature range. Such a nonlinear behavior of M(H T/ ) is
more typical for the SPM particles and can be explained
with the Langevin function [16]:
M T H M
H
k T
k T
H
SPM
s
SPM
B
B( , ) coth�
�
��
�
�
�� �
�
�
�
�
�
�
�
�
eff
eff
, (1)
where M s
SPM is the saturated magnetization of a SPM
phase and �eff is the average magnetic moment of the
SPM particles. Solid lines in Fig. 6 represent fitted
Langevin functions closest to the experiment data with a
variation of the average effective magnetic moment, �eff .
It is surprisingly that �eff greatly dependent on an applied
magnetic field through the empirical function of
�eff ~ /1 H. By taking the saturation magnetic moment for
bulk Nd Sr MnO0.5 0.5 3 in the FM state as 3.5 � B /Mn [17]
and assuming a spherical shape of the SPM clusters with a
volume of �D 3/ 6, we estimate their average diameter to
be D � 4.3 and 1.7 nm for 100 and 10000, respectively. In
principle, it is coincident with the ED data, particularly in
the case of a lowest applied magnetic field, and allow us
to conclude that the nanocrystalline disorder leads to for-
mation of the SPM state in the film. However, the size of
Nonclassical magnetic dynamics and negative exchange bias in Nd0.5Sr0.5MnO3 films
Fizika Nizkikh Temperatur, 2007, v. 33, No. 8 893
100 200
0
5
10
15
20
30
–1 0 1
–20
–10
0
10
20
3
2
1
TC
TB
T, K
Hc
in–plane
100 K
10 K
5 K
H, T
25
M
,
em
u
/c
m
3
M
,
em
u
/c
m
3
Fig. 5. Temperature dependence of the in-plane FC (solid sym-
bols) and ZFC (open symbols) of a magnetization for the
bilayered film measured under an applied magnetic field of
100 (1), 3000 (2), and 10000 (3) Oe. Lines are guides to the
eyes. Arrows indicate location of the blocking temperature. In-
set shows the in-plane hysteresis loops at different tempera-
ture. Arrows indicate the coercive field Hc.
0.02 0.04
10
–6
10
–5
10
–4
0 5 10
1
2
M
ag
n
et
ic
m
o
m
en
t,
em
u
T ,
–1
K
–1
3000 Oe
1500 Oe
500 Oe
H, kOe
10000 Oe
Fig. 6. M T( ) versus 1/ T plot for the nanoclustered film. Solid
lines are represent fitted Langevin functions closest to the ex-
periment data with a variation of the effective magnetic mo-
ment �eff . Inset displays the �eff ( )H dependence simulated on
the base of the Langevin approximation.
the SPM particle is not rigidly bound with the crystalline
cluster and is controlled by an applied magnetic field,
contradicting to the underlying principles of the theoreti-
cal models for an ensemble of the SPM particles. The sim-
ilar unusual FC M T( ) behavior was observed already in
the La 1�xCa xMnO 3 FM/AFM multilayers and explained
by the thermal instability of different exchange paths,
first of all, a spin-flop transition in AFM grains under an
applied magnetic field [18]. However, in our case both the
FC and ZFC M T( ) curves demonstrate an unsplitted ex-
ponential behavior, beginning from the lowest applied
magnetic field, except for the temperature range below
TB .
A following explanation on the unusual M H T( , ) be-
havior can be proposed. It is believable that the FM
ground state in the manganites is provided by the Zener’s
double exchange interaction by means of an electron
transfer from Mn
3+
to Mn 4
via the oxygen [1]. Because
any structural defects, such as grain or twin boundaries,
for example, block up the electron motion, the FM cou-
pling between the adjacent crystalline clusters will be
also suppressed. Consequently, the individual crystallites
in the nanoclustered film can be treated as the independ-
ent magnetic domains with the abnormally small
interdomain separation. Such type of the magnetic objects
lower their magnetostatic energy by forming the clo-
sure-domain [19] or the magnetic vortex [20] structure.
Therefore, the nanocrystalline clusters in this film are not
the FM single domains and their magnetic properties can
not to be correctly described in the framework of a classi-
cal Langevin statistics, expressed by Eq. (1). It is con-
firmed by the unusual shape of the hysteresis loops repre-
sented by the inset in Fig. 4. The loops have hysteretic
lobes (indicated by arrows) but are nonhysteretic as the
field crosses zero. Such kind of the hysteresis loop was
simulated recently for submicron Permalloy dot arrays,
utilizing the concept of the magnetic vortex (MV) [21].
Therefore, one can conclude that the nanoclustered film
demonstrates the magnetic properties, which are resem-
ble the behavior for an ensemble of the SPM particles,
however, each particle in this case must be consider as a
closure magnetic domain (or magnetic vortex) rather than
the single one.
Figure 4 displays that the FC and ZFC M T( ) curves
become splitted at T TB� . The similar effect is very often
observed in an ensemble of FM (or AFM) nanoparticles
and explained in framework of the N�el–Brown theory
[22,23]. According to this model, there is a critical
temperature, called the blocking one, given by TB �
� E kA B/ [ln ( / ) ]� �obs 0 , above which the FM nanopar-
ticles transform from the blocked to unblocked SPM
state. Here E A is the energy barrier between two direc-
tions of the magnetic moment in a single domain nano-
particle — along and against to an applied magnetic field,
�obs is the observation time, and � 0 is the characteristic
time constant, called the attempt frequency. On the other
hand, the magnetic vortex also has two different magnetic
states separated by an energy barrier, owing to two key
properties: chirality (clockwise or counterclockwise) and
polarity (positive or negative) [20]. It is reasonable to
suggest that below a certain temperature these states can
be blocked (or frozen) if the thermal energy becomes
smaller than the energy barrier between different states of
the magnetic vortex. Therefore, the observed splitting be-
tween the FC and ZFC M (T ) curves, represented by
Fig. 4, can be treated as a blocked–unblocked phase tran-
sition in an assembly of the SPM particles, and TB is the
blocking temperature. However, it worth to note, that in
contrast to the usual FM single domain nanoparticles the
M H( ) dependence measured at T TB� (see inset) mani-
fests the nonhysteretic loop as the magnetic field crosses
zero.
The self-organizing bilayered film, taking into account
a microstructure analysis, consist of the two layers with
the different microstructure and the same chemical com-
position. The temperature-dependent magnetization
curve in this case can be represented by superposition of
the FM and SPM terms: M T M T M TFM SPM( ) ( ) ( )�
.
The M TFM ( ) term belongs to the intermediate layer with
the perfect (but slightly strained) crystal structure while
M TSPM ( ) defines the magnetic properties of major part
of the film with the column-like structure. Figure 5 shows
that the intermediate layer undergoes only the FM transi-
tion at TC � 250 K with decreasing temperature, which is
good coincident with the published results. At the same
time, the part of the film with the column-like structure
demonstrates the M TSPM ( ) behavior similar to that was
observed for the nanoclustered one (see Fig. 4). The ex-
ponential growth of the M TSPM ( ) with decreasing tem-
perature is ended by the well-defined FC–ZFC splitting at
the high applied magnetic field (curves 2 and 3) or the
kink-like peculiarity when H is smaller of the coercive
field H c (curve 1). Inset in Fig. 5 reveals that H c �
� � 450 Oe at 100 K, which is larger than an applied mag-
netic field taken for measure of the curve 1 (H � 100 Oe).
Therefore, the observed large difference between the FC
and ZFC M T( ) experimental curves, starting closely be-
low TC , concerns to the FM transition only in the interme-
diate layer and can be explained by the lattice distortion
[14]. Therefore, the self-organizing bilayered film can be
treated as a hybrid magnetic system involving the SPM
and FM layers with a very sharp interface (see Fig. 2).
The blocking temperature in the this film (TB � 50 K)
turn out to be slightly larger than that observed in the
nanoclustered one (TB � 40 K). We assume that the ob-
served increase in TB is governed by an additional ex-
change coupling between the SPM and FM layers which
894 Fizika Nizkikh Temperatur, 2007, v. 33, No. 8
V.G. Prokhorov et al.
leads to increase of the energy barrier between different
state of the SPM particle.
Inset in Fig. 5 displays that the hysteresis loop of the
bilayered film greatly changes the shape with decreasing
temperature. It is seen the hysteresis loop is symmetrical
at 100 K with the coercive field Hc � � 450 Oe while
becomes greatly asymmetric at 10 and 5 K, H c �
1200
and �1800, and
1250 and �2500 Oe, respectively. This
effect is observed as a rule in the FM/AFM hybrid system
and explained by the appearance of an exchange-bias in-
teraction on the interface between the FM and AFM
phases [24]. Exchange bias is a measure of the shift in the
hysteresis loop along the applied magnetic field axis of
FM when it is magnetically coupled to AFM [25]. This is
commonly characterized by an exchange-bias field H EB
through which the center of the hysteresis loop of FM is
shifted from zero. In our case H EB � �300 and –625 Oe at
T � 10 and 5 K, respectively. Therefore, the self-organiz-
ing bilayered film demonstrates a negative exchange bias
which is typical for the FM(particles)/AFM(layer) hybrid
system in spite of that we deal with the FM(partic-
les)/FM(layer) one, in which this phenomenon is quite
unexpected. We suggest that the found contradiction is
connected with the closure-domain (or magnetic vortex)
nature of the nanoclusters which play a role of the SPM
particles in our film. In this case the SPM particle, form-
ing the magnetic vortex, can be treated as the AFM rather
than FM domain. Therefore, in fact, the bilayered film
represents the AFM(particles)/ FM(layer) hybrid system.
At high temperature (T TB� ) the AFM-like domains have
a behavior, which is similar to the freely moving SPM
particles without any interaction with FM intermediate
layer. In this case the hysteresis loop is symmetrical (see
the inset in Fig. 5). When a temperature decreases,
T TB� , the AFM-like domains transform in the blocking
state and induce exchange anisotropy in the FM layer
which shifts the hysteresis loop towards negative H val-
ues. Moreover, the exchange bias field H EB greatly in-
creases with decreasing temperature that absolutely
agrees with published results for the AFM/FM layered
systems.
6. Conclusions
In summary, the amorphous, the nanoclustered, and
the self-organizing bilayered films have been prepared by
the rf-magnetron sputtering using a so-called «soft» (or
powder) target. The amorphous film turn out to be a typi-
cal paramagnet with a freely moving of the individual Mn
spins, the magnetic properties of which is well described
in framework of the Curie–Weiss approximation. The
nanoclustered film manifests the magnetic properties
mimic to the ensemble of the SPM particles, however,
which can not be correctly described in the framework of
a classical Langevin approach. Taking into account, the
unique shape of the hysteresis loops, which have hyste-
retic lobes at high magnetic field but are nonhysteretic as
the field crosses zero, we suggest that each particle is a
magnetic vortex rather than the single domain. At the
same time, the unblocked to blocked transition was ob-
served in the film with decreasing temperature. This tes-
tify that the magnetic vortex, similar to the single-domain
particle, can transfer from a high-temperature SPM to a
low-temperature blocked (or «pinned») state at T TB� .
The self-organizing bilayered film demonstrates a nega-
tive exchange bias, which is typical for the FM/AFM hy-
brid system. The exchange bias interaction leads to the in-
crease of TB , the significant shift in the hysteresis loop
along the applied magnetic field axis, H EB � �625 Oe at
5 K, and the broadening of a hysteresis loop. All films did
not manifest the charge ordering in the whole temperature
range up to 87 K.
This work was supported by the KOSEF through the
Quantum Photonic Science Research Center.
1. For a review, see Colossal Magnetoresistance, Charge Or-
dering and Related Properties of Manganese Oxides,
C.N.R. Rao and B. Raveau (eds.), World Scientific, Singa-
pore (1998); Colossal Magnetoresistance Oxides, Y. To-
kura (ed.), Gordon and Breach, London (1999); E. Dagot-
to, T. Hotta, and A. Moreo, Phys. Rep. 344, 1 (2001).
2. R. Kajimoto, H. Yoshizawa, H. Kawano, H. Kuwahara, Y.
Tokura, K. Ohoyama, and M. Ohashi, Phys. Rev. B60,
6506 (1999).
3. T. Fujiwara and M. Korotin, Phys. Rev. B59, 9903 (1999).
4. Y. Ogimoto, M. Nakamura, N. Takubo, H. Tamaru, M. Izu-
mi, and K. Miyano, Phys. Rev. B71, 060403 (2005).
5. H. Wang, X. Zhang, M.F. Hudley, J.D. Thomoson, B.J.
Gibbons, Y. Lin, P.N. Arendt, S.R. Foltyn, Q.X. Jia, and
J.L. MacManus-Driscoll, Appl. Phys. Lett. 84, 1147
(2004).
6. Q. Qian, T.A. Tyson, C.-C. Kao, W. Prellier, J. Bai, A.
Biswas, and R.L. Greene, Phys. Rev. B63, 224424 (2001).
7. C. Cui, T.A. Tyson, Z. Chen, and Z. Zhong, Phys. Rev.
B68, 214417 (2003).
8. R.C. Yu, J. Tang, L.D. Yao, A. Matsushita, Y. Yu, F.Y. Li,
and C.Q. Jin, J. Appl. Phys. 97, 083910 (2005).
9. P. Levy, F. Parisi, G. Polla, D. Vega, G. Leyva, H. Lanza,
R.S. Freitas, and L. Ghivelder, Phys. Rev. B62, 6437
(2000).
10. V. Ponnambalam, S. Parashar, A.R. Raju, and C.N.R. Rao,
Appl. Phys. Lett. 74, 206 (1999).
11. V.G. Prokhorov, G.G. Kaminsky, V.A. Komashko, J.S.
Park, and Y.P. Lee, J. Appl. Phys. 90, 1055 (2001).
12. V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, V.L.
Svetchnikov, Y.P. Lee, and S.Y. Park, Fiz. Nizk. Temp. 30,
938 (2004) [Low Temp. Phys. 30, 705 (2004)].
13. P.M. Woodward, D.E. Cox, T. Vogt, C.N. R. Rao, and
A.K. Cheetham, Chem. Mater. 11, 3528 (1999).
14. Y.P. Lee, S.Y. Park, Y.H. Hyun, J.B. Kim, V.G. Prokho-
rov, V.A. Komashko, and V.G. Svetchnikov, Phys. Rev.
B73, 224413 (2006).
Nonclassical magnetic dynamics and negative exchange bias in Nd0.5Sr0.5MnO3 films
Fizika Nizkikh Temperatur, 2007, v. 33, No. 8 895
15. V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P.
Lee, S.Y. Park, Y.H. Hyun, V.L. Svetchnikov, K.W. Kim,
and J.Y. Rhee, Low Temp. Phys. 32, 128 (2006).
16. B.D. Cullity, Introduction to Magnetic Materials, Addi-
son-Wesley, New York (1972).
17. J. Geck, D. Buns, C. Hess, R. Klingeler, P. Reutler, M.v.
Zimmermann, S.-W. Cheong, and B. B�chner, Phys. Rev.
B66, 184407 (2002).
18. N. Moutis, C. Christides, I. Panagiotopoulos, and D. Niar-
chos, Phys. Rev. B64, 094429 (2001).
19. C. Kittel, Introduction to Solid State Physics, 7th ed. Wi-
ley, New York (1996).
20. V. Novosad, K. Yu. Guslienko, H. Shima, Y. Otani, S.G.
Kim, K. Fukamichi, N. Kikuchi, O. Kitakami, and Y. Shi-
mada, Phys. Rev. B65, 060402 (2002).
21. K.Yu. Guslienko, V. Novosad, Y. Otani, H. Shima, and K.
Fukamichi, Appl. Phys. Lett. 78, 3848 (2001).
22. L. N�el and C.R. Acad. Sci. URSS 228, 664 (1949).
23. W.F. Brown, Phys. Rev. 130, 1677 (1963).
24. W.H. Meilejohn and C.P. Been, Phys. Rev. 102, 1413
(1956).
25. For a review, see J. Nogu�s and I.K. Schuller, J. Magn.
Magn. Mater. 192, 203 (1999).
896 Fizika Nizkikh Temperatur, 2007, v. 33, No. 8
V.G. Prokhorov et al.
|