Journal of Magnetism and Magnetic Materials 323 (2011) 180–185
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Magneto-transport and magnetic properties of (1–x) La0.7Ca0.3MnO3 + xAl2O3 composites T.D. Thanh a, P.T. Phong a,b,n, N.V. Dai a, D.H. Manh a, N.V. Khiem c, L.V. Hong a, N.X. Phuc a a
Institute of Material Science, Viet Nam Academy of Science and Technology, Hanoi City, Viet Nam Nhatrang Pedagogic College, 1- Nguyen Chanh Str., Nhatrang City, Khanhhoa Province, Viet Nam c Department of Natural Sciences, Hongduc University, 307 -Le Lai Str. Thanhhoa City, Viet Nam b
a r t i c l e in f o
a b s t r a c t
Article history: Received 29 June 2010 Received in revised form 31 August 2010 Available online 7 September 2010
We report magneto-transport and magnetic properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3 composites synthesized through a solid-state reaction method combined with a high energy milling method. Most interestingly, the effective magnetic anisotropy is found to decrease with increase in the non-magnetic insulating Al2O3 phase fraction in the composites. In addition, we observed that the magnitude of low-field magnetoresistance arising from spin-polarized tunneling of conduction electrons, as well as that of high-field magnetoresistance, displays a Curie–Weiss law-like behavior. Finally, we found that the temperature dependence of low and high-field magnetoresistance is controlled predominantly by the nature of temperature response of surface magnetization of the particles. & 2010 Elsevier B.V. All rights reserved.
Keywords: Manganites composite Low field magnetoresistance Magnetic property Spin polarized tunneling
1. Introduction Magneto-transport and magnetic properties in manganites have been one of the most frequently studied topics in solid state physics since the discovery of colossal magnetoresistance (CMR) up to 1300% in LCMO thin film at T¼200 K [1]. CMR is restricted to a narrow range of temperature around the ferromagnetic– paramagnetic phase transition, and this makes it difficult to apply for electronic devices. Recently, another type of MR, extrinsic CMR in manganites, has been discovered. The extrinsic CMR, which is related to the grain boundaries, can be explained by spin-polarized tunneling [2]. The tunneling process takes place across the interfaces or grains separated by an energy barrier related to the magnetic disorder. Hence diluting with an insulating material in the manganites can adjust the barrier layer and thus influence the tunneling process. Since these extrinsic effects acted as pinning centers in demagnetization by domain wall displacement, a small field can align the neighboring ferromagnetic (FM) grains and as a result, enhanced MR response is achieved at low magnetic fields and low temperatures. The effect has been named as low field magnetoresistance (LFMR). Several groups have attempted to enhance LFMR by making a composite of these CMR oxides with a secondary phase like an insulating oxide, as well as with a hard FM material or also with a
n Corresponding author at: Nhatrang Pedagogic College, 1-Nguyen Chanh Str., Nhatrang City, Khanhhoa Province, Viet Nam. E-mail address:
[email protected] (P.T. Phong).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.08.060
polymer [3–14]. However, these studies focus only on enhancing LFMR values, paying less attention to the relationships between magnetic state of grain surface and magnetoresistance of composites, whereas spin misorientation at the magnetically virgin state of the system is crucial to obtain enhanced MR. In addition, the surface dislocations and the presence of extra phases in contact with the grain boundary change locally the anisotropy, thus providing pinning centers for the surface spin. When an external magnetic field is applied, the spin disorder is suppressed and Mn spins within the disordered region realign along the field direction. As a consequence, MR improvement is obtained. In a previous work [9], we have investigated the crystalline structures and LFMR properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3 (LCMO/Al2O3). However, the magneto-transport and magnetic properties of LCMO/Al2O3 have not clearly been shown. In this paper, we present a detailed study of magneto-transport and magnetic properties of LCMO/Al2O3 composites synthesized through a solid-state reaction method combined with a high energy milling method, especially its temperature and magnetic field dependence. Most interestingly, we observed that the effective magnetic anisotropy (Kan) decreases with increase in Al2O3 content, which is attributed to the increase in the nonmagnetic insulating Al2O3 phase fraction in composites. Further, we also observed that the magnitude of LFMR, arising from spinpolarized tunneling of conduction electrons, as well as that of high-field magnetoresistance (HFMR), displays a Curie–Weiss law-like behavior. In order to explore the basic physics behind the temperature dependence of LFMR, our data have been analyzed using a phenomenological model [15] based on the spin-polarized
T.D. Thanh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 180–185
tunneling of conduction electrons at the grain boundaries. Further, by analyzing the data in the following theoretical perspective proposed by Lee et al. [16], we found that the temperature dependence of MR is governed predominantly by the nature of temperature response on the surface magnetization (MS) of manganite composite particles.
2. Experiment The LCMO/Al2O3 (x¼0, 0.01, 0.02, 0.03, 0.04 and 0.05) composites were prepared in three steps. First, LCMO powder was synthesized by a conventional solid-state reaction method combined with a high energy milling method. High purity (99.99%) La2O3, CaCO3 and MnO powders were mixed in the appropriate stoichiometric ratio and ground. The well-mixed powders were preheated at a temperature of 1250 1C for 15 h. Subsequently, it was heated at 1300 1C for 10 h. Next the LCMO and Al2O3 powders were ground by the energy milling machine for 2 h. Finally the appropriate amounts of LCMO nanopowder and Al2O3 powder were mixed and the homogenous powder was pressed into pellets at a pressure of 10 MPa/cm2 and sintered at 900 1C for 3 h. Structural characterization was carried out by employing the X-ray diffraction (XRD) technique at room temperature in the 2y ˚ range of 20–751 with a step size of 0.031 using CuKa (l ¼ 1.5406 A) radiation and surface morphology was observed by scanning electron microscopy (SEM). Magnetic measurements were performed by utilizing a vibrating sample magnetometer (VSM) in the temperature range 80–300 K. Resistivity and magnetoresistance (MR–H) of all the composites were examined by Physical Property Measurement Systems (PPMS) in a magnetic field from 0 to 30 kOe at a temperature range from 5 to 300 K.
The paramagnetic (PM) to ferromagnetic (FM) phase transition temperature (TC) determined from the peak of dM/dT is almost independent of Al2O3 content and is about 250 K for all the samples. This is attributed to the fact that the PM–FM phase transition is an intrinsic and intragrain property. The general behavior of magnetization vs. temperature indicates long-range ferromagnetism . The observed constancy of TC also indicates that stoichiometry of LCMO phase within the grains remains essentially unchanged as Al2O3 is not accommodated within the perovskite structure and it occupies only the boundaries of the LCMO grains. The magnetic hysteresis loops recorded at 5 K for all samples are shown in Fig. 2. Magnetization of the samples increases rapidly at low field and then tends to saturate at higher field. The value of magnetization of composites again decreases with x because of reduced volume fraction of the LCMO phase. This demonstrates that ferromagnetic order is weakened and magnetic disorder increases with Al2O3 content. In order to understand the nature of field dependence of magnetization in the ferromagnetic regime, the magnetization curves were analyzed by using the ‘‘law of approach to the saturation’’ (LAS) of an assembly of particles with uniaxial anisotropy [17]: MðHÞ ¼ Ms ½1a=Hb=H2 þ wd H
ð1Þ
where Ms is the saturation magnetization, and a and b are suitable constants and wd is the high-field differential susceptibility. The best fits of the magnetization curves using Eq. (1) are shown in Fig. 3 for samples with x¼0, 0.03 and 0.05 by considering a, b, Ms and wd as free parameters. We define Kan as the magnetic
100
3. Results and discussion
x=0 x = 001 x = 0.02 x = 0.03 x = 0.04 x = 0.05
M (emu/g)
50
The XRD patterns of the composites clearly showed that the correlative peaks of LCMO do not shift. The perovskites phase of LCMO is preserved for all x weight fractions of Al2O3 considered, which would be indicative of the coexistence of two phases in the composite. The direct evidence of two phases also comes from SEM micrographs. Representative SEM micrographs of LCMO/ Al2O3 composites with x ¼0 and 0.03 are shown in Fig. 1. The Al2O3 regions present a different contrast, and exhibit dispersion of particles in LCMO. Moreover, energy dispersive X-ray (EDX) spectra of the doped composite for x ¼0.03 showed the aluminum peak along with La, Ca, Mn and O peaks, which also prove the presence of Al2O3 in the doped composites.
181
0
-50
-100
T=5K
-40
-20
0 H (kOe)
20
40
Fig. 2. Magnetic hysteresis loops at 5 K for all samples with various Al2O3 contents.
Fig. 1. SEM photograph of LCMO/Al2O3 samples with x¼ 0 (a) and x¼ 0.03 (b).
T.D. Thanh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 180–185
100 M (emu/g)
M (emu/g)
80 x=0
60 40
T=5K
20 0
0
10
20 30 H (kOe)
80
70
70
60 x = 0.03
60 50 40
40
M (emu/g)
182
x = 0.05
50 40
T=5K
30 0
10
20 30 H (kOe)
20
40
0
10
20 30 H (kOe)
40
Fig. 3. Fitting of magnetization curves of the samples with x¼ 0, 0.03 and 0.05 using Eq. (1).
0
Kan
2
1 0.5
MR
5
χ
-0.3
2.5
-0.4 0
0
1
2
3
4
5
-0.5 0
x (%)
0
x=0 x = 0.01 x = 0.02 x = 0.03 x = 0.04 x = 0.05
T=5K
-0.1 -0.2
1.5
χ (emug/Oe x 10 )
Kan (erg/cm3 x 107)
2.5
0
1
2
3
4
5 T = 250 K
x (%) Fig. 4. Effective magnetic anisotropy (Kan) dependence at 5 K on Al2O3 content. The line is a linear fit to the data. Inset: high-field differential susceptibility wd for the samples.
anisotropy energy (Kan) in all samples. For uniaxial systems [17] 1=2 105 bMs2 Kan ¼ ð2Þ 8 The effective magnetic anisotropy (Kan) and the high-field differential susceptibility wd are shown in Fig. 4 for LCMO/Al2O3. It is clear that the decrease in Kan is only due to the Al2O3 content present in the sample, while wd remains almost constant and, therefore, no further information about possible influence of Al2O3 content on magnetic ordering can be obtained from it. The value of Kan for x ¼0 obtained via Eq. (2) is about 2.39 107 erg/cm3 at 5 K. This magnetic anisotropy energy is much larger than the value of La0.7Sr0.3MnO3 reported in [17]. The magnetic field dependence of MR for all the samples studied at fixed temperatures 5 and 250 K measured in the magnetic field range of 0–30 kOe is shown in Fig. 5. MR behaves in an interesting manner. At low temperature (T¼5 K), MR value increases by nearly 15% on adding 5% of Al2O3 to pure LCMO. Nevertheless, MR at 250 K is the largest for x ¼0. The low field sensitivity (defined as the maximum slope of MR vs. H curve) also behaves in the same way. The decrease of MR at TC can be explained by taking into account the dilution of the FM magnetic phase and the DE mechanism around the paramagnetic to ferromagnetic phase transition temperature as a consequence of increasing Al2O3 content. In another interesting study, Hueso et al. [18] prepared nanocrystalline composites of LCMO/Al2O3 by the sol–gel method, which lead to LFMR of 40% at 77 K and 8 kOe at the percolation threshold of 10% Al2O3 content. They invoked that smaller grain sizes and good connectivity among neighboring grains caused by the dispersing particle are the reasons for LFMR enhancement in the LCMO/Al2O3 composites.
MR
-0.05 x=0 x = 0.01 x = 0.02 x = 0.03 x = 0.04 x = 0.05
-0.1
-0.15
-0.2
0
5
10
15 H (kOe)
20
25
30
Fig. 5. MR ratio as a function applied field for LCMO/Al2O3 composites at 5 and 250 K.
In order to further evaluate the property of MR behavior observed in this Al2O3-added composite, we survey the magnetic field dependence of MR for all samples using a phenomenological model that takes into account the spin-polarized tunneling at grain boundaries [15]. According to this model we get the expression for MR as Z H MR ¼ Au f ðkÞdkJHKH3 ð3Þ 0
Within the approximation of the model, in zero field the domain walls are pinned at the grain boundary pinning centers with the pinning strengths k. At the grain boundaries, the distribution of pinning strengths (defined as the minimum necessary field to overcome a particular pinning barrier) f(k) is given as f ðkÞ ¼ A expðBk2 Þ þ Ck2 expðDk2 Þ
ð4Þ
where all the adjustable fitting parameters A, B, C, D, J and K, with A0 absorbed in A and C, determined from a non-linear least-square fitting to calculate MRspt, which was defined as Z H f ðkÞdk ð5Þ MRspt ¼ 0
T.D. Thanh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 180–185
Differentiating Eq. (3) with respect to H and putting into Eq.(4), we get dðMRÞ ¼ A expðBH2 Þ þ CH2 expðDH2 ÞJ3kH2 dH
ð6Þ
In order to find the best-fit parameters at several temperatures, the experimental MR vs. H curves were differentiated and fitted to Eq. (4). Fig. 6(a) shows the differentiated curves and the best-fit function at T¼5 K for LCMO/Al2O3 composite samples. Next, these parameters were put in Eq. (1). Fig. 6(b) shows the theoretical curves from Eq. (3), which are quite matched to the experimental curves at several temperatures below TC. We observe that the total magnetoresistance is a monotonic function of temperature with a slow decrease at low temperature. The intrinsic contribution (MRint), however, follows the expected double exchange behavior with a steady increase in temperature. On the other hand, MRspt decreases steadily with increase in temperature.
183
According to Hwang et al. [2], the part of the MR most clearly identified with spin-polarized tunneling for La0.67Sr0.33MnO3 polycrystalline sample was prepared through a conventional solid-state reaction process in air. By this method the authors obtained sample grains of about micrometer size with a gradual increase with increasing calcining temperature. They had also observed that the temperature dependence of MRspt could be described quite well by an expression of type a+b/(c+T), which is characteristic of spinpolarized tunneling in granular ferromagnetic systems. Fig. 7 shows the best fit of MRspt(T) to the expression a +b/ (c + T). The fitted curves match well with the extracted values of MRspt from the model. However, our values of b and c for the best fit are much higher compared to those observed by Hwang et al. [2], although the TC value of our system is much smaller. This may be attributed to the structure of the composite. Al2O3 distribution at the grain boundaries can alter the size of the ferromagnetic grains, introducing artificial boundaries or defects, or diluting the ferromagnetic grains with a non-magnetic insulator. These procedures will have significant influence on the tunneling of conduction electrons and thus induce an enhancement of MR.
0
0
-0.02
-0.02
-0.01
-0.05
5K 30K 50K 70K 100K 120K
-0.05
MR
-0.1 -0.15 -0.2 -0.25
5K 30K 50K 70K 100K 120K
-0.05 -0.1 -0.15 -0.2 -0.25
x = 0.01
2
4 6 H (kOe)
8
-0.1 -0.15 -0.2 -0.25
x = 0.03
x = 0.05
-0.3 0
10
5K 30K 50K 70K 100K 120K
-0.05
-0.35
0
x = 0.05
-0.08 0
-0.3
-0.3
T = 5K
-0.04 -0.06
x = 0.03
-0.08 0
MR
-0.07 0
-0.35
T = 5K
-0.04 -0.06
-0.06
d(MR)/dH
T = 5K
-0.04
MR
d(MR)/dT
-0.03
d(MR)/dH
x = 0.01
-0.02
2
4 6 H (kOe)
8
-0.35
10
0
2
4 6 H (kOe)
8
10
Fig. 6. (a) Derivative of the experimental MR vs. H curve (dot) at 5 K in the magnetic field range 1–10 kOe for sample with x¼ 0.01, 0.03 and 0.05 and the fitted curve (line) using Eq. (6). (b) Experimental MR vs. H curve (dot) at various temperatures in the magnetic field range 0–10 kOe for sample with x ¼0.01, 0.03 and 0.05 and the fitted curve (line) using Eq. (3).
0.35
0.3 x = 0.01
0.15 0.1
0.25 0.2
20
40
60 80 100 120 140 T (K)
0.1
0.25 0.2
0.15 0
x = 0.05
0.3 MRspt
0.2
x = 0.03
0.3 MRspt
MRspt
0.25
0.35
0
20 40 60 80 100 120 140 160 T (K)
0.15
0
20 40 60 80 100 120 140 160 T (K)
Fig. 7. Best fit of MRspt at H ¼10 kOe to the function a+ b/(c + T) for 3 samples with x¼ 0.01, 0.03 and 0.05.
T.D. Thanh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 180–185
0.35
x = 0.01
0.4
0.3
0.35
0.25
0.25
0.2
0.2
MR
MR
0.3
0.35
0.15
x = 0.03
0.25
0.15
0.2 0.15
0.1
0.1
0.1
0.05
0.05
0.05
0
0
50
0
100 150 200 250 300 T (K)
x = 0.05
0.3 MR
184
0
50
0
100 150 200 250 300 T(K)
0
50
100 150 200 250 300 T(K)
Fig. 8. Magnetoresistance at H ¼10 kOe vs. temperature for LCMO/Al2O3 composites with x ¼0.01, 0.03 and 0.05 and the corresponding fitted curves (line) to a function a+ b/(c+ T).
1.4 1.3
1.5
1.2
1.4
1.6
x = 0.03
1.4
1.3
2
4 6 H (kOe)
8
1
10
1.3
x = 0.05
1.2 1.1
1.1 0
5K 30K 50K 70K 100K 120K 250K
1.5
1.2
1.1 1
5K 30 K 50 K 70 K 100 K 120K 250 K
1.6
x = 0.01
σ/σ0
1.5
σ/σ0
1.7
5K 30K 50K 70K 100K 120K 250K
σ/σ0
1.6
0
2
4 6 H (kOe)
8
1
10
0
2
4 6 H (kOe)
8
10
Fig. 9. Normalized conductivity as a function of magnetic field at several temperatures for LCMO/Al2O3 samples with x ¼0.01, 0.03 and 0.05.
25
25
15
25 S x 10-3
15
x = 0.03
20
x = 0.01
S x 10-3
S x 10-3
20
30
10
x = 0.05
20 15
10
5
10
0
5
5 0
50
100 150 200 250 300 T (K)
0
50
100 150 200 250 300 T (K)
0
50
100 150 200 250 300 T (K)
Fig. 10. Best fit of the high-field MC slope (S) at 10 kOe to a function a + b/(c + T) for the samples with x¼ 0.01, 0.03 and 0.05.
Fig. 8 shows temperature variation of the total experimental MR(T) at H¼10 kOe. Most interestingly, the temperature dependence of MR also is described quite well by an expression of the type a+b/ (c+T) similarly to the case of MRspt. In the case of polycrystalline materials grain boundaries provide defective sites, where the anisotropy energy of the surface spin is the lowest. At the disordered surface of polycrystalline grains, strong pinning of surface spins is expected. In the case of LCMO with the Al2O3 grain boundaries, these defects occur with high posibility at the grain surface. This is attributed to the magnetic dilution. On applying a magnetic field, the surface spins would be frozen as a consequence of the interactions between grain boundary pinning center (k) and magnetic field. However, when temperature increases up to a remakable extent, the
thermal energy is high enough to break the pinning of surface spin. As a result, MR varies with temperature following the relation a+b/(c+T), similarly to the case of MRspt. Futher, in order to clearly elucidate the temperature dependence behavior of LFMR and HFMR, we used the theoretical perspective proposed by Lee et al. [16]. Under the consideration of the present model, the slope of magnetoconductivity (MC) vs. H curve (correct form) at high field, i.e., H45 kOe, can be taken to be a measurement of the boundary spin susceptibility wb according to the equation MC
s 1 1 þ M2 þ 2wb HM 3 s0
ð7Þ
T.D. Thanh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 180–185
Here s is the conductivity in the presence of magnetic field H, s0 the zero-field conductivity, M the normalized magnetization of the bulk spin and wb the spin susceptibility of the boundary spins. Fig. 9 shows the high-field MC¼ s/s0 in H¼10 kOe, as a function of temperature and magnetic field for the three samples with x ¼0.01, 0.03 and 0.05. Fig. 10 shows the high-field (H¼10 kOe) MC slope (S), i.e., surface spin susceptibility (wb) as a function of temperature for the three samples. Very interestingly, we found that the temperature dependence of S follows an almost similar nature to that of temperature dependence of MRspt and MR of the respective samples, namely being described quite well by an expression of the type a +b/(c+ T). This highly correlated temperature dependence behavior of MRspt, MR and wb indirectly supports our understanding of the role of Ms. It is believed to be the controlling factor for temperature dependence behavior of MRspt and MR in manganites composites. It is worth noting that Dey and Nath. [19] and Mandal et al. [20] have also recently found that temperature dependence of MR is decided predominantly by the nature of temperature response of surface magnetization of these nanosize magnetic particles as proposed by Lee et al. [16] in La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3. 4. Conclusion In summary, a solid-state reaction method combined with high energy milling is used to prepare the LCMO/Al2O3 composites. XRD results show that no reaction between Al2O3 and LCMO takes place, and most Al2O3 is distributed at the grain boundaries. We have studied the effect of Al2O3 on magneto-transport and magnetic properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3. In spite of the increasing Al2O3 content, the ferromagenetic–paramagnetic transition temperature is still preserved. It has been found that the effective magnetic anisotropy (Kan) decreases with increase in Al2O3 content, while LFMR increases as the Al2O3 content increases. We have analyzed our experimental MR data following a phenomenological model to separate out the MR arising from spin-polarized transport, and from intrinsic contribution in our nanosize LCMO/Al2O3 composite samples. We also clearly show the major role played by the surface effects in an applied field. It is found that the temperature dependence of LFMR and HFMR displays a Curie–Weiss law-like behavior, i.e., MR(T)¼a + b/(c+T). This temperature dependence of MR is observed to be controlled predominantly by the nature of temperature response of surface
185
magnetization (Ms). It also requires deeper investigations of the Al2O3–manganites interaction, including the way in which Al2O3 disturbs the spin state at the grain boundaries of manganite.
Acknowledgments This work was performed using the facilities of the State Key Laboratories, Institute of Material Science (IMS), Vietnam Academy of Science and Technology (VAST). The financial support of the National Foundation for Science and Technology Development (Code: 103.02.45.09) is acknowledged. The second author is thankful to the Nhatrang Pedagogic College. References [1] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [2] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. ~ Solid State Commun. 145 [3] L.K. Gil, E. Baca, O. Mora´n, C. Quinayas, G. Bolanos, (2008) 66. [4] J. Kumar, Rajiv K. Singh, P.K. Siwach, H.K. Singh, Ramadhar Singh, O.N. Srivastava, J. Magn. Magn. Mater. 299 (2006) 155. [5] A. Gaur, G.D. Varma, Solid State Commun. 144 (2007) 138. [6] Bao-xin Huang, Yi-hua Liu, Xiaobo Yuan, Cheng-jian Wang, Ru-zhen Zhang, Liang-mo Mei, J. Magn. Magn. Mater. 280 (2004) 176. [7] Z.Y. Zhou, G.S. Luo, F.Y. Jiang, J. Magn. Magn. Mater. 321 (2009) 1919. [8] Z.F. Zi, Y.P. Sun, X.B. Zhu, C.Y. Hao, X. Luo, Z.R. Yang, J.M. Daii, W.H. Song, J. Alloys Compd. 477 (2009) 414. [9] P.T. Phong, N.V. Khiem, N.V. Dai, D.H. Manh, L.V. Hong, N.X. Phuc, Mater. Lett. 63 (2009) 353. [10] P.T. Phong, N.V. Khiem, N.V. Dai, D.H. Manh, L.V. Hong, N.X. Phuc, J. Alloys Compd. 484 (2009) 12. [11] V.P.S. Awana, R. Tripathi, S. Balamurugan, H. Kishan, E. Takayama-Muromachi, Solid State Commun. 140 (2006) 410. [12] C. Xiong, H. Hu, Y. Xiong, Z. Zhang, H. Pi, X. Wu, L. Li, F. Wei, C. Zheng, J.Alloys Compd. 479 (2009) 357. [13] Pham Thanh Phong, Nguyen Van Khiem, Vu Van Hung, Do Hung Manh, Le Van Hong, Nguyen Xuan Phuc, J. Phys. Conf. Ser. 187 (2009) 012070. [14] P.T. Phong, N.V. Dai, D.H. Manh, T.D. Thanh, N.V. Khiem, L.V. Hong va , N.X. Phuc, J. Magn. Magn. Mater. 322 (2010) 2737. [15] P. Raychaudhuri, K. Sheshadri, P. Taneja, S. Bandyopadhyay, P. Ayyub, A.K. Nigam, R. Pinto, J. Appl. Phys. 84 (1998) 2048. [16] S. Lee, H.Y. Hwang, Boris I. Shraiman, W.D. Ratcliff II, S.-W. Cheong, Phys. Rev. Lett. 82 (1999) 4508. [17] L.L. Balcells, J. Fontcuberta, B. Martinez, X. Obradors, J. Phys: Condens. Matter 10 (1998) 1883. [18] L.E. Hueso, J. Rivas, F. Rivaduddla, M.A. Lo´pez-Quintela, J. Appl. Phys. 89 (2001). [19] P. Dey, T.K. Nath, Phys. Rev. B 73 (2006) 214425. [20] S.K. Mandal, T.K. Nath, V.V. Rao, J. Phys.: Condens. Matter 20 (2008) 385203.
