CHAPTER 6. La doping effect in ferrimagnetic Ca 2 FeMoO 6 double perovskite. 6.1 Introduction

Transcription

1 CHAPTER 6 La doping effect in ferrimagnetic Ca 2 FeMoO 6 double perovskite 6.1 Introduction The experimental value of saturated magnetization in double perovskite structure A 2 FeMoO 6 is found to be lower ( 3.44, 3.52 and 3.84 in µ B /f.u for A = Ca, Sr and Ba, respectively) [1, 2] and non-integer in comparison with the theoretical predicted value (4µ B /f.u). The study of A 2 FeMoO 6 compounds with suitable non-magnetic substitution is essential for understanding the disorder effects in double perovskite structure. La and other non-magnetic substitutions have performed mainly on Sr 2 FeMoO 6 double perovskite [3]. The increase of La concentration (x) in Sr 2-x La x FeMoO 6 showed the decrease of magnetic moment, in addition to a non-monotonic increase of T C of the material. The results are explained in terms of decreasing site ordering of Fe 3+ and Mo 5+ ions (i.e., increasing ASD) in Sr 2- xla x FeMoO 6. There is some work on the structural and magnetic properties of Ca 2- xla x FeMoO 6 [4], which showed preliminary results on the limited concentration of La (x 0.3). Non-magnetic substitution in A 2 FeMoO 6 has opened another window to realize disorder effect. For example, replacement of Fe by non-magnetic (Al, Mg) in Sr 2 Fe 1-x Al x MoO 6 has decreased both magnetic moment and Curie temperature (T C ) [5, 6, 7]. The magnetic moment monotonically decreases with non-magnetic La and Nd doping in La x Sr 2 x FeMoO 6 [3, 8, 9], Nd x Ca 2 x FeMoO 6 [10, 11], La x Sr 2 x FeReO 6 [12], but T C of the material has shown a typical increasing trend at the early stage of doping. The decrease of magnetic moment has understood in terms of increasing ASD, which promotes strong antiferromagnetic Fe-O-Fe exchange interactions at the anti-phase boundary/grain-boundary region. It was even argued [10] that strong antiferromagnetic superexchange coupling between nearest-neighbor Fe Fe moments at the grain boundaries acts as the catalyst for further enhancement of ferromagnetic order and T C in electron-doped compounds. However, the nature of exchange interactions, whether double exchange ferromagnetic [11, 13, 14] or superexchange antiferromagnetic [10, 15], is still an open problem for A 2 FeMoO 6. 92

2 6.2 Sample preparation Polycrystalline samples of Ca 2-x La x FeMoO 6 (0.0 x 0.8) were prepared using solidstate sintering route. The stoichiometric mixture of high purity (>99.99%) La 2 O 3, CaCO 3, Fe 2 O 3, MoO 3 and Mo powders were ground together for 2 hours. La 2 O 3 was pre-fired at around C for 12 hours. The ground powder was made into pellets. The pellets placing in Platinum crucible were subjected to sintering at C (6 hours) and at C (12 hours) in a flow of Ar/H 2 (9:1) gas mixture with intermediate cooling - heating (@ 5 C/ minute) and grinding at room temperature. Final sintering of the pellets was carried out at C for 1 hour in the same gaseous atmosphere. The samples were cooled to room temperature at the rate of 50 0 C/hour. The samples with different La concentration were prepared under identical heat treatment and atmosphere. 6.3 Results and discussion Structure and surface morphology XRD pattern showed single phased La x Ca 2-x FeMoO 6 compound for various concentrations (x) of La. Fig. 6.1 shows the profile fit for selected (x = 0, 0.3 and ) samples. The crystal structure of La x Ca 2-x FeMoO 6 samples is adopted to the monoclinic structure with space group P2 I /n and consistent with earlier reports [10, 16]. Fig. 6.2 shows the variation of lattice parameters and cell volume (a, b, c and V) with La substitution. The overall increase of lattice parameters and cell volume support the fact that higher ionic radius La 3+ (1.16 Å) replaces the smaller ionic radius Ca 2+ (ionic radius ~ 1.12 Å) in the lattice structure. The electron injection due to La 3+ doping in Ca 2+ sites of double perovskite structure may also affect the cell expansion [3, 10]. The splitted of the Bragg reflections in Ca 2 FeMoO 6 suggest monoclinic superstructure. Fig. 6.3 shows the splitting of (112) and (312) peaks at about 2θ (in degrees) ~ and 58.0, respectively. The wing like minor splits of (112), (132) and (312) peaks exhibited a systematic decrease with La doping in Ca 2-x La x FeMoO 6. The splitting about (112) and (312) peaks are not visible for x. A gradual decrease of the peak intensity at 2θ ~19.86 (101) and ~ (211) indicates the suppression of supercell structure [3, 4, 15]. The suppression of supercell structure supports the increasing structural disorder (ASD) due to 93

3 E xperim ental data P rofile fit data D iffere nce of e xpt. a nd fit d ata B ragg position (a) C a 2 FeM o O 6 Intensity (arb. unit) (b) C a 1.7 L a 0.3 FeM o O 6 (c) C a 1.4 L a 0.6 FeM o O θ (d eg ree) Fig. 6.1 Profile fit of room temperature XRD pattern of selected samples b 236 Lattice Parameter (A 0 ) V c/2 a Volume (A 0 ) La concetration (x) 232 Fig. 6.2 Variation of the lattice parameters a, b, c and unit cell volume with La concentration in Ca 2 x La x FeMoO 6 compound. 94

4 // x = x = 0.1 x = Intensity (arb. unit) x = 0.3 x = x = x = // θ (degree) Fig. 6.3 Evolution of XRD pattern at about (112), (132) and (312) for Ca 2 x La x FeMoO 6 samples. The lines indicate the profile fit and Bragg positions of monoclinic structure. non-magnetic La substitution in the present Ca 2-x La x FeMoO 6 samples. The SEM pictures in Fig. 6.4 showed that the particles are well compacted (melted) to exhibit a relatively smooth (adhesive type) surface morphology for x = 0 sample. For this sample, one could expect a minimum grain boundary disorder in the lattice structure. In La doped samples (Fig. 6.4(b-c)) the micron size particles are better distinguished from each other and heterogeneously distributed and the surface is brittle type. The non-uniform shaped particles and rough surface morphology can obviously contribute a large grain boundary disorder in La doped samples. 95

5 Fig. 6.4 (d) represents a typical EDX spectrum for x = 0.3 sample. The EDX spectrum identified main constituent elements (Ca, La, Fe, Mo, and O) of the samples. The obtained elemental composition is found to be close to the expected value. The experimental data showed that the atomic ratio of Mo/Fe increases with the increase of La doping (e.g., 0.71, 0.77, 0.88 for x = 0, 0.3 and samples respectively). Slightly deficiency of Mo cations will increase the disorder in double perovskite structure. Fig. 6.4 SEM image of selected samples (a) x= 0, (b) x =0.3, (c) x= and a typical EDX spectrum for x = Magnetic properties AC susceptibility Fig.6.5 shows the temperature dependence of real (χ / ) and imaginary (χ // ) components of ac susceptibility for selected samples. The χ / (T) of x = 0 sample increases with the increase of temperature and exhibited a peak at about T P ~ 200 K. On further increase of temperature χ / 96

6 (T) rapidly decreases. The frequency response of χ / (T) in x = 0 sample is weak in comparison with the La doped samples. For x = sample, χ / (T) data are well separated below 175 K with the variation of frequency, but the frequency dependence is less in the temperature range 175 K to 300 K. There is no peak in χ / (T) for x = sample up to 300 K, indicating the χ / (T) peak may be at higher than 300 K. In contrast, χ / (T) data showed a well defined peak at about T P ~ 250 K and 220 K for x = and samples. On the other hand, x = 0 sample showed χ // (T) peak at T m ~100 K and the χ // (T) peak position is near to 100 K(±20 K) for x =, and samples. Interestingly, the T m position is strongly frequency dependent for La doped χ / (emu/mol/oe) χ / (emu/mol/oe) x = T(K) x = T m T m 1 Hz 10 Hz 100 Hz 1 khz 1 Hz 1 khz T P = 200 K T(K) 0.0 χ // (emu/mol/oe) χ // (emu/mol/oe) χ / (emu/mol/oe) χ / (emu/mol/oe) x = T m 1 Hz 10 Hz 100 Hz 1 khz T P = 250 K T(K) x = T m 1 Hz 10 Hz 100 Hz 1000 Hz T P = 220 K T(K) χ // (emu/mol/oe) χ // (emu/mol/oe) Fig. 6.5 Temperature dependence of χ / and χ // for La concentration x = 0,, and. (x =, and ) samples. In addition to increase the magnitude of χ // (T) peak at T m, the peak position is shifted to higher temperature with the increase of frequency. At ν = 1 Hz T m is ~110 K, 88 K, 123 K, 90 K and at ν = 1 κhz T m is ~120 K, 112 K, 148 K, 120 K for x = 0,, and samples, respectively. This gives the peak temperature shift per decade of frequency ( = T m /[T m (1 Hz) lnν]) ~ 0.013, 0.039, and for x = 0,, and 97

7 samples, respectively. The obtained value of the temperature shift per decade of frequency is larger than the typical value for classical spin-glasses like CuMn, but less than the typical value 0.1 for ideal superparamagnets [17]. The values of suggest the coexistence of cluster glass type phase in the samples. Coexistence of glassy phase indicates the existence of finite amount of internal disorder for x = 0 sample. The glassy feature in La doped samples can be understood from the analysis of Vogel-Fulcher law: ν= ν 0 exp[-e a /(T m -T 0 )]. Due to the lack of sufficient numbers of T m (ν) points a detailed analysis is not performed. However, a rough estimation provides the spin flip frequency (ν 0 ) in the range Hz to Hz, activation energy (E a ) in the range 0.14 ev to 4 ev by varying the interaction term (T 0 ) in the range 20 K- 30 K. The estimated parameters suggest that the coexisting glassy component belongs to the cluster glass regime [18]. The other interesting observation of La doping is that the magnitude of ac susceptibility (χ / and χ // ) of the material decreases with the increase of La concentration. This is due to magnetic dilution in double perovskite structure Field dependence of magnetization M(H) curves at 300 K in Fig.6.6(a) confirmed soft ferromagnetic nature of the samples. The samples showed a clear hysteresis loop and example is given in the inset of Fig. 6.6 (a) for x = and samples. The magnetic parameters (M R, M S ) showed a general trend of reduction with La doping (Table 6.1). The decrease of M S (from ~2.14 µ B /f.u for x = 0 to 0.12 µ B /f.u for x =0.7) suggests increasing magnetic dilution in double perovskite structure. The H C increases from ~33 Oe for x = 0 sample to ~168 Oe for x = 0.7 sample. Since ferromagnetic order retained for La doping up to x = 0.7, the increasing magnetic hardness could be related to increasing disorder or lattice defects either at grain boundary or inside the grains. M (H) data at 5 K (Fig. 6.6 (b)) also indicated typical soft (ferri) ferromagnetic nature of (x = 0,,, ) samples. Estimated M S 3.25 µ B /f.u for x = 0 sample is close to the reported value ( 3.44 µ B /f.u) at 4.2 K [3] and M S ( 3.25, 3.10, 2.75 and 1.30 in µ B /f.u for the x =, and, respectively) decreases with La doping. A portion of the loop at 5 K is shown in the inset of Fig. 6.6 for x = 0, and samples. Typical values of M R are ~ 0.16, 4, 1 and 0.14 µ B / f.u and H C are ~32, 78, 79 and 86 Oe for x = 0.0,, and samples, respectively. 98

8 M (µ B /f.u) 2 M (µ B /f.u) (a) H (Oe) x = Magnetic Field (Oe) M (µ B / f. u) x = 0 M (µ B / f. u) (b) La 0 La La H (Oe) Magnetic Field (Oe) Fig. 6.6 (a) Field dependence of Magnetization for La x Ca 2-x FeMoO 6 (x=0 to 7) samples at 300 K and inset shows the M(H) loop at 300 K for two samples. (b) M(H) data at 5 K for x = 0,,, samples. Inset shows a typical loop for x = 0, and samples. The decrease of M S and M R is consistent with the decrease of magnetic moment in La doped samples. The magnetic hardness, represented by H C, of the material also increases with the increase of La doping. The variation of magnetic parameters gives further support to the increasing disorder in the La doped samples Temperature dependence of magnetization Fig.6.7 (a) shows MZFC(T) and MFC(T) curves at 100 Oe. Magnetic irreversibility is seen below T irr ~330 K in all samples. MZFC curve of x = 0 sample exhibited maximum at about 300 K. MZFC is freezing on decreasing temperature down to 5 K. The MZFC curves of La doped samples showed similar behavior below T irr. MZFC maximum of these samples are more broadened and appeared at relatively lower temperatures than that of x =0 sample. MFC curve of all samples continuously increases down to 5 K. Fig. 6.7a shows that magnetic irreversibility (MFC-MZFC) decreases with the increase of La doping in La x Ca 2-x FeMoO 6 samples. The peak behavior of MZFC (T) curve for x = 0 sample is suppressed by increasing magnetic field to 500 Oe and 5 koe (Fig. 6.7(b)). Magnetic irreversibility between ZFC and 99

9 M (µ B / f. u) x 0.0 (a) M (µ B / f. u) 3 2 (b) Ca 2 FeMoO 6 (b) 100 Oe ZFC 100 Oe FC 500 Oe ZFC 500 Oe FC 5 koe ZFC 5 koe FC Temperature (K) Temperature (K) Fig. 6.7 Temperature dependence of Magnetization in Zero Field cooled (ZFC) and Field Cooled (FC) (a) measured at 100 Oe for selected x= 0.0,, and and (b) measured at different applied fields for Ca 2 FeMoO 6. M (µ B /f.u) x = First order derivative of M(T) data T C x T C K K K K T C 410 K 400 K 380 K x = K T (K) Ca 2-x La x FeMoO Temperature (K) Fig. 6.8 MZFC(T) at 1 koe for Ca 2-x La x FeMoO 6 samples. Inset shows the first order derivative of magnetization to estimate T C of the samples (marked in the inset). 100

10 FC curves decreases with increasing magnetic field. A small separation between ZFC and FC curves is noted below 130 K at 500 Oe. There is no magnetic irreversibility down to 5 K for 5 koe field. The temperature dependence of magnetization (300 K-600 K) at 1 koe (1 koe was applied to avoid the magnetic freezing) and its first order derivative (inset Fig. 6.8) have been used to determine ferromagnetic to paramagnetic ordering temperature (T C ) of the samples. Inset of Fig. 6.8 shows that T C of the material initially increases from 335 K for x = 0 sample to 410 K for x = sample and then decreases to 375 K on further increase of La content up to x = 0.7. Similar variation of T C with electron doping effect has been reported in double perovskite structure [3, 8-10], but the phenomenon is not unified with increasing electron doping in double perovskites. Table 6.1. The magnetic parameters (M S, H C ) at 300 K. Electrical parameters (ρ 25K : resistivity at 25 K, β is slope of lnρ vs. T curve, ρ 0, ρ 1 and ρ 2 ) were calculated using temperature dependence of resistivity curves. La (x) M S (µ B /f.u.) H C (Oe) β(10-4 K -1 ) T im (K) ρ 25K (mω cm) ρ 0 (mω cm) ρ 1 (x10 3 ) ρ 2 (x10 4 ) (mω cm/k 1/2 ) (mω-cm/k 3/2 ) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Electrical properties Fig. 6.9 shows the temperature dependence of normalized resistivity data [ρ(τ)/ρ(25 Κ)]. The summarized features are: (i) ρ(25 Κ) increases with the increase of La content (up to x = 0.8) (in Table 6.1), (ii) x = 0 sample shows a typical metallic conductivity with a continuous 101

11 increase of resistance with temperature up to 300 K, but ρ(τ) curve remarkably changed upon La doping. The resistivity for x = 0.1 to 0.8 samples initially (at lower temperature regime) decreases with the increase of temperature. At higher temperatures, the samples x = 0.1 to 0.7 regained metallic conductivity and persisted above room temperature [19]. A minimum in the ρ(τ) curve of La doped samples appeared due to the competition between low temperature up turn and high temperature metallic conductivity. We noted a substantial decrease of metallic conductivity in Ca 2 x La x FeMoO 6 samples. There is no signature of metallic type resistivity up to 300 K for x = 0.8 sample. The decrease of metallic character upon La doping is realized from the slope (β = lnρ/ T). The slope was estimated from the linear fit (shown in the inset of Fig. 6.9) of the lnρ(τ) vs T data at T > 150 K to 250 K. The slope (β), as shown in Table 6.1, systematically decreases and becomes negative for x = 0.8 sample where semiconductor signature appeared up to 300 K. The cross over temperature T IM below which a semiconductor type signature appeared is not straight forward to estimate due to broadening of resistivity minimum in the temperature scale. We have estimated T IM (shown in Table 6.1) from the loglog plot of ρ(t) data (Fig. 6.10). We do not find any signature of resistivity minimum down to 25 K for x = 0 sample. The resistivity minimum appeared for x 0.1 samples above 25 K. The typical value of T IM increases from 28 K for x = 0.1 sample to 220 K for 0.7 sample. There is no T IM below 300 K for x = 0.8 sample. This shows that La doping induces significant changes in the mechanism of electrical conductivity. In ferromagnetic manganites, different equations were proposed to explain various contributions in the low temperature upturn of resistivity curve [20-24], e.g., electron-electron interactions in term of T 1/2 dependence, spin dependent Kondo effect in term of lnt dependence, electron-magnon interactions in term of T 3/2 dependence, and electron-phonon interactions in terms of T p (p = 2-5) dependence. The theory for quantum correction and spin dependent Kondo effect in low temperature resistivity upturn is highly debatable [23, 24]. On the other hand, it has been realized that low temperature up turn and subsequently, resistivity minimum appeared due to grain boundary effect and magnetic spin disorder [20, 23]. Although there are many similarities in magnetic and electrical properties of manganites and double perovskites, but there is no proper equation to fit the low temperature resistivity upturn in double perovskite samples [1, 11, 15, 25-28]. 102

12 La (x) = 0 Ca 2-x La x FeMoO 6 0 x Normalisation data ρ/ (ρ in 25 K) ln[ρ(mω-cm) T (K) Temperature (K) Fig. 6.9 Temperature dependence of normalized resistivity of Ca 2-x La x FeMoO 6. Inset shows the ln ρ vs. T data and linear fit data (straight line) for selected samples. The experimental data, in our case, are well fitted with the expression: ρ(t) = ρ 0 + ρ 1 T 1/2 +ρ 2 T 3/2 (6.1) The temperature independent term ρ 0 is the residual resistivity in the material, contributed due to lattice disorder (ASD, APD) at grain boundaries. Second and third terms are due to electron-electron and electron-magnon interactions, respectively. The fitted data of equation (6.1) for x = 0, and 0.8 samples are shown in the inset of Fig and fit parameters (ρ 0, ρ 1, ρ 2 ) are shown in Table 6.1. The parameter ρ 0 increases rapidly with La doping up to x =. Then, increase of ρ 0 is slow for x = 0.7 and 0.8 samples. Negative value of the parameter ρ 1 signifies the decrease of resistivity with increasing temperature. The magnitude of ρ 1 is small for x = 0 sample. A rapid increase of ρ 1 with La content up to x = 0.8 indicates the increase of electronic interactions in the conduction band of La doped samples. The term ρ 2 is always positive and lower in magnitude than ρ 1 term. It increases with La doping up to x = and then decreases for further increase of La content. 103

13 0.1 L a (x) = ln[ρ(t)/ρ(25 K)] E xp e rim e n ta l d a ta : o p e n s ym b o l F it d a ta : C o lo u re d lin e 3 x = 0 a rro w g u id e s th e c h a n g e s o f T IM w ith L a c o n te n ts 0.7 ρ(t)/ρ(25 K) T (K ) ln [T (K )] Fig Temperature dependent ρ (T)/ρ(25 K) is used to determine the metal-insulator transition temperature (T IM ). The fit of equation (6.1) to the experimental ρ(t) data is shown in the inset for selected samples of Ca 2-x La x FeMoO 6. Viewing the opposite sign of ρ 1 and ρ 2, we understand that ρ 1 is mainly responsible for the low temperature increase of resistivity below T IM (semiconductor type signature in La doped samples) and ρ 2 is responsible for the metallic type resistivity. Metallic regime shifts to higher temperature by increasing La content. Significant weakening of spin dependent conduction process might be the reason of decreasing ρ 2 at x in our La doped material [21]. The decrease of Curie temperature (T C ) for x also supports the dilution of ferromagnetic order in Ca 2 x La x FeMoO 6 samples. The resistivity at 25 K (ρ 25K ) is roughly proportional to the grain boundary disorder of spins (i.e., canting or antiferromagnetic alignment). The increasing antiferromagnetic interactions are understood by the appearance of spin glass signature. Decrease of magnetic irreversibility with increasing applied magnetic field also indirectly indicated that the existing disorder in the material has magnetic (spin) origin and affecting the shape of resistivity curve [20, 23]. 104

14 6.3.5 Conclusions La doping in Ca 2 FeMoO 6 showed stability within single phased monoclinic crystal structure. Some structural changes, e.g., suppression of supercell structure, expansion of cell volume, stabilization of more Mo atoms in the crystal structure, loss of surface smoothness, and enhanced grain boundary contribution by increasing the number of grains were observed with the increase of La concentration. The ac susceptibility measurement was applied as an alternative technique to identify the increasing disorder and its role on reduction of magnetic moment and enhancement of Curie temperature in double perovskite structure. The appearance of cluster glass phase in the ferrimagnetic state, increasing magnetic hardness upon La doping and increasing resistivity in the metallic network provided the evidences of increasing disorder in La doped Ca 2-x La x FeMoO 6 double perovskite. The physical (i.e., magnetic and electrical) properties are strongly correlated with the internal disorder of crystal structure and morphology. There is a substantial decrease of both ferromagnetic ordering and metallic conductivity by increasing La content in Ca 2 x La x FeMoO 6 samples. The introduced disorder in the material by La doping has magnetic spin dependent origin. The electrical conductivity is strongly correlated to the disorder incorporated both in grains and grain boundaries. The decrease of metallic conductivity is related to the intrinsic property of the grains and modified band structure of the material. The increasing semiconductor type contributions are attributed to the modifications at metallic band structure of ferromagnetic grains due to non-magnetic and electronic doping effects of La atoms. The conduction band filling factor, in addition to the increasing hole concentration at the valence band edge, plays an important role in decreasing the mean field T C of the material at higher La doping in double perovskite structure. 105

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