Critical composition of La0.7-xDyxCa0.3Mn(Fe)O3 for high CMR

ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 311 (2007) 594–604 www.elsevier.com/locate/jmmm

Critical composition of La0:7x DyxCa0:3 MnðFeÞO3 for high CMR S.C. Bhargavaa,, Sher Singha, S.K. Malikb a

Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b Tata Institute of Fundamental Research, Colaba, Mumbai 400 005, India Received 13 July 2006; received in revised form 12 August 2006 Available online 27 September 2006

Abstract La0:7x Dyx Ca0:3 MnðFeÞO3 is studied extensively using Mo¨ssbauer spectroscopy, AC susceptibility, DC magnetization, and resistivity measurements. MR shows a cusp-like increase at the critical composition x ¼ 0:10. Based on the experimental results, we provide an explanation for the high MR. Several properties of the manganite that undergo changes at this composition as x is varied are found. Our study, thus, reveals a mechanism by which CMR in excess of 108 % can be obtained. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 75.30.Vn; 76.80.+y; 76.60.Es; 73.43.Qt Keywords: Colossal magnetoresistance; Phase coexistence; Ferromagnetic and spin glass phases; Critical composition

1. Introduction Manganite La1x Cax MnO3 shows a variety of properties as x is varied. Extensively studied is the phenomenon of metal–insulator (MI) transition when 0:25oxo0:5, which is associated with a colossal magnetoresistance (CMR) [1–7]. Initially, the double exchange theory was considered to adequately describe the phenomenon [8–10]. Subsequently, the roles of Jahn–Teller distortion [11], polaron formation [12–14], etc., were investigated. The role of the phase separation to CMR has been realized in recent studies. Separation of phases with different electronic densities has been known for a long time. Phase separation in manganite, which involves phases with equal densities, has been discussed [7] recently, and is the subject of the present study. The substitution of Mn or La with other cations in a manganite has been found to affect CMR and other properties. The two coexisting phases that are vitally important to the phenomena are ferromagnetic (FM) and spin glass (SG) phases. The magnetic and structural characteristics of the two phases differ substantially, Corresponding author. Tel.: +1 919 607 2864.

E-mail address: [email protected] (S.C. Bhargava). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.08.038

though compositionally they are similar. In the FM phase, the lattice distortion is insignificant, facilitating the hopping of the eg electrons between the ferromagnetically coupled Mn ions. The hopping is also responsible for the conductivity. Thus, there is an insulator metal transition along with magnetic ordering on lowering the temperature. Earlier study of the MI transition in La1x Cax MnO3 , 0:15pxp0:33, which is accompanied by a FM transition at T C , has shown that the tilting of MnO6 octahedra decreases with decreasing T above T C , undergoes an abrupt change at T C , and remains practically constant at ToT C [15]. Thus, there is an abrupt change of the matrix element describing the electron hopping between Mn sites at T C . Abrupt decrease in the lattice parameters of La0:75 Ca0:25 MnO3 at T C has also been observed [16–19], and these sharp anomalies were attributed to both the FM ordering as well as the change in the electronic properties. Similarly, a decrease of the unit-cell volume is found for x ¼ 0:33 [16,20] when long-range FM ordering sets in at low temperatures. Furthermore, it has been found that the unit cell of the structure shrinks significantly and continuously with the development of spin ordering induced by an external field [15], which also shows that the spin ordering is strongly coupled to the lattice [8,12,16,21].

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

595

The characteristics of the coexisting SG phase differs substantially from that of the FM phase. SG phase appears in the following way. The different size of the impurity ion can result in the distortion of the perovskite structure, which results in a deviation of the Mn3þ –O–Mn4þ angle from 180 . This angle is microscopically related to the transfer integral describing the electron hopping between Mn3þ and Mn4þ . The decrease in the angle reduces the strength of the FM coupling [15,20]. As a result T C decreases. When Mn–O–Mn angle becomes less than a critical angle yC , the FM interaction between Mn–Mn ions changes to an antiferromagnetic interaction. Compositions in which the average Mn–O–Mn angle is very close to the critical angle yC show magnetic frustration and order as a SG. They may show reentrant SG transition too. Furthermore, it has been shown [17] that not only is T C dependent on hrA i, but also on the random disorder of La3þ and Ca2þ cations, with different sizes, on the A sites of the perovskite structure. A large randomness in the Mn–O–Mn bond angle leads to a SG state. When the low-temperature phase is a SG phase, there is no MI transition and no significant change in the lattice characteristics at T C occurs. The effect of the substitution of La3þ with other cations, of different sizes, has been investigated in earlier studies. A phase diagram showing the effect of the substitution of La3þ by cation like Dy3þ [22], Tb3þ [20,23], Y3þ [16], Pr3þ [16], or Gd3þ [24] has been obtained, keeping the concentration of Ca2þ as 30%, as in the present study. In these studies, the phase coexistence was not taken into consideration. Thus, the behavior found represents the characteristics of the majority phase. Initially, when x is low, the low-temperature phase is predominantly a FM metallic phase. At higher x, SG phase is the majority phase. Only in the case of the substitution by Dy3þ , a reentrant SG behavior was concluded at x  0:25. At x40:25, the low-temperature phase is a SG phase in all the cases. In the present study, we investigate La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 in the range of x from 0.08 to 0.11. We show that the coexistence of FM and SG phases is vital to CMR. The study reveals remarkable relationship of CMR and the coexisting phases that throws light on its mechanism. We find that CMR shows a cusp at the critical composition xC , where FM loses its dominance to SG phase on increasing x. Furthermore, we find that when x is low and the FM phase is the dominant phase, the peak in CMR, as well as MI transition, occurs at a temperature close to T C . However, when x is close to xC , the CMR peak occurs at a temperature much lower than T C and MI transition is not observable. This observation is remarkable, because it is generally believed that T C is close to the MI transition temperature. Several properties that change sharply at the critical concentration xC are found.

CaCO3 , Dy2 O3 , 57 Fe2 O3 and MnO2 were mixed in the desired proportions in distilled acetone and heated at 1100  C, with an intermediate grinding, for 34 h. Subsequently, it was heated at 1300  C for 40 h, with an intermediate grinding. Resistance was measured using the standard four-probe method (a quantum design PPMS option) in the presence of fields up to 6 T. This instrument is capable of measuring resistance up to 4 MO. For higher resistance, a calibrated shunt of 2:7 MO across the sample is used. AC susceptibility (linear and non-linear), DC magnetization (FC and ZFC magnetization) and hysteresis loops were obtained using susceptometer (Quantum Design PPMS Model 6000). Mo¨ssbauer spectra in zero field have been obtained using a cryogenic set up described earlier [25–27]. A 57Co source in a Rh matrix, of 50 mCi strength, has been used in the measurements. The temperature is controlled to 0:1 with a carbon glass resistor and measured using a platinum resistance thermometer. The calibration spectrum of a Fe foil showed lines of  0:3 mm=s full-width. The velocity scale was linearized before the analysis of the Mo¨ssbauer data.

x

a (A˚)

c (A˚)

b (A˚)

2. Experimental

0.08 0.09 0.10 0.11

5.4557 5.45075 5.4469 5.4485

5.4594 5.4601 5.4605 5.4637

7.7018 7.6971 7.6941 7.6927

La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:07–0.12, have been prepared using the solid state reaction method. La2 O3 ,

3. Results The analysis of the X-ray diffraction pattern showed the formation of a single-phase orthorhombic perovskite structure in all the compositions. The cell constants are given in Table 1. a and b decrease and c increase with increase in x. 3.1. Resistance measurements The temperature at which MI transition occurs, T P , decreases and the peak resistivity increases with increase in x. When x is small, T P (Table 2) is close to T C (Table 4). The difference between the two temperatures becomes large and the change is drastic as we change from x ¼ 0:09 to 0.10, in absence of an external field. MI transition was not observed in zero field when xX0:10 even at 23 K. In the presence of a magnetic field of 3 or 6 T, however, MI transition is observed in all the compositions (Table 2) studied here. When MI transition is observable, the resistance tends to increase again at lower temperatures (Fig. 1). For any composition x, MI transition temperature, T P , increases when the applied magnetic field increases. The CMRs of all the compositions in the field Table 1 Results of X-ray analysis of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3

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7

Table 2 The peak temperature (in K) in the temperature dependence of resistivity of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 in magnetic fields of various strengths x

6

Magnetic fields (T) 0

3

6

107 78.5 66 o23 o49

126.5 100 89 87 64

142 114 101 98 78.5

10-8 CMR (in %)

0.07 0.08 0.09 0.10 0.11

3 tesla 6 tesla

5 4 3 2 1 0

25 Resistance (M.Ohm )

20 0 tesla

15

50

100

150

200

250

300

350

Temperature (K)

0.4 0.3

3 tesla

Fig. 2. Magnetoresistance of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:10, in presence of magnetic fields up to 6 T.

0.2 0.1 6 tesla

0.0 0

50 100 150 200 250 300 350

10

Temperature (K)

8 5

20 K

0 0

50

100

150

200

250

300

350

Temperature (K) Fig. 1. Resistivity of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:09, in presence of magnetic fields up to 6 T.

Table 3 Peak temperature of MR and CMR of La0:7x Dyx Ca0:3 MnðFeÞO3 x

Peak temperature (K)

CMR (in %)

0.07 0.08 0.09 0.10 0.11 0.12

104 72 64.5 o23 o41 o62

18 000 42 000 61 144 46:0  108 42:5  106 47:2  105

of 6 T are given in Table 3. It shows a cusp-like increase at x ¼ 0:1. CMR of x ¼ 0:10 in field of 6 T is greater than 6:0  108 and is the highest for this series of compositions (Fig. 2). To confirm the high value of CMR for this composition, we measured the field dependence of resistance at 20, 30, 40 and 50 K. The magnetic field dependencies of CMR at these temperatures thus obtained are shown in Fig. 3. The high value of CMR for x ¼ 0:1 is, thus, confirmed.

10-8 CMR (in %)

Resistance (M.Ohm)

0

0.5

6

4 30 K 2 40 K 0

50 K 0

20

40

60

80

100

Magnetic Field (in tesla) Fig. 3. Magnetoresistance of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:10, at various temperatures in fields up to 9 T.

3.2. Hysteresis loop Hysteresis loops of x ¼ 0:08, 0.09, 0.10, and 0.11 have been obtained as follows. The sample was demagnetized by taking it to the ambient temperature and cooled in zero field to the measurement temperature. The magnetic field was raised to 9 T, then decreased to 9 T, and then again increased to 9 T. At any field, the measurement was done by going into the persistent mode. Hysteresis loop of the composition x ¼ 0:09 at 2 K is shown in Fig. 4a. It shows anomalous behavior. Initial shape is typical of a noncollinear (NC) substance. At a critical field, H Cr , NC phase starts converting into FM phase. As can be seen, the transition is sharp at 2 K, as in a first-order phase

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

100

M (emu/gm)

50

0

T = 2K

-50

-100

-9

-6

-3

(a)

0 H (tesla)

3

6

9

100

M (emu/gm)

50

0

-50

-9 (b)

-6

-3

0

3

6

ture, to demagnetize, and cooled in zero field to 6 K. A DC field of 100 Oe was applied and ZFC magnetization measured while warming. The sample was again warmed to the ambient temperature, to demagnetize. It was cooled in presence of a DC field of 100 Oe to 6 K. The FC magnetization was measured by measuring the magnetization in the presence of 100 Oe while warming. The results obtained are shown in Fig. 5. The branching temperatures of FC and ZFC magnetizations of various compositions are given in Table 4. Remarkably, the branching temperatures are not related to the temperatures at which MR is maximum. The observation of ZFC and FC magnetizations for x ¼ 0:08, 0.09, 0.10, 0.11 enables us to derive the relation between branching temperature of FC and ZFC magnetizations and AC susceptibility peak temperature as will be seen below. This is important because it implies that even in x ¼ 0 composition, SG phase is likely to be present, as FC and ZFC magnetization of x ¼ 0 also show branching. Another remarkable feature of the result (Fig. 5) is the change that occurs as x is increased from 0.09 to 0.10. The lanthanide becomes predominantly SG phase when x ¼ 0:10. The present measurements show that the contribution of the SG phase decreases as x decreases. It is small when x ¼ 0:08, but is observable. It is not distinctly observable in the AC susceptibility when x ¼ 0:07. The presence of the SG phase in this composition at 50 K is, however, shown by the field dependence of magnetization (Fig. 6). 3.4. Linear AC-susceptibility

T = 2K

-100

597

9

Magnetic Field (tesla)

Fig. 4. Hysteresis loop of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , (a) x ¼ 0:09, (b) x ¼ 0:12, at 2 K.

transition (FOPT). It gets thermally smeared to a broad transition at higher temperatures. When the field is lowered, the FM phase does not reconvert back to the SG phase, even when the field is decreased to 9 T. The shape of the magnetization curve is typical of a FM phase when the field is taken from 9 to 9 T and back to 9 T. The anomaly is not present when x ¼ 0:07, but increases subsequently with x. The critical field, required for the transition from FM to SG phase, increases with x. Furthermore, the transition occurs in two steps when x ¼ 0:12 (Fig. 4b): the first transition occurs when the field reaches 4.5 T and the second one when the field reaches 6 T. The two-step transition is less striking at lower x. 3.3. Field cooled (FC) and zero field cooled (ZFC) magnetizations FC and ZFC magnetizations have been measured as follows. The sample was warmed to the ambient tempera-

In the present study, frequency, DC field, and x dependence of the AC susceptibility have been measured. As described below, they reveal several phenomena. 3.4.1. Field dependence For measurement of AC susceptibility in a DC field, the sample was first warmed to the ambient temperature, to demagnetize, and then cooled in zero field to 6 K before applying the DC field. AC susceptibilities of the compositions with x ¼ 0:08, 0.09, 0.1, and 0.11 in DC fields up to 3000 Oe are shown in Fig. 7a–d. The contribution of the FM phase, visible at lower temperatures, grows as x decreases. In the AC susceptibilities of the compositions x ¼ 0:09, 0.08 and 0.07, the FM phase is distinctly visible. It is insignificant when x ¼ 0:10 or higher. In zero field, a sharp peak appears at higher temperature in all the cases. As the DC field increases, this peak splits into two. One of the peaks moves to higher temperature and its amplitude decreases as the field increases. The other peak moves to the lower temperature and broadens as the field increases. The splitting is prominent even in low field when x ¼ 0:08 and 0:09: The part that moves to lower temperature merges with the FM part. The part which moves to higher temperature looks like a SG peak and is less prominent than the lower temperature part in these two compositions. The change in the behavior that occurs as we change from

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

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4

Table 4 Branching temperatures of FC and ZFC magnetizations, and temperature of AC susceptibility peak of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3

3

x

Br. temperature (K)

Peak temperature (K)

0.07 0.08 0.09 0.10 0.11 0.12

95 78 70 60 55 50

79 72 64 58 53

x = 0.11

2 1 0 7 6

100

5 4

80 0.10

3

M (emu/gm)

2

M

1 0

60

40

5 20

4

T = 50K

3 0

0.09

2

0

3

6

9

H (tesla) 1

Fig. 6. Field dependence of magnetization La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:07, at 50 K.

0

of

0.11. Thus, the measurements show distinctly different behavior of xp0:09 compositions than of xX0:10 compositions.

4 3 0.08

2 1 0 0

50

100

150

Temperature (K) Fig. 5. FC and ZFC magnetizations of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:08–0.11.

x ¼ 0:09 to 0.10 is remarkable. The part of the SG peak that moves to lower temperature becomes small, the part which moves to higher temperature and represents SG phase is relatively prominent. Secondly, the movement to the lower temperature is large when x ¼ 0:08 and 0.09, in comparison to the movement in cases when x ¼ 0:10 and

3.4.2. Frequency dependence AC susceptibility of the compositions x ¼ 0:08, 0.09, 0.10 and 0.11, in zero DC field and frequency up to 3333 Hz, are shown in Fig. 8a–d, respectively. The sharp changes that occur as we change from x ¼ 0:09 to 0.10 are indeed remarkable. There are three regions that can be distinctly seen in M 00 . A peak around 78 K, a peak around 50 K, and a region below 25 K. At low x, the peak around 78 K is distinctly visible. Remarkably, it changes from a large peak when x ¼ 0:09 to a insignificant peak when x ¼ 0:10. The peak at 50 K is pronounced when xX0:10, but insignificant below x ¼ 0:10. M 00 rises below 25 K when xp0:09, but becomes a well-defined peak when xX0:10. The frequency dependence is seen in all parts except in the peak at 78 K. The changes in M 0 with x are similar, though less pronounced. The frequency dependence of M 0 is characteristic of SG phase when xX0:10. The results show that the SG fraction rises below 20 K with the lowering of temperature when xp0:09. Thus, a part of the magnetic

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

0.6

0.35 0 Oe

400 Oe

0.21

600 Oe 800 Oe

0.14

1000 Oe

100 Oe M' (emu/gm)

M' (emu/gm)

200 Oe

0 Oe

0.5

100 Oe

0.28

200 Oe

0.4

400 Oe 600 Oe

0.3

800 Oe 1000 Oe

0.2

1200 Oe

1200 Oe

2000 Oe

0.07

2000 Oe

0.1

3000 Oe

3000 Oe 0.0

0.00 0

30

(a)

60 90 Temperature (K)

120

0

150

30

60

90

120

0.4 0 Oe

0 Oe

100 Oe

100 Oe

200 Oe 400 Oe

0.3

600 Oe 800 Oe

0.2

0.3 M' (emu/gm)

0.4

200 Oe 400 Oe 600 Oe

0.2

800 Oe

1000 Oe

1000 Oe

1200 Oe 0.1

1200 Oe

0.1

2000 Oe

2000 Oe

3000 Oe

3000 Oe

0.0

0.0 0

(c)

150

Temperature (K)

(b)

0.5

M' (emu/gm)

599

30

60

90

120

150

Temperature (K)

0 (d)

30

60

90

120

150

Temperature (K)

Fig. 7. AC susceptibility of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , (a) x ¼ 0:08, (b) x ¼ 0:09, (c) x ¼ 0:10, (d) x ¼ 0:11, measured, using AC field of 0.1 Oe and frequency 300 Hz, in presence of DC fields up to 3000 Oe.

lattice shows a reentrant SG behavior. The changes that occur when x is increased from 0.09 to 0.10 are remarkable. 3.5. Paramagnetic Mo¨ssbauer spectra The spectrum, for any x, at the ambient temperature shows two doublets even though Fe occupies equivalent sites. The paramagnetic spectrum is fitted with two symmetric doublets. The spectra of the composition x ¼ 0:09 are shown in Fig. 9a. The doublet with smaller relative intensity (RI) does not disappear (Fig. 9b) even when the temperature is lowered to T C . Quadrupole splittings (QS) of the two doublets show little temperature dependencies (Fig. 9c). The paramagnetic spectra of x ¼ 0:10 are shown in Fig. 10a. Unlike the behavior for xp0:09, the doublet with smaller intensity disappears as the temperature is lowered to T C . The temperature dependencies of the relative intensities are shown in

Fig. 10b. The paramagnetic spectrum of the composition with higher x shows behavior similar to that of x ¼ 0:1. In conclusion, the paramagnetic components in the Mo¨ssbauer spectra of the compositions xp0:9 and xX0:1 behaves differently. Thus, we show that the change in behavior of the component spectra occurs at the critical composition x ¼ 0:10: 4. Discussion The study shows that the SG and FM phases coexist in the manganite La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 below T C . As mentioned above, the two phases differ magnetically and structurally, even though they are similar compositionally. There is a sudden change in the lattice parameters as PM state changes to a FM state at T C , as is the case in a first-order transition. This is accompanied by MI transition. Such changes are absent when the PM state changes

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

600

15.5

8 7 M" (10-3 emu/gm)

M' (10-2 emu/g)

13.0 10.4 7.8 5.2 2.6

3333 Hz 1333 Hz 333 Hz 33 Hz

6 5 4 3 2

33 Hz

0.0

1

333 Hz

0

1333 Hz

8.3

0.35

3333 Hz

7.3

0.30 0.25

5.2 M' (emu/g)

M" (10-3 emu/g)

6.2

4.1 3.1 2.1 1.0

0.20 0.15 0.10 0.05

0.0

0.00

-1.0 0

50

(a)

100

0

150

(b)

Temperature (K) 20.3

40

80

120

160

Temperature (K) 4.6

17.8

-2

12.7 10.2

4.0

17.8 15.2 12.7

M" (10-3 emu/g)

M' (10 emu/g)

M' (10-2 emu/g)

15.2

10.2 7.6 5.1 20

7.6

40

60

Temperature (K)

5.1

2.6 2.0 1.3

2.5

0.7

0.0

0.0

3.6

19.9

3.0

16.6

2.0

M' (10-2 emu/g)

33 Hz 333 Hz 1333 Hz 3333 Hz

2.5 M" (10-3 emu/g)

3.3

1.5 1.0 0.5

33 Hz 333 Hz 1333 Hz 3333 Hz

13.2 9.9 6.6 3.3

0.0 0.0 -0.5 0

(c)

50

100

Temperature (K)

150

0

(d)

50

100

150

Temperature (K)

Fig. 8. AC susceptibility of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , (a) x ¼ 0:08, (b) x ¼ 0:09, (c) x ¼ 0:10, (d) x ¼ 0:11, measured, using AC field of 0.1 Oe and frequency up to 3333 Hz, in presence of zero DC field.

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

601

100 95 296 K 90

80

90

75

80

70

Relative Intensity (mm/s)

10- 4 Counts

85

22 21 80.5 K

20 19 18 17

70 60 50 40 30 20

16

component 1

10

15 -3 (a)

-2

-1

0

1

2

3

Velocity (mm/s)

50

100

150

200

250

300

Temperature (K)

(b)

1.6

Quadrupole Splitting (mm/s)

1.4

1.2

1.0 TC = 76 K 0.8

0.6

0.4

0.2 50 (c)

100

150

200

250

300

Temperature (K)

Fig. 9. (a) The paramagnetic Mo¨ssbauer spectra of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:09. The temperature dependencies of the (b) relative intensities and (c) quadrupole splittings of the two component spectra of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:09.

to a SG phase. A remarkable behavior is, however, shown by the SG phase. SG phase converts into a FM phase when an applied magnetic field exceeds a critical value (H Cr ). The transition is sharper at lower temperatures, as in a firstorder transition. This is shown by the hysteresis loop at a low temperature. H Cr gives a measure of the barrier

separating the free energy minima of the two phases. We can speculate, the free energy of the FM phase depends on the following: the magnetic energy due to the hopping of the eg electrons which results in the FM ordering, the energy due to the lattice contraction at T C , and the strain energy due to the fact that there is no significant bending of

ARTICLE IN PRESS S.C. Bhargava et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 594–604

602

27 26 25 24 23 22 21 20 19

296 K

100

80 76 K

28

Relative intensity (%)

10-4 Counts

30

26 24 22 18 17

74 K

60 TC (Mossbauer Spectroscopy) = 69 K 40

16 20

15

component 1

14 13 0 -3 (a)

-2

-1

0

1

Velocity (mm/s)

2

3

50 (b)

100

150 200 Temperature (K)

250

300

Fig. 10. (a) The paramagnetic Mo¨ssbauer spectra of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:10. (b) The temperature dependencies of the relative intensities of the two component spectra of La0:7x Dyx Ca0:3 Mn0:97 Fe0:03 O3 , x ¼ 0:10.

Mn–O–Mn bond, even though the substituted ions are of different size. The free energy of the SG phase depends on the following facts: there is no hopping of the eg electrons, the strain energy is low as the differing size of the impurity ions is able to cause Mn–O–Mn bond angle to become smaller than 180 , Jahn–Teller distortion energy, and the lattice contraction as a result of magnetic ordering is absent. The free energy minima of the two coexisting phases are separated by energy barrier (H Cr ). When the field is first applied, SG ordering is opposed by the applied field. Thus, the free energy of the SG phase increases. On the other hand, the increase in the magnetic field lowers the free energy of the FM phase, below the zero field free energy of the FM phase. SG phase changes into FM phase as soon as the increase in energy exceeds the barrier between the SG and FM phases. When the magnetic field is lowered back to zero, the system approaches the zero field free energy of the FM state. When field is taken to 9 T, the energy of the FM state lowers again, instead of increasing. Thus, the free energy of the FM state is unable to cross the barrier to go back to the SG state. The increase in the substitution of La3þ by Dy3þ increases the free energy of the FM phase and decreases the free energy of the SG phase. As a result, the volume fraction of the SG phase increases with x. The temperature at which the temperature dependence of resistivity shows a peak, T P , decreases as x increases. This is close to the magnetic ordering temperature when x is low and FM phase is the dominant phase below T C . Thus, when

x ¼ 0:09, magnetic ordering temperature is around 70 K, and T P is 66 K. However, when x changes from 0.09 to 0.10, FM phase looses its dominance, and SG phase becomes the dominant phase below T C . As a result, there is no MI transition even at temperature as low as 23 K, even though the magnetic ordering occurs at 60 K. In presence of a field as low as 3 T, however, there appears a MI transition and T P becomes as high as 87 K. In comparison, at higher x ¼ 0:11, zero field resistivity rises rapidly at 44 K, instead of at 23 K, and T P in 3 T show peak at 59 K, instead of at 87 K. Thus, the difference in the behavior between zero field and a field as low as 3 T is largest when x ¼ 0:10. This is the composition where the FM phase has just lost its dominance to the SG phase. This is shown by several measurements described above. As a result, CMR is largest for this composition and shows a cusp-like behavior at x ¼ 0:10. The application of a magnetic field increases FM phase and the effect is same as that of reducing x. In composition x ¼ 0:1, in which FM just lost its dominance to SG phase, the external field of 3 T is most effective in converting SG back to FM phase. As a result, MI transition, which was not observable in zero field even at 23 K, occurs at much higher temperature in presence of field of 3 T. The large difference between the behavior of MI transition in and without field results in a large CMR. Thus, we conclude that the value of x where FM and SG phases meet in the phase diagram is the critical concentration where CMR is maximum. At higher x, the effect of the magnetic field becomes less dramatic. Thus, CMR

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decreases again as x increases above the critical value of 0.1. The transition from predominantly FM phase to the predominantly SG phase of the manganite, as Dy content is increased, is a FOPT. As a result of increase in the substitution, Mn–O–Mn bond angle (y) and bond lengths decrease. There is a critical angle yC when FM and AFM interactions become comparable, resulting in frustration. Moreo et al. [7] have shown that sharp FOPT would occur at yC and 0 K in the absence of disorder. This implies a sharp FOPT at xC . The presence of disorder results in a range of x around xC , over which FOPT would occur. In the manganite under study, the range of x found is from 0.07 to 0.10. Hysteresis loop measurements also show that FOPT is sharper at lower temperatures. At low x (p0:07), small bubbles of SG phase coexists in predominantly FM manganite. The energy cost of such bubbles is proportional to the domain wall area. This is provided by disorder in yC , which in turn depends on the disorder in FM (hopping of eg electrons) and antiferromagnetic exchange couplings (between T 2g electrons) around the non-disorder values [7]. The present study shows that when the SG bubbles are small, the external field required to convert them to FM phase (H Cr ) is also small. As x increases, the size and stability of SG fraction increases. This increases H Cr too. At large x (X0:1), bubbles of FM phase exist inside SG phase. In the region of x between 0.09 and 0.10, there is a transformation from FM to SG phase domination in the manganite. It has been shown by Moreo et al. [7] that the cluster size depends on the extent of disorder from the non-disorder at the FOPT. When the disorder is small (large), cluster size is large (small). In the discussion below, we list other notable behavior observed in the present study. As we will see below, even though FM phase increases as x decreases, it is not fully FM at low x. Similarly, Mo¨ssbauer study showed that even though SG component increases with x, it is not fully SG phase even when x is high. Thus, we conclude that there is a co-existence of SG and FM phases even when x is low or high. In none of the compositions it is fully FM or SG like. AC susceptibility of the composition x ¼ 0:07, 0.08 or 0.09, in zero DC field, shows that the flat part representing the FM phase dips at a lower temperature, and possesses significant frequency dependence. This shows the presence of reentrant transition below 20 K. We can, therefore, visualize three phases: two of them (FM and SG phases) formed at the magnetic transition temperature. The third, a SG phase, forms at low temperature (around 20–30 K) and originates from FM part of the lattice at higher temperatures. It is found that the branching temperature in the FC and ZFC magnetizations and the temperature at which peak occurs in the AC susceptibility are related to each other (Table 4). This can be seen clearly in the compositions xX0:08, when the peak in the AC susceptibility is distinctly observable. This shows the relation between the onset of

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irreversibility in magnetization (branching of FC and ZFC magnetizations) and the SG transition. We may, therefore, conclude that the branching of FC and ZCF magnetizations shows the presence of SG component. The branching of FC and ZFC magnetization is observable even when x ¼ 0. It, thus, appears that the SG phase is present even when x ¼ 0, even though it is not visible in the AC susceptibility, even when x is as high as 0.07. 5. Conclusion We find increase in CMR as x is increased. It attains a maximum value (greater than 6  108 %) for x ¼ 0:10, and decreases again with further increase in x. Our extensive measurements show that the composition where FM phase has just lost its dominance to SG phase shows highest CMR and the behavior as a function of x shows cusp-like increase in CMR at the critical composition (xC ). At the critical point in the phase diagram, where FM and SG phases meet, the field causes drastic change in resistivity behavior, leading to maximum CMR. Several related changes that occur at xC have also been determined in the present study. The coexisting SG and FM phases are related by FOPT, which depends on x and H ext , and is sharper at lower temperatures [7]. The free energies of the two phases are separated by barrier H Cr . Small bubbles of SG phase coexist in the FM phase when xp0:07. These bubbles grow as x is increased. In xX0:1, small bubbles of FM phase exist in the SG phase. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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