- Email: [email protected]
Evidence of ferromagnetic domains in the (La0.757Dy0.243)0.7Ca0.3MnO3 perovskite
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1288–1289
Evidence of ferromagnetic domains in the (La0.757Dy0.243)0.7Ca0.3MnO3 perovskite S.M. Yusufa,*, K.R. Chakrabortya, R. Gangulyb, P.K. Mishrac, S.K. Paranjpea,1, J.V. Yakhmic, V.C. Sahnic a
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India c Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b
Abstract We have investigated the effects of 24.3 at% Dy substitution on the La site in the ferromagnetic perovskite La0.7Ca0.3MnO3. Results show a randomly canted ferromagnetic state below 65 K with a reduced metal–insulator transition. There is no breakdown of ferromagnetic domain structure at To65 K (covering the temperature range below 35 K where resistivity shows an upturn, indicating a reentrant transition to an insulating behaviour). Reduced geometrical tolerance factor and increased variance of the La-site ionic radius are responsible for the observed changes. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Lk; 75.60.Ch Keywords: CMR perovskite; Randomly canted ferromagnet
1. Introduction
2. Experimental
Extensive research has been done on metallic, ferromagnetic manganites of the type La0.7Ca0.3MnO3, through doping the La3+ sites with trivalent rare-earths of different sizes. Recently, Terai et al. [1] studied (La1xDyx)0.7Ca0.3MnO3 by DC magnetization, resistivity and magnetoresistance techniques. The x ¼ 0:243 compound was reported to show reentrant spin-glass behaviour where the high temperature insulating paramagnetic state transforms to a ferromagnetic metallic state at relatively lower temperatures and finally transforms to an insulating spin glass (with a collapse of ferromagnetism) at further lower temperatures. Here we present the results for the x ¼ 0:243 sample that provide strong evidence of the presence of ferromagnetic correlation in its low temperature insulating state contrary to what was concluded by Terai et al. [1].
Polycrystalline samples of (La1xDyx)0.7Ca0.3MnO3 (x ¼ 0 and 0.243) were prepared by solid-state reaction. X-ray diffraction patterns confirmed the single-phase formation of orthorhombic perovskite structure. DC magnetization measurements were carried out using a SQUID magnetometer (Quantum Design). Neutron diffraction and one-dimensional (z2z) neutron-depolarization measurements were carried out at Trombay [2].
*Corresponding author. Tel.: +91-22-25-595383; fax: +9122-25-505151. E-mail address: [email protected] (S.M. Yusuf). 1 Present address. International Atomic Energy Agency, Vienna, Austria
3. Results and discussion For the substituted sample, our earlier ac susceptibility study has shown a peak at 60 K and the resistivity study has shown an insulator to metal transition at 52 K and a metallic behaviour only down to 35 K [3] as found by Terai et al. [1]. At To35 K, an insulating behaviour reappears. Neutron diffraction pattern of the Dy substituted sample (at 15 K) could be fitted with only nuclear scattering intensities (Fig. 1). The absence of any
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.072
ARTICLE IN PRESS S.M. Yusuf et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1288–1289 X=0
1000
297 K
1.00 Polarization (P )
2000
Neutron Counts (arb. units)
0 2000
X=0
1000
15 K
0 1200
X = 0.243
800
297 K
1289
0.98 0.96
H = 7 Oe
0.94 0.92 0
20
40
60
80 100 120
Temperature (K)
400 0 1200
X = 0.243
800
15 K
Fig. 2. Transmitted neutron beam polarization as a function of temperature for the Dy substituted sample.
400 15
20
25
30
35
40
Scattering Angle (Deg.)
Fig. 1. Observed (open circles) and calculated (solid lines) ( for the x ¼ 0 and neutron diffraction patterns (l ¼ 1:094 A) 0.243. Bottom curves: difference patterns. The insets show combined peak intensity of the (1 1 0) and (0 0 2) Bragg peaks (2yE16:6 ) as a function of temperature.
(Magnetization) (emu g )
0
x=0 x = 0.243
H/M (Oe emu
magnetic intensity to the (1 1 0) and (0 0 2) nuclear Bragg peaks over 15–200 K (inset of Fig. 1) confirms the absence of long-range magnetic order at any temperature. It is therefore, suggested that the spins are randomly (local) canted. A canted ferromagnetic behaviour has been reported for other perovskites [4] and it was theoretically predicted by de Gennes within the framework of a model for competing ferromagnetic and antiferromagnetic interactions [5]. As shown in Fig. 2, the transmitted neutron beam polarization Pf shows a continuous decrease from B65 K, before attaining a constant value below B28 K confirming the presence of domains with net moments below 65 K. At To35 K, too, a ferromagnetic state is confirmed as reported for the ferromagnetic insulating compounds (La–Pr/Y)0.7Ca0.3MnO3 [6]. From the observed depolarization, the average size of domains was estimated using the expression (as used in Ref. [4]) d Pf ¼ Pi exp a /Fd S2 : D Here the spontaneous magnetization (MS ) 4.5 emu/g (B0:17 mB per formula unit, well below the detection limit B0:5 mB per Mn-site in diffraction experiments), obtained from the ‘‘Arrott’’ plot (Fig. 3), has been used to estimate the domain size (B5 mm at 15 K). Here the assumption of homogeneous magnetic state and the inaccuracy in estimating the MS may introduce errors [4]. Hence, the inferred domain size of B5 mm, only reflects the order of magnitude. Microscopically, the effects of the Dy substitution can be understood in terms of the reduced tolerance factor t (=0.916 and 0.908 for x ¼ 0; and 0.243, respectively)
g)
Fig. 3. Arrott plot for x ¼ 0 and 0.243 samples corresponding to the virgin magnetization at 15 K. Solid lines are the linear fit to the higher H=M data. Inset enlarges the extrapolated line over lower H=M for the x ¼ 0:243 sample.
and increased variance (a parameter corresponding to a random distribution of Mn–O–Mn bond angles) [1,3]. 4. Conclusion In the present work, the neutron depolarization study gives a robust demonstration of the existence of ferromagnetic domains in the randomly canted ferromagnetic state of the x ¼ 0:243 compound. There is no breakdown of ferromagnetic domain structure below 65 K (covering the temperature range below 35 K where resistivity shows an upturn, indicating a reentrant transition to an insulating behaviour), as opposed to what was concluded earlier by Terai et al. [1]. Our study essentially reflects how the average La-site ionic size /rR S and its variance s influence the magnetic order at different length scales [1,3].
References [1] T. Terai, et al., Phys. Rev. B 58 (1998) 14908. [2] S.M. Yusuf, L. Madhav Rao, Neutron News 8 (1997) 12; S.K. Paranjpe, Y.D. Dande, Pramana 32 (1989) 793. [3] S.M. Yusuf, et al., J. Alloys Compounds 326 (2001) 89. [4] S.M. Yusuf, et al., Phys. Rev. B 62 (2000) 1118; . . S.M. Yusuf, M. Sahana, K. Dorr, U.K. RoXler, K.-H. Muller, . Phys. Rev. B 66 (2002) 064414. [5] P.-G. de Gennes, Phys. Rev. 118 (1960) 141. [6] H.Y. Hwang, et al., Phys. Rev. Lett. 75 (1995) 914.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1288–1289
Evidence of ferromagnetic domains in the (La0.757Dy0.243)0.7Ca0.3MnO3 perovskite S.M. Yusufa,*, K.R. Chakrabortya, R. Gangulyb, P.K. Mishrac, S.K. Paranjpea,1, J.V. Yakhmic, V.C. Sahnic a
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India c Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b
Abstract We have investigated the effects of 24.3 at% Dy substitution on the La site in the ferromagnetic perovskite La0.7Ca0.3MnO3. Results show a randomly canted ferromagnetic state below 65 K with a reduced metal–insulator transition. There is no breakdown of ferromagnetic domain structure at To65 K (covering the temperature range below 35 K where resistivity shows an upturn, indicating a reentrant transition to an insulating behaviour). Reduced geometrical tolerance factor and increased variance of the La-site ionic radius are responsible for the observed changes. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Lk; 75.60.Ch Keywords: CMR perovskite; Randomly canted ferromagnet
1. Introduction
2. Experimental
Extensive research has been done on metallic, ferromagnetic manganites of the type La0.7Ca0.3MnO3, through doping the La3+ sites with trivalent rare-earths of different sizes. Recently, Terai et al. [1] studied (La1xDyx)0.7Ca0.3MnO3 by DC magnetization, resistivity and magnetoresistance techniques. The x ¼ 0:243 compound was reported to show reentrant spin-glass behaviour where the high temperature insulating paramagnetic state transforms to a ferromagnetic metallic state at relatively lower temperatures and finally transforms to an insulating spin glass (with a collapse of ferromagnetism) at further lower temperatures. Here we present the results for the x ¼ 0:243 sample that provide strong evidence of the presence of ferromagnetic correlation in its low temperature insulating state contrary to what was concluded by Terai et al. [1].
Polycrystalline samples of (La1xDyx)0.7Ca0.3MnO3 (x ¼ 0 and 0.243) were prepared by solid-state reaction. X-ray diffraction patterns confirmed the single-phase formation of orthorhombic perovskite structure. DC magnetization measurements were carried out using a SQUID magnetometer (Quantum Design). Neutron diffraction and one-dimensional (z2z) neutron-depolarization measurements were carried out at Trombay [2].
*Corresponding author. Tel.: +91-22-25-595383; fax: +9122-25-505151. E-mail address: [email protected] (S.M. Yusuf). 1 Present address. International Atomic Energy Agency, Vienna, Austria
3. Results and discussion For the substituted sample, our earlier ac susceptibility study has shown a peak at 60 K and the resistivity study has shown an insulator to metal transition at 52 K and a metallic behaviour only down to 35 K [3] as found by Terai et al. [1]. At To35 K, an insulating behaviour reappears. Neutron diffraction pattern of the Dy substituted sample (at 15 K) could be fitted with only nuclear scattering intensities (Fig. 1). The absence of any
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.072
ARTICLE IN PRESS S.M. Yusuf et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1288–1289 X=0
1000
297 K
1.00 Polarization (P )
2000
Neutron Counts (arb. units)
0 2000
X=0
1000
15 K
0 1200
X = 0.243
800
297 K
1289
0.98 0.96
H = 7 Oe
0.94 0.92 0
20
40
60
80 100 120
Temperature (K)
400 0 1200
X = 0.243
800
15 K
Fig. 2. Transmitted neutron beam polarization as a function of temperature for the Dy substituted sample.
400 15
20
25
30
35
40
Scattering Angle (Deg.)
Fig. 1. Observed (open circles) and calculated (solid lines) ( for the x ¼ 0 and neutron diffraction patterns (l ¼ 1:094 A) 0.243. Bottom curves: difference patterns. The insets show combined peak intensity of the (1 1 0) and (0 0 2) Bragg peaks (2yE16:6 ) as a function of temperature.
(Magnetization) (emu g )
0
x=0 x = 0.243
H/M (Oe emu
magnetic intensity to the (1 1 0) and (0 0 2) nuclear Bragg peaks over 15–200 K (inset of Fig. 1) confirms the absence of long-range magnetic order at any temperature. It is therefore, suggested that the spins are randomly (local) canted. A canted ferromagnetic behaviour has been reported for other perovskites [4] and it was theoretically predicted by de Gennes within the framework of a model for competing ferromagnetic and antiferromagnetic interactions [5]. As shown in Fig. 2, the transmitted neutron beam polarization Pf shows a continuous decrease from B65 K, before attaining a constant value below B28 K confirming the presence of domains with net moments below 65 K. At To35 K, too, a ferromagnetic state is confirmed as reported for the ferromagnetic insulating compounds (La–Pr/Y)0.7Ca0.3MnO3 [6]. From the observed depolarization, the average size of domains was estimated using the expression (as used in Ref. [4]) d Pf ¼ Pi exp a /Fd S2 : D Here the spontaneous magnetization (MS ) 4.5 emu/g (B0:17 mB per formula unit, well below the detection limit B0:5 mB per Mn-site in diffraction experiments), obtained from the ‘‘Arrott’’ plot (Fig. 3), has been used to estimate the domain size (B5 mm at 15 K). Here the assumption of homogeneous magnetic state and the inaccuracy in estimating the MS may introduce errors [4]. Hence, the inferred domain size of B5 mm, only reflects the order of magnitude. Microscopically, the effects of the Dy substitution can be understood in terms of the reduced tolerance factor t (=0.916 and 0.908 for x ¼ 0; and 0.243, respectively)
g)
Fig. 3. Arrott plot for x ¼ 0 and 0.243 samples corresponding to the virgin magnetization at 15 K. Solid lines are the linear fit to the higher H=M data. Inset enlarges the extrapolated line over lower H=M for the x ¼ 0:243 sample.
and increased variance (a parameter corresponding to a random distribution of Mn–O–Mn bond angles) [1,3]. 4. Conclusion In the present work, the neutron depolarization study gives a robust demonstration of the existence of ferromagnetic domains in the randomly canted ferromagnetic state of the x ¼ 0:243 compound. There is no breakdown of ferromagnetic domain structure below 65 K (covering the temperature range below 35 K where resistivity shows an upturn, indicating a reentrant transition to an insulating behaviour), as opposed to what was concluded earlier by Terai et al. [1]. Our study essentially reflects how the average La-site ionic size /rR S and its variance s influence the magnetic order at different length scales [1,3].
References [1] T. Terai, et al., Phys. Rev. B 58 (1998) 14908. [2] S.M. Yusuf, L. Madhav Rao, Neutron News 8 (1997) 12; S.K. Paranjpe, Y.D. Dande, Pramana 32 (1989) 793. [3] S.M. Yusuf, et al., J. Alloys Compounds 326 (2001) 89. [4] S.M. Yusuf, et al., Phys. Rev. B 62 (2000) 1118; . . S.M. Yusuf, M. Sahana, K. Dorr, U.K. RoXler, K.-H. Muller, . Phys. Rev. B 66 (2002) 064414. [5] P.-G. de Gennes, Phys. Rev. 118 (1960) 141. [6] H.Y. Hwang, et al., Phys. Rev. Lett. 75 (1995) 914.