Indian J. Phys.82(2), 155-161 (2008)
Effect of Ho and Nd substitution in La,
k^Ca
ft„MnO
U.o7 (1.33
CMR manganites
D K Mishra1, P K Mishra2, D R SahuJ, D Behera4 and B K Roul*1
Mnstitute of Materials Science, Planetarium Building, Bhubaneswai-751 013, Onssa, India 'Technical Physics & Prototype Engineering Division, Bhaba Atomic Research Center,
Mumbai-400 085, Maharashtra, India
department of Materials Science & Engineering, National Cheng Kung University Tainan 701, Taiwan
4National Institute of Technology, Rouikela-769 008 Onssa India E-mail ims(u lopb res in
Abstract Temperature dependent low field magnetization and electroresistive properties for the sample
Lau wHooo3Cao33Mn03& L ao64N cL} C ao33M n 0Th a v e b e G n s t u d , e c l T n e substitution of Nd3+ and HoJ+ in the site of La3+
in La0 6/Ca0 J3MnO^ (LCMO) lower the magnetic transition temperature (TJ in comparison to the main matrix of LCMO prepared by solid-state reaction route However, the substitution showed a direct relationship between Tc
and <rA> It is expected that decreasing the mean ionic radius <rA> influenced the reduction of Mn-O-Mn bond angle, which caused the reduction of the transition temperature The magnetic transition temperature (Curie temperature) T ' is well lying within metal-insulator transition temperature TM/'
Keywords Solid state reaction route, magnetic transition, metal-insulator transition PACS Nos. 71 30 Th, 73 43 Qt
1. Introduction
Much attention has been paid to the rare earth manganite perovskites, due to the colossal magnetoresistance (CMR) effect [1-5] in the compounds and their wide spectrum of physical properties. The CMR behavior of hole doped manganese perovskite occurs near the ferromagnetic (FM) transition temperature and this remarkable phenomena is attributed to the magnetic couplings between Mn3+ and Mn4+ ions as well as to the strong electron- phonon coupling arising from Jahn-Teller splitting of Mn 3d levels. It has also been noted that bond angle and bond length of Mn-O-Mn also play crucial role in controlling CMR properties of these manganites as the geometric quantity and the tolerance factor T are
Corresponding Author
©2008 I ACS
156 DK Mishra, P K Mishra, D R Sahu, D Behera and B K Roul modified when smaller ions are substituted in La site to fill the 3D network of MnC\
octahedra In several perovskites (^^^oe^Q^MnO^ w h e r e <R' R'> = L a» Y> N d' H o
etc, and A = Ca, Sr, Ba, etc, the overlap between Mn d orbitals and oxygen p type orbitals forms the electronically active band, and this overlap can be strongly influenced by the internal pressure generated by A-site substitution with ions of different radii [6-8]
However, the phase transition from insulating paramagnetic (PM) to the FM metallic state at the curie temperature varies appreciably with varying chemical pressure by incorporating tnvalent rare earth ions of different sizes into the primary perovskite structure without affecting the ratio of valency of Mn ions [6-10] Incidentally the tolerance factor, a geometrical index, is defined as t-\rA + rQ \j^}12(rMn ^ r0)], may be calculated from the empirical ionic radii [11], where <rA> is the average ionic radius of tnvalent and divalent cations, r0
is the ionic radius of oxygen ion and rMn is the average ionic radius of Mn3+ and Mn4+
This factor represents the microscopic distortion from the ideal cubic perovskite structure (t = 1) With decreasing f, Mn-O-Mn bond angles, which are microscopically related to transfer integral 'fc>* describing electron hopping between Mn3+ and Mn4+, decrease For decreasing <rA>, Tc decreases and the size of magnetoresistance increases drastically at the Tc The principal effect of decreasing <rA> is to decrease the Mn-O-Mn bond angle [8], there by reducing the transition temperature The magnetoresistive effect is indirectly influenced by the degree of distortion of the lattice, because it determines both the bond length and the Mn-O-Mn angle The degree of distortion of the lattice increases in small paticles, near major defects of the crystalline lattice and at the crystallite edges, or at dislocation zones. These lead to an increase of the mean deviation of the Mn-O-Mn angle and of the Mn-0 length The presence of rare earth ions with different ionic radii and their distribution modes in the lattice change the chemical degree of disorder and implicitly, the specific magnetization, the resistivity and the magnetoresistance [6-8, 12] With this notion in mind, we have doped Ho and Nd in the La067Ca03JMnO3 samples in which the average ionic radii of the A site <rA> is varied while keeping the Mn3+ / Mn4+ ratio fixed at 0 67 / 0.33 The doping of hole based ions in lesser comparable ionic radii (Nd3\ Ho3+) wrt La3+ is reducing the transition temperature from paramagnetic insulator to ferromagnetic metal in comparison to the main matrix LCMO [13,14].
2. Experimental
La064Nd003Ca033MnO3 and La064Ho003Ca033MnO3 were prepared by conventional solid- state reaction technique. The mixtures of the corresponding oxides of La, Ca, Mn and Nd/Ho were calcined at 950°C for several hours followed by repeated quenching and grinding The powders of above compositions were palletized (8 T/cm2) and sintered at 1450°C for 20 hours under controlled oxygen atmosphere. A standard four-probe a.c. technique (f = 17Hz, I = 1mA RMS) was used to measure the resistance from room temperature down to 25K. The zero field cooled (ZFC) and field cooled (FC) measurements were carried out from room temperature down to 150K using VSM.
Effect of Ho and Nd substitution in La0 67Ca0 33Mn03 CMR manganites 157
3. Results and discussions
With respect to the crystallography properties, the principal difference between the members of La064Nd003Ca033MnO3 and La0^Ho0 0 3Ca0 3 3MnO3 is the mean ionic radius <rA> of the A-site cations (La, Ca, Ho/Nd) The mean ionic radius decreases with decreasing cationic radius due to the difference in ionic radii of La3+ and Ho3+/Nd3+ This sugests that <rA> is the essential structural parameter responsible for the change of the Mn-O-Mn bond angle This assertion is supported by earlier investigations on the relationship between composition and structrue in perovskite manganites [6-8, 15] These authors also considered manganite systems with ~ 33% divalent A-site substitution and studied the effect of varying the mean radius of the A-site ions However, rather than changing the size of the divalent cation, their approach was to vary the size of the trivalent A-site member, / e they considered the series (IVxR\c)o67Ao33M n 03' w h e r e (R» R') = L a' Y> N d' H o etc > a n d A = Ca, Sr, Ba, etc The mean M n - 0 bond was nearly the same, i e - 0 196 nm, for all the members of the (R1_xR,x)067A033MnO3 series, and thus equivalent to the M n - 0 bond distance in the La067_x(Nd/Ho)xCa033MnO3 samples The mean Mn-O-Mn bond angle decreased with decreasing A-site ionic radius Regardless of the precise nature of the R, R and A ions, the compounds with (rA) < 0 124 nm were orthorhombic (Pbnm symmetry) while the compounds with \rA)>0 124 nm were rhombohedral (R-3c symmetry) This can be understood by involving the Goldsmith's tolerance factor, t The tolerance factor is 0 9176 for the mam matrix of LCMO but for La0 6 4N d0 0 3C a0 3 3M n 03 and La0 6 4Ho0 0 3Ca0 3 3MnO3 the tolerance factor is approximately equal to 0 917 and 0 916 respectively, which are less than the main matrix of LCMO due to decrease of mean cationic radius The tolerance factor can be regarded as a measure of the size mismatch between the radius of the A-site ions and the space between the MnOb octahedra [6, 8], where they reside t = 1 corresponds to a pefect size match and the Mn-O-Mn bond angle would be 180° When t < 1, the A-site ions are too small to fill the space between the Mn06 octahedra, which creates some internal pressure [6, 8]. This forces the octahedra to tilt and rotate in an attempt to redue the excess space around the A-site, by reducing the Mn-O-Mn bond distance and Mn-O-Mn bond angle Both the Mn-O-Mn bond distance and Mn-O-Mn bond angle are the basic structural parameters controlling the hybridization strength between Mn 3d and 0 2p states The samples with Pbnm space group symmetry have the tolerance factor t < 0 93 and t < 0 93 for R-3C symmetry [8, 16] According to the comparison of <rA> and t of La0 6 4Nd0 0 3Ca0 3 3MnO3 and La0 6 4Ho0 0 3Ca0 3 3MnO3 with the limiting value of <rA> and f, both the compounds exhibit orthorhombic structure with Pbnm space group symmetry The Curie temperature depends more specifically on the mean radius of the A-site ions than on composition parameter, x Even more specifically, the relationship between Tc and <rA> is due to the linkage between <rA> and Mn-O-Mn bond angle The magnetization dependence on tolerance factor is described in the later part
Figure 1 and Figure 2 shows the variation of magnetization (FC & ZFC) as a function of temperature at different magnetic field (upto 200 Oe) for La0 6 4Nd0 03Ca0 3 3Mn03 and
La0 64Ho0 03Ca0 33M n O3 s a mPl e sintered at 1450°C for 20 hours A large value difference in
158 D K Mishra, P K Mishra, D R Sahu, D Behera and B K Roul
magnetization is found in FC and ZFC cases of both the sample The broad magnetic transition from PM to FM in La064Nd003Ca033MnO3 and La064Ho003Ca033MnO3 are found to be 260°K and 220°K respectively, which are less than the transition temperature of LCMO [13-14]
1 5 - 0050-
0 04S- 0 040- O0J6- 0030' 0 02ft- 0020- 0 015- 0 010- 0 00ft- 0 000- -0 005-
FC V
—~«**A
— i — • i • i » i • i • — i »
- e - 5 0 O e - A — 100 Oe
=y>^, A it >
T • r • • ' ' i — * •
200 220 240 200 Temperature (K)
250 300 320
Figure 1. Variation of magnetization FC and ZFC as a function of temperature for two magnetic fields (50 and 100 Oe) for La064Nd00)Ca03^MnO3
220 Temperature (K)
Figure 2. Variation of magnetization FC and ZFC as a function of temperature for three magnetic fields (50, 100 and 200 Oe) for La0WHo00£a0T,MnO.,
The transition temperature determined from neutron, magnetic and resistive measurements confirm the strong dependence of Tc on <rA> or, equivalently, on the tolerance factor So when the lesser ionic radii of cations are doped into the LCMO matrix, the Mn-O-Mn bond length decreases and the bond angle between Mn-O-Mn deviates from 180°, which reduces the transition temperature, Tc [6-8, 17] It is also found a sharp increase of FC magnetization value at low temperature of 160°K in Nd and Ho doped LCMO because while cooling in an applied magnetic field, the spin tends to align parallel to each other and cause enhancement in ferromagnetic component, known as spin induced feromagnetism [18, 19] Even though the Mn-O-Mn bond angle and <rA> play decisive roles for the magnetic coupling strength in mixed-valence manganites these are not the only factors influencing Tc For instance, there is a tendency for the Curie temperature to decrease with decreasing <rA> and Mn-O-Mn bond angle will tend to strengthen the tg(Mn)~
2po{0)-tg(Mn) hybridization, which causes antiferromagnetic coupling between the Mn neighbours [8] This is mentioned to emphasize that the relationship between Tc and
<rA>, which is evident for mixed-valence manganite systems The broadening of the magnetic transition found in La0 64Nd0 03Ca0 3 3Mn03 and La0 64Ho0 03Ca0 3 3Mn03 could be attributed to the decreasing grain size associated with the doping of Nd and Ho in LCMO The SEM picture of Nd & Ho doped LCMO is shown in Figure 3 The average grain sizes are of the order of 6-8 urn, which is less than the average gram size (10-12 urn) of
L a0 67Ca0 33MnO3 sintered sample at 1450°C (shown in Figure 3(a)). A smaller grain size gives a larger proportion of surface-near spins, which may be weaker ferromagnetically coupled than spins in the bulk of the grains This could give a distribution of Curie
effect of Ho and Nd substitution in La0 67Ca0 33Mn03 CMR mangamtes 159
temperatures and thus a broadened magnetic transition A grain size dependent broadening of the Curie transition has previously been observed [20, 21]
(a) __. (b) (C)
*»*&**
:**''-'% mrm
^***&&,
Figure 3 S E M picture of (a) L a0 6 7C a0 ^ M n 03 (b) L a0 b 4N d0 0, C a0 3 3M n O , & (c) L a0 6 4H o0 ( HC a0 nM n O . ,
Figure 4 shows the M~H isotherms recorded on L a0 6 4N d0 0 3C a0 3 ?M n O j and La0b4Ho0(HCa0JJMnO3 at room temperature ( ~ 300K) upto a field of 1 2 T Both the samples didn't show saturated magnetization in the applied field of 1 2T in the PM region, which shows the evidence that there are no magnetic hysterysis effects
0 20' 0 15H 0 10 0 05 oooH - 0 05 - 0 10-4 - 0 15H - 0 20
0 02 ^ 0 0 0 -10000 0015^
_ ooioH
£ 0 005<
| 0 000- 5U0 005-
^ - 0 010- - 0 015-
-5000 i ' i ' i
5000 10000 15000 H(Oe)
L a0 64Nd0 0 3C au i 3M n° i
' I ' -1 • I "
-15000 -10000 -5000 0 H(Oe)
5000 10000 15000
Figure 4 Magnetization (M) vs Field (H) isotherm at 3 0 0 K for L a0 6 4N d0 0 3C a0 J JM n O3 a n d
L a064H O0 03C a0 3 3M n O3
400 350 300 250 -\
200 150 100 50 0
^ o e ^ o o a0^ ^0, • \
L^0 6 4Nd0 0 dC d0 J JM n O , l
0 50 100 150 200 Temperature (K)
250 300
Figure 5. Temperature d e p e n d e n c e resistance for
L a0 6 4N d0 0 3C a0 J JM n Oi a n d L a0 64HO0 03C a0 3 3M n O3
Sintered pellets were checked by resistance measurements to evaluate the magnitude of resistance and metal-insulator transition (TMI) temperature Figure 5 shows the variation of resistance as a function of temperature Qualitative substitutional effects are observed as compared to the resistive transition for both La064Nd003Ca033MnO3 (La/Nd) and
L ao 6 4H oo o 3C ao 3 3M n 03 (L a / H° ) !t , s n o t e d t h a t i n , d e n t , c a l processing condition, La/Nd (Tc =250K) substitution showed improved resistive transition than that of La/Ho (Tc =210°K)
160 D K Mishra, P K Mishra, D R Sahu, D Behera and B K Roul substitution stoichiometrically for these perovskite CMR manganites. This observation suggests that higher the mean ionic radii, higher the transition temperature. The transition from insulator to metal at 250°K in Nd doped LCMO and 210°K in Ho doped LCMO, which is close proximity to the Curie temperature Tc shown in Figure 1 & Figure 2.
4. Conclusions
The decrease of the <rA> due to the doping of Nd and Ho in place of La ion in the main matrix of LCMO leads to decrease the FM to PM transition temperature. Decrease of the tolerance factor, <rA> and the increase of the degree of distortion are corroborating with the decrease of the transition temperature. The Curie temperature and the metal insulator transition temperature were lying close proximity to each other.
Acknowledgment
One of the authors (DKM) is highly thankful to CSIR, New Delhi for providing financial support to carry out research work.
References
[1] S Yunoki, J Hu, A L Malvezzi, A Moreo, N Furukawa and E Dagotto Phys. Rev Lett 80 845 (1998)
[2] V Ravindranath, V S R Rao, G Rangarajan, Y Lu, J Klein, R Klingeler and S Uhlenbruck Phys. Rev B63 184434 (2001)
[3] A K Pradhan, B K Roul, Y Feng, Y Wu, S Mohanty, D R Sahu and P Dutta Appl Phys Lett 78 1598 (2001)
[4] G H Joneker and J H Van Santen Physics (Amsterdam) 16 337 (1950) [5] E O Wollan and W C Koehler Phys Rev 100 548 (1955)
[6] H Y Hwang, S W Cheong, P G Radaelli, M Marezio and B Batlog Phys Rev Lett 75 914 (1995) [7] H Y Hwang, S W Cheong, N P Ong and B Batlog Phys. Rev. Lett. 77 2041 (1996)
[8] J L Garcia-Munoz, J Fontcuberta, B Martinez, A Seffar, S Pinol and X Obradors Phys. Rev B55 R678 (1997)
[9] S Jin, T H Tiefel, M McCromack, R A Fastnatch, R Ramesh and L H Chen Science 264 413 (1994)
[10] K Khazeni, Y X Jia, L Lu, V H Crespi, M L Cohen and A Zettl Phys. Rev. Lett. 76 295 (1996) [11] R D Shanon Acta Crystallographica A32 751 (1976)
[12] N Cornei and M L Craus J. Alloys & Compounds 368 58 (2004)
[13] B K Roul, D R Sahu, S Mohanty and A K Pradhan Materials Chem & Phys. 67/1-3 267 (2001)
[14] A K Pradhan, B K Roul. J G Wen, Z F Ren, M Muralidhar, P Dutta, D R Sahu, S Mohanty and P K Patro Appl. Phys. Lett. 76 763 (2000)
[15] J L Garcia-Munoz, J Fontcuberta, M Suaaidi and X Obradors J. Phys. Condens Matter 8 L787 (1996)
[16] A R Dinesen Magnetocaloric and Magnetoresistive Properties of La067Ca0 33-xSrxMnO3 (ph- D- Thesis) RisQ National Laboratory, p.103, August (2004).
Effect of Ho and Nd substitution in La0 67Ca0 33Mn03 CMR manganites 161
[17] T G Reddy, P Y Reddy, V R Reddy, A Gupta, M Gupta and K R Reddy Solid State Communication 133 77 (2005)
[18] D S Rana, K R Mavani, C M Thaker, D G Kberkar, D C Kundaliya and S K Malik J Magn. Magn.
Materials 271 215 (2004)
[19] D S Rana, K R Mavani, C M Thaker, D G Kberkar, D C Kundaliya and S K Malik J AppI Phys 95 4934 (2004)
[20] N Zhang, W Ding, W Zhong, D Xing and Y Du Phys. Rev. B56 8138 (1997) [21] N Zhang, F Wang, W Zhong and W Ding J. Phys Conden. Matter 11 2625 (1999)
