Surface and interfacial effects in ceramic manganites

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Journal of Magnetism and Magnetic Materials 196-197 (1999) 479-480

Journalof mnad gneusm magnetic materials

Surface and interfacial effects in ceramic manganites B. Martinez*, L1. Balcells, M. Respaud, F. Sandiumenge, J. Fontcuberta, X. Obradors lnstitut de Cikncia de Materials de Barcelona (CSIC), Campus Universitat Autdnoma de Barcelona, E-08193 Bellaterra, Catalunya, Spain

Abstract Low-(LFMR) and high-field (HFMR) magnetoresistance of ceramic Laz/3Srl/3MnO 3 samples with different grain sizes (20 nm < ~ < 10 p.m) have been investigated. A limiting L F M R of about ~ 30% is obtained for ~3 ,~0.5 rtm with no increase in further reductions of the grain size. Neverthless, the H F M R progressively rises when reducing the grain size. Experimental results suggest that H F M R originates from a non-collinear ferromagnetic surface layer. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Low-field magnetoresistance; High-field magnetoresistance; Manganese perovskites

The low-field magnetoresistance response (LFMR) of manganese perovskites is known to be closely connected to the existence of interfaces and grain boundaries [1-5]. However, the precise origin of the L F M R remains unclear. Two basic approaches have been considered so far, the so-called tunnel magnetoresistance (TMR) among grains [1] and the common mechanism of spin-dependent scattering at interfaces (as in giant magnetoresistance G M R granular films) [4,5]. Inspection of the reported magnetoresistance across grain boundaries, however also reflects another striking feature which has not been considered so far: following the initial drop of resistivity p(H) up to fields of about ~ 5 kOe, there is also a substantial high-field magnetoresistance, that leads to further decrease of p(H). For fields of few teslas, this high-field magnetoresistance, (HFMR) contribution is even larger than the low-field one [1-3]. In this high-field region, although the sample magnetization is almost saturated, there is still an extraordinary change of resistivity. This H F M R response could not be related to the overall sample magnetization since it does not exist in single crystals and thus it should also reflect an interface response. It has been suggested

*Corresponding author. Tel.: + 34-9-3580-1853; fax: + 3493-580-57-29; e-mail: [email protected].

that the H F M R could also be due to the existence of a non-ferromagnetic surface layer whose nature is a key ingredient in the mechanism of the magnetoresistance at interfaces, as it constitutes the barrier that carriers should cross or tunnel. Ceramic Laz/3Sra/3MnO3 samples with particle size differing more than three orders of magnitude, prepared by the standard solid state reaction [6], have been used to carefully analyse the H F M R contribution. We have found that this contribution systematically increases when reducing the grain size and significantly, it is accompanied by a substantial reduction of the saturation magnetization. As shown in Fig. 1, samples annealed at higher temperatures, thus having larger particle size, exhibit a metallic like behavior all the way from Tc down the lowest measured temperature. When reducing the particle size the resistivity, at any temperature, becomes higher and a clear insulating-like behavior develops. We note that resistivity spans more than six orders of magnitude. It is observed (Fig. 2) that when reducing the particle size the L F M R increases up to a limiting value of about 30% and no further enhancement is obtained. Of major relevance is the substantial enhancement of H F M R when reducing ~ , while Ms significantly decreases, even all the samples have the very same To. Therefore, the H F M R still increases while the L F M R is already saturated at its higher value, thus revealing the distinct origin of both

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contributions. The most obvious explanation for this behavior is the presence of a non-collinear ferromagnetic surface layer, whose relative thickness t / ; g increases when reducing the grain size. We suggest that the H F M R is strongly connected with the existence of a magnetically disrupted surface layer, whose thickness can be controlled by the grain size. This is clearly revealed by the H F M R versus Ms plot of Fig. 2b. The fact that M R can be increased from roughly a 30% in the low-field region up to 50-60% at 50 kOe without indication of saturation would indicate that this surface layer is magnetically hard [7]. Measurements of magnetic anisotropy by ferromagnetic resonance corroborate this conclusion [8]. In summary from the analysis of the magnetoresistance of a number of ceramic L S M O samples having averaged grain size ranging from 20 nm to 10 ~tm, we have shown that: (a) the L F M R can be pushed up to ~ 3 0 % for the ~ 4 0 0 n m but no larger L F M R is obtained by further reduction of the grain size down to the nanometric range. (b) Inversely the H F M R progressively rises when reducing the grain size. (c) This observation correlates with a significant reduction of the saturation magnetization in the smallest samples indicating the existence of a non-collinear surface layer at the grains.

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Fig. 2. (a) Low-field magnetoresistance LFMR (right) (open circles) and high-field magnetoresistance HFMR (left) (solid circles) versus grain size ~ . Rhombi are the corresponding data taken from Ref. [5]. (b) HFMR data versus the saturation magnetization Ms (10 K, 5 T). We would thank Prof. F. Guinea for useful discussions. Financial support by the C I C Y T (MAT97-0699)and the C E E - O X S E N projects and the Generalitat de Catahmya (GRQ95-8029) are acknowledged. References

[1] H.Y. Hwang, S-W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [2] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N. Rao, Appl. Phys. Lett. 68 (1996) 2291. [3] R.D. Stinchez et al., Appl. Phys. Lett. 68 (1996) 1. [4] X.W. Li, A. Guptm Gang Xiao, G.Q Gong, Appl. Phys. Lett. 71 (1997) 1124. [5] Gupta et ai., Phys. Rev. B 54 (1996) R15629. [6] J. Fontcuberta, B. Martinez, V. gaukhin, LI. Balcells, X. Obradors, C.H. Cohenca, R. Jardim, Phil. Trans. Roy. Soc. A 356 (1998). [7] L1. Balcells, J. Fontcuberta. B. Martinez, X. Obradors, J. Phys. C 10 (1998) 1883. [8] M. Respaud et al., in preparation.