Lattice Strain Accompanying the Colossal Magnetoresistance Effect inEuB6

Abstract

The coupling of magnetic and electronic degrees of freedom to the crystal lattice in the ferromagnetic semimetal EuB6, which exhibits a complex ferromagnetic order and a colossal magnetoresistance (CMR) effect, is studied by high-resolution thermal expansion and magnetostriction experiments. EuB6 may be viewed as a model system, where pure magnetism-tuned transport and the response of the crystal lattice can be studied in a comparatively simple environment, i.e., not influenced by strong crystal-electric field effects and Jahn-Teller distortions. We find a very large lattice response, quantified by (i) the magnetic Grüneisen parameter, (ii) the spontaneous strain when entering the ferromagnetic region and (iii) the magnetostriction in the paramagnetic temperature regime. Our analysis reveals that a significant part of the lattice effects originates in the magnetically-driven delocalization of charge carriers, consistent with the scenario of percolating magnetic polarons. A strong effect of the formation and dynamics of local magnetic clusters on the lattice parameters is suggested to be a general feature of CMR materials.Materials, in which the resistivity exhibits drastic changes in response to an external magnetic field, are of great interest both from a fundamental as well as a technological point of view. Those anomalous magnetotransport effects are particularly strongly pronounced close to a combined magnetic and insulator-metal transition, where a large or even a colossal magnetoresistance (CMR) can be observed. Prominent examples include magnetic semiconductors, rare-earth chalcogenides, silicides and hexaborides, Mn-based pyrochlors, as well as the mixed-valent rare-earth manganites [1][2][3][4][5]. One route for describing the CMR effect involves the formation of magnetic polarons (MPs). This concept was first suggested based on experiments on Eu 1−x Gd x Se [1] and shortly thereafter given a theoretical foundation [6]. MPs are formed when it is energetically favorable for the charge carriers to localize and spin polarize the surrounding local moments over a finite distance. With increasing magnetic field, these ordered clusters may grow in size, accompanied by a progressive alignment of the spins outside the ordered clusters thereby facilitating the charge transport. The existence of magnetic clusters (tantamount to MPs) in some manganites has been demonstrated by a concomitant lattice distortion, the field dependence of which closely follows the magnetoresistivity [7]. Despite considerable efforts to understand the inter- * Present address: Dept. of Physics, Indian Institute of Technology, New Dehli, India † Present address: IGCE, Unesp.-Univ. Estadual Paulista, Dept.de Física, Rio Claro, Brazil ‡ Email: j.mueller@physik.uni-frankfurt.de play between spin, charge and lattice degrees of freedom in the CMR effect for the various materials, see e.g. [7][8][9][10][11][12], no general picture has evolved yet. For the manganites, in particular, the reason for that may be related to their complexity due to t...

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