Intergranular Giant Magnetoresistance in a Spontaneously Phase Separated Perovskite Oxide

Abstract

We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite La(1-x)Sr(x)CoO(3) phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.

Enhancement of giant magnetoresistance effect in the Ruddlesden–Popper phase Sr3Fe2−xCoxO7−δ: predominant role of oxygen nonstoichiometry and magnetic phase separation

“…For the perovskite cobaltites, small-angle neutron scattering experiments have shown [7] that the material segregates into F metallic clusters embedded in a non-F insulating matrix, being a naturally occurring analog of the artificial granular-GMR materials [20]. We suggest that the magnetic phase separation in Sr 3 Fe 2-x Co x O 7-δ creates a similar heterostructure.…”

“…the F behavior becomes predominant with increasing cobalt and/or oxygen contents, and the enhanced F interaction leads to deterioration of the negative MR effect. Within the granular heterostructure model [7], the dominance of ferromagnetism in Co-rich and/or O 2 -annealed samples may reflect the increased size or volume fraction of F clusters, as deduced from the increase in τ 0 (Fig. 10).…”

“…All these cobaltites exhibit a negative giant magnetoresistance (GMR) at low temperatures, concomitantly with a spin glass (SG) behavior. In fact, small angle neutron scattering experiments on La 1-x Sr x CoO 3 by Wu et al [7] show a granular heterostructure which is the likely source of the enhanced negative GMR, in contrast to the CMR effect in manganites, which originates from the double-exchange interaction mechanism. This cobaltite is believed to segregate into metallic ferromagnetic (F) clusters embedded in a non-F insulating matrix, corresponding to a "naturally-created" intergranular GMR material.…”

Low-temperature interactions of magnetic excitons inLaCoO3

“…3-7͒; or a LS to high spin ͑HS͒, S = 2 state, t 2g 4 e g 2 transition [8][9][10] resulting in a LS-HS stable matrix of 1:1 ratio. [11][12][13] Moreover when hole doped with Sr ions, La 1−x Sr x CoO 3 , perovskite cobaltite becomes an excellent system in which to study magneto-electronic phase separation, [14][15][16] a property ubiquitous in complex magnetic oxides, which can lead to exotic behavior such as colossal magnetoresistance.…”

“…La 0.97 Sr 0.03 CoO 3 has been used as a comparison as it is well below the known percolation limit of hole-induced ferromagnetic clusters ͑x = 0.18͒ and the spin-glass concentration level ͑x = 0.09͒. 28 When hole doped, Co 4+ ions are expected to induce a moment in surrounding Co 3+ ions forming "nanodroplets" of ferromagnetism, 14,16 the exact mechanism for interaction between the clusters at low doping levels is not well understood. The high-field high-temperature susceptibility of La 0.97 Sr 0.03 CoO 3 does not show the spin-state transition of pristine LaCoO 3 and no further magnetic transition is obvious.…”