Determination of the Upper Limit for the Cluster Moment Contributing the Giant Magneto Resistance in Laser Deposited Gra
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*Departamento de Fisica, Universidad Piiblica de Navarra, E-31006 Pamplona, Spain. #Department of Condensed Matter Physics. Royal Institute of Technology, S- 10044 Stockholm, Sweden. ABSTRACT We have fabricated granular Cu 95Co5 thin films by laser ablation-deposition. Within a regime of annealing temperatures, these samples exhibit Giant Magneto Resistance (GMR), typically 5% in 0.5 Tesla at 5 K. The magnetic hysteresis loops are found to show finite coercive fields in the whole temperature range 2 K - 300 K. Below 9 K, the field dependence of the MR shows a split maximum. We interpret the data in terms of coercivity arising from blocking phenomenon of single domain superparamagnetic Co clusters. A quantitative determination of the upper limit for the cluster moment contributing to GMR is estimated to be 17000 gB (a cluster size of 5 nm). INTRODUCTION Granular solids exhibiting giant magnetoresistance (GMR) are fabricated by a variety of techniques, both in the form of thin films and bulk materials, including several microns thick meltspun ductile long ribbons1". Post fabrication annealing of the as quenched materials produces a fine dispersion of magnetic precipitates in the metallic host matrix. The overall magnetic behaviour of such a material is superparamagnetic, concomitant with the phenomenon of GMR'-8 . It is thus of considerable interest to study the evolution of the magnetic behaviour of such magnetic granules: the size dependence of the total spin per cluster, the intra- intercluster interactions, as well as the magnetic properties of the metallic host the clusters are embedded into. Clearly, a technique of fabricating materials that enables good control of the size and separation
distance of magnetic particles will provide us an insight into the GMR phenomenon which has become a fundamental problem in condensed matter physics.
This work is an attempt to demonstrate the feasibility of producing metallic thin films by pulsed
laser ablation of the immiscible elements, wherein a fine dispersion of magnetic particles is made to precipitate by subsequent annealing. A correlation between GMR and the magnetic properties of Cu95 Co, is presented by estimating the distribution of magnetic moments attained by this
deposition technique. EXPERIMENTAL
Thin films, with a nominal composition of Cu95 CO, have been fabricated by pulsed laser deposition technique (Nd:YAG laser, 1=1024 nm, 20 Hz repetition rate, 4 ns pulse duration, energy I mJ per pulse, 2 mm in diameter beam spot size on target). The circular target used consisted of one (n/l0) circular sector of pure Co, and one (197r/10) circular sector of pure Cu, and was rotated at an angular speed of (703) rad/s. The films were deposited onto ordinary glass substrates at room temperature, under a vacuum of 10.6 mbar, and had a total thickness of 100 rim. The samples have been subsequently annealed at different temperatures between 300'C and 400'C for 10 minutes in an inert atmosphere. Microstructural measurements have been carried out in a X-ray diffractometer
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