Evolution of Nanoscale Ferromagnetic Particles in Co-Cr and Cr-Fe Alloys Observed by Atom Probe Field Ion Microscopy
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concentration depth profiles by collecting an extended number of ions. The lateral spatial resolution of such analysis is determined by the probe hole diameter and a monoatomic layer resolution is obtained in the depth direction. In a three dimensional atom probe, the mass to charge ratios and the coordinates of individual ions are determined simultaneously [2]. By collecting such information from an extended number of atoms, two dimensional element mappings can be constructed with a subnanometer spatial resolution. As atoms always ionized from the specimen surface, atom number corresponds to the depth of the specimen. Scaling appropriate depth distance as a function of the number of detected ions, three dimensional element mappings can be constructed [3]. The details of atom probes used in the present study are described elsewhere [4,5]. The Co-Cr film was deposited by rf magnetron sputtering from a Co-22at.%Cr alloy target on a Cu coated Si wafer substrate at an Ar pressure of 1.1 Pa. The film thickness was estimated to be approximately 3 Rtm. During deposition, the substrate temperature was kept at approximately 500 K. The specimens for atom probe analyses were prepared by using a combination of photolithography and micro-electropolishing. A detailed description of this method is given elsewhere [4]. It should be noted that the FIM tips were prepared in a planar direction of the film, and hence atom probe analyses were conducted in this direction. The specimens were etched symmetrically during electropolishing so that the analysis region corresponded approximately to the middle of the film, i.e., about 1.5 pim from both substrate and the surface. Cr-Fe alloy ingots with different compositions were prepared by argon arc melting. Small square rods of approximately 0.5 x 0.5 x 12 mm were cut out of the button ingot. These samples were solution heat treated in a vacuum sealed quartz tube at 1300 K for I h, then quenched into water (referred to as ASQ). Solution treated specimens were subsequently annealed in vacuum at 773 K for periods of 400 and 2600 h (hereafter, referred to as 400 h and 2600 h, respectively). Magnetoresistance (MR) was measured at 5.8 kOe at three different temperatures of 4.2, 77 and 300 K by the standard four-point probe method. Magnetic field is applied parallel to the electric current for all the MR measurements. PHASE DECOMPOSITION IN Co-Cr SPUTTERED THIN FILMS Co-Cr alloy is the fundamental system employed in current longitudinal high density recording media, such as Co-Cr-Ta and Co-Cr-Pt [7,8]. Co-Cr alloy thin film are also considered to be one of the most promising media for perpendicular recording [9]. It is well known that the magnetic properties of the thin film recording media depend on sputtering parameters, such as bias voltage, gas pressure and substrate temperature. One such example is the increase in coercivity and the decrease in media noise when Co-Cr based films are deposited on heated substrates [10,11]. The underlying mechanisms of such magnetic property changes, however,
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