Determination of para- and ferromagnetic components of magnetization and magnetoresistance of granular Co/Ag films (invited)

Abstract

Several series of thin films (∼3000 Å) were fabricated by coevaporation of Co and Ag in a dual e-beam UHV system at substrate temperatures, TS, between 300 and 500 K. The composition was varied between 23 and 54 at. % Co. The maximum measured magnetoresistance was 31% at room and 67% at nitrogen temperature. Magnetization and Rutherford backscattering measurements showed that the Co and Ag atoms are completely segregated for films deposited at TS≥400 K. The magnetoresistance (MR) and magnetization curves have been analyzed taking into account both the paramagnetic (PM) and ferromagnetic (FM) contributions that are observed to be present. The films having ≤32 at. % Co are entirely paramagnetic. The fraction of Co atoms in the ferromagnetic component increases monotonically with increasing TS and/or at. % Co. The average radii of the PM granules in the films having an MR effect of 25%–31% are ∼20–22 Å with a minimum average separation of ∼10 Å. The large MR values attained in both the PM and FM components are attributed to the small effective domain sizes, ∼40–50 Å. The MR values are large because the MR effect varies inversely as the mean-free-path for magnetic boundary scattering which is the average distance between domain boundaries. The variation of the resistive and magnetic behavior of the films was also studied as a function of the annealing time for an annealing temperature of 300 °C. A striking behavior seen in these films is the long relaxation times when a FM component is present.

Magnetic transitions in disorderedGdAl2

“…In contrast, the hysteresis loops of the 400-h milled sample are a superposition of a hysteretic ͑h͒ and a nonhysteretic ͑nh͒ contribution at all temperatures. The nonhysteretic part is due to paramagnetic or superparamagnetic behavior and is modeled by a Langevin function, while the hysteresis is modeled by a phenomenological form 19 described by…”

Structural Characteristics and Magnetic Properties of Al2O3 Matrix-Based Co-Cermet Nanogranular Films

“…Dominant features in the change in the magnetic phases from paramagnetic-to superparamagnetic-and ferromagnetic-like behaviors with increasing Co content, which correspond to < 0.12, 0.12 ≤ < 0.37, and ≥ 0.37, respectively, were observed for all Co x (Al 2 O 3 ) 1−x granular thin films prepared. For systems with Co granules immersed in nonmagnetic matrixes, the ferromagnetic behavior begins to appear at room temperature with a concentration at ∼ 35 Co at% [27]. Figure 4 Co atoms or with very small atomic agglomerates.…”

“…Thus, the magnetization process for such systems should occur in two steps: (i) in lowfield region, about several tens of Oersted, orientation of magnetization of the large ferromagnetic masses and (ii) in higher-field region, orientation along the applied field of the disorder magnetization of the clusters/particles that are dispersed in the matrix and/or in the ferromagnetic masses. These two steps will correspond with two magnetization components that can be separated for each ferromagnetic and paramagnetic contribution [27].…”

Interparticle interactions mediated superspin glass to superferromagnetic transition in Ni-bacterial cellulose aerogel nanocomposites

“…Hence the overall magnetization is due to a combination of large weakly ferromagnetic Ni nanoparticles of size > 75 nm, small superparamagnetic Ni nanoparticles with size <75 nm and the paramagnetic BC. The resulting magnetization variation of the composites therefore has been fitted using the following equation [29,30],…”