Thermodynamic analysis for the determination of anisotropy constants in polycrystalline nanowires with cubic phase.

D.R. CORNEJO; K. MONTERO REY; M.S. VIQUEIRA; S.E. URRETA; G. POZO LÓPEZ; A.M. CONDÓ; L.M. FABIETTI

Gramado

Congreso; XVI Brazil MRS Meeting; 2017

Ni and CoxPd1-x nanowires (11≤x≤60) were prepared by electrodepositing the metal ions within the pores of an anodized aluminum oxide (AAO) membrane. The nanowires are polycrystalline, with mean grain size values smaller than the wire diameter. The small grains (with fcc crystalline structure) are equiaxed and randomly oriented.Magnetization measurements, as a function of the angle θ between the main axisof the nanowires and the applied field, were performed at room temperature, by using a Lake Shore VSM system. As expected, the samples exhibit strong shape anisotropy. The results showed that a major fraction of the grainsf,pa, show coercivity improved by the shape anisotropy and tend to keep their magnetic moments aligned parallel to the major axis of the nanowires. The remaining part, fpe (fpa+fpe = 1), has low coercivity and shows a tendency to maintain their magnetic moments aligned perpendicular to the main axis of the wires. These fractions were quantitatively determined by the behavior of the relative remanence S. We show that the experimental data of S vs θ are properly adjusted by a function related to the effective anisotropy of the system,FAN = K1 sin2θ (4 cos2θ + Ku/K1), where K1 is the magneto-crystalline anisotropy constant and Ku is the anisotropy constant due to the shape anisotropy.Using a thermodynamic analysis of irreversible processes, we show that we can determine these anisotropy constants measuring initial magnetization curves from two different AC demagnetized states: the first one with the magnetic field applied parallel to the nanowires, and the second one with the magnetic field applied perpendicular to the wires. The values obtained for Ni are in agreement with previous results. In the case of CoPd, our results are consistent with the few studies that exist on these alloys. To best of our knowledge, the method developed in this work to determine anisotropy constants in ordered nanowire systems is completely new.