Finite-size effects in nickel nanowire arrays

Abstract

Nickel nanowire arrays with diameters in the range 30-500 nm have been fabricated by electrochemical deposition into nanoporous, single-crystal mica templates, which allow measurements of the magnetic properties of nickel nanowire arrays at high temperatures. The Curie temperature is found to be reduced by as much as 51 K for the 30 nm diameter nanowires. The Curie temperature shift with wire diameter follows the finite-size scaling relation with ϭ0.94 and 0 ϭ22 Å.The influence of reduced physical dimensions on magnetic entities is of both fundamental and technological interest. In bulk magnetic systems, the correlation length increases with temperature and diverges at the bulk transition temperature T C (ϱ). When one or more dimensions in the system are small, the growth of will eventually be limited by the smallest dimension d and the system displays a reduced transition temperature T C (d) due to finite-size effects. Technologically, as the feature sizes of magnetic structures continue to decrease, the influence of dimensionality on their magnetic properties has also become an important issue. To date, reports of finite-size effect in magnetic systems have been largely limited to ultrathin films and quasi-twodimensional systems. Measurements on quasi-onedimensional systems have been hampered by the difficulty in fabricating structures that are sufficiently small and stable over the required temperature range.In quasi-two-dimensional systems, thickness dependent phase transition temperatures have been measured in ultrathin ferromagnetic films of Ni, 1-6 Fe, 7,8 Co, 9 and Gd ͑Refs. 10-12͒ and CuMn spin glass films. 13,14 Recently, finite-size effects have also been reported in antiferromagnetic CoO ͑Refs. 15 and 16͒ and Cr ͑Ref. 17͒ thin films. In quasizero dimensional systems, such as granular thin films, phase transitions are not observed due to the onset of superparamagnetism when the particle size is very small. 18 Electrochemical deposition of metals into porous polymer films has been used to fabricate quasi-one-dimensional nanowire arrays 19-22 and the ferromagnetic properties of Ni and Co have been studied both at low temperatures and at room temperature. 22-25 However, measurements of magnetic phase transitions are not possible in these structures since the Curie temperatures for elements such as Ni, Co, and Fe are much higher than the temperature range of polymer templates. In this paper we describe the fabrication of quasi-onedimensional nickel nanowire arrays in porous mica films and show that this approach can be used to determine the temperature dependence of the magnetic properties at elevated temperatures.Nickel nanowire arrays were fabricated by electrochemical deposition into porous single-crystal muscovite mica templates. Mica is chemically stable up to 770 K, well above the Curie temperature for bulk Ni ͑ϳ630 K͒. The porous templates were fabricated by nuclear track etching. Particle tracks were formed in 5 m thick mica wafers by exposure to Ϸ6 meV ␣ particles from a 100 CiCf-252 source in a c...

Effect of size reduction on the ferromagnetism of the manganite La1−xCaxMnO3(x=0.33)

“…As an example of finite-size effects on the magnetic transitions in different nanomaterials, the value of λ = 0.94-0.98 with ξ 0 = 2.2-3.35 nm for Ni nanowires [36]. Finite-size effects have been investigated in oxide films [38] From these data we find that λ = 0.6 and ξ 0 = 3.4 nm.…”

“…In conventional elemental ferromagnetic nanowires and nanoparticles, this effect has been well established. The value of the correlation length at zero temperature ξ 0 sets the scale for the finite-size effect and for most conventional ferromagnetic systems ξ 0 ≈ 2-4 nm [36,37]. Generally, the finite-size effect that leads to depression in T C in magnetic systems is the relation [35] …”

Weak ferromagnetism and spin-glass state with nanosized nickel carbide

“…The blocking temperature is expected even lower. In addition, the Curie temperature of a 2.8 nm Ni particle would reduce from the bulk value of 631 K down to 130 K according to the finite size effect [29]. Hence, T B is expected further reduced from 0.48 K accordingly.…”

“…The experimentally observed weak ferromagnetism at 300 K by the FC-ZFC and M(H) measurements is therefore inconsistent with this picture. On the other hand, for a Ni nanoparticle or a nanocluster embedded inside the Ni 3 C particle to exhibit ferromagnetism at T > 300 K, the diameter is calculated to be at least 21 nm by assuming ΔN = 0.1 with the finite size effect [29] and the temperature dependent shape anisotropy effect of Ni accounted for [27]. If the possibly incomplete reaction process leaves the unreacted Ni in the form of a large particle with the diameter D > 21 nm, then, it is difficult to interpret the appearance of the dip showing up in H C (T) around the freezing temperature by a simple magnetic core-shell structure of the Ni nanoparticles.…”

Effect of temperature-dependent shape anisotropy on coercivity for aligned Stoner-Wohlfarth soft ferromagnets

“…One is the geometric confinement on the correlation length near the critical regime. It becomes significant for nano-sized particles, causing a reduction in the ferromagnetic ordering temperature, T C , according to the power law, [18][19][20][21][22]. In the expression, T C (∞) is the Curie temperature for the bulk and T C (d), for nanoparticles with diameter d, ξ 0 is the correlation length and λ = 1/ν with ν the critical exponent.…”