M. Ciureanu

Magnetic anisotropy in arrays of Ni, CoFeB, and Ni/Cu nanowires

An effective field model based on intrawire and interwire dipolar interactions has been developed in order to describe the magnetic anisotropy in arrays of homogeneous and multilayer nanowires. Variable angle ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) characterization techniques were used to determine the effective interaction field acting on Ni, CoFeB, and Ni/Cu nanowires. FMR spectra are well described by a rigid magnetization model and VSM data are in rough agreement with a mean longitudinal field model. FMR and VSM values of the effective fields are mutually consistent and in fair agreement with the values calculated with the model. The results show that the anisotropy of our arrays is strongly dominated by the dipolar interactions.

First-Order Reversal Curves Diagrams of Ferromagnetic Soft Nanowire Arrays

First-order reversal curves (FORC) diagrams map the statistical distributions of magnetic hysterons, based on their critical fields (H c) and local interaction fields (Hu), throughout the magnetization reversal. This technique has been adapted to soft nanowire arrays deposited into alumina templates, with axes perpendicular to the template plane (diameter=50--175 nm, length=5--60 mum). From the topology obtained, the intrinsic physical properties of the array can be determined, notably the coercivities of individual nanowires and the interaction field at saturation. However, the FORC diagram may also yield information on other properties, such as the existence of a distribution of wire lengths

Reversible and quasireversible information in first-order reversal curve diagrams

Two methods for extracting information from first-order reversal curves (FORCs) obtained on low coercivity samples are presented. The proportion of reversibility as a function of applied field can be extracted by calculating the ratio of the initial slope of each FORC to the susceptibility on the major hysteresis loop upper branch at the same field. This gives us the part of the reversal process, a process occurring with zero coercivity, that is, where H=Hr, during the magnetization reversal. In order to be able to see the nonperturbed trace coming from the irreversible processes with a small coercivity compared to the FORC domain, some points have to be added in the H<Hr area in a way that minimizes the discontinuities near H=Hr. This is done by using two functions characterizing the behavior of the magnetization on the H=Hr axis (“extrapolated FORCs”). These methods were used to characterize a CoFeB nanowire array with the applied field perpendicular to the nanowire axis.

Microwave studies of magnetic anisotropy of Co nanowire arrays

The effect of magnetocrystalline anisotropy and dipolar interactions in Co nanowire arrays is studied by ferromagnetic resonance (FMR). Microwave measurements performed by the microstripline method are reported for two series of crystalline hcp Co (with the c axis nominally perpendicular [Co(c⊥)] and parallel to the wires [Co(c∥)]) and an amorphous alloy with Co as the main component—Co94Fe5B1. Extrapolation of the high field linear part of the resonance curve (frequency versus dc field) permitted an evaluation of the effective anisotropy fields for saturated samples, as well as of the intrinsic fields HK, showing that the great differences between the three series are due to the magnetocrystalline anisotropy. The HK values for the two series of Co are discussed in terms of a model which accounts for the effect of the distributions of the c axis orientation in systems of uniaxial ferromagnets. The observed dependence of the effective anisotropy fields on the array geometry (wire length and diameter) is in...

Measurement of Complex Permeability of Ferromagnetic Nanowires using Cavity Perturbation Techniques

The complex permeability of nanowire arrays of different materials was measured at X-band using a cavity perturbation technique. Compared with ferromagnetic films of the same materials, a relatively high permeability with a low loss is obtained. Our results indicate that ferromagnetic nanowire arrays have important potential technological applications as magnetodielectrics for microwave devices such as variable attenuators, phase shifters, modulators, stop-band filters and power absorbing terminals

Permeability Characterization of Ferromagnetic Nanowires Arrays From Transmission-Line Measurements

A broad-band transmission-line technique suitable for measuring magnetic and dielectric parameters of ferromagnetic nanowire arrays is discussed. Complex permittivity and permeability are obtained from analytical equations applied to S-parameter measurements of a coplanar line propagating the dominant mode. Measurements were made on crystalline Co and an amorphous alloy (Co94Fe5B1) in the 0.05-40 GHz frequency range. The particular geometrical and electromagnetic properties of ferromagnetic nanowire arrays offer good stopband performances in the frequency range of 12-40 GHz, which cannot by achieved by classical microwave materials. Our results indicate that ferromagnetic nanowire arrays could have important technological applications for microwave devices, such as variable attenuators, phase shifters, modulators, stop-band filters and power absorbing terminals.