Origins of Giant Magnetic Impedance Effect in Magnetic Nanostructures in Millimeter Waveband

Abstract

The dependence of a complex transmission coefficient of the electromagnetic wave passing through multilayered and granular nanostructures demonstrating giant magnetic impedance phenomenon as a function of an external magnetic field has been explored in the millimeter waveband ( f =34 GHz). The magnetic multilayer film Fe6(Co1/Cu2)16 and magnetic granular film Co51.5Al19.5O29 have been investigated. The maximum magnitudes of the relative module of the complex transmission coefficient 2 % and its phase shift 1 T φ Δ ≈ have been detected. The interrelation between complex impedance Z • and the effective conductivity of magnetic nanostructure is analyzed. Magnetic nanostructures demonstrating Giant Magnetic Impedance (GMI) and Tunnel Magnetic Impedance (TMI) are of great interest today from the point of view of their applicability in modern technologies. Particularly the opportunity to use such objects in megahertz band as magnetic fields sensors [1], spintronic devices (diodes, transistors) [2] is under intensive study. However, the strong demand of modern technology is a permanent increasing the operating frequencies of the corresponding devices. Thus, the examination of applicability of the magnetic nanostructures for higher frequencies, namely, up to Extra-High Frequencies (EHF)-band (millimeter waveband) is a very important contemporary problem [3-5]. One of the principal questions is – whether the GMI phenomenon is defined only by active losses in nanostructure or some reactive processes take place as well. 1285 ISSN 0040-2508 © 2006 Begell House, Inc. S.V. NEDUKH, M.K. KHODZITSKY, AND S.I. TARAPOV The high-frequency absorption and dispersion were investigated previously for some kinds of magnetic nanostructures, namely, microwires [6-8] and thin layers [9] at the frequencies which not exceed 18 GHz. Namely, the real part ' μ and imaginary part μ of dynamic permeability were studied there as a function of various parameters such as frequency, magnetic field amplitude, layer thickness, etc. Besides the low-field microwave impedance in amorphous microwires (FeSiBC, CoMnSiB, CoSiB) at 9.8 GHz and 32.5 GHz was investigated in [7]. As well, the magnetoimpedance in micron-size powders of La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 at 3.8-11.9 GHz was studied in [10]. In the majority of these nanostructures the GMI effect nature is defined by the Natural Ferromagnetic Resonance (NFMR) phenomenon. Authors [7] showed that magnetization processes, which are revealed in the direction of the magnetic component of high-frequency field, can play the main role in GMI phenomenon for some specimens of microwires. The GMI effect in the nanostructures under study in this paper has another origin than for the structures mentioned above. This nature is the spin-depending scattering of conductivity electrons by the interface magnetic/nonmagnetic metal for multilayer (ML) structures, or by the interface metal/dielectric for granular structures [11,15]. Let us note that when we are dealing with the effective conductivity we keep in mind the conductivity of the whole nanostructure but not the conductivity of its components. The given work represents the attempt to separate the contribution of static and dynamic magnetization to GMI effect at EHF band. For this purpose the millimeter waveband experiments ( f =34 GHz) on separate registration of absorption and dispersion components of impedance (which is usually called as the GMI effect for ML-structures Co/Cu [3-5,11] and TMI effect for granular structures CoAlO [12-14]) have been carried out. The GMI has been measured by the well-approved technique [5]. The plane electromagnetic wave fell on the plane specimen normally. The vector of DC magnetic field was oriented in the plane of the specimen. The magnetic component of high-frequency (AC) field was oriented normally to the DC magnetic field and lied in the specimen’s plane as well. Multilayered Co/Cu film Fe6(Co1/Cu2)16 [3-5,11] and granular-layered film Co51.5Al19.5O29 [12,13] are well studied magnetic nanostructures. In articles [3,11,14] the GMI effect has been detected in these samples at 20-120 GHz and a detailed study of the Electron Spin Resonance has been carried out. We have applied the vector Agilent Network Analyzer PNA-L N5230A to detect the module and phase of the transfer coefficient T • for the electromagnetic wave passing through the sample separately and simultaneously. As well, the EHF impedance Z Z iZ • ′ = + ′′ of the circuit including the sample (placed in the