Pelos resultados apresentados, é evidente a formação de estrutura cristalina em substrato amorfo, sendo que estes materiais apresentam possibilidades de aplicações no ramo da Spintrô- nica e nas demais áreas de aplicações tecnológicas. Com a obtenção desse tipo de estrutura, tem-se a fase desordenada A2 com surgimento da fase B2 das ligas de CFA. Para estes ma- teriais, nota-se ainda a presença de anisotropias uniaxial, que podem está associadas ao baixo amortecimento de Gilbert presente nestes tipos de ligas.
Com base nos resultados apresentados, em trabalhos futuros, pretende-se trabalhar com ligas de Heusler e investigar diferentes efeitos que possam ocorrer a partir de tratamentos tér- micos realizados e, utilizando novas técnicas como MR e FMR e seus possíveis efeitos, além da caracterização por meio de microscopia eletrônica de varredura (MEV).
Outro fato, é associar a possibilidade do CFA ter um fator de amortecimento baixo com a certeza que a multicamada tem fator de amortecimento baixo,e que a multicamada por natureza gera um fator de amortecimento baixo, e o CFA pode ter fator de amortecimento baixo. Se con- seguirmos associar essas duas coisas podemos ter resultados dinâmicos bastante importantes.
Giant magnetoimpedance effect in Co
2FeAl single layered
and Co
2FeAl/Ag multilayered
films in amorphous substrates
A.M. Silvaa, V.M. Escobara, G.L. Callegarib, K. Agraa, C. Chesmana, F. Bohna, M.A. Corrêaa,n
aDepartamento de Física, Universidade Federal do Rio Grande do Norte, Natal, RN 59078-900, Brazil bDepartamento de Física, Universidade Federal de Santa Maria, Santa Maria, RS 97105-900, Brazil
a r t i c l e i n f o
Article history: Received 21 March 2015 Accepted 28 April 2015 Available online 8 May 2015 Keywords: Spintronics CFA Magnetoimpedance Ferromagneticfilms a b s t r a c t
We investigate the structural, quasi-static and dynamic magnetic properties of a Co2FeAl single layered
film and a Co2FeAl/Ag multilayer, both grown onto amorphous substrate. We verify that bothfilms have
A2 structure and anisotropic magnetic behavior, as well as present the well-known magnetoimpedance behavior of anisotropic systems. The magnetization dynamics is discussed in terms of the mechanisms that govern the magnetoimpedance effect in a wide range of frequencies, as well as of the orientation between the external magneticfield and electrical current with respect to the magnetic anisotropy. The single layer has the largest magnetoimpedance variations values of 50% for 3.0 GHz, while the multilayer present considerable higher values, reaching to 200% at the same frequency. The association of kind of sample structure, anisotropic magnetic behavior and low damping parameterα of the CFA alloy opens possibilities to reach higher magnetoimpedance performances. The results place the CFA full-Heusler alloy as a promising candidate for applications in magnetoimpedance-based sensors and radio frequency range devices.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
Materials traditionally employed in spintronics applications use to present common features as spin-polarized current, low damp- ing magnetic behavior, and tunable magnetic and electrical prop- erties[1–3]. In this sense, the Heusler alloys become a natural candidate for application in technological devices based in spin- tronics effects [4], since, theoretically, they present energy gap around the Fermi level, which makes possible the achievement of 100% spin polarized currents[5].
In particular, a full-Heusler material can be described by A2BC,
where A and B are transition metals and C is a sp-element. The Co2FeAl (CFA) is a noticeable example of these alloys, presenting large
magnetic moment and high Curie temperature [2], although its magnetic and electrical properties are strongly dependent on the chemical disorder. The CFA presents three well-known crystallo- graphic structures (L21, B2 and A2), relative to the type of occupation
of the three available lattice sites, such as (0, 0, 0) and (1/2, 1/2, 1/2) for Co; (1/4, 1/4, 1/4) for Fe; and (3/4, 3/4, 3/4) for Al[6].
The Heusler alloys have attracted strong interest due to the wide application in shape memory devices, electrodes in magnetic tunnel junctions[7]. At the same time, the possibility to produce
samples with low damping magnetic parameter makes attractive its use in radio frequency (rf) applications [8,9], such as spin torque oscillators, and biosensors.
At the frequency range considered in this work, the magnetization dynamics is governed by the Landau–Lifshitz–Gilbert equation, given by[10] dM→ dt¼ γ M → H→ef f γα M M → M→ H→ef f h i ; ð1Þ
where M!is the magnetization vector, H!eff is the effective magnetic
field, andγ¼ jγGj =ð1þα2Þ, in whichγGis the gyromagnetic ratio and
αis the phenomenological Gilbert damping constant. In particular, the effective magneticfield H!eff presents two contributions and can be
written as H!eff ¼ ð H!þ H!ξÞþ h
!
ac. Thefirst term, ð H!þ H!ξÞ, corre-
sponds to the static component of thefield. It contains the external magneticfield (H!) and the internal magneticfield H!ξ¼ ∂ξ=∂M!
due to different contributions to the magnetic free energy densityξ
[11]. On the other side, the second term corresponds to an alternate magneticfield h!ac.
An important tool to characterize the magnetization dynamics at rf is the magnetoimpedance (MI) effect. The MI effect corresponds to the changes of the electrical impedance of a soft magnetic conductor when submitted to an external magneticfield. The origin of this effect resides in the variations of the transverse magnetic permeability when the material is submitted to an external magneticfield, related to Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/matlet
Materials Letters
http://dx.doi.org/10.1016/j.matlet.2015.04.147
0167-577X/& 2015 Elsevier B.V. All rights reserved. nCorresponding author. Tel.:þ558436068089.
E-mail address:marciocorrea@dfte.ufrn.br(M.A. Corrêa).
modifications of the penetration depth of the drive current[12], as well as the MI performance is strongly dependent onfield alignments during the MI measurement[13].
In this work, we perform a systematic investigation of the structural, quasi-static and dynamic magnetic properties of a Co2FeAl single layered film and a Co2FeAl/Ag multilayer, both
grown onto amorphous substrate. The magnetization dynamics is discussed in terms of the mechanisms that govern the magne- toimpedance effect in a wide range of frequencies, as well as of the orientation between the external magnetic field and electrical current with respect to the magnetic anisotropy. We verify that the films present very good magnetoimpedance performance. In this case, the relevant MI% values are a direct consequence of the kind of sample structure, anisotropic magnetic behavior and low damping parameter αof the CFA alloy [9]. The results place the CFA full-Heusler alloy as a promising candidate for applications in MI-based sensors and rf range devices.
2. Experiment
We produce a CFA single layered film with a thickness of 500 nm and a [CFA(10 nm)/Ag(2 nm)] 20 multilayer. Similar thickness values have already been considered in papers pre- viously published by our group[14,15]. The samples are deposited onto amorphous glass substrates with dimensions of 6 6 mm2,
previously covered by a 5 nm-thick Ta buffer layer.Fig. 1(a and b) shows a schematic representation of the produced samples. The deposition process is carried out with the following parameters: base pressure of 6 107Torr, Ar pressure during disposition of
2 103Torr, current of 10 mA in the DC source for the Ta layer,
and 20 W and 10 W set in the rf power supply for the CFA and Ag layers, respectively. Using these parameters, the deposition rates obtained through low-angle X-ray diffraction measurements are 0.37 nm/s, 0.07 nm/s and 0.44 nm/s for Ta, CFA and Ag, respec- tively. In order to induce a magnetic anisotropy and define an easy magnetization axis, a constant H¼1 kOe magnetic field is applied in the substrate during the deposition.
The structural properties of the samples are obtained through high-angle X-ray diffraction (XRD) measurements performed using a Rigaku MiniFlex system. The quasi-static magnetic beha- vior is obtained via magnetization curves acquired with a
LakeShore Model 7407 vibrating sample magnetometer, with maximum external in-plane magneticfield of 7350 Oe.
The dynamic magnetic behavior is investigated through MI measurements. The MI measurements are performed using an impedance analyzer Agilent model E4991, with E4991A test head connected to a stripline, i.e., a microstrip in which the sample is the central conductor, as shown inFig. 1(c). The impedance (Z) is measured over a wide range of frequencies from 100 MHz up to 3 GHz, in a linear regime with 1 mW (0 dBm) constant power, with magneticfield varying between 7350 Oe. We perform the mea- surements with both probe current and external magnetic field applied along (ϕ¼ 01) and perpendicular (ϕ¼ 901) to the direction defined by the magnetic field applied during the deposition.
3. Results and discussion
Fig. 2(a) shows the XRD results obtained for the single layered and multilayered films. For the single layered one, the pattern clearly indicates the (022) texture of the CFA alloy, assigned by the well-defined and high-intensity peak identified at 2θ¼ 44:731. Peaks related to the (111) and (002) CFA are not observed, suggesting the A2 structure associated with the complete chemical disorder among the Co, Fe, and Al sites.
Similar A2 structure is verified for the multilayer. However, the XRD pattern indicates the polycrystalline state, in which the (022), (004) and (422) orientations are present. The distinct structural character verified between the samples is related to the stress stored during the deposition. In particular, when considered the multilayer, the CFA peaks are shifted to smaller angles, a fact associated to the compressive residual stress in thinner layers. It is important to point out that the structural features observed here for films grown onto amorphous substrates are similar to those already verified for CFA films produced with oriented substrates
[2,16].
Fig. 2(b and c) show the magnetization curves, measured with the external in-plane magnetic field oriented both along and perpendicular to the direction defined by the magnetic field applied during the deposition. The angular dependence of the magnetization curves for both samples indicates an uniaxial in- plane magnetic anisotropy, induced by thefield applied during the deposition process. The anisotropy fields are around 30 Oe and 40 Oe for the single layeredfilm and multilayer, respectively. It is well-known that quasi-static magnetic properties play a funda- mental role in the dynamic magnetic response and MI behavior. Thus, the soft magnetic properties, high anisotropy induction, and the dynamic magnetic properties of the CFA alloy enable us to obtain interesting MI curves and very good MI performance.
Regarding the magnetization dynamics, the MI performance is associated with the well-known magnetoimpedance ratio MI(%) defined by
MIð%Þ ¼ZðHÞZðHmaxÞ
ZðHmaxÞ 100; ð2Þ
where Z(H) is the impedance for a given external magnetic field value and ZðHmaxÞ is the measured impedance for the magnetic
saturation state. Thus,Fig. 3shows the MI(%) curves, for selected frequencies and different orientations between drive current and external magnetic field with respect to the induced anisotropy direction.
The shape and the amplitude of the magnetoimpedance curves are dependent on the magnitude of the external magneticfield, probe current frequency, orientation of thefield and probe current with respect to the magnetic anisotropies[17,18], as well as are related to the mechanisms responsible for the transverse magnetic
Fig. 1. (a and b) Schematic representation of the produced samples, in which the H indicates the direction of the magnetic field applied during the deposition. (c) Experimental setup for the MI experiment.
permeability changes: skin effect and ferromagnetic resonance (FMR) effect[19].
For ϕ¼ 01, as a consequence of the alignment of the easy magnetization axis with the ac current and magnetic field applied during the MI measurement, the MI curves for both samples present a single peak structure at the intermediate frequency range, up to 1:5 GHz, and a double peak structure for higher frequencies. The evolution of the shape of the MI curves reflects the modification of the main mechanism responsible for the MI variations. In particular, at low and intermediate frequency ranges, the skin effect acts changing the transverse magnetic permeability, while the FMR effect, associated to the skin effect, becomes the mechanism with the increase of the frequency.
Forϕ¼ 901, both samples exhibit a double peak behavior for the whole frequency range, a signature of the perpendicular alignment of the externalfield and ac current with the easy magnetization axis. Up to 1:0 GHz, not shown, the position of the peaks remains unchanged close to the anisotropy fields, indicating that the skin effect governs the magnetization dynamics. For frequencies above this value, besides the skin effect, the FMR effect also becomes an important mechanism responsible for variations in MI effect, a fact evidenced through the change of the peaks position in the magnetic field as the frequency is increased.
However, the most striking finding resides in MI performance. Although the single layeredfilm and the multilayer have similar MI behavior, the amplitude of the MI variations is noticeable distinct. The similarity in shape and contrast between the MI variations verified for single films and multilayers are in concordance with theoretical predictions previously reported by our group[11]. In particular, the
single layer has the largest MI(%) values of 50% for 3.0 GHz, while the multilayer presents considerable higher values, reaching to 200% at the same frequency, irrespective on theϕconsidered in the MI measurement. In this case, the relevant MI% values are a direct consequence of the kind of sample structure, anisotropic quasi-static magnetic behavior and low damping parameterαof the CFA alloy[9]. The association of these features opens possibilities to reach higher MI performances and fast response under rf magnetic fields.
4. Conclusions
In summary, in this work we perform a systematic investiga- tion of structural, quasi-static and dynamic magnetization proper- ties of a CFA single layeredfilm and a CFA/Ag multilayer, grown onto amorphous substrates.
We verify that both produced samples have the A2 structure, although the singlefilm presents the (022) texture while the multi- layer is polycrystalline, with the (022), (004) and (224) orientations. The structural features observed forfilms grown onto amorphous substrates are similar to those verified for CFA films produced with oriented substrates [2,16]. Moreover, irrespective of the sample structure, the films present uniaxial in-plane magnetic behavior, induced by thefield applied during the deposition.
The MI results present the well-known MI behavior of aniso- tropic systems, including the Z behavior for distinct orientation of the anisotropy with external magneticfield and probe current, as well as the its dependence with frequency. On the other hand, the association of the kind of sample structure, anisotropic quasi-static
Fig. 2. (a) XRD results for the single layered and multilayeredfilms, with the indexed CFA and Ag peaks. The patterns are obtained in ICDD (ICDD 01-071-5670 (red dotted lines) and ICDD 03-065-2871 (blue dotted lines)). (b and c) Normalized magnetization curves for the (b) single layeredfilm and (c) multilayer, acquired for ϕ ¼ 01 and ϕ ¼ 901. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
A.M. Silva et al. / Materials Letters 156 (2015) 90–93 92
magnetic behavior and low damping parameterαof the CFA alloy enables us very good MI performance. The single layer presents MI (%) values of 50% for 3.0 GHz, while the multilayer has con- siderable higher values, reaching to 200% at the same fre- quency. Thus, the results place the CFA full-Heusler alloy as a promising candidate for applications in MI-based sensors and rf range devices.
Acknowledgments
The research is supported by the Brazilian agencies CNPq (Grant nos. 306362/2014-7, 441760/2014-7, 306423/2014-6, and 471302/2013-9), CAPES, and FAPERN (Pronem no. 03/2012). M.A.C. and F.B. acknowledge financial support of the INCT of Space Studies.
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