DOI 10.1007/s10948-012-1536-y R E V I E W PA P E R
Magnetic Properties and Electronic Structure of Ni
81Fe
19/Zr Multilayer Films
Z. Yamkane·H. Lassri·N. Omari·E.K. Hlil
Received: 18 February 2012 / Accepted: 22 March 2012 / Published online: 14 April 2012
© Springer Science+Business Media, LLC 2012
Abstract Magnetic properties of NiFe/Zr multilayer films, prepared by DC magnetron sputtering, have been studied by vibrating sample magnetometer and ferromagnetic res- onance (FMR). Spin-wave resonances were observed in Ni81Fe19/Zr multilayer films in FMR experiments, and the spin wave was found to be sustained by both the whole films. Estimated small interlayer coupling constant shows weak exchange coupling effect between NiFe layers across Zr spacers. The FMR linewidth, in parallel geometry, of the uniform mode was found to increase with decreasing NiFe thickness (20 Å≤tNiFe ≤120 Å) indicating that it corre- sponds to an interfacial effect. In addition, electronic and magnetic properties of multilayers are investigated by self- consistent ab initio calculations based on Korringa–Kohn–
Rostocker (KKR). Spin polarized within the framework of the Coherent Potential Approximation (CPA) is considered for calculations.
Keywords NiFe/Zr multilayers·Ferromagnetic resonance·Spin-waves·Interlayer coupling·Electronic structure calculations
Z. Yamkane ()·H. Lassri·N. Omari
Laboratoire de Physique des Matériaux, Micro-électronique, Automatique et thermique, Faculté des Sciences Ain Chock, Université Hassan II, B.P. 5366 Mâarif, Route d’El Jadida, km-8, Casablanca, Morocco
e-mail:[email protected] N. Omari·E.K. Hlil
Institut Néel, CNRS—Université Joseph Fourier, BP 166, 38042 Grenoble, France
1 Introduction
Magnetic layered systems have attracted much attention, both for their fundamental interest as well as for possible technological applications, because they can demonstrate in- teresting magnetic properties that are absent in bulk materi- als or single thin films. Magnetic multilayers with Ni81Fe19
layers have been intensively studied since the discovery of their giant magnetoresistance (GMR) effect [1,2]. In addi- tion, multilayers with Ni81Fe19 layers are nowadays one of the most common systems studied due to their soft magnetic properties, which are convenient for a technological applica- tion in magnetic sensors [3–5]. But in other cases, the goal has been the investigation of the magnetic properties that appear in the interface regions, such as: magnetic moment, interfacial anisotropy and interfacial roughness [6–8].
In this work, we report some results on Fe19Ni81/Zr mul- tilayers. Amorphous NiFeZr (a-NiFeZr) phase also displays soft magnetic properties and is seldom reported in films [9,10]. Magnetization and ferromagnetic resonance mea- surements were used to investigate the magnetic proper- ties of the Ni81Fe19/Zr multilayers. Magnetic properties and electronic structure of their interfaces, considered as chemi- cal disordered systems, are investigated as well as by ab ini- tio calculations based on the appropriate KKR-CPA method.
2 Experimental Details
Ni81Fe19/Zr multilayers were prepared by dc magnetron sputtering, using high purity Ni81Fe19 and Zr targets. The initial pressure in the chamber, before the deposition, was roughly at 6×10−8Torr, while the sputter gas (ultrahigh pu- rity argon—5 nines) pressure was kept constant at 2 mTorr.
Film deposition was done onto a water-cooled Si (100) sub- strate, being kept at a temperature of 20 °C. The Ni81Fe19/Zr multilayers were prepared with NiFe layer thickness (tNiFe) varying from 20 Å to 120 Å while the Zr thickness (tZr) was fixed at 15 Å. The number of periods N varied from 10 to 20. Zr buffer and capping layers of 100 Å and 50 Å thicknesses, respectively, were deposited. Low angle x-ray diffraction (XRD) of all the samples revealed peaks typical of the modulated structure and the XRD in the high angle showed the existence of fcc NiFe (111) peak. Magnetiza- tion measurements were done with a vibrating sample mag- netometer (VSM). The ferromagnetic resonance measure- ments were performed using a spectrometer with X-band microwave frequency of 9.6 GHz.
3 Electronic Structure Calculations
The KKR-CPA method [11], with the parameterization of Vosko, Wilk, and Nusair (VWN) [12] was considered. Us- ing such method consists to solve the DFT (density func- tional theory) one-particle equations, multiple-scattering theory. The form of the crystal potential is approximated by a muffin-tin potential, and the wave functions in the respective muffin-tin spheres were expanded in real har- monics up to l =2, where l is the angular momentum quantum number defined at each site. Higher K-points up to 144 in the irreducible part of the first Brillouin zone are considered. In the present calculations, we used the MACHIKANEYAMA2002v08 package produced by H. Akai of Osaka University [13].
4 Results and Discussion
The magnetic properties of the multilayer films were found to be very dependent ontNiFe. The in-planeM–H loops are all rectangular. The coercivity first increases slightly from 20 to 26 Oe when tNiFe decreases from 120 to 80 Å, then decreases strongly to 5 Oe whentNiFedecreases to 20 Å, at 300 K. This decrease could be attributed to the change in the microstructure. The change of the coercive field is com- parable to that resulting from the structural transition of the NiFe sublayers between the amorphous or nanocrystalline and polycrystalline states (see [10]). An additional contribu- tion to the coercivity behavior could result from the inter- layer exchange coupling of the NiFe sublayers across the Zr spacers.
The saturation magnetization expressed (MS) in terms of total volume of NiFe decreases from 733.7 emu/cm3for tNiFe=120 Å to 115.3 emu/cm3fortNiFe=20 Å (Table1).
This could be explained in terms of a magnetically dead layer of NiFe at each interface due to alloying effects. It is
Table 1 The saturation magnetization per unit volume of Ni81Fe19/Zr multilayer films
tNiFe(Å) MS(emu/cm3)
20 115.3
30 381.2
40 401.8
60 524.6
80 613
120 733.6
Fig. 1 Resonance fieldHresin perpendicular geometry versusn2at 300 K
known from the dead layer model that the saturation mag- netization of multilayer can be expressed as:MS=M0(1–
2δ/tNiFe), whereM0is the bulk NiFe value andδis the dead layer thickness at each interface. The values ofM0andδare found to be equal to 850 emu/cm3(1μB) and 9.5 Å, respec- tively, for all multilayer films [10].
In FMR measurement, we observed spin wave modes suggesting the interlayer coupling between Ni81F19 layers.
The multilayer becomes a single coupled system, the spin waves may propagate through the nonmagnetic layers and the standing spin-wave modes are sustained by the whole film. The observed spin wave field positions for the sample are plotted versusn2 in Fig.1. The presence of even and odd spin wave resonance (SWR) modes implies an inhomo- geneous distribution of magnetization perpendicular to the film plane, and an asymmetrical spin pinning at the two sur- faces and interfaces of the Ni81Fe19layer [14,15].
A model for spin waves in ferromagnetic/weak ferromag- netic multilayer proposed by van Staple et al. [16] was ex- tended to the case of ferromagnetic/nonmagnetic multilay- ers by Wang et al. [17]. In perpendicular geometry, for a single magnetic layer in multilayers, the spin wave disper-
sion relation can be expressed as ω
γ =Hres⊥ −4π Meff+2Ak2 MS
, (1)
whereHres is the resonance magnetic field, 4π Meff is the effective magnetization, A is the exchange coupling con- stant in the magnetic layer, andk is the spin wave number (k=nπ/L).Land integern, are the total thickness of the magnetic film sustaining the spin waves and the spin wave mode number, respectively. When the magnetic layers cou- ple to each other by interlayer exchange interactions, a col- lective spin wave mode may appear with overall wave vec- tor K. K andkare related by the dispersion relation [16]
cos(ktNiFe)=cos(KtNiFe)+
A
tNiFeAg
ktNiFesin(ktNiFe), (2) wheretNiFeis the thickness of a single magnetic layer and Agis the interlayer exchange coupling constant (per area).
Within the approximation for small ktNiFe and ktNiFe, we have
K=k
1+
2A
tNiFeAg
. (3)
Then the spin wave dispersion relation of the multilayer film can be expressed by:
ω
γ =Hres⊥ −4π Meff+ 2A MS
1
(1+tNiFe2AAg)
K2, (4)
K depends on the boundary conditions. For an ideal pin- ning boundary condition and for an ideal free boundary, NKtNiFe=mπ, wheremis also an integer. Thus, the spin wave spectra should satisfy a n2 law. In addition, we can estimate the interlayer coupling Ag by analyzing the ex- perimental results shown in Fig. 1 with Eq. (4). In order to determine the interlayer exchange coupling constantAg, we assumed that the fcc-NiFe layers in multilayers have the same exchange coupling constant as a fcc-NiFe single layer film (A=0.8×10−6erg/cm). Using this value, we obtained the interlayer coupling constantAg≈0.1 erg/cm2 fortNiFe=40 Å. A positive sign ofAgmeans ferromagnetic coupling and agrees with our expectation.
The FMR linewidthH|| is the sum [18] of two contri- butions: an inhomogeneous width corresponding to a distri- bution ofHresand an homogeneous width associated to the intrinsic relaxation rate of the magnetization vector. The par- allel geometry linewidthH||reflects essentially the intrin- sic damping of the magnetic layer. For the Ni81Fe19/Zr mul- tilayers, a linear variation withtNiFe−1 of the linewidthH||is observed (Fig.2) indicating that it corresponds to an inter- facial effect. The physical origin of the interfacial increase
Fig. 2 ThetNiFe−1 dependence of theH||at 300 K
Fig. 3 Total DOS from KKR-CPA calculations for disordered Ni81Fe19
of the magnetic layer damping can be associated to the con- tribution due to the spin-lattice relaxation of the conduction electrons in the Zr and Ni81Fe19layers.
Now, we will be interested in self-consistent ab initio cal- culations, based on Korringa–Kohn–Rostocker KKR-CPA, to shed light on both electronic and magnetic behaviors at Ni81Fe19/Zr multilayer interfaces considered as chemi- cal disordered systems. For this aim, total Density of States (DOS) of Ni81Fe19, deduced from band structure calcula- tions is reported in Fig.3. It mainly originates from the Ni atom contribution since concentration is 4 times higher for Ni than for Fe. Thel-decomposed DOS ofs, p or d like- states provide a more detailed picture and allow concluding that both Ni and Fe contribution have mainly ad character
Fig. 4 Total DOS from KKR-CPA calculations for disordered Ni80Fe15Zr5
Fig. 5 Total, Ni and Fe Magnetic moment versus Zr content in Ni80Fe20−xZrx
as expected. The computed value of both Fe et Ni atoms are 2.7μBand 0.67μB, respectively; values being very close to magnetic moment value in pure metals. Total magnetization of the disordered Ni81Fe19 is computed as well and found equal to 1μB. This result is in accordance with the measured saturation magnetization (1μB) for bulk Ni81Fe19alloy. It is worthy to notice that this estimated magnetic moment value, for bulk Ni81Fe19 alloy, is very different compared to mo- ment value deduced from saturation magnetization measure- ments in Ni81Fe19/Zr multilayer films. Such differences are reasonably explained in terms of a magnetically dead layer
Fig. 6 Total, Ni and Fe Magnetic moment versus Zr content in Ni80−xZrxFe20
of NiFe at each interface due to alloying effects and assert that NiFeZr alloys taking place at the interface.
DOS calculations versus Zr content are also performed for Ni80Fe20−xZrx as well as for Ni80−xZrxFe20 in order to check the magnetization sensitivity to Zr alloying taking place at Ni81Fe19/Zr interfaces. As example, total DOS for Ni80Fe15Zr5is reported in Fig.4. Analysis gives evidence that no dramatic change, induced by this low Zr content, is observed. In addition, thel-decomposed DOS underlines that the Zr contribution to both occupied states and unoccu- pied states is mainly from Zr(d) like-states. Computed mag- netization and magnetic moments carried by both Ni and Fe atoms are found to decrease versus Zr content as seen in Figs. 5 and6. Comparing to data experimental, this mag- netic reduction indicates that NiFeZr alloys takes place at interfaces.
5 Conclusion
In conclusion, we have studied Ni81Fe19/Zr multilayers pre- pared by DC magnetron sputtering. The magnetizations of Ni81Fe19/Zr multilayer films are found to decrease with de- creasing NiFe layer thickness. The spin-waves resonance modes were observed in perpendicular geometry. The in- terlayer exchange constants were determined. The reso- nance linewidthH||are found to increase with decreasing Ni81Fe19 layer thickness. Electronic structure calculations of disordered Ni81Fe19shed light on magnetic structure. In- deed, computed total magnetization for disordered Ni81Fe19
is found in accordance with the measured saturation mag- netization for bulk Ni81Fe19 alloy while the computed total
magnetic in Ni80Fe20−xZrxalloys is found to decrease ver- sus Zr content. Such magnetic reduction points out to the existence of noticeable quantity of NiFeZr alloys at each in- terface of multilayer films as concluded from experimental data.
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