Magnetic properties of Ni/V multilayers

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*Corresponding author.

Magnetic properties of Ni/V multilayers

M. Abid , H. Ouahmane, H. Lassri *, A. Khmou, R. Krishnan

Laboratoire de Physique des Mate&riaux et de Micro-e&lectronique, Faculte& des Sciences Ain Chock, Universite& Hassan II, B.P. 5366, Route d'El Jadida, km-8, Casablanca, Morocco

Laboratoire de Physique des Solides, Universite&moulay Isman(l, Faculte& des Sciences de Meknes, B.P. 4010, Morocco Departement de Physique, Faculte&des Sciences et Techniques, Errachidia, Morocco

Laboratoire de Magne&tisme et d'Optique, URA 1531, 45 Avenue des Etats Unis, 78035 Versailles Cedex, France Received 11 August 1998; received in revised form 26 February 1999

Abstract

The magnetization and anisotropy of Ni/V multilayers prepared by RF sputtering are presented. The magnetization decreases with Ni layer thicknesst

, and the analysis of the results at 5 K indicates the presence of dead Ni layer about 10 As thick. The Curie temperature¹

!decreases as a function of"lm thickness in the same way as is usually found from

"nite-size e!ects in other ferromagnets. The ferromagnetic resonance (FMR) spectra are obtained with the applied magnetic"eld parallel and perpendicular to the"lm plane at 300 K. From FMR, a negative value for Ni/V interface anisotropy is obtained. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Multilayers; FMR; Magnetization; Anisotropy

1. Introduction

The properties of magnetic metallic superlattices, multilayered or compositionally modulated "lms are of great interest because of the insight they provide into the fundamental nature of magnetism.

Owing to its technological importance, inherent two-dimensional nature and the lack of clear cut theoretical understanding, the surface anisotropy in these materials is of particular interest [1]. In gen- eral, we assume that the surface anisotropy energy constant K

1 could be treated as originating from several e!ects which alter the surface spins at their interfaces such as mis"t strain anisotropy [2,3], surface roughness [4,5] and NeHel's anisotropy [6].

When there is some interdi!usion between the

magnetic layer and non-magnetic layer, roughness e!ects may greatly alter the magnetic surface anisotropy. We have shown recently that in Ni/Pt multilayers perpendicular magnetization is stabil- ized for Ni layers thinner than about 18 As, though the Curie temperature is relatively low [7].

Uniaxial anisotropy has been reported in Ni/Pd multilayers [8,9]. In the search for new materials with uniaxial anisotropy, we investigated multilayers in which V was the non-magnetic layer. In this paper we describe our studies on Ni/V multilayers prepared by RF sputtering.

2. Experimental

The multilayers were deposited onto water- cooled glass substrates by RF diode sputtering. The

PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 3 5 9 - 5

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Fig. 1. Thet

, dependence of the productM;t

, at 300 and 5 K.

chamber was "rst evacuated to a pressure of 1}2;10\Torr using a turbomolecular pump. Ar- gon of 5 N purity was used as the sputter gas and its pressure was kept constant at 6;10\Torr. The RF power density was 2.1 W/cm. The"nal thickness was monitored by quartz oscillators, pre-calib- rated by using a pro"lometer of V and magnetization measurements for Ni layers, respectively.

The Ni layer thicknesst

, varied from 18 to 240 As and that of V layer t4 was "xed at 20 As . The numberNof bilayers were in the range of 3}15. All the samples were grown on vanadium bu!er layers 100 As thick. In all the cases the "rst and the last layers were V. In what follows, the growth para- meters of the samples will be indicated as (t

, /t 4)

,. Low-angle X-ray di!raction studies were made to check the periodic structure and to obtain the layer thickness. Low-angle X-ray di!raction of all the samples revealed peaks typical of the modulated structure and the thickness calculated

from these peaks agree within 3% with that obtained from the quartz oscillator after calibration.

Magnetization 4pM

1 was measured using a vibra- ting sample magnetometer (VSM) with a precision better than $1% under magnetic "elds up to 18 kOe and in the temperature range from 5 to 300 K. Ferromagnetic resonance (FMR) studies were carried out at 9.8 GHz with the static "eld applied both perpendicular (H

,) and parallel (H_#) to the"lm plane. We calculated the e!ective magnetization 4pM"4pM1!H) where H) is the perpendicular anisotropy"eld, thegfactor, and the linewidth.

3. Results and discussion

The magnetization decreases strongly with a decrease in Ni layer thickness. This could be explained in terms of a magnetically dead layer of Ni

336 M. Abid et al./Journal of Magnetism and Magnetic Materials 202 (1999) 335}341

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Fig. 2. Temperature dependence of the magnetization of Ni/V multilayers with varying Ni thicknesses.

at each interface due to alloying e!ects. It is known from the dead layer model that the magnetization of multilayer (M) can be expressed as

M"M (1!t

/t

, ), (1)

whereM

is the bulk Ni value andt

/2 is the dead layer thickness at each interface. The thickness of such a dead layer can be estimated as shown in Fig. 1, where we have plotted the productM;t, as a function of t, at both 300 and 5 K. The slope which corresponds to the magnetization yields 490 and 501 emu/cmat 300 and 5 K, respectively. The intercept on the abscissa gives the value oft

which is found to be 15.5 and 10 As at 300 and 5 K, respectively.

Fig. 2 shows the temperature dependence of M for several values of t

, thicknesses. It can be noticed that the Curie temperature ¹

! decreases

when t

, decreases as is to be expected due to a decrease in the exchange interaction arising from less Ni neighbors. The low-temperature magnetization was studied in detail for a few samples. For three-dimensional magnetic "lms, the magnetization has a¹dependence due to the classical spin-wave excitations. In such cases, according to spin-wave theory, the temperature dependence should follow the relation

*M(¹)/M"[M!M(¹)]/M

"B

¹#C¹. (2) In all cases this behavior is observed for temper- atures as high as ¹

!/3. The spin-wave constant B decreases from 92;10\ for t

, "18 As to 26.2;10\for t

, "96 As . These values are much larger than the value of 7.5;10\K\found for bulk Ni.

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Fig. 3. Curie temperature as a function of"lm thicknesst , .

As the saturation magnetization decreases with decreasing layer thickness, so does the Curie temperature¹

!. These values of¹

!are much smaller than the bulk Ni value of 630 K. For ferromagnetic

"lms the decrease of ¹

!(t

, ) as the layer thickness t, is reduced and is usually described in the frame- work of "nite-size scaling, which predicts that

¹!(t

, ) scales witht

, via a shift exponentj[10]

[¹

!(R)!¹

!(t, )]/¹

!(R)"(t/t)\H. (3)

The"t further results in a&shift exponent'j"1.39,

which should be connected with the critical expo- nentlof the correlation length byj"1/l[8]; our results, obtained from multilayers, are in accord- ance with the valuel"0.72 of the 3D Ising model.

Finally, the"t results in t

"10 As, corresponding to a thickness at which¹

!(t

)"0 (Fig. 3).

FMR measurements have been carried out at 300 K and f"9.8 GHz. The FMR spectra for

H for all the samples show a single absorption mode. But the resonance"eld varies as a function of the orientation of the in-plane"eld direction show- ing a uniaxial in-plane anisotropy. The value of this anisotropy obtained from FMR agrees well with that obtained from M}H loop. Such in-plane anisotropy could be explained in terms of in- homogeneous stresses that are introduced during sputtering. The presence of uniaxial anisotropy has been observed in many multilayers by many authors [11}14]. However the origin of this is not yet well understood. As regards the H

, FMR spectra, they show an intense mode followed by either one or two modes of much less intensity. Fig.

4 shows the typical examples whent

, "24, 48, 96 and 240 As . Such multiple absorption is character- istic of multilayer structures. We have considered only the strongest mode to calculate 4pM

and the gfactor. Thegfactor for all the samples lies in the

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Fig. 4. FMR spectra for (a) (24 As/20 As)

, (b) (48 As/20 As)

, (c) (96 As/20 As)

and (d) (240 As/20 As)

. ("") Applied DC"eld along the"lm

plane, and (N) applied DC"eld perpendicular to the"lm plane at 300 K.

Fig. 5. Thet

, dependence of the e!ective magnetization 4pM

(䊏) and the linewidth*H

,(;) in the parallel geometry at 300 K.

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Fig. 6. Variation ofH)versus 1/t, for Ni/V multilayers at 300 K.

range 2.18}2.20, in agreement with the classical value of 2.20 for Ni. Incidentally, this justi"es the choice of the absorption mode for our calculation.

The e!ective anisotropy was deduced by using the following expressions where the resonance "eld Hcorresponds to the main mode:

For the perpendicular geometry we have (u/c),"H,!4pM. (4) For parallel geometry we get

(u/c)#"H_#(H_##4pM

) (5)

As displayed in Fig. 5, the 4pM

and the linewidth *H_# in the parallel geometry of Ni/V multilayers change correlatively. With increasing t, the e!ective magnetization increases and linewidth decreases respectively.

According to previous studies [1,15], an interface magnetic anisotropy of multilayers can be deduced

through the dependence of the perpendicular anisotropy on the thickness of the magnetic layer, if the interface anisotropy essentially enhances the

"rst-order anisotropy energyK

. Then the perpendicular anisotropy"eld (excluding the demagneti- zation term) can be written as

H)"H3#2H1/t, , (6)

i.e.

H)"4pM

1!4pM "H

3#2H 1/t

, . (7) The perpendicular anisotropy "eld H)"2K/M1 includes two terms, a homogeneous volume anisotropy H3"2K3/M1, and a surface-induced one H1"2K

1/M 1.

When the demagnetizing energy is included, Eq.

(5) can be written as K"K

4#2K 1/t

, , (8)

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where K4"K

3!2pM1, K "K

!2pM1, (9) KandK

4are the e!ective perpendicular anisotropy energy and e!ective volume anisotropy energy, respectively (including the demagnetizing energy). Fig. 6 shows a plot ofH

)as a function of 1/t, for the multilayers with di!erent Ni layer thicknesses and a "xed V layer thickness (t4"20 As). A linear variation ofH)versus 1/t, is shown. From the slope of the straight line, the value of the interface anisotropy constantK1is deduced to beK

1"!0.085 erg/cm. Also, by extrapolating the straight line to 1/t

, "0, we obtained H 3"

!152 Oe (K

3"!3.72;10erg/cm). HereK 1is a negative value, which means that the interface anisotropy con"nes the magnetization to the"lm plane. This value corresponds to an interface anisotropy that is lower in magnitude than the one (K1"!0.2 erg/cm) determined experimentally for Ni"lms coated with metal Cu, Pd or Re [16].

The negative K

1 value for Ni/V multilayers may arise from the cancellation between the NeHel interface anisotropy and the mis"t dislocation anisotropy at the interface through magnetostriction [2,3]. The smallK3value for Ni/V multilayers may arise from the magnetocrystalline anisotropy energy of cubic Ni (!4.8;10erg/cm).

In conclusion, the magnetic anisotropy of the Ni/V multilayers has been investigated by FMR

and VSM measurements. A negative interface anisotropy constant con"nes the magnetization to the

"lm plane.

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