influence of the thickness of titanium and cobalt layer on the interface and magnetization of co/ti...

phys. stat. sol. (a) 161, 125 (1997)

Subject classification: 75.70.Cn; 68.65.+g; S1.2

Influence of the Thickness of Titaniumand Cobalt Layer on the Interfaceand Magnetization of Co/Ti Multilayers

Ping Wu1), E. Y. Jiang, H. L. Bai, H. Y. Wang, and C. D. Wang

Department of Applied Physics, Tianjin University, Tianjin 300072,People's Republic of China

(Received September 5, 1996; in revised form November 18, 1996)

Two series of Co/Ti multilayers with different layer thicknesses are prepared by dual facing targetsputtering (DFTS) at a lower temperature (�30 �C). The microstructure, composition modulationand the magnetic properties of the films are examined by various methods. The samples are amor-phous and have periodic layered structures and in-plane easy axes. In series A with constant Tilayer thickness dTi � 23 �A, the saturation magnetization Ms of the film is found to increase withincreasing Co layer thickness dCo, but to be lower than that of a pure Co film because of the forma-tion of a nonmagnetic layer d0 at the interfaces. The thickness of d0 is about 5.5 �A, which differedsignificantly from the value of d0 (�11.3 �A) in other Co/Ti studies. The perpendicular magnetiza-tion has an increasing tendency when dCo decreased. In series B with dCo fixed at 18 �A, the Ms isfound to decrease with dTi and approaches a constant value when dTi is thick enough. This is inter-preted as due to the changes at the interface of the multilayers.

1. Introduction

Ultrathin films and multilayers have been studied because their properties differ signifi-cantly from those of the bulk ferromagnets. In recent years, both experimental and theo-retical studies have uncovered a number of unusual magnetic properties due to reduceddimensions and the increasing dominance of interfaces [1]. In Co/nonmagnetic metalMLFs, many authors have found that their saturation magnetization value per unitvolume Ms is somewhat smaller than the Co bulk value. The reduction of Ms may be, inpart, due to the nonmagnetic portion of the Co layer (called ªdead layerº) formed at theinterfaces. The thickness of the dead layer was reported to be 7 �A in Co/Mn MLFs [2],5.6 �A in Co/Al MLFs [3]. But in Co/Ti MLFs [4], a large thickness of the dead layer(�11.3 �A) was reported. Recently the novel properties of magnetic multilayers arisingfrom the changes of magnetic and nonmagnetic layer thicknesses further promoted inter-est in widespread research. In Co/Al MLFs, their Ms tended to be lower as the Al layerbecame thicker. But in Fe±Si/Cr MLFs, an oscillatory function of Ms versus the thick-ness of Cr layers with a period of about 10 �A was obtained [5]. The origin of the non-magnetic layer formed at the interface and the relationship between Ms and the mag-netic and nonmagnetic layer thicknesses are under further investigation.

Co/Ti is an interesting system because, from the Co±Ti equilibrium phase diagram[6], Co and Ti can form several kinds of alloys according to the compositions of Co and

1� Corresponding author.

Ping Wu et al.: Influence of Ti and Co Layer on Co/Ti Multilayers 125

Ti above 600 �C. Taking advantage of its very limited interdiffusion at lower tempera-tures, Co/Ti multilayers can be prepared by controlling the substrate temperature.

In this paper, we present our experimental results on the magnetic properties ofCo/Ti multilayers.

2. Experimental Details

Samples were prepared by depositing alternately cobalt (purity 99.99%) and titanium(purity 99.99%) at rates of 0.46 and 1.14 �A/s in a DFTS system with a vacuum ofabout 2� 10ÿ5 Pa. The sputtering rate of cobalt and titanium was normalized by lowangle X-ray diffraction (LAXRD) in order to obtain the accurate thickness of the layer.The films were deposited onto 20� 20� 0:2 mm3 microscope cover glass plates in Ar(purity 99.998%) with a sputtering pressure of about 4� 10ÿ1 Pa. The substrate holderwas cooled by water. Two series of multilayers, A and B, each with 30 bilayers, wereprepared. A Ti buffer layer of 50 �A was used between the glass substrate and the multi-layer to improve the quality of the superlattice interfaces and to make sure that the firstCo layer is in the same condition as the other Co layers. In the twelve samples of seriesA, the thickness of the nonmagnetic layer was kept constant dTi � 23� 2 �A and dCo

varied between 2.3 and 69 �A. In series B, dCo was kept constant dCo � 18� 2 �A and dTi

varied from 3.2 to 70 �A.The crystal structure and composition modulation were examined by high and low

angle XRD and transmission electron microscopy (TEM) techniques. The magnetic mea-surements were made by using a vibrating sample magnetometer (VSM) at room tem-perature (RT). Samples for magnetic measurements were diced into pieces, each with an

area of 5 � 5 mm2, using a diamondsaw. The magnetic field, H, in therange �5 kOe, was applied paralleland perpendicular to the plane ofthe film. Hysteresis curves were re-corded.

3. Results and Discussion

3.1 Microstructure of the films

Fig. 1 shows the LAXRD pattern forfour films of series A. The LAXRDanalysis of the multilayers displays

126 Ping Wu, E. Y. Jiang, H. L. Bai, H. Y. Wang, and C. D. Wang

Fig. 1. Typical LAXRD patterns forglass/Ti(50 �A)/[Co(t �A)/Ti(23 �A)]30 mul-tilayers. The Co layer thicknesses are 4.6,6.9, 9.2, and 13.8 �A

the first-order superlattice peak, showing a good layer structure. The intensity of thepeak increases with dCo indicating that the thicker dCo is, the sharper the interface is.Though Co and Ti are miscible above 600 �C, multilayers with a good layer structurewere obtained because the substrate temperature was controlled to a very low value(�30 �C). Only when dCo is so thin that the sharpness and continuity of the interfaceare not good, the intensity of the LAXRD peak decreases.

High angle XRD measurements shown in Fig. 2 revealed that no crystalline peakswere found for any of the Co/Ti MLFs. The TEM diffraction pattern did not reveal anyrings other than a broad one showing that all samples were in the amorphous or micro-crystalline state, which confirmed the HAXRD results.

3.2 The Co layer thickness dependence of the magnetization

The magnetization and magnetic anisotropy were determined by VSM at RT. The sam-ples with Co layer thickness less than �11 �A showed no measurable net magnetization.For those samples with Co layer thickness greater than 11 �A, both in-plane and perpen-dicular hysteresis loops were shown in Fig. 3 and 4, respectively. The samples had in-plane easy axes and none of the perpendicular loops achieved saturation. Fig. 4 revealedthat there was an increasing tendency for perpendicular magnetization in the Co/TiMLFs.

The relationship between MsdCo and dCo is shown in Fig. 5. The results can be de-scribed by a straight line Ms�dCo� �M0�1ÿ 2d0=dCo� [4], [7], where Ms is the in-planesaturation magnetization, M0 is the Co magnetization, d0 is the dead layer thickness.From the experimental points in Fig. 5, we can obtain M0 � 1398 emu/cm3, just the

Influence of Ti and Co Layer on Interface and Magnetization of Co/Ti Multilayers 127

Fig. 2. Typical HAXRD patterns forglass/Ti(50 �A)/[Co/Ti]30 multilayers. (a)[Co(18 �A)/Ti(25.2 �A)]30 (b) [Co(18.4 �A)/Ti(23 �A)]30 (c) [Co(23 �A)/Ti(23 �A)]30

9 physica (a) 161/1

same as the magnetic moment measured for a pure Co film with a thickness of 1500 �A.For Co/Ti multilayers of series A, the thickness of the dead layer is about 5.5 �A. Thisresult differed significantly from the value of d0 reported in literature [4]. One possiblesource of the difference is that our Co/Ti multilayer films are in the amorphous state, sothere is no serious Co lattice deformation at the interfaces, and the nonmagnetic layerthickness formed by interdiffusion or alloying at the interfaces is thinner. The multilayerhas a weaker interdiffusion in the amorphous state than in the crystalline state because,for crystallization and grain growth processes, advancing interfaces normal to the sub-strate would provide high-diffusivity paths, resulting in a larger interdiffusivity [8]. An-


Fig. 3. The hysteresis loops of [Co(t �A)/Ti(23 �A)]30 multilayers measured in a magnetizing fieldparallel to the film plane. The Co layer thicknesses are (a) 23 �A; (b) 18.4 �A, (c) 13.8 �A, (d) 11.5 �A

Fig. 4. The hysteresis loops of [Co(t �A)/Ti(23 �A)]30 multilayers measured in a magnetizing fieldperpendicular to the film plane. The Co layer thicknesses are (a) 18.4 �A, (b) 13.8 �A

other possible reason for the difference is that the constant thickness of Ti is different�dTi � 49� 3 �A in literature [4], dTi � 23� 2 �A in our films), which could influence thedead layer thickness. But, in our opinion, it could not give rise to the great change of d0,especially when dTi is thicker than 20 �A.

3.3 The Ti layer thickness dependence of the magnetization

A plot of Ms versus dTi is shown in Fig. 6. It is clear that Ms is not constant or en-hanced when dCo is kept constant. Instead, Ms decreases with increasing dTi. Supposingthat there was no interlayer coupling in the Co/Ti multilayers [9], the change of Ms

might be caused by the change of the nonmagnetic portion of the Co layer with increas-ing dTi. From the experimental points in Fig. 6, we can see that Ms approaches the Comagnetization when dTi approaches zero, Ms approaches a constant value (�560 emu/cm3)when dTi exceeded 20 �A. For dTi > 20 �A, the thickness of the magnetic Co layer was18� �560=1398� � 7:2 �A. Therefore d0 � �18ÿ 7:2�=2 � 5:4 �A, which is consistent withthe result in series A. The correlation between dTi and the interface structure is thatwhen dTi is thicker than 20 �A, the thickness of the nonmagnetic portion of the Co layerseemed to have a constant value of 5.4 �A per Co/Ti interface. The thickness of thenonmagnetic Co layer became thinner as the dTi is thinner than 20 �A.

One simple model which accounts for the observed decrease in Ms±dTi dependence isthat at the interface with Ti, each Co layer is divided into two nonmagnetic layers atthe Ti interface and a magnetic layer at the center of the Co layer. Then the decrease ofMs as dTi is increased is usually described as

Ms�dTi� �M0�1ÿ 2d0�dTi�=dCo� :What is different from the analysis of series A is that in series B dCo is constant and d0

is dependent on dTi. In particular, the correlation between d0 and dTi is probably of a


Fig. 5. Plot of MsdCo as a function ofdCo for [Co/Ti(23 �A)]30 multilayers

9*

structural origin and would therefore be also dependent on the preparation conditions,such as sputtering rate, substrate temperature, etc. But in series B, the sputtering rateand substrate temperature of the samples are the same, so the decrease in magnetizationis most likely an interface effect, although its exact origin is not immediately clear.

4. Conclusion

Co/Ti MLFs with different layer thicknesses have been prepared by a DFTS system.The samples had periodic layered structures in the amorphous or microcrystalline statesand had in-plane easy axes. The magnetization was examined as a function of dCo anddTi with VSM. In series A with constant dTi � 23 �A, the dead layer thickness isd0 � 5:5 �A, which differed significantly from the value of d0 (�11.3 �A) in other Co/Tistudies. This is attributed to the amorphous structure of our films, which leads to lessCo lattice deformation and interdiffusion or alloying at the interfaces. The in-plane Ms

of the films is weakened with decreasing dCo and became zero when dCo is less than 11 �Adue to the formation of the dead layer. There was an increasing tendency for perpendic-ular magnetization in the films when dCo decreased. In the series B with constantdCo � 18 �A, Ms decreases with increasing dTi due to the change of the effective Co layerthickness. The single model used in this paper well explains the experiment results.Although the interfacial effect is too complicated to be clear, our results indicate thatwhen dTi and dCo change, there are different interfacial variations, which may lead tothe variation of Ms of the films.

Acknowledgements This work is supported by the National Natural Science Founda-tion of China. The authors are grateful to the researchers of the VSM Laboratory ofPeking University and the Analysis Centre of Nankai University for their help.


Fig. 6. Saturation magnetization ofCo as a function of the thickness ofthe Ti spacer layer for [Co(18 �A)/Ti]30 multilayers

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influence of the thickness of titanium and cobalt layer on the interface and magnetization of co/ti...

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