Influence of seed layers on the microstructure of NiFe layers

Influence of seed layers on the microstructure of NiFe layers

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 272–276 (2004) e1495–e1496 Influence of seed layers on the microstructure of NiFe layers...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) e1495–e1496

Influence of seed layers on the microstructure of NiFe layers B. Warota,*, J. Imriea, A.K. Petford-Longa, J.H. Nickelb, T.C. Anthonyb a

Department of Materials, University of Oxford, Parks road, OxfordOX1 3PH, UK b HP Labs, 1501 Page Mill Road, Palo Alto, CA 94304, USA

Abstract Magnetic tunnel junctions are of interest for their application in magnetic random access memory. Structural parameters can influence the homogeneity of the nanojonctions and we report on the influence of the seed layer on the structural and magnetic properties of a NiFe layer in a (Si/Al2O3/seed layer/NiFe/Al2O3/TaN) multilayer where the seed layers are Ta, TaN and Al2O3. All the NiFe layers are crystalline although the texture, the grain size and the roughness vary with the seed layer. Magnetic measurements indicate small differences in the saturation magnetisation for the films with the different seed layers. r 2003 Elsevier B.V. All rights reserved. PACS: 68.37.Lp; 68.55. a; 75.70.Ak Keywords: Transmission electron microscopy; Seed layers; NiFe; Magnetic properties

The use of seed or buffer layers can improve the magnetic properties of sputtered thin films by inducing structural changes. Seed layers under NiFe layers have been used to control the grain size in the layer and thus increase the coercivity. The grain size in the NiFe layer is also an important parameter for magnetoresistance. Large grain size induces lower film resistance, as there is less scattering of conduction electrons at the boundaries, and appropriate seed layers can affect this parameter too. There are various reports in the literature of studies describing the effects of a seed layer on the growth of NiFe layers. MgO, Si and Ta seed layers have been used as seeds under 6 to 100 nm thick NiFe layers and the magnetic properties depend not only on the texture of the ferromagnetic film but also on the grain size and roughness [1,2]. NiCrFe is also widely used as a seed layer for NiFe [3,4]. In this study, we have investigated the effect of the seed layer on the magnetic properties of Ni0.80Fe0.20 layers. Structural studies have been carried out to assess crystallinity, interface roughness and grain size.

*Corresponding author. Tel.: +33-562257970; fax:+33562257999. E-mail address: [email protected] (B. Warot).

Three different seed layers have been studied: Ta(2 nm), Al2O3(5 nm) and TaN(5 nm). A 4 nm thick NiFe layer was deposited on top of the seed layers and capped with alumina(2 nm) and TaN(15 nm). All the layers were deposited by sputtering on an alumina underlayer layer deposited on a silicon substrate. The thin alumina layer on top of the NiFe layer was formed by aluminum deposition and exposure to oxygen plasma, and was present to simulate the presence of the tunnel barrier in a spin tunnel junction structure. Transmission electron microscopy (TEM) experiments were carried out on the as-grown samples and on the annealed samples. Cross-section samples were prepared by mechanical polishing and the slice was then thinned further using a precision ion polishing system (PIPS). The high-resolution TEM (HRTEM) analysis was carried out using a JEOL 4000EX operated at 400 kV (point-to-point resolution 0.16 nm). vibrating sample magnetometry (VSM) measurements were made to measure the saturation magnetisation of the continuous films prior to patterning. Fig. 1a shows a cross-sectional HRTEM image of a Ta/NiFe/Al2O3/TaN sample; the Ta seed layer is 2 nm thick and the ferromagnetic NiFe layer is 4 nm thick. The seed layer is amorphous and the NiFe layer has a high /1 1 1S crystallographic texture with large grain

0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.284

ARTICLE IN PRESS B. Warot et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1495–e1496

200

TaN (5 nm)

600

Al2O 3 (5nm)

1000

Ta (2nm)

Satu ration Magne tisation (emu/cc)

e1496

Fig. 2. Saturation magnetisation measured at 20 Oe.

Fig. 1. HRTEM image of Ta/NiFe/Al2O3/TaN (a), Al2O3/ NiFe/Al2O3/TaN (b) and TaN/NiFe/Al2O3/TaN stacks (c).

size (20 nm). The Al2O3 layer on top of the NiFe layer is continuous and flat. Fig. 1b shows a cross-sectional HRTEM image of an Al2O3/NiFe/Al2O3/TaN sample, in which the seed layer is covered by a 4 nm thick NiFe layer. On top of the amorphous Al2O3 layer, the NiFe layer has a random texture with smaller grain size than the previous sample (5 nm). The layers in the stack do not appear rough and the main difference when compared with the previous sample is the grain size. Fig. 1c is a cross-sectional HRTEM of a TaN/NiFe/ Al2O3/TaN sample. The amorphous TaN seed layer is covered by a 4 nm NiFe layer with a random texture and an average grain size of 5 nm. The alumina/TaN interface is very rough and the roughness continues throughout the sample, with the NiFe/on/Al2O3 interface for this sample being the roughest observed in this study. Magnetic measurements (Fig. 2) on these samples indicate that the TaN-seeded sample has the lowest saturation magnetisation.

This study shows the influence of the seed layer on the microstructure of thin NiFe layers. Different stack structures have been observed with different roughness, texture and grain size. Flat interfaces throughout the stack are obtained with an alumina or a tantalum seed layer whereas very rough interfaces are observed when a TaN seed layer is used. The roughness arises from the way in which TaN grows on alumina. TaN layers have already been reported to be rougher than Ta layers when deposited on SiO2 [5] and the same appears to be true when the TaN is grown on alumina. The seed alumina layer is flat as usually observed when alumina is deposited on silicon. In all these cases, the seed layer/ NiFe and NiFe/Al2O3 interfaces reproduce the roughness of the Al2O3 underlayer/seed layer interface. This conformal growth has previously been observed when a NiFe layer is deposited on a rough top Ta surface deposited on a rough and large grain-sized Al layer [2]. No chemical diffusion is expected at the TaN/NiFe and Al2O3/NiFe interfaces but some interdiffusion has been reported at the Ta/NiFe interface [6]. This diffusion could be responsible for the lower magnetisation of the Ta/NiFe sample compared to the Al2O3/NiFe sample. The NiFe layers on TaN and Al2O3 have the same grain size and the roughest interface of the TaN seeded sample could explain the loss of magnetisation as the NiFe atoms are locally surrounded by non magnetic atoms.

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