Influence of underlayers on the soft properties of high magnetization FeCo films

Abstract

A remarkable reduction in coercivity Hc was found in sputtered Fe65Co35(=FeCo) films on Cu, NiFe, Ru, Ta/Cu, Ta/NiFe, or Cu/IrMn underlayers. A decrease in Hc from 120 to 7–12 Oe was observed for Cu, NiFe, and Ru underlayers as thin as 2.5 nm but less for Ta. A Cu underlayer significantly reduced the maximum anisotropy fields from 2 kOe to 40 Oe, resulting in a well-defined in-plane average uniaxial anisotropy field Hk∼30 Oe. The saturation magnetostriction with Cu was (47±4)×10−6, independent of Cu and FeCo thicknesses. In-plane tensile film stress decreased with underlayer thickness tUL from 2 to 0.2 GPa but much less rapidly than the reduction in Hc. All underlayers induced a (110) texture in FeCo, which was strongest with Ta. Transmission electron microscopy of cross-sections showed an unusually long range coherence with low angle grain boundaries in the FeCo without an underlayer. Clear columnar grains were visible with all underlayers with an average grain size of ∼50 nm with Ta dropping to 9–10 nm for Cu, NiFe, and Ru. This alone is sufficient to explain quantitatively the reduction in Hc using Hoffmann’s ripple theory.

Underlayer effect on structural and magnetic properties of Co₉₀Fe₁₀ thin films

“…This texture change might result in stress relief [17]. So, our results are similar with recent studies which reveal that texture change in crystal structure accompanies coercivity reduction [3,9,11,12,15]. Overall, according to GI-XRD results, it is not hard to say that film texture is tailored by underlayers.…”

“…Much effort has done for lowering coercivity of FeCo alloys to improve soft magnetic properties in recent years. Underlayers, adding additives, and using different deposition techniques are frequently used methods for lowering coercivity [4,5,[8][9][10][11][12]. The reduction of coercivity in these studies generally suggested that consequence of reducing grain size according to Hoffman's ripple theory or change on preferred crystal orientation [4,8,13,14].…”

“…Choosing a suitable underlayer for FeCo alloy films is an effective method for lowering coercivity and improving soft magnetic properties of films. In these studies, using an underlayer generally results in a structural change [3,5,12,[15][16][17]. Structural changes also affect some other magnetic properties like magnetic anisotropy, which was effective on soft magnetic properties [18,19].…”

Growth rate effects in soft CoFe films

“…It is clear that samples A1 and A2 with small grain size are magnetically soft and in-plane anisotropic, while samples B1 and B2 with large grains are magnetically harder with coercivities exceeding 120 Oe. These samples are in-plane isotropic [1]. The observed change in the coercivity with grain size is in agreement with the random anisotropy model [5], [6].…”

“…Controlling both and of CoFe is difficult and usually involves the use of additives or seed layers [1], which are inconvenient in terms of cost and final device capabilities. In this paper, we demonstrate the preparation of CoFe thin films with control of and over a broad range of values.…”