Advancements in Electroplated Co-Pt Thick Film Micromagnets

There are enormous opportunities to employ permanent magnets to establish strong magnetic fields or field gradients in microsystem applications such as microactuators, microsensors, biological cell manipulation, micro power generators, etc. [1]. These applications motivate the development of microfabrication methods to realize high-remanence, high-coercivity magnetic structures in a dimensional “gap” between traditional bulk materials (usually >1 mm) and thin films (usually <1 mm).  

In this work, we summarize advancements in electroplated Co-Pt alloys as candidate materials for wafer-level integration into magnetic microsystems. As a fabrication technology, electrodeposition offers advantages of speed, cost, and process integrability. Compared to other magnetic materials, Co-Pt alloys offer good energy density, high Curie temperature, and high corrosion-resistance. Specific attention is focused on the Co-rich composition (Co₈₀Pt₂₀) as well as the equiatomic composition (Co₅₀Pt₅₀) in the ordered L1₀phase, as shown in Figure 1. 

Electroplated Co₈₀Pt₂₀ affords hard magnetic properties in the as-deposited magnetic state, without the need for any high-temperature processing steps.  The material exhibits an hcp structure with large magnetocrystalline anisotropy, and by careful selection of the seed layer, significant out-of-plane texture (perpendicular anisotropy) can be achieved.  For example, we have shown films up to 18 mm thick with H_ci = 330 kA/m (4 kOe), B_r = 1.0 T, and BH_max = 130 kJ/m³(16 MGOe) [2].  One potential drawback of this material is high sensitivity to temperature [3].

In contrast, L1₀ CoPt requires a high-temperature annealing step (~700°C) to induce a transformation from a disordered fcc atomic arrangement to an ordered L1₀ structure. This transformation results in strong magnetocrystalline anisotropy, but the material exhibits an overall isotropic behavior due to the random arrangement of the polycrystalline grains.  For example, we have demonstrated 15-mm-thick photolithographically patterned magnets with H_ci = 800 kA/m (10 kOe), B_r = 0.8 T, and BH_max = 150 kJ/m³(19 MGOe) [4].  

This talk will expound on the electrodeposition of these two alloys and the resultant magnetic properties.  The usage of these films in example microsystem applications will also be presented.

[1] O. Cugat, J. Delamare, and G. Reyne, “Magnetic micro-actuators and systems (MAGMAS),” IEEE Trans. Magn., 39, 3607-3612 (2003)

[2] O. D. Oniku and D. P. Arnold, “Microfabrication of high-performance thick Co₈₀Pt₂₀ permanent magnets for microsystems applications,” ECS Trans., 50(10), 167-174 (2012)

[3] A. Garraud, E. Shorman, O. D. Oniku, B. Qi, and D. P. Arnold, “Influence of temperature on the magnetic properties of electroplated Co-rich Co–Pt thick films,” J. Micromech. Microeng., 24(1), 017002 (2014)

[4] O. D. Oniku, B. Qi, and D. P. Arnold, “Electroplated L1₀ CoPt thick-film permanent magnets,” J. Appl. Phys., 115(17), 17E521 (2014)