Ololade D. Oniku

Batch Patterning of SubMillimeter Features in Hard Magnetic Films Using Pulsed Magnetic Fields and Soft Magnetizing Heads

In this paper, we report a method to impart spatially defined magnetic patterns into hard magnetic films using pulsed magnetic fields. Differing from prior investigations on magnetic patterning, a single magnetizing head is used rather than a sandwich structure with two magnetizing heads. Arrays of 250μm-wide and 125μm-wide magnetic stripes with alternating out-of-plane polarity (perpendicular magnetization) are patterned in a 10μm-thick Co-rich Co-Pt alloy (Co80Pt20) film using three different microstructured, 2-mm-thick, Fe-Co magnetizing heads and pulsed magnetic fields ranging from 0.1 to 1 T. A magneto-optical imaging film is used to view the magnetization patterns at the surface of the Co-Pt film. Scanning Hall probe measurements of the resulting magnetic structures indicate a ~5mT peak magnetic field 100 m above the surface which compares favorably with numerical simulations of the field patterns at the same height.

Imprinting of fine-scale magnetic patterns in electroplated hard magnetic films using magnetic foil masks

This paper presents a batch-fabrication approach to imprint complex magnetic pole patterns (perpendicular north/south poles) into hard magnetic films using fine-scale, laser-machined iron foil “masks.” In this approach, pulsed magnetic fields varying between 0.1 and 1.0 T are used in conjunction with the field-concentrating effect of the iron foil masks to selectively reverse the magnetization of selected regions of 10-μm-thick Co80Pt20 films (Br = 1 T, Hci = 340 kA/m). Simple stripe array patterns are systematically studied both experimentally and via simulation, achieving patterned feature sizes (poles) down to 30 μm. Double exposures are also explored to create checkerboard patterns from simple stripe-like masks. Finally, masks with more complex patterns are used to demonstrate the viability of the approach to imprint arbitrary features.

Assessment of Laser-Induced Damage in Laser-Micromachined Rare-Earth Permanent Magnets

The effects of micromachining bulk rare-earth magnets are investigated by measurements of the moment of sub-millimeter scale micromachined structures. A model representing these effects is also presented to provide a basis for interpreting the results. Laser-induced damage to the magnetic material, manifesting as a loss of magnetic properties, is estimated to reach 10-20 μm from the magnet edge in the lateral dimension. The results indicate the affected volume due to laser micromachining bulk rare-earth magnets is limited to less than 25 μm, laterally through the material, for the rare-earth magnetic materials tested.

Influence of temperature on the magnetic properties of electroplated L10 CoPt thick films

This paper reports the magnetic properties of 2 μm thick electroplated isotropic L10 CoPt films on silicon at temperatures ranging from 300 K to 790 K, as well as the room-temperature properties of the films after various thermal cycles. Electroplated equiatomic CoPt layers require a post-deposition annealing typically at ∼973 K to induce L10 ordering so that they exhibit hard magnetic properties at room temperature. However, the influence of temperature on these post-deposition annealed films is an important consideration for their utility in end applications. Here, a reversible temperature coefficient of remanence of −0.11% K−1 is measured along with a maximum operating temperature of 400 K (recovery to 95% of the initial remanence). The maximal energy density of the films is reduced by 50% at a temperature of 550 K. However, the original room-temperature magnetic properties are shown to be fully recoverable by remagnetization after various thermal cycles—800 K for 15 min in Ar, 373 K for 168 h in air, ...

Influence of temperature on the magnetic properties of electroplated Co-rich Co–Pt thick films

In this paper, the magnetic properties of 10 µm thick Co-rich Co–Pt films (~80:20 atomic ratio) hard magnetic films electroplated on silicon substrates are characterized at temperatures from 300 to 1000 K. With an increase in temperature up to 500 K, the coercivity and remanence of the films both decrease rapidly, dropping to only 10% of their respective as-deposited values. Above 500 K, the coercivity and remanence continue declining but at a slower rate, reaching nearly zero at 1000 K. Conversely, the saturation magnetization reduces by only 15% at 800 K and 40% at 1000 K from the as-deposited value. In addition to these measurements at elevated temperature measurements, thermal cycling tests are performed to examine the influence of various thermal exposures. Cycles up to 500 K are shown to have little impact on the film morphology but notably reduce the layer adhesion. Most importantly, a thermal cycle to just 400 K is shown to essentially destroy the hard magnetic properties. Because of these temperature sensitivities, Co-rich Co–Pt films may be significantly limited for end applications.

Effect of current density on electroplated CoPt thick films

In this paper, we report on the influence of electrodeposition current density on the structural and magnetic properties of plated equiatomic CoPt magnetic thick films. Films are electroplated using constant current density, ranging 25-200 mA/cm2, and subsequently annealed at 700°C for 40 min. It is found that electroplating current density has a strong influence over the plating rate, surface roughness, grain size, and order parameter. However, current density does not strongly affect the atomic composition of the plated film and only modestly influences the magnetic properties, namely coercivity. The films exhibit quasi-isotropic magnetic properties with the best results for a sample plated at 100 mA/cm2, yielding Hci = 900 kA/m, and in-plane squareness of 0.95 for a 3-μm-thick film having an rms surface roughness of 20 nm.

Mitigation of interfacial silicide reactions for electroplated CoPt films on Si substrates

We report in this paper the influence of film thickness on the material and magnetic properties of electroplated CoPt permanent magnets. Layers of CoPt magnets with film thicknesses ranging from 0.5 μm to 5 μm are deposited into photoresist molds (3.5 mm x 3.5 mm square and 5 μm x 50 μm arrays) on a (100)Si substrate coated with 10 nm/100 nm Ti/Cu adhesion/seed layer. Results show an unexpected reduction in magnetic properties for films below 2 μm thick. This effect is determined to be a consequence of metal-silicide reactions at the substrate interface during annealing leading to the formation of a non-magnetic layer at the interface. Subsequently, a TiN diffusion-barrier layer is added to inhibit the silicide reaction and thereby maintain strong magnetic properties (Hci ~800 kA/m, Mr/Ms = 0.8) in micron- thick electroplated CoPt layers.

Microscale magnetic patterning of hard magnetic films using microfabricated magnetizing masks

We present a batch-fabrication process to imprint microscale magnetic pole patterns (perpendicular north/south poles) into hard magnetic films using field-shaping, soft magnetic “magnetizing masks”. Using 7-μm-thick, electroplated Fe-Co magnetizing masks, magnetic stripes with widths down to 50 μm have been imprinted into both 15-μm-thick Co-Pt films and 5-μm-thick Nd-Fe-B films. These patterned films exhibit a sinusoidal stray magnetic field pattern with ~4 and ~7 mTpk-pk variations and corresponding field gradients of 80 and 140 T/m, respectively. We also demonstrate the ability to transfer more complex patterns by showing magnetization of various geometric shapes.