P131 - Modification of magnetic properties in thin films by substrate roughness

6. Magnetic thin films, multilayers, surface and interfaces
Michal Belusky1 , Serban Lepadatu2, John Naylor3, Melvin Vopson1
1 University of Portsmouth
2 University of Central Lancashire
3 Kurt J. Lesker Company

Some of the most influential thin film technologies are based on
magnetic films, which are indispensable in magnetic data storage,
magnetic sensing or actuation devices. The ability to manipulate their
magnetic and structural properties is therefore of great interest. One
of the ways in which we can alter the behaviour of a film is through
the preparation method. This is typically achieved by adjusting base
pressure of the system, rate of deposition or angle of substrate with
respect to the deposition source. More elaborative methods include
growing films under influence of external magnetic field thus setting
up preferred magnetisation or by using a seed layer in order to
control nucleation process. In this work, we studied how substrate
roughness affects the thin film properties. Three substrates of
different average roughness were chosen, two of which are mechanically
flexible. Films of varied thickness were plasma sputtered onto each
substrate and their magnetic properties studied by Magneto Optic Kerr
effect and atomic force microscopy.

We have found some unexpected results connecting film’s magnetic
properties to the substrate roughness. When the film thickness values
are similar to substrate roughness, the magnetic coercive field is at
the highest value. We measured remarkable increases in coercive fields
of 15 nm films of various magnetic materials coated on rough substrate
compared to the same films coated on a smooth one. This effect was
backed by an observed emergence of magnetic anisotropy, even for
materials not normally possessing magnetocrystalline anisotropy, such
as Permalloy.

To back our results we have assessed our finding by modified
micromagnetics calculations that included surface roughness. Modelling
confirmed that the observed anisotropy and coercive field increases
result from spatial anisotropy of surface roughness leading to an
effective in-plane magnetic anisotropy.

We believe that roughness effect is not only a scientifically
interesting phenomenon but it also provides a simple and inexpensive
way of modifying magnetic properties of thin films. We further remark
that our best results were achieved with flexible substrates, which
are the key players on electronic market today and they bring a new
dimension to magnetic thin film technologies.