Neutron reflectometry reveals the spiral magnetic phase in thin film Erbium

By Tom Sharp on January 9, 2017

The magnetic ordering inside bulk crystals of rare-earth ferromagnets is now well understood. Upon reducing their dimensionality to thin films, the magnetic state changes due to the additional influence of the surfaces and strain. In their recent Journal of Physics: Condensed Matter paper, Nathan Satchell et al. showed that even in films as thin as 6 nm, the rare-earth ferromagnet erbium can still form a magnetic spiral. Read on to find out more from the authors.

Magnetism in the rare earth element Erbium arises from complicated competition between the indirect exchange interaction and the crystalline structure, which want to align the magnetism in perpendicular directions. In bulk crystals, this results in Erbium undergoing several magnetic phase transitions with temperature (or applied field). These are characterised (broadly) into paramagnetic, antiferromagnetic, helical or conical magnetic states. In sufficiently thick films (200 nm), we recently found that all bulk-like magnetic states can be retained.

When reducing the film thickness to only a few tens of atomic layers (in order to become device applicable), it is likely many of the bulk magnetic states become suppressed, or even destroyed completely. In our recent Journal of Physics: Condensed Matter paper, we studied the magnetic state of such a thin film using the polarised neutron reflectometry (PNR) technique. This allowed us to extract a depth dependent magnetic profile of our film. We performed our work on the PolRef beamline at the ISIS neutron source.

We have found that with careful fitting of the PNR signal, (see figure), we obtain a magnetic profile (model 1 (c)) which is consistent with the Erbium layer forming a magnetic spiral. For comparison model 1 (b) shows a forced homogeneous magnetisation profile, which returned an inferior fit to the experimental data (a).

Interestingly, the repeat distance of the spiral is inconsistent with any known bulk magnetic phase of Erbium, confirming our hypothesis that reducing the film thickness would impact the magnetic ordering. This finding also demonstrates the power of the PNR technique and the importance of performing such analysis on thin films before incorporating new materials into devices.

We also performed SQUID magnetometry to determine the film’s response to large magnetic fields. We found that when a large enough magnetic field was applied it was possible to completely unzip the spiral. When the field was removed, the spiral re-zipped. This behaviour is markedly different to Erbium’s neighbour in the periodic table, Holmium, which once unzipped, does not reform its spiral phase.

Recently, advances in combining superconductors with ferromagnetic materials with inhomogeneous magnetic texture has led to the new field of superconducting-spintronics, where the spin aligned Cooper pair (generated at the interface) forms the basis of a new class of spintronic devices. The magnetic textures of the rare-earths are perfectly suited for this application, as they are periodic with approximately the same length scale as the Cooper pair. In our future work we plan to incorporate Erbium into such a device in order to study the superconducting proximity effect in this complicated magnetic system.

Nathan Satchell is a postdoctoral researcher in the nano-magnetism group at the ISIS neutron and muon spallation source, and has recently been awarded his PhD from the University of Leeds under the supervision of Dr Gavin Burnell. His current research interest is the interface between superconducting and ferromagnetic materials with either tuneable or intrinsic magnetic inhomogeneity.

James Witt is a postdoctoral bon viveur based at the nexus of folly and whim. His interests revolve around his insides like fresh laundry in a tumble-dryer.

Christy Kinane is the principle instrument Scientist on the PolRef beamline at the ISIS neutron and muon spallation source, where he has worked on polarised reflectometers for the last nine years. His research interests are based around studying interface effects in magnetic or nonmagnetic thin films via scattering techniques.

Professor Sean Langridge is head of the multidisciplinary Diffraction and Materials Division at the Rutherford Appleton Laboratory (RAL) and an STFC Fellow. SL’s research interests cover the magnetism and superconductivity of nanoscale systems utilising neutron, muon and x-ray probes.

Jos Cooper is a beamline scientist on the OFFSPEC reflectometer at the ISIS neutron and muon source. He completed his PhD in thin film magnetism and electrodeposition in Cambridge before moving to ISIS to work on thin film superconductor/ferromagnet proximity effects. Following this he took the position as instrument scientist and now works on instrument development, electrochemistry and magnetism.

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