Magnetic imaging: Micro-flowers increase the local magnetic field

Materials with magnetic nanostructures have many potential applications such as in spintronics. To explore such materials, nanoscale magnetic-sensitive imaging techniques are very useful, but up to now only weak magnetic fields could be applied during the imaging process. Now an international collaboration led by Dr. Sergio Valencia, HZB, has developed an approach that overcomes this limitation. The team designed tiny magnetic flux concentrators (MFCs), into which the sample is placed. The geometry of the MFCs resembles a flower with a number of petals which focus the applied magnetic field into its center. This greatly expands the magnetic field range available during imaging, and so the range of magnetic systems that can be investigated. The micro-flowers, enhancing magnetic fields locally, can find application in different nanometric magnetic microscopy techniques.

Materials with magnetic nanostructures have a wide range of potential applications. One area of application is so-called spintronics with devices that encode information in magnetic domains. These magnetic bits can be written, read and erased in a more energy-efficient way than bits in current semiconductor devices. Spin textures and magnetic domains in such materials can be investigated using nanoscale magnetic imaging techniques, for example photoemission electron microscopy (PEEM), coupled with a magnetically sensitive detection mechanism. However, observing the behaviour of materials under larger magnetic fields is difficult if not impossible, because the photoelectrons emitted by the sample and detected by the microscope are strongly deflected by the so-called Lorentz force which appears when a magnetic field is present. Up to know, only very weak magnetic fields of up to 30 millitesla (mT) could be applied during imaging, meaning that only soft ferromagnetic systems could be studied, whilst semi- and hard ferromagnetic systems remained inaccessible for in-field imaging.

Magnifying glass

In a collaboration with research teams from Spain, Belgium, UK and China, HZB physicist Dr Sergio Valencia has now developed an approach that overcomes this limitation. To achieve this, the team has designed tiny magnetic flux concentrators (MFCs) made of ferromagnetic materials, into which the nano- or micro-structures to be investigated are integrated. The geometry of the MFCs resembles a flower with a number of petals. This geometry does focus the applied magnetic field into a central region where the sample is located. It increases the local magnetic field, akin to what a magnifying glass does with sun light.

Factor 5

‘In 2025, we were able to show that such micro-flowers greatly enhance the sensitivity of magnetic sensors placed at their center. Now, in a new step, we have used them to locally amplify an applied magnetic field within a tiny region where the sample to be investigated is located. And it works. We now can image magnetic domains up to at least 150 mT, so the local field is way larger than our 30 mT limit. The reason is that this field is so confined that electrons experience almost no deflection,’ says Valencia. The MFCs amplified the local magnetic field by a factor of 5; theoretically, even increases by factors of up to 30 are possible. ‘By adjusting the geometry of the MFC, we can precisely control how the magnetic field is amplified and adapt it to the specific sample geometry,’ says Valencia.

Test with two different samples

As a demonstration Valencia’s team examined two different magnetite samples of biological origin at the PEEM station at BESSY II: A chain of magnetic nanoparticles with diameters of around 45 nanometres, naturally synthesised by magnetotactic bacteria, and a 60 million years old fossil approximately 2 micrometres in size. Polarised X-ray light was used to have magnetic sensitivity during imaging via X-ray magnetic circular dichroism (XMCD-PEEM). Besides demonstrating the approach to locally increase magnetic fields, the experiments revealed new insights too: in the giant magnetofossil, the evolution of the magnetic domain structure  was observed for the first time.

New insights into quantum materials

This work represents an enormous step forward for magnetic imaging with PEEM. By enlarging the accessible range of magnetic fields, it expands the number of applications and systems that can be investigated like new nanoscale systems with field- and temperature-dependent magnetic phase transitions, artificial spin ice, magnetic nanoparticles and nanostructures, as well as antiferromagnetic spintronic devices such as spin valves and tunnel magnetoresistance junctions, including 2D van der Waals magnets. 

Notably, MFCs could also be used to locally generate stronger magnetic fields in other electron-based microscopy techniques, as well as in techniques were spatial constraints limit the size of conventional systems to generate magnetic fields. To this respect, techniques such as X-ray transmission microscopy, X-ray ptychography and X-ray laminography could also benefit from the micrometre-scale dimensions of the MFCs and their direct sample integration.