Magnetism Under Pressure: Much-Studied Ceramic Material Reveals New Secrets

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An international team of researchers led by DTU Energy has discovered that the oxide material strontium titanate, conventionally thought to be nonmagnetic, can exhibit an intricate magnetic order when a local force is applied to it. 

Strontium titanate (SrTiO3) or STO is a transparent material which does not conduct electricity. Its remarkable light dispersion abilities made it at one point in time highly sought after as a synthetic alternative to diamond gemstones. Scientists have studied it for many years both due to its many interesting properties and the fact that one can make atomically flat surfaces of STO. This makes it the perfect substrate to grow very thin ceramic layers on. Using, e.g., a process called pulsed laser deposition (PLD) one can put down perfectly defined atomic layers, one at a time, on top of STO.

Such artificial heterostructures can have very different properties than that of the constituent materials. A striking example is that putting a very thin insulating layer on top of the insulating STO can make the interface electrically conducting. And combining two nonmagnetic materials can create magnetism (see  PhD defense: Dennis discovers magnetism when non-magnetic materials are put together). A group at DTU Energy led by Professor Nini Pryds have studied these systems for several years.

But pure STO also holds a few surprises, as Dennis Christensen discovered during his PhD work at DTU Energy. He found that both STO heterostructures but also STO on its own can display a magnetic order. By measuring the microscopic landscape of the magnetic state using a technique called scanning SQUID microscopy, Dennis could show that STO can exhibit magnetic stripes which reflect the orientation of the different crystallographic domains in the material. The stripes are at most a few micrometers in width and can only be detected with great care. A scanning SQUID microscope consists of a very thin tip on which a small superconducting circuit (a SQUID – superconducting quantum interference device) is mounted that can measure extremely small magnetic fields.

Not only did Dennis discover a magnetic order in STO, he also found that the order can be manipulated mechanically: By using the tip to gently press down on the surface of the STO and create a local force he could change the configuration of the magnetic stripes drastically. The existence of such strain-tunable magnetic order provides a key missing piece in the puzzle of understanding STO on a fundamental level. And it is an important step towards realizing nanoscale ‘spintronics’ – circuits in which the information is not carried by electrical charge as in conventional silicon chips but by magnetic spins. Some researchers speculate that spintronics may provide a path to significantly faster computers.

Dennis undertook his study in collaboration with Stanford University in the US and Bar-Ilan University in Israel which are home to some of the best scanning SQUID groups in the world. As often happens in science, he was looking for something else when he started out. “It has been an interesting journey that started in 2015 with the purpose of using scanning SQUID microscopy to detect whether the motion of the electrons differs between STO heterostructures with high or low conductivity, but after almost four years it unexpectedly ended up with the discovery of strain-tunable magnetic stripes”, Dennis says. His results have just been published in one of the world’s leading physics journals, Nature Physics.