Spin mixing in ferromagnets revealed

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For the first time, Uppsala researchers and international colleagues were able to measure the spin mixing in a ferromagnetic material through experiments and theory. Through the experimental measurements, they discovered that a common factor in the spin equations that had been in use since the 1950s was significantly underestimated.

In addition to the known electrical charge, electrons have a magnetic moment, also known as spin. The spin can be simplified as the micro-level equivalent of the macro-level bar magnet, which has a north pole and a south pole. An important phenomenon in quantum mechanics is that the spin can take on several values ​​at the same time, a so-called spin mixture, which is in stark contrast to classical mechanics. This can be compared with Schrödinger’s thought experiment, which lives and dies until an investigation is started.

“Spin mixing is a bridge that allows spin to change direction quickly and could be important for future spin electronics. You want a low spin mix if you want the spin to be unchanged when the spin is used as an information carrier. Instead, you want a high degree of spin mixing if you want to quickly influence the spin, as is the case with the ultrafast demagnetization of a material, ”says Ronny Knut, researcher at the Institute for Physics and Astronomy.

Can be estimated from calculations

The mixing of different spin states in a material can be estimated from calculations, but so far there has been no good experimental method to measure this effect.

What the Uppsala researchers found is that equations from the 1950s that are used today to describe spin behavior in a microwave field have underestimated the importance of spin mixing and a correctly fitted equation can be used for measurements instead.

The researchers came to the results by using two different measurement methods with different results, Ferromagnetic Resonance (FMR) at NIST in Boulder and X-ray Magnetic Circular Dichroism (XMCD), carried out at Synchrotron BESSY II in Berlin. The answer to the differences between the two measurement methods results from the previously neglected factor in the theoretical model. It has now been realized that it still plays a role and that the difference between FMR and XMCD is directly related to the spin mix.

“This work will enable methodical studies on spin mixing that will drive the development of spin electronics,” says Ronny Knut.

Equation used in modern magnetism

The equation developed by physicist Charles Kittel is one of the most well-known relationships in modern magnetism and the basis for thousands of publications on magnetic phenomena. When Kittel was making his equation, he decided to ignore a post that he believed had no practical meaning at the time. The neglected contribution is an extension of Kittel’s original application of the laws of quantum physics, which assumes that the behavior of electrons and their spins is described by wave functions that follow strict quantum mechanical rules.

Uppsala University contributed both with experiments (Ronny Knut and Olof Karis) and with theory (Yaroslav Kvashnin, Erna K. Delczeg-Czirjak and Olle Eriksson). The research was carried out in collaboration with researchers from NIST in Boulder (USA), the University of South Florida in Florida (USA), Örebro University, Trinity College in Dublin (Ireland) and the Helmholz Center Berlin (Germany).

Elin Backstrom


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