Physicists measure spin nutation in ferromagnets

Kumar Neeraj et al. / Nature Physics, 2020

European scientists succeeded in experimentally detecting spin nutation in ferromagnets with high accuracy and directly (using stimulated resonance) to measure its frequency and relaxation time. The effect itself was recently predicted by introducing an inertial term into the Landau - Lifshitz - Hilbert equation, which, in particular, describes the process of a forced nanosecond spin flip in magnetic storage media. The data obtained on the spin dynamics in ferromagnets not only broaden the ideas of scientists about the fundamental mechanisms of ultrafast magnetism, but can also lead to the creation of significantly more energy efficient and fast magnetic information carriers. The article was published in the journal Nature Physics .

Much information is now stored in the form of tiny magnetic bits on thin-film materials in hard drives. In such bits, the role of zeros and ones is played by the arrangement of the magnetic moments (spins) of the atoms of the carrier substance, and information is recorded due to intense and highly localized magnetic fields acting on time scales of the order of nanoseconds. The dynamics of spins in such processes, in turn, is described by the Landau - Lifshitz - Hilbert (LLG) equation, and until recently it was believed that this equation contains the entire physics of magnetic dynamics, and the optimization of information recording processes can be based only on it.

However, in 1996, the existence of processes of spin dynamics with time scales of several picoseconds, which cannot be described by the LLG equation , was experimentally confirmed , which led to the birth of a new direction in physics to study the so-called ultrafast magnetism. One of the ways to solve the emerging discrepancy between observations and theory was to introduce an inertial term into the LLG equation, which would lead to spin nutation - its frequent oscillations in the vicinity of its standard precession trajectory , like oscillations of a gyroscope when trying to unbalance it. But for experimental measurements of such a phenomenon, physicists did not have enough magnetic field sources with a sufficiently high frequency.

The Landau - Lifshitz - Hilbert equation in the classical version (1) and with an additional inertial term (2). / Kumar Neeraj et al. / Nature Physics, 2020

Now the generation of a periodic magnetic field with a frequency of up to terahertz has become possible at installations for the acceleration of short electron beams with a high total charge, for example, at TELBE at the Center. Helmholtz Dresden-Rossendorf in Dresden. It was this setup that served as a source of radiation for the work of Kumar Neeraj from Stockholm University, who, together with colleagues, irradiated ferromagnetic samples and observed their stimulated resonance by the polar magneto-optical Kerr effect... Physicists really managed to see resonance at a frequency of 0.5 - 0.6 terahertz for all samples, with a nutation-specific dependence of the effect amplitude on the relative position of the intrinsic magnetic moment and external high-frequency magnetic radiation: the effect is maximum when they are perpendicular. Scientists also managed to measure the relaxation time of such oscillations, which turned out to be 10 picoseconds.

Differences in modeling the behavior of the magnetic moment in the absence (a) and presence (b) of the inertial term in the LLG equation. / Kumar Neeraj et al. / Nature Physics, 2020

The experiment also revealed a number of unexpected features of the effect. Thus, the value of the nutation frequency turned out to be weakly dependent on the crystal structure and chemical composition of the ferromagnetic sample, while theoretical predictions indicated a possible strong dependence of the nature of spin nutation on the structure of the substance. In addition, the observed resonance itself turned out to be significantly wider than expected, and also for all samples: the researchers verified that this effect was not a consequence of measurement errors, but left it without explanation. Finally, the measured relaxation time turned out to be an order of magnitude higher than the values ​​indirectly measured in spectroscopic experiments on observing ferromagnetic resonance, but the authors of the work are inclined to believe that their methods made it possible to obtain more accurate and realistic data.

Discovered resonances for (top to bottom) CoFeB, epitaxial NiFe, and polycrystalline NeFe. Left - experimental values, right - theoretical predictions. / Kumar Neeraj et al. / Nature Physics, 2020

The authors also note that the detected high-frequency magnetic excitations could not be caused by the spin wave modes: the thickness of the samples for the excitation of such resonances was too large. All this suggests that scientists really have achieved direct experimental confirmation of the existence of spin nutation in ferromagnets. Although physicists are not yet clear on the nature of the inertia that leads to the observed nutation, the data obtained already help to better understand the nature of the mechanisms of ultrafast magnetism, and they can also be used to create a faster and more energy efficient technology for magnetic information recording.

Research into the spin dynamics of atoms can help not only improve the efficiency of existing storage media, but also create more spacious analogs: we recently talked about how physicists found a way to write two bits in one atom. And the information recording density can be improved with the help of microwave generators.

#Physics #Science #Ferromagnets #Experiment

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