Future information technology: Microscopic insight into processes when magnets suddenly heat up

Magnetic solids can demagnetize upon heating. Despite decades of research, it has so far been unclear how this process works in detail. Now, for the first time, an international group has observed in a step-by-step manner how sudden heating affects the magnetic order of a ferrimagnetic insulator. The result: The magnetic order changes on two time scales. The first process is surprisingly fast and takes only one picosecond, while the second process takes 100,000 times longer. This insight could help to increase the switching speed in magnetic storage media by at least a factor of 1000.  The work is published in Science Advances.

Magnetic solids have always fascinated mankind. Today's information society with its great demands for faster and higher capacity data storage would be inconceivable without such magnetic materials. Ferrimagnets are the largest known class of magnets. They have a crystal structure in which the magnetic moments (spins) of the atoms are alternately antiparallel. However, they do not fully cancel each other out, as the spins are having different magnitudes. The total magnetization is the sum of all spins of type 1 (M1, blue arrows) and type 2 (M2, green arrows). Due to the opposite direction, the total magnetization is M1-M2.

When a insulating ferrimagnet is heated, the heat is eventually distributed  within the crystal lattice: The atoms vibrate and can also tumble around their equilibrium position. As a result, the original magnetic order is gradually lost; the total magnetization (M1-M2) decreases and disappears as soon as the so-called Curie temperature is exceeded.

But how this actually happens in detail was a mystery so far. Even for the very well studied ferrimagnet yttrium-iron-garnet (YIG) it was completely unclear how long it takes for the heated atomic lattice and the spins to come into balance with each other. Estimates range from picoseconds (10^-12s) to microseconds (10^-6s).  Now teams from Berlin, Dresden, Uppsala (Sweden), St. Petersburg (Russia) and Sendai (Japan) have been able to observe this process step by step on a YIG sample for the first time.

"In order to heat the atomic lattice of a YIG film instantaneously and selectively, we used ultra-short laser light bursts in the terahertz range at the Fritz Haber Institute. Immediately afterwards we probed the sample with another laser pulse in the visible range. This enabled us to follow the evolution of the spins step by step. Essentially, we are recording stop-motion film of how magnetization evolves," says Dr. Sebastian Maehrlein from Fritz Haber Institute Berlin, who conducted the experiments.

The HZB guest scientist Dr. Ilie Radu from Max-Born.Institute, who had the initial conceptual idea, summarizes the results: "The sudden heating of the atomic lattice changes the magnetic order of the ferrimagnet on two different time scales: Certain processes are incredibly fast, in the picosecond range, while other processes take about 100,000 times longer and require around 100 nanoseconds".

Illustration of the complex processes with a pot of water in the oven

The two physicists illustrate this process with a pot of water in a hot oven with the lid closed. The hot air in the furnace corresponds to the hot atomic lattice, while the water in the pot represents the spins (see figure). When the atomic lattice is heated, the random oscillations of the atoms drive the transfer of magnetic order from spin type 1 to spin type 2, so the two magnetic moments M1 (blue arrows in field B) and M2 (green arrows) shrink by exactly the same amount (red arrows), and thus the total magnetization M1-M2 does not change. This process develops on the fast time scale being mediated by exchange interaction, which is the strongest force in magnetism. The atomic spins are forced to heat up with constant total magnetization, just like water in a closed pot with a fixed volume.

In order for the total magnetization M1-M2 to change, part of the spin (i.e. angular momentum) must be transferred to the atomic lattice. For the pot in the oven, the pressure increases until the lid is not tight anymore, so that the angular momentum can be slowly released outwards (see plate C). This also happens in the Ferrimagnet on the longer timescale, since coupling between spins and lattice is very weak.

Outlook: future applications

"We now have a clear picture of how the hot atomic lattice and the cold magnetic spins of a ferrimagnetic insulator equilibrate with each other," Radu emphasizes. The rapid transfer of energy creates a new state of matter in which the spins are hot but have not yet reduced their entire magnetic moment. This "spin overpressure" is released by much slower processes that allow the angular momentum to be transmitted to the lattice.

"Our results are also relevant for data storage applications," adds Sebastian Maehrlein. "If we want to switch between 0 and 1 in a magnetic storage medium, both angular momentum and energy must be transmitted between atomic lattice and spins." And Radu adds: "With these new insights it could be possible to drive magnetization switching phenomena at THz rates i.e. more than a thousand times faster compared to the master-clock frequencies of nowadays computers".

Zur Publikation in Science Advances (2018): Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation. S. F. Maehrlein, I. Radu, P. Maldonado, A. Paarmann, M. Gensch, A. M. Kalashnikova, R. V. Pisarev, M. Wolf, P. M. Oppeneer, J. Barker, T. Kampfrath

DOI: 10.1126/sciadv.aar5164