op, centre: While the magnetization of gadolinium (red arrow) has not yet changed, the magnetization of iron (blue arrow) has already reversed. Large picture: The laser pulse (pink) triggers magnetic reversal, while the X-ray pulse (blue) measures it. (Credit: HZB/Radu)
A newly discovered magnetic phenomenon could accelerate data storage by several orders of magnitude. The researchers have not only proven that magnetic reversal can take place in femtosecond timeframes, they have also derived a concrete technical application from it: “Translated to magnetic data storage, this would signify a read/write rate in the terahertz range. That would be around 1000 times faster than present-day commercial computers,” says Radu.
The physical limit to the recording speed of magnetic storage media has remained largely unresearched. In experiments performed on the particle accelerator BESSY II of Helmholtz-Zentrum Berlin, Dutch researchers have now achieved ultrafast magnetic reversal and discovered a surprising phenomenon.
In magnetic memory, data is encoded by reversing the magnetization of tiny points. Such memory works using the so-called magnetic moments of atoms, which can be in either “parallel” or “antiparallel” alignment in the storage medium to represent to “0” and “1”.
The alignment is determined by a quantum mechanical effect called “exchange interaction”. This is the strongest and therefore the fastest “force” in magnetism. It takes less than a hundred femtoseconds to restore magnetic order if it has been disturbed. One femtosecond is a millionth of a billionth of a second. Ilie Radu and his colleagues have now studied the hitherto unknown behaviour of magnetic alignment before the exchange interaction kicks in.
The researchers needed an ultra-short laser pulse to heat the material and thus induce magnetic reversal. They also needed an equally short X-ray pulse to observe how the magnetization changed. This unique combination of a femtosecond laser and circular polarized, femtosecond X-ray light is available in one place in the world: at the synchrotron radiation source BESSY II in Berlin, Germany.
In their experiment, the scientists studied an alloy of gadolinium, iron and cobalt (GdFeCo), in which the magnetic moments naturally align antiparallel. They fired a laser pulse lasting 60 femtoseconds at the GdFeCo and observed the reversal using the circular-polarized X-ray light, which also allowed them to distinguish the individual elements. What they observed came as a complete surprise: The Fe atoms already reversed their magnetization after 300 femtoseconds while the Gd atoms required five times as long to do so. That means the atoms were all briefly in parallel alignment, making the material strongly magnetized. “This is as strange as finding the north pole of a magnet reversing slower than the south pole,” says Ilie Radu.
Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10–100 femtoseconds), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.