HZB scientists have identified a mechanism with which it may be possible to develop a form of magnetic storage that is faster and more energy-efficient. They compared how different forms of magnetic ordering in the rare-earth metal named dysprosium react to a short laser pulse. They discovered that the magnetic orientation can be altered much faster and with considerably less energy if the magnetic moments of the individual atoms do not all point in the same direction (ferromagnetism), but instead point are rotated against each other (anti-ferromagnetism). The study was published in Physical Review letters on 6. November 2017 and on the cover of the print edition.
Dysprosium is not only the atomic element with the strongest magnetic moments, but it also possesses another interesting property: its magnetic moments point either all the same direction (ferromagnetism) or are tilted against each other, depending on the temperature. This makes it possible to investigate in the very same sample how differently oriented magnetic moments behave when they are excited by an external energy pulse.
Magnetic-order perturbation examined at BESSY II
Physicist Dr. Nele Thielemann-Kühn and her colleagues have now investigated this problem at BESSY II. The BESSY II X-ray source is one of the few facilities worldwide that enables processes as fast as magnetic-order perturbations to be observed. Her finding: the magnetic orientation in antiferromagnetic dysprosium can be much more easily toggled using a short laser pulse than in ferromagnetic dysprosium.
“This is because the magnetic moments at the atomic level are coupled to angular momenta like that of a gyroscope”, explains Thielemann-Kühn. Tipping a rotating gyroscope requires force because its angular momentum must be transferred to another body. “Albert Einstein and Wander Johannes de Haas showed in a famous experiment back in 1915 that when the magnetisation of a suspended bar of iron changes, the bar begins to rotate because the angular momenta of the atomic-level magnets in the suspended bar are transferred to it as a whole. If the atomic-level magnetic momenta are already pointing in different directions initially, their angular momenta can interact with one another and cancel each other out, just as if you were to combine two gyroscopes rotating in opposite directions”, clarifies Dr. Christian Schüssler-Langeheine, head of the group.
Antiferromagnetic order is perturbed faster
The transfer of angular momentum takes time, though. Antiferromagnetic order, for which this transfer is not required, should therefore be able to be perturbed faster than ferromagnetic order. The empirical evidence for this conjecture has now been delivered in this study by Thielemann-Kühn and her colleagues. Moreover, the team also discovered that the energy needed in the case of the antiferromagnetic momenta is considerably lower than in the case of ferromagnetic order.
From this observation, the scientists have been able to suggest how materials could be developed with a combination of ferromagnetic and antiferromagnetic aligned spins that are suitable as magnetic storage media and might be switched with considerably lower energy expenditure than material made from conventional magnets.
SOURCE: Helmholtz-Zentrum Berlin (HZB)