Room-temperature ferromagnetism of nickel at multimegabar pressure

28-01-2014

The extension of nuclear resonance scattering techniques to high energies has made it possible to follow the magnetic properties of nickel metal at pressures up to 260 GPa. This is the highest pressure at which magnetism has been observed in any material. Nickel plays a key role in an astonishingly wide range of fields in science, spanning biology, microelectronics and Earth sciences. Knowledge of the origin of its magnetism, as well as the interplay of pressure and its magnetic and electronic structure, is of great importance for the basic understanding and future technological applications.

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It is generally observed that high pressure leads to a suppression of the magnetism in matter. The critical pressure where a material becomes non-magnetic is of importance for the fundamental understanding of the origin of the magnetism. It is known that other simple 3d-transition metals such as Fe and Co become non-magnetic below 20 and 200 GPa, respectively [1]. However, in these cases, the collapse of ferromagnetism is associated with structural transitions. In Ni, on the other hand, there is no structural transition at least up to 200 GPa [2]. Thus, Ni is a good candidate for the study of the effect of pressure on the magnetic properties.

The development of nuclear forward scattering (NFS) for the 67.4 keV nuclear transition in 61Ni at ID18, the nuclear resonance beamline, has opened the way for the study of high pressure magnetism in Ni [3]. This technique overcomes several technical and fundamental problems that exist in other spectroscopy techniques such as Mössbauer (low brilliance and short life time of the gamma-ray source) and XMCD (sensitivity to only ferromagnetism, weak XMCD signal for Ni).

In this work, the magnetic hyperfine splitting of Ni metal has been observed by NFS at room temperature for pressure values up to 260 GPa (see Figure 1), which confirms that Ni stays ferromagnetic up to this pressure, the highest pressure where magnetism has been observed so far in any material. The peculiar pressure dependence of the magnetic hyperfine field observed was explained by relativistic ab initio calculations.

 

Time evolution of the nuclear forward scattering for Ni measured at room temperature and various pressures

Figure 1. Time evolution of the nuclear forward scattering for Ni measured at room temperature and various pressures. The solid lines show a fit. The period of oscillations of the signal is inversely proportional to the magnetic hyperfine splitting.

The efficiency of the technique benefits a lot from the application of high pressure because of the hardening of the material and, consequently, the huge increase of the Lamb-Mössbauer factor.  Thus, the highest pressure observed in this work is not limited by the technique but by the diamond anvil cells. With a recent breakthrough in high pressure technology [4], the technique can be applied to study Ni and Ni containing compounds at pressures above 500 GPa.

The measured magnetic hyperfine field has a peculiar pressure dependence as shown in Figure 2. It increases up to the maximum between 100 - 225 GPa and slightly decreases at higher pressure. This result seems to be in contradiction to the observed [2] and theoretically predicted continuous decrease of the magnetic moment, which is supposed to be proportional to the magnetic hyperfine field. The theoretical explanation of this behaviour, however, shows the importance of relativistic effects. The hyperfine field consists of two main contributions of different sign: a negative contribution due to the Fermi contact interaction and a positive contribution due to the induced orbital magnetic moment of the 3d electrons. The magnitude of both contributions decreases with pressure, similar to the magnetic moment. Their balance, however, which determines the total hyperfine field, increases up to 100-180 GPa reproducing well the experimentally observed behaviour.

 

Pressure dependence of the magnetic hyperfine field in Ni from the experiment and from fully relativistic ab initio calculations.

Figure 2. Pressure dependence of the magnetic hyperfine field in Ni from the experiment and from fully relativistic ab initio calculations. Positive and negative contributions to the calculated field are shown as well as scalar relativistic calculations (mainly due to the Fermi contact interaction) and 3d orbital contribution (inset).

This work is of importance for two reasons. Firstly, the breakthrough in the experimental technique enables the study of magnetism in Ni containing compounds at high and ultra-high pressures. Secondly, the so far unexplored interplay of pressure, magnetism and electronic structure is important for the fundamental understanding of condensed matter.

 

Principal publication and authors
Hyperfine splitting and room-temperature ferromagnetism of Ni at multimegabar pressure, I. Sergueev (a), L. Dubrovinsky (b), M. Ekholm (c), O. Yu. Vekilova (d), A.I. Chumakov (e), M. Zajac (e), V. Potapkin (e,b), I. Kantor (e), S. Bornemann (f), H. Ebert (f), S.I. Simak (d), I.A. Abrikosov (d), R. Rüffer (e), Physical Review Letters 111, 157601 (2013).
(a) Deutsches Elektronen-Synchrotron, Hamburg (Germany)
(b) Bayerisches Geoinstitut, Universität Bayreuth (Germany)
(c) Swedish e-Science Research Centre (SeRC), Department of Physics, Chemistry and Biology (IFM), Linköping University (Sweden)
(d) Department of Physics, Chemistry and Biology (IFM), Linköping University (Sweden)
(e) ESRF
(f) Department of Chemistry, Ludwig-Maximilians-Universität München (Germany)

 

References
[1] R. Torchio et al., Phys. Rev. B 84, 060403 (2011).
[2] R. Torchio et al., Phys. Rev. Lett. 107, 237202 (2011).
[3] I. Sergueev et al., Phys. Rev. Lett. 99, 097601 (2007).
[4] L. Dubrovinsky et al., Nature Communications, 1163 (2012).

 

Top image: Nuclear forward scattering for nickel at multi-megabar pressure.