Scientists found a new magnetic state in YbRh2Si2, driven by nuclear magnetism at 1.5 mK, adding a new twist to the interplay between magnetism and superconductivity.

Molar heat capacity in zero field of the compound YbRh2Si2.

Experiments show that the heavy fermion compound YbRh₂Si₂ undergoes a spectacular electro-nuclear phase transition into a modulated magnetic state at a temperature as low as 1.5 mK.

In general the magnetic properties of solids are entirely determined by the electrons. In magnetic materials these electrons carry large magnetic moments μ_e, which can orient themselves below a critical temperature to form a magnetically ordered state. Depending on whether the moments are aligned  parallel to each other or in different directions, this state is known as ferromagnetic (FM), like e.g. in permanent magnets, or antiferromagnetic (AFM).

In contrast, the magnetic moment μ_I of an atomic nucleus is about a thousand times smaller than the electronic moment. Accordingly, these tiny nuclear magnetic moments usually have no influence on the macroscopic properties of a solid. But Yb is distinguished by a large hyperfine interaction which couples nuclear and electronic magnetism. And YbRh₂Si₂ is a heavy fermion compound in which local electronic moments are entangled with conduction electrons by the Kondo effect. So how might nuclear magnetism enter the game?

In a just published paper [1], an international team of scientists from the UK (Royal Holloway University of London) and Germany (Goethe University Frankfurt, MPI-CPfS Dresden) demonstrated that in YbRh₂Si₂, at T_A = 1.5 mK, the tiny nuclear moments profoundly change an electronic magnetic state formed at a much higher temperature  T_N = 70.5 mK.   This was shown by heat capacity measurements with remarkably high-resolution and within a challenging temperature range of 180 µK - 80 mK (see figure). This kind of measurements requires a precise determination of the temperature which is very difficult to achieve below 10 mK. Thus, these experiments were only possible because the scientific team developed and strongly improved over the last years a specialized measurement technique known as current sensing noise thermometry. It has set new standards for such kind of measurements.

How can the tiny nuclear magnetic moments profoundly change the state of the three orders of magnitude larger electronic moments? Previous measurements showed that in YbRh₂Si₂ the electronic order established at T_N is weak AFM, with a tiny ordered moment, a few thousandths of μ_e.  Furthermore this state is extremely fragile, being strongly influenced by an unusually weak magnetic field. Thus with increasing field a parallel electronic moment develops up to about one tenth μ_e. Ultimately at a modest field, corresponding to the tiny magnetic energy of only around 2 mK, the AFM order is fully suppressed. 

By contrast, at T_A = 1.5 mK, a new strongly antiferromagnetic electronic state appears, driven by nuclear magnetism.

The low temperature heat capacity shown in the figure arises from the nuclear moments of the two Yb isotopes, ¹⁷¹Yb and ¹⁷³Yb. The hyperfine interaction between the nuclear moments and the electrons favors states with large magnetic polarization of both the electrons and the nuclear moments. At T_A this interaction drives a nuclear assisted transition into a state in which the electronic magnetism appears to be both stronger, around one tenth μ_e, and spatially modulated. The polarization of the nuclear moments results in a spectacular signature in their heat capacity, as shown in the figure.

Nuclear magnetism is only carried by the isotopes ¹⁷¹Yb and ¹⁷³Yb, with total abundance of only 30% , while the remaining isotopes have no nuclear moment. Nevertheless the gain in magnetic polarization energy of these tiny Yb nuclear moments packs enough punch to drive the formation of a new electronic state.

In addition to the rich magnetic behaviour reported in this work, YbRh₂Si₂ exhibits unconventional superconductivity below 10 mK. The distinct magnetic states above and below T_A may host different superconducting order parameter, adding a new twist to the fundamental question of the interplay between magnetism and superconductivity. Perhaps the most interesting open question is whether YbRh₂Si₂ is a spin triplet crystalline topological superconductor, a class of materials long sought after as a companion to superfluid ³He and because of their technological significance.

[1] Electronuclear Transition into a Spatially Modulated Magnetic State in YbRh2Si2
J. Knapp, L. V. Levitin, J. Nyéki, A. F. Ho, B. Cowan, J. Saunders, M. Brando, C. Geibel, K. Kliemt, and C. Krellner
Phys. Rev. Lett. 130, 126802 (2023)

This news item can also be consulted from the Max Planck Institute's website (link 1, link 2).