A Smooth Ferromagnetic Transition
Apply enough pressure to the right ferromagnetic material and you can reduce its Curie temperature—the temperature above which its magnetism vanishes—to nearly absolute zero. It was long thought doing that for metals would unavoidably change the ferromagnetic transition from a continuous, second-order transition into an abrupt, first-order one. After studying the problem for more than 20 years, Theodore Kirkpatrick, at the University of Maryland, College Park, and Dietrich Belitz, at the University of Oregon, Eugene, showed that this assumption doesn’t always hold. By including previously neglected spin-orbit interactions in their calculations, the researchers predict that first-order transitions are suppressed in certain metal crystal structures, and they suggest that previous experiments show hints of this behavior.
Classical phase transitions are driven by thermal fluctuations, which disappear near absolute zero. Quantum ones, on the other hand, are driven by ever-present quantum fluctuations and appear near absolute zero. An example of such fluctuations are excitations of a material’s conduction electrons, which in most metals trigger a first-order ferromagnetic transition that overtakes the second-order one. Lattice defects or magnetic fields stave off the first-order transition by interfering with these excitations. Kirkpatrick and Belitz show that strong spin-orbit coupling in crystal systems without inversion symmetry produce the same interference effect while simultaneously preventing new excitations from arising. This behavior preserves the second-order transition in defect-free metals without an external field.
Experiments with noncentrosymmetric compounds and , in which high pressure reduces the Curie temperature, have already demonstrated a second-order ferromagnetic transition down to 1 K. Kirkpatrick and Belitz say experiments at even lower temperatures, and with other noncentrosymmetric ferromagnets, are needed to confirm their prediction.
This research is published in Physical Review Letters.
–Marric Stephens
Marric Stephens is a freelance science writer based in Bristol, UK.