A new process of magnetic fragmentation
To date, the ability of an entity being two mutually exclusive states was thought to be specific to quantum mechanics with the emblematic example of the Schrödinger cat - both alive and dead. However, it is now apparent that an equally counterintuitive superposition of states, albeit of fundamentally different natures, could also form in classical contexts by collective effects.
A recent study conducted at the Institut Laue-Langevin (ILL), combining neutron scattering experiments and magnetic measurements on a frustrated magnet, discovered a new mechanism whereby the same magnetic degree of freedom is simultaneously involved into two antinomic thermodynamic states, namely a crystal and a fluid.
Spin ice was discovered in the 1990s in pyrochlore compounds containing magnetic moments distributed on a network of corner-sharing tetrahedra. Each magnetic moment is constrained by magnetocrystalline anisotropy to align along the direction joining the corner it occupies and the centres of the tetrahedra sharing this corner. With ferromagnetic interactions, two of the four magnetic moments in each tetrahedron must be oriented inward and the two others outward. This ‘2 in 2 out’ local constraint can be achieved in a number of ways that grow exponentially with the number of tetrahedra so that no order can occur. A spin ice should therefore be viewed as a fluctuating fluid of correlated moments, despite its name being inherited from a form of crystalline ice.
When a single magnetic moment is flipped, 2 out’ local constraint is broken in the two tetrahedra that share this moment, giving rise to ‘1 in 3 out’ and ‘3 in 1 out’ configurations. These excited local configurations can be separated far from each other with almost no energy cost by suitable successive flips of moments, thus providing an analogue of Dirac magnetic monopoles connected by opposite pairs through Dirac strings. They can organise themselves according to subtle balance between magnetic moment interactions to form a monopole crystal in a spin ice. This defines a novel state of matter where an Ising magnetic moment – a single degree of freedom – fragments thermodynamically into a part sustaining a solid, the monopole crystal and a part sustaining a fluid, the spin ice. However, drastic conditions are needed to achieve this in pyrochlores.
A recent multi-partner study has for the first time experimentally shown that the physical phenomenon of magnetic moment fragmentation can be induced in a very clean and simple way by an additional ingredient, a local magnetic exchange field, which acts as a so-called ‘magnetic charge injector’. This local field, which varies non-uniformly at the atomic scale, is actually produced by other magnetic moments naturally present in the structure. As they order magnetically, they create monopoles in the underlying magnetic vacuum, which leads to an almost perfectly fragmented crystal.
The study, titled ‘Fragmentation in spin ice from magnetic charge injection’ was recently published in Nature Communications - a collaboration between the ILL, Institut Néel (IN, CNRS-UGA) and the Laboratoire Léon Brillouin (LLB, CEA).
Neutron diffraction studies were conducted at the ILL on the D1B diffractometer and at LLB on the G4.1 diffractometer. Inelastic neutron scattering (INS) experiments were performed on the IN6 and IN4 neutron spectrometers at the ILL. IN6 is a cold neutron, time-focussing, time-of-flight spectrometer designed for quasielastic and inelastic scattering, and IN4 is a thermal neutron spectrometer designed to study magnetic excitations at high energies. These experiments were complemented by DC and AC magnetic measurements and confirmed the occurrence of the fragmentation phenomenon. To better understand this remarkable state, let us imagine that it corresponds to a water molecule that would participate at the same time to both liquid and solid states of water. This raises the interesting question of what pouring a glass of water would look like; will it flow like liquid or fall like ice cubes?
Emilie Lefrançois, PhD student of the ILL and Université Grenoble Alpes (UGA) and principle investigator of this study says: “The spin ice state was discovered in the 1990s and ever since it has raised a strong interest in the scientific community. The fragmented state developing on it is actually a very recent proposal. However, studying it in depth has been restricted by the inability to induce it in controlled environments thus far. Our study has found that the fragmented state can, in fact, be induced with the help of a second interpenetrating magnetic lattice, and the main impact of this is that we can now investigate and further understand the physics that underpin it.”
Dr Jacques Ollivier, ILL Scientist says: “Neutrons were decisive in this study as they allowed us to observe the magnetic order as well as the diffuse scattering, from which we could confirm the magnetic fragmented state produced here through a new mechanism. The use of neutron scattering was suitable for the given observations, including diffraction and the inelastic neutron scattering from which the iridium staggered local field could be inferred. Microscopic measurements such as this require neutrons to be observed.”
Re.: Fragmentation in spin ice from magnetic charge injection. E. Lefrançois, et al., Nature Communications (2017).
FIG. 1. Staggered eld in a pyrochlore lattice and magnetic fragmentation.
Notes for editors:
About ILL – the Institut Laue-Langevin (ILL) is an international research centre based in Grenoble, France. It has led the world in neutron-scattering science and technology for almost 40 years, since experiments began in 1972. ILL operates one of the most intense neutron sources in the world, feeding beams of neutrons to a suite of 40 high-performance instruments that are constantly upgraded. Each year 1,200 researchers from over 40 countries visit ILL to conduct research into condensed matter physics, (green) chemistry, biology, nuclear physics, and materials science. The UK, France and Germany are associates and major funders of the ILL.