Augsburg/FL/MH – Breaking a magnet into two pieces, reveals a north and a south pole in each of them. Independent magnetic monopoles have been known as emergent excitations in one special class of magnetic crystals only. Researchers of the University of Augsburg together with international collaboration partners found a new flavor of magnetic monopoles in a material which is even electrically conducting. This is the first realization of a state called "kagome spin ice".

Crystals have an ordered arrangement of atoms. Ice, i.e., frozen water, represents an exception. Here, the hydrogen atoms can occupy different orientations with one condition: they have to fulfill the “ice-rule”. Accordingly, each oxygen atom must have two strong and two weak bonds to its four nearest neighbor hydrogen atoms. The ice-rule restricts the orientational degrees of freedom for hydrogen only partially, but not fully.

“Systems with such constraint degrees of freedom can show interesting novel properties”, says Prof. Dr. Philipp Gegenwart from the University of Augsburg. One example is “Kagome spin ice” which up to now has only been a theoretical concept.

Kagome denotes a braided structure, realized in typical Japanese baskets. The Kagome lattice features corner-shared triangles. Placing magnetic atoms with magnetic moment, called spin, on such a lattice can lead to interesting physics. The reason is, that the triangular structure does not allow the ordinary antiparallel arrangement of nearest neighbors. In “Kagome spin-ice” the spins have to point inwards or outwards the center of each triangle and must fulfill a special constraint: either two spins point in and one out of the triangular center, or vice versa. The name “spin-ice” is used in analogy to the configurational constraint for hydrogen atoms in frozen water.

Kagome spin ice shows a unique state that can be described as if there were no spins at each triangular corner but rather one individual new object, called magnetic monopole at the triangular center. A magnetic monopole represents an independent magnetic south or north pole, analogous to an electric charge. “The expression ‘as if’ indicates, that this monopole description is a model only”, emphasizes Philipp Gegenwart. “Instead of being true elementary particles, these monopoles can be named as ‘quasiparticles’. Here, they are used as concept to simplify the description of the complex magnetic state in Kagome spin ice”.

More than 10 years ago, first indication for such monopole quasiparticles was obtained in one special material class, featuring tetrahedral magnetic units. These materials were electrical insulators. Initiated by Dr. Kan Zhao at the University of Augsburg, an international collaboration now found a new setting, namely the first realization of Kagome spin ice. In their study, published in Science, the intermetallic compound HoAgGe was investigated. This material contains spins on a Kagome-like lattice.

The researchers studied this substance at different temperatures in the presence of a magnetic field. Dependent on its strength, different spin configurations were realized, which all obey the Kagome spin-ice rule. The experiments were accompanied by theoretical simulations. This allowed the researchers not only to determine the magnetic interactions in HoAgGe, but also to uncover fine details in contrast to the current theory for Kagome spin ice. Furthermore, the material, conducts electrical current, i.e., it is a metal. This will allow to study the interaction between electrical charges and magnetic monopoles. “Such interaction can enable new magnetoelectric effects, perhaps even with application potential”, speculates Philipp Gegenwart.

The work was partially supported by the German Science Foundation (DFG) through the collaborative research center TRR 80 (“From electronic correlations to functionality”, spokesperson: Prof. Dr. Philipp Gegenwart) and the priority program SPP 1666.

Original publication:
Kan Zhao, Hao Deng, Hua Chen, Kate A. Ross, Vaclav Petříček, Gerrit Guenther, Margarita Russina, Vladimir Hutanu und Philipp Gegenwart: Realization of the kagome spin ice state in a frustrated intermetallic. Science, DOI: 10.1126/science.aaw1666
https://science.sciencemag.org/content/367/6483/1218