LNS - Scientific Highlights

Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds and magnetic molecules to artificial metal structures formed on surfaces and metalloproteins.

Magnetic insulators have proven to be usable as quantum simulators for itinerant interacting quantum systems. In particular the compound (C₅H₁₂N)₂CuBr₄ (for short: (Hpip)₂CuBr₄) was shown to be a remarkable realization of a Tomonaga-Luttinger liquid (TLL) and allowed us to quantitatively test the TLL theory.

In geometrically and/or exchange frustrated materials spin fluctuations may endure down to lowest accessible temperatures - the phenomenon known as persistent spin dynamics. Since spin fluctuations hinder the onset of extended static correlations, persistent spin dynamics and long-range magnetic order are generally considered as mutually exclusive. Remarkably, their coexistence has been found in several frustrated magnetic systems but was lacking a suitable explanation.

Skyrmions are topologically protected magnetic spin 'whirls' that form a hexagonal 2D lattice in non-centrosymmetric magnets. Until recently, skyrmions had only been observed in itinerant metallic alloys such as MnSi, where they can also be manipulated by applied electric currents.

Spin correlations with power-law decay are usually associated with a critical point, but stable phases with power-law correlations may exist in frustrated magnets. Such phases are interesting, because they represent model materials where short-range interactions and local constraints lead to emergent symmetries and fractional quasiparticles.

The interest in BaCuSi₂O₆ is motivated by its extraordinary phase diagram with field-induced Bose-Einstein condensation. Being a quantum paramagnet at zero magnetic field down to the lowest temperatures, the system displays a quantum phase transition into a magnetically ordered state at the critical value of magnetic field of ~23.5 T.

Magnetism has been predicted to occur in systems in which dipolar interactions dominate exchange. We present neutron scattering, specific heat, and magnetic susceptibility data for LiErF₄, establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition in applied field H_c|| = 4.0 ± 0.1 kilo-oersteds.

In many heavy fermion materials the quantum critical point is masked by superconductivity and it can only be detected by use of a local probe. In the noncentrosymmetric heavy fermion CeRhSi₃ the ground state at ambient pressure is antiferromagnetically ordered and superconductivity sets in above 12 kbar coexisting with antiferromagnetism. We have unraveled a magnetic quantum critical point hidden deep inside the superconducting state of CeRhSi₃.

We report on the synthesis and characterization of multiresponsive hybrid microgel particles. The particles consist of ellipsoidal silica-coated maghemite cores subsequently coated with thermoresponsive poly (N-isopropylacrylamide) (PNIPAM) shells. The PNIPAM shell enables the hybrid particle to alter its size and ratio of long to small axis with increasing temperature while the core morphology remains unchanged.

The olivine compound Mn₂GeO₄ is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism.

We introduce a novel method for local structure determination with a spatial resolution of the order of 0.01 Å. It can be applied to materials containing clusters of exchange-coupled magnetic atoms. We use neutron spectroscopy to probe the energies of the cluster excitations which are determined by the interatomic coupling strength J.