Investigating the Magnetic Properties of Frustrated Spin Systems: Square and Breathing Pyrochlore Lattices

Abstract

Frustrated magnets have long served as a fertile ground for exploring unconventionalground states in magnetic systems, such as the elusive quantum spin liquid phase. This work extends the frontiers of frustrated magnetism through the investigation of new materials with distinct anisotropic frustrated lattices, advancing both experimental and theoretical understanding. A central focus is the Shastry-Sutherland lattice (SSL), a twodimensional orthogonal dimer network that hosts strong frustration due to competing interactions, realized in rare-earth melilite compounds RE2Be2GeO7 (RE = Er, Tm, Tb), which exhibit strong spin anisotropy. Additionally, we explore the breathing pyrochlore lattice, a three-dimensional network of corner-sharing tetrahedra with alternating bond strengths (expanded and contracted tetrahedra), exemplified by the compound Ba3Tm2Zn5O11. These systems host remarkable emergent phenomena, including unconventional magnetization plateaus, exotic magnetic structures, and intriguing low-temperature thermodynamic properties with potential applications in magnetocaloric cooling (MCE) and adiabatic demagnetization refrigeration (ADR). A key achievement of this research is the successful synthesis of single crystals—essential for probing anisotropic magnetic systems. We report the first high-quality single crystals of these materials, grown via solid-state reactions for polycrystalline precursors followed by the floating-zone technique. This breakthrough enabled precise magnetic characterization under oriented applied fields. Comprehensive experimental studies combining in-house SQUID magnetometry, Physical Property Measurement System (PPMS) analyses, neutron scattering, and x-ray diffraction at national laboratory facilities revealed detailed insights into their crystal structures, magnetic orders, low-energy excitations, and crystal electric field (CEF) energy levels. For Ba3Tm2Zn5O11, neutron scattering uncovered dispersionless crystal electric field (CEF) excitations and a single-ion singlet ground state. Inelastic neutron scattering further revealed low-energy modes at 0.8meV and 1meV pointing to novel magnetic dynamics in this breathing pyrochlore system. The Er2Be2GeO7 Shastry-Sutherland compound hosts a zero-field anisotropic Ising-dimer antiferromagnetic state. Magnetization measurements reveal fractional plateaus at 14 and 12 of saturation, deviating from the Ising SSL model’s predicted 13 plateau. This discrepancy motivated the development of an Anisotropic Shastry- Sutherland Lattice Ising Model (ASSLIM) incorporating spatially anisotropic interactions, supported by single-crystal neutron diffraction observations of orthorhombic distortion. Tb2Be2GeO7exhibits in-plane single-ion anisotropy, manifesting a magnetization plateau at M Ms = 12 , thereby enriching the understanding of fractional plateaus in rare-earth Shastry- Sutherland systems. Lastly, Tm2Be2GeO7 shows no long-range magnetic ordering down to 55mK, positioning it as a promising candidate for ADR applications.