Destruction of long-range magnetic order in an external magnetic field and the associated spin dynamics in Cu2GaBO5 and Cu2AlBO5 ludwigites

• Published in: arXiv.org 2020-11
• Main Authors: Kulbakov, A A, Sarkar, R, Janson, O, Dengre, S, Weinhold, T, Moshkina, E M, Portnichenko, P Y, Luetkens, H, Yokaichiya, F, Sukhanov, A S, Eremina, R M, Schlender, Ph, Schneidewind, A, H -H Klauss, Inosov, D S
• Format: Article
• Language: English
• Subjects: Aluminum compounds; Antiferromagnetism; Copper; Crossovers; Crystal structure; Inelastic scattering; Ladders; Long range order; Low temperature; Ludwigite; Magnetic fields; Muon spin relaxation; Neutron diffraction; Neutron scattering; Neutrons; NMR; Nuclear magnetic resonance; Spin dynamics; Spin glasses; Two dimensional models; Unit cell
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2011.05819

The quantum spin systems Cu\(_2\)M'BO\(_5\) (M' = Al, Ga) with the ludwigite crystal structure consist of a structurally ordered Cu\(^{2+}\) sublattice in the form of three-leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu\(^{2+}\) and nonmagnetic Ga\(^{3+}\) or Al\(^{3+}\) ions. A microscopic analysis based on density-functional-theory calculations for Cu\(_2\)GaBO\(_5\) reveals a frustrated quasi-two-dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low-temperature \(^{11}\)B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below \(T_\text{N}\) \(\approx\) 4.1 K and ~2.4 K for the Ga and Al compounds, respectively. From neutron diffraction, we find that the magnetic propagation vector in Cu\(_2\)GaBO\(_5\) is commensurate and lies on the Brillouin-zone boundary in the (H0L) plane, \(\mathbf{q}_\text{m}\) = (0.45 0 -0.7), corresponding to a complex noncollinear long-range ordered structure with a large magnetic unit cell. Muon spin relaxation is monotonic, consisting of a fast static component typical for complex noncollinear spin systems and a slow dynamic component originating from the relaxation on low-energy spin fluctuations. Gapless spin dynamics in the form of a diffuse quasielastic peak is also evidenced by inelastic neutron scattering. Most remarkably, application of a magnetic field above 1 T destroys the static long-range order, which is manifested in the gradual broadening of the magnetic Bragg peaks. We argue that such a crossover from a magnetically long-range ordered state to a spin-glass regime may result from orphan spins on the structurally disordered magnetic sublattice, which are polarized in magnetic field and thus act as a tuning knob for field-controlled magnetic disorder.