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  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, P.; Schneidewind, A.; Klauss, H.H.; Inosov, D.S.: Destruction of long-range magnetic order in an external magnetic field and the associated spin dynamics in Cu₂GaBO₅ and Cu₂AlBO₅ ludwigites. Physical Review B 103 (2021), p. 024447/1-15

10.1103/physrevb.103.024447
Open Access Version

Abstract:
The quantum spin systems Cu2M'BO5 (M'=Al,Ga) with the ludwigite crystal structure consist of a structurally ordered Cu2+ sublattice in the form of three-leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu2+ and nonmagnetic Ga3+ or Al3+ ions. A microscopic analysis based on density-functional-theory calculations for Cu2GaBO5 reveals a frustrated quasi-two-dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low-temperature 11B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below TN approximation 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 Cu2GaBO5 is commensurate and lies on the Brillouin-zone boundary in the (H0L) plane, qm=(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.