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

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 disorde