Destruction of long-range magnetic order in Cu2GaBO5 and Cu2AlBO5 ludwigites by an external magnetic field

Abstract: The quantum spin systems M22+M′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 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 ≈ 4.1 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 of 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.

Host: D. Inosov

REC/C213 (audience capacity: 11)

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