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

Datum
24.11.2020
Zeit
14:50 - 16:20
Sprecher
Anton Kulbakov
Zugehörigkeit
Technische Universität Dresden
Serie
TUD Physik-IFMP Institutsseminar
Sprache
en
Hauptthema
Physik
Andere Themen
Physik
Beschreibung

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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