Ferromagnetism inCo7(TeO3)4Br6

Abstract

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound Co7(TeO3)4Br6 has been investigated by powder and single crystal neutron diffraction, magnetization and ac susceptibility techniques. Anisotropy of susceptibility extends far into the paramagnetic temperature range. A principal source of anisotropy are anisotropic properties of the involved octahedrally coordinated single Co 2+ ions, as confirmed by angular-overlap-model calculations presented in this work. Incommensurate antiferromagnetic order sets in at TN =34 K. Propagation vector is strongly temperature dependent reaching k1=(0.9458(6), 0, 0.6026(5)) at 30 K. A transition to a ferrimagnetic structure with k2=0 takes place at TC=27 K. Magnetically ordered phase is characterized by very unusual anisotropy as well: while M − H scans along b-axis reveals spectacularly rectangular but otherwise standard ferromagnetic hysteresis loops, M − H studies along other two principal axes are perfectly reversible, revealing very sharp spin flop (or spin flip) transitions, like in a standard antiferromagnet (or metamagnet). Altogether, the observed magnetic phenomenology is interpreted as an evidence of competing magnetic interactions permeating the system, first of all of the single ion anisotropy energy and the exchange interactions. Different coordinations of the Co 2+ -ions involved in the low-symmetry C2/c structure of Co7(TeO3)4Br6 render the exchange-interaction network very complex by itself. Temperature dependent changes in the magnetic structure, together with an abrupt emergence of a ferromagnetic component, are ascribed to continual spin reorientations described by a multi-component, but yet unknown, spin Hamiltonian.

Slow magnetic dynamics and hysteresis loops of a bulk ferromagnet

“…Ferromagnetic component of magnetic order of Co 7 (TeO 3 ) 4 Br 6 sets in accompanied by extremely sharp and sizable peak 13,14 in ac susceptibility. Surprisingly, the peak in imaginary susceptibility (χ ) was found, depending on the strength and frequency of the applied field, significantly bigger than the peak in real susceptibility (χ ).…”

“…The measurements were performed on single crystals of a new, cobalt-based oxi-halide compound Co 7 (TeO 3 ) 4 Br 6 13,14 . Magnetism of this system is characterized by strong magnetic anisotropy extending far into the paramagnetic temperature range.…”

“…Standard phenomenology of ferromagnetism characterizes measurements along this axis only 13 -along other axes standard phenomenology of antiferromagnetic order, like spin-flip (or flop) transition, persists below T c .…”

“…The elementary switching volume V 0 for Co 7 (TeO 3 ) 4 Br 6 can be readily estimated from E a = KV 0 , where K represents the magnetocrystalline anisotropy energy. Estimating crudely K from the Stoner-Wohlfarth approximation, H c = 2K/M , and the measured DC susceptibility values 13 for H c and M one gets V 0 = 5.5 • 10 −23 m 3 . The edge length of the corresponding cube equals 38 nm, comprising approximately 1500 Co-ions.…”

“…In this study we focus on the low-frequency, domain wall-related, magnetic dynamics of a new oxo-halide system Co 7 (TeO 3 ) 4 Br 6 13,14 . Owing to rare combination of parameters determining its magneto-dynamics, conveniently matching the dynamical hysteresis' experimental window, we show that the experimental hysteresis can be quantitatively reproduced within a simple Debyerelaxation-based model for thermally activated spin reversals.…”