Investigation of magnetic structure and phase transitions in Co₃¹¹B₇O₁₃Br boracite by neutron diffraction

Abstract

Neutron diffraction investigations of orthorhombic Co 3 11 B 7 0 13 Br at 293 K and in the temperature range 1.5-30 K revefl a two step onset of magnetic ordering, starting at the the second order transition to a weakly ferromagnetic phase at TN = 17.5 K (Nee( type) and followed at about T, = 10.5 K, by a smooth transformation to a three-dimensional canted magnetic structure (magnetic space group Pc'a2,'(Shl~'), magnetic modes Cx( + + --), F,( ++++),A,(+ --+ ). The three Co sites display completely different moment values and directions, arising from frustration effects associated with the formation of isolated Co,Br groups with a triangular arrangement of the three Co sites.

Magnetic and structural phase transitions of multiferroic boracitesM3B7O13X(M=

“…Neutron diffraction studies on Co 3 11 B 7 O 13 Br [181] revealed a two-step onset of magnetic ordering, starting at the second-order PT at T N = 17.5 K and followed below about T = 10.5 K by a smooth transformation to a threedimensionally canted magnetic structure of space group P c a2 1 . The transformation coincides with a minimum of magnetic birefringence n γ,β (T ) at ≈10 K and with a maximum of the ME coefficient α 32 at 10 K [177].…”

“…These anomalies are also reflected in other physical properties, for which we compile data in Table III. Neutron diffraction has been performed on 11 B enriched polycrystalline samples of Co-Cl [180], Co-Br [181], and Co-I [149].…”

Investigation of the crystal and magnetic structures of the trigonal multiferroic iron-boracite Fe3B7O13(OH)

“…For M = Co and X = Br, despite the presence of the expected large orbital momentum contribution, the third site exhibits a magnetic moment of 1.7(1) µ B while the other 2 show respectively a magnetic moment of 4.7(2) and 4.0(2) µ B . 14 For Mn 3 B 7 O 13 I, only one site shows a saturated magnetic moment with 5.4(2) µ B while the other two sites are reported with a magnetic moment of 3.8(2) µ B . 12 Similar results are reported for the other compositions investigated by neutron diffraction.…”

“…These results are in contrast with the reported literature on the magnetic ground state of the boracites. 8,[12][13][14][15] Fe 3 B 7 O 13 (OH) crystallizes in the trigonal space group R3c which is potentially ferroelectric but does not exhibit any weak ferromagnetic component in contrast to all the other reported boracites. 8 Even Co 3 B 7 O 13 Cl which also exhibits the trigonal symmetry R3c at room and low temperature, changes of symmetry giving rise to a weak ferromagnetic component below T N = 12 K. 25 It would be of interest to investigate the other compositions exhibit the trigonal R3c symmetry (X = OH, NO 3 for instance) at room temperature in order to investigate further whether Fe 3 B 7 O 13 (OH) is the exception to the rule or not.…”

“…11 Despite the large number of studies dedicated to this family and its wide chemistry, few studies have been dedicated to the determination of their magnetic ground states using neutron diffraction. [12][13][14][15] Recently, a new composition with M = Fe and X = OH has been reported. 16 It has been shown that this boracite crystallizes in the space group R3c (No.…”