Magnetic structure in iron borates RFe₃(BO₃)₄(R = Y,Ho): a neutron diffraction and magnetization study

Abstract

The layered perovskites RCr(BO3)2 (R = Y and Ho) with magnetic triangular lattices were studied by performing ac/dc susceptibility, specific heat, elastic and inelastic neutron scattering, and dielectric constant measurements. The results show (i) both samples' Cr 3+ spins order in a canted antiferromagnetic structure with TN around 8-9 K, while the Ho 3+ ions do not order down to T = 1.5 K in HoCr(BO3)2; (ii) when a critical magnetic field HC around 2-3 T is applied below TN , the Cr 3+ spins in the Y-compound and both the Cr 3+ and Ho 3+ spins in the Ho-compound order in a ferromagnetic state; (iii) both samples exhibit dielectric constant anomalies around the transition temperature and critical field, but the Ho-compound displays a much stronger magnetodielectric response. We speculate that this is due to the magnetostriction which depends on both of the Cr 3+ and the Ho 3+ ions' ordering in the Ho-compound. Moreover, by using linear spin wave theory to simulate the inelastic neutron scattering data, we estimated the Y-compound's intralayer and interlayer exchange strengths as ferromagnetic J1 =-0.12 meV and antiferromagnetic J2 = 0.014 meV, respectively. The competition between different kinds of superexchange interactions results in the ferromagnetic intralayer interaction.

Ho and Fe magnetic ordering in multiferroic HoFe3(BO3)4

“…12, normalized to their saturation values at 5 K. The 50%-50% ratio agrees with saturation magnetization values of about 5 μ B for both magnetic sublattices. 12 The simulated intensity is in agreement with the experimental data down to 22.5 K (see Fig. 4).…”

“…The small ab component of the Ho moments below T SR in neutron diffraction measurements has been determined from an additional magnetic peak appearing below T SR . 12 We were unable to measure this reflection at the Ho L 3 resonance due to beam heating, and it was too weak to be detected by HEXMS. At this point we can conclude that the ordering of the Fe moments in the ab plane is probed through nonresonant magnetic x-ray scattering, and the observation of zero polarization 8 below T SR is due to nearly coinciding moment directions of Fe and Ho.…”

“…4, means that all Fe as well as Ho moments undergo a spin reorientation, easy plane to easy axis, below T SR . We would like to mention that in neutron diffraction, 12 below T SR , Ho moments are observed as oriented by 60 • with respect to each other along the c axis, instead. The small ab component of the Ho moments below T SR in neutron diffraction measurements has been determined from an additional magnetic peak appearing below T SR .…”

“…2,3 However, recent observations of improper ferroelectricity, 4,5 anomalous even within the realm of magnetoelectricity, require an even better understanding of the subject. Rare earth iron borates RFe 3 (BO 3 ) 4 , where R = rare earth, [6][7][8][9] are multiferroics, characterized by long-range magnetic wave vectors q c*, [10][11][12][13][14][15] and show a large magnetoelectric effect ( P ∼ 100 μC m −2 ). The RFe 3 (BO 3 ) 4 crystal structure 16,17 allows a dominant Fe-Fe exchange interaction, which is reflected by a T N in a narrow temperature range (30-40 K) for compounds with different R. The rare earth exchange interaction takes place via O and B, i.e., R-O-B-O-R.…”

“…To understand the magnetic structure of these compounds neutron powder diffraction on R = Y, Pr, Nd, Tb, Ho, and Er, [12][13][14][15] resonant magnetic x-ray scattering on R = Gd, 11 and nonresonant magnetic x-ray scattering on R = Nd, Gd, Tb, and Y 10 have been performed. From neutron powder diffraction it has not been possible to determine the spin direction in the basal plane, since these compounds have a trigonal crystal structure.…”