A -cation control of magnetoelectric quadrupole order in A(TiO)Cu4(PO4)4

Abstract

Ferroic magnetic quadrupole order exhibiting macroscopic magnetoelectric activity is discovered in the novel compound A(TiO)Cu4(PO4)4 with A = Pb, which is in contrast with antiferroic quadrupole order observed in the isostructural compounds with A = Ba and Sr. Unlike the famous lone-pair stereochemical activity which often triggers ferroelectricity as in PbTiO3, the Pb 2+ cation in Pb(TiO)Cu4(PO4)4 is stereochemically inactive but dramatically alters specific magnetic interactions and consequently switches the quadrupole order from antiferroic to ferroic. Our firstprinciples calculations uncover a positive correlation between the degree of A-O bond covalency and a stability of the ferroic quadrupole order. I. INTORODUCTIONEarlier works demonstrated that the usage of specific elements with characteristic chemical properties is effective to realize desired ferroic order. For example, lonepair stereochemical activity of a heavier post-transition metal cation with an s 2 electron configuration such as Pb 2+ and Bi 3+ , which we call an s 2 -cation, is known as a driving force for ferroelectric order [1,2], as discussed in perovskite oxides PbTiO 3 [3], BiMnO 3 [4], and BiFeO 3 [5]. The stereochemically active s 2 -cations are surrounded by "hemidirected" local coordination, in which there is a void in the distribution of bonds to the ligands [1]. The origin for this directional bonding is explained by the hybridization between nominally empty metal p states with anti-bonding states formed by filled metal s states and ligand p states [2]. Such a hybridization is possible only when the inversion-symmetry at the cation site is broken. This is a driving force for off-center distortion and thus ferroelectric order.Another potential role of s 2 -cations is an impact on magnetism in insulating magnetic oxides. There, dominant magnetic superexchange interactions are usually mediated by O 2p orbitals near Fermi energy (E F ) [6]. As exemplified by the comparison between PbTiO 3 and BaTiO 3 [3], s 2 -cations tend to exhibit strong orbital hybridization with O ions, which should significantly affects superexchange interactions. Note that s 2 -cations are not necessarily stereochemically active; there are comparable number of compounds containing such cations located in "holodirected" local environment without a void in the * kentakimura@edu.k.u-tokyo.ac.jp ligand bond distribution [1]. In this case, substituting s 2 -cations can be a promising way of fine-tuning magnetic interactions without large distortion of the original structure.Among various ferroic orders, a particular class of magnetic order with broken space-inversion and time-reversal symmetries has recently attracted considerable interest because it can exhibit symmetry-dependent unique phenomena, such as magnetoelectric (ME) effects [7][8][9][10][11][12][13][14][15] and unconventional nonreciprocal electromagnetic responses [16][17][18]. From a symmetry point of view, it is known that ferroic order of magnetic multipole moments (toroidal, monopole, and quadrupole m...

Magnetic structural unit with convex geometry: A building block hosting an exchange-striction-driven magnetoelectric coupling

“…3(a) and 3(g), the application of B X induces a sharp peak in ε along the Y axis (ε Y ), accompanying the onset of a finite P along the same direction (P Y ). This behavior is consistent with the previous report [27], which results from the above-mentioned linear mangetoelectric effect due to the quadrupole type spin arrangement. As we will see in the following subsections, the exchange striction mechanism given by Eq.…”

“…A peculiar non-coplanar spin arrangement was recently found in the family of square cupola based antiferromagnets A(TiO)Cu 4 (PO 4 ) 4 (A = Ba, Sr, and Pb) (see, Fig. 2) [25][26][27]. The ab-plane spin component can be regarded as a magnetic quadrupole moment which is a source of linear magnetoelectric effects -linear induction of electric polarization (magnetization) by a magnetic field (electric field).…”

“…A peculiar non-coplanar spin arrangement was recently found in the family of square cupola based antiferromagnets A(TiO)Cu 4 (PO 4 ) 4 (A = Ba, Sr, and Pb) (see, Fig. 2) [25][26][27]. The ab-plane spin component can arXiv:1807.10457v1 [cond-mat.mtrl-sci]…”

Magnetic structure of the square cupola compound Ba(TiO) Cu4(PO4)4

“…Single crystals of Ba(TiO)Cu 4 (PO 4 ) 4 were grown with the flux method by Kimura et al 2,9 . The sample crystal used in the experiments sized 1.9 x 2.0 x 3.9 mm and weighed 61.84 mg. NMR measurements were conducted using the spectrometer MAGRes2000 attached to a B = 4.7 T superconducting magnet, 31 P resonance frequency 80.97 MHz.…”

The spin-1/2 coupled tetramer system Ba(TiO) Cu4(PO4)4 probed by magnetization, specific heat, and 31P NMR

“…Our fit (see Fig.4) yields J/k B ∼ 47 K and g ∼ 2.23. Although the simple tetramer model fits the χ(T ) data well even a little below the broad maximum, non-zero intralayer and interlayer interactions among Cu 2+ ions were inferred from the analysis of the inelastic neutron scattering data 4,9 . Therefore a complete analysis which includes other intra-tetramer/cupola and inter-cupola interactions is needed.…”