Rich magnetoelectric phase diagrams of multiferroic single-crystal α−NaFeO2

Abstract

The magnetic and dielectric properties of the multiferroic triangular lattice magnet compound α-NaFeO 2 were studied by magnetization, specific heat, dielectric permittivity, and pyroelectric current measurements and by neutron diffraction experiments using single crystals grown by a hydrothermal synthesis method. This work produced magnetic field (in the monoclinic ab-plane, B ab , and along the c * -axis, B c ) versus temperature magnetic phase diagrams, including five and six magnetically ordered phases in B ab and along B c , respectively. In zero magnetic field, two spin-density-wave orderings with different k vectors-(0,q, 1 2 ) in phase I and (q a ,q b ,q c ) in phase II-appeared at T = 9.5 and 8.25 K, respectively. Below T = 5 K, a commensurate order with k = (0.5,0,0.5) was stabilized as the ground state in phase III. Both B ab 3 T and B c 5 T were found to induce ferroelectric phases at the lowest temperature (2 K), with an electric polarization that was not confined to any highly symmetric directions in phases IV ab (3.3 B ab 8.5 T), V ab (8.5 B ab 13.6 T), IV c (5.0 B c 8.5 T), and V c (8.5 B c 13.5 T). In phase VI c , within a narrow temperature region in B c , the polarization was confined to the ab plane. For each of the ferroelectric phases, the k vector was (q a ,q b ,q c ), and noncollinear structures were identified, including a general spiral in IV ab an ab cycloid in IV c and V c , and a proper screw in VI c , along with a triclinic 11 ′ magnetic point group allowing polarization in the general direction. Comparing the polarization direction to the magnetic structures in the ferroelectric phases, we conclude that the extended inverse Dzyaloshinskii-Moriya mechanism expressed by the orthogonal components p 1 ∝ r ij × (S i × S j ) and p 2 ∝ S i × S j can explain the polarization directions. Based on calculations incorporating exchange interactions up to fourth-nearest-neighbor (NN) couplings, we infer that competition among antiferromagnetic second NN interactions in the triangular lattice plane, as well as weak interplane antiferromagnetic interactions, are responsible for the rich phase diagrams of α-NaFeO 2 .

The effect of octahedral distortions on the electronic properties and magnetic interactions in O3 NaTMO2 compounds (TM = Ti–Ni & Zr–Pd)

“…1(c) and (d)] preventing the stabilization of the ferromagnetic phase. 9,11,32,33 If a distortion, however, breaks the symmetry and brings the TM-O-TM angle closer to 90 , ferromagnetic superexchange can prevail. One should note that, as indicated in Table 1, the octahedral distortions in these compounds basically bring the O-TM-O angle closer to 90 .…”

“…6,7 Such contradictions oentimes stem from coarse structural characterization, restricted by instruments' range and resolution, which falls short in capturing the delicate structural details that dictate the magnetic ground state in these compounds. 8,9 This study therefore presents a detailed and focused density functional insight into one important family of such layered materials, namely O3 NaTMO 2 compounds in which TM is a fourth or h row transition metal element.…”

Incommensurate atomic and magnetic modulations in the spin-frustrated β−NaMnO2 triangular lattice

“…On the other hand, on approaching the lower transition temperature at T N2 , the 1/T 1 of the paramagnetic PM component is rapidly enhanced, suggesting the onset of critical fluctuations. A phenomenological fit of the critical model 1/T 1 =A+B(T-T N2 ) -p to the PM data in the temperature range between T N2 and 110 K, yields the critical exponent p = 0.45 (10) for A = 66(5) s -1 and T N2 = 95.0(5) K (Figure 3b). Such critical enhancement demonstrates that the magnetic fluctuations that govern the transition at T N2 cannot be filtered out anymore at the Na site.…”

“…These are rock-salt derivatives of the family A + Me 3+ O 2 (A= alkali metal, Me= 3d transition metal) delafossites 5, 6 that have attracted considerable interest due to their physical and chemical properties. They include, transparent conducting oxides, such as the CuAlO 2 7 , superconductors, like the hydrated variant Na 0.3 CoO 2 •1.3H 2 O 8 of the P2-Na y CoO 2 bronzes 9 , multiferroics as AFeO 2 (A= Na, Ag) 10,11 , and cathodic materials for high-capacity Na-ion re-chargeable batteries, like P2-Na y Mn 1-x M x O 2 (x, y ≤ 1, M = Ni, Mg, Li) 12 . Such intercalation materials show high structural flexibility upon alkali metal insertion or extraction and give rise to a rich phase diagram.…”

Single-phase multiferroics: new materials, phenomena, and physics

“…These observations are similar to the measured results on Ni 3 V 2 O 8 and MWO 4 , and the existence of multiple multiferroic phases may be utilized for unusual ME memory applications [ 53 , 54 ]. A long list of other type-II multiferroics, including α-NaFeO 2 [ 55 ], Mn 2 O 3 [ 56 ], (La, Bi)Mn 3 Cr 4 O 12 [ 57 , 58 ], Ni 3 TeO 6 [ 59 , 60 ], (Fe, Mn) 2 Mo 3 O 8 [ 61–63 ], Co 4 Nb 2 O 9 [ 64 ], Mn 2 MnWO 6 [ 65 ], RFe 3 (BO 3 ) 4 [ 66–68 ], KCu 3 As 2 O 7 (OD) 3 [ 69 ], NaFeSi 2 O 6 [ 70 ], In 2 NiMnO 6 [ 71 ], and so on. Certainly, this list should not exclude materials other than oxides, and compounds with other anions such as S and Se have been revealed to show type-II multiferroicity too, including CaOFeS [ 72 ], MnSb 2 S 4 [ 73 ], BaFe 2 Se 3 [ 74 ], Cu 3 Bi(SeO 3 ) 2 O 2 Cl [ 75 ], CsCuCl 3 [ 76 ], and CuBr 2 [ 77 ].…”