PHYSICAL REVIEW B 99, 184444 (2019) Magnetic inversion symmetry breaking and spin reorientation in Tb2MnNiO6: A polar strong ferromagnet

Jose Luis García-Muñoz,1,* Javier Blasco,2 Xiaodong Zhang,1 and Oscar Fabelo3 1Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus Universitario de Bellaterra, E-08193 Bellaterra, Spain 2Instituto de Ciencia de Materiales de Aragón, Departamento de Física de la Materia Condensada, Consejo Superior de Investigaciones Científicas (CSIC)—Universidad de Zaragoza, E-50009 Zaragoza, Spain 3ILL-Institut Laue Langevin, 71, Avenue des Martyrs, 38042 Grenoble Cedex, France

We report a description and comprehensive study on four successive magnetic transitions in ferromagnetic Tb 2 MnNiO 6 double perovskite. In the ground state (P2 1 ), the moments of magnetic A and B sites order according to different nonpolar magnetic modes, but the coupling between them generates an overall polar symmetry which makes this oxide potentially multiferroic due to magnetic trilinear coupling, and therefore ferromagnetic and ferroelectric in its ground state. Its macroscopic magnetization is large (5 μ B /f.u) and not related to a weak ferromagnetic component induced by Dzyaloshinskii-Moriya interaction. However, a sharp and severe spin reorientation of the ferromagnetic transition-metal moments has been observed which opens the door to the magnetic switching of the ferroelectric state in this perovskite, and conversely to the control of the magnetization direction by electrical fields applied parallel to b. We also anticipate that in this material the direction of the magnetization (towards c/a) could be used as the key to switch the polar/nonpolar (ferroelectric/antiferroelectric) transformation.

I. INTRODUCTION
Unlike the proper ferroelectrics, in improper ferroelectrics the polarization (P) does not arises from the condensation of a polar lattice distortion associated to a zone-center instability but it is a secondary effect induced by some other primary distortion D that introduces an odd coupling of the polar and nonpolar modes (free-energy term of the form F = αPD n ). If n is odd the polarization can switch by inverting the distortion mode D. Let us recall first that the appearance of ferroelectricity induced by antiferromagnetic transitions, which double the unit cell, has attracted attention for a long time [1]. Like other effects described by tensors which are invariants under time reversal, it was predicted to appear accompanying the reduction of symmetry associated with magnetic transitions. Therefore, it is known that in materials such as perovskites, the presence of magnetic atoms at special Wyckoff positions can favor a polar magnetic symmetry [2]. For example, in compounds with nonsymmorphic space groups, if the magnetic atoms sit at special Wyckoff positions (e.g., with pointgroup symmetry −1, as the monoclinic R 2 MnMO 6 [M: Ni,Co] series), some propagation vectors at the zone boundary of the paraelectric phase favor polar symmetries and the occurrence of type II multiferroicity. An example is the collinear E phase in monoclinic P2 1 /n double perovskites (DPs) presenting upup-down-down (↑↑↓↓) spin chains along a particular crystallographic direction (e.g., R 2 MnCoO 6 , R = Yb, Lu [3]). The presence of a magnetic propagation vector k = (0, 0, 1/2) implies that the glide plane is necessarily broken. The system can either keep the twofold screw rotation axis or the inversion * garcia.munoz@icmab.es center, but not both. The propagation vector hosts the antitranslation {1 |0 0 1} in the parent setting [(1 |0,0,1/2) in the doubled magnetic cell], and the presence of opposite spins in contiguous parent unit cells is not compatible with the glide plane. The pristine point symmetry −1 at the 2c and 2d sites (B, B ) occupied by transition-metal atoms (M) is lost as the E-type ordering requires both −11 and −1 symmetry, depending on the considered spin.
However, this effective mechanism favoring the symmetry break into polar symmetries cannot be generalized to the cases in which the primary magnetic mode or cooperative atomic displacement R occurs at the point k = 0 (zone center) in the reciprocal space of the paraelectric phase, without an enlargement of the unit cell. In this case, the absence of changes in the translational symmetry precludes an antitranslation of the type {1 |0 0 n} (n: integer, in the parent setting) able to break the point symmetry −1. In addition, it is worth remembering that conversely the cases with k = 0 can be more favorable to the presence of ferromagnetic (FM) configurations.
Secondly, the presence of magnetic atoms at the A site (R = Tb in the present case) and the coupling between the A and B spin subsystems in ABO 3 like perovskites and their derived structures are cause for a variety of exciting effects [4]. In RMO 3 compounds, the transition-metal spins order first because they interact stronger, although the spins of rare-earth ions can be much more anisotropic. The interactions between transition-metal and rare-earth spins are attracting interest due to the their important effects in multiferroic and magnetoelectric materials [5]. In DyFeO 3 a strong linear magnetoelectric response activates due to the interplay between the ordered spins of Fe and Dy ions [6]. In GdFeO 3 orthoferrite the polar distortion producing spontaneous polarization results from the combination of the rotational Pbnm distortion and the simultaneous G-AFM magnetic order of the independent Gd and Fe spin sublattices [4]. In the multiferroic TbMnO 3 the coupling between the Mn spins produces a spin-density wave and later a spiral ferroelectric state. The ordering of Ising-like Tb spins below ∼7 K produces a mutual locking of the Mn and Tb spins and leads to a significant enhancement of the electric polarization induced by the spiral [7][8][9].
In this work, we report a description and comprehensive study on four successive magnetic transitions in the ferromagnetic Tb 2 MnNiO 6 DP driven by zone-center modes of the parent paramagnetic phase. In the ground state, the moments of magnetic A and B sites order according to different nonpolar magnetic modes. However, the coupling between them generates an overall polar symmetry which makes this oxide potentially multiferroic due to magnetic trilinear coupling, and therefore a ferromagnetic ferroelectric in its ground state. A sharp spin reorientation of the ferromagnetic transitionmetal moments has been observed which opens the door to the magnetic switching of the ferroelectric state in this perovskite.

II. EXPERIMENTAL SECTION
The Tb 2 MnNiO 6 double perovskite was prepared by the conventional solid-state reaction method. Stoichiometric amounts of Tb 4 O 7 , NiO, and Mn 2 O 3 with nominal purities not less than 99.9% were mixed, ground, and heated for 15 h at 900°C. They were then ground, pressed into pellets, and sintered several times in air at intermediate temperatures. The final sintering was done at 1250°C for 24 h, followed by very slow cooling (0.2°C/min) down to 300°C in order to improve the cationic ordering at the perovskite B site and the oxygen stoichiometry [10,11].
X-ray diffraction patterns were collected at room temperature (RT) using a Siemens D-5000 diffractometer and Cu Kα radiation, and were used to verify that our specimen was single phase with the expected peaks for a perovskite phase.
The magnetic response to dc and ac magnetic fields was measured using a superconducting quantum interferometer device (SQUID) and a Physical Properties Measuring System (PPMS) from Quantum Design. The ac magnetization was measured at the frequency value of ν = 333 Hz and 10 Oe of amplitude.

III. RESULTS AND DISCUSSION
For determining the proportion of antisites (misplaced cations) high-resolution neutron diffraction was used, confirming a nearly perfect Mn/Ni ordered superstructure. In the Supplemental Material [17], Fig. S1 shows the refinement of Magnetic and neutron measurements disclose the presence of several magnetic transitions at low temperature. The susceptibility of Tb 2 MnNiO 6 deviates from a Curie-Weiss evolution below ∼200 K (not shown), and the presence of ferromagnetism is easily identified below T C1 ∼ 120 K in the magnetization. Figure 1(a) displays the temperature dependence of dc magnetization under a magnetic field of 1 kOe in zero-field-cooled (ZFC) and field-cooled (FC) conditions. These measurements reveal a tiny ZFC-FC magnetic irreversibility, in which the two branches meet and match one over the other below ∼115 K, coinciding with the ferromagneticlike transition at T C1 . This ferromagnetic transition temperature is higher than in the homologous Tb 2 MnCoO 6 compound [18]. Compared to the dc magnetization, the sensibility to the magnetic behavior is higher in the ac magnetic response displayed in Fig. 1(b). Both the in-phase and out-ofphase signals are shown in this figure, which reveal a more complex scenario of magnetic phase transitions beyond the simple ferromagnetic ordering of Mn/Ni spins. In addition to the first maximum peaked at 110 K [ Fig. 1(b)], the in-phase ac susceptibility (χ ) displays a second (very broad) maximum that in the out-of-phase component (χ ) extends over the 15-90 K range and it is peaked around 50 K. The presence on several kinks below T C1 foresees probable successive reorganizations of the spin ordering, as has been confirmed by neutron diffraction. Figure 2 plots the evolution of the integrated intensity of four selected magnetic reflections between 1.5 and 120 K obtained after normalizing neutron data collected on D1B (λ = 2.52 Å) warming the sample. In Fig. 3 we plot a T-2θ projection of the temperature dependence for the neutrondiffracted intensities around four characteristic magnetic reflections [(001) ± k Tb , (100), (110), and (112)]. k Tb in Figs. 2 and 3 reflects a very slight modulation (0.063 [1], 0.069 [1], 0.0387 [2]) associated to defects in the arrangement of Tb moments (more details are given in the last paragraph of the current Sec. III). Figure 3 (top) shows the evolution in the high-temperature range (120-5 K), and in Fig. 3 (bottom) we show the corresponding evolution in the extended lowtemperature range, between 1.5 and 12 K.
In order to describe the magnetic evolution one can distinguish two regimes: (i) the high-temperature one, in the 20-150 K temperature range, mainly governed by the magnetic metals at the B sites, and (ii) the changes occurring in the low-temperature range, below 15 K, where we expect the decisive intervention of the terbium moments. As we will show below, this almost perfectly ordered DP displays up to four different magnetic regimes. To explore possible symmetry breaks we used the tools freely available online at the Bilbao Crystallographic Server ([19]; also see [13][14][15]).

A. Evolution of the ferromagnetism in B/B sites down to 15 K
FM1 phase. The structural description in the P2 1 /n setting is displayed in the Supplemental Material [17] (see Table  SI). The corresponding standard setting of this space group (SG) (No. 14) is P2 1 /c, so that our nonstandard description of the parent space group is given in the nonconventional setting (a, b, a + c; 0, 0, 0). After examining possible magnetic space groups (MSGs) compatible with the gray group P2 1 /c1 that keep unchanged the dimensions of the magnetic cell (k = 0), the magnetic intensities can be well reproduced by the maximal subgroup P2 1 /c (No. 14.79 in BNS notation [20]). The magnetic ordering follows the irreducible representation (irrep) mGM 2 + , compatible with a magnetic arrangement of the F x C y F z type in Bertaut's notation [21]. Figure 4 displays the refinement of the neutron pattern collected at 85 K, from which the ordered Mn moment FM2 phase. The FM1 order is stable within the [T C1 , T C2 ] = [120 K, 70 K] temperature interval. Below ∼70 K neutron patterns evidence a gain of magnetic intensity in reflections of the type (0kl) and (hhl) with l = 0 even, consistent with the strengthening of the F x component as temperature is further reduced. In Fig. 3(c), top, the FM intensity in the (110) peak develops below T C1 ≈ 120 K but reaches a maximum around 70 K, coinciding with the appearance of a second maximum in the ac susceptibility, and then systematically decreases. This evolution illustrates first the emergence of the F z component of the FM1 phase, but secondly, it shows that below ∼70 K there is a progressive and systematic transference of intensity from F z to F x . In this way, the aforementioned maximum in the (110) magnetic peak [red color in Fig. 3(c), top] signals the entering of the rotating FM2 phase. As the main feature of the FM2 regime, the FM moments progressively rotate towards the a axis upon cooling, causing a marked decrease of the (110) Figure 5 plots the evolution of the (110) and (002) neutron magnetic intensities originated by, respectively, the F z and F x components, illustrating the strengthening of (002) when the former starts decreasing.
In this regime the single active mGM 2 + irrep is preserved and the neutron patterns were correctly reproduced by considering the P2 1 /c MSG. Experimentally the y component is negligible at the B/B sites. Mn and Ni exhibit a collinear order with their spins making an angle θ with the c axis. The evolution of the tilting angle θ down to 1.5 K is plotted in Fig. 6, where the evolution of the refined magnetic moments is depicted as well. In Fig. 4(b) we display the neutron pattern refined at 15 K and the extracted magnetic structure is shown in Table I. The tilting angle of the FM moments strongly depends on temperature below 60 K: At 15 K we found θ = 46(2) • . A schematic view of the evolution of the ferromagnetic structure and the anisotropic magnetization is shown as  and the c axis. The evolution of the angle ϕ Tb that Tb moments form with the a axis in the ab plane is also shown (blue unfilled triangles). In the inset we show the schematic view of the P2 1 /c magnetic order in FM1 (above 80 K, with θ small and constant) and FM2 phases. In the rotating FM2 phase the inclination angle (θ) systematically increases due to a significant ferromagnetic Tb-(Mn,Ni) interaction, which attracts metal moments towards the ab plane. The evolution of the canting in this phase is indicated in its corresponding figure. insets in Fig. 6(b). Here we stress the fact that the Tb sublattice plays a key role on the evolution followed by the tilting angle (θ ) of the magnetization. Figure 7 discloses the emergence of significant short-range ferromagnetic correlations between Tb moments, which are clearly visible below around 70 K, coinciding with the appearance of the rotating FM2 phase, the apparent augmentation of (002) magnetic intensity, and diminution of the (110) signal. So, these observations suggest and probably bear out that a ferromagnetic Tb-(Mn,Ni) interaction (J Tb-M > 0) is at the origin of the continuous rotation of the magnetization below T C2 ≈ 70 K. This interaction acts by attracting the metal moments towards the ab plane. Remarkably, as Fig. 6(b) discloses, the magnetization rotates ∼50°in the short interval 5-70 K.
Concerning the magnetism in the A site, Figs. 3 and 6 illustrate that slightly below 15 K the long-range ordering (LRO) of Tb moments is at play (T N3 ≈ 15 K).

B. Successive magnetic phases with magnetic order at A and B sites below 15 K
In the two precedent ferromagnetic regimes (first with a very small stable canting, and later with the magnetization rotating with temperature) magnetic LRO is limited to B/B sites. Two additional magnetic configurations were observed at lower temperatures that also involve the ordering of terbium moments at the A sites of this monoclinic perovskite. As we will see below, in both phases with ordered moments in A and B sites the resultant magnetization is non-null.
FM3 phase. The exchange field created by the 3d metals at the A sites promotes the partial LRO of terbium moments below T N3 ≈ 15 K. Ostensible changes were observed in a set of reflections such as the (001)-forbidden by the structural P2 1 /n symmetry-(100), (110), or (020) whose evolution is depicted in Figs. 2 and 3. So, for example, one can observe in Fig. 2 that the integrated intensity of (100) and (020) significantly grow on cooling between ∼15 K down to 6 K, by contrast with a marked decrease of the (110) signal. Note that the temperature evolution of the intensities does not follow a Brillouin curve, bearing out that the partial polarization of terbium moments is due to the FM exchange field created by the Mn/Ni sites. The refined neutron pattern at 6 K is shown in Fig. 7, and we show the details of the magnetic configuration in Table II. The evolution of magnetic order in the low-temperature range is depicted with great detail in Fig. 8. The tilting angle θ of the metals, the refined moment in Tb atoms (perpendicular to c), and its deviation with respect to the a axis (given by ϕ Tb ) are shown down to 1.5 K. According to the symmetry dictated by the mGM 2 + irrep (or the P2 1 /c MSG) the Tb moments adopt the permitted Bertaut F x C y F z configuration [21]. The previous P2 1 /c magnetic symmetry is preserved, in which all Tb sites are related by magnetic symmetry operations. Namely, Tb occupies a single independent position (Table II) and therefore the two Tb magnetic layers parallel to ab (at z ≈ 0.25 and z ≈ 0.75) are identical. The point magnetic symmetry is 2 /m (5.5.16), a nonpolar symmetry because this FM3 magnetic structure conserves the spatial inversion.
As can be deduced by inspection of the FM3 magnetic structure, the J Tb-M exchange produces a mutual polarization: In addition to the partial polarization of Tb moments along the a axis, the tilting of the metal moments towards the ab plane becomes much more prominent below T N3 . So, we observe a significant increase of the canting angle θ after entering the FM3 phase, which finally reaches a maximum of θ max = 70 • around 4 K, before decreasing again (Fig. 8).
FM4 phase. In Tb 2 NiMnO 6 the genuine magnetic ground state enters below T N4 ≈ 3.7 K, although its signature can be observed at higher temperatures in the form of short-range ordering (SRO) correlations. New magnetic reflections are visible, first as SRO and later as well-defined visible peaks around the positions (001), (011), (100), etc. (Figs. 2 and 3).
Accurate structural and magnetic refinements of neutron patterns collected below T N4 were obtained using the P2 1 184444-5  (a, b, a + c (4.9) MSG and the spin configuration described in Table II, which corresponds to the magnetic refinement of the neutron pattern collected at 1.5 K [ Fig. 7(b)]. P2 1 (4.9) is a direct subgroup (translationgleiche) of the P2 1 /c (14.79) MSG, the common symmetry to the FM1, FM2, and FM3 ferrimagnetic phases preceding the ground state of Tb 2 MnNiO 6 . Interestingly, the inversion symmetry operation connecting Tb magnetic ions at different TbO 2 layers is lost. As a result of the P2 1 /c → P2 1 phase transition, the only Tb position in P2 1 /c is split to give two independent Tb sublattices at low temperature (each of the Tb1 and Tb2 orbits corresponds to a different layer in the unit cell). Within the individual Tb layers the magnetic coupling is identical to the FM3 phase FIG. 7. Rietveld refinement (solid line) of the neutron patterns for Tb 2 MnNiO 6 collected on D1B at (a) 6 K (FM3 phase), and (b) 1.5 K (FM4 phase), using, respectively, the P2 1 /c and P2 1 MSGs. generating a strong ferromagnetic component parallel to the a axis. What changes is that terbium moments in successive TbO 2 layers (split sublattices) are now flipped, producing the cancellation of the ferromagnetic component (F x C y → A x G y ). As a result, the ferromagnetic exchange field from Tb layers on the transition-metal layers (which was substantial in FM3) suddenly vanishes below T N4 . This cancellation of the 4 f -3d exchange field felt by the B/B metals permits a reentrant rotation back towards the c axis of the FM Mn/Ni spins, and so the angle θ suddenly drops back to its position of comfort. θ sharply decreases from ∼70°down to ∼27°, primarily dictated by the magnetic anisotropy of Mn 4+ (t 3 2g e 0 ) and Ni 2+ (t 6 2g e 2 ) metal ions. Remarkably, the canting of the magnetization in the ground state (1.5 K) adopts the same value (27°) as the initial value in the rotating FM2 phase [ Fig. 6(b)]. Furthermore, Fig. 8 illustrates that across the FM3-to-FM4 phase transition the B cations experience strong changes in the competition between the anisotropic exchange (with origin at the FM Tb layers) and the single-ion anisotropy terms. As shown in this figure, this transition develops in two steps. First, the P2 1 /c → P2 1 symmetry breaking takes place by the inversion of half of the Tb layers (F x C y → A x G y Tb modes), producing a sudden jump ( m ≈ 4μ B /Tb) in the ordered Tb moment. Then a second step occurs which raises the Tb moments up to 7.5 μ B /Tb). Most interestingly, whereas the metal moments do not rotate across the first step, the second step is concurrent or is activated only when metal spins initiate the reentrant spin reorientation towards the c axis. Then, the magnetization in the Mn/Ni sublattices describes a sharp spin reorientation θ ≈-43°in only 1.5 K (on warming from 3 K down to 1.5 K).
The transition temperature T N4 ≈ 3.7 K coincides with the AF ordering temperature of terbium in other perovskites such as TbCoO 3 (T N = 3.3 K [22]). Interestingly, the absence of moments at the B site in TbCoO 3 (in the ground state Co 3+ atoms are in LS, S = 0) indicates that (apart from anisotropy terms) J Tb-Tb must be very similar in the cobaltite and in Tb 2 MnNiO 6 . As in TbCoO 3 the observed magnetic arrangement of Tb moments in the FM4 phase (ground state) reflects the Ising nature of the terbium moments associated with the lowest crystal field energy levels. The electronic configuration of Tb 3+ ions (4 f 8 ) determines its non-Kramers nature and strong anisotropy in low-symmetry sites. The FIG. 8. Evolution of the magnetic order at A and B sites in the low-temperature range for Tb 2 MnNiO 6 . Evolution down to 1.5 K of the tilting angle θ of the metals (filled circles, left axis), the refined moment in Tb atoms (filled squares, right axis), and the deviation ϕ Tb of Tb moments with respect to the a axis (unfilled circles, left axis). Lines are guides to the eye. Mn and Ni moments are fully ordered in this temperature range. A schematic view of the successive magnetic structures is also shown. point-group symmetry of Tb in 4e site is 1. As free ion its lowest-lying term is 7 F 6 (L = 3, S = 3, J = 6). The low symmetry of Tb sites completely removes the degeneracy into 13 singlets [23,24], but the ground and first excited states of Tb 3+ ions form an accidental quasidoublet, very close in energy ( = 0.025 meV for TbAlO 3 ) [24], which involves two conjugate J = 6 states of the type |6, +6 + |6, −6 and |6, +6 − |6, −6 . Any small external magnetic field or in this case the internal exchange field created by B/B -site moments mix the two first eigenstates strongly favoring one of the |6, +6 and |6, −6 ionic states. As consequence the character of the Tb 3+ moments is strongly Ising-like and they exhibit a very large local anisotropy.
In the polar P2 1 phase (FM4) J Tb-Tb interactions promote an AF coupling between successive TbO 2 layers and, as mentioned above, terbium moments exhibit strong Ising anisotropy within the x-y plane. In this phase Tb 3+ moments are ordered following the irrep (mGM 2 − ) whereas metal moments preserve the mGM 2 + magnetic mode. The Tb-M ferromagnetic coupling expressed through the common F x component in FM3 is overcome by Tb-Tb interaction producing the transformation of the F x C y Tb configuration (FM3) into A x G y (FM4), keeping the Tb 3+ moments always in the x-y plane due to the Ising anisotropy. In FM4 Tb 3+ moments in the same x-y plane are noncollinear with their moments displaying almost 90° (Table II). On one hand, starting from the paramagnetic gray P2 1 /n group, the activation of the magnetic mGM 2 + mode in Co/Ni sublattices generates a P2 1 /n (14.79) nonpolar symmetry. On the other hand, the mGM 2 − mode adopted by Tb 3+ moments below T N4 produces a P2 1 /n (14.77) symmetry, also nonpolar. However, even if these two distortion magnetic modes adopted, respectively, by the rare-earth and metal sites generate nonpolar symmetries (P2 1 /n and P2 1 /n separately), the intersection of both groups produces the final P2 1 (4.9) MSG in which the loss of the n plane (c plane in the P2 1 /c setting) causes the splitting of the Tb site (and each of the three oxygen positions) in two orbits and the disappearance of the inversion center {-1|0} at the origin of the parent phase. Therefore, the final ferrimagnetic FM4 configuration shown in Fig. 8 is polar, and its magnetic point symmetry 2 (3.3.8) permits the appearance of ferroelectricity with spontaneous electrical dipoles parallel to the unique axis b.
An additional remarkable feature in the diffraction patterns below T N4 is the presence of defects in the stacking of ferrimagnetic Tb layers, so it is necessary to include a small modulation k Tb = (0.063 [1], 0.069 [1], 0.0387 [2]) associated to the ordering of Tb 3+ moments. This modulation matches well the angular position of the magnetic intensities nearby (001) in the FM4 ground state. This smooth magnetic modulation superimposed to the primary A x G y mode of Tb moments has a very long period of ∼200 Å at 1.5 K perpendicular to the ferrimagnetic x-y Tb layers. Within these layers the period of the modulation is around ∼80-90 Å and it is rather isotropic. This accidental extra modulation of Tb moments is very soft or light, and it does not imply a significant alteration of the average and primary P2 1 magnetic symmetry of the FM4 phase in Tb 2 MnNiO 6 , whose tensor properties are dictated by the polar 2 point symmetry.

IV. CONCLUDING REMARKS
Tb 2 MnNiO 6 undergoes four magnetic transitions below room temperature. Two inverse spin reorientation transitions of the metal moments were observed with decreasing temperature, always keeping the ferromagnetic mGM 2 + configuration at the B sites: One is very slow and develops in the interval 70-4 K. The second, very sharp and opposite to the former, is concurrent to the activation of the A x G y mode of Tb moments during their mGM 2 + to mGM 2 − transition (T N4 ≈ 3.7 K). The ground state is ferrimagnetic and exhibits polar P2 1 symmetry so that ferroelectricity (induced by symmetric magnetostriction) and macroscopic magnetization can coexist in this perovskite. Spontaneous electric polarization is anticipated as a secondary effect induced by the superposition of primary nonpolar magnetic modes associated to the zone center in the reciprocal space. A large ferromagnetic component is preserved in the system (5 μ B /f.u), not related to a weak ferromagnetic (wFM) component induced by DM interaction. Although ferroelectricity and ferromagnetism would not be induced by the same instability, the overlay of the mGM 2 + (B site) and mGM 2 − (A site) modes generating the polar symmetry is accompanied by a sudden change in the anisotropy of the resulting ferromagnetism. A sharp reorientation of the magnetization concurs with the entering of the polar phase. Accordingly we can assure that the same instability (the substitution of mGM 2 + by mGM 2 − mode in the Tb sublattice) is finally responsible for the activation of the polar P2 1 symmetry and the severe rotation of the ferromagnetic vector. Consequently, the ferroelectric state could be switched off (on) by applying magnetic fields along a (along c), and reversely, the orientation of the spontaneous magnetization could be severely modified by the application of voltages along the b axis (polar axis).