Manganite oxides family RMn2O5 (R – rare-earth element) represents example of a multiferroics with extremely interesting and close relationship between magnetism and ferroelectricity [1-3]. The particularities of the magnetic alignment seems to play the decisive role in this M-E coupling. In order to go deep in the understanding in explicit the microscopic mechanisms responsible for spin-driven ferroelectricity in these compounds the investigations of the magnetic ordering in the Nd0.8Tb0.2Mn2O5 single crystal were performed. The magnetic phase diagram in both parent compounds is rather complicate, and is different for them. TbMn2O5 is characterized by several successive phases - high-temperature incommensurate, at TCP &lt; T &lt; TN (TCP = 35 ÷ 37 K, TN = 45K), commensurate at TLTIC &lt; T &lt; TCP (TLTIC = 20 ÷ 24 K) and low-temperature incommensurate, at T &lt; TLTIC [4]. The ferroelectric ordering in TbMn2O5 can also be represented as successive ferroelectric phases in the same temperature ranges. In TbMn2O5, chiral scattering was detected in all magnetically ordered phases [4]. In contrast to TbMn2O5, chiral scattering was observed in multiferroic NdMn2O5 only below the temperature T ~ 20 K, i.e., below this temperature, the transition from a incommensurate non-chiral magnetic phase to a chiral one occurs [5]. The temperature of this chiral transition corresponds to the temperature of the ferroelectric transition TCE [6], while the value of the ferroelectric polarization is extremely small. The temperature and electric field dependences of magnetic ordering in single crystal Nd0.8Tb0.2Mn2O5 were investigated using diffraction of unpolarized and polarized neutrons. The temperature dependence of the propagation vectors of the observed magnetic phases is shown at Fig. 1. Our results demonstrate that two magnetic phases coexist in a broad temperature range, indicating magnetic phase separation. One of them bears features of TbMn2O5, another – of NdMn2O5. Thus, phase 1 or “Tb-phase” has higher temperature of alignment TN ≈ 37 K, but it is still lower than that of TbMn2O5. The propagation vector k1 = (0.5 0 kz1) is commensurate along a-axis and incommensurate along c-axis at all temperatures of alignment, while for TbMn2O5 propagation vector k = (½-δx, 0, ¼+δz) is non-commensurate in both a and c directions in two incommensurate phases – HTIC and LTIC. It was possible to change the ratio of "right" and "left" spirals in Nd0.8Tb0.2Mn2O5 crystals by an external electric field. The features of phase 2 – “Nd-phase” – are close to those of magnetic order at NdMn2O5. It has transition temperature the same as TN = 30 K in NdMn2O5, the propagation vector k2 = (0.5 0 kz2) has the same component along a* and kz2 value is close to that of NdMn2O5. It is difficult to say whether this “Nd-phase” is chiral in whole temperature range of existence or it is chiral below some definite temperature TCh, which takes place in NdMn2O5, with its TCh = 20 K. Compared to NdMn2O5, the system Nd0.8Tb0.2Mn2O5 exhibits even greater ability to generate ordered magnetic phases of the same type. This leads to the fact that, in the narrow temperature range of 20 K – 30 K two more magnetic incommensurate phases were observed. Such phenomena were observed in the compounds of this family like TbMn2O5 in narrow temperature range [7, 8], in NdMn2O5 – in a broad temperature range [5, 9]. This feature should be connected with the magnetic inhomogeneity of the system. This inhomogeneity could follow from the non-rigidity of the Mn3+ - Mn4+ bonding at RMn2O5 family. From another side, at RMn2O5 series the bond lengths and angles in the Mn3+ and Mn4+ polyhedra are in the critical position for the superexchange interactions or close to it [10]. The small deviation in the geometrical arrangement of Mn3+ - Mn4+ bonding provide different magnetic interaction strength. The coexistence of two rare earth ions with different types of ground state: Kramers-like Nd3+ and non-Kramers one Tb3+, could lead to different types of site anisotropies. The system energy lowers by phase separation state for two main phases with their characteristic resembling those of phases of pure compounds. The broadening of the incommensurate reflections at low temperature (Fig.2) could be connected with the existence of several propagation vectors, distributed around the two main values k1,2. At the temperature region 22 K – 30 K, the competing interactions lead to more phases with similar vectors k3,4. Approximately at the same temperature region competing interactions stabilize the commensurate structure in TbMn2O5. The studied system Nd0.8Tb0.2Mn2O5 at this temperature range corresponds to the least homogeneous state, when the energy minimum achieved by the separation on three and four magnetic phases. This fact can be one more indication that CM phase in RMn2O5 is not really commensurate. Propagation vector component kz in this phase could be not precisely commensurate and it arises as not very stable result of interplay of different interaction. Recent work [8] witnesses in favor of that where kz component of propagation vector in TbMn2O5 changed value in magnetic field from 0.25 to 0.32. As another argument can be the “reverse” transition from CM to LTIC phase. The work was supported by the Russian Foundation for Basic Research grant # 19-52-12047, and DFG grant # SA 3688/1-1.

**References:**

1. N. Hur, S. Park, P. A. Sharma, et al. Nature 429, 392 (2004). 2. Y. Noda, H. Kimura, M. Fukunaga, et al. J. Phys.: Condens. Matter 20, 434206 (2008). 3. G. R. Blake, L. C. Chapon, P. G. Radaelli, et al. Phys. Rev. B 71, 214402 (2005). 4. I.A. Zobkalo, S.V. Gavrilov, A. Sazonov, et al. J. Phys.: Condens. Matter 30, 205804 (2018). 5. I.A. Zobkalo, A.N. Matveeva, A. Sazonov, et al. Phys. Rev. B 101, 064425 (2020). 6. S. Chattopadhyay, V. Baledent, et al., Phys. Rev. B 93, 104406 (2016). 7. I.A. Zobkalo, S.V. Gavrilov, V.A. Sanina, et al. Physics of the Solid State 56, 51 (2014). 8. T. Chatterji, P. J. Brown. J. Appl. Phys. 116, 203904 (2014). 9. I. A. Zobkalo, S. V. Gavrilov, et al., J. Magn. Magn. Mater. 354, 85 (2014). 10. L. M. Volkova, D. V. Marinin.J. Phys.: Condens. Matter 21, 015903 (2009).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/07c5c5ebce767eccbe77e3ed92b7d489.jpg)
Temperature dependence of propagation vectors for four magnetic phases in heating mode

![](https://assets.underline.io/uploads/markdown_image/1/image/07e625659faaba34826e7cd116e02b03.jpg)
Temperature dependence of peak width of satellites (0 1 0)+k1 and (0 1 0)+k2 in heating mode

 Magnetic phases and chirality control in magnetic multiferroics Nd0.8Tb0.2Mn2O5 by the neutron scattering

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Magnetic phases and chirality control in magnetic multiferroics Nd0.8Tb0.2Mn2O5 by the neutron scattering

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