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Peng Zhao, Shao-ying Zhang, Zhao-hua Cheng, Hong-wei Zhang, Ji-rong Sun, Bao-gen Shen, and R. A. Dunlap. Abstract—The effects of Ti substitution for Sn ...



Structure and Magnetic Properties of TbMn6Sn6 xTix(x = 0–0:9) Compounds Peng Zhao, Shao-ying Zhang, Zhao-hua Cheng, Hong-wei Zhang, Ji-rong Sun, Bao-gen Shen, and R. A. Dunlap

Abstract—The effects of Ti substitution for Sn on the structure and magnetic properties of TbMn6 Sn6 Ti ( = 0–0 9) compounds have been investigated by means of x-ray diffraction (XRD), magnetization measurements and 119 Sn Mössbauer spectroscopy. The substitution of Ti for Sn results in an increase in the lattice constants and the unit-cell volumes. The magnetic ordering temperature decreases monotonically with increasing Ti content from 423 K for = 0 to 376 K for = 0 9. At room temperature, the easy magnetization direction changes from the -axis for 0 3, to the cone for 0 3 0 9 and then to the ab-plane for = 0 9. This variation implies that the substitution of Ti for Sn leads to a decrease in the -axis anisotropy of the Tb sublattice. An increase in the nonmagnetic Ti concentration results in a monotonic decrease of the spontaneous magnetization Ms at both 5 K and room temperature. Since there are three nonequivalent Sn sites, 2 (0 33 0 67 0), 2 (0 33 0 67 0 5) and 2 (0 0 0 34) in the TbMn6 Sn6 Ti compounds, the 119 Sn Mössbauer spectra of the TbMn6 Sn6 and TbMn6 Sn5 4 Ti0 6 compounds can be fitted by three sextets. The hyperfine fields, HFs, decrease in the order of HF(2 ) HF(2 ) HF(2 ), which is in agreement with the magnetic structure. Index Terms—Magnetic properties, rare-earth transitional metal compounds.





XTENSIVE studies have been focused on the magnetic structures and magnetic properties of the series of rare earth element and X Sn or Ge) comRMn X (R pounds since they were discovered in the late 1980s [1]. These compounds show various magnetic behaviors because the arrangement of the Mn magnetic moments is sensitive to the R elements and interatomic distances [2], [3]. Among these compounds, TbMn Sn has many interesting magnetic properties, such as a relatively high ordering temperature (Curie tempera), the occurrence of a high coercive field at ture T at 4.2 K) and a spin low temperature (coercivity H K [4]. The TbMn Sn reorientation transition at T compound crystallizes in the HfFe Ge -type structure (space group: P6/mmm) which can be described as alternating (00l) planes containing magnetic Tb and Mn atoms stacking along Manuscript received May 22, 2000. This work was supported by the State Key Project of Fundamental Research and National Natural Sciences Foundation of China. The work of Z.-h. Cheng and R. A. Dunlap was supported by the Killam Foundation and the Natural Sciences and Engineering Research Council of Canada for financial support. P. Zhao, S.-Y. Zhang, Z.-H. Cheng, H.-W. Zhang, J.-R. Sun and B.-G. Shen are with the State Key Laboratory of Magnetism, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China (e-mail: [email protected] R. A. Dunlap is with the Department of Physics, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada. Publisher Item Identifier S 0018-9464(01)07255-7.

the -axis with the sequence -Mn-Tb-Mn-Mn-Tb-Mn-. The magnetic structure of TbMn Sn is characterized by ferromagnetic (001) Mn planes with parallel intraplanar Mn moments, which result in the relatively high T of this compound. It is noteworthy that the interplanar Mn-Mn coupling remains ferromagnetic while the interplanar Tb-Mn coupling is antiferromagnetic throughout the range of the magnetic ordering temperature. The strong Tb-Mn coupling causes the Tb and Mn atoms to order simultaneously so that the compound attains a ferrimagnetic structure [4], [5]. The magnetic moments (Tb and Mn) align along the -axis below 330 K and lie in the basal plane from 330 K to T . Like other RMn X -type compounds, the Mn-Mn and R-Mn atomic distances strongly influence the intraplanar and interplanar interactions, thus ultimately determining the magnetic behaviors of these compounds. In the present paper, we reporte the effects of Ti substitution for Sn on the structure and magnetic properties of Ti compounds. By replacing TbMn Sn nonmagnetic Sn atoms, the effect of the Mn-Mn and R-Mn interatomic distances on the magnetic behavior of TbMn Sn can better be studied. It is unnecessary to consider the dilution of the magnetic sublattice in the compound, resulting from substituting the magnetic elements Tb or Mn as reported in previous work [6]. To simplify the various exchange interactions in the 1:6:6-type compounds, only the main coupling of the intraplanar Mn-Mn and that of the nearest interplanar Mn-Mn and R-Mn are considered in order to investigate the influence of the atomic distances on the exchange interactions of the magnetic elements in this series of compounds. II. EXPERIMENT TbMn Sn Ti polycrystalline samples were obtained by arc melting the constituent elements in a highly purified Ar atmosphere. The purity of the elements was at least 99.9%. A 5% excess of Tb and a 15% excess of Mn were added to compensate for the mass loss during melting. The ingots were remelted at least three times to ensure homogeneity and annealed in an evacuated quartz tube at 1043 K for 240 hours, then rapidly quenched in water. In order to investigate the magnetocrystalline anisotropy, magnetically oriented samples were prepared by mixing powders with epoxy resin and then aligning in a magnetic field of 1 T at room temperature (RT). The crystal structure was confirmed by room temperature X-ray diffractometer (XRD) scans using CuK radiation. A superconducting quantum interference device (SQUID) magnetometer was used to measure the magnetic properties of all samples below 400 K. Thermomagnetic

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Fig. 1. Dependence on Ti content of the lattice constants a and c and the Ti compounds. unit-cell volume v of TbMn Sn

Fig. 3. Dependence on Ti content of the Curie temperature (T ) and the spontaneous magnetization at 5 K and 293 K for TbMn Sn Ti compounds.

Fig. 2. The thermomagnetic behavior under a magnetic field of 500 Oe for the Ti compounds. TbMn Sn

curves above 400 K were measured by a vibrating sample Sn Mössbauer magnetometer (VSM) in a field of 0.05 T. spectroscopy was used to investigate the hyperfine fields at the three nonequivalent Sn sites. III. RESULTS AND DISCUSSION XRD patterns show that all samples are almost single phase with the hexagonal HfFe Ge -type structure. The substitution of Ti for Sn does not change the crystal structure of this series of compounds, while the values of the lattice constants a and c and the unit-cell volume v are all increased due to the larger atomic radius of Ti compared to Sn (Fig. 1). Therefore the atomic distances of intraplanar Mn-Mn, interplanar Mn-Mn and R-Mn are all increased with increasing Ti content. Ti compounds ThethermomagneticbehaviorofTbMn Sn has been measured in a magnetic field of 500 Oe and the results are shown in Fig. 2. Both the magnetic ordering transition and the spin-reorientation transition can be observed in

the whole series of compounds and both transition temperatures decrease gradually with increasing Ti content. The magnetic ordecreases monotonically dering temperature of 423 K for . The spin-reorientation transition beto 376 K for comes progressively less well defined and decreases from 330 K to 280 K for . The compositional depenfor dence of T is shown in Fig. 3. It has been widely accepted that the relatively high ordering temperature mainly results from the strong Mn-Mn intraplanar couplings. Because the substitution of Ti for Sn increases the distance between intraplanar Mn-Mn in the (001) plane), the strong ferroatoms (Mn Mn magnetic interaction of intraplanar Mn-Mn atoms is weakened and consequently lowers the magnetic ordering temperature of these compounds. For all samples, the isothermal magnetization curves at 5 K and 293 K were measured and the spontaneous magnetization . The spontawas derived by extrapolating the curves to H neous magnetization at both 5 K and 293 K decreases abruptly with a small amount of Ti substitution, then decreases linearly with further increasing nonmagnetic Ti concentration. These results are in agreement with the decrease of T with increasing Ti . Ti content in TbMn Sn Fig. 4 shows the XRD patterns of magnetically oriented Ti samples at RT. Similar to that of the TbMn Sn TbMn Sn compound, the presence of the (00l) peaks for the shows that the alignment of magnetic sample with , both the (00l) moments is parallel to the -axis. When and the (hk0) peaks appear, indicating a conical structure of , only (hk0) peaks magnetic moments. Eventually at exist in the XRD patterns. The substitution of Ti for Sn results in the easy magnetization direction changing gradually from the -axis to the ab-plane at RT. In the TbMn Sn compound, the Tb sublattice has an axial anisotropic contribution, while the Mn sublattice has a planar anisotropic contribution. A



Fig. 4. XRD patterns of the magnetically oriented TbMn Sn Ti samples with x = 0:1 (easy c-axis), x = 0:5 (cone) and x = 0:9 (easy ab-plane).

spin reorientation transition occurs when the axial anisotropic contribution of the heavy rare-earth element Tb competes with the planar anisotropy of the Mn sublattice [4]. In previous work, Chafik El Idrissi et al. assumed that a negative charge associated with the axial Sn atoms at the sites of TbMn Sn yields a second-order crystal-field parameter and concluded that the low temperature axial behavior of TbMn Sn is due to the effective charge of the Sn ions at the sites and that distance [4]. this effect is reinforced by the short Tb-Sn Our recent x-ray diffraction Rietveld analysis for the sample of TbMn Sn Ti indicated that Ti atoms preferably occupy the sites and enlarge the Tb-Sn distance [7]. This preferable occupation changes the electrostatic potential around the Tb site, thus weakening the -axis anisotropy induced by the crystalline electric field acting on the Tb site. Therefore, the easy magnetization direction changes from easy -axis to planar and the spin reorientation temperature decreases with increasing Ti content. Sn Mössbauer spectra of TbMn Sn Fig. 5 illustrates the and TbMn Sn Ti compounds. Since there are three , nonequivalent Sn sites, and in TbMn Sn Ti , we fit the spectra with three subspectra. The hyperfine fields corresponding to these three subspectra are 25.1 T, 24.2 T and 9.2 T for TbMn Sn and 21.5 T, 13.8 T and 6.5 T for TbMn Sn Ti , respectively. The addition of Ti leads to a decrease of the hyperfine fields at these three nonequivalent sites due to the decrease of the saturation magnetization and the Curie temperature. Since Sn is a nonmagnetic atom, the hyperfine fields at the Sn sites are



Fig. 5. Sn Mössbauer spectra of TbMn Sn and TbMn Sn Ti at RT. (Lines: the calculated subspectra and a fit of the sum of these spectra, Points: the experimental data).

transferred from neighboring magnetic atoms and are proportional to the number of magnetic atoms and their magnetic moments. All of the three Sn sites have six near neighbor Mn atoms, while the Sn atoms at , and sites have three, zero and one Tb atoms as neighbors. Due to the anti-ferromagnetic coupling between Tb and Mn ions, the hyperfine fields, HF, HF HF . The decrease in the order HF intensities of the three subspectra for TbMn Sn Ti imply that the Ti atoms prefer the site, which is in agreement with the x-ray diffraction results. REFERENCES [1] B. Malaman, G. Venturini, and B. Roques, “New ternary stannides: MMn Sn (M=Sc, Y, sm, gd-tm, Lu) and ScFe Sn ,” Mat. Res. Bull., vol. 23, pp. 1629–1633, 1988. [2] G. Venturini, B. Chafik El Idrissi, and B. Malaman, “Magnetic properties of R Mn Sn (R=Sc, Y, Gd-Tm, Lu) compounds with HfFe Ge type structure,” J. Magn. Magn. Mater., vol. 94, pp. 35–42, 1991. [3] G. Venturini, R. Welter, and B. Malaman, “Crystallographic data and magnetic properties of RT Ge compounds (R=Sc, Y, Nd, Sm, Gd-Lu; T=Mn, Fe),” J. Alloys Comp., vol. 185, pp. 99–107, 1992. [4] B. Chafik El Idrissi, G. Venturini, and B. Malaman, “Magnetic structures of TbMn Sn and HoMn Sn compounds from neutron diffraction study,” J. Less-Common Met., vol. 175, pp. 143–154, 1991. [5] B. Malaman, G. Venturini, R. Welter, J. P. Sanchez, P. Vulliet, and E. Ressouche, “Magnetic properties of RMn Sn (R=Gd-Er) compounds from neutron diffraction and mössbauer measurements,” J. Magn. Magn. Mater., vol. 202, pp. 519–534, 1999. [6] Y. G. Wang, F. M. Yang, C. P. Chen, N. Tang, J. L. Wang, X. F. Han, H. G. Pan, J. F. Hu, K. W. Zhou, R. W. Zhao, and Q. D. Wang, “Structure and magnetic properties of TbMn Ti Sn ,” J. Phys.: Condens. Matter, vol. 8, pp. 1851–1856, 1996. [7] Z. H. Cheng, P. Zhao, S. Y. Zhang, R. A. Dunlap, and B. G. Shen, unpublished.

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