Magnetic phase transitions in the Gd1 − xTmxMn6Sn6 compounds

Magnetic phase transitions in the Gd1 − xTmxMn6Sn6 compounds

Journal of Alloys and Compounds 416 (2006) 31–34 Magnetic phase transitions in the Gd1 − xTmxMn6Sn6 compounds O. Cakir a , I. Dincer a , A. Elmali a ...

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Journal of Alloys and Compounds 416 (2006) 31–34

Magnetic phase transitions in the Gd1 − xTmxMn6Sn6 compounds O. Cakir a , I. Dincer a , A. Elmali a , Y. Elerman a,∗ , H. Ehrenberg b , H. Fuess b a b

Faculty of Engineering, Department of Engineering Physics, Ankara University, Be¸sevler, Ankara TR-06100, Turkey Institute for Material Science, Darmstadt University of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany Received 17 July 2005; received in revised form 12 August 2005; accepted 16 August 2005 Available online 22 September 2005

Abstract The magnetic properties of the HfFe6 Ge6 -type Gd1 − x Tmx Mn6 Sn6 (0 ≤ x ≤ 0.9) compounds have been investigated in the temperature range of 5–600 K. The compounds with x ≤ 0.8 are ferrimagnetic in the whole temperature range. The ferrimagnetic Curie temperatures decrease with increasing Tm content from 435 K (x = 0) to 344 K (x = 0.9). The samples with x = 0.85 and 0.9 show paramagnetic–ferrimagnetic–antiferromagnetic– ferrimagnetic phase transitions with decreasing temperature. For the x = 0.85 compound, the metamagnetic phase transition from antiferromagnetism to ferrimagnetism is induced by an applied field 20 kOe. © 2005 Elsevier B.V. All rights reserved. Keywords: Rare-earth manganese stannides; Ferrimagnetism; Antiferromagnetism; Metamagnetic phase transition

1. Introduction Recently, a large magnetoresistance effect has been observed in HfFe6 Ge6 -type (space group: P6/mmm) RMn6 Sn6 compounds and their derivative, YMn6 Sn5.8 Ga0.2 , Y0.5 Ho0.5 Mn6 Sn6 , Y0.7 Ce0.3 Mn6 Sn6 [1–6]. The rare-earth R and Mn atoms occupy the 1b and 6i positions, respectively, while the Sn atoms occupy the three crystallographic positions: 2c, 2d and 2e. The lattice has an intrinsic layered structure. In these compounds, the rare-earth sublattice orders together with the Mn sublattice somewhat above room temperature [1–9]. The competition between different interlayer exchange interactions results in variety of magnetic structures. The interlayer Mn moments through the Mn–Sn–Sn–Mn slab are always parallel while the arrangement within the Mn–(R, Sn)–Mn slab depends on the R elements. The Gd, Tb, Dy and Ho compounds are characterized by a ferrimagnetic behaviour below 435, 423, 393 and 376 K, respectively [2]. Furthermore, except GdMn6 Sn6 , these compounds exhibit an additional transition (at Tt = 330, 240 and 180 K) which could be related to a change in their axis direction. Strong Mn–R antiferromagnetic coupling causes the rare-earth and Mn sublattices to order simultaneously and leads to collinear ferrimagnetic arrangements in the whole ordered range. On the

other hand, the compounds with Sc, Y and Lu display antiferromagnetic structure or helimagnetic arrangements [1–3]. According to earlier studies [1,2], GdMn6 Sn6 is characterized by a ferrimagnetic behaviour below TC = 435 K. Among the RMn6 Sn6 compounds, GdMn6 Sn6 exhibits easy plane behaviour in its whole ordered state [1,2]. TmMn6 Sn6 orders antiferromagnetically below TN = 347 K and displays a collinear antiferromagnetic structure above 330 K while a helical structure occurs in the 2–330 K [1,10]. The second transition at 58 K could be correlated to an antiferromagnetic ordering of the Tm sublattice [1,10]. The Mn–Mn and R–Mn atomic distances strongly influence the intraplanar and interplanar interactions, thus ultimately determining the magnetic behaviors of these compounds [2]. In order to better understand these magnetic behaviors, we decided to investigate the effects of Tm substitution for Gd on the structure and magnetic properties of Gd1 − x Tmx Mn6 Sn6 (0 ≤ x ≤ 0.9). Since the antiferromagnetic ordering in RMn6 Sn6 (R: Sc, Y, Er, Tm and Lu) is very sensitive to the external magnetic field and metamagnetic phase transitions can be induce by the low critical external field, we performed the temperature dependent magnetization measurements for this system in low applied fields. 2. Experimental



Corresponding author. Tel.: +90 312 212 6720; fax: +90 312 212 7343. E-mail address: [email protected] (Y. Elerman).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.08.029

The Gd1 − x Tmx Mn6 Sn6 samples with x = 0, 0.7, 0.8, 0.85, 0.9 were prepared by arc melting furnace under Ar atmosphere

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O. Cakir et al. / Journal of Alloys and Compounds 416 (2006) 31–34 Table 1 Refined coordinates of Gd0.15 Tm0.85 Mn6 Sn6

Fig. 1. Observed (circle) and calculated (solid line) X-ray diffraction patterns for Gd0.15 Tm0.85 Mn6 Sn6 . Positions for the Bragg reflections are marked by vertical bars. Differences between the observed and calculated intensities are shown below the bars.

in a water-cooled copper boat. The purity of the elements was 99.9% for Gd, 99.9% for Tm, 99.98% for Mn and 99.9% for Sn, respectively. An excess of R and Mn elements has been used in order to avoid the formation of impurity. The polycrystalline ingots were turned over and remelted several times to ensure homogeneity. The structure was analyzed by powder Xray diffraction on a STOE powder diffractometer using Mo K␣1 ˚ radiation, and the powder diffraction pattern of (λ = 0.7093 A) Gd0.1 Tm0.9 Mn6 Sn6 is given in Fig. 1. The magnetic properties of Gd1 − x Tmx Mn6 Sn6 compounds were studied in the temperature range 5–350 K with a SQUID magnetometer from quantum design. The samples were first heated to 350 K and subsequently cooled to 5 K in zero external magnetic field. The measurements were performed in a zerofield-cooled (ZFC) and field-cooled (FC) sequence in an external field of 50 Oe. The temperature dependence of the magnetization in the range 300 K ≤ T ≤ 600 K and in external magnetic field of 100 Oe was measured using vibrating sample magnetometer. 3. Results and discussion The X-ray patterns confirm the existence of a primitive hexagonal main phase having the HfFe6 Ge6 -type structure with a minor impurity phase (Snβ ). A summary of the crystallographic properties is given in Table 1. For the x = 0.85 compound, the result of Rietveld refinements obtained at room temperature is shown in Fig. 1. The substitution of Tm for Gd leads to a slight decrease of the lattice parameters a and c while the ratio c/a remains approximately constant 1.63. The refined lattice parameters are listed in Table 2. In the GdMn6 Sn6 compounds, Mn and Gd sublattices favor an easy planar behavior [1], and the magnetocrystalline anisotropy is determined by competing contributions from the manganese and the rare-earth sublattice anisotropies. The magnetic ordering of GdMn6 Sn6 is ferrimagnetic, due to an antiferromagnetic coupling of ferromagnetic Gd and Mn sublattices [1,2]. Figs. 2 and 3 show the temperature dependence of the magne-

Atom

Position

x

y

z

Gd Tm Mn Sn1 Sn2 Sn3 Rbrag (%) Rf (%) χ2

1a 1a 6i 2c 2d 2e 9.0 7.18 3.91

0 0 0 1/3 1/3 0

0 0 1/2 2/3 2/3 0

0 0 0.2449(4) 0 1/2 0.3373(3)

tization for the Gd1 − x Tmx Mn6 Sn6 compounds in temperature range for 300 K ≤ T ≤ 600 K and 5 K ≤ T ≤ 350 K, respectively. For x ≤ 0.9, all data exhibit a sharp rise with decreasing temperature around the ferrimagnetic Curie temperatures TC . The TC values were determined approximately from the inflection point of the magnetization curve. The compounds with x ≤ 0.8 exhibit ferrimagnetic ordering below the ferrimagnetic Curie temperatures (see Figs. 2 and 3). The strong Mn–Gd antiferromagnetic coupling causes the magnetic ordering of the two sublattices simultaneously and yields a collinear ferrimagnetic arrangement for these compounds. The x = 0.8 sample is ferrimagnetic, but the weak antiferromagnetic exchange is additionally introduced between 150 and 250 K as seen in Fig. 3. This leads to a mixture of ferrimagnetic and antiferromagnetic ordering. The temperature dependence of magnetization for the samples with x = 0.85 and 0.9 show several magnetic transitions with the decreasing temperature. Above TC , they are paramagnetic and become ferrimagnetic below TC . The decreasing on the M(T) curves of these compounds shows the antiferromagnetic ordering with the Neel temperatures TN are 280 and 300 K, respectively. There is an increasing on the M(T) curves of these compounds at low temperatures with the temperature decreasing. This increasing is due to the ferrimagnetic coupling between R and Mn magnetic moments as in TmMn6 Sn6 [2]. For these compounds, the transition temperatures TN and TCR are derived with the same method as used for TC . The reduction in ferrimagnetic coupling continues as the Tm concentration increases and becomes an antiferromagnetic state at a low enough temperature for them. These samples undergo ferromagnetic–antiferromagnetic–ferrimagnetic transitions below the transition temperatures. These different Table 2 The lattice constants and the magnetic transition temperatures of Gd1 − x Tmx Mn6 Sn6 x

˚ a (A)

˚ c (A)

c/a

0 0.7 0.8 0.85 0.9 1

5.524(1) 5.514(1) 5.514(2) 5.513(1) 5.511(1) 5.511(1)

9.023(2) 9.006(2) 9.005(3) 9.003(2) 9.001(2) 8.999(2)

1.633 1.633 1.633 1.633 1.633 1.633

TCR (K)

81 51 58

TN (K)

280 300 347

The related values of the x = 1 sample are taken from Ref. [10].

TC (K) 435 370 363 358 344

O. Cakir et al. / Journal of Alloys and Compounds 416 (2006) 31–34

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Fig. 2. Temperature dependence of magnetization in the temperature range of 300–600 K for Gd1 − x Tmx Mn6 Sn6 in a magnetic field of 100 Oe.

magnetic behaviours can be explained in terms of temperature dependence of Mn–Mn and R–Mn exchange interactions. It is believed that below TCR , the R–Mn strong antiferromagnetic coupling dominates and causes the R and Mn sublattices to order simultaneously, resulting in ferrimagnetic arrangement. With increasing temperature, the R–Mn exchange interaction weakens and Mn–Mn coupling dominates, leading to an antiferromagnetic behaviour similar to the TmMn6 Sn6 in this temperature range [10].

To better understand the effect of the external magnetic field on magnetic properties, we measured the magnetization of the x = 0.85 sample at various magnetic fields for the powder sample (see Fig. 4). The magnetization is proportional to the external field in the antiferromagnetic region, and shows a steep increase around 20 kOe, suggesting that the magnetic field induces transitions from antiferromagnetic state to the ferrimagnetic state, which is consistent with the metamagnetic phase transitions in TmMn6 Sn6. A similar metamagnetic phase transition

Fig. 3. Temperature dependence of magnetization in the temperature range of 5–50 K for Gd1 − x Tmx Mn6 Sn6 in a magnetic field of 50 Oe.

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O. Cakir et al. / Journal of Alloys and Compounds 416 (2006) 31–34

¨ ber 20010705044) and TUBITAK-BMBF Bilateral Programme ¨ (Grant Numbers MISAG-JULICH-1 and WTZ 42.6.BOA.6.A). References

Fig. 4. Temperature dependence of magnetization for Gd0.15 Tm0.85 Mn6 Sn6 at various external magnetic fields.

is also observed in Y0.5 Ho0.5 Mn6 Sn6 , Er1 − x Dyx Mn6 Sn6 and Gd1 − x Erx Mn6 Sn6 [11–14]. Acknowledgments Y. Elerman and A. Elmali thank Alexander von Humboldt Foundation for support. This work was further supported by the University of Ankara Research Funds (Grant Num-

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