Magnetic and specific heat properties of a new Gd-doped ZnCr2Se4

Materials Chemistry and Physics 168 (2015) 187e192

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Magnetic and specific heat properties of a new Gd-doped ZnCr2Se4 a, * _  c, E. Macia˛ zek , M. Karolus b, M. Kubisztal b, I. Jendrzejewska a, R. Sitko a, T. Gron c c  , M. Fijałkowski A. Slebarski a

University of Silesia, Institute of Chemistry, Szkolna 9, 40-006 Katowice, Poland w, Poland University of Silesia, Institute of Material Science, 75 Pułku Piechoty 1A, 41-500 Chorzo c University of Silesia, Institute of Physics, Uniwersytecka 4, 40-006 Katowice, Poland b

h i g h l i g h t s
ZnCr2Se4 spinel compounds doped by Gd were successfully synthesized.
All compounds show AFM order below TN~22 K and spin-glass behaviour below 7.3 K.
Gd-substitution weakens the FM magnetic interactions, causing the magnetic frustration.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 June 2015 Received in revised form 29 October 2015 Accepted 11 November 2015 Available online 28 November 2015

The series of chromium selenides Gd-doped ZnCr2Se4 (from 0.07 Gd to 0.21 Gd) was synthesized in polycrystalline form using ceramic method from stoichiometric amounts of elements. The phase and structure determination by X-ray diffraction showed the main cubic normal spinel structure and traces amount of phases ZnSe and the selenium. Magnetic measurements showed an antiferromagnetic order l temperature TN~22 K, a change of slope at the first critical field Hc1 of for all compositions with the Nee about 11.5 kOe for T ¼ 2 K characteristic for a metamagnetic transition, a breakdown of the helical spin arrangement at the second critical field Hc2 of about 60 kOe for T ¼ 2 K, and splitting of the zero-fieldcooling and field-cooling susceptibilities below the freezing temperature Tf ¼ 7.3 K suggesting magnetic frustration. Specific heat measurements exhibited first-order anomalies at TN. These effects are interpreted in terms of the superexchange integrals for the first two coordination spheres including structural defects and non-stoichiometry. © 2015 Elsevier B.V. All rights reserved.

Keywords: Chalcogenides X-ray diffraction Magnetic properties Specific heat

1. Introduction Commercial applications of thermoelectric devices entailed interest in new materials. Seleno-spinels are promising compounds for this purpose [1] due to rather large cubic unit cell (about 10 Å), ease of substitution and synthesis and vacant octahedral holes. Pure ZnCr2Se4 combines p-type semiconducting conductivity and a heel temperature lical antiferromagnetism (AFM) below the Ne TN z 20 K with a strong ferromagnetic (FM) component evidenced by a large positive CurieeWeiss temperature (q) of 115 K [2,3]. The helical structure has a FM arrangement in the (0 0 1) planes with a turning angle of 42 between the spins in adjacent (0 0 1) planes. It crystallizes in the cubic spinel ZnCr2Se4-type structure (space

* Corresponding author. _ E-mail address: [email protected] (E. Macia˛ zek). http://dx.doi.org/10.1016/j.matchemphys.2015.11.020 0254-0584/© 2015 Elsevier B.V. All rights reserved.

group Fd-3m) [4], with lattice parameter: a ¼ 10.4970 Å (ICDD: 03065-0689). It has a normal cation distribution with zinc ions located at the tetrahedral sites and chromium ions in octahedral ones. Substitution of one of the elements can strongly influence on properties of a parent compound [5e9]. Paramagnetic gadolinium ion is very rewarding element because it contains only the spin contribution. Moreover, there are many works on doping Gd to modify the physical properties of other compounds. In the cubic and tetragonal phases of EuO the effect of Gd doping and O deficiency on the electronic structure, exchange interaction, and Curie temperature has been reported. Among other things, the Curie temperature was found to exhibit a distinct maximum as a function of the defect concentration [10]. In turn, the zinc oxide (ZnO), a prototypical wide-band-gap oxide, doped with Gd showed the Kondo effect, while ZnO is not a conventional material for manifesting the Kondo physics [11]. Current paper is a continuation of our earlier work aiming in

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2.4. Magnetic and specific heat measurements

exploring an influence of substitutions onto physical and chemical octahedral sites. We present results of structural analysis of the new Gd-doped ZnCr2Se4 compounds using X-Ray powder diffraction, Rietveld refinement and magnetic and specific heat measurements.

Static magnetic susceptibility in the zero-field-cooling (ZFC) and field-cooling (FC) mode and magnetization were measured in the temperature range 2e300 K using a Quantum Design Physical el temperature Properties Measurement System (QD-PPMS). The Ne and the critical fields, were determined as the temperature corresponding to the extreme of dc/dT vs. T and dM/dH vs. H, respectively. The effective pffiffiffi magnetic moment was calculated from the meff ¼ 2:83 C mB equation, where C is the Curie constant [12]. The superexchange integrals for the first two coordination spheres J1 and J2 were calculated with the aid of the Holland and Brown equations: TN ¼ e5J1 þ 10J2 and q ¼ 15J1 þ 90J2 [13]. Specific heat was measured with a Quantum Design Physical Properties Measurement System (QD-PPMS) with heat capacity option in temperature range 1.9e250 K and at magnetic field up to 90 kOe.

2. Experimental 2.1. Synthesis The samples were obtained using ceramic method from high e purity elements in a form of powders: Zn- spectrally pure, Cr-99.9%, Gd-99.9% and Se-99.5%. The weighs were prepared with the assumption that gadolinium ions substitute chromium and occupy octahedral sites. The stoichiometric amounts of elements were weighed, the total mass of a sample was equal 6 g. Mixtures were loaded in 25 mm outer diameter, approx 250 mm long quartz ampoules. Then they were evacuated to 105 Torr and sealed using torch. Samples were sintered in chamber furnace for six days: first day at 473 K, second day at 673 K and the remaining days at 1073 K. After that, the samples were furnace cooled, opened in air, pulverized in an agate mortar, re-sealed in quartz ampoules and sintered again. This procedure was repeated maximum of four times: the last two times the sintering temperature of 1173 K was applied. Powder diffractograms were recorded after the final reaction and in between heatings in order to control the progress.

3. Results and discussion 3.1. Phase and analytical analysis The X-ray phase analysis showed that all obtained samples have the main phase in a spinel structure ZnCr2Se4 (ICDD PDF 03-0650689) (see Fig. 1). The lattice parameters of the spinel phase varied slightly (Table 2) and increase with the increase of gadolinium doping as the ionic radius for gadolinium in the sixfold coordination r ¼ 93.8 pm is bigger than for chromium r ¼ 62 pm [14]. Analytical measurements confirmed gadolinium presence in the obtained samples and determined its solubility in ZnCr2Se4 as 6.5 weigh percent (Table 1). Table 2 summarizes crystal data and refinement results for Gd-doped ZnCr2Se4 spinel system. There is evidence that the presence of gadolinium influences the lattice parameters in the spinels under study. The final structure parameters for Gd-doped ZnCr2Se4 showed that the presence of gadolinium do not influence the anion parameters (Table 3).

2.2. Analytical measurements The chemical analysis of powder materials was determined using energy-dispersive X-ray fluorescence (EDXRF) spectrometer e Epsilon 3 (Panalytical, Almelo, The Netherlands) with a Rh target X-ray tube operated at max. voltage of 30 keV and max. power of 9W. The spectrometer is equipped with thermoelectrically cooled silicon drift detector (SDD) with 8 mm Be window and resolution of 135 eV at 5.9 keV. The quantitative analysis was performed using Omnian software based on fundamental parameter method. The results are shown in Table 1.

3.2. Magnetic studies The results of magnetic measurements of the spinels under

2.3. X-ray measurements Powder X-ray diffraction were collected at room temperature, in the Bragg-Brentano geometry using Siemmens D5000 powder diffractometer with Ni filtered CuKa1 radiation. The diffraction patterns were collected in the 2q range of 10e140 with 0.05 step, constant slit and time mode. The main, spinel phase of ZnCr2Se4 structure (ICDD: 03-065-0689) was identified in all diffraction patterns. The slight lines belonging to other compounds were identified as ZnSe (ICDD: 01-071-5978) and Se (ICDD: 00-0511389) phases. The phase analysis was done using the ICDD PDF4þ2014. The qualitative phase analysis and the Rietveld refinement were performed using the PANalitycal HighScore Plus package.

Table 1 Results in % (m/m) of the EDXRF chemical analysis for Gd-doped ZnCr2Se4 polycrystals. No

1 2 3

Fig. 1. XRD of ZnCr2Se4 spinels Gd-doped.

Determined concentration of elements

Obtained composition

Zn

Cr

Gd

Se

14.1 ± .0.55 12.5 ± .0.61 11.9 ± 0.43

21.4 ± 0.91 19.7 ± 0.75 19.5 ± 0.83

2.15 ± 0.09 5.01 ± 0.22 6.50 ± 0.26

62.3 ± 2.0 62.8 ± 2.5 62.2 ± 1.9

Zn1.09Cr2.09Gd0.07Se4 Zn0.96Cr1.96Gd0.16Se4 Zn0.92Cr1.90Gd0.21Se4

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Table 2 Structural data collection and structure refinement details for Gd-doped ZnCr2Se4 spinels. Structural data Chemical formula M (g/mol) Cell setting, space group a (Å) V (Å3) Z, radiation d (g/cm3) Refinement R factors (%) Goodness of fit Profile function No. of parameters (a, SOF, xyz, UVW)

Zn1.09Cr2.09Gd0.07Se4 506.79 Cubic, Fd-3m 10.4926(1) 1155.18(1) 8, CuKa1 5.943

Zn0.96Cr1.96Gd0.16Se4 505.69 Cubic, Fd-3m 10.4936(8) 1155.53(6) 8, CuKa1 5.792

Zn0.92Cr1.90Gd0.21Se4 507.81 Cubic, Fd-3m 10.4949(4) 1155.95(1) 8, CuKa1 5.831

Rwp ¼ 12, Rexp ¼ 10, Rp ¼ 10 S ¼ 1.44 Pseudo-Voit 13

Rwp ¼ 12, Rexp ¼ 10, Rp ¼ 10 S ¼ 1.55 Pseudo-Voit 13

Rwp ¼ 19, Rexp ¼ 10, Rp ¼ 12 S ¼ 1.94 Pseudo-Voit 13

Table 3 Site occupation factors (SOF) and anion parameter (u) for Gd-doped ZnCr2Se4 spinels. Compound Zn Cr Gd Se u

SOF SOF SOF SOF

Zn1.09Cr2.09Gd0.07Se4

Zn0.96Cr1.96Gd0.16Se4

Zn0.92Cr1.90Gd0.21Se4

0.99 0.88 0.09 1 0.2407(3)

0.95 0.85 0.10 1 0.2407(3)

0.92 0.82 0.12 1 0.2407(3)

Atomic positions are given in standard setting for space group Fd-3m (No. 227). Zn 8b (3/8.3/8.3/8), Cr/Gd 16c (0,0,0), Se 32e (u, u, u).

study are depicted in Figs. 2e5 and in Table 4. The dependence of the magnetic susceptibility versus temperature T, c(T), both in ZFC and FC mode shows AFM behaviour below TN ¼ 23 K for all measured spinels (Fig. 2) and paramagnetic state above TN. The latter is also visible on the magnetization curve, M(H), (Fig. 3). For

0.3 T = 300 K Zn0.92Cr1.90Gd0.21Se4 Zn0.96Cr1.96Gd0.16Se4 Zn1.09Cr2.09Gd0.07Se4

M (μB/f.u.)

0.2

0.1

0.0

0

15

30

45

60

75

H (kOe) Fig. 3. Magnetization M vs. magnetic field H at 300 K for the spinels under study.

Fig. 2. Magnetic susceptibility c recorded at the ZFC and FC mode as well as 1/cZFC vs. temperature T for Zn1.09Cr2.09Gd0.07Se4 (a), Zn0.96Cr1.96Gd0.16Se4 (b) and Zn0.92Cr1.90Gd0.21Se4 (c). The solid (black) line, (T-q)/C, indicates a CurieeWeiss el and freezing temperatures, behaviour. TN and Tf indicated by arrows are the Ne respectively.

confirmation of these findings the superexchange integral for the first coordination sphere (J1) is negative and it does not change significantly as the Gd content increases (Table 4). Next, the product of cZFC,T versus T in Fig. 4 achieves almost zero, decreasing continuously and going through the inflection point upon cooling, which suggests fully compensated AFM interactions between the metal centres and the metamagnetic transition visible in the M(H) curve (Hc1 in Fig. 5). It is worthwhile noticing that the first critical field (Hc1) poorly depends on the Gd substitution. It means that the el temperature, are long-range AFM interactions, defined by the Ne strong and the paramagnetic Gd ions do not substantially take part in the exchange interactions. In a case of the short-range FM interactions, represented by the positive value of the CurieeWeiss temperature (q), a slight decrease of q as the Gd content increases, especially in comparing to the matrix ZnCr2Se4, is observed. In the

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Fig. 4. Product cZFC,T and effective magnetic moment meff vs. temperature T for Zn1.09Cr2.09Gd0.07Se4 (a), Zn0.96Cr1.96Gd0.16Se4 (b) and Zn0.92Cr1.90Gd0.21Se4.

6

3.3. Specific heat studies

5 Hc2

M (μB/f.u.)

4 3 Hc1

2

T=2K Zn0.92Cr1.90Gd0.21Se4

1

Zn0.96Cr1.96Gd0.16Se4 Zn1.09Cr2.09Gd0.07Se4

0

same way behaves the superexchange integral for the second coordination sphere (J2), which also slightly decreases with increasing Gd-content. The relatively small span of q refers to the samples of similar content of magnetic ions and a certain amount of structural defects as the spinels under study are non-stoichiometric (Table 4). It may mean that the FM cluster structure in a random manner above ordering temperature is formed during the synthesis process. Reducing of the second critical field (Hc2) with the rise in the content of gadolinium can suggest a weakening of the FM shortrange interactions, since the breakdown of the helical spin arrangement in lower magnetic fields is observed (Table 4 and Fig. 5). Here, the paramagnetic Gd ions can play a role of the spin defects as it in case of Al ions in the ZnxCryAlzSe4 spinel series [15,16]. In both cases mentioned above a disappearance of Hc2 is not observed, as is the case for ZnCr2Se4 doped with indium [17]. The calculations of the magnetic parameters in Fig. 4 and in Table 4 (including data of ZnCr2Se4 [15e18] for comparison) are presented. The effective magnetic moment meff vs. T (Fig.4) undergoes through a broad peak close to the ordering temperature and slowly decreases in the CurieeWeiss region, a little more than the effective number of Bohr magnetons, peff, (Table 4). This divergence between meff and peff can mean, that with increasing of the Gd3þ ions the mixed valence of Cr3þ and Cr4þ, caused both by the structural defects and non-stoichiometry can occur. The superexchange integrals for the first two coordination spheres, J1 and J2, calculated based on the Holland and Brown equations [13], show the strong competition between AFM and FM exchange interactions, since the splitting of the ZFC-FC susceptibilities below the freezing temperature Tf ~ 7.3 K is observed.

0

15

30

45

60

75

H (kOe) Fig. 5. Magnetization M vs. magnetic field H at 2 K for the spinels under study. Critical fields Hc1 and Hc2 are indicated.

l temperature (Table 4) specific heat Due to the comparable Nee measurements were made only for the spinel Zn1.09Cr2.09Gd0.07Se4. The results of these measurements are depicted in Figs. 6e9. Fig. 6 shows a not sharp peak at TN for H ¼ 0 in both the curve C(T) and C(T)/T, unlike the sharp peak observed for the ZnCr2Se4 matrix [9]. A similar effect was observed for the In-doped ZnCr2Se4, caused probably by the complex magnetic structure (e.g., mixed or lowspin (LS) states) [19]. TN in Fig. 7 is shifted to lower temperatures by external magnetic fields and finally is fully suppressed by a field of 90 kOe, where the helical spin arrangement is completely destroyed. However, the specific heat plotted in the representation C/T vs. T (Fig. 8) exhibits another broad feature with a maximum at ~5 K under the field 30 kOe, while for the fields larger than 30 kOe the effect is not observed. This weak maximum could mean the Z Tof magstate of the spin freezing visible in the ZFC and FC curves CðTÞ netic susceptibility (Fig. 2). Fig. 9 shows entropy SðTÞ ¼ dT T 0 obtained at different magnetic fields. At the ordering temperature TN ¼ 23 K and magnetic field H ¼ 0 the value of magnetic and phonon contributions to the entropy is only ~4.3 J/(mol,K), i.e., much lower than the magnetic contribution Sm ¼ Rln(2S þ 1) ¼ 11.52 J/(mol,K) calculated per one Cr3þ ion with

Table 4 el, freezing and CurieeWeiss temperatures, respectively, meff is the Magnetic parameters of ZnCr2Se4 spinels doped with Gd: C is the Curie constant, TN, Tf andq are the Ne effective magnetic moment, peff is the effective number of Bohr magetons, J1 and J2 are the superexchange integrals for the first two coordination spheres. Experimental data for ZnCr2Se4 were taken from Refs. [10e12] for comparison. Spinel

C (emu,K/mol)

TN (K)

Tf (K)

q (K)

meff (mB/f.u.)

peff

Hc1 (kOe)

Hc2 (kOe)

J1 (K)

J2 (K)

ZnCr2Se4 Zn1.09Cr2.09Gd0.07Se4 Zn0.96Cr1.96Gd0.16Se4 Zn0.92Cr1.90Gd0.21Se4

4.057 4.642 4.668 5.116

21 23 22 22

12 5.8 5.0 7.3

86 65.4 71.5 63.9

5.700 6.097 6.115 6.401

5.477 5.980 6.283 6.460

10.0 11.4 13.1 10.0

65.0 62.9 57.7 60.4

1.72 2.36 2.11 2.24

1.24 1.12 1.15 1.08

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200

191

0.9

1.5

100 Zn1.09 Cr2.09Gd0.07Se4

50

0

0.5

H = 0 kOe C C/T 0

50

100

150

2

1.0

C/T [J/(mol·K )]

TN

2

C [J/(mol·K)]

150

C/T [J/(mol·K )]

Zn1.09Cr2.09Gd0.07Se4

0.6 H: 0 10 kOe 30 kOe 50 kOe 70 kOe 90 kOe

0.3

0.0 250

200

T (K)

0.0

0

9

18

Fig. 6. Temperature dependence of the specific heat of Zn1.09Cr2.09Gd0.07Se4 plotted as C and C/T vs. T in zero magnetic field.

36

T (K) Fig. 8. Temperature dependence of the specific heat of Zn1.09Cr2.09Gd0.07Se4 plotted as C/T vs. T.

32

Zn1.09Cr2.09Gd0.07Se4

250

H:

24

0 10 kOe 30 kOe 50 kOe 70 kOe 90 kOe

16

Zn1.09Cr2.09 Gd0.07Se 4 H= 0 H = 90 kOe

200

S [J/(mol·K)]

C [J/(mol·K)]

27

8

150 100 50

0

0

9

18

27

36

T (K)

0

0

50

100

S ¼ 3/2 and neglecting the magnetic contribution of Gd ions”. Figs. 7 and 8 show that the magnetic anomaly above 2 K is fully suppressed by the field of 90 kOe. Therefore, to estimate the magnetic contribution to the entropy we subtracted the S data obtained at H ¼ 0 by the entropy S(T) measured under magnetic field of 90 kOe. This procedure gives Sm z 3.5% of the full magnetic entropy expected of the magnetic contribution of Cr3þ ions. It is the smallest contribution to the magnetic entropy compared to ZnCr2Se4 doped with gallium, indium and cerium [17]. However, a very similar and strong reduction is characteristic of the doped [17] and parent [18] ZnCr2Se4 spinels. This anomaly can be interpreted as a result of spin fluctuations present in the paramagnetic regime above TN. 4. Conclusions The single phase region of the solubility limit of gadolinium in ZnCr2Se4 was found to be about 6.5%, which can be attributed to

200

250

T (K)

Fig. 7. Specific heat, C, vs. temperature T taken at different external magnetic fields for Zn1.09Cr2.09Gd0.07Se4.

3þ

150

Fig. 9. Entropy S vs. Zn1.09Cr2.09Gd0.07Se4.

temperature

T at

different

magnetic

fields

H

for

large mismatch between ionic sizes of Gd and Cr ions. All of the samples revealed cubic spinel as the dominant component with slight presence of additional phases. The change of lattice parameters of the cubic phase with increased nominal Gd content may indicate that the gadolinium is incorporated in the spinel lattice even for larger concentrations but the solid solution phase separates on cooling. Magnetic measurements and calculations of the magnetic parameters showed antiferromagnetic order and the spin-glass behaviour connected with the strong competition between AFM and FM exchange interactions visible in the splitting of the ZFC-FC susceptibilities. For all samples a metamagnetic transition at Hc1 and the breakdown of the helical spin arrangement at Hc2 were observed. The anomalies in the magnetic susceptibility and specific heat observed close to TN are in good agreement and become strongly dependent on the stoichiometry and structural defects.

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Acknowledgement This paper is funded from science resources for years 2011e2014  and MF thank as a research project (project No. N N 204 151940). AS the National Science Centre (NCN) for financial support, on the basis of Decision No. DEC-2012/07/B/ST3/03027. References [1] G.J. Snyder, T. Caillat, J.P. Fleurial, Mat. Res. Innov. 5 (2001) 67. [2] R. Plumier, J. Phys. 27 (1966) 213. Paris. [3] F.K. Lotgering, in: Proceedings of the International Conference on Magnetism, Nottingham, Institute of Physics, London, 1965, p. 533. [4] R. Kleinberger, R. de Kouchkovsky, Comp. Rend. Acad. Sci. 262 (1966) 628. _  , T. Mydlarz, A. Kita, J. Alloys Comp. 442 (2007) [5] E. Macia˛ zek, H. Duda, T. Gron 183. [6] I. Jendrzejewska, P. Zajdel, T. Goryczka, J. Goraus, A. Kita, T. Mydlarz, J. Alloys Compd. 520 (2012) 153. [7] I. Jendrzejewska, P. Zajdel, J. Heimann, J. Krok-Kowalski, T. Mydlarz,

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