Absence of ferromagnetism in Mn- and Fe-stabilized zirconia nanoparticles

ARTICLE IN PRESS Physica B 403 (2008) 4264–4268

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Absence of ferromagnetism in Mn- and Fe-stabilized zirconia nanoparticles J. Yu, L.B. Duan, Y.C. Wang, G.H. Rao  Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

a r t i c l e in f o

a b s t r a c t

Article history: Received 29 July 2008 Received in revised form 29 August 2008 Accepted 17 September 2008

In order to experimentally check the recent theoretical prediction regarding the Mn- or Fe-doped cubic ZrO2 as potential spintronic materials and to search for high-temperature ferromagnetic spintronic material, a series of Zr1xMnxO2 (x ¼ 0.15–0.35) and Zr1xFexO2 (x ¼ 0.15–0.40) nanoparticles were prepared by a coprecipitation method. The crystal structure and magnetic properties of the compounds were investigated by means of X-ray diffraction, transmission electron microscopy (TEM) and magnetic measurements. Detailed structural analysis confirmed that single phase of Mn- and Fe-stabilized cubic zirconia was obtained. The solubility limit of Mn (or Fe) in ZrO2 is about x ¼ 0.27 (or 0.36). Magnetic measurements showed that all the samples were paramagnetic at room temperature and 5 K. Existence of antiferromagnetic interactions in the samples was inferred from the fitting of magnetization data to the Curie–Weiss law. The absence of ferromagnetism could plausibly be attributed to an excess of oxygen vacancies in the Mn- or Fe-stabilized cubic zirconia. & 2008 Elsevier B.V. All rights reserved.

PACS: 61.90.i 75.90.+w 65.90.+i Keywords: Doped zirconia Crystal structure Magnetization Spintronic materials

1. Introduction In recent years, spintronics has been one of the most important fields of research [1–5]. The charge and spin of an electron are expected to be utilized for carrying information and for storing data, respectively. Spintronic devices are anticipated to have lots of advantages over conventional semiconductor devices, such as the nonvolatility, higher data processing speed and lower electric power consumption. To be used in these devices, it is desirable for the spintronic materials such as diluted magnetic semiconductors or half-metal ferromagnets to have an above-the-room-temperature Curie temperature and be compatible with conventional semiconductors and metals [6]. Mn- or Fe-doped ZrO2 has been predicted as potential spintronic material according to the recent ab initio electronic structure calculations [7]. The theoretical work showed that cubic Zr1xMxO2 (M ¼ Mn, Fe or Co) remains ferromagnetic to reasonably high temperatures (4500 K for x ¼ 0.25) and this ferromagnetism is robust to oxygen vacancy defects. On the one hand, TC of Zr1xMnxO2 increases with the increase of the Mn content and is well above room temperature (RT) when the Mn content is higher than 0.05. On the other hand, TC of Zr1xMnxO2 decreases with the increase of oxygen vacancies.

ZrO2 can crystallize in three different forms with monoclinic (space group: P21/c), tetragonal (space group: P42/nmc) and cubic (space group: Fm3¯m) structures at normal atmospheric pressure [8]. For bulk pure zirconia, the monoclinic phase is the thermodynamically stable one up to about 1400 K. It transforms to the tetragonal phase at ca 1400 K via a first-order displacive martensitic phase transition and then to the cubic phase at ca 2650 K upon increasing the temperature [8]. However, the cubic phase can be stabilized at RT by doping with appropriate amount of some elements (such as Y, Mn, Fe, etc.) [9]. For nano materials, the cubic ZrO2 could be stable at RT owing to the effect of surface free energy [10]. Although ZrO2 is widely used as a catalytic material, its magnetic properties have rarely been studied up to now. To verify the theoretical prediction of Ostanin et al. experimentally, Clavel et al. recently synthesized zirconia doped with up to 5% of manganese by a benzyl alcohol route and found no ferromagnetism even at low temperature [11]. They speculated that an increase in the manganese concentration was required to achieve the ferromagnetism. In this article, we synthesized a series of cubic Zr1xMnxO2 (x ¼ 0.15–0.25) and Zr1xFexO2 (x ¼ 0.15–0.35) samples and studied their structure and magnetic properties to search for a novel ferromagnetic spintronic material with high Curie temperature.

2. Experiment  Corresponding author.

E-mail address: [email protected] (G.H. Rao). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.09.015

Zr1xMxO2 (M ¼ Mn or Fe and x ¼ 0.15–0.45) nanoparticles were prepared using a coprecipitation method. An appropriate

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3. Results and discussion The XRD patterns of the Zr1xMnxO2 (x ¼ 0.15–0.30) at room temperature are shown in Fig. 1(a). The XRD pattern of the sample doped with up to x ¼ 0.25 exhibits only diffraction peaks of the cubic ZrO2 structure (space group: Fm3¯m) and does not show any evidence of impurity phases. Rietveld refinement result of the XRD pattern for Zr0.75Mn0.25O2 is shown in Fig. 2(a). The solid curve and crosses represent calculated and experimental patterns, respectively. The vertical bars indicate the expected Bragg reflection positions and the lowest curve is the difference between the observed and calculated patterns. As the amount of Mn doping increases, the XRD peaks clearly shift to higher angle, indicating a decrease of the unit cell parameter. Evolution of cell constant a derived from the Rietveld refinement with the Mn content x is shown in Fig. 3. The cell constant a decreases linearly with x from x ¼ 0.15 to 0.25, which conforms to the Vegard’s law. Trace amount of Mn2O3 as a secondary phase was found in the sample with x ¼ 0.3 as marked by the downward arrow in Fig. 1(a). Therefore, we conclude that the solubility limit of Mn ions in ZrO2 under our synthesis condition is about x ¼ 0.27 and

Intensity (arb.unit)

Zr0.75Mn0.25O2

Zr0.75Fe0.25O2

Rwp:12.8 %

Rwp:9.68 %

20

40

60 80 2θ (degree)

100

Fig. 2. Observed (crosses) and calculated (solid curve) XRD patterns of Zr0.75Mn0.25O2 (a) and Zr0.75Fe0.25O2 (b). The vertical bars at the bottom indicate the expected Bragg reflection positions, and the lowest curve is the difference between the observed and the calculated XRD patterns.

5.09 Zr1-xMnxO2 Zr1-xFexO2

5.08 5.07 a (Å)

amount of analytical reagent-grade 50% solution of Mn(NO3)2 (or Fe(NO3)3  9H2O) and ZrOCl2  8H2O were mixed and the hydroxides were precipitated by a slow addition of ammonia during stirring. The precipitations were calcined at 873 K in air for 1 h. The phase composition and crystal structure of the samples were examined by X-ray powder diffraction (XRD) using a PANalytical X’pert PRO Alpha-1 diffractometer with Cu Ka1 radiation (l ¼ 1.5406 A˚) and a high-resolution transmission electron microscope (HRTEM, JEM-2010). The XRD data were analyzed by the Rietveld refinement technique using the program FULLPROF [12,13]. Magnetic measurements were performed on a superconducting quantum interference device (SQUID, Quantum Design MPMS 7). For the thermogravimetric tests, a TA Instruments SDT Q600 was used. The mass of specimens for the TGA analysis was in the range of 14–17 mg. All samples were heated at a constant rate of 20 1C/min from ambient temperature to 750 1C after being isothermal at 100 1C for 20 min. High purity Ar was used as purging gas at a constant flowing rate.

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J. Yu et al. / Physica B 403 (2008) 4264–4268

5.06 5.05 5.04 5.03 0.15

0.20

0.25

0.30

0.35

0.40

x Fig. 3. Evolution of cell constant a of Zr1xMnxO2 (or Zr1xFexO2) nanoparticles as a function of the Mn (or Fe) content x.

Intensity (arb.unit)

x = 0.30 0.25 0.22 0.20 0.15

Intensity (arb.unit)

x = 0.40 0.35 0.30 0.25 0.20 0.15

20

40

60 80 2θ θ (degree)

100

Fig. 1. XRD patterns of Zr1xMnxO2 (a) and Zr1xFexO2 (b) nanoparticles. The downward arrows mark the reflection positions of Mn2O3 in (a) and Fe2O3 in (b).

the smaller Mn ions replace Zr ions randomly in ZrO2 for x ¼ 0.15–0.27. The XRD patterns of Zr1xFexO2 (x ¼ 0.15–0.40) are shown in Fig. 1(b). The samples with the Fe content up to x ¼ 0.35 are of single phase with the cubic ZrO2 structure. Fe2O3 peaks, marked by the downward arrow, were detected in the XRD pattern as a secondary phase for the samples with higher dopant concentration xX0.4. The cell constant a of the Fe-doped ZrO2 also decreases linearly with x from x ¼ 0.15 to 0.35 due to the random replacement of Zr ions by the smaller Fe ions (Fig. 3). The solubility limit of Fe ions in ZrO2 is about x ¼ 0.36. As an example, Fig. 2(b) shows the refinement results of the XRD pattern for Zr0.75Fe0.25O2. Typical HRTEM images of Zr0.75Mn0.25O2 and Zr0.75Fe0.25O2 nanoparticles are shown in Fig. 4(a and b), respectively. HRTEM analyses were carried out on different regions of the samples. The images reveal no segregation of secondary phases and a uniform microstructure. A spacing of (2¯ 0 2¯) planes in Zr0.75Mn0.25O2 is about 1.80 A˚ in Fig. 4(a). The bottom-right inset shows the

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M (emu/g)

600

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[001]

150 0.00

d = 1.79 Å

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0 0

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150 T (K)

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Fig. 5. M vs T and 1/w vs T curves of Zr1xMnxO2 (a) and Zr1xFexO2 (b) in a field of 1000 Oe after cooling in zero field (ZFC).The dashed lines are the theoretical fits to the Curie–Weiss law.

5 nm

Fig. 4. TEM images of Zr0.75Mn0.25O2 (a) and Zr0.75Fe0.25O2 (b) nanoparticles. The insets shows the selected area Fast Fourier Transform.

Table 1 The results derived from the fitting to Curie-Weiss law of the w–T curve in high temperature range Zr1xMnxO2

selected area fast Fourier transform (FFT) pattern. A spacing of (2¯ 2 0) planes in Zr0.75Fe0.25O2 is about 1.79 A˚ in Fig. 4(b). The top inset shows the selected area FFT pattern. Fig. 5 shows plots of magnetization and inverse susceptibility 1/w as functions of temperature for Zr1xMnxO2 (a) and Zr1xFexO2 (b) acquired on warming in a field of 1000 Oe after cooling in zero field (ZFC). The plots of 1/w versus T are almost linear in hightemperature region and exhibit some curvature, small for Zr1xMnxO2 and distinct for Zr1xFexO2, in low temperature region. The high-temperature part (between 200 and 300 K) of 1/w vs. T displays Curie–Weiss behavior and the data can be fitted by the Curie–Weiss law, w ¼ C0x/(T+Y) [14]. The effective magnetic moment meff and the effective spin angular momentum S are derived by meff ¼ [3kBC0x/NA]1/2 ¼ g[S(S+1)]1/2mB, where g is the Lande´ factor and for transition metal ions it is reasonable to take g ¼ 2 [14]. The fitting results are shown in Table 1. The negative sign of the Curie–Weiss temperature Y indicates the presence of antiferromagnetic (AFM) interactions in all the samples. The negative value of Y decreases with the increase of the Mn (or Fe) content, suggesting that AFM interactions in the samples are strengthened with the increase of the Mn (or Fe)

Y (K)

meff (mB) S

Zr1xFexO2

x ¼ 0.15

x ¼ 0.20

x ¼ 0.25

x ¼ 0.15

x ¼ 0.25

x ¼ 0.35

18.6 4.58 1.84

29.3 4.52 1.81

42.1 4.68 1.89

112.5 5.14 2.12

163.1 4.87 1.99

222.1 4.74 1.92

content x. For Zr1xMnxO2 (x ¼ 0.15, 0.20 and 0.25), the derived values of meff and S are comparable to the value for a high-spin Mn3+ ion (S ¼ 2, m ¼ 4.9mB [15]), whereas the derived S of Zr1xFexO2 (x ¼ 0.15, 0.25 and 0.35) is close to the value for a high-spin Fe2+ ion (S ¼ 2). X-ray photoelectron spectroscopy (XPS) revealed the Mn3+ valence state in nanocrystalline Zr1xMnxO2 [16], in consistence with our inference from the Curie–Weiss law fitting to the magnetization data. However, the same authors speculated that the Fe is in the Fe3+ valence state in the nanocrystalline Zr1xFexO2 based on the electron paramagnetic resonance (EPR) experiments [17]. For the samples investigated in the present work, if both Mn and Fe ions were in trivalent state, the lattice constant of Zr1xMnxO2 and Zr1xFexO2 should be almost the same for the

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same doping level x, since the high-spin Mn3+ and Fe3+ have almost the same ionic radii (rMn3+ ¼ rFe3+ ¼ 0.645 A˚, CN ¼ 6 [18] and the data of CN ¼ 8 for Mn3+ is not available therein). For the cubic ZrO2 structure (fluorite-type), there is no adjustable atomic parameters and the cell parameter a is exclusively determined by the Zr–O bondlength. By requiring the bond valence sum around oxygen to equal to its chemical valence [19], it is easy to derive that the ideal lattice parameter a(Zr1xMn3+O2y)Ea (Zr1xFe3+O2y)4a(Zr1xFe2+O2y), in consistence with our observations (Fig. 3). Extensive studies revealed that the formation of oxygen vacancies plays a crucial role for the stabilization of the tetragonal and cubic phases of zirconia [8,20]. Substitution of low-valence ions such as Y3+, Ca2+, Mg2+, Mn3+, etc. for high valence Zr4+ ions is effective to stabilize the cubic zirconia, which tends to introduce oxygen vacancies in the structure due to the charge compensation. Fig. 6 shows the TGA traces of the Zr0.75Mn0.25O2 and Zr0.75Fe0.25O2 samples upon heating to 750 1C in flowing Ar atmosphere. Fig. 7 shows XRD patterns of the samples at 773 K, which indicates no phase transformation in both the samples up to 773 K. Therefore, the weight loss in both the samples can be attributed to the generation of oxygen vacancies upon heating.

99.0 Zr0.75Mn0.25O2

98.5

Zr0.75Fe0.25O2

Weight (%)

98.0 97.5 97.0 96.5 96.0 100

200

300 400 500 Temperature (°°C)

600

700

800

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The fact that the weight loss of Zr0.75Mn0.25O2 is larger than that of Zr0.75Fe0.25O2 is indicative of more oxygen vacancies in the assynthesized Zr0.75Fe0.25O2 and a lower valence state of Fe than Mn3+. According to the calculation of Ostanin et al. [7], the existence of oxygen vacancies is detrimental to the ferromagnetism of the Mn-stabilized cubic zirconia. The calculation showed that the Curie temperature TC is around 600 K for Zr0.75Mn0.25O2, decreases rapidly to 200 K for Zr0.75Mn0.25O1.875 and becomes antiferromagnetic for Zr0.75Mn0.25O1.85. Provided that the Fe is divalent in Zr1xFexO2 and Mn is trivalent in Zr1xMnxO2 and bearing in mind the strong dependence of TC on the oxygen vacancies, it is readily understandable that the negative Curie–Weiss temperature Y decreases with the doping level (Fe or Mn content) and the absolute value of Y of Zr1xFexO2 is much larger than that of Zr1xMnxO2 (see Table 1), which indicates a stronger AFM interaction in the Zr1xFexO2 due to a higher concentration of oxygen vacancies than in Zr1xMnxO2. Therefore, a subtle compromise of the Mn content and oxygen vacancy content seems crucial to stabilize the cubic phase and to achieve high TC ferromagnetic zirconia. The absence of ferromagnetism in Zr1xFexO2 and Zr1xMnxO2 investigated in this work could be attributed to excess oxygen vacancies in the investigated samples. Magnetization (M) at 300 and 5 K of Zr1xMnxO2 and Zr1xFexO2 as a function of magnetic field (H) up to 5 T is shown in Fig. 8(a–d). Magnetic measurements show no hysteresis, no remanence and no saturation, indicative of no evidence for ferromagnetism even at T ¼ 5 K. The solid curves are fitting results of the experimental data at 5 K using a Brillouin function expressed as [21,22]        a  x 2S þ 1 ð2S þ 1Þa 1 M ¼ eff Sg mB coth coth  , (1) 2S 2S x 2S 2S where a ¼ SgmB(SH/kBT), S ¼ 2, kB is the Boltzmann constant, and xeff is the effective Mn (or Fe) content that contributes to the net magnetization compared to the fully saturated magnetization of the Mn (or Fe) spin. For Zr1xMnxO2 (x ¼ 0.15, 0.20 and 0.25), the fitting value of xeff/x is 0.35, 0.29 and 0.25, respectively. For Zr1xFexO2 (x ¼ 0.15, 0.25 and 0.35), the xeff/x ¼ 0.22, 0.12 and 0.07, respectively. The xeff/x decreases with the increase of x, indicative of an increase of the average AFM interaction between the doped magnetic ions.

Fig. 6. Thermal gravity (TG) plots of Zr0.75Mn0.25O2 (dashed line) and Zr0.75Fe0.25O2 (solid line) in Ar atmosphere.

Intensity (arb.unit)

4. Conclusion Zr0.75Mn0.25O2 Rwp:13.4 %

Intensity (arb.unit)

773 K

Zr0.75Fe0.25O2 Rwp:12.8 % 773 K

20

40

60 80 2θ (degree)

100

Fig. 7. XRD patterns of Zr0.75Mn0.25O2 and Zr0.75Fe0.25O2 at 773 K.

Cubic Zr1xMnxO2 and Zr1xFexO2 samples with high doping concentration (Mn and Fe content x ¼ 0.15–0.25 and x ¼ 0.15–0.35, respectively) were synthesized. The crystal structure and magnetic properties of the doped samples have been investigated. The solubility limit of Mn (or Fe) in ZrO2 is about x ¼ 0.27 (or 0.36). The results of magnetic measurements showed no evidence of long-range ferromagnetic ordering between 5 and 300 K. All the samples are paramagnetic in the investigated temperature range. The fitting of magnetization data to Curie– Weiss law indicates the presence of the AFM interactions between the doped magnetic ions and the AFM interactions are enhanced with the increase of Mn (or Fe) content x. Detailed analysis of crystal structure and magnetization data indicates the Mn3+ and Fe2+ valence states in the Zr1xMnxO2 and Zr1xFexO2, respectively. The absence of the theoretically predicted high TC ferromagnetism in Mn- or Fe-stabilized zirconia could be attributed to excess oxygen vacancies in the investigated samples because of the opposite contributions of the magnetic ion concentration and oxygen vacancy concentration to the predicted ferromagnetism.

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1.2 0.8

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Fig. 8. M vs H curves of the Zr1xMnxO2 (a, c) and Zr1xFexO2 (b, d) samples at 300 and 5 K respectively. The solid lines are the fitting curves using the modified Brillouin function.

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