Conduction mechanism and magnetic behavior of dysprosium strontium iron garnet (DySrIG) nanocrystals

Materials Chemistry and Physics 126 (2011) 780–785

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Conduction mechanism and magnetic behavior of dysprosium strontium iron garnet (DySrIG) nanocrystals M.A. Ahmed a,∗ , Samiha T. Bishay b , S.I. El-dek a a b

Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egypt Phys. Dept., Faculty of Girls for Art, Sc. and Education, Ain Shams Univ., Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 29 June 2010 Received in revised form 15 August 2010 Accepted 13 December 2010 Keywords: Dy garnet Nanoparticles D. Crystal structure D. Transport properties Insulator to metal transition Exchange interaction

a b s t r a c t Garnet nanoparticles Dy2.8 Sr0.2 Fe5 O12 (DySrIG) were prepared by citrate auto-combustion method and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and differential thermal analysis (DTA). The suitable formation of this garnet in single phase was at 1100 ◦ C with crystallite of size 95 nm. The Curie temperature of DySrIG is obtained at 610 K. The effective magnetic moment eff was calculated experimentally and theoretically and they are compatible with each other. The dielectric constant ε increases from order 102 at room temperature to 104 at 850 K passing by four transition temperatures. The temperature dependence of the resistivity of DySrIG at different frequencies (f) 100 kHz ≤ f ≤ 5 MHz indicates the presence of 5 transition temperatures which are slightly different from those of ε data. The resistivity data are frequency independent at f < 1 MHz. The transition height is decreased by increasing the temperature from ≈5 M cm at 320 K and 200 kHz to ≈20  cm at 700 K. Accordingly, Dy2.8 Sr0.2 Fe5 O12 (DySrIG) is recommended for the use in phase shifter, circulators and microwave applications. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Rare earth ferrite garnet R3 F5 O12 (where R is a rare earth element) have attracted great attention for applications in both microwave devices and magnetic recording media [1,2]. Recently, garnets are widely used in microwave communication through mobile and satellite and are of significant interest for numerous applications including magnetic materials, laser, phosphorescent sources and electrochemical devices [3]. Yttrium iron garnets YIGs are scientific importance because the wide range of variety of magnetic properties that can be obtained in substituting yttrium by a rare earth metal [4–7]. Ferromagnetic garnets crystallized in cubic structure (space group Ia3d) every cell contains eight R3 3+ Fe5 3+ O12 molecules, Fig. 1 [8]. R3+ cannot occupy the octahedral and tetrahedral site because of large ionic radius, so R3+ ions can only occupy dodecahedral site of large space. The ionic distribution in garnet structure can be represented by formula as {R3 }[Fe2 ](Fe3 )O12 ; {}, [] and () are represented as 24C dodecahedral, 16A octahedral and 24D tetrahedral, respectively. Caffarena et al. [9] prepared Sm3 Fe5 O12 by the co-precipitation method and submitted to sintering in the range of 1200–1400 ◦ C to establish a correlation of its magnetic properties with morphology.

∗ Corresponding author. E-mail address: [email protected] (M.A. Ahmed). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.12.044

Haitao et al. [10] prepared nanoparticles of Y3−x−y Cex Gdy Fe5 O12 ; x = 0.0–0.1, y = 0.0–0.1 by sol–gel-method with particle size ranged from 42 to 70 nm. They reported that the saturation magnetization increased with increasing the particle size as well as Ce content and decreased with increasing Gd content in a linear manner. Hemeda et al. [11] synthesized Dy3−x Nix Fe5 O12 ; 0.0 ≤ x ≤ 0.5 by the standard ceramic technique. They studied the dependence of the dielectric parameters on the temperature for the prepared samples in the temperature range 300–700 K. The dielectric constant in many insulating magnets pronounced changes at magnetic ordering temperatures or with application of external magnetic field [12]. Dy3 Fe5 O12 garnet exhibits a large Faraday rotation of 1 × 105◦ cm−1 , which may be developed as high-density magnetooptical recording media [13]. Few reports exist on the preparation of DyIG uptill now. Therefore, one decided to use citrate auto combustion to minimize the crystal size of such garnet as well as to study its physical properties. The aim of the present study was to describe our attempts to synthesize nanocrystalline Dy2.8 Sr0.2 Fe5 O12 (DySrIG) by citrate auto-combustion method. The determination of the optimum growth condition for the garnet phase by changing the heat treatment temperatures was one of our important goals. The samples have been thoroughly characterized using X-ray, IR spectra analysis, transmission electron microscope, DTA analysis, magnetization measurement as a function of temperature up to the Curie point. Also, the dielectric parameters were determined in a wide range

(800)

(840) (842) (664)

(444) (640) (642)

781

(611)

(321) (400)

Intensity

(521)

(420)

o

1100 C

(422)

M.A. Ahmed et al. / Materials Chemistry and Physics 126 (2011) 780–785

60

70

o

1080 C o

950 C o

700 C

ICDD 73-1378

Fig. 1. Crystal structure of rare earth garnet. The figure shows 1/8 of the elementary cell of a 3Fe5O12 crystal (R is rare earth or/and Y). Oxygen ions are located at the vertexes of the polyhedron.

20

30

40

50

80

2θ Fig. 2. XRD pattern of the sample DySrIG annealed at different temperatures.

of temperature 300 K ≤T ≤ 900 K as a function of the applied frequency 100 kHz ≤ f ≤5 MHz in order to reach the limit at which the materials become more applicable. 2. Experimental Sample of the chemical formula Dy2.8 Sr0.2 Fe5 O12 (DySrIG) was prepared using the citrate-nitrate auto combustion method. The molar ratio of metal nitrates to citric acid was 1:1. A small amount of ammonia was added to the solution to adjust the pH value at 2. The mixed solution was heated on a magnetic stirrer (200 ◦ C) and transformed into a xerogel. When ignition points were observed, the dried gel burnt in a self propagating combustion manner until all the gel was burnt out completely to form loose powder. In fact, the powder was heated at temperatures (700, 950, 1080 and 1100 ◦ C)for 3 h, to check on single phase formation. Only single garnet phase exists uniquely at 1100 ◦ C or above. X-ray diffraction analyses for the as prepared sample was carried out using Scin˚ to identify the preparation of the tag X-ray diffractometer with Cu K␣ ( = 1.5418 A) samples in proper. The particle size was calculated using Scherrer formula. The (DySrIG) sample was annealed at different temperatures (700, 950, 1080 and 1100 ◦ C) for a constant time 3 h until crystallization in single phase garnet without any traces of DyFeO3 . Differential thermal analysis (DTA) was carried out in nitrogen atmosphere using Shimadzu thermal analysis model DTA-50 with rate flow 30 ml min−1 , at a heating rate of 10 ◦ C min−1 . Transmission electron microscope JEOL-TEM-1230 was used to investigate the size and the morphology of particles of the sample annealed at 1100 ◦ C. The dc magnetic susceptibility measurements were carried out using  Faraday s method at three different magnetic field intensities where the measurements were performed from room temperature up to 850 K. In this method, a very small amount of the powdered sample was inserted in a cylindrical glass tube at the point of maximum gradient. For electrical conductivity measurements, the two surfaces of pellet were coated with silver paste and checked for good electrical contact. The dielectric measurements in a wide range of temperature 300 K≤ T ≤ 900 K were carried out at different applied frequencies 100 kHz ≤ f≤ 5 MHz using RLC bridge Hioki model 3531 Japan. The temperature of the sample was measured using K-type thermocouple connected to Digi-sense thermometer (USA) with junction in contact with the sample.

3. Results and discussion 3.1. Structural analysis XRD pattern of the sample of Dy2.8 Sr0.2 Fe5 O12 (DySrIG) prepared by citrate auto-combustion method treated at different temperatures (700 ◦ C ≤ T ≤ 1100 ◦ C) is shown in Fig. 2. The figure is giving rise to intermediate compounds (DyFeO3 and Sr3 Fe2 O7 ), a feature meaning that greater time and temperatures are needed, in order to achieve full conversion of the material to true iron garnet [14]. The figure shows that as the heat-treatment temperature increased, the volume fraction of the garnet phase increased. On the other hand, the volume fraction of the other foreign phases decreased considerably. It is possible that at high temperature, the perovskite and Fe2 O3 phases react with each other to give the garnet phase [15]. In fact, for the dysprosium precursor, only single garnet phase exists

uniquely at 1100 ◦ C or above. The crystallization of the sample in a single garnet phase structure is more complete as the annealing temperature is increased. The major diffraction peaks are marked at 1100 ◦ C at which a complete crystallization was obtained as indexed and compared with ICDD card No. 73-1378. This suitable degree of formation of the single garnet phase structure by autocombustion method is lower than 1600 ◦ C by standard ceramic technique [11,16] and 1400 ◦ C by the co-precipitation method [9]. The lattice parameter was calculated on the basis of cubic symmetry and found to a = 12.4005 A˚ which is slightly less than that ˚ ICDD card 73-1378 and (12.52 A) ˚ reported for the bulk (12.404 A) for Dy3 Fe5 O12 and YIG [11,17]. This may be due to doping with Sr which led to cell contraction. This was also associated with the increase in the average oxidation state of Fe ions and thus decreasing size. However, the garnet structure is quite complex, it consists of 160 atoms that are not closely packed. Thus, direct correlation between lattice volume and ionic size may be oversimplified [18]. The theoretical density was calculated from (Dx = ZM/NV), where Z is the number of molecules per unit cell, M is the molecular weight, N is Avogadro’s number and V is the unit cell volume. The theoretical density was found to be =6.659 g cm−3 which is in a good agreement with the ICDD card 73-1378 of 6.672 g cm−3 . The crystal size was calculated from the XRD line broadening of the peak (4 2 0) using the classical Scherrer relation, ˚ k is a conLh k l = k/B cos , were Lh k l is the particle diameter in A, stant (shape factor) with a value of 0.9, B is the full width at half maximum, and  is the target wave length. The obtained data show that, the crystallite size increases (from 18 to 40 to 75 to 95 nm) as the treatment temperature is increased from (700 to 950 to 1080 to 1100 ◦ C) respectively because the crystallization of the samples becomes more and more perfect with the disappearance of the secondary phase DyFeO3 by increasing the treatment temperature. The specific surface area S for the sample was 5.766 m2 g−1 as calculated from S = 6000/Dx L, where L is the average crystal size in nm. The TEM image for (DySrIG) annealed at 1100 ◦ C is illustrated in Fig. 3. The image shows a homogeneous distribution of particles and the crystallites are of spherical shape appeared agglomerated due to the magneto-optical properties of the garnet itself. The particle size (90 nm) agrees with that calculated from XRD. The inset of Fig. 3 shows the selected area electron diffraction SAED for (DySrIG) sample which is dominated by diffraction rings indicating the polycrystalline state of the nanoprepared sample. Also, the SAED pattern for (DySrIG) further, exhibits a clear specific orientation of the particles.

M.A. Ahmed et al. / Materials Chemistry and Physics 126 (2011) 780–785

M(emu/g)

782

1340Oe 1660Oe 1990Oe 16

12

8 300

400

500

600

700

800

T(K) Fig. 3. TEM micrograph of DySrIG annealed at 1100 ◦ C.

Fig. 5. Dependence of the magnetization on absolute temperature at different magnetic field intensities for DySrIG.

3.2. Thermal analysis The DTA thermogram and its derivative for the sample annealed at 1100 ◦ C is illustrated in Fig. 4. The derivative shows endothermic peaks at 325 K, 465 K, 515 K, 690 K and 800 K. This may be due to the creation of the oxygen vacancies through heating this type of rare earth garnet. This expectation is enhanced with the resistivity temperature spectrum as discussed below. 3.3. Magnetic measurements

Exo

Fig. 5 shows the dependence of the magnetization on the absolute temperature as a function of magnetic field intensity for the Dy2.8 Sr0.2 Fe5 O12 . The data reveals the normal trend of ferrimagnetic with a characteristic hump after which it decreases reaching the Curie temperature. In the first temperature range, the magnetization increases slightly up to near 425 K and then start to decrease. The decrease in the magnetization after that is due to the increase in the thermal energy which increases the entropy as well as the randomness and decreases the magnetocrystalline anisotropy. Heat capacity anomalies have been observed [19] for all garnets in the temperature range 530–560 K, resembling the type transition. From the reported data [19] on the rare-earth iron garnets (RIG), it has been observed that the phase transition is of second order in nature and involves magnetic order–disorder transition from ferrimagnetic to paramagnetic state characterized by 0.01

5

0.005

-15 0 -25 -0.005

Endo

-35

-45

-0.01

DTA(uV) DrDTA

-55 250

350

450

550

650

750

-0.015 850

T(K) Fig. 4. DTA thermograph for DySrIG annealed at 1100 ◦ C.

DrDTA(uV/min)

DTA(uV)

-5

the Curie temperature (TC ). The Curie temperature is obtained at 610 K as determined from the plot of dM/dT vs T; where M is the magnetization; T is the absolute temperature; and it was found to be slightly larger than the reported values for the un-doped sample [19]. The difference is mainly due to the doping of Dy3 Fe5 O12 garnet lattice by Sr2+ ions on the dodecahedral sites. The Fe3+ ions on the A sites are strongly coupled antiferromagnetically to those of the D sites [20–25]. The resultant magnetization of these two sublattices is antiferromagnetically coupled to the spins of the R3+ ions on the C sites. The magnitude of the Fe–Fe interaction is stronger than the R–Fe interactions. The coupling between the A and D sites is responsible for the position and shift in the Curie temperature which is approximately the same for all RIG. Doping DyIG with Sr2+ ions could lead to one of the following probabilities: The first is the occurrence of oxygen deficiency to compensate the charge difference between Dy3+ and Sr2+ . The second one is that some of the Fe3+ will be forced to change their valences to Fe4+ . The amount of Fe4+ ions is assumed here to be equal to the Sr2+ content to preserve charge neutralization. Same assumption was suggested and reported in rare earth orthoferrites by Ahmed et al. [26,27]. The effective magnetic moment was calculated from the plots of M −1 √ vs T using the relation eff = 2.83 C, where C is the Curie constant. The value of eff was found to be (36 B.M.) which is relatively large. The net magnetic moment is given by the equation [19] net = 3 ␮(R3+ )C − 3 ␮(Fe3+ )D + 2 ␮(Fe3+ )A ; where C, D and A denoted dodeca, octa and tetrahedral sites respectively and in our case, R = Dy with 10.5 B.M. The effective magnetic moment calculated from the above theoretical relation for the DySrIG is (24.32 B.M). The difference here between the experimental and theoretical values of eff is mainly due to two main reasons: the first one is the 4f–3d interactions as Dy3+ has 4f9 with 7 spins parallel and 2 anti-parallel and 3d5 of the Fe3+ ions where all spins are parallel. The second reason is the quenching of the orbital momentum especially for the Fe3+ ions which requires a strong crystalline field. Here, one expected that R–Fe interactions contribute to the magnetic properties altogether with the strong and predominant Fe–Fe interactions on A and D sites [19]. 3.4. Dielectric spectrum and resistivity study Fig. 6(a and b) illustrates the dependence of the dielectric constant ε on the absolute temperature 300 K ≤ T ≤ 850 K as a function of applied field frequency 100 kHz ≤ f≤5 MHz. It is clear that ε increases from order 102 at room temperature to 104 at 850 K passing by four transitions (435 K, 495 K, 682 K and 800 K) increasing in height with increasing the temperature. At high temperature, the

M.A. Ahmed et al. / Materials Chemistry and Physics 126 (2011) 780–785

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8.E+05

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0.E+00 300

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700

800

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500

600

700

T (K)

800

Fig. 7. The dependence of the resistivity on absolute temperature for DySrIG; (a) low frequencies, (b) high frequencies.

T (K) Fig. 6. The dependence of the real part of the dielectric constant (ε ) on absolute temperature for DySrIG; (a) low frequencies, (b) high frequencies.

dielectric constant increases with temperature; due to increasing the contribution resulting from ionic mobility and crystal defect mobility [11]. From another point of view, this increase corresponds to the jump orientation and space charge effects resulting from the increased concentration of charge carriers in the paramagnetic region. Fig. 7(a and b) illustrates the temperature dependence of the resistivity of DySrIG at different frequencies 100 kHz ≤ f ≤5 MHz. The data shows that there are 5 transition temperatures for the investigated sample at (325 K, 450 K, 500 K, 700 K and 800 K) which agree well with those obtained from DTA thermograph with small differences. It is known that in YIG, the iron ions on the octahedral sites form a body center cubic lattice and the nearest neighbor ˚ [28]. The nearest neighbor disdistance are relatively large (5.7 A) tances will slightly increase for the considered substituted samples by Sr2+ ions. Based on Austin and Mott model [29], the charge carriers hopping on the octahedral sites move most easily by jumping indirectly through tetrahedral sites. According to this model, two types of centers (A octahedral) and (D tetrahedral) are separated by a constant energy u < kT and that the conduction is due to hopping between A and D sites. However, in each case, the carriers must hop over an intermediate D site. Such a model could also explain the frequency independent resistivity in the first frequency range Fig. 7a.

According to Abo el Ata et al. [16] two parallel mechanisms may participate in the conduction process in the temperature region (400–600 K). The first one is the thermal activated hopping and the second is the conduction through the impurity levels. In the intermediate temperature region, two main interactions take place. The first one is the antiferromagnetic ordering between the moments of Fe3+ ions on the octahedral and those on the tetrahedral sites, which causes a splitting of the energy levels [30]. The second interaction is the electrostatic interaction between the electrons of cations and anions. The predominant conductivity in the last temperature region is due to the ionic conduction [31] through the oxygen vacancies, which act as donor centers [32]. Oxygen vacancies may be created during sintering process [33] and/or due to the substitution of Sr2+ into DyIG [34]. Another interpretation for the conduction mechanism of DySrIG is due to hole hopping between Fe4+ and Fe3+ . The presence of Fe4+ is related to the presence of Sr2+ ions. In Fig. 7(b), at T > 550 K, the resistivity increases with Table 1 The transition temperatures for the DySrIG sample as depicted from &T, ε &T, M&T and DTA, respectively. T (K)

Tε (K)

325 450 500 700 800

– 435 495 682 800

TM (K) – 425 – 610 –

TDTA (K)

Assignment

325 465 515 690 800

TR–Fe TFe–Fe TI–M TC –

784

M.A. Ahmed et al. / Materials Chemistry and Physics 126 (2011) 780–785

Table 2 Material Data sheet of DySrIG date: 20-6-2010 R.T. = room temperature.

Chemical formula Material State Color Odor Solubility Method/temperature preparation Melting point pH Thermal stability Crystal symmetry Space group ˚ a (A) Dx (g cm−3 ) Molecular weight Particle size (nm) XRD Particle size (nm) TEM R.T. dc ( cm) R.T. ac ( cm) at 5 MHz ac ( cm) at 800 K, 5 MHz R.T. ε at 5 MHz ε at 800 K, 5 MHz R.T. tan ı at 5 MHz tan ı at 800 K, 5 MHz R.T. Z () – 5 MHz Z () 800 K, 5 MHz

(␮s) M (emu g−1 ) at 1990 Oe TC (K) eff (exp.) B.M. Recommended applications

DySrIG

Reported

Reference

Dy2.8 Sr0.2 Fe5 O12 DySrIG Disks, fine particles Olive Green (RGB: 51 51 0) Odorless Water insoluble Citrate auto combustion/1100 ◦ C >1300 ◦ C Neutral 300–600 K Cubic Ia3d 12.4005 6.659 954.6 95 90 2.29 × 108 2.08 × 104 63 69 1.15 × 104 0.25 0.49 2.6 × 104 14.54 7.7 16.5 610 36 Phase shifter, circulators, microwave component

Dy3 Fe5 O12

ICDD 73-1378 – – – – – – – –

increasing temperature due to the electron–phonon interaction (lattice scattering). The decrease in the mobility of charge carriers decreases the conductivity. At T > 550 K, the sample behaves as metallic. Accordingly, one could assign this transition as insulatormetal transition. The transitions obtained in the DySrIG sample are reported in Table 1 as depicted from the –T, ε –T, M–T and the derivative of the DTA plots. The 1st transition at 325 K which appears as a peak in the –T plot could be ascribed to the compensation temperature of rare earth and iron sublattices (TR–Fe ). At this temperature, the spontaneous magnetization of the C sublattices are exactly opposite like in a normal antiferromagnet. Since R-Fe interactions are weak as compared with Fe–Fe one, therefore the 2nd transition at about 435 K is an indication of a 2nd compensation temperature related to the iron sublattices namely A and D sites respectively. At this temperature, the magnetic phase transition appears as a broad hump in M vs T. This compensation temperature (TFe–Fe ) looks like that existing in ferrimagnets [35–38]. There is sometimes a temperature below the Curie temperature at which the two sublattices have equal moments, resulting in a net magnetic moment of zero; this is called the magnetization compensation point [35]. This is the result of a different temperature dependence of the two sublattice magnetizations and represents the point where the magnetizations of these two antiparallel-coupled sublattices are equal and cancel each other. This compensation point is observed easily in garnets and rare earth – transition metal alloys and it is a crucial point for achieving high speed magnetization reversal in magnetic memory devices. The 3rd transition at about 500 K could be ascribed to the insulating-metal transition (I–M) as it is clear from –T plot which is followed by the transformation of the sample into a paramagnetic state. Above this transition (TI–M ), the resistivity increased with increasing temperature as an opposite

–

12.405, 12.413 6.672

– – ICDD 73-1378 [39] ICDD 73-1378

1 × 1012

[40]

12 4 × 104 >1 25 – – – 5, 6 553 –

[40] [11] [11] [11]

[39,41] [42]

situation to that before transition. Consequently, the Curie temperature observed in the magnetic susceptibility corresponds to the change in the spin ordering which lead to a transformation of the garnet from insulating state to a metallic like state. All the obtained characteristic data as compared with parent compound (DyIG) for our prepared new nano garnet (DySrIG) are reported in Table 2. 3.5. Summary and conclusion From the above studies it can be concluded that 1. The suitable formation of the prepared garnet (DySrIG) in single phase structure by citrate auto-combustion method is 1100 ◦ C with crystal size of about 90 nm. This annealing temperature for the crystallization is lower than the previously reported data for garnets. 2. Curie temperature for DySrIG is obtained at 610 K, which is slightly larger than the reported values for the un-doped sample. 3. The effective magnetic moment eff was calculated and found to be (36 B.M.). 4. The dielectric constant ε increases from order of 102 at room temperature to 104 at 850 K. 5. The temperature dependence of the resistivity of DySrIG at difference frequencies 100 kHz ≤ f ≤ 5 MHz indicates that there are 5 transitions (325 K, 450 K, 500 K, 700 K and 800 K) and are approximately frequency independent for f < 1 MHz. The transition height is decreased by increasing temperature for f = 200 kHz from ≈ 5 M cm at 320 K to ≈ 20  cm at 700 K. 6. We offered a new DySr iron garnet (DySrIG) with versatile properties for multipurpose especially in high frequencies due to its negligible losses.

M.A. Ahmed et al. / Materials Chemistry and Physics 126 (2011) 780–785

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