Observation of superparamagnetism in rf-sputtered films of zinc ferrite

~

ELSEVIER

Journal of Magnetismand MagneticMaterials 146 (1995) 291-297

Jeurnal of InagneUsm aH magnetic materials

Observation of superparamagnetism in rf-sputtered films of zinc ferrite J. Chen

G. Srinivasan

a

a,*

S. Hunter

a

V. Suresh Babu

b

M.S. Seehra b

~' Department of Physics, Oakland University, Rochester, MI 48309-4401, USA b Department of Physics, West Virginia University, Morgantown, WV 26506-6315, USA

Received 15 August 1994; in revised form 17 November 1994

Abstract

Structural and magnetic properties of rf-sputtered zinc ferrite, ZnFe204, have been studied using X-ray diffraction, susceptibility, and high-field magnetization measurement techniques. Films prepared by radio-frequency sputtering in argon, oxygen or mixed argon-oxygen atmospheres contain a microcrystalline phase, most likely nonstoichiometric wustite FezO. Argon-sputtered samples show spin freezing below 60 K. For samples deposited in pure or partial oxygen atmospheres, data on temperature and field dependence of susceptibility and magnetization provide strong evidence for a superparamagnetic character. Anomalous variations are observed in the ferromagnetic resonance line width at temperatures close to the spin freezing temperature. Ferrimagnetically ordered clusters are suggested to cause the superparamagnetic behavior.

I. Introduction

In crystalline oxides with the inverted spinel structure, M 2+ Fe 3+ 042- , where M is Fe, Co, or Ni, for example, the divalent M ions occupy the octahedral B sites and the trivalent iron ions occupy both the B sites and the tetrahedral A sites. The A - A and A - B superexchange interactions between magnetic ions are antiferromagnetic and the 90 ° superexchange between the octahedral divalent ions is often ferromagnetic [1]. Depending on the strengths of various exchange interactions, one observes a collinear or canted ferrimagnetic or antiferromagnetic order for the spinels. This study is concerned with the structural and magnetic properties of amorphous counterparts of crystalline sp.inels. The key

* Corresponding author.

objective is to investigate the effects of crystalline disorder on the superexchange interactions and the resulting magnetic structure. Sugimoto et al. [2-4] reported the observation of ferrimagnetism in amorphous spinels in phosphate glasses. A relatively small saturation magnetic moment was observed in the amorphous state even though the Curie temperature was close to that for the crystalline spinels. The ferrimagnetic ordering was also evident from M6ssbauer measurements at room temperature. The reason for the low saturation moment in the amorphous state was not understood. Recently, Okuno et al. [5] examined the magnetic structure of amorphous cobalt- and zinc ferrites prepared by ion-beam deposition techniques. A spinglass-type ordering for the films was inferred from susceptibility and M6ssbauer studies. We report here on the magnetic properties of noncrystalline zinc ferrite films prepared by rf spur-

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01688-7

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J. Chen et al. /Journal of Magnetism and Magnetic Materials 146 (1995) 291-297

tering. The crystalline ZnFe204 is a normal spinel with Zn in A-sites and Fe occupying B-sites, and the oxide orders antiferromagnetically with a N6el temperature TN of 9 K due to the negative superexchange between Fe 3+ ions [6]. Since the compound contains only one type of magnetic ion, viz. iron, the system is appropriate for studying the effects of superexchange interactions among Fe ions in disordered spinels. We inferred the possible presence of microcrystals of FezO in rf-sputtered films. Data on dc susceptibility for argon-sputtered films reveal spin freezing at about 60 K. A superparamagnetic character with a relatively high spin freezing temperature Tf is observed in films deposited in pure 02 or mixed O2-Ar atmospheres. The value of Tf ranges from 100 to 200 K, depending on the strength of the applied static magnetic field H. Data on the H dependence of the magnetization for temperatures below Te show symmetric hysteresis loops for both zero-field-cooled and field-cooled conditions, and are indicative of superparamagnetism in the samples. X-band ferromagnetic resonance line width data show anomalous changes in the line width at 120-140 K. Details of structural and magnetic studies on the films are presented in the following.

2. Experiment Films of ZnFe204 were prepared by the technique of rf sputtering using a single-target sputtering system with provisions for deposition in controlled atmospheres. A 2 inch diameter polycrystalline sintered target prepared by standard ceramic techniques from appropriate chemicals was used. The deposition system was evacuated to a base pressure of 0.01 mTorr and the sputtering was performed in Ar, 02 or a mixed argon-oxygen atmosphere at a total pressure of 2 mTorr and an rf power of 100-200 W at 13.6 MHz. Films were deposited in a sputter-down configuration on Coming glass or silicon substrates mounted on a rotating unheated platform placed under the target. The chemical composition of the films was determined by energy dispersive X-ray spectroscopy. Samples with the composition 0;42 ZnO • 0.58 Fe203 were obtained. Thus the films were rich in iron, but deficient in zinc. Films with a thickness of 1 I~m

0 O o3

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20 (Degrees) Fig. 1. X-raydiffractiondata for films of ZnFe204 preparedby ff sputtering in pure argon and mixed argon-oxygenatmospheres. The verticallines denotethe expectedintensitypeak positionsfor crystallinewustite, FeO. The films are free of any othercrystalline phases, includingZnFeaO4, Fe304, of ~/-Fe203. were used for structural and magnetic characterization. A Cahn Model 27 electrobalance was used to determine the film mass with an accuracy of 0.01 mg. The film mass ranged from 0.2 to 0.3 mg. Studies on the crystal structure were done by X-ray diffractometry (XRD). Fig. 1 shows XRD data for samples sputtered in pure argon and in a 50% oxygen +50% argon atmosphere. The diffraction pattern show well defined intensity peaks for the Ar-sputtered film and weak broad peaks for A r - O 2sputtered samples. The data were compared with the expected peak positions for a number of crystalline Fe- or Zn-based compounds including Fe304, Fe203, FeO, and ZnFe204. Even though neither the peak positions nor the intensity ratios of the lines for these compounds perfectly match the diffractogram in Fig. 1, the possible presence of microcrystalline nonstoichiometric wustite FezO is evident from the data. One notices a reasonable match between the two primary lines for the films and the peak positions for ferrous monoxide indicated by sticks in Fig. 1. Since FeO often crystallizes in the non-stoichiometric state FezO, the observed discrepancy in the peak position and the intensity ratios could be due to a highly non-stoichiometric wustite in the films. The absence of other FeO lines in Fig. 1 for the films may be due to the preferential orientation of the microcrystals. It is also clear from the data that the volume fraction of

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J. Chen et al. /Journal of Magnetism and Magnetic Materials 146 (1995) 291-297

the microcrystalline phase is relatively small in films sputtered in partial oxygen atmospheres. The most important inference from the data is that crystalline ferrimagnetic compounds such as ",/-Fe20 3 or Fe304 are not precipitated in the films.

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Magnetic measurements were done with a Faraday susceptibility balance. A SQUID magnetometer was used for high-field susceptibility studies. Ferromagnetic resonance studies were performed at 9.4 GHz using a Varian ESR spectrometer. Films sputtered in pure argon are found to be paramagnetic with a room-temperature susceptibility X of 3 × 10 -4 e m u / g . In order to investigate the magnetic structure at low temperatures, the films were initially zerofield-cooled (ZFC) to 4.2 K and X was measured at some field H as the sample temperature T was increased to 300 K. Following the ZFC measurements, the sample was cooled back to 4.2 K in a field H and X data for the field-cooled (FC) condition were obtained as T was increased. Such data for H = 1 kOe for a film on silicon substrates are shown in Fig. 2. The most important observations from Fig. 2 are the following. (i) As T is decreased, ZFC and FC curves separate below 60 K and is indicative of spin freezing. (ii) The presence of a sharp peak centered at 60 K in the ZFC curve is probably due to microcrystals or clusters of uniform size. A wide distribution in the cluster size is expected to result in a broad susceptibility peak. (iii) Below 60 K, both FC and ZFC X values decrease with decreasing T down to 2 0 - 3 5 K. (iv) A rapid increase in X with decreasing T is seen at very low temperatures for both ZFC and FC conditions and could be due to a coexisting paramagnetic phase in the sample. Fig. 3 shows the field dependence of the magnetization for the film at 5 K and 300 K. The data were obtained for both increasing and decreasing H. At 300 K, M varies linearly with H, as expected for a paramagnetic material. The room-temperature X value of 3 X 10 -4 e m u / g is a factor of two larger than the paramagnetic susceptibility for FezO [7] and could be attributed to additional unidentified para-

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Fig. 2. Susceptibility versus temperature for ZFC and FC conditions for a film sputtered in a pure argon atmosphere. The maximum in X centered at 60 K is attributed to spin freezing associated with the crystalline phase, most likely non-stoichiometric wustite, identified from XRD data in Fig. 1.

magnetic clusters in the film. One does not observe any hysteresis in the data at 300 K. The absence of hysteresis in the magnetization is noticed down to 5 K, well below the spin freezing temperature of 60 K (for H = 1 kOe), even though M appears to depart from the linear dependence on H. The magnetization does not saturate even at fields as high as 30 kOe. Based on the XRD data in Fig. 1 and magnetic susceptibility and high-field magnetization data in Figs. 2 and 3, we propose for the following reasons

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Fig. 3. Static field dependence of the magnetization at 300 and 5 K for a film sputtered in a pure argon atmosphere.

294

J. Chen et al. /Journal of Magnetism and Magnetic Materials 146 (1995) 291-297

that Ar-sputtered samples of zinc ferrite contain clusters of antiferromagnetic or random antiferromagnetic (speromagnetic) wustite, FezO. First, XRD data show peaks due to a microcrystalline phase at d-values of intensity close to the values expected for crystalline wustite. Second, the absence of any hysteresis in the M versus H data for temperatures well below the spin freezing temperature implies that the microcrystalline phase is neither superparamagnetic nor a spin glass. The crystalline FezO orders antiferromagnetically and the N6el temperature depends sensitively on the z-value [7]. In general, T~ decreases with increases in z, from about 199 K for z = 0.93 to 192 K for z = 0.95. The observed drastic reduction in the spin ordering temperature, from about 190-200 K in the crystalline state to about 60 K in the microcrystals, could be attributed to a relatively high z-value and to a long-range crystalline disorder in the sputtered film. A similar reduction in the ordering temperature was reported for sputtered films of noncrystalline ferric oxide and yttrium iron garnet [8,9].

3.2. Pure oxygen- and mixed oxygen-argon-sputtered films As mentioned in Section 2, XRD data for films sputtered in oxygen show evidence for the precipitation of a microcrystalline phase, most likely FezO. Data on the temperature dependence of the low-field magnetization M measured with a Faraday susceptibility balance for an oxygen-sputtered sample are shown in Fig. 4. The M values were measured for both ZFC and FC conditions for a series of H values in the range 60-500 Oe. At 300 K, the susceptibility ranges from 10 × 10 - 3 to 15 × 10 - 3 e m u / g and it decreases with increasing H. It is obvious from Fig. 4 that ZFC and FC magnetization curves overlap for T >_ Tf. A decrease in Tf, from 200 K for H = 60 Oe to 130 K for H = 500 Oe, is also evident from the data, and the dependence of Tf on H is well described by the expression Tf = A H - ' , where A is a constant. A plot of In Tf versus In H yields a value of 0.2 for the exponent n, which is in agreement with the values reported for similar noncrystalline oxide compounds [10,11]. Below the spin freezing temperature, the ZFC and FC curves separate.

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Fig. 4. Magnetization as a function of temperature for ZFC and FC conditions for static fields of 100, 200 and 500 Oe for a film sputtered in a pure oxygen atmosphere.

Similar magnetization data are shown in Fig. 5 for films deposited in 50% 02 + 50% Ar atmospheres. One essentially observes in the data features similar to those mentioned earlier for O2-sputtered films, such as spin freezing at 100-200 K. Now we compare the magnetic parameters for 02, O2-Ar and pure argon-sputtered films. The largest room-temperature susceptibility, (10-30) × 10 -3 e m u / g depending on the H values, is observed for the film deposited in O2-Ar. For samples deposited in a pure or 20

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Fig. 5. Similar data as in Fig. 4 for a film sputtered in a mixed a r g o n - o x y g e n atmosphere.

J. Chen et al. /Journal of Magnetism and Magnetic Materials 146 (I995) 291-297 12: I

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Fig. 6. Variation of the magnetization with the static magnetic field at 300 K for a film deposited in pure oxygen. The data do not show any hysteresis.

partial oxygen atmosphere: (i) X is larger by a factor of 3 - 1 0 than for pure argon-sputtered samples; (ii) the spin freezing temperature is relatively high; and (iii) the peak in the ZFC magnetization is very broad and could be due to a wide distribution in the size of clusters or microcrystals. In order to examine the origin of magnetization above the spin freezing temperature in the sputtered films, we measured the H dependence of the magnetization. Such data at room temperature for an oxygen-sputtered film are shown in Fig. 6. A nonlinear M versus H in the figure clearly indicates the absence of a purely paramagnetic phase. Since hysteresis or remanence is not observed in the data, one can rule out the possibility of a long-range ferromagnetic or ferromagnetic order. However, a nonlinear M versus H and the absence of hysteresis are expected characteristics for superparamagnetic clusters. Superparamagnetism arises in materials consisting of single-domain ferromagnetic or ferromagnetic particles when the thermal energy is large enough to cause the magnetization to undergo random fluctuations [12]. Such a system is expected to behave like a Langevin paramagnet and the magnetization M must scale as H/T. Fig. 7 shows the variation of M with H/T for an oxygen-sputtered film for temperatures T above the spin freezing temperature. The data for T = 200-300 K were obtained from the M versus T

295

data as in Fig. 4. It is evident from Fig. 7 that the criterion for superparamagnetism is satisfied for T > 200 K. One also needs to address the question of whether a collection of single-domain particles behaves like a superparamagnet or a spin-glass. It is possible to arrive at a conclusion on the nature of ordering from the M versus H data. For temperatures below Tf, the hysteresis loop for ZFC and FC conditions for a superparamagnet must be symmetric about the origin. However, for a spin glass the ZFC hysteresis loop is expected to be symmetric whereas the FC loop is displaced from the origin for temperatures below the glass transition temperature [13]. Fig. 8 shows the ZFC and FC hysteresis loops at 85 K for an oxygen-sputtered film. The loops are symmetric about H = 0 in both cases, and the data provide additional evidence for superparamagnetic clusters in the films. Ferromagnetic resonance measurements at X-band frequencies were performed to study the effects of spin freezing on high-frequency magnetic parameters for the films. Data on FMR line width for a representative sample sputtered in A r - O 2 are shown in Fig. 9, which shows the variation of the line width A H as a function of temperature for the static field H

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Fig. 7. Magnetization versus H / T for a pure oxygen-sputtered film. The data were obtained from M versus T for a series of H values as in Fig. 4 for temperatures ranging from 200 to 300 K. A superparamagnetic behavior for the film is evident from these results.

J. Chen et al. /Journal of Magnetism and Magnetic Materials 146 (1995) 291-297

296 40 A O)

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Fig. 8. M versus H at 85 K for zero-field-cooled (ZFC) and field-cooled at 5 kOe (FC) samples of ZnFe204 film sputtered in a pure oxygen atmosphere.

1200 [] []

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Fig. 10. Variation of ferromagnetic resonance fields for the static field parallel and perpendicular to the sample plane for an argonoxygen-sputtered film.

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parallel (A Htl) and perpendicular (A H ± ) to the film plane. A gradual decrease in AHII with increasing T is evident from the data. A relatively large A H is measured for H perpendicular to the film. Of particular interest in the data in Fig. 9 is the observation of anomalous variation with temperature of A H 1 over the range 80-200 K with a minimum centered at 120 K. However, it is not clear whether the minimum in AH± is related the spin freezing at low temperatures as seen in the magnetization in Fig. 5. We also studied the temperature dependence of the FMR resonance fields for the films. In Fig. 10 data on the variation with temperature of the resonance field for static fields parallel (HII) and perpendicular ( H ± ) to the film plane are shown for an argon-oxygen-sputtered film. The resonance field H± is larger than HII at all temperatures. One observes an increase in the resonance fields as the temperature is increased. The resonance fields Htl and H± are related to the angular frequency to of the microwave radiation, effective magnetization 4 ~ M for the film and gyromagnetic ratio 3, by the expressions ( t o / y ) = H± - 4"rrMeff,

(1)

and

400 50

100

1 0

2 0

250

300

Temperature (K)

Fig. 9. Temperature dependence of the ferromagnetic resonance (FMR) line width at 9.2 GHz for an argon-oxygen-sputtered film. The data are for the static magnetic field perpendicular and parallel to the film plane.

( t o / / ' y ) 2 = HII(HII q- 4 ' r r m e f f ) .

(2)

Since the magnetization decreases with increasing T and there is only a small change in the operating frequency, from 9.315 GHz at 80 K to 9.285 GHz at

J. Chen et al. /Journal of Magnetism and Magnetic Materials 146 (1995) 291-297

300 K, one expects a decrease in H ± and an increase in HII as T is increased. The field HII increases with increasing T as predicted. However, the observed H 1 versus T behavior could be due to a strong temperature dependence of the gyromagnetic ratio T for the oxide. The estimation of Y from data on the resonance fields is complicated by the fact that the magnetization for the films is not saturated even at high fields and values for the parameter 4 r r M in Eqs. (1) and (2) are not the same. The temperature dependences of the resonance fields shown in Fig. 10 are interesting and merit further studies. We now compare the magnetic parameters for sputtered films with results of Okuno et al. [5] for amorphous ZnFe20 4 prepared by ion beam sputtering. In their study films were deposited in a reactive oxygen atmosphere. Data on M versus T for films with a room-temperature X of 1.6 X 10 - 4 e m u / g showed spin freezing at 60 K. M/Sssbauer spectra showed detectable broadening at temperatures slightly higher than Tf and the hyperfine field increased rapidly with decreasing T. From anomalous X-ray scattering studies, Zn and Fe ions were suggested to occupy both tetrahedral and octahedral sites in the disordered state [14]. Magnetic parameters and Tf for our Ar-sputtered films are in agreement with observations in ion-beam-sputtered noncrystalline films. Finally, a few comments are in order here regarding the nature of microcrystals or clusters that result in a superparamagnetic behavior for oxygen or argon-oxygen-sputtered films. The behavior cannot be attributed to clusters of zinc ferrite since (i) XRD data do not indicate the precipitation of the phase; (ii) the crystalline phase is antiferromagnetic with T N = 9 K and one does not expect superparamagnetism due to clusters with such an ordering; and (iii) the films have a relatively high spin freezing temperature, of the order of 100-200 K. One could also rule out the possibility of superparamagnetism due to FezO clusters since (i) the XRD data imply a low concentration of Fe,O in the films, in agreement with room-temperature susceptibility values which are an order of magnitude larger than for crystalline FezO; (ii) clusters of FezO in sputtered films are expected to show spin freezing at a much lower

297

temperature of about 60 K, as in the case of argonsputtered samples; and (iii) clusters with antiferromagnetic or random antiferromagnetic order are not expected to result in a superparamagnetic behavior. It is very likely that films sputtered in oxygen or o x y g e n - a r g o n atmospheres show superparamagnetism and spin freezing due ferrimagnetically ordered clusters. M6ssbauer studies are necessary for information regarding the nature of magnetic order in clusters. One also needs to study the films with other structural characterization techniques such as small-angle X-ray diffraction and EXAFS to obtain additional information on the local coordination for magnetic ions and the size and composition of the clusters.

Acknowledgements The work was supported by a grant from the Petroleum Research Fund, administered by the American Chemical Society.

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