Sol-gel as a Method to Tailor the Magnetic Properties of Co1+yAl2-yO4

The magnetic properties of mesoscopic materials are modified by size and surface effects. We present a sol-gel method used to tailor these effects, and illustrate it on Co1+yAl2-yO4 spinel. Nanocomposites made of spinel oxide Co1+yAl2-yO4 particles dispersed in an amorphous SiO2 matrix were synthesized. Samples with various mass fractions -x of Co1+yAl2-yO4 in composite, ranging from predominantly SiO2 (x = 10 wt%) to predominantly spinel (x = 95 wt%), and with various Co concentrations in spinel y were studied. The spinel grain sizes were below 100 nm with a large size distribution, for samples with predominant spinel phase. Those samples showed Curie-Weiss paramagnetic behavior with antiferromagnetically interacting Co ions (θ ≈ -100 K). The grain sizes of spinel stays confined in 100 nm range even in the spinel samples diluted with as low as 5 wt% concentration of amorphous SiO2. For the samples with predominant SiO2 the crystalline nanoparticles are well separated and of size of around 100 nm, but with presence of much smaller spinel nanoparticles of about 10 nm. The magnetic properties of the samples with predominant silica phase showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of θ and consequently lower exchange constant.


Introduction
Co 1+y Al 2-y O 4 belongs to the group of magnetic spinel oxides (represented with the general formula AB 2 O 4 ).The spinel crystal lattice is a cubic with a face centered (f.c.c.) packing of oxygen atoms which form tetrahedral (A) and octahedral (B) holes (sites) occupied by cations.In the so-called normal spinels, the A site is occupied by divalent cation (M), while the B site is filled by trivalent cation (N).Generally, the cations distribution is random and can be represented by the formula (M 1-δ N δ ) tet [M δ N 2-δ ] oct O 4 where δ denotes the inversion parameter and can take any value 0 ≤ δ ≤ 1.In an inverse spinel, δ = 1.Both, A and B sites, can accommodate magnetic cations and in that case there is the competition between various types of superexchange interaction (J AA , J BB , and J AB ).Hence, variation of the magnetic cations distribution over the A and B sites can provoke different magnetic behavior in spinel compounds.
Bulk Co 3 O 4 is the normal spinel (δ = 0) with the magnetic moment equal to magnetic moment of Co 2+ in A position.Co 3+ in B position is in low-spin state with no magnetic moment [1].In bulk Co 1+y Al 2-y O 4 , Co preferentially takes the A position [2].Polycrystalline CoAl 2 O 4 shows spin-glass (SG), while CoCo 2 O 4 shows antiferromagnetic (AF) ground state at the lowest temperatures.Intermediate CoCoAlO 4 shows spin-liquid state [2][3][4].
Behavior of magnetic materials in reduced dimensions is a very interesting topic both for fundamental research and for application in spintronics.Magnetic properties of nanoparticles are greatly determined by surface effects, particularly antiferomagnetics.By decreasing particle size to nano dimensions, the finite size effects give rise to surface spin canting, superparamagnetism (SPM), and/or site disorder effect.Many factors determine magnetic behavior of nanosized system, like particle size and shape, size distribution, agglomeration, cation distribution, interparticle interactions.As magnetic nanoparticles usually tend to agglomerate due to their large surface energy and/or strong magnetic interactions, it is useful to study their magnetic properties in the form of nanocomposites.In this work we used sol-gel method to explore the influence of different level of dilution in amorphous silica matrix, ranging from predominantly SiO 2 (x = 10 wt%) to predominantly spinel (x = 95 wt%), where x is mass fraction of Co 1+y Al 2-y O 4 in composite.Various Co concentration in spinel, y, were also studied.

Experimental details
Nanocomposites were synthesized by the sol-gel technique.The basic idea, considering different silica:spinel ratio, is illustrated in Fig. 1.The left branch illustrates synthesis with predominant silica and right branch illustrates nanocomposite with predominant spinel.
Tetraethylorthosilicates (TEOS) [Si(C 2 H 5 O) 4 ] was used as a precursor for the SiO 2 , whereas cobalt nitrate hexahydrate [Co(NO 3 ) 2 ⋅6H 2 O] and aluminium nitrate nonahydrate [Al(NO 3 ) 3 ⋅9H 2 O], were used as precursors for the Co 1+y Al 2-y O 4 nanoparticles.A cold mixture of HCl and water in the molar ratio 0.01:1 is added dropwise to a cold mixture of TEOS and ethanol in the volume ratio 1:1, under vigorous stirring.The molar water : TEOS ratio was set to be 4:1.So obtained silica sol was used as a silica precursor for the next step of synthesis.In the next step, the mixture of Co(NO 3 ) 2 ⋅6H 2 O and Al(NO 3 ) 3 ⋅9H 2 O solutions in deionised water was added dropwise to the obtained silica sol solution corresponding to various x and y ratios (see Table I).Further, the total amount of water was adjusted to be 8 molecules of water per one molecule of TEOS monomer.
The 2 M solution of ammonia [NH 4 OH] was dropped inside of this mixture until the pH value was set up to be 7. NH 4 OH enable transformation of Co precursor into corresponding Co-ammino complexes ([Co(NH 3 ) 6-n (H 2 O) n ] 2+ , [Co(NH 3 ) 6-n (H 2 O) n ] 3+ ) and Al precursors to Al(OH) 3 .The point of zero charge of Al(OH) 3 is at about pH = 9.7 (maximum zeta potential of aluminum hydroxide is at about 4.5) and higher values of pH cause its faster precipitation [5].Yet again, the higher concentration of ammonia has opposite impact on solubility of Co complexes.
Therefore, the amount of added NH 4 OH should be adjusted to be as more as close to balanced kinetics of precipitation of Al(OH) 3 and Co ammine complex.The value of pH = 7 seems to be optimal for balanced precipitation.The formation of single Co_Al oxide phase without phase separation, diluted inside of SiO 2 phase, confirms that the given pH is indeed chosen properly.The mixture was then placed in an oven at 50°C for 24 h in order to complete the gelling.Before final gelling, a reaction of Co amino complex degradation (caused by ammonium evaporation) occurs.This process is followed with slight change of color from ocher to the dark reddish brown, due to partial change of valence of Co ions.As obtained complex gel was then dried at 150°C for three days, and annealed at 550°C for 5 hours in air.
The compositions of obtained samples were determined by atomic absorption method.The samples were labeled according to mass fraction x of Co 1+y Al 2-y O 4 in composite, and y as Sx_y (Table I).
Powder X-ray diffraction (XRD) spectra were recorded on a Philips 1710 diffractometer using CuK α radiation without energy filters, with a step size (2θ) of 0.05° at a slow scan rate of 40 s per step.The magnetization measurements were carried out on a SQUID magnetometer (MPMS XL-5, Quantum Design).The dc magnetization measured at H = 8, 50, 100, 1000 and 10000 Oe under zero-field-cooled (ZFC) and field-cooled (FC) regimes, were recorded in the range of temperature from 2 K to 300 K.The ac magnetization measurements were made under an ac exciting field of 6.5 Oe, in the frequency range from 1 to 1500 Hz.The hysteresis loop, M(H) curves, were recorded at 2, 5, 10, 20 and 50 K.Transmission electron microscopy (TEM) micrographs were obtained using a JEOL 2010 F microscope operating at 200 kV coupled with an EDXS microanalysis system (LINK ISIS EDS 300).Finely ground samples were dispersed in ethanol, then submitted to ultrasonic bath and the as-obtained suspension was placed on carbon coated copper grids.Atomic force microscopy (AFM) was carried out in AC-tapping mode in ambient air using a JSPM-5200 JEOL instrument.The samples were obtained by pressing powder into pellets.

Morphology and microstructure
The morphology of the samples was imaged using atomic force microscope, AFM, phase-contrast technique (in tapping mode and in ambient air).AFM systems are able to detect intermolecular forces in the order of 10 -11 -10 -13 N which belong to the class of van der Waals type forces, usually modeled by Lennard-Jones potentials (LJP).The variations in the slopes of LJP curves (typical for each chemical species) determine different intermolecular forces acting on the AFM sensor tip.That difference modulates the vibration frequency of the AFM cantilever, creating a higher-harmonics in the feedback signal of AFM system.The varying magnitudes of higher-harmonics are the signals detected and displayed in phasecontrast images.In our case, they depict the difference between force fields of matrix material (SiO 2 ) and nano-structured filling material, thus visualizing the grain border which enables us to deduce the sizes of different grains.
Observed grain sizes show significant variation in the range below 100 nm (20 -100 nm) (Fig. 2).The sizes are 20-60 nm for the sample with 56 wt% of the spinel, 30-100 nm for the 75 wt% and 30-110 nm for the sample with 95 wt% concentration of spinel.The fraction of grains ≥100 nm is small and subject to possible merging of smaller grains.It can be concluded that grain sizes are larger for the higher concentration of spinel.It is interesting to note that grain sizes of spinel stays confined in 100 nm range even in the samples diluted with as low as 5 wt% concentration of amorphous SiO 2 .Regarding the fact that silica has a density of 2.2 g/cm 3 , much lower than that of Al-substituted Co 3 O 4 (~ 5.6 g/cm 3 ), the corresponding volume percentages of spinel are lower.XRD patterns (Fig. 3) of the studied samples show the broad reflection centered at 21.5° originating from amorphous silica matrix, and the narrow diffraction peaks that correspond to cubic spinel phase (space group Fd3-m).Presence of any impurity phase was not found.The spectra were fitted using FullProf-program based on the Rietveld method (Fig. 3).Linear interpolation with 19 points was used to describe background.Relatively high background comes from diffraction from amorphous SiO 2 and from fluorescence when Cu cathode is used (no energy filter was used).Peak profiles were delineated by Pseudo-Voigt peak shape function.The crystallite sizes calculated from the XRD line broadening range from 30 to 90 nm.The width of the diffraction lines is illustrated with enlarged peak (511) shown in Fig. 3

Magnetic measurements
Magnetic susceptibility at higher temperatures was analyzed using the Curie-Weiss (CW) equation, ( 2), where N is the number of Co 1+y Al 2-y O 4 molecules per unit mass, θ is the Curie-Weiss temperature and p eff is the effective magnetic moment per Co 1+y Al 2-y O 4 molecule.Fig. 4 shows inverse susceptibility χ -1 vs temperature for various samples.Three of the samples, S5_09, S9_13 and S7_12, with higher concentration of Co 1+y Al 2-y O 4 in SiO 2 , behave as Curie-Weiss paramagnets with antiferromagnetically interacting Co ions (θ ≈ -100 K).The negative θ points to prevailing antiferromagnetic interaction.These samples show similar χ(T) dependencies (Fig. 4).Other three samples, with higher concentration of silica S1_13, S5_03 and S5_11 show lower absolute value of θ (Table I).
The value of the Curie-Weiss temperature θ represents the strength of the leading exchange interaction.The obtained lower absolute values of θ (Table 1) of the samples with higher concentration of silica S1_13, S5_03 and S5_11 than reported values [1,6,7,8] points to decrease in the strength of the antiferromagnetic exchange interactions between the Co ions or onset of ferromagnetic interaction that competes with antiferromagnetic.ZFCFC curves of the samples S1_13, S5_03 and S5_11 show bifurcation (Fig. 5(a)).Bifurcation of the FC and ZFC branches in the dc magnetic susceptibility is characteristic of most of the spin-glass systems, but also of the interacting and non-interacting superparamagnetic nanoparticles.Absence of the ZFCFC bifurcation and hardly detectable ac susceptibility peaks for the samples S5_09, S9_13 and S7_12, verify them as primarily Curie-Weiss paramagnetic down to 2 K.
Figs. 5 (b,c and d) show the temperature dependence of the imaginary part of the ac magnetic susceptibility χ''(T) for all the samples at frequencies f of ac applied magnetic field.Imaginary part of all the samples shows more or less distinctive peaks at very low temperatures (<12K).The positions of peaks observed in χ''(T) moves to higher temperatures with increasing frequency f .We used the empirical parameter Φ = ΔT f /(T f Δlog f ) to classify the origin of the observed maxima [9].The values of the parameter Φ are presented in Table 1.The values are about 0.08 (0.07-0.10) except for the samples S9_13 and S7_12 which show higher values because peaks merge and disappear for frequencies higher than 50 Hz and positions cannot be determined precisely.If we waive the values for f > 50 Hz, we also get value of Φ S9_13 to be 0.08.These values are consistent with those for semiconducting and insulating spin glasses [9].Fig. 6 shows magnetization curves at a) T = 2 K and b) T = 20 K.The same grouping of results (intensity of magnetization), considering concentration of Co 1+y Al 2-y O 4 in nanocomposite, can be observed, as were observed in static χ(T) and in dynamic χ''(T) magnetic susceptibility data.The magnetic frozen states and values of θ influence the M(H) behavior (M = f(T + θ)) and therefore is substantial difference of magnetization curves between various samples, especially between paramagnetic samples, S5_09, S9_13 and S7_12, and spin-glass samples, Fig. 6.
The magnetization curves M(H) at 50 K show Brillouin like curvature.At lower temperatures (≤20 K) the curves deviate from the Brillouin function.The M(H) curves show hysteresis at lower temperatures, below about 10 K.The sample S1_13 exhibits appreciable hysteresis with coercive field H c around 1000 Oe at T = 2 K, and sample S5_11 shows coercive field around 540 Oe.All the other samples show an order of magnitude lower H c (Table 1).At higher temperatures (≥5 K) the values of H c are lower than 50 Oe for all the samples.
Pure bulk Co 3 O 4 is antiferromagnetic with the Néel transition temperature T N = 30 K [1,6].The χ(T) and χ''(T) dependencies of our samples show Curie-Weiss behavior and does not exhibit magnetic phase transition at 30 K. It is a system with strong frustration effect and emergence of frozen magnetic ground state is expected [2,4].The results suggest existence of magnetic ground states below temperature of magnetic ground state of bulk Co 3 O 4 , and even more, existence of multiple magnetic ground states that can form at various temperatures.Analysis suggests that these states are of spin-glass type.
The strength and positions of the ac susceptibility peaks correspond to ZFCFC bifurcation extent and particularly to the intensity and position of the peaks at ZFC branch; (compare b) and c) to a) of Fig. 5).The sample with predominant silica, S1_13, show the strongest ac susceptibility peaks.The peaks of samples with medium concentration of silica, S5_03 and S5_11 are order of magnitude weaker and the samples with predominant Co 1+y Al 2- y O 4 , S5_09, S9_13 and S7_12 show two orders of magnitude weaker ac susceptibility peaks.The samples with low intensity ac susceptibility peaks, S5_09, S7_12 and S9_13, do not show ZFCFC bifurcation.
The samples with more distinguished magnetic frozen state characteristic, (S1_13, S5_03, S5_11), also have markedly lower absolute values of θ and higher intensities of magnetization at the lowest temperature (Table 1, Fig. 6).Lower absolute values of negative θ point to decrease in the strength of the antiferromagnetic exchange interactions between the Co ions or onset of ferromagnetic interaction that competes with antiferromagnetic.Correlation of M and θ with x suggests surface physics as significant.We suggest a solid-state surface electrochemical reaction, described by Martin-Gonzales et al. [10] as responsible for behaviour of the magnetic properties of this material; Surface Co ions at A position may interact ferromagnetically at Co 1+y Al 2-y O 4 -SiO 2 interface, due to solid-state surface electrochemical reaction where SiO 2 reduces Co at B position from Co 3+ oxidation state, that has zero magnetic moment, to Co 2+ , with magnetic moment, and consequently the non-zero moment may appear due to A-O-B-O-A interaction.
The variation of Al concentration in polycrystalline Co 1+y Al 2-y O 4 , with insignificant inversion, changes the Néel temperature T N , but does not have substantial influence on θ [4].In our samples no correlation between θ or T f to inversion or Al concentration could be established.

Conclusion
Magnetic spinel oxide Co 1+y Al 2-y O 4 dispersed in an amorphous SiO 2 matrix synthesized by a sol-gel technique were studied.The used sol-gel method provided size confinement of the Co 1+y Al 2-y O 4 nanoparticles to be ≤ 100 nm even for the sample with as low as 5 wt% of silica.The Co and Al ions occupation of cation positions in spinel were close to random.The samples, with higher concentration x of Co 1+y Al 2-y O 4 in SiO 2 were Curie-Weiss paramagnets with antiferromagnetically interacting Co ions (θ ≈ -100 K).Other samples, with lower x, showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of θ associated with lower exchange constant.
(b).The crystallite sizes obtained from XRD agree well with grain sizes observed using AFM.The occupation factors were fitted taking into account Co-Al ratio determined by atomic absorption method.The occupation factors can be accurately determined because relative intensity of diffraction peaks greatly depends on occupation factors.The obtained occupation factors of Co in A position -Co/A, in Table I.The samples do not show significant preference of Co to A position.The fraction of Co ions at A position, (Co/A)/(1+y), range from 0.33 to 0.38, which is close to random (0.33).Tab.I Sample labels, y -composition of Co 1+y Al 2-y O 4 , x -mass fraction of Co 1+y Al 2-y O 4 in composite,T f -positions of peaks of χ''(T) at f = 1 Hz, Φ = ΔT f /(T f Δlog f ) -frequency shift of respective values from column T f , Co/A, Co/B, (Co/A)/(1+y) -fraction of Co at specified position, p eff -effective magnetic moment per Co 1+y Al 2-y O 4 molecule, θ-Curie-Weiss temperature, M(50 kOe, 2 K) -magnetization at H = 50 kOe and T = 2 K, H c -coercive field at T

Fig. 5 .
Fig. 5. a) ZFCFC M(T) curves of samples S1_13, S5_03 and S5_11.b), c), d) Imaginary part of the ac magnetic susceptibility χ'' per mole of Co 1+y Al 2-y O 4 as a function of temperature b)for sample S1_13, c) for samples S5_03 and S5_11 and d) for S5_09, S7_12 and S9_13.