Structural and magnetic properties of holmium substituted cobalt ferrites synthesized by chemical co-precipitation method

Journal of Magnetism and Magnetic Materials 324 (2012) 3773–3777

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Structural and magnetic properties of holmium substituted cobalt ferrites synthesized by chemical co-precipitation method Irshad Ali a, M.U. Islam a, M. Ishaque a, Hasan M. Khan a, Muhammad Naeem Ashiq b,n, M.U. Rana a a b

Department of Physics, Bahauddin Zakariya University, Multan, Pakistan Department of Chemistry, Bahauddin Zakariya University, Multan, Pakistan

a r t i c l e i n f o

abstract

Article history: Received 15 December 2011 Received in revised form 2 June 2012 Available online 19 June 2012

CoHoxFe2  xO4 ferrites (x ¼ 0.00–0.1) were prepared by the co-precipitation technique and the effect of holmium substitution on the magnetic properties was investigated. X-ray diffraction reveals that the substituted samples show a second phase of HoFeO3 along with the spinel phase. The magnetic properties such as the saturation magnetization (Ms), coercivity (Hc) and remanence (Mr) are obtained from the hysteresis loops. It is observed that the Ms decreases while Hc increases with Ho3 þ substitution. The decrease of saturation magnetization is attributed to the weakening of exchange interactions. The coercivity increases with increase of the Ho3 þ concentration, which is attributed to the presence of an ultra-thin layer at the grain boundaries that impedes the domain wall motion. Low field AC susceptibility was also measured over the temperature range 300–600 K at the frequency of 200 Hz. It decreases with the increase of temperature following the Curie–Weiss law up to the Curie temperature. Above the Curie temperature it shows paramagnetic behavior. The increase in coercivity suggests that the material can be used for applications in perpendicular recording media. & 2012 Elsevier B.V. All rights reserved.

Keywords: Soft ferrite Co-precipitation Susceptibility Coercivity Magnetic interaction

1. Introduction The spinel ferrites are technologically an important class of magnetic oxides due to their versatile magnetic and electrical properties [1]. Rare earth substituted ferrites are nowadays under extensive investigations in order to enhance the saturation magnetization, permittivity and permeability. Experimental and theoretical investigations of the exchange interactions in the rare earth-transition metal (4f–3d) elements containing spinel oxides have received more attention in the recent years [2–4]. Special attention has been given to cobalt containing spinel ferrites due to its promising applications as a magnetic material in recording media [5,6]. Low field AC susceptibility is used to determine the magnetic properties of soft ferrites and hence their applications in electronic industry. The temperature dependent AC susceptibility determines the magnetic phase in the magnetic oxides viz multidomain (MD), single domain (SD), and super-paramagnetic (SP) [7]. Magnetic properties of spinel ferrites are mainly due to interaction of the species on tetrahedral A- and octahedral B-sites. In case of rare earth substituted ions the contribution to the net magnetization is due to both the orbital moment and spin moment of 4f level [8]. The aim of present investigation is to enhance the coercivity of the material to make it useful for

n

Corresponding author. Tel.: þ92 61 9210092; fax: þ 92 61 9210068. E-mail address: [email protected] (M. Naeem Ashiq).

0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.008

applications in perpendicular recording media (PRM). For this purpose Ho3 þ is substituted instead of iron in cobalt ferrites and their AC magnetic properties are also studied.

2. Experimental The CoHoxFe2  xO4 (x¼00, 0.04, 0.06, 0.08, 0.10) ferrites were prepared using the chemical co-precipitation method. The stoichiometric amounts of FeCl3  6H2O, C4H6CoO4  4H2O were dissolved in deionized water whereas Ho2O3 was first dissolved in HNO3 in order to obtain holmium nitrate and then mixed with the solution. The solution was heated to 60 1C with continuous stirring and a mixture of NaOH and Na2CO3 (with ratio 1:5) was added to the solution as a precipitating agent and to maintain the pH value equal to 12. The dropwise addition of NaOH and Na2CO3 resulted in brown colored precipitates. These precipitates were washed several times with deionized water till the pH of the substance reached 7.0. The precipitates were filtered by using a Whatman filter paper with the help of a suction flask operating on a vacuum pump and finally washed with deionized water in order to remove chlorides, nitrates and acetate ions. The presence of chloride ions in the filtrate was confirmed with a silver nitrate (AgNO3) test. The wet brown colored slurry was dried in an oven at 1001 C for 12 h to remove water content from the slurry. The prepared powder was pressed into a pellet of 8 mm diameter and thickness of 2 mm. The pellets were finally

I. Ali et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3773–3777

sintered at 10001 C for 10 h followed by air quenching. The phase of sintered materials was confirmed by X-ray diffraction (XRD) technique using a Schimadzu X-ray diffractometer (XD5A) ˚ Each equipped with CuKa as a radiation source (l ¼ 1.5424 A). sample was scanned for 2y range (15–751) with a step size of 0.021. The X-ray tube was excited at 40 kV and 30 mA. The MH loops were measured at room temperature using a vibrating sample magnetometer (VSM) model Lake Shore, new 7400 series, USA. The magnetic susceptibility of the CoHoxFe2  xO4 spinel system was measured over the temperature range 300–600 K on a susceptometer using the low field AC mutual inductance technique at a frequency of 200 Hz. A Lock-in amplifier model LI 570 (NF Electronics) was used for this purpose. The apparatus was calibrated in the temperature range 300–600 K using paramagnetic salt Fe(NH4)2(SO4)2  6 H2O with known Curie constant (3.78 emuK/g). The sensitivity factor was calculated which was equal to 0.3704 emu/V. This factor was used to convert the voltage obtained from the Lock-in amplifier into susceptibility.

3. Results and discussion 3.1. Structural properties Fig. 1 shows the X-ray diffraction patterns of all the CoHoxFe2  xO4 (x¼0.0, 0.04, 0.06, 0.08, 0.10) ferrites. It is observed that the pure CoFe2O4 (x¼ 0.0) shows the spinel phase matched with standard pattern (ICSD 00-001-1121) and a second phase of HoFeO3 is also observed along with spinel phase in all substituted samples. The lattice constant was calculated by using the Nelson Relay function [9]. The lattice constants as functions of Ho concentrations are shown in Fig. 2. It can be seen that the lattice constant increases up to x¼0.04 and then decreases. It may be possible that the spinel lattice is compressed by the intergranular secondary phase due to the differences in the thermal expansion coefficients [4]. Hence the decrease of ‘‘a’’ with increasing concentration of Ho3 þ may suggest the existence of a solubility limit for Ho3 þ ions. Therefore, the small amount of Ho3 þ introduced in CoHoxFe2  xO4 affects not only the phase composition but also the size of the spinel matrix. 3.2. Magnetic properties 3.2.1. Hysteresis loops Fig. 3(a) and (b) depicts the MH-loops for both in-plane (H applied parallel to the sample surface) and out-of-plane (H applied perpendicular to the sample surface) orientations for CoHoxFe2  xO4 (x¼0.0, 0.04, 0.06, 0.08, 0.10) ferrites. It is shown

Fig. 1. XRD patterns for CoHoxFe2  xO4 ferrites (x ¼0.0, 0.04, 0.06, 0.08, 0.10) (* represents the secondary phase HoFeO3).

8.37 8.36 Lattice parameter a(Å)

3774

8.35 8.34 8.33 8.32 8.31 8.3 0

0.02

0.04

0.06

0.08

0.1

Ho concentration (x) Fig. 2. Lattice parameter a versus Ho concentration x for CoHoxFe2  xO4 ferrites (x ¼0.0, 0.04, 0.06, 0.08, 0.10).

from Fig. 3(a) and (b) that the holmium substituted cobalt ferrites have a small area under the curve. It can be observed that in both cases the saturation magnetization (Ms) and remanence magnetization (Mr) decrease with the increase of Ho3 þ concentration except at x¼0.08 as shown in Fig. 4(a) and (b), respectively. The decrease in saturation magnetization and remanence may be due to weakening of AB-exchange interactions since there are 3 types of negative exchange interactions [10] between the unpaired electrons of two ions lying in A- and B-sites. The A–B interaction heavily predominates over A–A and B–B interactions. The net magnetic moment of whole lattice is the difference between the moments of B- and A-sublattice i.e. M ¼[MB–MA], where MA and MB are the magnetic moments of the A and B sites, respectively. It is a well-known fact that CoFe2O4 ferrite adopts the inverse structure. In CoFe2O4, most of the cobalt ions occupy octahedral sites (B-sites) and the Fe3 þ ions are distributed on both octahedral and tetrahedral sites (A-sites) [11]. The magnetic moment of each composition depends on the magnetic moments of ions involved. The magnetic moment of Fe3 þ and Co2 þ are 5 mB and 3 mB, respectively while holmium is paramagnetic at room temperature. The substitution of rare earth with Fe3 þ ions takes place on octahedral sites due to the larger ionic radius [12]. Therefore the substitution of holmium of ionic radii (0.901 A˚ ) with Fe3 þ (0.64 A˚ ) ions takes place on octahedral sites due to the larger ionic radius and the possibility of occupying tetrahedral sites is very rare. Hence it is expected that the number of magnetic moments on the octahedral site will decrease. Thus the magnetic moment of B-sublattice decreases and consequently magnetization also decreases. Hence, net magnetization of the sample decreases. In the present samples, decrease of magnetization with Ho contents is in good agreement with results reported by other researchers [13,14]. A slight increase in saturation magnetization for x ¼0.08 may be due to the migration of some Fe3 þ ions to B-sites from A-sites. Coercivity measured at in-plane and out-of-plane orientations of CoHoxFe2  xO4 ferrite system increases as a function of Ho3 þ content as shown in Fig. 4(c). The increase in coercivity may be attributed to the presence of an ultra-thin layer at the grain boundaries which impedes the domain wall motion [15]. The saturation magnetization is related to Hc through the Brown’s relation [16] Hc ¼2K1/moMs where K1 is magnetocrystalline anisotropy, mo is vacuum susceptibility, Ms is saturation magnetization, and Hc is coercivity. Here Hc is inversely proportional to Ms, this is consistent with our experimental results and with the results reported earlier [16]. The coercivity (Hc) increases rapidly with Ho content but at x¼ 0.08 a small decrease in Hc has been observed; this may be due to unquenched orbital angular momentum of Ho3 þ ions [16].

I. Ali et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3773–3777

80

80

M (emu/g)

100

M (emu/g)

100

60 40 20

-10000

0

-5000

-20

3775

60 40 20

0

5000 H (Oe)

0

10000

-10000

-5000

-20

-40

0

10000

H (Oe)

-40

-60

5000

-60

-80

-80

-100

-100 Fig. 3. (a) In-plane MH-loop for CoHoxFe2  xO4 ferrite (x ¼0.00, 0.04, 0.06, 0.08, 0.10). (b) Out-of plane MH loop for CoHoxFe2  xO4 ferrites (x ¼0.00, 0.04, 0.06, 0.08, 0.10).

34

100 90

29

80 Mr (emu/g)

Ms (emu/g)

70 60 50 40 30 20

24 19 14 9

10 0

4 0

0.02

0.04 0.06 0.08 Ho concentration (x)

0.1

0

0.02

0.04 0.06 Ho concentration (x)

0.08

0.1

1200 1100 1000 Hc (Oe)

900 800 700 600 500 400 0

0.02

0.04 0.06 Ho concentration (x)

0.08

0.1

Fig. 4. (a) In-plane and out-of- plane saturation magnetization versus Ho concentration (x) for CoHoxFe2  xO4 ferrites (x¼ 0.0, 0.04, 0.06, 0.08, 0.10). (b) In-plane and out-ofplane remanence versus Ho concentration (x) for CoHoxFe2  xO4 ferrites (x ¼0.0, 0.04, 0.06, 0.08, 0.10). (c) In-plane and out-of- plane coercivity versus Ho concentration (x) for CoHoxFe2  xO4 ferrites (x ¼0.0, 0.04, 0.06, 0.08, 0.10).

In conventional longitudinal magnetic recording (LMR), the magnetization in the bits is directed circumferentially along the track direction. In perpendicular recording media (PRM), the

‘‘magnetic bits’’ point up or down perpendicular to the disk surface. The well-liked explanation for the advantage of perpendicular recording is that it can deliver more than 3 times the

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I. Ali et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3773–3777

heat content of the system is enough to disturb all the aligned moments giving rise to complete para-magnetism [20]. That is why the value of magnetization decreases with the increase of temperature and the behavior of the reciprocal of susceptibility versus temperature verifies the Curie–Weiss law above the curie temperature Tc and below the curie temperature it shows deviation from the Curie–Weiss law. From the linear portion of 1/w versus T plots, the Curie constant (emu K/g) was calculated using the Curie–Weiss law [21]

storage density of traditional longitudinal recording. Magnetic samples with higher coercivity are inherently thermally more stable which is proportional to the product of volume times the uniaxial anisotropy constant Ku; the product is of course larger for higher coercive material. PRM requires a high coercivity medium because of the fact mentioned above. In the present study the investigated samples which are spinel ferrites can be used in PRM due to large value of coercivity 1000Oe which is comparable to the those of hard magnetic materials.

w ¼ C=T þ y

3.2.2. Squareness ratio Squareness ratio (Mr/Ms) ranging 0.33–0.46 for in-plane (0.17– 0.48 out-of-plane) is well below the typical value  1 for single domain isolated ferromagnetic particles. The squareness ratio is 0.5 for randomly oriented uniaxial anisotropic ferromagnetic particles [17]. The deviation of Mr/Ms from typical single domain class value may be assigned to the interaction amongst the grains which are affected by the core–shell structure of grains and grain size distribution in material. The anisotropy constant (K ¼HcMs/2) was calculated using the given relation and assuming the magnetic particles to be isolated (exchange interacting spin) single domains [17]. The corresponding values of K are listed in Table 2. The values of K are almost 100 times less than that reported in RE doped CoFe2O4 [18,19]. This shows that grains are not single domains and anisotropy contribution is not uniaxial, but it may be cubic magneto-crystalline anisotropy and hence strong grain to grain interactions exist in these materials.

where C (emu K/g) is the Curie constant which is just the reciprocal of the slope of 1/w versus temperature plot and is given as C ¼ Ng 2 mB SðS þ 1=3K B Þ

0.01

2500

0.008

2000

0.006

1500

1/γ (g/emu)

χ (emu/g)

ð2Þ

where N is the total number of magnetic moments within the sample. The values of Tc are listed in Table 1. Tc decreases with the increase of Ho3 þ content. This is because the Ho3 þ produces a secondary phase which increases the coercivity (Hc) and reduces the saturation magnetization. The decrease in Curie temperature may be explained on the basis of exchange interactions (ABinteraction), which are weakened due to the Ho3 þ substitution [22]. Effective magnetic moment, Peff was calculated using the formula Peff ¼g[s(s þ1)]1/2 where, g is the Lande g-splitting factor and s is spin of the electron and the corresponding values of Peff are listed in Table 1. It is clear that Peff decreases with increasing Ho content [23] which is attributed to the weakening of the magnetic interactions. The characteristic temperature ‘‘y’’ values are negative and the values are listed in Table 1 which shows that the samples CoHoxFe2  xO4 are ferromagnetic. The molecular field constant (l) was calculated using the relation l ¼Tc/C, where C is the curie constant and values of l are listed in Table 2. It is observed that the values of molecular field constant (l) increased with increasing holmium content. The values of magnetic moment (nB) are listed in Table 2. The behavior of the magnetic moment (nB) follows the saturation magnetization (Ms) as both decrease with increasing holmium content, may be due to the weakening of AB interactions. The values of exchange interaction (J) were calculated using the formula [22] pffiffiffi J ¼ 3kB T c =2znsðs þ1Þ ð3Þ

3.2.3. AC magnetic susceptibility Low field magnetic susceptibility of the CoHoxFe2  xO4 (x¼ 0.0, 0.04, 0.06, 0.08, 0.1) spinel system was measured over the temperature range 300–600 K using the mutual inductance technique at the frequency 200 Hz. The plots of the susceptibility versus temperature are shown in Fig. 5(a) which shows that the susceptibility decreases as the temperature increases and above Tc it shows paramagnetic behavior. The reciprocals of the susceptibilities versus temperature are shown in Fig. 5(b). The susceptibility of the CoHoxFe2  xO4 ferrites appears to follow the Curie–Weiss behavior at (high) temperature with marked deviation from this behavior for the entire temperature range below the Curie temperature. One can divide this variation into two different regions: the first one is considered as a ferrimagnetic region in which the thermal energy was not quite sufficient to disturb the aligned moments of the spins. In the second region the

0.004

1000

500

0.002

0 300

ð1Þ

350

400

450

500

Temperature (K)

550

600

650

0 300

350

400

450

500

550

600

650

Temperature (K)

Fig. 5. (a) Plot of susceptibility versus T (K) for CoHoxFe2  xO4 system. (b) Plot of reciprocal of susceptibility versus T (K) for CoHoxFe2  xO4 system.

I. Ali et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3773–3777

Table 1 Effective magnetic moment ( Peff ), characteristic temperature (Y), Curie. temperature (Tc ) for CoHoxFe2  xO4 (x¼ 0.00, 0.04, 0.06, 0.08, 0.10 ) ferrites. Ho content x C (emuK/mol) g

Peff (lB) H (K)

Curie Temp. Tc (K)

0.0 0.04 0.06 0.08 0.10

9.92 9.40 7.65 6.96 6.408

538 529 512 501 491

0.428 0.352 0.355 0.321 0.303

2.5 2.37 1.93 1.75 1.61

 37.14  43.58  66.67  100  150

3777

due to the substitution of Ho ions in the present samples. It can be concluded that the effective magnetic moment (Peff), magnetic moment (nB) and saturation magnetization (Ms) decrease as a function of Ho3 þ content; hence the results of the magnetic properties are consistent with the exchange interactions (J).

4. Conclusions

Table 2 Molecular field constant (l), Magnetic moment (nB) and Anisotropy constant (K) For CoHoxFe2  xO4 (x ¼0.00, 0.04, 0.06, 0.08, 0.10) ferrites. Ho content x

k

nB (lB)

K (erg/cm3)

0.0 0.04 0.06 0.08 0.10

1457.00 1502.84 1439.99 1558.66 1620.30

3.64 1.96 1.55 1.96 0.55

1.87  104 1.96  104 1.85  104 2.25  104 0.64  104

1.36

The Ho substitution in CoFe2O4 ferrite increases the lattice constants up to x ¼0.04 and it decreases for x4 0.04 which may be the solubility limit of Ho3 þ in the CoFe2O4 lattice. Both the inplane and out-of-plane magnetic properties show that there is a small anisotropy involved whereas squareness ratio indicates that the incorporation of Ho3 þ gives rise to the cubic magnetocrystalline anisotropy and hence strong grain–grain interaction exists. The AC susceptibility reveals the dilution of the magnetic interactions (J) and the other magnetic properties are consistent with the result of magnetic interactions. In the present study the investigated samples which are spinel ferrites can be used in PRM due to the large value of coercivity  1000Oe which is comparable to those of the hard magnetic materials.

1.34

References

J (meV)

1.32 1.3 1.28 1.26 1.24 1.22 1.2 0

0.02

0.04

0.06

0.08

0.1

Ho concentration (x) Fig. 6. Magnetic interactions versus Ho concentration (x) for CoHoxFe2  xO4 ferrites (x ¼0.0, 0.04, 0.06, 0.08, 0.10).

where Z ¼8, S ¼7/2 for the present samples and kB is the Boltzmann constant. Fig. 6 shows the variation of exchange interaction, J, as a function of Ho3 þ content. It is known that the magnetic behavior of the ferromagnetic oxides is largely governed by the Fe–Fe interaction (the spin coupling of the 3d electrons). By introducing rare earth (Ho3 þ ) ions into the spinel lattice, the R–Fe interaction also appears (3d–4f coupling) which can lead to small changes in the magnetization [22]. In the present work it is observed that the exchange interactions decrease with increasing Ho3 þ content. It can be explained on the basis of the larger Ho3 þ content does not enter the spinel lattice but forms aggregates on the grain boundary, forming a secondary phase and affecting directly by reducing exchange interaction. Such a decrease is due to weaker Fe–Ho interactions on B-sites than Fe–Fe interaction. The dominant exchange interaction (J) between B-sites decreases indicating that magnetic properties have been diluted

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