Magnetic and Resonance Study of Double doped M-type hexaferrite Ba1-x(CuxAlxFe12-x)O19

T. Ravindra Reddy¹, P. Narayana Reddy², S. Lakshmi Reddy³*, Yoshinobu Nakamura⁴, Satoru Kaneko⁵, Paolo Mele⁶, Taras Polek⁷, Tamio Endo^8
1Department of Physics, AITS, Cuddapah 516003, India
²Department of Physics, Government College for Men, Cuddapah 516001, India
³Department of Physics, SV Degree College, Cuddapah 516003, India
⁴The University of Tokyo, 7 Chome-3-1 Hongo, Bunkyo, Tokyo, Japan
⁵Kanagawa Institute of Industrial Science and Technology, Shimo-Imaizumi, Ebina, Kanagawa 2430435, Japan
⁶Shibaura Institute of Technology, Toyosu Campus, SIT Research Laboritiries, 3-7-5 Toyosu, Koto-ku 4th Floor, Tokyo 1358548, Japan
⁷Institute of Magnetism of the NAS of Ukraine and MES of Ukraine, 36b Vernadsky Avenu, Kyiv 03142, Ukraine
⁸Japan Advanced Chemicals, 3007-4 Kamiechi, Atsugi, Kanagawa, Kanagawa 2430801, Japan
Frontier Research Today, 2019, 2, 2007
Published Online: 02 October 2019 (Article)
DOI: 10.31716/frt.201902007
*Corresponding Author. Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Nanoparticles of Ba_1-x(Al_xCu_xFe_12-x)O₁₉ (x varies from 0 to 1) were prepared by solid-state reaction route method. The structural and morphological characterization of samples has been investigated by X-ray diffraction and TEM. Electron Spin Resonance (ESR), Mossbauer, VSM techniques were used to study the magnetic properties of the compounds. Change in the magnetic state of nanoparticles due to doping was also examined. Mossbauer studies suggest that the sample is ferrimagnetic when x equals 0 and completely paramagnetic when x euqals 1. ESR studies reveal splitting of resonance lines at certain concentrations (x) of Cu/Al. VSM and ESR studies reveal the destruction of collective magnetism when doping is increased.

Keywords

Doped barium ferrite; Eelectron spin resonance; Mossbauer spectroscopy; Magnetic properties

Introduction

Hexagonal ferrites are one of the most important materials for modern industrial world. These materials have been employed in the manufacture of simple permanent magnets, data storage and microwave devices¹. Ba – hexaferrite, also known as M-type [(Ba, Sr)Fe₁₂O₁₉], has attracted particular attention because of its significant properties like magnetic magnetocrystalline anisotropy, high coercive field, and Curie temperature². Being M-type hexaferrite, it has a complex structure where Fe³⁺ ions have nonequivalent positions and are separated by O^2- ions. The spins of opposing iron ions are linked by interactions which take place via the intermediate oxygen atom, and this is termed as super-exchange¹. In general, the magnitude of exchange depends on distance in Metal-Oxygen-Metal chain angle. The smallest interaction occurs at 90^o and maximum exchange takes place at 180^o. So the distortions in iron environments (oxygen polyhedrons) result in super-exchange and hence consequently change the magnetic properties. Thus the behavior of hexaferrites is similar to substituted manganites where magnetic ions of manganese interact with each other by double exchange³.

Doping is an effective method to change properties. Ba²⁺ (1.34 Å) and Fe³⁺ (0.64 Å) ions have different ionic radii. Accordingly, substitutions in different positions cause a different response. Also, Fe³⁺ ions of barium hexaferrites are distributed over five different sub-lattices¹. There are many reports on substituted barium hexaferrites by different elements with different oxidation states and ionic radii^4-10. In a number of studies, it is observed that the substitution of Al³⁺ in the Fe³⁺-site leads to an increase in coercive force (H_C)^4,11,12 and change in magnetic properties. Aluminum ions replace the iron ions and reduce the volume of the unit cell since ionic radius of Al³⁺ (0.54 Å) is less than that of Fe³⁺ (0.64 Å). This leads to a distortion of the oxygen polyhedrons and as a consequence, the super-exchange interaction is affected. The partial substitution of Fe³⁺ by Al³⁺ in barium hexaferrite has been studied by varying the concentration of x (0≤ х ≤4)¹². These results suggest that saturation magnetization (M_S) reduces and similarly coercive field growth increases with increase of aluminum concentration. Similar, results are also reported by others¹³. The increase of H_C was established when concentration of Al is increased from 2.8 kOe at x = 0 to 4.7 kOe at x = 2. The change of magnetic properties with small and equal amount of either Al or In in Ba ferrite has been studied⁴. These studies reveal that H_C is 14 times greater in indium doped ferrite than that in Al doped Ba ferrite. Further the total magnetic moment of samples is reduced from 20 μ_B (for pure Ba ferrite) to 19 and 16.7 μ_B for Al and In doped Ba ferrites. The authors reported that in In doped sample, distortion occurs in crystal structure due to the replacement of iron by indium ions (In³⁺ = 0.79 Å)⁴ which have greater ionic radii thus creating frustrated magnetic structure in the sample.

Copper doped hexagonal barium ferrite [BaFe_12-xCu_xO₁₉] was examined¹⁴. Magnetic properties were investigated and found that when x = 0.03, the low coercive change (≈ 0.64 %) and saturation magnetization increase sharply¹⁵. Further increase of Cu doping increases saturation magnetization, Curie temperature and coercivity. Hexaferrite properties are sensitive to the method adopted in the synthesis¹. Since Ba ion has large ionic radius, doping by ion having more ionic radius (≈ 1 Å) such as Ce³⁺, Pr³⁺, La³⁺ etc.^6,16,17 can replace Ba in hexaferrite. Double doping in hexaferrite causes unexpected structural and magnetic properties^5,6,18,19. So far, dual substitution of ions with large and small ionic radii of Al and Cu ions in hexaferrite was not reported. Therefore, we propose to synthesize Ba_1-x (Cu_xAl_xFe_12-x)O₁₉ ferrite via solid state route method by varying x from 0.00 to 1.00 in steps of 0.25 to study the influence of concentration of Cu/Al on the magnetic properties of the compound.

Experimental

Barium hexagonal ferrite Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ (x = 0.0, 0.25, 0.5, 0.75, 1.0) samples were synthesized by employing solid state route method. Generally in synthetic ferrite systems, Ba composition remains constant and is at its regular lattice site while Fe has been replaced by Cu/Al. But in the present synthesis, compositions of Ba, Cu, Al and Fe are varied.

X-ray diffractometry was performed by Philips X-ray diffractometer using Cu-K_α radiation (λ = 1.54060 Å) at room temperature. Information about morphology and particle size were obtained on Philips CM 200 transmission electron microscope operating at 200 kV and resolution 0.23 nm. ESR spectra of all the samples were recorded from 150 K to 300 K on Carl JES X3 series ESR spectrometer operating at X band frequency (υ = 9.198 GHz). ⁵⁷Fe and ⁵⁵Fe Mossbauer absorption spectra were recorded in transmission geometry at room temperature using a multichannel analyzer with a drive in constant acceleration mode; ⁵⁷Co(Rh) source with initial activity of 20 mCi was used. Magnetic properties of the compounds were measured using vibration sample magnetometer (VSM). The magnetization loops (M versus H) of all samples were carried out at 5, 150, 200, 255 and 300 K using VSM in a fields up to 80 kOe.

Result and Discussion

Micro and crystal structure

Figure 1 presents the X-ray diffraction patterns of all barium ferrite samples of Ba_1-x (Cu_xAl_xFe_12-x)O₁₉. The X-ray diffraction patterns of all the species are similar with very sharp peaks which indicate that all have hexagonal crystal structures. No additional phases and impurities are found.

Using X-ray data, lattice constants are calculated and are in the range of 5.8827 Å to 5.777 Å and 23.195 Å to 23.00 Å for a and c respectively. The lattice constant a is found to decrease with increase of Cu/Al concentration (x), the lattice constant c decreases when x is increased up to 0.25 and then remains constant at concentration x = 0.50, 0.75 and 1.00. This indicates that the change of the minor axis (a-axis) is larger than that of major ‘c’ axis for the substitution of Cu²⁺/Al³⁺ ions. This behavior is attributed to the substitution of Fe³⁺ ions which are of larger ionic radii (0.064 nm) compared with Al³⁺ ions (0.054 nm), and the substitution of Ba²⁺ ions which are of larger ionic radius (0.134 nm) compared with Al³⁺ ions (0.054 nm) and Cu²⁺ ions (0.078 nm). The ionic radii sum of Al³⁺ ions and Cu²⁺ ions are 0.132 nm and is almost the same as Ba²⁺ ionic radius. Thus in addition to normal doping of Al³⁺ in Fe³⁺, double doping of Al and Cu into Ba site takes place. Hence the noticeable value of cell size decreases and resulting low cell constant. Using Debye-Scherrer equation, the particle size is evaluated from the line broadening of the peak (203) which changes from 125 to 100 nm.

 

Figure 1. XRD pattern of Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ compound.

 

Figure 2(a-c) shows TEM images of the un-doped (x = 0.00), doped (0.50) and maximum doped (x = 1.0) samples respectively. It is noticed that the hexaferrite particles are well defined with polygonal octahedral, tetrahedral and bipyramidal shapes (shown in Figure 2(a) at x = 0.0) and are well joined. From nano-chemistry point of view, the adjacent particles which have common crystallographic orientation collide each other leading to coalescence. Adjacent primary particles attach to each other and form aggregates.

 

Figure 2. TEM images synthesized Ba_1-x(Cu_xAl_xFe_1-x)O₁₉.

 

It is evident from Figure 2(b) (x = 0.50) that the particle size decreases with doping when compared to undoped ferrite. When doping is maximum, x = 1.0, as shown in Figure 2(c), the particle size is further reduced. The particles prepared from higher Cu/Al concentration are discrete. The decrease in agglomeration is due to the replacement of Fe³⁺ ions by Al³⁺ ions. Further replacement of Ba²⁺ ion by both Cu²⁺ and Al ³⁺ ions leads to decrease in radii of cations. This results in the decrease of grain size. Hence the lattice constant decreases. This suggests that adjacent particles which do not have common crystallographic orientation do not collide with each other and thus they are appearing as separate particles. Also it is seen that the particle size decreases with increase of Cu/Al concentration. From Figure 2, it is observed that the particle size is drastically affected with increase of Cu/Al composition. The particle size distribution is broader and indicates the polycrystalline nature. The particles size varies from 98 nm, 95 nm, 69 nm, 78 nm and 61nm when x is increased from x = 0.00 to 1.00 in steps of 0.25. The particle size is large at x = 0.0 and small at x = 1.0. The ratio of particle size is about 10:6 (without doping vs maximum doping).

Magnetic properties

Bulk hexaferrite BaFe₁₂O₁₉ is a classic magnetic material with Tc = 725 K and exhibits very high uniaxial anisotropy. When the same compound is in nanoscale form, its magnetic state changes drastically^1,20.

Figure 3 shows magnetization (M emu/g) versus temperature (T/K) behavior of Ba_1-x(Cu_xAl_xFe_12-x) O₁₉ (x= 0.0 to 1.0) samples under both Zero-Field-Cooled (ZFC) and Field-Cooled (FC) conditions in an applied magnetic field of 80,000 Oe. ZFC curves were obtained by cooling the sample from 300 K to 5 K without applying an external magnetic field. In the case of FC temperature of the sample after reaching 5 K, an 80,000 Oe magnetic field was applied and the magnetization was recorded as the temperature was increased. From these curves blocking temperature is known. This is the temperature at which branching of ZFC and FC curves merge together or where a maximum in magnetization in the ZFC data is seen.

 

Figure 3. Temperature dependence of M(T) of substituted Ba-ferrite.

 

In the present study nearly temperature independent magnetization behavior have been seen between 300 K to 150 K except for x = 0.50. For this compound raise in the magnetization below 250 K was observed and attributed to paramagnetic phase. Below T_B magnetic susceptibility decreases with decrease of temperature. This behavior is characteristic of super paramagnetism present in the compound.

As can be seen from Figure 3, the un-doped sample (x=0.0) shows a high magnetization when compared to the doped samples; it changes slightly with increase of temperature. A similar trend is observed in doped sample when x = 0.25. In this compound the magnetization decreases by about 40% relative to the pure ferrite. In this compound T_B is noticed as 150K. The magnetization is zero beyond T ≥ 150 K. When x ≥ 0.5, a qualitatively different dependence inherent to super-paramagnetic state is observed. In these compounds T_B is found to be approximately 25 K, 20 K and 10 K for x= 0.50, 0.75 and 1.00 respectively. These results suggest that T_B decreases with the increase of Cu/Al content. At room temperature, no magnetic property is noticed with x= 0.75 and 1.0.

In the present case, when x = 0.75, the magnetization is almost zero at T ≥ 200 K. Previous reports¹² indicate that substitution of Al even up to x = 4.0 in the compound results in weak magnetization. Using Mossbauer spectroscopy, they explained that Al ions preferentially occupy the octahedral site 4f₂, 2a,12k and 4f₁ at x ≤ 2.0.

The magnetization measurements (M verses H) of all Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ (x= 0 to 1.0) nano samples are carried out at 5, 100, 200, 255 and 300 K using vibrating sample magnetometer (VSM) in a field of 80000 Oersted. The measured hysteresis loops for Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ samples as a function of the applied magnetic field are shown in Figure 4(a-b). Magnetization is normalized to the maximum value for each curve at 30 kOe for better data comparison. From Figure 4(a), we notice that there are three different cases: For undoped BaFe₁₂O₁₉ sample (x=0.0), M(H) curve shows no hysteresis and its residual magnetization is an indication of superparamagnetic state ²¹. Completely doped sample (x =1.0) M(H) show typical behavior of paramagnetic nature and its magnetization linearly increases with increase of magnetic field. In all other cases, the compound shows combined behavior. Thus, on one hand, the symmetric loop of hysteresis is clearly visible and on the other hand, the magnetization does not reach to saturation even at high field. When temperature increases to 300 K, there are no qualitative changes in M – H curves. When x = 0.50, there is a decrease in residual magnetization. Further when x = 0.25, the residual magnetism H_c is 0.3 kOe at 100 K and it decreases to 0.15 kOe at 300 K.

 

Figure 4. Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ hysteresis loops for different temperatures.

 

In BaFe₁₂O₁₉ nano ferrite system, there are five non-equivalent sub-lattices, of which three are octahedral (2a,12k, and 4f₂), one tetrahedral (4f₁) and one trigonal bipyramidal (2b)²². In the Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ nano compounds, Fe³⁺ ions are partially replaced by an equal amounts of Cu²⁺ and Al³⁺ ions, similarly Ba²⁺ ions are replaced by both Al³⁺ and Cu²⁺ ions. Now when the diamagnetic Al³⁺ ions are substituted by Fe³⁺ ions, it results in decrease of the magnetic saturation (M_s) value because of the super exchange interaction (Fe³⁺-O-Fe³⁺) since the Al³⁺ ions preferentially occupy the octahedral 2a site followed by 12k and 4f₂ sites²³. Introduction of Cu²⁺ and Al³⁺ into the tetrahedral site does not change the net magnetization of the ferrite compound but replacement of Cu²⁺ ions in octahedral sites resulted in the variation of the net magnetization of the materials²⁴. Thus all the magnetic properties are found to decrease with the increase of Cu/Al contents, which is due to the occupation of the doped cations at the octahedral sites (12k and 2a) having a spin of electrons in an upward direction.

Previous reports²⁵ indicate that substitution of Cu/Al in tetrahedral and octahedral sites in cobalt ferrite lowers the magnetization values. In our compounds, similar trend is noticed suggesting that Cu/Al doping enters both tetrahedral and octahedral sites which results in decreasing of the magnetic properties.

It is well known that the size of nanoparticles has a strong influence on the magnetic properties²⁶. In the case of our samples, it has been found that substitution affects the size of nanoparticles. This gives the possibility of partially eliminating the size effect in magnetic properties. For further investigation of the magnetic state, the method of electron spin resonance is used. This powerful method, in some cases, allows us to separate the magnetic phases of magnetic materials including complex oxides^27-29.

Resonance study

The results of VSM measurement are confirmed by Electron Spin Resonance spectroscopy. ESR spectra recorded for barium ferrite powder Ba_1-x (Cu_xAl_x Fe_12-x O₁₉) in the range 150 K to 300 K are shown in Figure 5. Fe³⁺, a d⁵ ion with ⁶S_5/2 spin state splits into three Kramer’s doublets, separated by nearly 1.0 cm^-1. In this case, only the lowest doublet is populated and one can observe resonances at g = 9.0^14,15,30,31. On the other hand, if the middle Kramer’s doublet is also populated, a g value of 4.29 is expected. If third doublet is also populated, g values ranging from 0.3 to 4.29 can be seen.

ESR spectra recorded for nano powder (BaFe₁₂O₁₉) in the range 150 K to 300 K is shown in Figure 5(a). It is seen that only a resonant signal with strongly symmetric resonance at all temperatures with same g value of 2.12, varying line intensity and width is noticed. It is attributed to the ions Fe(III) distributed in octahedral coordinate sites. It is ascribed to the ferrimagnetic resonance exhibited by Fe(III) cations in the ferrite at different interstitial sites that couple anti-ferromagnetically due to a super exchange interaction^17,29,32. The line-width of EPR signal may be due to magnetic dipole interactions among the particles or super exchange interactions between the magnetic ions through the oxygen ions. If dominant dipole interactions are present in the compound large line-width and g-factor occurs, whereas super exchange interactions are dominant small-line-width and lower g-values are observed^19,33. Occurrence of narrow peak-to-peak line-width at room temperature in the sample may be due to dominant super exchange interaction. As Cu/Al doping progresses, line splitting is observed. The similar results were observed by the authors²⁷.

 

Figure 5. EPR spectrum of nano Ba_1-x(Cu_xAl_xFe_12-2x)O₁₉ powder.

 

EPR spectra of nano (Cu_0.50Ba_0.50)(Al_0.50Fe_11.50)O₁₉ recorded in the range 150 to 300 K is shown in Figure 5(b). The spectrum is characteristic of Fe(III) and Cu(II). The calculated g values are 4.0, 2.25 are ascribed to Fe(III) and Cu(II) respectively. In particular at 250 K the observed g values are 2.74 and 2.18 which may be due to the superimposition of Fe(III) resonant signal with g ≅ 2.18.

EPR spectra of nano (Cu_0.75Ba_0.25)(Al_0.75Fe_11.25)O₁₉ recorded in the range of 150 K -300 K is shown in Figure 5(c). The spectrum is resolved with characteristic of Fe(III) and Cu(II) signals. At 250 K, the observed resonant signals give g values 5.67, 3.65 and 2.25 which are ascribed to Fe(III) and Cu(II) respectively. This suggests that as Cu/Al concentration increases the nano compound reducing its ferromagnetic character and slowly converting into super paramagnetic nature. Further we notice that the Cu is entering into tetragonal distortion³⁴.

EPR spectrum of completely doped Cu(AlFe₁₁)O₁₉ nano compound recorded in the temperature range of 150 K to 300 K is shown in Figure 5(d). The spectrum is well resolved with five resonant signals ranging from low to high field. These five resonant signals indicate that the compound may have five Fe(III) sites. These signals are due to Fe³⁺ and Cu²⁺. Thus the compound is super-paramagnetic in nature. This is explained as follows: It is assumed that Ba²⁺ is totally replaced by Cu²⁺ and only one Fe³⁺ is replaced by diamagnetic Al³⁺ ions in the unit cell. Thus paramagnetic behavior of Cu²⁺ is dominated. From the spectrum, the g values obtained for Cu²⁺ are g₁₁ = 2.70 and g_⊥ = 2.27 which indicate that Cu²⁺ is in tetragonal distortion. Tetragonal cupric complexes with D_4h generally have g₁₁ (corresponding to the magnetic field along the Z axis of the complexes) > g_⊥ >2.04 have the ground state d_x2-y2^35-37. In the present case g₁₁ > g_⊥ > 2.00 (2.70 > 2.27 > 2.0036) and hence the ground state is ²B_1g (d_x2-y2). Using this data, a new parameter G=(g₁₁ - g_e)/(g_⊥ - g_e) is defined³⁸ and calculated as ≅ 2.61. If G value falls in between 3 and 5, the unit cell contains magnetically non-equivalent ions. If G value is less than 3, the exchange coupling among the magnetically non equivalent Cu(II) ions in the unit cell is not very strong. If G is greater than 5, a strong exchange coupling takes place among the magnetically non equivalent Cu(II) ions in the unit cell^39-42 . In the present case G is less than 3.0 (observed values is 2.61), which indicates that the unit cell contains magnetically non-equivalent ions and the coupling is not very strong. As the ferrite contains a very high percentage of iron, the resonant signals due to Fe³⁺ are also noticed with g values at 9.0, 4.96, 2.04. Thus the EPR results indicate that the CuAlFe₁₁O₁₉ ferrite nano-crystal is in distorted octahedron structure. The resonant signals observed at higher magnetic field shown in Figure 5(d) have g values of 1.20 and 0.90. Such a low g values may not be due to Cu(II). Since the compound contains Cu, Al, Fe and O^2-. Generally a free radical O^2- gives a resonant signal around DPPH. Also Al is diamagnetic. Hence the resonant values are not due to Al, O^2- and Cu(II). Thus these are assigned to Fe(III) which is occupied by third Kramer’s doublet given in the theory earlier. Thus the compound with x = 1.0 is completely paramagnetic. This result confirmed the magnetic measurements shown in Figure 4. The sample with complete substitution has zero magnetization at 200 K and as the concentration of Cu/Al increases, saturation magnetization (M_s) also increases.

From Figure 4(b) (х = 0.5), the temperature evolution of the magnetic state is observed. At a low temperature, the almost symmetric resonance line is observed. However, as temperature progresses the appearance of an additional signal is observed. At 300 K, two clear lines are observed. This suggests that the samples exhibit a weak ferromagnetic nature and significant paramagnetic nature (remanence is equal to 7 % of normalized M). But from Figure 4(a), it is observed that the ferromagnetic behavior is large (30 %) at low temperature. Thus these results suggest that coexistence of two magnetic phases in the compound.

₅₅Fe Mossbauer absorption spectra of hexagonal ferrite system Ba_1-x(Cu_xAl_xFe_12-x)O₁₉ for x = 0.0 and 1.0 were recorded at room temperature. The result is shown in Figure 6. It can be seen from Figure 6(b) that the spectrum of BaFe₁₂O₁₉ exhibits a superimposition of two Zeeman sextets, which indicates the ferrimagnetic behavior of the sample. The sample is polycrystalline in nature and the presence of any texture is unlikely; the widths and heights of the six lines of each sextet are constrained during fitting so that the area ratio of the lines (1,6): (2,5):(3,4) becomes 3:2:1. One sextet (outer) corresponding to a higher magnetic field is attributed to Fe³⁺ ions in the octahedral (B)-site and the other sextet (inner) corresponding to the lower magnetic field is attributed to Fe³⁺ ions in the tetrahedral (A)- site. For the sample, BaFe₁₂O₁₉, the sextet with lowest isomer shift (IS) (0.34 mm/s) is attributed to Fe³⁺ ions in tetrahedral sites. The other sextet with higher IS (0.45 mm/s) is attributed to Fe³⁺ ion in the octahedral site (B). The isomer shift of octahedral sites is greater than that in the tetrahedral site and is in agreement with the reported data⁴³. Further, the quadrupole splitting (QS) observed for the tetrahedral site is greater than that of the octahedral site, which indicates that the tetrahedral site is higher asymmetric for Fe³⁺. Also the observed values (0.45 and 0.34 mm/s) of isomer shift (δ) are significantly less than the expected value, 0.5 mm/s for the Fe²⁺ ions⁴⁴. Hence, the Mossbauer spectrum of pure Ba ferrite is not affected by the relaxation process. Thus the presence of Fe²⁺ ions in the present ferrite system is ruled out. Therefore the electron exchange interaction (Fe²⁺↔ Fe³⁺+ e^-) does not occur and hence the oxidation state of Fe³⁺ remains unchanged during the synthetic process. Decrease in isomer shift may indicate lowering of coordination number of Fe³⁺ ions which is usually small⁴⁵. But the increase (0.17, 0.24 mm/s) in the quadrupole splitting indicates a loss of symmetry of Fe³⁺ ions to the highly distorted octahedral environment. Quadrupole splitting increases with increase of Cu/Al. These results indicate that Ba ferrite lattice shows a distortion. The area ratio of Fe³⁺ octahedral to the tetrahedral site is obtained from the Mossbauer spectra BaFe₁₂O₁₉ is 39.7/60.9 = 0.6519 which is in good agreement with that calculated from X-ray intensities (not discussed in the XRD analysis). It is observed that this ratio decreases with increasing Cu/Al concentration suggesting a decrease in ferrimagnetic behavior and converting the same into paramagnetic¹².

Figure 6(a) shows the Mossbauer spectrum (MS) of maximum substituted Cu/Al concentration in the ferrite. The presence of doublet indicates the characteristic features of paramagnetic behavior. The relaxation of paramagnetic nanoparticles at a faster rate than that of MS measurement time (10^-9 s) causes in double. A significant change in the isomer shift (0.24 mm/s) of ferrite is observed with progressive doping of Cu²⁺/Al³⁺ ions. This indicates that the s-electron charge distribution of Fe³⁺ ion is influenced by Al substitution. The sample with x = 1.0 is paramagnetically relaxed, and the relaxation in these sample decreases while intensity of the paramagnetic doublet increases. This may be due to the interaction of the electric field gradient (EFG) with the quadrupole moment of ⁵⁷Fe nucleus and the reduction of interaction between Fe ions and octahedral Al³⁺ ions. The analysis of the data shows that quadrupole splitting (0.66 mm/s) increases with the increase of doping. The quadrupole doublet pattern clearly shows that the sample exhibits paramagnetism. The results obtained are in consistent with the results of the vibrational sample magnetometer and electron paramagnetic resonance spectroscopy.

 

Figure 6. Mossbauer absorption spectra of system (a) BaFe₁₂O₁₉ and (b) CuAlFe₁₁O₁₉.

 

Conclusions

Nano-size particles of Cu/Al substituted barium hexaferrite Ba_1-x(Cu_xAl_xFe_12-x)O₁₉, х = (0.0 – 1.0) have been synthesized by solid-state route method. X-ray diffractometry and TEM showed that the size of synthesized nanoparticles is about 100 nm and the doping by Cu/Al leads to a large decrease in their sizes. Magnetic properties are found to decrease with the increase of Cu/Al content which is due to the occupation of the doped cations at the octahedral and tetrahedral sites. The magnetization of the un-doped sample is high and is very slightly dependent on temperature. At the same time, magnetization of maximum-doped (x = 1.0) sample is weak. The study of M(H) dependence of BaFe₁₂O₁₉ showed that there is no residual magnetization in the sample and shows almost no hysteresis. When x = 1.0 the sample is completely paramagnetic. This is confirmed by the linear dependence of magnetization and EPR studies which has a low value. At intermediate concentrations (0 < x < 1), they show a small loop of hysteresis where coercivity (H_C) was up to 300 Oe, but they do not reach saturation in large fields. This suggests the coexistence of strongly (ferrimagnetic) and weakly (paramagnetic) behavior which is confirmed by ESR. Mossbauer spectroscopy further confirmed that nanoparticles of un-doped samples have strong-magnetic properties (ferrimagnetic) and completely doped (x = 1.0) samples have weak magnetic properties (paramagnetic). Considering the results and literature data, we assume that the double substitution greatly changes the magnetic properties of individual nanoparticles. This leads to the fact that the ensembles of these particles change their behavior (Super PM – PM/FM – PM).

Notes

The authors declare no competing financial interest.

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