Structural, magnetic and thermo-magnetic properties of NiMn Y-Type strontium nano-hexaferrites

Journal Pre-proof Structural, magnetic and thermo-magnetic properties of NiMn Y-Type strontium nanohexaferrites Vijay V. Warhate, D.S. Badwaik PII:

S0925-8388(19)34076-9

DOI:

https://doi.org/10.1016/j.jallcom.2019.152830

Reference:

JALCOM 152830

To appear in:

Journal of Alloys and Compounds

Received Date: 20 August 2019 Revised Date:

25 October 2019

Accepted Date: 27 October 2019

Please cite this article as: V.V. Warhate, D.S. Badwaik, Structural, magnetic and thermo-magnetic properties of NiMn Y-Type strontium nano-hexaferrites, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152830. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical Abstract

20 15

Moment, M (emu/gm)

10 5 0

x = 0.0 x = 0.5 x = 1.0 x = 1.5 x = 2.0 x = 2.5

-5 -10 -15 -20 -15

-10

-5

0

Field, H (kOe)

5

10

15

Structural, Magnetic and Thermo-magnetic Properties of NiMn Y-Type Strontium Nano-hexaferrites Vijay V. Warhate1, D. S. Badwaik2 1

Department of Physics, S. N. Mor College, Tumsar, Maharashtra, India 441912

2*

Department of Physics, Kamla Nehru Mahavidyalaya, Nagpur, Maharashtra, India 440025 *

Corresponding author: [email protected] ; Phone No. : +917588883515

1

ABSTRACT Six chemical compounds of poly-crystalline NiMn Y-type strontium nano hexaferrites doped with hybrid Ti4+Co2+, having chemical formula Sr2NiMnFe12X(TiCo)X/2O22

(0 ≤ x ≤ 2.5 and ∆x = 0.5) synthesized by sol-gel auto-combustion through

microwaves and calcined at 950 ºC for 5 hr. The refined XRD analysis shows compounds are in single Y-type hexagonal phase. The lattice parameter ‘a’ slightly increases and easy magnetized ‘c’ axis undergoes more expansion with the content of TiCo. The grain size measured from XRD data is in the range of 41 nm to 71 nm. The microstructure was visualized and studied by SEM, TEM, HRTEM and SAED. TEM images show that the compounds are in hexagonal shape with grain size in the range of 42 nm to 89 nm. Saturation magnetization (Ms), Retentivity (Mr) and Coercivity (Hc) were observed through vibrating sample magnetometer (VSM). All the six compounds are found to be ferrimagnetic at the room temperature and remain so up to the transition temperature (TM). Above this transition temperature the compounds start becoming paramagnetic. The transition temperature (TM) was determined from the derivative of the thermo-magnetic susceptibility curve. The strong (negative) derivative peak confirmed a single magnetic phase and the narrow peak profile predicts magnetic homogeneity of the compounds.

Keywords: Y-type hexa-ferrites; Sol-Gel Auto-combustion; XRD, HRTEM and SAED; Magnetic properties; Transition temperature.

2

1.

Introduction The research advancement for ferrites is increased because of its technological

applications started initially to co-fine magnetic flux and guide through ferrite [1]. Ferrites are divided into two types depending on magnetic coercivity. Soft ferrites have low coercivity while hard ferrites have high coercivity and difficult to demagnetize. Poly-crystalline hexaferrites exhibit outstanding magnetic and dielectric properties which depend on chemical composition, processing conditions, sintering time and temperature. On the basis of crystal structure, hexaferrites are classified into six (M, W, X, Y, Z, U) categories. Among them, Y-type ferrites (Sr2Me2Fe12O22) composed of hexagonal M type and cubic spinel ferrites unit [2]. These are well-suited for widespread applications viz. Ferrite cores, Multilayer ferrite chip inductor, refrigerator magnets, loudspeakers, small electric motors, magnetic latches and magnetic levitation, wave absorber and magnetic recording media. Research organizations and researchers continuous focus is to develop a revolutionary type of Magneto-electric Memory (data storage) device (MMD) using Nanoscale multiferroic systems (NMS) to achieve improved storage density and performance, enhanced power output, energy efficient, thermal stability and physical size reduction. Perpendicular magnetic recording (PMR-2005) bits align vertically and provide a significant increase in storage density as compared to conventional longitudinal magnetic recording (LMR) technology. The combination of PMR media and shielded magneto resistive head technology enables multiterabyte drives with densities approaching to one trillion bits/sq. inch. The main disadvantage is to stabilize the magnetization of the material medium due to thermal fluctuations [3]. To use ferrites in a high density recording medium, a high coercivity, high anisotropy, high values of remnant magnetization, nearly squared M-H loop, better thermal stability and single domain particles are the basic requirements [4]. The microwave assisted sol-gel auto-combustion method was used to synthesize the samples because this method has many advantages such as energy efficient, short reaction rate, low calcinations temperature, easy operation, better distribution of particle size, excellent chemical homogeneity and ultra-fine powder of nano-size [5]. As size is reduced to the nanoscale, it can affect the optical, electrical and magnetic behavior of materials. Due to nano size, it is possible to form the structures to achieve specific properties.

3

In present work, Sr2NiMnFe12-X(TiCo)X/2O22 (0 ≤ x ≤ 2.5 and ∆x = 0.5) synthesized by sol-gel auto-combustion through microwave and calcined at 950 ºC for 5 hr. This research focuses on the structural, magnetic and thermo-magnetic properties of NiMn Y-type strontium hexaferrites and the thermo-magnetic effect of TiCo substitution. The magnetic parameters such as saturation magnetization (Ms), coercivity (Hc), retentivity (Mr) and squareness ratio (Mr/Ms) values are obtained and discussed. To understand the temperature dependence of the magnetization, temperature of maximal magnetization i.e. Transition temperature (TM) and Curie temperature (TC) was determined. The transition temperature was determined from the derivative of the thermo-magnetic curve. The Curie temperature value is obtained from the usual inverse susceptibility curve. The similar Curie temperature is also obtained from the novel 5th polynomial fit second order derivative of the χ-T curve. 2.

Experimental details

2.1.

Synthesis Six polycrystalline chemical compounds of Sr2NiMnFe12-X(TiCo)X/2O22 (0 ≤ x ≤ 2.5

with increment of 0.5) were prepared by microwave assisted sol-gel auto combustion route. The starting chemicals used were high purity analytical reagent grade Sr(NO3)2 (99.99 %, Aldrich), Ni(NO3)26H2O (99.99 %, Emsure-Merck KGaA), Mn(NO3)24H2O (99.99 %, Emsure-Merck KGaA), Fe(NO3)29H2O (99.99 %, Emsure-Merck KGaA), Co(NO3)26H2O (99.99 %, Emsure-Merck KGaA), titanium tetra chloride TiCl4 (99.99 %, Emsure-Merck KGaA) and CO(NH₂)₂ ( ≥ 99.5 %, Emsure-Merck). The initial composition derived from the stoichiometric amount of the metal nitrate and urea were calculated by using the total oxidizing and reducing valences of the components which provide as numerical coefficients for stoichometric balance so that the equivalent ratio is unity and the energy generated by the combustion is at the maximum. Formation of Sr2NiMnFe12-X(TiCo)X/2O22 by the exothermic redox reaction of the metal nitrates with fuel (urea) can be represented as 12 Fe(NO3)3·9H2O + 2 Sr(NO3)2 + Ni(NO3)2·6H2O + Mn(NO3)2·4H2O + 36.666 CH4N2O 6 Fe2O3 + 2 SrO + NiO + MnO + 36.66 CO2 + 58.666 N2 + 191.33 H2O Sr2NiMnFe12O22 + 36.66 CO2 + 58.666 N2 + 191.33 H2O + ∆Q↑ ------- (1)

4

Stoichiometric amount of metal nitrates and fuel were dissolved one by one in 30 ml of triple distilled water to prepare a solution. It is then heated with continuous stirring at 95 ºC to form gel. Then it was irradiated at 2.45 GHz into a microwave domestic oven. After a while due to self-propagating combustion, dark brown fumes started coming out and gel got fired into moderate flames and yielded into a foamy dark brown powder. The yielded samples were crushed in a pestle mortar for about two hours. The milled powder pressed to pellets and then calcined at 950 ºC for about five hours in furnace and allowed the furnace to cool slowly to room temperature. The calcined pellets crushed and milled in a agate mortar and pestle for about one hour, to get Sr2NiMnFe12-X(TiCo)X/2O22 Ytype hexagonal ferrite samples ready for characterization. 2.2.

Characterization PANalytical X’Pert Pro X-ray diffractometer with filtered Kα radiation (λ = 1.5406

Å) is used for recording the X-ray diffraction pattern. To estimate the shape and size of the particles and crystallinity, SEM (JEOL JSM - 6390LV), TEM, high-resolution TEM (HRTEM) and SAED-TEM (JEOL JEM-2100) measurements were performed. The field dependent magnetization was measured at room temperature on vibrating sample magnetometer (VSM: Lake Shore 7404) with a maximum applied field of 12 kOe. The samples are characterized by Gouy’s balance for thermo-magnetic susceptibility measurement with uniform heating from room temperature up to 750 ºC carefully using temperature controller unit. 3.

Results and discussions

3.1.

Structural and phase analysis The indexed XRD patterns of calcined samples with different TiCo concentration are

shown in Fig. 1. The observed peaks of the samples were compared with standard data file ICSD # 024575 (PDF number- 732035) of Y-type (Ba2Zn2Fe12O22) compiled by- ICSD (Inorganic Crystal Structure Database) and the entire observed pattern is indexable for single phase [6, 7]. All XRD patterns exhibit sharp and well-defined diffraction peaks which identify the formation of crystallized Y-type hexaferrite structure. It also reveals that substitution of Ti and Co ions into the hexaferrite crystal has no considerable effect on the phase formation and purity of obtained hexaferrites. All the six compounds belong to the

R 3 m (no.166) space group. 5

For the hexagonal crystal structure, the unit cell is characterized by the values of lattice parameters ‘a’ and ‘c’. The inter planer spacing ‘d’ is related to the dimension of unit cell by the following equation [8].

1 4(h 2 + hk + k 2 ) l 2 = + 2 2 d hkl 3a 2 c

(2)

The lattice parameters (‘a’, ‘c’), Experimental density (Db), X-ray density (Dx), porosity and crystallite size of the samples were calculated from XRD data. The compositional variation of lattice constants ‘a’ and ‘c’ are represented in Fig. 2. It is observed that the lattice constant ‘a’ slightly increases and easy magnetized ‘c’ axis undergoes more expansion with the content of TiCo. This increase in ‘a’ and ‘c’ is explained on the basis of the ionic radii. The ionic radius of the Ti4+ ion is (0.74 Å) and Co2+ ion is (0.72 Å). Since ionic radius of Ti4+ and Co2+ is more than that of Fe3+ (0.64 Å), the increase in lattice constants with TiCo substitution is expected. Hexaferrites are anisotropic because it is easier to orient the spin directions along c-axis which is perpendicular to the hexagonal base plane. Due to this, there is considerable variation in lattice constant c than a. The lattice parameters, a = 5.88Å, c = 43.32Å are comparable with other Y-type hexagonal ferrites [8, 9]. The variation of X-ray density, experimental density and porosity with content of TiCo is presented in Fig. 3. It is observed that the x-ray density decreases with TiCo content with minute inconsistency. This decrease in density with increasing TiCo content can be explained on the basis of the atomic density. Since the average atomic density of substituent Ti (4.506 g/c3) and Co (8.90 g/c3) is less than that of Fe (7.874 g/c3) therefore decrease in density is expected. The value of x-ray density, 5.2 g/cm3 for all compounds is comparable with other ferrites [10]. Experimental density is calculated from mass and sample dimension. It is observed that experimental density is always less than X-ray density. This is due to the fact that pores are generated during calcination of the samples. The experimental density increases with TiCo content up to x = 1.0 and then decreases. This increase in density may be due to the formation of solid solution. The formation of solid solution is also confirmed by increase in the lattice constant ‘c’ with TiCo content, leading to an increase in the rate of cation interdiffusion [11]. According to Coble and Burke, the density decreases as intra- granular 6

porosity increases [12]. The compositional variation of Porosity has the opposite trend to that of experimental density shown in Fig. 3. The crystallite size calculated by well-known Debye Scherrer formula for different intense peaks are found to be in the range of 41 nm to 98 nm and for most intense peak of (119) plane is found to be in the range of 41 nm to 71 nm shown in Table 1. 3.2.

Microstructure SEM micrographs of samples are shown in Fig 4. It shows that hexaferrite particles

formed are well agglomerated to form the clusters of different sizes and shapes. High surface to volume ratio gives high surface energy and to minimize it, nano-particles get agglomerated [13, 14]. Similar results have been observed for Sr2Co2Fe12O22 [15]. TEM images (Fig 5) show that the prepared samples are in hexagonal shape with grain size in the range of 42 nm to 89 nm which is in good agreement with the values estimated by Scherrer formula. HRTEM grain images of prepared hexaferrite samples are shown in Fig. 6. The inter-grain spacing ‘d’ values calculated from HRTEM images are in good agreement with the same observed from XRD and calculated from TEM SAED pattern. Indexed TEM SAED patterns of all samples are shown in Fig 7. 3.3.

Magnetic Properties The magnetic hysteresis loops of all the compounds are shown in Fig. 8 indicate that

all the samples exhibit steep rise in magnetization at low magnetic field followed by slow variation at high field. The magnetic parameters such as saturation magnetization (Ms), coercivity (Hc), retentivity (Mr) and squareness ratio (Mr/Ms) values were obtained from these loops are listed in Table 1. The values of saturation magnetization (Ms) were calculated by the law of approach to saturation (LAS) [16-18] using the following equation. a b   M = M S .1 − − 2  + χ hf .H ----------- (3)  H H  Where, Ms is the saturation magnetization, a- is in-homogeneity parameter representing the contributions of inclusions and micro-stress, χ hf is the high field susceptibility

and b is the magneto-crystalline anisotropy parameter. For hexagonal crystal structure, b can be expressed as

7

b = H a / 15 = 4 K 1 / 15 .M S 2

2

2

------------------------- (4)

Where, Ha is anisotropy field and K1 is anisotropy constant. It has been experimentally confirmed that ferromagnetic materials follow the law of approach to saturation (equation-3) [19]. When applied field is sufficient for magnetic saturation, LAS is best fit technique to understand the magnetic material. If applied magnetic field is not sufficient for magnetic saturation, then extrapolation plays an important role. If magneto-crystalline anisotropy (b) is dominant, an extrapolation of 1/H2 gives the saturation magnetization and if inclusions and micro-stress are dominant, an extrapolation of 1/H gives the saturation magnetization. Fig. 9 shows the LAS fit saturation magnetization for all samples. The saturation magnetization Ms was also obtained by extrapolating M(1/H) and M(1/H2) curves to 1/H = 0 and 1/H2 = 0.

Table 2 shows observed values of Ms along with estimated values of Ms by Law of approach to saturation and by extrapolation of 1/H and 1/H2. From Fig. 8 and Fig. 9, it is observed that the samples with x = 0.5 and x= 2.0 are magnetized up-to saturation and LAS is best fit for these samples. For samples x = 0.0, x = 1.5 and x= 2.5 (where inclusions and micro-stress are dominant; a > b), extrapolation of 1/H gives the saturation magnetization. For sample x = 1.0 (where magneto-crystalline anisotropy b is dominant; b > a), extrapolation of 1/H2 gives the saturation magnetization. The Bohr magneton number (Magnetic moment per formula unit in Bohr magneton) was calculated by using Eq. (5) and listed in Table 1.

nB =

M .M S M .M S = ----------------(5) µ B .N A 5585

Where, M is the molecular weight of the sample and Ms is the observed saturation magnetization in unit emu/g. Saturation magnetization decreases (Table 2) with the substitution of Co2+ and Ti4+ ions from x = 0.0 (17.87 emu/g) to x= 2.0 (6.43 emu/g) and then increases with further substitution (x = 2.5). The metal ions distribution at different sites in both S and T block is responsible for this variation of saturation magnetization (Ms). The sub-lattices sites of 3bVI (+), 18hVI (+), 3aVI (+), and 6cVI (-) are located at octahedral interstitial sites, whereas 6cIV (-) and 6c∗IV (-) are located at tetrahedral interstitial sites. Actually, the presence of nonmagnetic 8

ions in the 3bVI, 18hVI, and 3aVI sub-lattices sites with spin-up direction i.e. weakening of super-exchange interaction between Fe3+ cations in the octahedral and tetrahedral sites is responsible for the decrease of total magnetic moments and saturation of magnetization [20]. It is worth noting that inside the T block, three octahedral ions, belonging to 6cVI and 3bVI sub-lattices, both these sites lay on a vertical threefold axis, the central 3bVI ion sharing two faces of its coordination figure with the adjacent 6cVI ions. As a consequence, a nonmagnetic or less magnetic Me2+ ion with a marked preference for either 6cVI or 3bVI may cause drastic changes in the magnetic configuration, that leads to the cancellation of the antiferromagnetic 3bVI -6c∗IV, interaction which is the strongest one in the Y-structure [20]. The super exchange interaction plays a crucial role in the magnetic ordering of Sblock magnetization between octahedral 3aVI and tetrahedral 6cIV sites of metal ions. The octahedral site is larger than the tetrahedral site and allows more electronegative ion [21]. Crystal Field Theory states, “tetrahedral sites are occupied by d1, d2, d3 and d4 ions and octahedral sites are occupied by d6, d7, d8, d9 ions and d0, d5, d10 configuration ions have no site preference” [22]. Co2+ ions with d7 configuration prefer to occupy octahedral site. Ti4+ ions have no site preference due to d0 configuration and can occupy octahedral or tetrahedral or both sites. Site occupancy also depends on nature of participating cation viz. electronegativity and compressibility. Ti4+ ion with 3p6 configuration is more compressible than Co2+ ion (3d7 configuration), thus can occupy tetrahedral site. Besides this, more electronegative ion tends to occupy octahedral site and the electro-negativity of Ti4+ ion is more than that of Co2+ ion, thus Ti4+ ion can also occupy octahedral site [23]. By taking into account the octahedral site preference of Ti4+, Ti4+ and Co2+ both ions preferred octahedral 3aVI sites. Therefore, replacement of Fe3+ ions by Ti4+Co2+ ions consequently causes weakening of super exchange interaction of type FeA3+–O– FeB3+ between 3aVI and 6cIV sites, leading to collapse of magnetic collinearity of the lattice. This is confirmed from measured values of the Curie temperature. Table 3 shows reduction in the Curie temperature from 908 K to 820 K with the substitution. In other word substitution of Ti4+Co2+ in place of Fe3+ at octahedral 3aVI site dilute the magnetization of this site, whereas the magnetization of tetrahedral 6cIV site remains constant. As net magnetization is equal to M(3aVI)oct - M(6cIV)tet, so saturation magnetization is found to decrease with substitution from x = 0 to x = 2.0 similar results were reported earlier for Y – type hexaferrite [24]. However,

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saturation magnetization increased for x = 2.5, this is due to the fact that Ti4+ may acquire tetrahedral site, leading to increase in saturation magnetization. Day et al., proposed in their report which is known as Day diagram that large values of Squareness ratios (0.5 < Mr/Ms < 1) mean the material is more anisotropic, hard and single domain. Further, it is stated that 0.05 < Mr/Ms < 0.5 is for the particle interact by magnetostatic couplings with pseudo single domain and while Mr/Ms < 0.05 is for randomly oriented multi-domain nanoparticles [25, 26]. Squareness ratios of all samples listed in Table 1 are ranging from 0.42 to 0.58 which depict that the nanoparticles are of single domain in nature. The coercivity of the doped NiMn Y-type Strontium hexagonal ferrites is shown in

Table 1. The magnetic hysteresis loops of all the compounds are shown in Fig. 8. With the increase of dopant concentration, the width of the hysteresis loop decreases. As Ti-Co concentration increases the value of coercivity decreases. This decrease in coercivity can be attributed to the following reason. In XRD calculations it has been observed that with increase in doping concentration, porosity of the samples decreases. Kersten and Neel support linear behavior of coercivity and porosity [27, 28]. Economos has reported similar behavior where coercivity decreases in Mg ferrites with decrease in porosity values [29]. The second effect accompanying reduction in coercivity is extrinsic, causing increasing grain size with substitution, the same behavior is observed in TEM result. Data analysis of XRD, SEM and TEM shows almost inverse nature of coercivity with grain size. With TiCo substitution, inter-granular pores decrease, thereby grain size increases results into decrease in coercivity [30]. If the coercivity is high enough above 1.2 kOe, then hexaferrite materials are useful for their applications in perpendicular recording media (PRM) which is an emerging technology in the magnetic recording medium [31, 32]. The observed value of Hc of the investigated sample (for x = 0.0, 0.5 and 1.0) are greater than 1.2 kOe. There is 75.72 % reduction in coercivity occurred from compound x = 0.0 (3143 Oe) to x = 2.0 (763 Oe) and the similar decrease by 88% in TiCo substituted Ba-Sr hexagonal ferrite [11]. The net magnetic moment per formula unit is in the range of 1.50 B- 4.19 B. Hysteresis (M-H) loops reveal linear relationship from 8.2 kOe to 12 kOe in all the samples. Thus, a/H and χhf terms from Eq. (3) can be eliminated and b can be calculated from the slope of straight-line M = Ms (1- b / H2) against 1/H2. Table 1 shows rapid fall in Ha by 75.17 % from x = 0.0 to x = 2.0 whereas coercivity lowers down by 75.72 % at the same

10

substitution level. The analogy between Hc and Ha (Table 1) indicates that anisotropy field (Ha) is the dominant factor affecting magnetization process of ferrite [12]. 3.4.

Thermo-magnetic analysis The samples were characterized by Gouy’s balance with uniform heating from room

temperature to 750 °C carefully using temperature controller unit. The critical transition temperature TM (temperature of maximal magnetization) is determined from the temperature dependence of the molar magnetic susceptibility (magnetization) curve obtained at a fixed applied magnetic field of 1.65 kOe.

Fig. 10 shows the temperature dependence of the molar magnetic susceptibility (magnetization) for all samples. With increase of temperature, the change in mass increases and becomes maximum and starts decreasing with the transition from antiferromagnetic (ferrimagnetic) to a paramagnetic state, the sample "loses" mass and this loss being terminated at the end of the complete transition to paramagnetic state. All the six compounds are found to be ferrimagnetic at the room temperature and remains so up to the temperature of maximal magnetization i.e. Transition temperature (TM). The compounds start becoming

paramagnetic

above

this

transition

temperature.

The

thermo-magnetic

susceptibility curve for these samples shows ferromagnetic-paramagnetic phase transition indicated by the drop of the magnetization between 553 K and 653 K. The transition temperature (TM) was determined from the derivative of the thermomagnetic susceptibility curve. In which, the strong (negative) peak confirmed that the sample was dominated by a single magnetic phase, and the narrow peak profile exhibits magnetic homogeneity of the sample [33].

Fig. 10 shows the thermomagnetic susceptibility curve for all samples. The presence of a Hopkinson peak at transition temperature in the χ-curve is associated with superparamagnetic relaxations [33]. The Transition temperature (TM) and Curie temperature (TC) values presented in Table 3 show decreasing trend in the range 653-563 K and 908-820 K (for x = 0 to x = 2.0) respectively, however for x = 2.5 the value of TM and TC increases. The decrease in the transition temperature of the Magnetization phase is correlated with the reduction of the superexchange interactions as a consequence of the TiCo-substitution [34]. The

earlier

reported

results,

transition

temperature

and

Curie

temperature

of

Ba0.5Sr1.5Mg2Fe12O22 Y-type nanohexaferrite show TM at 400 K and TC at 670 K and seen that

11

the TM and TC increase with increasing Sr content in Ba2-xSrxMg2Fe12O22 [35], TC of Ba0.5Sr1.5Zn2(Fe0.96Al0.04)12O22 is 712 K [36], TC of SrFe12O19 is 733 K [1], 853 K [37]. The ferro-magnetic to paramagnetic complete transition (Curie temperature, TC) is identified at the temperature where the magnetic susceptibility is significantly dropped [35]. As soon as the sample reaches the Curie temperature, a clear deflection of position of Gouy’s tube with change in the weight vs temperature is identified visually and the exact Curie temperature value is noticed. According to Juan C De Jesus the best way to identify the exact Curie temperature value is by taking a derivative of the χ-T curve. The Curie temperature value is obtained from the usual inverse susceptibility curve. The similar Curie temperature is also obtained from the novel 5th polynomial fit second order derivative of the χ-T curve that is shown in Fig. 10. The phase transition (ferro-magnetic to paramagnetic) up to the TC was confirmed by the presence of Hopkinson peak at thermo-magnetic susceptibility curve [38]. The presence of a Hopkinson peak at the χ verses T curve also confirms that the material is composed of single domain particles [38].

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4.

Conclusions: The XRD patterns of prepared samples Sr2NiMnFe12-x(TiCo)x/2O22 elucidate a single-

phase Y-type hexaferrites belong to the R 3 m ( No. 166) space group. The lattice constant ‘a’ slightly increases and easy magnetized ‘c’ axis undergoes more expansion with the content of TiCo. The crystallite size obtained from the most intense peak of (119) plane is in the range of 41 nm to 71 nm. HRTEM grain images show that the inter-grain spacing ‘d’ values are in good agreement with the values calculated from XRD and TEM SAED pattern. The observed saturation magnetization decreases from 17.87 emu/gm (at x = 0) to 6.44 emu/gm (at x = 2.0), then increases and becomes 17.40 emu/gm (at x = 2.5). The Squareness ratio is ranging from 0.42 to 0.58 which depict that the Sr2NiMn-Y nanoparticles are of single domain in nature. As Ti-Co concentration increases from x = 0 to x = 2.0, the value of coercivity decreases sharply from 3143 Oe to 763 Oe. This decrease in coercivity by 75.72 % is analogues with the rapid fall in anisotropy field Ha by 75.17 % from x = 0.0 to x = 2.0, indicates that anisotropy field (Ha) is the dominant factor affecting magnetization process of ferrite. All compounds are found to be ferrimagnetic at room temperature and remains so up to the transition temperature (TM). Above this transition temperature the compounds start becoming paramagnetic. The transition temperature (TM) was determined from the derivative of the thermomagnetic susceptibility curve. The strong (negative) derivative peak confirmed that the sample was dominated by the single magnetic phase and the narrow peak profile indicates magnetic homogeneity of the sample. The Curie temperature is obtained from the novel 5th polynomial fit second order derivative of the χ-T curve. The Transition temperature (TM) and Curie temperature (TC) values show decreasing trend (for x = 0 to x = 2.0) in the range 653-563 K and 908-820 K respectively. However, for x = 2.5 the value of TM and TC increases. The high coercivity single domain materials with smaller grain sizes and larger thermal stability may be useful as perpendicular recording media (PRM).

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16

List of Tables Table 1 The grain size and magnetic parameters of Sr2NiMnFe(12-x)(TiCo)x/2O22 measured at room Temperature.

Table 2 Observed and Estimated Saturation Magnetization (emu/gm) of Sr2NiMnFe(12x)(TiCo)x/2O22.

Table 3 Transition Temperature (TM) & Curie Temperature (TC) of Sr2NiMnFe12-X (TiCo)X/2O22

List of Figures Fig. 1 Indexed XRD patterns of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites. Fig. 2 Variation in Lattice Constants ‘a’ and ‘c’ of Sr2NiMnFe(12-x)(TiCo)x/2O22. Fig. 3 Variation of X-ray density, experimental density and porosity with content of TiCo Fig. 4 SEM micrographs of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 1.0 and c) x = 2.0.

Fig. 5 TEM images of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5.

Fig. 6 HRTEM grain images of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5.

Fig. 7 Indexed TEM SAED pattern of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5.

Fig. 8 The Magnetic Hysteresis M-H loop of Sr2NiMnFe(12-x)(TiCo)x/2O22. Fig. 9 Fitted curves of Ms calculated by law of approach to saturation for Sr2NiMnFe(12x)(TiCo)x/2O22

with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and

f) x = 2.5.

Fig. 10 Variation of Molar Susceptibility (emu/gm) versus Temperature for Sr2NiMnFe(12x)(TiCo)x/2O22 Y-type

hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x

= 1.5, e) x = 2.0 and f) x = 2.5. -----------------------------------------------------------------------------------17

Tables Table: 1 The grain size and magnetic parameters of Sr2NiMnFe12-X (TiCo)X/2O22 measured at room Temperature Sr. No.

Ti-Co content, (x)

Grain Size, D (nm)

HC (kOe)

MS (emu/g)

Mr (emu/g)

Mr/Ms

nB (µB)

Anisotropy field, Ha (kOe)

1)

0.0

41.60

3.143

17.87

09.78

0.58

04.19

1.313

2)

0.5

62.39

2.857

06.39

03.67

0.57

01.50

0.519

3)

1.0

55.45

1.985

11.73

06.36

0.54

02.75

0.545

4)

1.5

62.49

0.931

10.37

04.75

0.46

02.43

0.346

5)

2.0

49.89

0.763

06.44

02.72

0.42

01.50

0.326

6)

2.5

71.26

0.854

17.39

07.57

0.43

04.07

0.302

Table: 2 Observed and Estimated Saturation Magnetization (emu/gm) of Sr2NiMnFe12-X (TiCo)X/2O22 Sr. No.

Ti-Co content, (x)

Ms (Experimental)

Ms (LAS fit)

Ms f(1/H)

Ms f(1/H2)

1

0.0

17.8651

17.92328

22

19.8

2

0.5

6.39241

6.369737

7.58

7.04

3

1.0

11.7277

11.66399

13.75

12.77

4

1.5

10.3662

10.37105

11.9

11.14

5

2.0

6.43926

6.410996

7.08

6.75

6

2.5

17.3937

17.37595

19.35

18.22

Table: 3 Transition Temperature (TM) & Curie Temperature (TC) of Sr2NiMnFe12-X (TiCo)X/2O22 Composition, x

0.0

0.5

1.0

1.5

2.0

2.5

TM (K)

653

603

588

583

563

623

TC (K)

908

852

893

845

820

870

Fig. 1 Indexed XRD patterns of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexa-ferrites

6.2

43.2

a c

43.1

6.0

43.0

5.9

42.9

5.8

42.8

5.7

42.7

5.6

42.6

5.5

42.5

5.4

42.4

5.3

42.3

5.2

42.2 0.0

0.5

1.0

1.5

2.0

c (Å)

a (Å)

6.1

2.5

Composition (x)

Fig. 2 Variation in Lattice Constants ‘a’ and ‘c’ of Sr2NiMnFe(12-x)(TiCo)x/2O22

5.5 34 32

3

Dx Db P

4.5

30 28 26

4.0 24 3.5

Porosity, P (%)

Density: D x , Db (g/cm )

5.0

22 20

3.0 0.0

0.5

1.0

1.5

2.0

2.5

Ti-Co content (x)

Fig. 3 Variation of X-ray density, experimental density and porosity with content of TiCo

Fig. 4 SEM micrographs of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 1.0 and c) x = 2.0

Fig. 5 TEM images of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5

Fig. 6 HRTEM grain images of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5

Fig. 7 Indexed TEM SAED pattern of Sr2NiMnFe(12-x)(TiCo)x/2O22 Y-type hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5

20 15

Moment, M (emu/gm)

10 5 0

x = 0.0 x = 0.5 x = 1.0 x = 1.5 x = 2.0 x = 2.5

-5 -10 -15 -20 -15

-10

-5

0

5

10

15

Field, H (kOe)

Fig. 8 The Magnetic Hysteresis M-H loop of Sr2NiMnFe(12-x)(TiCo)x/2O22

6.5

18

b:- x = 0.5

a:- x = 0.0 6.0

M (emu/gm)

M (emu/gm)

16

14

12

5.0

4.5

Experimental LAS fit: a= 1.735, b= 0.115, χ hf= 0.00 LAS fit: a= 1.735, b= 0.115, χ hf= 0.222

10

5.5

Experimental LAS fit: a= 1.253, b= 0.018, χ hf= 0.00 LAS fit: a= 1.253, b= 0.018, χ hf= 0.0538

4.0

3.5

8 2

4

6

8

10

2

12

4

6

10

12

10.5

12

c:- x = 1.0

d:- x = 1.5

10.0

11

9.5 9.0

10

M (emu/gm)

M (emu/gm)

8

H (kOe)

H (kOe)

9

8

Experimental LAS fit: a= 0.885, b= 2.05, χ LAS fit: a= 0.885, b= 2.05, χ

7

hf

8.5 8.0

Experimental LAS fit: a= 0.955, b= 0.008, χ LAS fit: a= 0.955, b= 0.008, χ

7.5 7.0

= 0.00

6.5 hf

= 0.0822

hf

hf

= 0.00 = 0.0692

6.0

6 2

4

6

8

10

2

12

4

6

8

10

12

H (kOe)

H (kOe)

7

18

f:- x = 2.5

e:- x = 2.0 16

6

14

M (emu/gm)

M (emu/gm)

5

Experimental LAS fit: a= 0.705, b= 0.008, χ hf = 0.00 LAS fit: a= 0.705, b= 0.008, χ hf = 0.0292

4

3

12 10

Experimental LAS fit: a= 0.785, b= 0.008, χ hf = 0.00 LAS fit: a= 0.785, b= 0.008, χ hf = 0.0942

8 6 4

2

2 0

2

4

6

H (kOe)

8

10

12

0

2

4

6

8

10

12

H (kOe)

Fig. 9 Fitted curves of Ms calculated by law of approach to saturation for Sr2NiMnFe(12-x)(TiCo)x/2O22

Fig. 10 Variation of Molar Susceptibility (emu/gm) versus Temperature for Sr2NiMnFe(12-x)(TiCo)x/2O22 Ytype hexaferrites with Ti-Co content at a) x = 0.0, b) x = 0.5, c) x = 1.0, d) x = 1.5, e) x = 2.0 and f) x = 2.5

Research Highlights •

Sr2NiMnFe12-x(TiCo)x/2O22 hexaferrite samples were synthesized by sol-gel method.

•

Structural effects of TiCo substitution were investigated by XRD, SEM and TEM.

•

Single phase Y-type nano-hexaferrites were obtained at low temperature of 950 ºC.

•

The magnetic and thermo-magnetic properties of these samples were investigated.

•

The high coercivity single domain materials with smaller grain sizes and larger thermal stability may be useful as perpendicular recording media (PRM).

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

….. no known competing financial interests ……

Yours Sincerely Dr. Dilip S. Badwaik Department of Physics, Kamla Nehru Mahavidyalaya, Nagpur 440024, India Tel.: +917588883515 E-mail: [email protected]

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