High temperature magnetic properties of nanocrystalline Sn0∙95Co0∙05O2

Structural and magnetic properties of Sn_0?95Co_0?05O₂ nanocrystalline and diluted magnetic semiconductors have been investigated. This sample has been synthesized by co-precipitation route. Study of magnetization hysteresis loop measurements infer that the sample of Sn_0?95Co_0?05O₂ nanoparticle shows a well-defined hysteresis loop at 300 K temperature, which reflects its ferromagnetic behaviour. We confirmed the roomtemperature intrinsic ferromagnetic (FM) semiconductors by ab initio calculation, using the theory of the functional of density (DFT) by employing the method of Korringa–Kohn–Rostoker (KKR) as well as coherent potential approximation (CPA, explain the disorder effect) to systems. The ferromagnetic state energy was calculated and compared with the local-moment-disordered (LMD) state energy for local density approximation (LDA) and LDA–SIC approximation. Mechanism of hybridization and interaction between magnetic ions in Sn_0?95Co_0?05O₂ is also investigated. To explain the origin of ferromagnetic behaviour, we give information about total and atoms projected density of state functions.     

Magnetic properties of Zn1−xCoxO bulks prepared by hot pressing

Zn_1−xCo_xO-diluted magnetic semiconductor bulks have been prepared by hot pressing. Mixed powders of pure ZnO and CoO were compacted under pressure of 10 MPa at the temperature of 1073 K. Then, the samples were annealed in vacuum at the temperature range from 673 K to 873 K for 10 h. The crystal structure and magnetic properties of Zn_1−xCo_xO bulks have been investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). X-ray photoelectron spectroscopy (XPS) and was used to study chemical valence of zinc and cobalt in the samples. The results showed that Zn_1−xCo_xO samples had c-axis-oriented wurtzite symmetry neither cobalt nor cobalt oxide phase was found in the samples if x was less than 0.15. Zn and Co were existed in Zn_0.9Fe_0.1O sample in Zn²⁺ and Co²⁺ states. The results of VSM experiment proved the room temperature ferromagnetic properties (RTFP) of Co-doped ZnO samples. The saturation magnetization and the coercivity of Zn_0.9Co_0.1O sample, observed in the M–H curve, were about 0.22 emu/g and 300 Oe, respectively.     

Tailoring of Magnetic Properties of Magnetostatically-Coupled Glass-Covered Magnetic Microwires

We report on tailoring of magnetic properties of Fe- and Co-rich microwires through magnetostatic coupling among them. We studied hysteresis loops of the arrays containing different number of the Co₆₇Fe_3.9Ni_1.5B_11.5Si_14.5M_0.6 and Fe₇₄B₁₃Si₁₁C₂ amorphous microwires. Fe₇₄B₁₃Si₁₁C₂ microwires have rectangular hysteresis loop, while Co₆₇Fe_3.9Ni_1.5B1_1.5Si_14.5M_0.6 with vanishing magnetostriction constant posses inclined hysteresis loop with low coercivity. The presence of neighboring microwire (Fe either Co-based) significantly modifies hysteresis loop of whole microwire array. In a microwire array containing Fe-based microwires, we observed splitting of the initially rectangular hysteresis loop with a number of Barkhausen jumps correlated with number of Fe-rich microwires. In Co?CCo arrays, we observed a change of inclination of overall hysteresis loop, and consequently magnetic anisotropy field under influence of the additional of Co-based microwire. In the case of mixed arrays containing Fe and Co-rich microwires, we were able to obtain irregular hysteresis loops with unusual shape. In this case, considerable increase of harmonics has been observed. Magnetic field amplitude and frequency affect the behavior of all studied arrays. Increasing the amplitude the shape of hysteresis loop of microwire array containing Fe-based microwires transforms from multi-step to single above certain magnetic field amplitude. In the array with Co-based microwires, we observe a change of coercivity. Observed dependences have been attributed by us to the magnetostatic interaction between the microwires with different magnetic domain structure. Together with the conventional method, such as thermal treatment, designing of arrays containing different types of microwires can serve for tailoring of their magnetic properties.     

Combination of Chemical and Physical Methods to Produce High-Quality Epitaxial Zn1−x Mn x O Thin Films

We have carefully analyzed the epitaxial growth and magnetic properties of Zn_1?x Mn _x O (x=0–0.1) thin films fabricated by pulsed laser deposition on c-Al₂O₃(0001) substrates. The ablation targets are prepared by pressing nano-sized Zn_1?x Mn _x O (x=0–0.1) powders which were previously synthesized by the polymeric precursor method. The films show excellent crystalline quality with full width at half-maximums varying from ～0.07–1^°. Some films show well-defined magnetic hysteresis loops both at 300 and 5 K, which suggests the presence of ferromagnetic order. Nevertheless, we find that the ferromagnetism in the samples depends strongly on the oxygen partial pressure in the deposition chamber. This finding suggests that the observed ferromagnetism in Mn-doped ZnO is not an extrinsic property of the system. It is probable that unpaired electron spins, which have their origin in the oxygen vacancies, are responsible for the observed ferromagnetic behavior.     

Unusual Hysteresis Loop of (Tb0.2Pr0.8)(Fe0.4Co0.6)1.9−x C x in Pulsed High Magnetic Field

The magnetization properties of (Tb_0.2Pr_0.8)(Fe_0.4Co_0.6)_1.9?x C _x ( $x=0$ –0.5) were investigated in a pulsed high magnetic field. Obvious unsymmetry magnetic hysteresis loops were observed, in which the magnetization measured in the second(negative) magnetic field route was larger than that measured in the first(positive) one. Meanwhile, the unsymmetry of the hysteresis loops was C doping levels, magnetic fields and temperatures dependent. The experimental results suggest that the unusual hysteresis phenomenon result mainly from the collapse of the pinning center and the formation of ferromagnetic(FM) pseudo-domain.     

Cu-doping effect on structure and magnetic properties of Fe-doped ZnO powders

Fe-doped and Cu, Fe co-doped ZnO diluted magnetic semiconductors powders were synthesized by sol–gel method. The x-ray diffraction (XRD) results showed that Zn_0.97−xFe_0.03Cu_xO (x ≤ 0.02) samples were single phase with the ZnO-like wurtzite structure. X-ray photoelectron spectroscopy (XPS) showed that Fe²⁺ and Fe³⁺ existed in Zn_0.97Fe_0.03O, while Fe²⁺, Fe³⁺and Cu⁺, Cu²⁺ were found in Zn_0.95Fe_0.03Cu_0.02O. Both Zn_0.97Fe_0.03O and Zn_0.95Fe_0.03Cu_0.02O exhibited ferromagnetic performance at room temperature. But the Cu incorporation reduced the saturation magnetization of Fe-doped ZnO diluted magnetic semiconductors.     

Study of Magnetic Properties in Spinels Co x Zn1?x Cr2O4 systems

The exchange interactions J _AA(x) and J _AB(x) are calculated by using a probability law for the Co _x Zn_1?x Cr₂O₄. The magnetic properties of a diluted ferromagnetic spinels Co _x Zn_1?x Cr₂O₄ system are investigated by using the high-temperature series expansions combined with the Padé approximants. The magnetic phase diagram, i.e., T _N versus?x, and the critical exponent associated with the magnetic susceptibility (??) are deduced.     

Effect of oxygen content on magnetic properties of bulk Zn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>O<Subscript>1±δ</Subscript> synthesized by high pressure and high temperature technique

Polycrystalline Zn_0.9Co_0.1O_1±δ (Zn_0.9Co_0.1O_0.9, Zn_0.9Co_0.1O and Zn_0.9Co_0.1O_1.033) bulk samples have been prepared by directly adjusting the proportion of the starting materials under high pressure and high temperature. Structure analysis revealed that Co is incorporated into the lattice as Co²⁺ substituting Zn²⁺ ions, forming a wurtzite structure. Magnetization measurements clearly showed all samples in the absence of ferromagnetism and exhibit an antiferromagnetic behavior at 5 K. And the antiferromagnetic couple between Co atoms through oxygen evidenced that the magnetism is related to oxygen content. By combination of analysis, the effect of oxygen content on the nature and origin of an antiferromagnetism was investigated. This is further supported by a simple model, which shows the decrease of oxygen content decreases the chance of the antiferromagnetic super-exchange interaction. The results suggested oxygen content show no significant effect on ferromagnetic, however have a certain influence on antiferromagnetic in our Co-doped ZnO system.     

Fabrication and characterizations of Zn1?xCoxO bulk ceramics prepared by solid state reaction combined with spark plasma sintering

Zn_1?xCo_xO (x?=?0.01, 0.05 and 0.1) bulk ceramics were prepared through a two-step, solid state reaction method combined with spark plasma sintering technique. The single phase Zn_1?xCo_xO powders were synthesized using ZnO and Co₃O₄ at 935?°C in air for 3?h. The Zn_1?xCo_xO bulks were prepared at sintering temperature from 900 to 1,100?°C for 5?min by SPS. The relative density of Zn_1?xCo_xO bulk ceramics sintered at 1,100?°C is higher than 99% of the theoretical value. The Structure, composition analysis, optical absorption, Raman and XPS measurements revealed that the Co²⁺ substituted Zn²⁺ ions and was incorporated into the lattice of ZnO in both of the single phase Zn_1?xCo_xO powders and bulk ceramics. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior and that the paramagnetic Co amount is smaller than the nominal Co concentration for all of Zn_1?xCo_xO samples at 4?K. The paramagnetic magnetism of bulk ceramics is apparently larger than that of powders with the same composition. The electrical properties measurements reveal that the Co concentration has a slight influence on the electrical properties of Zn_1?xCo_xO bulk ceramics. The carriers concentration is about 1?×?10²⁰?cm^?3 and with the Co concentration increases the resistivity slightly increases from 3.56?×?10^?3 (x?=?0.01) to 5.58?×?10^?3 (x?=?0.1) Ωcm.     

The magnetic properties of Bi0.9Ba0.1Fe0.81M0.09Ti0.1O3 solid solutions (M = Co, Mn, Sc, Al)

Solid solutions with general formula Bi_0.9Ba_0.1Fe_0.81M_0.09Ti_0.1O₃ (M = Co, Mn, Sc, Al) together with parental Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ were prepared by the traditional solid state reaction method. Their structural, room temperature magnetic, and dielectric properties were investigated. X-ray diffraction analysis indicated that all samples maintained original R3c space group. M–H hysteresis loop of Co³⁺ doped sample saturated at an applied field of 1 T with spontaneous magnetization of 1.735 emu/g, while Mn⁴⁺ substitution enhanced the magnetization of Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ less strongly; addition of Sc³⁺ helped decrease magnetic coercive field while Al³⁺ modified sample exhibited paramagnetic M–H hysteresis loop. Differential scanning calorimetry was applied to determine the Neel temperature (T_N) and the T_N for undoped, Co³⁺, Mn⁴⁺, Sc³⁺, Al³⁺ doped solid solutions were 318.1, 324.3, 335.7, 293.9 and 295.8 respectively. Sc³⁺ substitution had little influence on the dielectric properties of Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ while Al³⁺ doping improved its dielectric constant. In contrast, Co³⁺, Mn⁴⁺ doped samples showed decreased permittivity but inhibited tan δ at frequencies larger than 30 kHz.     

Cobalt substituted ZnO thin films: a potential candidate for spintronics

Cobalt doped zinc oxide thin films have been deposited using spray pyrolysis method. These single phasic films exhibited [100] preferential texture and small decrease in the lattice parameter on cobalt substitution. The films having different Co concentration have almost similar surface morphology and microstructure. These Zn_1?x Co _x O (x ≤ 0.10) thin films distinctly showed ferromagnetic character at room temperature. The optical transmission measurements of these films clearly proved that in these films Co substitutes for Zn²⁺ and exists in +2 state. Based on the optical, structural and magnetic measurements, the possibility of occurrence of ferromagnetic ordering due to cobalt clustering is ruled out in these spray-pyrolyzed films. A correlation of the observed ferromagnetic behavior in these Zn_1?x Co _x O films with structural change resulting from the addition of Co is presented in this paper.     

Structural and Magnetic Properties of Zn and Co Substituted Nickel Ferrites Prepared by the Citrate Sol–Gel Method

Three different ferrites with nominal compositions, NiFe₂O₄, Ni_0.50Zn_0.50Fe₂O₄, and Ni_0.50Zn_0.25Co_0.25 Fe₂O₄, were synthesized by wet chemical citrate sol–gel process, mainly to understand how doping with different cations affects the magnetic properties. X-ray diffraction (XRD) confirmed the existence of a single phase without detectable impurities. XRD analysis using the Rietveld refinement technique showed that the samples crystallize in a cubic spinel structure with the Fd-3m space group and the lattice constants were evaluated. Scanning electron microscopy of the calcined samples revealed particles that were approximately 100 nm size. The elemental analysis by energy dispersive spectroscopy showed that the element concentrations were at the stoichiometric ratio in all samples. The magnetic properties measured at room temperature using a vibrating sample magnetometer revealed enhancement in the magnetic properties of nickel ferrite when doped with zinc and cobalt. The high field regimes of the hysteresis loops were modeled using the Law of Approach to Saturation to determine the cubic anisotropy coefficient (K). The anisotropy coefficient increased with doping Zn and Co, which could be explained in terms of the site occupancy of the dopants in the cubic spinel lattice.     

Synthesis,Characterization and ESR Studies of Zn1−x Co x O Nanoparticles

Zn_1?x Co _x O polycrystalline nanoparticles with different (x=0.03, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) compositions were synthesized using the sol-gel technique. The effects of doping ratio and annealing temperature on structure and magnetic properties were investigated systematically. The phase, crystal structure, and microstructure of the Zn_1?x Co _x O nanoparticles were characterized using X-Ray diffraction and scanning electron microscope. Electron spin resonance spectra of Zn_1?x Co _x O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz.     

Bismuth effect in the structural, magnetic and dielectric properties of CoZn ferrite

Nano particles of Co_0.5Zn_0.5Bi_xFe_(2−x)O₄, with x varying from 0.0 to 0.3 in steps of 0.1 were synthesized using the chemical co-precipitation method. The powder X-Ray diffraction pattern confirms the formation of spinel phase for all prepared samples. The lattice parameters are calculated by powder X-Ray diffraction, and it is observed that the values of the lattice parameter are less than those of bulk materials. The saturation magnetization is found to decrease with the increase of concentration of Bi ion up to x = 0.2 and then increases for x = 0.3. The hysteresis loop for concentration x = 0.0, 0.1 and 0.3 shows almost zero coercivity and remanance at 300 K, implying that the samples behave as superparamagnetic at this temperature; whereas for the concentration at x = 0.2 the coercivity was found to be 32 Oe at 300 K. The dielectric measurements were carried out in the Co_0.5Zn_0.5Bi_xFe_(2−x)O₄ system over the temperature range from 300 to 700 K as a function of frequency, from 5 to 5 MHz. The variation of dielectric constant and dielectric loss factor for the prepared samples are explained on the basis of Maxwell–Wagner interfacial polarization.     

Room-Temperature Ferromagnetism in Co-doped ZnO Prepared by Microemulsion

Zn_1?x Co _x O (x=0, 0.005, 0.01, 0.02, 0.03, 0.06, 0.12) powders have been synthesized by the microemulsion method. XRD patterns confirm the pure wurtzite structure without any impurity phase. TEM images illustrate that Zn_1?x Co _x O particles are formed by aggregation of small crystallites. The magnetization measurements show clear room-temperature ferromagnetism in Zn_1?x Co _x O (x=0, 0.005, 0.01), and the saturated ferromagnetic magnetization increased with increasing Co concentration to 0.01. However, the ferromagnetic magnetization decreased drastically with further increasing Co concentration (x>0.01). PL spectra clearly show an increasing concentration of oxygen vacancies with increasing Co concentration to 0.01, and strong suppression of oxygen vacancies with further increasing Co concentration in Zn_1?x Co _x O. We conclude that the high concentration of oxygen vacancies is the key origin for the observed ferromagnetism in Co-doped ZnO.     

Magnetic and dielectric properties of Co–Zn ferrite

Nanocrystalline powders of Co substituted Zn ferrite with the chemical formula Co_xZn_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) were synthesized by sol–gel autocombustion method using tartaric acid as fuel agent. The samples were sintered in static air atmosphere for 7 h at 773 K, 7 h at 973 K and 10 h at 1173 K. The organic phase extinction and the spinel phase formation were monitored by means of Fourier transform infrared spectroscopy. The X-ray diffraction patterns analysis confirmed the spinel single phase accomplishment. Crystallite size, average grains size, lattice parameter and cation distribution were estimated. Magnetic behavior of the as-obtained samples by means of M-H hysteresis measurements was studied at room temperature. Permeability and dielectric permittivity at room temperature versus frequency was the subject of a comparative study for the Co_xZn_1−xFe₂O₄ series. In agreement with the proposed cation distribution the sample with Co_0.8Zn_0.2Fe₂O₄ formula exhibits the optimal magnetic and dielectric properties.     

Characterizations of ferromagnetic Zn1−xCoxO thin films grown on Al2O3 (0001) by reactive radio-frequency magnetron sputtering coupled with post-growth annealing

High-quality ferromagnetic Zn_1−xCo_xO thin films were deposited on a sapphire (0001) substrate at 600 °C by using reactive radio-frequency magnetron sputtering coupled with post-annealing treatment for 1 h at 580 °C under an Ar atmosphere. High resolution X-ray diffraction patterns show that hexagonal wurzite crystal structures of undoped ZnO film were maintained even after Co doping up to 4.5 at.% without forming Co clusters or oxides. X-ray photoelectron spectroscopy spectra represent the energy difference of 15.42 eV between Co2p_3/2 and Co2p_1/2, which is different from 15.05 eV of Co clusters. The characteristic absorption bands near 658, 616, and 568 nm wavelengths out of UV-VIS-IR spectroscopy spectra are correlated with the d-d transitions of tetrahedrally coordinated Co²⁺ ions. The low temperature photoluminescence spectrum for undoped ZnO shows a strong near-band edge (NBE) emission peak of 3.42 eV without deep level emission peaks. But, Co content increases in Zn_1−xCo_xO film, the NBE emission peak intensity decreases and another emission peak at 3.37 eV as well as a broad green emission peak at around 2.5 eV starts to appear with larger intensity due to the more actively creating oxygen vacancies. The emission peak at 3.37 eV proves the interaction between Co ions and the hydrogenic electrons in the impurity band and also supports the typical ferromagnetic hysteresis curves obtained by superconducting quantum interface device magnetometry at 300 K for Zn_1−xCo_xO films. High insulator characteristics are observed for as-grown Zn_1−xCo_xO films whereas it exhibits n-type characteristics with the increased carrier concentration, mobility, and resistivity after post-growth annealing. The spintronic devices could be fabricated with the utilization of Zn_1−xCo_xO films grown by the economically feasible reactive radio-frequency magnetron sputtering coupled with the post annealing treatment.     

Magnetic transition and magnetic entropy changes of La0.8Pb0.1MnO3 and La0.8Pb0.1Na0.1MnO3

In the present work we analyze the magnetic entropy change ΔS_M of La_0.8Pb_0.1MnO₃ (LPMO) and La_0.8Pb_0.1Na_0.1MnO₃ (LPNMO). Using Arrott plots, it was found that the phase transition for both samples is of second-order. The presence of short-range magnetic order (SRMO) in the paramagnetic phase of these samples greatly depresses the magnetic entropy: the values are well below the values expected without considering the effect of SRMO. Moreover, the analysis of the MCE using Landau theory of phase transition shows that the contributions to the free energy from the presence of ferromagnetic clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.     

Effect of nickel on the structural and magnetic properties of nano structured CoZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript>

Nano particles of Co_(0.5–x)Ni_xZn_0.5Fe₂O₄ (x = 0–0.3) were synthesized by chemical coprecipitation method. Powder X-ray diffraction pattern confirms the formation of spinel phase for all the prepared samples. The lattice parameters were calculated from powder X-ray diffraction data and the result reveals that the values are less than that of bulk values. The saturation magnetization decrease for the concentration x = 0.1 and increases for all the other concentration. Further, the hysteresis loop for the concentration x = 0 and 0.1 shows that there is no coercivity and remanance at 300 K and hence the samples behaves as superparamagnetic at 300 K. The field-cooled (FC) and zero-field-cooled (ZFC) curve for the concentration x = 0 and 0.1 separates at 150 and 167 K respectively hence the prepared samples behaves as superparamagnetic at room temperature.     

Explanation of Co Magnetic Moment Enhancement in (Co,Cu)-Doped ZnO by First-Principle Calculations

By ab-initio calculations on Zn_0.95?x Co_0.05Cu _x O, we study the variations of magnetic moments versus Cu concentration. The electronic structure is calculated by using the Korringa?CKohn?CRostoker (KKR) method combined with coherent potential approximation (CPA). We show that the total magnetic moment and magnetic moment of Co increase with increasing Cu content. From a density of state (DOS) analysis, we propose an explanation of the enhancement of the Co magnetic moment versus Cu concentration.