Room-temperature ferromagnetism in N+-implanted ZnO:Mn thin films

Mn–N co-doped ZnO films with wurtzite structure were fabricated by RF magnetron sputtering together with the ion-implantation technique. Then a post-annealing at 650 °C for 10 min in a N₂ atmosphere was performed to activate the implanted N⁺ ions and recover the crystal quality, and a p-type ZnO:Mn–N film with a hole concentration of about 2.1×10¹⁶ cm^?3 was obtained. It is found that the Mn mono-doped ZnO film only exhibits paramagnetic behavior, while after N⁺-implantation, it shows ferromagnetism at 300 K, and the magnetization of the ZnO:Mn–N films can be further enhanced by thermal annealing due to the activation of the N acceptors. Our experimental results confirm that the codoping N acceptors are favorable for ferromagnetic ordering of Mn²⁺ ions in ZnO, which is consistent with the recent theoretical calculations.     

Effect of annealing on the magnetic properties of Cu-capped ultrathin Co films

We report the effect of intermixing of Cu on the magnetic properties of ultrathin Co films deposited on Si(1 0 0). Rutherford backscattering was employed to determine the extent of intermixing, which increased from 7% in an as-grown sample to nearly 23% when annealed at 400 °C. The as-grown sample showed a higher value of magnetization of 530 emu/cm³, which dropped to 20 emu/cm³ on annealing at 400 °C. The low temperature magnetization behavior of the as-grown Co films showed the presence of both positive and negative exchange bias due to the formation of antiferromagentic domains in parallel with ferromagnetic domains. This behavior is explained using the Malozemoff Random Field Model, which predicted values of exchange bias closely matching the present experimental findings.     

Micro Raman,Mossbauer and magnetic studies of manganese substituted zinc ferrite nanoparticles: Role of Mn

A series of Mn–Zn Ferrite nanoparticles (<15 nm) with formula Mn_xZn_1−xFe₂O₄ (where x=0.00, 0.35, 0.50, 0.65) were successfully prepared by citrate-gel method at low temperature (400 °C). X-ray diffraction analysis confirmed the formation of single cubic spinel phase in these nanoparticles. The FESEM and TEM micrographs revealed the nanoparticles to be nearly spherical in shape and of fairly uniform size. The fractions of Mn²⁺, Zn²⁺ and Fe³⁺ cations occupying tetrahedral sites along with Fe occupying octahedral sites within the unit cell of different ferrite samples are estimated by room temperature micro-Raman spectroscopy. Low temperature Mossbauer measurement on Mn_0.5Zn_0.5Fe₂O₄ has reconfirmed the mixed spinel phase of these nanoparticles. Room temperature magnetization studies (PPMS) of Mn substituted samples showed superparamagnetic behavior. Manganese substitution for Zn in the ferrite caused the magnetization to increase from 04 to18 emu/g and Lande's g factor (estimated from ferromagnetic resonance measurement) from 2.02 to 2.12 when x was increased up to 0.50. The FMR has shown that higher Mn cationic substitution leads to increase in dipolar interaction and decrease in super exchange interaction. Thermomagnetic (M–T) and magnetization (M–H) measurements have shown that the increase in Mn concentration (up to x=0.50) enhances the spin ordering temperature up to 150 K (blocking temperature). Magnetocrystalline anisotropy in the nanoparticles was established by Mossbauer, ferromagnetic resonance and thermomagnetic measurements. The optimized substitution of manganese for zinc improves the magnetic properties and makes these nanoparticles a potential candidate for their applications in microwave region and biomedical field.     

Observation of ferromagnetism and anomalous Hall effect in laser-deposited chromium-doped indium tin oxide films

We report on the structural, magnetic, and magnetotransport characteristics of Cr-doped indium tin oxide (ITO) films grown on SiO₂/Si substrates by pulsed laser deposition. Structural analysis clearly indicates that homogeneous films of bixbyite structure are grown without any detectable formation of secondary phases up to 20 mol% Cr doping. The carrier concentration is found to decrease with Cr ion addition, displaying a change in the conduction type from n-type to p-type around 15 mol% Cr doping. Room temperature ferromagnetism is observed, with saturation magnetization of ∼0.7 emu/cm³, remnant magnetization of ∼0.2 emu/cm³ and coercive field of ∼30 Oe for 5 mol% Cr-doped ITO. Magnetotransport measurements reveal the unique feature of diluted magnetic semiconductors, in particular, an anomalous Hall effect governed by electron doping, which indicates the intrinsic nature of ferromagnetism in Cr-doped ITO. These results suggest that Cr-doped ITO could be promising for semiconductor spin electronics devices.     

Dielectric and magnetic properties of Bi1−xYxFeO3 ceramics

Y doped BiFeO₃ polycrystalline ceramics were prepared by sol–gel method. Crystal structure examined by X-ray diffraction indicates that the samples were single-phase and crystallize in rhombohedral structure. An anomaly in the dielectric constant and dielectric loss in the vicinity of the antiferromagnetic Neel temperature (T_N) was observed. Saturated magnetization loops were observed for all sample with saturated magnetization M_s=0.678 emu/g and remnant magnetization M_r=0.084 emu/g for x=0.3.     

First order Raman scattering analysis of transition metal ions implanted GaN

Transition Metal (TM) ions V, Cr, Mn and Co were implanted into GaN/sapphire films at fluences 5×10¹⁴, 5×10¹⁵ and 5×10¹⁶ cm⁻². First order Raman Scattering (RS) measurements were carried out to study the effects of ion implantation on the microstructure of the materials, which revealed the appearance of disorder and new phonon modes in the lattice. The variations in characteristic modes 1GaN i.e. E₂(high) and A₁(LO), observed for different implanted samples is discussed in detail. The intensity of nitrogen vacancy related vibrational modes appearing at 363 and 665 cm⁻¹ was observed for samples having different fluences. A gallium vacancy related mode observed at 277/281 cm⁻¹ for TM ions implanted at 5×10¹⁴ cm⁻² disappeared for all samples implanted with rest of fluences. The fluence dependent production of implantation induced disorder and substitution of TM ions on cationic sites is discussed, which is expected to provide necessary information for the potential use of these materials as diluted magnetic semiconductors in future spintronic devices.     

Microwave-induced combustion synthesis of Ni–Zn ferrite powder and its characterization

Ni–Zn ferrite powders were successfully synthesized by microwave-induced combustion process. The process takes only a few minutes to obtain calcined Ni–Zn ferrite powders. The resultant powders were investigated by XRD, SEM, VSM, TG/DTA and surface area measurements. The as-received product shows the formation of cubic ferrite with saturation magnetization (M_s)≈23 emu/g, whereas upon annealing at 850°C for 4 h, the saturation magnetization (M_s) increased to ≈52 emu/g.     

Accumulating evidence for the two-step magnetic ordering in the borocarbides DyNi2B2C and DyCo2B2C

Accumulating evidence for a two-step magnetic ordering in the borocarbide DyNi₂B₂C is summarized, including earlier overlooked evidence for the initial magnetic transition and a recent magnetization study of polycrystalline samples. The two-step ordering involves initial two-dimensional ferromagnetic ordering in the DyC (basal) planes at T_N (=16.3 K), gradual build-up of the three-dimensional (3D) alternate stacking of ferromagnetic planes, and a final 3D ordering in the AF–I-related structure at a lower temperature T_o (=10.4 K), depicting a first-order transition. Supporting evidence for the two-step magnetic ordering in DyNi₂B₂C comes from point-contact spectroscopy measurements in the normal state for DyNi₂B₂C–Ag contact, and from similar behaviour of PrNi₂B₂C and (Pr_0.91Dy_0.09)Ni₂B₂C. In the isostructural borocarbide DyCo₂B₂C the two magnetic transitions (at 7.8 and 2.6 K) deduced from the specific-heat measurements are also attributed to a two-step magnetic ordering.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

Preparation of superparamagnetic magnetite nanoparticles by reverse precipitation method: Contribution of sonochemically generated oxidants

Magnetic iron oxide nanoparticles were successfully prepared by a novel reverse precipitation method with the irradiation of ultrasound. TEM, XRD and SQUID analyses showed that the formed particles were magnetite (Fe₃O₄) with about 10 nm in their diameter. The magnetite nanoparticles exhibited superparamagnetism above 200 K, and the saturation magnetization was 32.8 emu/g at 300 K. The sizes and size distributions could be controlled by the feeding conditions of FeSO₄ · 7H₂O aqueous solution, and slower feeding rate and lower concentration lead to smaller and more uniform magnetite nanoparticles. The mechanisms of sonochemical oxidation were also discussed. The analyses of sonochemically produced oxidants in the presence of various gases suggested that besides sonochemically formed hydrogen peroxide, nitrite and nitrate ions contributed to Fe(II) ion oxidation.     

Synthesis of Co nanocapsules coated with BN layers by annealing of KBH4 and [Co(NH3)6]Cl3

Cobalt (Co) nanocapsules coated with boron nitride (BN) layers were synthesized by annealing of ammine complex. KBH₄ and [Co(NH₃)₆]Cl₃ were used as starting materials, and annealed these powders at 500–1000 °C with flowing nitrogen gas. Formation of fcc-Co nanocapsules coated with BN layers was observed from X-ray diffraction patterns and high-resolution electron microscopy. Particle size of fcc-Co prepared at 1000 °C with flowing 100 sccm N₂ gas was approximately 40 nm, and the values of saturation magnetization and coercivity were 74.5 emu/g and 88 Oe, respectively. Good oxidation- and wear-resistances were obtained by encapsulating Co nanoparticles with BN layers.     

Defects induced ferromagnetism in Mn doped ZnO

Single phase Mn doped (2 at%) ZnO samples have been synthesized by the solid-state reaction technique. Before the final sintering at 500 °C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 h). The grain sizes of the samples are very close to each other (～32±4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value for saturation magnetization (0.11 μ_B/Mn atom) was achieved for 96 h milled sample. Electrical resistivity has been found to increase with increase in milling time. The most resistive sample bears the largest saturation magnetization. Variation of average positron lifetime with milling time bears a close similarity with that of the saturation magnetization. This indicates the key role played by open volume vacancy defects, presumably zinc vacancies near grain surfaces, in inducing ferromagnetic order in Mn doped ZnO. To attain optimum defect configuration favorable for ferromagnetism in this kind of samples proper choice of milling period and annealing conditions is required.     

Large magnetocaloric effect in HoFeO3 single crystal

Magnetocaloric properties of HoFeO₃ single crystal are investigated along the direction [100]. Magnetic field dependent magnetization isotherms at different temperatures undergo a metamagnetic transition, entropy change as large as 19.2 J/kg K and 15.8 J/kg K are obtained at 7 T in the vicinity of antiferromagnetic ordering temperature of Ho³⁺ and the metamagnetic transition, respectively. The coupling of Ho and Fe spins generates the compensation behavior at 6.5 K, separating the two large magnetic entropy change. Its refrigeration capacity (RC) value, as high as 220 J/kg, is appreciable and can be considered as a promising magnetic refrigerant. New evidence for spin reorientation of Fe³⁺ in HoFeO₃ is also provided by the change of magnetic entropy.     

N2 emission-channel change in NO reduction over stepped Pd(211) by angle-resolved desorption

A sharp change in the N₂ emission channel from N₂O(a) → N₂(g) + O(a) to N(a) + N(a) → N₂(g) has been found at around 500 K in a steady-state NO + D₂ reaction over stepped Pd(211) = [(S)3(111) × (100)] by means of angle-resolved desorption. The desorbing N₂ is highly collimated at around 30° off normal toward the step-down direction below about 500 K due to the intermediate N₂O decomposition, whereas, above 500 K, the near normally directed desorption due to the recombination of N(a) is relatively enhanced. The N₂O decomposition channel is promoted when the reaction is carried out with hydrogen (deuterium) and the channel change is accelerated by quick changes of the amounts of surface hydrogen and oxygen (or NO(a)) into the opposite directions, and enhanced nitrogen removal as ammonia on the resultant hydrogen-rich surface. In the steady-state NO + CO reaction, the N₂ emission channel gradually changes above 500 K toward recombination. A model for the off-normal N₂ emission is briefly described.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Magnetic properties and formation of Sr ferrite nanoparticle and Zn,Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σ_s and coercive field strength H_c are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σ_s=74 emu/g and coercive field strength H_c=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe₂O₃ and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn²⁺, Ti⁴⁺ and Ir⁴⁺ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)_x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)_x substituted phase.     

Magnetic properties of rare-earth transition metal aluminides R6T4Al43 with Ho6Mo4Al43-type structure

The magnetic properties of 53 aluminium-rich intermetallic compounds R₆T₄Al₄₃ with R=rare-earth elements and T=Ti, V, Nb, Ta, Cr, Mo, W were investigated using polycrystalline samples and a SQUID magnetometer in the temperature range from 2 to 300 K with magnetic flux densities up to 5.5 T. The yttrium and lutetium compounds are Pauli paramagnetic, indicating that the transition metal atoms do not carry magnetic moments. The samarium compounds show van Vleck behavior and antiferromagnetic order with Néel temperatures of less than 12 K. Of these Sm₆Ti₄Al₄₃ becomes metamagnetic. The ytterbium compounds show a mixed or intermediate valent behavior and no magnetic order down to 2 K. All other compounds obey the Curie–Weiss law above 30 K. Their effective magnetic moments correspond to the theoretical moments of the rare-earth ions. They show ferromagnetic or metamagnetic behavior with ordering temperatures all below 20 K. The magnetization curves of most compounds (recorded up to 5.5 T) reach about 50% of the theoretical magnetization already at 0.5 T. The gadolinium compounds are exceptional in that they reach at 0.5 T only about 10% of their theoretical magnetization. The crystal structures of the isotypic compounds Yb₆V₄Al₄₃ and Yb₆Ta₄Al₄₃ were refined from single-crystal X-ray data.     

Neutron diffraction study of the CeFeSi-type RTiGe compounds (R=Pr,Nd, Tb–Er)

Neutron diffraction experiments were carried out on polycrystalline RTiGe (R=Pr, Nd, Tb–Er) samples. These compounds crystallise with the tetragonal CeFeSi-type structure (space group P4/nmm). This structure is closely related to the ThCr₂Si₂-type and can be described as “BaAl₄ blocks” connected via R–R contacts (“W blocks”). All the compounds are antiferromagnetic. PrTiGe and NdTiGe are characterised by an easy-plane sine-modulated structure characterised by a wave vector k=0,0,q_z=0.242 and 0.334 below 62 and 128 K, respectively. Below 80 K, NdTiGe exhibits a commensurate arrangement, which consists on ferromagnetic (0 0 1) Nd layers coupled antiferromagnetically along the c direction with the sequence ++−−. This commensurate magnetic ordering also occurs in TbTiGe, DyTiGe, HoTiGe and ErTiGe below 312, 185, 124 and 36 K, respectively. In the first three compounds (R=Tb−Ho), the magnetic moment is aligned along the c-axis whereas it is localised in the basal plane in ErTiGe. In all the RTiGe compounds, the magnitude of the ordered moments at 2 K amounts nearly to the free ion magnetic moment (gJ) values for the respective R³⁺ ions.     

Comparative investigation of CuFe2O4 nano and microstructures for structural,morphological, optical and magnetic properties

CuFe₂O₄ nanocrystals were synthesized by the sol–gel method (SGM) and microwave method (MM) by using sucrose as a fuel. The structural, morphological, optical and magnetic properties of the products were determined and characterized in detail by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), photoluminescence (PL) spectroscopy and vibrating sample magnetometer (VSM). The XRD results confirmed the formation of cubic phase CuFe₂O₄. The formation of CuFe₂O₄ nano and microstructures were confirmed by HR-SEM. Photoluminescence emissions were determined by PL spectra, respectively. The relatively high saturation magnetization (78.22 emu/g) of CuFe₂O₄-MM shows that it is ferromagnetic and low saturation magnetization (35.98 emu/g) of CuFe₂O₄O-SGM confirms the super paramagnetic behavior.     

Evolution of CoSi2 buried structures created by high temperature Co+ ion implantation into Si(1 0 0) during post-implantation annealing

The evolution of buried structures of cobalt disilicide, which are formed in a Si(1 0 0) matrix by 400 keV Co⁺ ion implantation at 875 K substrate temperature with subsequent rapid thermal annealing at 1275 K was studied by cross-sectional transmission electron microscopy (X-TEM). The analysis of identical samples with successive variations of the implanted doses and annealing times allows a detailed observation of the role of defects, created by the ion flux, on the process of ripening and growth of CoSi₂ precipitates. We found that transport of the implanted material along diffusive links leads to the formation of a secondary CoSi₂ distribution between the main layer and the surface. Post-implantation annealing results in the evolution of defects into dislocations, which affects the mobility and therefore the growth of CoSi₂ precipitates. Increasing the annealing time leads to the separate growth of precipitates in each layer. The result is not the formation of a single uniform buried layer because the distance between the individual layers is too large due to a screening effect, which operates during the ripening stage.