Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

Magnetic and transport properties of double distorted perovskites CaCuMn6O12 and CaCu2Mn5O12

Magnetic and transport properties of double distorted perovskites CaCuMn₆O₁₂ and CaCu₂Mn₅O₁₂ are studied in a range 2–300 K. The leading role in magnetism of these compounds belongs to antiferromagnetic exchange interaction of Cu²⁺ in square coordination with Mn³⁺/Mn⁴⁺ in octahedral coordination. The values of saturation magnetization indicate that Mn³⁺ ions in square coordination are coupled ferromagnetically with Mn³⁺/Mn⁴⁺ in octahedral coordination. The colossal magnetoresistance in the pellet samples is due assumingly to intergranular spin-polarized tunneling of current carriers.     

Structural and Mössbauer studies of nanocrystalline Mn<Superscript>4+</Superscript>-doped Li<Subscript>0.5</Subscript>Fe<Subscript>2.5</Subscript>O<Subscript>4</Subscript> particles prepared by mechanical milling

The structure and magnetic properties of spinel-related Mn⁴⁺-doped Li_0.5Fe_2.5O₄ nanocrystalline particles of the composition Li_0.5Fe_2.25Mn_0.1875O₄, prepared by milling a pristine sample for different times, were investigated. The average crystallite and particle size, respectively, decreased form ～40 nm to ～10 nm and ～2.5 μm to ～10 nm with increasing milling time from 0 h to 70 h. Rietveld refinement of the XRD data of the non-milled sample show the Mn⁴⁺ dopant ions to substitute for Fe³⁺ at the octahedral B-sites of the spinel-related structure. The Mössbauer spectra of the milled ferrites indicate that more particles turn superparamagnetic with increasing milling time. The Mössbauer data collected at 78 K suggest that while in the non-milled sample the Mn⁴⁺ ions substitute for Fe³⁺ at the octahedral B-sites, this is reversed as milling proceeds with doped Mn⁴⁺ ions, balancing Fe³⁺ vacancies and possibly Li⁺ ions progressively migrate to the tetrahedral A-sites. This is supported by the slight increase observed in the magnetization of the milled samples relative to that of the non-milled one. The magnetic data suggest that in addition to the increasing superparamagentic component of the milled particles, thermal spin reversal and/or spin canting effects are possible at the surface layers of the nanoparticles.     

Structure and magnetic properties of a new single-molecule magnet [Mn11Fe1O12 (CH3COO)16(H2O)4].2CH3COOH.4H2O

A new single-molecule magnet [Mn₁₁Fe₁O₁₂ (CH₃COO)₁₆(H₂O)₄]?2CH₃COOH?4H₂O (Mn₁₁Fe₁) has been synthesized.The structure has been studied by the single crystal x-ray diffraction. The difference of Jahn--Teller distortion between Fe³⁺ and Mn³⁺ ion reveals that Fe³⁺ ion substitutes for Mn³⁺ ion on the Mn(3) sites in the Mn₁₂ skeleton. The temperature dependence of the magnetization gives a blocking temperature T_B=1.9K for Mn₁₁Fe₁. Based on the magnetization process analysis of the crystal at T=2K, we suggest that Mn₁₁Fe₁ has the ground state with a total spin S= 11/2.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

Influence of doping concentration on room-temperature ferromagnetism for Fe-doped BaTiO3 ceramics

Ba(Ti_1−xFe_x)O₃ ceramics (x=7, 30 and 70 at%) were prepared by solid-state reaction. All samples are single-phase with 6H-BaTiO₃-type hexagonal perovskite structure. Mössbauer spectra show all Fe atoms to be present as Fe³⁺ in BaTiO₃ lattice, occupying M1 octahedral and pentahedral sites. Room-temperature ferromagnetism is exhibited and saturation magnetization gradually decreases with increasing Fe content. The observed ferromagnetism is considered to be an intrinsic property of Ba(Ti_1−xFe_x)O₃, originating from super-exchange interactions between Fe³⁺ in different occupational sites associated with oxygen vacancies. The variation in magnetization with Fe content is related to the ratio of pentahedral to octahedral sites and oxygen vacancies.     

Structural, thermal and magnetic properties of Ni1−xMnxFe2O4 nanoferrites

In this paper, the structural, thermal and magnetic properties of Ni_1−xMn_xFe₂O₄ are presented. It is observed that high concentration of Mn²⁺ ions into NiFe₂O₄ tends to reduce the particle size. Calcination at 500 °C has resulted in the growth of Ni_1−xMn_xFe₂O₄ nanoparticles, but the calcination at 900 °C has led to the evaporation of the majorities of the polyvinyl alcohol. After calcination at 900 °C, crystallographically oriented NiMnFe₂O₄ nanoparticles are formed. These Ni_1−xMn_xFe₂O₄ nanoparticles show hysteresis behaviour upon magnetization. On the other hand, saturation magnetization (Ms) values decreases with increasing Mn content in ferrite due to the influence of Mn²⁺ ion in the sub lattice.     

On the existence of tetragonally deformed octahedrons in the cubic part of the system MnxFe3-xO4

The mean square amplitude of lattice vibrations has been measured in the cubic part of the Mn _x Fe_3-x O₄-system. The observed increase of the -value with increasing content of Mn ions in the system is interpreted as that, with gradual occupation of more and more octahedral sites by Mn³⁺ ions towards higher values ofx, the number of locally distorted Mn³⁺O ₆ ^2– octahedrons increases.     

Magnetic properties of the mixed spinel Mg1+xMnxFe2-2xO4

The structural and magnetic properties of the mixed spinel Mg_1+xMn_xFe_2-2xO₄ system for 0.1<= x <= 0.9 have been studied by means of X-ray diffraction, magnetization, a.c. susceptibility and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Mn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Mg²⁺ ions substitute for A-site Fe³⁺ ions. All samples are magnetic at 12 K displaying Mössbauer spectra that have magnetic sextets coexisting with a central doublet that increases in population with increasing Mn concentration, indicating the presence of short range ordering (clustering). The Mössbauer intensity data show that Mn possesses a preference for the B-site of the spinel over the whole range of concentration. As expected, the hyperfine field and Curie temperature determined from a.c.susceptibility data decrease with increasing Mn content. Magnetization results indicate that on increasing dilution x, the collinear ferrimagnetic phase breaks down at x = 0.3 before reaching the ferrimagnetic percolation limit (x=0.6), as a result of the presence of competing exchange interactions, which is well supported by Mössbauer results. From all the above results, it is proposed that with increasing Mn content from x=0.6 to 0.9, the frustration and disorder increase in the system suppressing the ferrimagnetic ordering, and the system approaches to a cluster spin glass type of ordering at x=0.8 as reflected in the a.c.susceptibility and Mössbauer spectrum.     

Mössbauer effect study of Ni1−xCuxMnyFe2−yO4 system

The Mössbauer effect technique has been employed for the study of magnetic properties of spinel series Ni_1?xCu_xMn_yFe_2?yO₄ with 0.0≤x≤1.0, and y=0.6. The substitution of Mn³⁺ and Cu²⁺ ions results in a slight decrease of the hyperfine field at B‐ as well as A‐sites. The area ratio of Fe³⁺ ions at the A‐ and B‐site at 77 K indicates that Cu²⁺, Ni²⁺ and Mn³⁺ ions occupy the octahedral sites in an evidence for complete inverse spinel in this system. The temperature dependence of the hyperfine parameters has been studied for composition with x=0.5 where Nèel point T_N and Debye temperature θ_D are found to be 650 and 679 K, respectively. The temperature dependence of the sublattice magnetization σ(T) obeys a one‐third‐power law in the range 0.5N<0.99.     

The magnetic anisotropy of manganate ferrites

The anisotropy constants K₁ for systems Mn_xFe_3–xO₄, with 1x 1·8, are calculated on the basis of the one-ion model and it is shown that the anomalous temperature dependence of the constant K₁ can be explained by the presence of Mn³⁺ ions in octahedral positions. The influence of the spin order on the magnetic anisotropy and the uniaxial anisotropy in systems Mn_xFe_3–xO₄ are discussed.     

NMR investigations of ion distribution in real structure of manganese-zinc ferrites

Using the nuclear magnetic resonance, X-ray and electromagnetic methods we investigated structure and properties of the single- and polycrystalline manganous and manganese-zinc ferrites: MnFe₂O₄, Mn_0.8Zn_0.2Fe₂O₄, Mn_0.6−xCu_xZn_0.3Fe_2.1O₄ (0 ≤x ≤ 0.3). The real structure of the manganous and manganese-zinc ferrites is found to contain simultaneously vacancies in cationic and anionic sublattices. The analysis of the NMR spectra of⁵⁵Mn,⁵⁷Fe,^63,65Cu and⁶⁷Zn ions allowed us to find the magnetic and valence states of all ions and their distribution in the real spinel structure. It was shown that the Fe³⁺ paramagnetic ions, Zn²⁺ diamagnetic ions and cationic vacancies in tetrapositions are distributed statistically. The Mn³⁺ and Cu²⁺ Jahn-Teller ions and presumably Fe²⁺ in octahedral positions are distributed nonstatistically forming cluster-type mesoscopic inhomogeneities. The correlation between distribution of ions and vacancies in the real structure of manganese-zinc ferrites and their functional electromagnetic properties was found.     

Electric and magnetic properties of the perovskites Ba2(FeMo)O x (5.88 ≤ x 6.01)

The purpose of this work is to establish the relation between the magnetic, electric, and magnetotransport properties and the oxygen nonstoichiometry of the compounds Ba₂(FeMo)O _x (5.88 ≤ x ≤ 6.01). The investigations established the behavior of the magnetization, resistance, and magnetoresistance of samples in this series. It is shown that the behavior of the magnetization can be described by assuming that the iron ions become divalent (Fe³⁺→ Fe²⁺) as a result of the reduction of the samples and the molybdenum ions become hexavalent (Mo⁵⁺ → Mo₆₊) as a result the oxidation of the samples. It is established that there are two contributions to the magnetoresistance which arise as result of magnetic ordering of the intragranular interlayer and intergranular transfer of spin-polarized charge carriers. It is inferred that electric transport in samples of this series is determined by percolation processes between granules with metallic conductivity separated by a dielectric interlayers.     

Structural and magnetic investigations of sub-nano Mn-Mg ferrite prepared by wet method

Manganese-magnesium ferrite nanoparticles Mn_1−xMg_xFe₂O₄; 0≤x≤0.25 were prepared by the co-precipitation route. The samples were characterized by X-ray diffraction (XRD), which confirms the single phase spinel structure. Crystallite size, calculated from the (3 1 1) peak using the Scherrer formula, was found to increase with increasing Mg²⁺ concentrations and was found to be within the range 3-6 nm. TEM was also used to characterize the microstructure of nanosized Mn_1−xMg_xFe₂O₄. Nominal composition of the samples was determined by Atomic Absorption analysis (AA). Hysteresis loops of manganese-magnesium ferrite were obtained at room temperature and revealed lower saturation magnetization values associated with nanocrystalline Mn_1−xMg_xFe₂O₄ particles. This behavior was attributed to structural distortion of surface spins compared to that of the bulk one.     

Mössbauer and magnetic study of Mn2+- and Cr3+-substituted spinel magnesioferrites of the composition Mg1−xMnxFe2−2xCr2xO4

Chromium and manganese co-substituted spinel magnesioferrites of the composition Mg_1?x Mn _x Fe_2?2x Cr_2x O₄ (x?=?0.0, 0.1, 0.2, 0.3, and 0.5) were investigated with X-ray diffraction (XRD), Mössbauer spectroscopy and magnetic measurements. The cation distribution inferred suggests that Mn²⁺ and Cr³⁺ ions dominantly occupy the A- and B-sites respectively. The gradual decrease of the hyperfine fields and Curie temperatures with increasing x reflects a gradual weakening in the AB exchange interaction. Mössbauer data of the sample with x = 0.5 is suggestive of cation clustering and/or superparamagnetism. The magnetization data is suggestive of Yafet-Kittel-type canted magnetism.     

Site occupancy and magnetic study of Al and Cr co-substituted Y3Fe5O12

Single-phased polycrystalline Y₃Fe_5−2xAl_xCr_xO₁₂ garnet samples (x=0, 0.2, 0.4 and 0.6) have been prepared by the conventional ceramic technique. Rietveld refinement of X-ray diffraction patterns of the samples shows them to crystallize in the Ia3d space group and the corresponding lattice constant to decrease with increasing Al³⁺ and Cr³⁺ contents (x). Mössbauer results indicate that Cr³⁺ substitutes for Fe³⁺ at the octahedral sites whilst Al³⁺ essentially replaces Fe³⁺ at the tetrahedral sites. This result indicates that co-doping of Y₃Fe₅O₁₂ does not affect the preferential site occupancy for separate individual substitution of either Cr³⁺ or Al³⁺. The magnetization measurements reveal that the Curie temperature (T_c) monotonically decreases with increasing x while the magnetic moment per unit formula decreases up to x=0.4 and then slightly increases for x=0.6. This reflects a progressive weakening of the ferrimagnetic exchange interaction between the Fe³⁺ ions at octahedral and tetrahedral sites due to co-substitution. The magnetic moment was calculated using the cations distribution inferred from the Mössbauer data and the collinear ferrimagnetic model, and was found to agree reasonably with the experimentally measured value. The phenomenological amplitude crossover, characterized by the temperature T*, has also been observed in the doped YIG and briefly discussed.     

Li0.5Fe2.5−xMnxO4 ferrite sintered from microwave-induced combustion

Li_0.5Fe_2.5−xMn_xO₄ (0≦x≦1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, ferric nitrate, manganese nitrate and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Mn-substituted lithium ferrite powders. The resultant powders annealed at 650 ^°C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the Mn content were strongly influenced the magnetic properties and Curie temperature of Mn-substituted lithium ferrite powder. As for sintered Li_0.5Fe_2.5−xMn_xO₄ specimens, substituting an appropriate amount of Mn for Fe in the Li_0.5Fe_2.5−xMn_xO₄ specimens markedly improved the complex permeability and loss tangent.     

On the ferromagnetic perovskite with potential GMR properties: The manganite La1−x K x MnO3

We investigate structural features (cell distortions), magnetic and electric properties versus temperature for La_1-x K _x MnO₃ (0.05 < x < 0.2) perovskites. All the phases crystallize in a rhomboedral symmetry (R3c) and are ferromagnetic. Curie temperatures are strongly dependent of x, with a maximum around 308 K for La_0.8K_0.2MnO₃. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, characteristic of a conventional Brillouin type curve. The average magnetic moment of Mn is about 88 e.m.u/g at 5 K for the composition La_0.8K_0.2MnO₃. The transition from ferromagnetic-metallic to paramagnetic-semiconducting states is explained from the suggested distribution of the cations (La³⁺ _1-x K⁺ _x )_A(Mn⁴⁺ _1-2x Mn⁴⁺ _2x )_BO₃ by the double exchange of Mn³⁺-Mn⁴⁺ pairs at the B-sublattice. Preliminary magnetoresistance measurements on bulk ceramic samples display an MR effect of the same amplitude as in the case of the alkaline earth substituted La manganites.     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Mössbauer and optical investigation of Co3−x Fe x O4 thin films grown by sol–gel process

Structural transformation and the related variation in magnetic and optical properties of Co_3?x Fe _x O₄ thin films grown by a sol–gel method have been investigated as the Fe composition varies up to x?=?2. The normal spinel phase is dominant below x?=?0.55 and the inverse spinel phase grows as x increases further. Conversion electron Mössbauer spectroscopy (CEMS) measurements indicate that the normal spinel phase have octahedral Fe³⁺ ions mostly while the inverse spinel phase contain octahedral Fe²⁺ and tetrahedral Fe³⁺ ions. For higher Fe composition (x?>?1.22), Co²⁺ ions are found to substitute the octahedral Fe²⁺ sites. The measured optical absorption spectra for the Co_3?x Fe _x O₄ films by spectroscopic ellipsometry support the CEMS interpretation.