Non-magnetic Al3 Research Articles

Influence of Al3+ doping on the magnetism of one-dimensional frustrated Ca3CoMnO6 compound

Previous reports depict that the nonstoichiometric Co/Mn ratio greatly affects the physical properties of Ca3CoMnO6, but few papers focus on the Co/Mn site doping effect. In order to investigate the influence of the substitution on the magnetization, partial Mn4+ ions were replaced with Al3+ ions and polycrystalline Ca3CoMn1-xAlxO6 (x = 0, 0.05, 0.1, 0.15, 0.2) samples were prepared. All the samples crystallized into rhombohedral structure without existence of impurities. SEM patterns showed that the mean particle size decreased with the introduction of Al element. Due to the dilution effect of nonmagnetic Al3+ ions to the spin chains, the competing short-range antiferromagnetic/ferromagnetic order caused by the “order by disorder” phenomenon was suppressed. The high-temperature spin freezing peak disappeared gradually and the doped samples presented a faster dynamic behavior, indicating a more robust long-range antiferromagnetic order. The time-dependent magnetization showed a nonmonotonic relaxation in a field of 5T, implying the possible existence of quantum tunneling of magnetization.

Magnetic phase transitions in Ba0.5Sr1.5Zn2Fe11.92Al0.08O22 hexaferrites

We report studies on the effect of substituting the magnetic Fe3+ cations with nonmagnetic Al3+ cations in Y-type hexaferrite Ba0.5Sr1.5Zn2Fe11.92Al0.08O22 powders on their magnetic properties and especially on the magnetic phase transitions responsible for observing the magnetoelectric effect. In this research, the Y-type hexaferrite powders were synthesized by citric acid sol-gel auto-combustion. After the auto-combustion process, the precursor powders were annealed at 1170 °C in air to obtain the Y-type hexaferrite materials. The effects of Al substitution on the structural, microstructural properties and phase content were investigated in detail using X-ray powder diffraction and scanning electron microscopy. Hysteresis measurements were performed by a physical-property-measurement-system (PPMS) (Quantum Design) at 4.2 K and at room temperature. Dc-magnetic measurements of the temperature dependence of the magnetization at magnetic fields of 50 Oe, 100 Oe and 500 Oe were used to determine the effect of applying a magnetic field on the temperature of magnetic-phase transitions. We demonstrated that the helical spin state can be modified further by varying the magnetic field.

Influence of addition of Al3+ on the structural and solid state properties of nanosized Ni–Zn ferrites synthesized using malic acid as a novel fuel

Abstract In this work, the synthesis of Ni0.7Zn0.3Fe2-xAlxO4 (x = 0.0–0.6) nanoferrites by combustion method using malic acid as a novel fuel is being reported for the first time. The structural analysis of the nanosized ferrites was carried out by using XRD and IR studies. The cation distribution of all the ferrites was predicted using the Rietveld refinement data and was well supported by the Mossbauer analysis. The XRD parameters were calculated using this predicted cation distribution. The theoretical lattice constant matched exactly with the lattice constant obtained from the Rietveld refinement confirming the predicted cation distribution. The morphology of all the ferrites was studied using the SEM microscopy from which the particle size distribution was calculated along with the EDS mapping which confirmed the presence of all the constituent elements in all the ferrite samples. The dielectric properties of all the ferrites as a function of frequency as well as temperature were studied. The influence of substitution of magnetic Fe3+ by non-magnetic Al3+ in the ferrite series was also studied. It was observed that the addition of non-magnetic Al3+ in ferrites decreases the lattice parameter, density, saturation magnetization, and the Curie temperature. The FC-ZFC magnetization curve revealed a decrease in blocking temperature (TB) with an increase in Al3+ concentration in Ni0.7Zn0.3Fe2-xAlxO4.

Effect of aluminium substitution in magnetically affluent inverse spinel ferrites studied via 57Fe-Internal field NMR

Ferro/ferrimagnetic materials are of fundamental interest due to their variety of applications. The structural and magnetic properties change significantly with different synthesis procedures. Here, we report the synthesis, X-Ray Diffraction (XRD), Vibrating Sample Magnetometry (VSM), and Nuclear Magnetic Resonance (NMR) studies of spinel nickel ferrites doped with non-magnetic cations like cadmium and aluminium. The spinel ferrites like Ni0.7Cd0.3Fe2-xAlxO4 (x = 0, 0.1, 0.2) are synthesized using one step auto combustion technique. The X-ray diffraction measurements confirm the formation of these systems in pure phase. In the present study we have used the modified home-built NMR spectrometer to study 57Fe NMR in these ferro magnetic materials. The difference in the Fe3+ bonding at the octahedral (B-site) and the tetrahedral (A-site) sites result in the different hyperfine fields yielding Internal Field (IF) NMR frequencies at two different frequencies. The substitution of the non-magnetic Al3+ results in increasing in the line width of the NMR spectra corresponding to the octahedral site (B-site). Further, the NMR spectra corresponding to A-site decreases (both in terms of line width and area) due to the decrease in the ferrimagnetic contribution at that site. Change in the local environment around Fe3+ ion present at B-site is very well observed using 57Fe NMR technique.

Influence of trivalent Al–Cr co-substitution on the structural, morphological and Mössbauer properties of nickel ferrite nanoparticles

Here, we report the influence of nonmagnetic Al3+ and magnetic Cr3+ co-substitution on the structural, morphological, magnetic and Mössbauer properties of nickel ferrite nanoparticles synthesized via sol-gel auto combustion route. Citric acid was used as a fuel and metal nitrate to fuel ratio was chosen to be 1:3. The resultant powder was sintered at 550 °C for 4 h and used for further characterizations. Single phase formation and nanocrystalline nature was confirmed through X-ray diffraction analysis. The crystallite size of all the samples calculated through Debye-Scherrer’s formula found to be in the range of 18–55 nm. The lattice constant calculated from XRD data show decreasing trend. Scanning electron microscopy technique was used to study the surface morphology of typical samples (x = 0.0 and 0.4). The average grain size determined through SEM images found to be 70 nm–35 nm respectively for x = 0.0 and 0.4 samples. The magnetic properties were investigated through magnetization and Mössbauer spectroscopy technique. The saturation magnetization measured from M-H hysteresis plot show decreasing trend with increase in Al–Cr content x. Using Mössbauer spectra the relative area of a sextet (T), Isomer shift (δ), Hyperfine Field (Hf), Quadrupole Splitting (Δ) and Line width (Γ) were calculated. Overall, the co-substitution of Al–Cr has significantly influenced the structural, morphological and magnetic properties of nickel ferrite nanoparticles.

Investigation on structural and magnetic properties of Al3+ and Ce3+ doped hexaferrites

Hard magnetic ferrites or Hexa ferrites (AB12O19, A-Ba, Sr, Ca, Pb) are widely used as permanent magnets, in refrigerator seal gaskets, microphones, and speaker gaskets. They can also be used in small motors for cordless appliances and portable magnetic storage devices. In the present research, Al and Ce doped hexaferrites (AB12O19) were chosen and synthesized by combustion technique using the precursors (Sr(NO3)2, Ba(NO3)2, Ca(NO3)2, Fe(NO3)3, Al(NO3)3, Ce(NO3)3) and fuels (C6H8O7, C12H22O11). The prepared powder was calcined at different temperature (800 °C, 900 °C, 1100 °C) for 12 h. Phase identification was done by using X-Ray Diffraction (XRD) technique by which phases were confirmed and also the morphological behavior was studied by using Scanning Electron Microscopy (SEM). Elemental analysis was done by using Energy Dispersive Spectroscopy. The intrinsic magnetic properties were studied by using Vibrating Sample Magnetometer, and the hexaferrites are found to be affected by partial substitution of Sr and Fe sites by rare earth elements (RE3+) and non-magnetic trivalent Al3+ ions respectively, and it is same in case of BaFe12O19 and CaFe12O19. The insertion of Ce3+ and Al3+ into hexagonal lattice enhances the coercivity due to increase in magneto-crystalline anisotropy or grain refinement. Thus, the hexaferrites are doped by Ce and Al to achieve Sr0.9Ce0.1Fe10Al2O19, Ba0.9Ce0.1Fe10Al12O19, Ca0.9Ce0.1Fe10Al12O19.

Intrinsic Magnetic and Ferroelectric Behaviour of Non-magnetic Al3+ Ion Substituted Dysprosium Iron Garnet Compounds

Synthetic garnets are a group of oxide materials that play a vital role in the development of solid state lasers, magnetic and optic-electronic devices. The analysis on the solubility of rare-earth elements in the well-crystallized system has led to the discovery of peculiar oxide materials with multifunctional properties. By following the above importance, Aluminium ion (Al3+) substituted Dysprosium Iron Garnet compounds with the chemical formula of Dy3Fe5−xAlxO12 (x = 0–0.5) have been synthesized by the solid state reaction method and the effect of substitution of non-magnetic ions into the magnetic sub-lattices have been analyzed. The Rietveld refinement of powder x-ray diffraction patterns confirms that the pure Dy3Fe5O12 compound crystallizes with cubic (Space group: Ia3d) superstructure whereas all the Al3+ substituted samples exhibit the coexistence of cubic (Ia3d) and trigonal Fe2O3 (R3c) phases. The energy gap values of the prepared compounds is found to be around 1.6 eV which reveals the semiconducting nature and the decreasing trend of band gap values may be due to the growth factor of crystallites, structural disorder and distortion introduced into the crystal lattice. From the Micro-Raman analysis, it is found that the substituted Al3+ ions starts filling into both tetrahedral and octahedral positions and the assignments of vibrational modes observed from Raman spectra confirm the incorporation of Al3+ ions into the Dy3Fe5O12 garnet structure. From the magnetization analysis, it is found that the response of super-exchange interaction between Fe3+ ions in the a and d sites of Dy3Fe5O12 compound leads to net magnetic moment and the substitution of Al3+ ions preferably replaces Fe3+ ions in d sites and suggests the decrease in net magnetization values. From the photoluminescence studies, it is noticed that the luminescence behavior of Al3+ ions substituted Dy3Fe5−xAlxO12 compounds are due to the superposition of a broad emission band and reveals the variation in concentration of Al3+ ions in the prepared compounds. An interesting point to note is that, a well saturated “soft” ferroelectric hysteresis loop is obtained in both pure and Al3+ substituted Dy3Fe5−xAlxO12 compounds, and the observed electric hysteresis loops are found to be influenced by the factors such as capacitance nature, resistivity effects and leakage current of the compounds. Hence, the study on effect of trivalent non-magnetic ion substitution in a hard magnetic Dy3Fe5O12 system leads to interesting intrinsic magnetic and ferroelectric properties and found suitability for the fabrication of energy storage and optoelectronic devices.

Resonance Effect of Ionic Valences on the Structural and Magnetic Properties of Dy2NiMnO6 Induced by Nonmagnetic Al Ion Doping

Magnetic and structural properties of nonmagnetic Al3+ doping in Dy2NiMnO6 compounds have been investigated systematically. The X‐ray diffraction shows that all the samples are crystallized in double perovskite. The UV–vis absorption spectra present a non‐monotonous dependence of optical band gap on the Al3+ doping content, which is attributed to the local structural distortion induced by Al3+ doping. Magnetization curves show three magnetic transitions corresponding to the temperatures TC1, TC2, and TN, which are attributed to ferromagnetic interactions of Mn4+–O–Ni2+, Mn3+–O–Ni3+, and antiferromagnetic interactions of the Dy3+ with the Ni(Al)/Mn moments, respectively, changing with the Al3+ doping content. A resonance effect of ionic valences has been observed while Al3+ doping at Ni site (B site), i.e., not only the Mn valence decreasing, but also the Ni valence increasing, without destroying the long‐range Ni–O–Mn orders. The Al3+ doping causes the weakening of the super‐exchange interaction between Mn4+ and Ni2+ ions and the Ni/Mn ordered degrees. Meanwhile, the amount of Mn3+ and Ni3+ ions and the strengthening of the Mn3+–O–Ni3+ ferromagnetic interactions increase with trivalent Al ion doping. As a result, TC1 decreases, TC2 and TN increase first and then decrease, showing a non‐linear change, with the Al3+ doping content increase. The resonance effect of ionic valences provides a new way to understand and adjust the magnetic properties for the application.

Thermal stability and electrical properties of BiFe1−xMxO3 (M = Al3+, Ga3+) ceramics

BiFeO3 (BFO) is a fascinating multiferroic material, exhibiting ferroelectric and G-type antiferromagnetic characteristics simultaneously. In this work, non-magnetic Al3+ and Ga3+ doped BFO (BFAO and BFGO) ceramics were synthesized via sol–gel and conventional sintering methods. Structural, thermal stability and electrical properties of samples were analyzed in detail. X-ray diffraction (XRD) patterns of powder and ceramic samples demonstrated efficient crystallization, consisting of rhombohedral structures with R3c space group for small amounts of added dopant. Thermal analysis exhibited that BFO decomposes into Bi25FeO39 and Bi2Fe4O9 at 950 °C. It is found that Al3+ and Ga3+ doping readily contribute to decomposition, as supported by calculations from first-principles. BiAlO3 and BiGaO3 are unstable and would spontaneously decompose, if they could be synthesized using ordinary technology. As a result, decomposition temperatures of doped powders decreased to ~ 680 °C. Dielectric behavior can be explained through the Maxwell–Wanger model and Koop’s theory. Dielectric loss decreased with increasing substitution. Leakage current density of doped ceramics became 2–3 orders of magnitude lower than that of BFO ceramic, improving performance and championing applications of modified BFO in future.

Effects of Nd, Al Doping on the Structure and Properties of BiFeO3

The BiFeO3(BFO) and Bi0.95Nd0.05Fe1-xAlxO3 (x = 0, 0.03, 0.05, 0.075, 0.1) powders were prepared at 200 °C for 24 h by hydrothermal method. The effects of Al doping content on the structure, morphology, magnetic, and photocatalytic properties were studied. X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FTIR) demonstrated the compounds are distorted rhombohedral perovskite structure without any other heterogeneity and structural transition. Field emission scanning electron microscope (FESEM) reflected surface of compounds is a dense, agglomerated sphere. As the concentration of Al3+ increases, a small part of the spherical crystallites eventually becomes cauliflower shape. Energy-dispersive X-ray (EDS) showed the Bi0.95Nd0.05Fe0.95Al0.05O3 sample mainly consists of five elements (Bi, Fe, O, Nd, Al) and atom radio matched well with the formula. Vibrating sample magnetometer (VSM) integrated in a physical property measurement system (PPMS-9) illustrated introducing Nd3+ ions into BFO will enhance its magnetism at room temperature. However, with the increase of non-magnetic ion Al3+ concentration in Bi0.95Nd0.05Fe1-xAlxO3 (x = 0, 0.03, 0.05, 0.075, 0.1), the network structure of Fe-O-Fe was destroyed, which led to the decrease of its net magnetization, so that the hysteresis loop shows paramagnetism. The photocatalytic performance of BFO increased initially and decreased afterwards as Al3+ concentration increased, and the best catalytic performance was achieved at x = 0.05.

Structural and magnetic properties of Al-doped yttrium iron garnet ceramics: 57Fe internal field NMR and Mössbauer spectroscopy study

The structural and magnetic properties of Al substituted yttrium-iron garnet (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powders synthesized using solution combustion method were investigated. Post combustion, the samples were calcination at 1045 °C for 6 h and subsequently at 1200 °C for 6 h to obtain phase-pure garnets. X-ray diffraction (XRD) results confirm the formation of garnets with Ia3¯d structure. The occupancy of Y3+ ions in the dodecahedral site and the distribution of Al3+ and Fe3+ ions in the tetrahedral and octahedral sites in the bcc structure of the garnet were confirmed by Rietveld refinement of XRD patterns, Mössbauer spectroscopy and 57Fe internal field NMR spectroscopy. For low Al content, Al3+ ions have preference to occupy tetrahedral (Td) sites than the octahedral (Oh) sites. At higher Al content the distribution of Al tends towards a ratio of 3:2 at the tetrahedral:octahedral site. Increase in Al doping results in the decrease in the lattice parameter due to smaller size of Al3+ as compared to Fe3+ ion. All the studied samples show coral-network-like surface morphology. The saturation magnetization (MS) values decrease from ∼26.94 emu/g to ∼ 0.17 emu/g with increase in Al content from 0.0 to 1.8. Further addition of Al makes the sample paramagnetic at RT. Substitution of non-magnetic Al3+ reduces the saturation magnetization rapidly due to the decrease in the superexchange interaction in the crystal.

Effect of non-magnetic Al3+ doping on structural, optical, electrical, dielectric and magnetic properties of BiFeO3 ceramics

Pure BiFeO3 (BFO) and Al doped BFO samples were synthesized via citrate precursor method and sintered at 500°C for two hours. Effect of Al doping on the structural, optical, electrical, dielectric and magnetic properties were investigated. X-ray diffraction (XRD) confirmed the distorted rhombohedral structure without any merging of peaks which indicates no structural transformation. Average crystallite size was found to be in the range 28–39nm. Field emission scanning electron microscopy (FESEM) images illustrated the dense, agglomerated, spherically shaped with reduced grain size nanoparticles. Increased value of dielectric constant with low dielectric tangent loss was observed for the Al doped BFO samples. The value of dielectric constant was found to be 51 at 100kHz for x = 0.1 sample. Temperature dependent dielectric constant showed a dielectric anomaly, indicating the antiferromagnetic transition. The remanent polarization and the corresponding coercive field for x = 0.1 was found to be 0.0625µC/cm2 and 56.154kV/cm at an operating voltage of 1000V. The improved electrical properties with low leakage current density were ascribed to the stabilization of the pervoskite structure and reduced oxygen vacancies.

Influence of Al-substitution on structures, resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 (x=3y, y=0.05, 0.1, 0.15, 0.2)

Two series of single-phased LaxSr(2−x)Fe(1+y)Mo(1−y)O6 and LaxSr(2−x)Fe(1+0.5y)Al0.5yMo(1−y)O6 (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al3+ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10K, 1T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10K, 1T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420K and 405K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.

Impact of Al doping on structural and Magnetic Properties of Co-Ferrite

Nanocrystaslline Al doped cobalt ferrite CoAlyFe2-yO4 (y = 0.5, 1.5) powders have been synthesized by microwave assisted sol-gel auto combustion method and characterised using X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The XRD revealed that the powders obtained are single phase with spinel structure and crystallises to Fd3m space group. The calculated grain size from XRD data have been verified using TEM. TEM photograph shows that powder consists of manometer sized grains. It was observed that as Al3+ content increases the grain size and lattice constant decreases which attributed towards the influence of non-magnetic Al3+ concentration on the grain size. The magnetic properties of the synthesized samples is characterised by using VSM. The saturation magnetisation (Ms), coercivity (Hc) were found to decreases with increases of aluminium content which is due to nonmagnetic Al3+ ion and weakened interaction between sub lattices.

Magnetic Nanoparticles: Synthesis, Characterization and Magnetic Properties of Cobalt Aluminum Ferrite.

Nanoparticles of the ferrite system CoFe(2-x)Al(x)O4 (x = 0.0, 0.3, 0.7 and 1.0) were synthesized through the co-precipitation technique. Thermal decomposition process and formation of a single crystalline phase were followed using thermal differential analysis technique (DTA). X-ray powder diffraction patterns of the samples confirmed the formation of a nano-size single spinel phase. The average crystallite size was found to be in the range 20-63 nm for all samples. This was further confirmed by TEM of one of the samples, with concentration x = 1.0 which was found statistically to be 27 nm. This agrees well with the value of 24 nm deduced by means of X-ray diffraction method for the same sample. A considerable decrease in the intensity of the octahedral bands is observed as the aluminum concentration increases, and even vanishes completely at x = 1.0 indicating the migration of cations between the octahedral and tetrahedral sites. The magnetic hysteresis loops at room temperature showed decrease in both, coercivity and saturation magnetization as the non-magnetic Al3+ ions content increases. The relative values of M(r0/M(s) were found to be between 0.44 and 0.31 for the samples with a remarkable change in the squareness of the loops. This is highly beneficial for the microwave and memory devices applications of these nano sized ferrite system.

Electrical and Dielectric Properties of Non-magnetic Al3+ Substituted Ni–Zn Nano Ferrites for High Frequency Applications

The effect of Al substitution on electrical and dielectric parameters of Ni–Zn ferrite has been discussed in the present work. The phase identification, surface morphology was studied using X-ray diffractometer (XRD), scanning electron microscope (SEM), respectively. The XRD patterns confirm the single-phase formation of these ferrites. With Al3+, substitution lattice parameter decreases due to smaller Al3+ ions replacing Fe3+ ions. The average grain size obtained from SEM results are in the range of 390–27 nm. The DC resistivity was observed to increase with increasing Al3+ ions concentration due to the unavailability of Fe3+ ions. Dielectric constant ( $$\upvarepsilon ^{\prime }$$ ) and dielectric loss tangent (tan δ) have been studied as a function of frequency (1 kHz–10 MHz) and temperature (50–300 °C). The observed results are explained on the basis of interfacial polarization as predicted by Maxwell and Wagner.

Effect of non-magnetic and magnetic trivalent ion substitutions on BaM-ferrite properties synthesized by hydrothermal method

Two species trivalent transition metal ions, nonmagnetic Al3+, Bi3+ and magnetic Cr3+, Mn3+ substituted barium hexaferrite powders are synthesised via dynamic hydrothermal method then calcinated at different temperatures. Powder XRD patterns analysis and Rietveld refinement indicate magnetoplumbite-type crystalline phase. The crystallite size calculated using Scherer's formula varies from 153 to 248 nm as it is affected by the ionic radius of trivalent metal ions and calcination temperatures. The morphological features show agglomerations of spherical-shaped particles with an average size of 400 ± 50 nm for BaBiFe11O19 and hexagonal-shaped particles 1–2 μm for the other substitutions. Magnetic studies show a ferromagnetic behaviour for all samples. The nonmagnetic Al3+, Bi3+ substituted materials have lower saturation magnetization (Ms) and remanence (Mr) than the magnetic substitutions Cr3+, Mn3+. These results reveal that Ms and Mr are related to the magnetic moment of each metallic ion and its distribution in the lattice. For magnetic substitution, Ms of manganese substitution (61.10 emu/g) is found higher than that of chromium (60.33 emu/g) since Mn3+ has a magnetic moment of 5μB greater than Cr3+ with 3μB. Coercivity Hc and maximum energy product (BH)max determined from M−H loops exhibit an increasing trend with calcination temperature as they are related to the crystallite size. Hc and (BH)max of BaMnFe11O19 are found maximum 0.426 T and 7.47 kJ m−3, respectively.

Studies on charge transport in Al–doped La0.7Ca0.3Mn1−xAlxO3 manganites

In this communication, we report the results of the studies on the effect of non-magnetic Al3+-doping on structure and properties of La0.7Ca0.3Mn1−xAlxO3 (LCMAO) manganites synthesized by conventional solid state reaction (SSR) route. The Rietveld refinement of the X-ray diffraction (XRD) data confirms the single phasic nature of the samples without any detectable impurities. All the samples exhibit metal to insulator transition (TP) which decrease with increase in Al3+ doping concentration while it increases with applied magnetic field. To understand the nature of charge transport in metallic and insulating regions of resistivity, various models and mechanisms have been used to fit the observed experimental data.

Site occupancy and magnetic properties of Al-substituted M-type strontium hexaferrite

We use first-principles total-energy calculations based on density functional theory to study the site occupancy and magnetic properties of Al-substituted M-type strontium hexaferrite SrFe12−xAlxO19 with x = 0.5 and x = 1.0. We find that the non-magnetic Al3+ ions preferentially replace Fe3+ ions at two of the majority spin sites, 2a and 12k, eliminating their positive contribution to the total magnetization causing the saturation magnetization Ms to be reduced as Al concentration x is increased. Our formation probability analysis further provides the explanation for increased magnetic anisotropy field when the fraction of Al is increased. Although Al3+ ions preferentially occupy the 2a sites at a low temperature, the occupation probability of the 12k site increases with the rise of the temperature. At a typical annealing temperature (>700 °C) Al3+ ions are much more likely to occupy the 12k site than the 2a site. Although this causes the magnetocrystalline anisotropy K1 to be reduced slightly, the reduction in Ms is much more significant. Their combined effect causes the anisotropy field Ha to increase as the fraction of Al is increased, consistent with recent experimental measurements.

Reversing ferroelectric polarization in multiferroic DyMn2O5 by nonmagnetic Al substitution of Mn

The multiferroic RMn2O5 family, where R is rare-earth ion or Y, exhibits rich physics of multiferroicity which has not yet well understood. DyMn2O5 is a representative member of this family. The ferroelectric polarization of DyMn2O5 is claimed to be magnetically relevant and have more than one component. Therefore, the polarization reversal upon the sequent magnetic transitions is expected. We investigate the evolution of the ferroelectric polarization upon a partial substitution of Mn3+ by nonmagnetic Al3+ in order to tailor the Mn3+-Mn4+ interactions and then to modulate the polarization in DyMn2−x/2Alx/2O5. It is revealed that the polarization can be successfully reversed by Al-substitution via substantially suppressing the Mn3+-Mn4+ interactions, while the Dy3+-Mn4+ interactions can sustain against the substitution until a level as high as x = 0.2. In addition, the independent Dy spin ordering is shifted remarkably down to an extremely low temperature due to the Al3+ substitution. The present work unveils the possibility of tailoring the Mn3+-Mn4+ and Dy3+-Mn4+ interactions independently, and thus reversing the ferroelectric polarization.