Ferromagnetic Signal Research Articles

Mn valence state mediated room temperature ferromagnetism in nonpolar Mn doped GaN

In this work, we demonstrate room temperature ferromagnetism in nonpolar, m-plane manganese doped gallium nitride (m-GaN:Mn) epitaxial thin films synthesized using plasma-assisted molecular beam epitaxy on nonpolar, m-plane sapphire (Al2O3) substrates. We observed that Mn doping concentration plays a key role in stabilizing the valence state of Mn ions thereby influencing the magnetic and transport properties. Chemical characterization by X-ray photoelectron spectroscopy, X-ray absorption near edge spectra, and energy dispersive X-ray spectroscopy affirm homogeneous valence state Mn+3 in lightly doped samples, while higher doping levels cause a heterogeneous Mn+0 and Mn+2 mixed valence states. Magnetic measurements indicate that films with low doping levels show the signature of room temperature ferromagnetism, while films with higher doping show antiferromagnetic/spin glass behavior besides the indication of MnGa alloys. Temperature dependent resistivity measurements show that both the samples are semiconducting with deep level defect states. Our findings suggest the possibility of incorporating spin functionality in non-polar optoelectronic devices.

Exchange bias and spin glass in La2FeMnO6 nanoparticles

The aim of this work is to study the La2FeMnO6 double perovskite system from an experimental point of view. The nanoparticles were prepared through the ionic coordination reaction method and have a mean particle size of 36 nm, as determined by MEV analysis. The crystal structure is monoclinic with P21/n space group. The 57Fe Mössbauer spectroscopy (MS) study reveals that Fe ions species have a dominant magnetic spectra at temperatures smaller than 90 K, and above 180 K the spectra are dominated by a paramagnetic component. The magnetization versus temperature (MxT) measurement showed a fast increase of magnetization at temperatures below 130 K and seems to be related to the Fe moments. The M−1 × T data follows a linear trend for T > 591 K indicating a magnetic transition at this temperature. The MS and the magnetic data reveal that for T > 180 K the ferromagnetic signal is mainly due to Mn3+OMn+4 clusters. Studies on AC susceptibility as a function of frequency suggested a cluster spin glass transition at 98.9 K, and the field cooling MxH loops depicted an exchange bias effect below 20 K. Both results indicate the complexity of this system making it of great interest in basic research and practical applications.

Structural and Magnetic Properties of Fe, Mn Co-doped BaTiO3 Synthesized Using Ball Milling Technique

Nano composite, monophasic nano-crystalline as well as bulk single phase materials of Fe, Mn co-doped BaTiO3 compounds are successfully synthesized through two different strategies using high energy meachnical attritor and muffle furnace. Significant changes in the structural properties such as formation of coexisting tetragonal, cubic and orthorhombic phases are brought out by varying post-processing temperature. A weak ferromagnetic signature is observed for nanocomposites and it ceased to exist with increasing post heating temperatures. Interestingly, superparamagnetic state is observed for nano-composites with blocking temperature of 263 K for a measurement field of 500 Oe. Further, this nanocomposite found to exhibit magnetic ordering well above room temperature which facilitates the possible interplay of multiple ferroic phases.

The carriers of AMS in remagnetized carbonates. Insights for remagnetization mechanism and basin evolution

Magnetic fabrics are usually studied to unravel the evolution of sedimentary basins, mainly focusing the attention in paramagnetic minerals. However, since basins are sometimes affected by burial-related chemical remagnetizations, magnetic fabrics can also be carried by authigenic ferromagnetic minerals related to remagnetization processes. Consequently, the study of the different types of fabrics can give complementary and valuable information about the evolution of sedimentary basins. Here, we explore, in the Jurassic carbonates of the Central High Atlas (Morocco), the role in magnetic fabrics of authigenic magnetite that grew during the Cretaceous (ca. 100 Ma) widespread remagnetization event.Magnetic fabrics are studied in 53 sites using the anisotropy of the magnetic susceptibility measured at room temperature (RT-AMS) and comparing results with sub-fabrics that alternatively enhance the paramagnetic and the ferromagnetic signal. Furthermore, an innovative analysis is proposed, comparing the magnetic fabrics before and after bedding correction (the common procedure) with fabric orientation after partial bedding correction. We use the paleomagnetic information to reconstruct the attitude of bedding at the remagnetization time and to restore the magnetic fabrics at this time.The performed analysis allows interpreting RT-AMS in terms of the contribution of different subfabrics, and the relationship between these, tectonic processes and the magnetic mineralogy. Four RT-AMS types are defined: T1 is carried by superparamagnetic magnetite that grew during the remagnetization stage and shows a horizontal lineation parallel to the extension direction at this time; T3 and T4 are carried by paramagnetic minerals and show compressive fabrics with the magnetic lineation parallel to intersection (bedding-cleavage) lineation; finally, T2 shows a mix between ferrimagnetic and paramagnetic fabrics.

Modulation of magnetization dynamics in an epitaxial Heusler ferromagnet due to pure spin current in a laterally configured structure

Using a laterally configured structure, we demonstrate modulation of the magnetization dynamics of an epitaxial Heusler ferromagnet, Fe3Si, by nonlocally injecting pure spin currents. From the ferromagnetic resonance signals, the effective Gilbert damping constant () of the epitaxial Fe3Si nanomagnet is estimated to be ∼0.003 at room temperature. By using the lateral and nonlocal spin injection, the value of the epitaxial Fe3Si nanomagnet can be modulated from 0.0034 to 0.0024. This study will open a way for control of the magnetization dynamics of high-performance ferromagnetic Heusler alloys due to pure spin currents even in laterally configured structures.

Manipulating superconducting phases via current-driven magnetic states in rare-earth-doped CaFe2As2

Inhomogeneous superconductivity in rare-earth (RE)-doped CaFe2As2 (Ca122) compounds leads to a novel state of matter in which the superconducting and magnetic states can be simultaneously controlled by using an electric current (I). Both La- and Ce-doped Ca122 single crystals show a very broad superconducting transition width (ΔTc) due to their non-bulk nature. Surprisingly, ΔTc becomes sharper or broader after an electric current larger than a threshold value (It) is applied, with a concomitant change in the normal-state magnetism. The sharpened (broadened) ΔTc is accompanied by a decrease (an increase) in the amplitude of the ferromagnetic signals. The sensitive changes in the superconductivity and magnetism that occur when an external current is applied are related to the inhomogeneous electronic states that originate from the Fe magnetic state and/or self-organized superconducting/magnetic composites in Ca122 compounds. These discoveries shed new light on the role of Fe in Fe-based superconductors and will provide new ideas for the design of novel superconducting devices.

Ferromagnetism and spin-dependent transport at a complex oxide interface

Complex oxide interfaces are a promising platform for studying a wide array of correlated electron phenomena in low-dimensions, including magnetism and superconductivity. The microscopic origin of these phenomena in complex oxide interfaces remains an open question. Here we investigate for the first time the magnetic properties of semi-insulating NdTiO$_3$/SrTiO$_3$ (NTO/STO) interfaces and present the first milli-Kelvin study of NTO/STO. The magnetoresistance (MR) reveals signatures of local ferromagnetic order and of spin-dependent thermally-activated transport, which are described quantitatively by a simple phenomenological model. We discuss possible origins of the interfacial ferromagnetism. In addition, the MR also shows transient hysteretic features on a timescale of ~10-100 seconds. We demonstrate that these are consistent with an extrinsic magneto-thermal origin, which may have been misinterpreted in previous reports of magnetism in STO-based oxide interfaces. The existence of these two MR regimes (steady-state and transient) highlights the importance of time-dependent measurements for distinguishing signatures of ferromagnetism from other effects that can produce hysteresis at low temperatures.

Exploring ferroelectric and magnetic properties of Tb-substituted m = 5 layered Aurivillius phase thin films

Here, we report the effect of A-site substitution of Tb at the expense of Bi on the ferroelectric and magnetic properties in m = 5 layered 2-D Aurivillius Bi6Ti3Fe2O18 thin films. The nominal stoichiometry of the prepared compound is Tb0.40Bi5.6Fe2Ti3O18, Tb0.90Bi5.1Fe2Ti3O18, and Bi6Ti3Fe2O18. Phase examination reveals that only 0.40 mol. % is successfully substituted forming Tb0.40Bi5.6Fe2Ti3O18 thin films. Lateral and vertical piezoresponse switching loops up to 200 °C reveal responses for Bi6Ti3Fe2O18, Tb substituted Tb0.40Bi5.6Fe2Ti3O18, and Tb0.90Bi5.1Fe2Ti3O18 thin films along the in-plane (±42.31 pm/V, 88 pm/V and ±134 pm/V, respectively) compared with the out-of-plane (±6.15 pm/V, 19.83 pm/V and ±37.52 pm/V, respectively). The macroscopic in-plane polarization loops reveal in-plane saturation (Ps) and remanence polarization (Pr) for Bi6Ti3Fe2O18 of ±26.16 μC/cm2 and ±22 μC/cm2, whereas, ±32.75 μC/cm2 and ±22.11 μC/cm2, ±40.30 μC/cm2 and ±28.5 μC/cm2 for Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 thin films, respectively. No ferromagnetic signatures were observed for Bi6Ti3Fe2O18 and Tb0.40Bi5.6Fe2Ti3O18. However, a weak response was observed for the Tb0.90Bi5.1Fe2Ti3O18 at 2 K. Microstructural analysis of Tb0.90Bi5.1Fe2Ti3O18 revealed that it contains 4 vol. % Fe:Tb rich areas forming FexTbyOz, which accounts for the observed magnetic moment. This study demonstrates the importance of thorough microstructural analysis when determining whether magnetic signatures can be reliably assigned to the single-phase system. We conclude that Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 samples are not multiferroic but demonstrate the potential for Fe-RAM applications.

Effects of rapid thermal annealing on the local environment, electronic structure and magnetic properties of Mn doped TiO2 thin films

In this report, nanostructure thin films of Ti0.97 Mn0.03 O2−δ were deposited on Si (001) substrate by pulsed laser deposition (PLD) technique, followed by rapid thermal annealing (RTA) in different ambient gases; O2, N2 and Ar. The RTA treatment dramatically affected the surface defects, local environment, electronic structure and magnetic properties of the films, where, annealing of the film in O2 gas recovered the dislocated atoms of oxygen vacancy (Vo) at the surfaces and induced a diamagnetic phase, whereas the annealing of film in N2 gas introduced Vo, reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) and induced ferromagnetic (FM) signal. Therefore, the change in the local atomic defects of Vo at the surface texture is likely to be responsible for the magnetic response. The presence of oxygen vacancy has been traced by means of Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. The NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L3,2 and O-K edges. The spectral feature at Ti L3,2 edges revealed the rutile phase of TiO2. The asymmetry of t2g and eg bands at the O-K edge confirmed the formation of Vo and reflected the modification in the O co-ordination around Ti4+cations. The Mn L3,2 edges revealed the incorporation of Mn ions in the TiO2 network with Mn2+ and Mn3+ mixed valence states. This different valance states (Mn2+ and Mn3+) is the basis for the ferromagnetism induced by Stoner spin-splitting of the local density of defects state at Fermi level (EF). The change in the magnetic moment of the films after RTA process is related to the change in the local density of defects band. Crystal field and charge transfer parameters were extracted roughly by using atomic multiplet calculations (theory).

Low temperature magnetic properties of Nd2Ru2O7

We present magnetic susceptibility, heat capacity, and neutron diffraction measurements of polycrystalline Nd2Ru2O7 down to 0.4 K. Three anomalies in the magnetic susceptibility measurements at 146, 21 and 1.8 K are associated with an antiferromagnetic ordering of the Ru4+ moments, a weak ferromagnetic signal attributed to a canting of the Ru4+ and Nd3+ moments, and a long-range-ordering of the Nd3+ moments, respectively. The long-range order of the Nd3+ moments was observed in all the measurements, indicating that the ground state of the compound is not a spin glass. The magnetic entropy of Rln2 accumulated up to 5 K, suggests the Nd3+ has a doublet ground state. Lattice distortions accompany the transitions, as revealed by neutron diffraction measurements, and in agreement with earlier synchrotron x-ray studies. The magnetic moment of the Nd3+ ion at 0.4 K is estimated to be 1.54(2)µB and the magnetic structure is all-in all-out as determined by our neutron diffraction measurements.

Annealing effect on the ferromagnetism of MoS2 nanoparticles

Novel properties of two-dimensional (2D) materials have drawn extensive attention in past decade. Magnetic property is one of the research focuses since it can add extra spin freedom for the manipulation of 2D material properties. The magnetism in 2D materials can be originated from defects (i.e. vacancies, zigzag defects). In this work, MoS2 nano particles were annealed at various temperatures (from 600 till 1000 °C) in an argon environment to explore the post-annealing effect on the magnetic properties. The results indicate that annealing may induce tunable ferromagnetism associated with the annealing temperature. The ferromagnetic signal is enhanced with the increase in annealing temperature which may attribute to the defects pertaining to the sulfur vacancies and edge terminated structure induced by annealing.

Sintering process effect on the magnetic behavior of vanadium-doped ZnO ceramics

V-doped ZnO ceramics were prepared using conventional sintering and spark plasma sintering from milled nanopowders. Prior to the sintering process, some powders were subjected to thermal treatment under reducing atmosphere in order to create oxygen vacancies. The sample sintered in air (with as-milled powders) exhibited the highest magnetization with a magnetic moment of 2.1 μB per V5+ ion and for a vanadium concentration of 4 at%. The samples sintered in vacuum or in Ar atmosphere (from reduced powders) exhibit lower magnetizations despite the formation of oxygen vacancies. Secondary phases were observed on these latest samples as the diamagnetic ZnV2O4 and the paramagnetic Zn3(VO4)2 phases, which reduce the magnitude of the ferromagnetic signal.

Influence of Ga vacancies, Mn and O impurities on the ferromagnetic properties of GaN micro- and nanostructures

We present a study of the influence of gallium vacancy (VGa) point defects on the ferromagnetic properties of GaN:Mn and GaN:Mn,O micro- and nanostructures. Results demonstrate that the generation of these point defects enhances the ferromagnetic signal of GaN:Mn microstructures, while incorporation of oxygen as an impurity inhibits this property. XPS measurements revealed that Mn impurities in ferromagnetic GaN:Mn samples mainly exhibit a valence state of 2+. Cathodoluminescence (CL) spectra from Mn-doped GaN samples displayed emissions centered at about 1.97 eV, attributed to transitions between the 4T1-6A1 states of the Mn2+ d orbitals, and emissions centered at 2.45 and 2.9 eV, associated with the presence of VGa. CL measurements also revealed a blue shift of the GaN band-edge emission generated by the expansion of the wurtzite lattice due to Mn incorporation, which was confirmed by XRD measurements. These latter measurements also revealed an amorphization of GaN:Mn due to the incorporation of oxygen as impurities. The GaN:Mn samples were synthesized by thermal evaporation of GaN and MnCO3 powders onto Ni0.8Cr0.2/Si(100) in a horizontal furnace operated at low vacuum. The residual air inside the system was used as a source of oxygen during the synthesis of Mn and O co-doped GaN nanostructures. Mn and O impurities were incorporated into the nanostructures at different concentrations by varying the growth temperature. Energy Dispersive Spectroscopy, XRD, and XPS measurements confirmed that the obtained samples predominantly consisted of GaN.

Synthesis and Characterization of Er-Doped Nano ZnO Samples

Pure and erbium-doped ZnO nanoparticles (Zn1−xErxO), (0.00 ≤ x ≤ 0.10), were synthesized by wet chemical co-precipitation method. The structural, optical, and magnetic properties of the prepared samples were investigated using x-ray powder diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV), Fourier transform infrared spectroscopy (FTIR), M-H magnetic hysteresis, and electron paramagnetic resonance (EPR). XRD studies exhibit the presence of a single ZnO wurtzite hexagonal crystal structure for 0.00 ≤ x ≤ 0.06. A secondary phase of Er2O3 appears for x > 0.06. This means that the solubility limit for doping Zn2+ ions by Er3+ ions is about x = 0.06 under our preparations condition. The lattice parameter a is not affected by the Erbium doping. On the other hand, the lattice parameter c and the unit cell volume V increase with the increase of x up to x = 0.06. This is attributed to the larger ionic size of Er3+ ions (0.88 A) compared to Zn2+ ions (0.74 A). Both c and V decrease for x > 0.06. TEM micrographs indicate that the shape and the size of the ZnO nanoparticles are modified by changing the doping level of Er. The UV measurements point out that band gap energy Eg decreases with the increase of x up to x = 0.06. Then, it increases for both x = 0.08 and 0.10. FTIR spectra confirm the presence of O–H and Zn–O stretching modes at 3451.963 and 428.901 cm− 1, respectively, in pure and doped ZnO samples. The Zn–O stretching mode shifts toward a lower wavenumber for x = 0.06 and toward a higher wavenumber for x = 0.10. M-H hysteresis analysis, at room temperature, reveals that the pure ZnO has a ferromagnetic signal combined with diamagnetic and paramagnetic contributions. This ferromagnetism is reduced for the doped samples up to x = 0.02, and an antiferromagnetic alignment appears for 0.04 ≤ x ≤ 0.10. The saturation magnetization (Ms), the coercivity (Hc), the retentivity (Mr), the anisotropy constant (Ka), and the magnetic moment (μm) were estimated and discussed in terms of erbium doping for the different samples. EPR spectra for Zn1−xErxO were measured at room temperature in order to study the effect of Er substitution on the g value, resonance field (Hr), peak to peak line width (ΔHpp) and spin–spin relaxation time constant (T2).

Proteic sol-gel synthesis, structure and magnetic properties of Ni/NiO core-shell powders

Core-shell structured magnetic Ni/NiO powders were prepared by a proteic sol-gel route. Commercial gelatin and nickel nitrate were used as precursor materials. The synthesized material was calcined in air at 500°C and further investigated by XRD, VSM and TEM. In order to investigate the effects of the structure on the magnetic properties, NiO powders were synthesized by three other methods for sake of comparison: citrate method, nitrate calcination and combustion method. XRD results revealed that the core-shell structured material is composed of 84.8wt% NiO and 15.2wt% Ni, while the samples from other methods are single phase. Hysteresis loop at room temperature showed a strong ferromagnetic behavior for samples prepared by proteic sol-gel and citrate methods. Powders from nitrate calcination and combustion showed weak ferromagnetic behavior most likely attributed to unpaired moments in their nanoparticles. The overall results showed that the proteic sol-gel method is a versatile chemical way to prepare Ni/NiO core-shell powders with high ferromagnetic signals.

Influence of oxygen vacancies on magnetic and transport properties of thin Zn1-xCoxOy films

The features of the structural, transport and magnetic properties of thin Zn1-xCoxOy films (x=0-0.45), fabricated on С-sapphire substrates by the pulsed laser deposition method are studied. It is found that the transport and ferromagnetic properties of the wurtzite Zn1-xCoxOy films nonmonotonously depend on Co concentration at room temperature. For the Zn0.87Co0.13Oy film, the strongest ferromagnetic signal is observed that is caused by formation of the greatest number of metallic Co clusters. A further increase of doping impurity concentration in the films leads to the oxidation of metallic Co and formation of the paramagnetic Co3O4 phase, in consequence of which the ferromagnetic signal subsides.

Formation and magnetic properties of InFeP:Ag nanorods fabricated with noble metal Ag using an ion milling method

The formation, including the density and height of the InFeP:Ag nanorods doped with noble metal Ag using an ion milling method, was preponderantly determined from transmission electron microscopy and x-ray diffraction analyses. We investigate, in particular, the enhanced ferromagnetism of the well-aligned InFeP:Ag nanorods. Auger electron spectroscopy and x-ray photoelectron spectroscopy measurements were carried out in order to investigate the incorporation of Ag and to verify the local chemical bonding of the InFeP:Ag nanorods. The variation of FWHM for the double-crystal x-ray rocking curve and triple-axis diffraction peaks demonstrates that noble metal Ag is incorporated into the InFeP:Ag nanorods. The noticeable ferromagnetic signature (M-H curve) of the InFeP:Ag nanorods is observed and Tc persists up to almost 350 K (3.9 × 10−4 emu g−1), as determined by temperature-dependence magnetization (M-T curve) measurements. This study suggests that the InFeP:Ag nanorods should be a potential candidate for the application of spintronic devices.

Phase diagram of Sm and Mn co-doped bismuth ferrite based on crystal structure and magnetic properties

Sm and Mn co-substituted BiFeO3 (BFO) ceramic has been synthesized by the tartaric acid modified sol-gel method to understand the effect of structural modification on the magnetic properties. The secondary phases (i.e., Bi2Fe4O9, Bi25FeO39 etc.) present in the parent compound BiFeO3 have been significantly reduced with increase in the substitution percentage of Sm and Mn at Bi and Fe sites of BFO respectively. The Rietveld refinement of the X-ray diffraction (XRD) patterns reveals a structural transition from rhombohedral (R3c, space group) to orthorhombic (Pbnm, space group) crystal symmetry with the concentration of Sm and Mn in the sample. The chemical state and elemental ratio of the samples have been studied by analyzing the X-ray photoelectron spectroscopy (XPS) data. Crystallite size obtained from the transmission electron microscopy analysis is in well comparable with the XRD analysis. Fourier transform infrared spectroscopy (FTIR) analysis supports the XRD results. A change in the phonon frequency and line widths in Raman spectra confirms the lattice distortion in the co-substituted samples which are in agreement with the XRD analysis. Ferromagnetic signature has been observed in Sm and Mn co-substitution. The magnetic hysteresis loop at room temperature has been analyzed by considering both ferromagnetic and antiferromagnetic contribution. The saturation magnetization for ferromagnetic phase increases up to 10% substitution of Sm and Mn in BFO and, decrease with the further increase of percentage of substituents. A correlation between crystal structure and magnetic behavior has been observed for Sm and Mn co-substituted sample. Phase diagram of crystal symmetries percentage, saturation magnetization (MS), remnant magnetization (Mr) and coercivity (HC) of Bi1-xSmxFe1-xMnxO3 for x = 0.000, 0.025, 0.050, 0.075, 0.100 and 0.150 ceramics.

Interplay between localization and magnetism in (Ga,Mn)As and (In,Mn)As

Ion implantation of Mn combined with pulsed laser melting is employed to obtain two representative compounds of dilute ferromagnetic semiconductors (DFSs): Ga1-xMnxAs and In1-xMnxAs. In contrast to films deposited by the widely used molecular beam epitaxy, neither Mn interstitials nor As antisites are present in samples prepared by the method employed here. Under these conditions the influence of localization on the hole-mediated ferromagnetism is examined in two DFSs with a differing strength of p-d coupling. On the insulating side of the transition, ferromagnetic signatures persist to higher temperatures in In1-xMnxAs compared to Ga1-xMnxAs with the same Mn concentration x. This substantiates theoretical suggestions that stronger p-d coupling results in an enhanced contribution to localization, which reduces hole-mediated ferromagnetism. Furthermore, the findings support strongly the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions even on the insulator side of the metal-insulator transition.

Magnetic and electric properties of Ni-doped ZnO nanoparticles exhibit diluted magnetic semiconductor in nature

Zn1-xNixO (x = 0.0, 0.03, 0.06, 0.09, 0.12, 0.15) nanoparticles were prepared by chemical synthesis route using hydroxyoxalate type precursors and thermal decomposition process. All the samples crystallize in hexagonal wurtzite structure. Nickel doping into the ZnO structure was confirmed by Energy Dispersive X-ray (EDX) spectroscopy, High Resolution Transmission Electron Microscopy (HRTEM) and UV-VIS spectroscopy. The absorption spectra show the increase in band gap with increasing doping parameter. Fluorescence (FL) spectra exhibits the excitonic peak at 354 nm relating to near band edge emission for undoped and nickel doped ZnO nanoparticles. The signature of ferromagnetism of the Zn0.91Ni0.09O sample has been evidenced by SQUID. Dielectric properties and magnetoelectric (ME) coupling of the samples have also been discussed.