Bloch's Law Research Articles

Insights into crystal structure, temperature-dependent magnetization and dielectric relaxation mechanism in Bi substituted samarium iron garnet

The single phase Sm3−xBixFe5O12 with x=0.0,0.2,0.4 and 0.6 samples are synthesized by solid-state reaction method. We have systematically studied the structural, morphological, temperature dependent magnetic and electric properties. All the samples crystallize in simple cubic crystal structure and belongs to Ia3‾d space group. The lattice constant and bond angle between Fe(a)−O−Fe(d) network is enhanced with Bi substitution. As a result, the ferrimagnetic transition temperature is also enhanced from 565K to 569K. We have examined the applicability of Bloch's and Cojocaru's laws to our temperature-dependent magnetization data and find that Bloch's T3/2 law is not suitable due to geometrical asymmetry, long wavelength excitation, and local atomic disorders. However, Cojocaru's law which accounts for the geometrical aspects, provides a better fit to our magnetization data. Further, the impedance plots at room temperature exhibit no relaxation which is attributed to the limited mobility of charge carriers. On the other hand, the high temperature impedance data represents dielectric relaxation at a particular frequency. This frequency is used to determine the relaxation time which is fitted to Arrhenius law and activation energy is evaluated. The activation energy lies in between 0.24−0.32eV which corresponds to the singly ionized oxygen vacancies. The conduction mechanism is analyzed with Variable range hopping model and the average hopping length and hopping energies are also determined for these garnet samples.

Elucidation of annealing effects on austenite–martensite and magnetic phase transitions in Mn2Ni1.6Sn0.4 ribbons synthesized from bulk alloys prepared by RF induction melting

We investigate the consequences of annealing on the structure, microstructure, magnetic, and electrical transport of Mn2Ni1.6Sn0.4 ribbons. The 40–60 μm thick ribbons were formed by melt spinning of the bulk ingot synthesized by RF induction melting. Ribbons were annealed at 600 °C for 12 hrs and the other at 800 °C for 2 hrs in vacuum. All ribbons crystallize into a cubic symmetry (space group F4¯3m). The orthorhombic phase appears as the secondary one. In ribbons annealed at 600 °C, the crystallite size decreases from ∼ 47 nm to ∼ 29 nm, and annealing at 800 °C doubles the crystallite size to ∼ 96 nm. The least amount of microstrains (17x10−4) in the as-synthesized ribbon is consistent with the occurrence of the superlattice peaks (111) and (200). Lower intensities of the superlattice peaks in 600 °C and 800 °C ribbons are consistent with higher microstrains (25x10−4 and 24x10−4). Hence, a higher degree of Ni-Mn ordering improves magnetic properties in annealed samples. The as-synthesized ribbon has an austenite-phase Curie temperature of TCA∼303K, which enhances to TCA∼320K and TCA∼310K after annealing at 600 °C and 800 °C. The ribbon annealed at 600 °C has the highest saturation moment. All the ribbons show the exchange-bias (EB) effect, and the most pronounced effect is seen in the 800 °C annealed ribbon. Microstructure uniformization and relative strengths of the Ni-Mn and Mn-Mn interactions improve the magnetics. Near TC, MT=M0TC-Tβ is satisfied (β = 0.31, 0.36, and 0.37, for as-synthesized, 600 °C and 800 °C annealed ribbons). The low-temperature spontaneous magnetization obeys the Bloch law MT=M01-AT32, with a deviation in 600 °C annealed ribbon due to spin freezing. The ρ-T has been fitted in the different temperature ranges using the various combinations in ρT=ρ0+AphT+BsdTα. At T < 85 K, the resistivity of the as-synthesized ribbon follows ρT=ρ0+BsdTα, with α≈1.47, while for the 600 °C annealed ribbon, α≈1.49. The exponent α∼1.5 supports the existence of the spin glass state in the lower temperature region of as-synthesized and 600 °C annealed ribbons. In the high-temperature range, the linear ρ-T behavior is described by ρT=ρ0+AphT and consistent with the electron–phonon scattering.

Determination method for magnetic momentum compensation temperature of ferrimagnets by extrapolating from narrow temperature range

We propose here an experimental method to determine the magnetization compensation temperature of ferrimagnets. Our method utilizes the first-order derivative of the spontaneous net magnetization with respect to the temperature and then analyzes the results within the context of the Bloch's law of ferrimagnetic materials. This method enables us to extrapolate the compensation temperatures, even when they exceed the measurement limit. Consequently, a wide range of compensation temperatures can be determined using a single experimental setup. The present method provides a versatile tool for analyzing huge compensation temperature in the emerging field of ferrimagnetic and antiferromagnetic spintronics.

Evaluation of magnetic inhomogeneities in non-stoichiometric Mg0.5Ca0.5Fe2O4 nanoferrite

Present work reports a systematic study on the evaluation of magnetic inhomogeneities in non-stoichiometric Mg0·5Ca0·5Fe2O4 nanoferrite (MCNF) by conducting exhaustive dc-magnetization, ac-susceptibility and 57Fe Mössbauer spectroscopic measurements and exchange bias investigations using training protocol down to 6 K. Rietveld fitting to PXRD established the formation of anticipated spinel fcc phase of MCNF (non-stoichiometric) along with a minute impurity phase of calcite. Scherrer method and HRTEM micrographs illustrated broad size distribution of MCNF nanoparticles with an average nanocrystallite size of ∼15 nm. Combined 57Fe Mössbauer spectroscopic and dc-magnetization analysis establishes coexistence of ferrimagnetic (67 %) & superparamagnetic (33 %) states at 300 K with notable Ms = 22 emu/g, Mr = 4 emu/g & Hc = 130 Oe and blocking of most of the nanoparticles of MCNF below 300 K. The coercivity followed the size-modified Kneller law for ferrimagnetic nanoparticles and the saturation magnetization abides the Bloch law. Moreover the frequency-dependent ac-susceptibility investigations revealed two magnetic transitions: (i) A transition at ∼ 330 K in the low frequency data attributed to the relaxation of blocked particles of bigger sizes under the superparamagnetic (SPM) regime and (ii) an irregularity at low temperatures is assigned to surface spin glass freezing. Surface spin glass freezing was further affirmed by the ageing experiments and dynamic scaling law. Furthermore, even the best fit to the dynamic scaling couldn't assert the existence of conventional spin glass phase due to slower spin-flip time of surface spins. A soft ferrimagnetic core of MCNF is enveloped with disordered surface spins, which manifest spin glass state. Concurrently, the findings of exchange bias at 30 K and training effect at 6 K affirmed that MCNF nanoparticles are presenting themselves as FM core- SG shell system. Our experimental findings suggested magnetic inhomogeneities comprised of superparamagnetism, ferrimagnetism and disordered surface spin glass state in the non-stoichiometric MCNF.

Modulating the magnetic anisotropy in SmCo/Co nanocomposites through alteration in spin–orbit coupling

In the present work, one-dimensional (1-D) as-synthesized SmCo/Co magnetic nanocomposites were prepared through magnetically-assisted chemical reduction method. The samples were prepared with the varying reaction times between 1 and 10 h labelled as SC1, SC2, SC5, SC7 and SC10. The Rietveld refinement of XRD patterns revealed the presence of Sm(OH)3 (P63/m), hcp-Co (P63/mmc), cubic-Co (Fm-3 m) and Sm2Co17 (P63/mmc) phases with varying phase fractions. The nanocomposites were found to possess the highest aspect ratio (∼7) with lowest crystallite size (∼50 nm) along with the highest developed lattice strain in SC5. The decisive influence of the reaction time was observed on the magnetic response of the samples indicating highest saturation magnetization of ∼ 150 emu/g and effective magnetic anisotropy of ∼ 1.5 x 106 erg/cm3 in SC5. The generalized form of Bloch law was employed to study the origin of magnetic anisotropy determined as spin–orbit coupling which was found to be highest in SC5. It was suggested that the enhanced magnetic properties of the nanocomposites are attributable to the high fraction of magnetic phases with the soft magnetic character and coupling between multiple phases.

Effect of hydrazine on structural, morphological and magnetic properties of SmCo-Co nanocomposites

In the present work, one-dimensional (1-D) SmCo-Co magnetic nanocomposites are prepared in a single-pot chemical synthesis in the presence of external magnetic field with varying amounts of hydrazine hydrate with samples named as: SC-2 mL, SC-4 mL, SC-6 mL and SC-8 mL. The Rietveld refinement of XRD patterns revealed the formation of Sm2Co17 (P63/mmc), Sm(OH)3 (P63/m) and hcp-Co (P63/mmc) phases. The 1-D nanocomposites are found to possess the highest aspect ratio (∼6.3), lowest crystallite size (∼49 nm) and highest developed strain (∼4.76 x 10−3) corresponding to SC-4 mL. The magnetic response of the samples is found to be affected by the hydrazine amount showing highest saturation magnetization (∼156 emu/g) and effective magnetic anisotropy (∼1.2 x 106 erg/cm3) for SC-4 mL with highest exchange coupling observed using Bloch law fitting. The results indicate suppressing behaviour of hydrazine amount to be utilized up to a certain limit.

Optical and temperature-dependent magnetic properties of Mn-doped CoFe2O4 nanostructures

Understanding and untangling magnetic materials are essential to implanting these materials for practical industrial applications. This work demonstrates how manganese (Mn) doping can significantly advance cobalt ferrite's magnetic and optical characteristics. We applied the hydrothermal approach to synthesize cobalt ferrite nanostructures, followed by Mn doping, to acquire CoMnxFe2−xO4 (0 ≤ x ≤ 0.1) nanostructures. The crystal structure of all the specimens confirms the formation of Fd-3 m space growth with a typical cubic structure. The estimated crystallite size values vary between 19.8 and 25.8 nm, and the lattice parameter significantly enhances from 8.31 Å to 8.38 Å as a result of Mn substitution. The fast Fourier transform infrared spectroscopy (FTIR) was performed between 400 and 4000 cm-1. The lattice vibrations ranging υ1 = 614–607 cm-1 and υ2 = 428–426 cm-1 were assigned to tetrahedral and octahedral sites, respectively. The high-resolution transmission electron microscopy (HRTEM) analysis further confirms the crystalline structure of CoMnxFe2−xO4. The room temperature magnetic measurements investigate that the magnetic coercivity of CoMn0.2Fe1.8O4 nanostructures is maximum (∼ 800 Oe) which was opted for temperature-dependent measurements. The low-temperature magnetic characteristics agree with Bloch's and Kneller's laws for saturation magnetization and coercivity of CoMn0.2Fe1.8O4 nanostructures. The optical properties confirm a blueshift and slight increment in bandgap energy of CoMnxFe2−xO4 nanostructures.

Enhancement of room-temperature unidirectional spin Hall magnetoresistance by using a ferromagnetic metal with a low Curie temperature

The unidirectional spin Hall magnetoresistance (USMR) effect is useful to detect the direction of magnetization in a ferromagnetic metal/nonmagnetic metal bilayer, which is the typical geometry used in a spin-orbit torque switching device. In this Letter, we demonstrate the enhancement of the USMR effect by reducing the Curie temperature ${T}_{\mathrm{C}}$ of the ferromagnetic (FM) layer in a FM/platinum bilayer. The USMR ratio was maximized when the thickness of each FM layer was consistent with the spin diffusion length. We found that the maximum USMR ratio can be doubled by replacing ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (${T}_{\mathrm{C}}=854$ K) with ${\mathrm{Ni}}_{85}{\mathrm{Cu}}_{15}$ (${T}_{\mathrm{C}}=527$ K) as the FM. This enhancement of the USMR effect is attributed to an increase of electron-magnon scattering in accordance with Bloch law. We also found that the use of ferromagnets with face-centered-cubic structures, such as Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{85}{\mathrm{Cu}}_{15}$, increased the USMR effect relative to that of body-centered-cubic Fe.

Room and low-temperature magnetic characterization of Cr doped CoFe2O4 nanostructures

The macroscopic magnetism in nanoparticulated systems is the consequence of numerous contributions, ranging from the spatial arrangement of nanoparticles to their intrinsic nanostructure. Understanding and unraveling these temptations is essential to scientifically and technologically introduce nanomaterials. In the present work, we investigate how the magnetic properties of Co ferrite can be significantly ameliorated by Cr substitution. At first, applied the co-precipitation strategy to synthesize stoichiometric Co ferrite nanoparticles followed by the Cr substitution to achieve CoCrxFe2-xO4 (0 ≤ x ≤ 1) nanoparticles. The phase and structural analysis by X-ray diffraction identified the formation of spinel structure with the Fd-3m space group. An increasing behaviour in X-ray density with Cr substitution in Co ferrite has been observed due to lattice contraction. Field-emission scanning electron microscopic images confirmed that the particle size ranged from 14 to 17 nm. Room temperature magnetic investigations showed that the magnetic coercivity of the synthesized nanoparticles increased up to 40% Cr substitution, followed by a monotonic decrease in the magnetic properties. Low-temperature magnetic properties were carried out for 40% Cr substituted Co ferrite. The increase in saturation magnetization with a reduction in temperature followed Bloch's law, and the trend of temperature-dependent coercivity was confirmed with Kneller's law.

Dielectric and temperature dependent magnetic studies of Al3+ substituted Ba0.4La0.1Sr0.5AlxFe12-xO19 hexaferrite for microwave application

The Al3+ substituted polycrystalline Ba0.4La0.1Sr0.5AlxFe12-xO19 (0 ≤ x ≤ 1) hexaferrite sample were prepared by the solid-state method. The structural, morphology, dielectric, and temperature dependent magnetic properties are studied systematically. The XRD and Rietveld refinement analysis revealed that all the samples have magneto plumbite hexaferrite structure with space group P6/mmc. The lattice parameters a and c are decreased from 5.888 Å to 5.874 Å and 23.109 Å to 23.085 Å, respectively. The c/a ratio is found to be between 3.92 and 3.93, which is ideal for M−type hexaferrite. It is observed that all the Raman peaks are shifted along the higher wavenumber with increasing Al3+ substitution. The surface morphology of these samples has uniform grain sizes and is closely packed. Also, all the constituent elements are present in proper composition except barium, which has a little higher atomic percentage than the calculated value. The complex dielectric constant (ε', ε“) decreased with Al3+ insertion (for x = 0: ε' = 3.44 k, ε” = 0.993 k; x = 1: ε' = 0.744 k, ε“ = 0.211 k @ 1 MHz). The hysteresis isotherm curves were analyzed by the law of approach to saturation magnetization in the temperature range of 10 K − 300 K. Further, the saturation magnetization and magnetic anisotropy constant K1(T) are found to be decreased, whereas anisotropy field Ha(T) and coercivity HC(T) are rises with an increase in temperature as well as Al3+ concentration. The Bloch’s spin wave theory is used to analyze temperature dependent saturation magnetization, and all the samples get deviated from Bloch’s T3/2 law as well as modified Kobler’s Bloch law. It might be due to finite size effects, long wavelength excitation, and the introduction of constant energy gap into spin wave spectrum, geometrical asymmetries, and other atomic disorders. Finally, Dyson’s approximation which includes T5/2 terms is perfectly fitted with the experimental data. The analysis makes it possible to predict the curie temperature and visualize the nature of the ferromagnetic coupling with the increasing Al3+ concentrations. Above that, the temperature dependent magnetic anisotropy constant K1(T) is linearly fitted with cubic of temperature dependent saturation magnetization MS(T), which confirmed that all the samples have uniaxial hexagonal crystal symmetry. The combination of magnetic and dielectric responses made the material suitable for permanent magnets and microwave applications.

Structural and temperature-dependent magnetic characteristics of Ho doped CoFe2O4 nanostructures

The magnetism in low-dimensional materials is the consequence of several factors, including the type of crystal structure and spatial distribution of nanoparticles. Understanding and exploring these challenges are vital to introduce nanomaterials for industrial applications. The present study reveals the substitutional effect of Holmium on the magnetic properties of cobalt ferrite. The nanostructures are prepared by co-precipitation procedure for the stoichiometric growth of CoHoxFe2-xO4 (0 ≤ x ≤ 0.1); Δx = 0.025 ferrite compositions. The crystal structure and phase determination of all the samples identify a typical cubic structure (Fd-3m); however, a secondary phase, appearing at x = 0.1, originates due to the formation of HoFeO3. An increasing trend in lattice constant, X-ray density and hopping lengths with Ho doping in Co ferrite is observed in the synthesized nanoparticles. The structural analysis by high-resolution transmission electron microscopy confirms the crystal structure of spinel ferrite. The magnetic characteristics measured at room temperature depict that the coercivity, saturation magnetization, and magneton number increase due to the impact of Ho3+ substitution. The saturation magnetization of CoHo0.05Fe1.95O4 nanoparticles as a function of temperature satisfies Bloch's law, and the coercivity of the sample remained in good agreement with Kneller's law.

Study of structural, electronic, magnetic and vibrational properties of CoFeTiSi quaternary Heusler alloy

We have synthesized CoFeTiSi quaternary Heusler alloy and experimentally measured its structural and magnetic properties. Detailed ab-initio first principle calculations have been performed to investigate the half-metallic behavior of this quaternary Heusler alloy and the effect of disorder on its half-metallicity. The saturation magnetization and Curie temperature are obtained as 1.243 μB/f.u. and 583 K respectively. The spinwave-stiffness constant (D = 3.259 nm2-meV) is obtained from temperature dependence according to Bloch's law. Dynamical stability of the alloy is confirmed from the phonon dispersion curve of the alloy. The first principle calculations show that the alloy is 100% spin-polarized at the Fermi level and the spin-polarization is resistant to the uniform lattice distortion up to a large extent. The effect of anti-site disorder of atoms on the total magnetic moment and the electronic structure is also explored and the moment (and the half-metallicity) is found to be nearly independent (resistant) for the disorders due to Co–Fe and Ti–Si atomic swaps whereas it is affected in case of Co–Si and Fe–Ti atomic swaps. This study reveals that the alloy may be a viable candidate for spintronic applications at room temperature.

Effects of carbon implantation on the low temperature magnetism behavior in amorphous Fe93Zr7 films

We present a detailed investigation on the effects of C-implantation on low temperature magnetic behavior in Fe93Zr7 amorphous films using spin wave theory, law of approach to saturation, and Alben and Becker's theory. Significant increase in Curie temperature with a decrease in coercivity were observed upon C-implantation. Bloch's law was used to analyze the temperature dependence of magnetization, and several important parameters were extracted such as the stiffness constant of the spin wave, and the exchange constant. The local random anisotropy constant decreases from 0.35 to 0.19 MJ/m3 with increasing carbon concentration from 0 to 11%. The ferromagnetic correlation length for which the anisotropy directions are assumed to be arbitrarily oriented increases significantly with increasing carbon content. By the use of the theory of Alben and Becker, we also extracted the local anisotropy from coercivity, and reasonable agreement was found with random magnetic anisotropy approach.

Enhanced Ferromagnetic Interaction at Mnp/Fe Interface: A Bloch Law Based Study for Temperature and Magnetic Field Evolution of Magnetization

Enhanced Ferromagnetic Interaction at Mnp/Fe Interface: A Bloch Law Based Study for Temperature and Magnetic Field Evolution of Magnetization

Large Tunneling Magnetoresistance in van der Waals Ferromagnet/Semiconductor Heterojunctions.

2D layered chalcogenide semiconductors have been proposed as a promising class of materials for low-dimensional electronic, optoelectronic, and spintronic devices. Here, all-2D van der Waals vertical spin-valve devices, that combine the 2D layered semiconductor InSe as a spacer with the 2D layered ferromagnetic metal Fe3 GeTe2 as spin injection and detection electrodes, are reported. Two distinct transport behaviors are observed: tunneling and metallic, which are assigned to the formation of a pinhole-free tunnel barrier at the Fe3 GeTe2 /InSe interface and pinholes in the InSe spacer layer, respectively. For the tunneling device, a large magnetoresistance (MR) of 41% is obtained under an applied bias current of 0.1 µA at 10 K, which is about three times larger than that of the metallic device. Moreover, the tunneling device exhibits a lower operating bias current but a more sensitive bias current dependence than the metallic device. The MR and spin polarization of both the metallic and tunneling devices decrease with increasing temperature, which can be fitted well by Bloch's law. These findings reveal the critical role of pinholes in the MR of all-2D van der Waals ferromagnet/semiconductor heterojunction devices.

Optimization of structure-property relationships in nickel ferrite nanoparticles annealed at different temperature

In this report, a detail analysis of the impact of annealing temperature on the structural, elastic, morphological, optical, and magnetic behavior of NiFe2O4 nanoparticles prepared by the citrate sol-gel method is presented. Analyzing the XRD patterns by the Rietveld method confirms that all the annealed samples have been crystallized to cubic spinel structure belonging to Fd3−m space group with a single phase. Rietveld analysis demonstrates the change in structural and microstructural parameters and movement of cations from tetrahedral to octahedral sites and vice-versa upon annealing. The quantitative estimation of Ni2+ & Ni3+ and Fe2+ & Fe3+ has been carried out using XPS analysis. Decreases in peak broadening and shift of five Raman active peaks towards higher frequency upon annealing have been analyzed using the phonon confinement model. The variation in elastic parameters with annealing temperature has been assessed by FTIR analysis. The UV analysis reveals the increase of the optical energy band gap and the decrease of Urbach energy with annealing temperature enhancement. A noticeable sharp absorption band at 748 nm in UV spectra is attributed to A32g(3F)→T31g(3F) electronic transition. Room temperature magnetic hysteresis loops exhibit an increase of saturation magnetization upon annealing which is discussed with reference to finite size effects and disorderly surface spins. The estimated value of magnetocrystalline anisotropy constant by Law of Approach to saturation (LAS) theory as well as coercivity value elucidates the annealing effect in changing the magnetic single domain state of the particle to a multidomain state. Analysis of ZFC and FC magnetization curve measured at 100 Oe in the temperature range 400 K–60 K reveals the significant impact of annealing temperature on magnetic anisotropy, inter-particle interaction, and blocking temperature. Exploring the magnetic hysteresis loop measured in the temperature range 60–400 K over field strength of ± 3 T demonstrates the significant role of annealing on magnetic exchange interaction. Temperature dependent behavior of saturation magnetization and coercivity has been analyzed using modified Bloch's law and Kneller's relation. The magnetic heating efficiency examined by the induction heating system reveals that the sample has enough potential for hyperthermia application.

Spin-Valve Effect in Fe3GeTe2/MoS2/Fe3GeTe2 van der Waals Heterostructures.

The van der Waals (vdW) materials offer an opportunity to build all-two-dimensional (all-2D) spintronic devices with high-quality interfaces regardless of the lattice mismatch. Here, we report on an all-2D vertical spin valve that combines a typical layered semiconductor MoS2 with vdW ferromagnetic metal Fe3GeTe2 (FGT) flakes. The linear current-voltage curves illustrate that Ohmic contacts are formed in FGT/MoS2 interfaces, while the temperature dependence of the junction resistance further demonstrates that the MoS2 interlayer acts as a conducting layer instead of a tunneling layer. In addition, the magnitude of the magnetoresistance (MR) of 3.1% at 10 K is observed, which is around 8 times larger than that of the reported spin valves based on MoS2 sandwiched by conventional ferromagnetic electrodes. The MR decreasing monotonically with increasing temperature follows the Bloch's law. As the bias current decreases exponentially, the MR increases linearly up to a maximum value of 4.1%. Our results reveal the potential opportunities of vdW heterostructures for developing novel spintronic devices.

The limited capacity of visual temporal integration in cats.

It has been long known that prolonging stimulus duration may increase the perceived brightness of a visual stimulus. The interaction between intensity and duration generally follows a rule, such as that described in Bloch's law. This visual temporal integration relationship has been identified in human subjects and in non-human primates. However, although auditory temporal integration has been extensively studied in the cat, visual temporal integration has not. Therefore, the goal of this study was to examine visual temporal integration in the cat. We trained five cats to respond when a brief luminance change was detected in a fixation dot. After training, we measured the success rate of detecting the luminance change with varying durations at threshold, subthreshold, and suprathreshold luminance levels. Psychometric functions showed that prolonging stimulus duration improved task performance, more noticeably for stimuli below 100 ms than beyond. Most psychometric functions were better fit to an exponential model than to a linear model. The gradually saturated performance observed here, as in previous studies, can be explained by the “leaky integrator” hypothesis, that is, temporal integration is only valid below a critical duration. Overall, we developed a task whereby visual temporal integration was successfully demonstrated in the cat. The effect of stimulus duration on detection success rate displayed a pattern generally consistent with previous human and non-human primate findings on visual temporal integration.

Sustained coherent spin wave emission using frequency combs

We demonstrate sustained coherent emission of spin waves in NiFe films using rapid demagnetization from high repetition rate femtosecond laser pulse trains. As the pulse separation is shorter than the magnon decay time, magnons having a frequency equal to a multiple of the 1 GHz repetition-rate are coherently amplified. Using scanning micro-Brillouin Light Scattering (BLS) we observe this coherent amplification as strong peaks spaced 1 GHz apart. The BLS counts vs. laser power exhibit a stronger than parabolic dependence consistent with counts being proportional to the square of the magnetodynamic amplitude, and the demagnetization pulse strength being described by a Bloch law. Spatial spin wave mapping demonstrates how both localized and propagating spin waves can be excited, and how the propagation direction can be directly controlled. Our results demonstrate the versatility of BLS spectroscopy for rapid demagnetization studies and enable a new platform for photo-magnonics where sustained coherent spin waves can be utilized.

Oxidation-controlled magnetism and Verwey transition in Fe/Fe3O4 lamellae

Abstract The structural and magnetic properties of Fe/Fe3O4 nanocomposites, synthesized by combined high energy ball milling and controlled oxidation, have been studied. An X-ray diffraction analysis of the crystal structure of the nanocomposites confirmed the coexistence of Fe and Fe3O4 phases. An increase of the oxygen concentration during oxidation process led to the formation of a higher fraction of the Fe3O4 phase with good crystallinity and stoichiometry. The morphology of the nanocomposites revealed a lamella-like structure with a thickness of about 30 nm. The saturation magnetization decreased when the phase fraction of Fe3O4 increased. The coercivity was enhanced at low temperatures (≤100 K) but decreased at high temperatures, due to thermal fluctuation effects on the anisotropy in the Fe3O4 phase. Interestingly, the lamellae exhibited a sharp Verwey transition near 120 K, which is often suppressed or absent in nanostructured Fe3O4 due to the poorly crystalline, off-stoichiometric characteristic. The temperature dependence of high-field magnetization of the lamellae is analyzed by the modified Bloch law. Our study demonstrates the possibility of tuning the magnetism in iron/iron oxide nanosystems through controlled oxidation.