Ac Initial Susceptibility Research Articles

Ferrofluid clustering driven by dilution: An alternating current susceptibility investigation

The influence of sample dilution upon cluster disruption/formation within a ferrofluid sample is investigated by monitoring the temperature dependence of the initial AC susceptibility. The effective magnetic response of the ferrofluid sample is described by a combination of Langevin’s functions modulated by the relative content of monomers and clusters. Deviations from the linearity found in the inverse susceptibility versus temperature (χ-1-T) data were successfully described via the disruption of clusters into monomers within the approach of a second order phase transition at the critical temperature T*. We found T* increasing monotonically from 386 K to 412 K as the stock ferrofluid sample is diluted up to a factor of 5. In the same dilution range, we found the normalized relative content of clusters increasing from about 38% up to 42%, whereas the average effective magnetic moment of the clusters increased by a factor of 1.7.

Quenching of initial ac susceptibility in single-domain Ni nanobars

The ac susceptibility measurement probes the dynamic properties of a magnetic material, which is believed to consist of magnetization rotation and domain wall motion contributions. Here we report the observation of a complete quenching of the initial ac susceptibility for a single-domain Ni nanobar array, when the ac field is aligned with the long axis of the bars. The vanishing of the susceptibility in one direction is a unique nanoscale phenomena, allowing an unambiguous determination of the magnetic state of the nanostructure and a clean separation of different contributions to its dynamic properties. For example, an unambiguous determination of the temperature-dependent surface anisotropy energy is obtained when the field is applied perpendicular to the long axis, even when the size of the nanobar is still large and the surface anisotropy does not dominate the magnetic energy.

Structural, magnetic and magnetostrictive properties of Co-doped Tb1-xHoxFe2 (0 ≤ x ≤ 1.0) alloys

Structural, magnetic, and magnetostrictive properties of Tb1-xHox(Fe0.8Co0.2)2 (0 ≤ x ≤ 1.0) alloys have been investigated by means of x-ray diffraction, ac initial susceptibility, an alternating gradient magnetometer and a standard strain gauge technique. A single (Tb,Ho)(Fe,Co)2 Laves phase with a cubic MgCu2-type structure is formed when 0.50 ≤ x ≤ 1.0, while the secondary phase with a PuNi3-type structure occurs at higher Tb content for 0 ≤ x ≤ 0.40. The easy direction of magnetization (EMD) is observed toward 〈111〉 when 0 ≤ x ≤ 0.75, accompanied by a rhombohedral distortion with large spontaneous magnetostriction coefficients λ111. The abnormal λ100 of –550 ppm for x = 0 and as larger than 300 ppm for 0.60 ≤ x ≤ 0.75 were observed, which can be ascribed to the filling of the 3d band due to Co substitution for Fe. The 20 at.% Co substitution for Fe moves the anisotropy compensation point to the Tb-rich side, and increases the Curie temperature TC with about 30 K as compared to the Co-free Tb1-xHoxFe2 alloys. The Tb0.25Ho0.75(Fe0.8Co0.2)2 alloy has a large magnetostriction at a relatively low magnetic field and a low anisotropy accompanied by the minimum coercivity, and may make it a promising magnetostrictive material.

Microstructure and magnetic studies of Mg–Ni–Zn–Cu ferrites

Soft Mg–Ni–Zn–Cu spinel ferrites having general chemical formula Ni x Mg 0.5− x Cu 0.1Zn 0.4Fe 2O 4 (where x = 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by standard double sintering ceramic method. The samples were characterized by X-ray diffraction at room temperature. The X-ray diffraction (XRD) study revealed that lattice parameter decreases with increase in Ni content, resulting in a reduction in lattice strain. The electrical and magnetic properties of the synthesized ferrites have been investigated as a function of temperature. The variation of initial permeability and AC susceptibility with temperature exhibits normal ferrimagnetic behavior. The variation of initial permeability with frequency is studied. The Curie temperature ( T C) in the present work was determined from initial permeability and AC susceptibility. The Curie temperature increases with Ni content.

Temperature-driven spin reorientation transition of magnetron sputtered nickel thin film

The temperature-driven spin reorientation transition of magnetron sputtered Ni/Si (111) systems has been studied. The relationship between ac initial susceptibility and temperature of nickel films with different thicknesses shows that the magnetization orientation changes from in-plane to out-of-plane with the increase of temperature. The temperature dependence of magnetoelastic, magneto-crystalline, and magnetostatic anisotropies determines the direction of the reorientation transition. The temperature-driven spin reorientation transition is supported by Hall coefficient measurements which show that its temperature dependence is similar to that of susceptibility.

Magnetic orderings and temperature dependence of the hysteresis loops of Nb 1−xFe 2+x

Investigations of the magnetic properties of NbFe 2 were performed by measuring the temperature dependence of hysteresis loops, initial AC-susceptibility and zero field cooled (ZFC)–FC magnetizations in a wide field range of 50 Oe⩽ μ 0 H⩽8000 Oe. Two well-defined peaks, at 4.8 and 21.4 K, were found in the AC-susceptibility curve, and only the first peak is sensible to a frequency change of the applied field. The peak changes to high temperature and its intensity decreases with increasing frequency, thus showing a relaxation phenomenon. In ZFC magnetization at 50 Oe, the peaks occur at 3.4 and 21.4 K. With increasing DC field, both peaks are shifted to low temperatures; however, the first peak disappears at fields higher than 100 Oe and a field higher than 1250 Oe suppresses the second peak. A third maximum merges from the magnetization curve at around 7 K in a field of 500 Oe, in which case, the maximum is suppressed by an external field higher than 4000 Oe. One can suggest that the ordered state of the stoichiometric NbFe 2 is a spin density wave (SDW) and both ferro- and antiferromagnetic spin fluctuations coexist.

Magnetic transitions in melt-spun nanocrystalline Fe–Si–Nb alloys

Magnetic transitions of the melt-spun Fe 70Si 20Nb 10 and Fe 77.9Si 12.1Nb 10 ribbons were investigated by means of temperature dependence of the initial AC-susceptibility measurements on these ribbons, and additionally, on stoichiometric and off-stoichiometric NbFe 2. The coexistence of nanocrystalline BCC Fe–Si and hexagonal Nb(Fe 1+ y Si y ) 2 could be evidenced from these measurements and also from Mössbauer experiment.

Structural, magnetic and magnetostrictive properties of Tb0.2Pr0.8(Fe0.4−xCo0.6+x)1.93 alloys

Structural, magnetic and magnetostrictive properties of Tb0.2Pr0.8(Fe0.4−xCo0.6+x)1.93 (0 ⩽ x ⩽ 0.2) alloys have been investigated by means of x-ray diffraction, ac initial susceptibility, a superconducting quantum interference device magnetometer and a standard strain gauge technique. A single (Tb, Pr)(Fe, Co)2 Laves phase with a cubic MgCu2-type structure is formed when 0.05 ⩽ x ⩽ 0.2. Increasing the Co content in the Tb0.2Pr0.8(Fe0.4−xCo0.6+x)1.93 alloys reduces the magnetocrystalline anisotropy constant K1 and improves the magnetostrictive properties at relatively low magnetic fields at room temperature. The Tb0.2Pr0.8(Fe0.35Co0.65)1.93 alloy with a single Laves phase has a spontaneous magnetostriction λ111 as high as 1900 ppm, a large magnetostriction λa (= λ∥ − λ⊥) at a relatively low magnetic field and a large λa/K1, and may make it a promising magnetostrictive material.

Magnetic properties of nanogranular Co xCu 1− x structures

CoCu nanogranular specimens with Co concentration of 5, 9 and 15 at.% were produced by annealing the parent melt-spun ribbon at 450 and 600 °C for 1 h. Their magnetization curves were quantitatively analyzed for the amount of superparamagnetic and ferromagnetic phases in the temperature of measurement ranging from 5 to 300 K. For the two lower concentrations and lower temperature of annealing, the great part of particles became already superparamagnetic within this temperature range. The temperature dependencies of both, magnetization measured in ZFC and FC regimes and initial AC susceptibility, indicated the presence of mictomagnetic state in the studied specimens at low temperatures. Analyses of the latter dependencies pointed to antiferromagnetic interactions existing between particles. Temperature dependencies of the coercivity for samples annealed at 600 °C were consistent with those published in the literature, whereas these dependencies for samples annealed at 450 °C indicated non-monotonous shape possibly reflecting interactions among superparamagnetic particles.

Oscillatory Curie temperature in ultrathin ferromagnets: experimental evidence

We determined the Curie temperature TC for 2–3 monolayers of Co/Cu(100) as a function of Cu-cap thickness by means of temperature-dependent measurements of the initial zero-field AC susceptibility. We see a strong reduction of TC that we discuss in terms of hybridization effects and modifications of the electronic structure of Cu. In addition, we found a small oscillatory contribution that reflects an indirect exchange interaction Jcap in the ultrathin Co films mediated by quantum-well states in the nonmagnetic Cu overlayer. We estimate that Jcap is of the order of 2μeV/atom which corresponds to an oscillation amplitude of 4K as observed in our experiment.

Harmonics of the ac susceptibility in ultrathin film ferromagnets

The initial ac susceptibility χ(ω) is used to monitor the odd harmonics in the response function. For an ultrathin film (Fe2/V5)50 superlattice we record for the first time harmonics χn(ω) up to 11ω in ultrathin films. A detailed Fourier analysis of χn(ω,T) yields the full temperature dependence of the hysteresis including the coercive field HC and the saturation magnetization MS in the vicinity of TC for 0.96<ε=T/TC<1.02.

Higher harmonics of theacsusceptibility: Analysis of hysteresis effects in ultrathin ferromagnets

The initial ac susceptibility is used to monitor the higher harmonic susceptibilities ${\ensuremath{\chi}}_{n}(T,{\ensuremath{\omega}}_{0})$ as a function of temperature. To our knowledge, for the first time we present ${\ensuremath{\chi}}_{n}(T,{\ensuremath{\omega}}_{0})$ up to $n=11$ for a $({\mathrm{Fe}}_{2}/{\mathrm{V}}_{5}{)}_{50}$ superlattice in the ultrathin-film limit. A detailed analysis of ${\ensuremath{\chi}}_{n}(T,{\ensuremath{\omega}}_{0})$ yields the full temperature-dependent hysteresis loops, including the coercive field ${H}_{C}(T)$ and the saturation magnetization ${M}_{S}(T),$ with high accuracy close to the Curie temperature ${T}_{C}.$ We show that this type of analysis allows for an independent determination of ${T}_{C}.$ In addition, the ${\ensuremath{\chi}}_{n}(T,{\ensuremath{\omega}}_{0})$ are calculated in the framework of a mean-field theory which compares well with the experimental data.

Harmonics of the ac susceptibility in ultrathin film ferromagnets

Abstract The initial ac susceptibility χ(ω) is used to monitor the odd harmonics in the response function. For an ultrathin film (Fe2/V5)50 superlattice we record for the first time harmonics χn(ω) up to 11ω in ultrathin films. A detailed Fourier analysis of χn(ω,T) yields the full temperature dependence of the hysteresis including the coercive field HC and the saturation magnetization MS in the vicinity of TC for 0.96

Structure, magnetic properties and magnetostriction of Dy 1− xPr x(Fe 0.9B 0.1) 1.93 (0≤ x≤0.8) alloys

The structure, magnetic properties and magnetostriction of Dy 1− x Pr x (Fe 0.9B 0.1) 1.93 (0≤ x≤0.8) alloys have been investigated using X-ray diffraction, ac initial susceptibility, vibrating sample magnetometry and standard strain gauge technique. The (Dy, Pr)(Fe, B) 2 magnetostrictive phase with cubic MgCu 2-type structure exists up to x=0.6. The magnetization of the alloys with 0≤ x≤0.5 decreases with increasing Pr content, due to the antiparallel alignment of Pr and Dy moments in the (Dy, Pr)(Fe, B) 2 phase. The coercivity remains relative low values in the alloys without Nd 2Fe 14B-type phase and increases pronouncedly with the emergency of this phase. The Pr solubility limit in the (Dy 1− x Pr x )(Fe, B) 2 phase is limited to the range 0.5≤ x≤0.6 determined by analysis of structure and magnetic properties. The magnetostriction λ a= λ ||− λ ⊥ of the Dy 1− x Pr x (Fe 0.9B 0.1) 1.93 alloys with 0≤ x≤0.3 increases with increasing Pr content, due to increment of the saturation magnetostriction λ s contributed by the large λ 111 of PrFe 2 and partial compensation of the anisotropy.

Structure, magnetic properties, and magnetostriction of Sm0.5R0.5(Fe1−xCox)2 compounds (R=Nd,Pr)

The structure, magnetic properties, and magnetostriction of Sm0.5R0.5(Fe1−xCox)2 compounds (R=Nd,Pr) have been investigated by using x-ray diffraction analysis, ac initial susceptibility, vibrating sample magnetometer, and standard strain techniques. It was found that the (Sm0.5Nd0.5)Fe2 compound with MgCu2-type cubic structure possesses low magnetocrystalline anisotropy and high ratio of magnetostriction to magnetocrystalline anisotropy. When Co is substituted for Fe, the Curie temperature of (Sm0.5Nd0.5)(Fe1−xCox)2 alloys exhibit a peak at x=0.25 and the saturation magnetization increases with increasing Co content when x&amp;lt;0.25 and decreases when x⩾0.5. The microstructure of (Sm0.5Pr0.5)(Fe1−xCox)2 alloys is nearly a single phase in the range of 0.75⩽x⩽1. The saturation magnetization and magnetostrictiom of the alloys exhibit a peak at x=0.25 and x=0.5, respectively.

Effect of boron on structure and magnetic properties of magnetostrictive compound Dy0.7Pr0.3Fe2

Crystal structure, magnetic properties and magnetostriction of Dy0.7Pr0.3(Fe1−xBx)2 (0⩽x⩽0.20) have been investigated by means of x-ray diffraction, ac initial susceptibility, a vibrating sample magnetometer, and a standard strain technique. The matrix of Dy0.7Pr0.3Fe2 alloy consists of (Dy,Pr)Fe2 phase with a cubic MgCu2-type structure and some amount of (Dy,Pr)Fe3 phase with a rhombohedral PuNi3-type structure. The introduction of boron effectively restrains the emergence of the iron-rich phase and thus decreases the amount of (Dy,Pr)Fe3 phase. Dy0.7Pr0.3(Fe1−xBx)2 alloy with x=0.05 contains small amount of (Dy,Pr)(Fe,B)3 phase, and those with x=0.10 and x=0.15 are essentially of a single (Dy,Pr)(Fe,B)2 phase. An unidentified minor phase appears when x=0.20. The lattice parameter of (Dy,Pr)(Fe,B)2 phase decreases monotonically with the boron substitution up to x=0.20, indicating that the boron atoms occupy the substitutional sites. The Curie temperature for (Dy,Pr)(Fe,B)2 phase obviously increases compared with the boron free one. The saturation magnetization at room temperature increases with increasing boron content for the alloys studied, suggesting that the partial boron substitution is beneficial to the increment of the exchange interactions in the Dy0.7Pr0.3Fe2 system. The increment of the magnetization originates from the decrement of iron content, because the Fe moment that aligns antiparallel with the Dy moment almost keeps constant in this system. Boron substitution for iron increases the lattice distortion and anisotropy, thus causes the decrease of the linear magnetostriction λa=λ∥−λ⊥ at the room temperature.

Thermoremanence and zero-field-cooled/field-cooled magnetization study ofCox(SiO2)1−xgranular films

A systematic study of $\mathrm{Co}({\mathrm{SiO}}_{2})$ granular films by means of transmission electron microscopy (TEM), dc and ac initial magnetic susceptibility, and thermoremanent magnetization (TRM) is presented. The experimental results are compared with simulations of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length \ensuremath{\Lambda}, which results in a larger average ``magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ``particle clusters'' of volume ${\ensuremath{\Lambda}}^{3},$ involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature $〈{T}_{B}〉$ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

Soft Magnetic Properties of Amorphous CoFeSiB Wire

Thermomagnetic analysis of the initial AC susceptibility of amorphous CoFeSiB wire shows strong stabilization of the domain structure. The initial susceptibility of the as-cast sample drastically decreases in the temperature range (100-250)°C due to directional reordering. A special form of the hysteresis loop is a consequence of large internal stresses introduced in the wire by the rapid melt quenching. It was shown that in the as-cast state the stabilization leads to the reduction of the number of Barkhausen jumps into one large Barkhausen jump at temperatures above 100°C. After stabilization at 150°C, the sample shows almost bistable behaviour even at room temperature.

Influence of aluminum additions on exchange interaction and magnetostriction of Sm/sub 0.9/Pr/sub 0.1/Fe/sub 2/

Structure, exchange interaction, magnetic properties and magnetostriction of series of cubic C15 MgCu/sub 2/-type rare-earth transition-metal (R-T) compounds of Sm/sub 0.9/Pr/sub 0.1/(Fe/sub 1-x/Al/sub x/)/sub 2/ (0/spl les/x/spl les/0.2) have been studied by X-ray diffraction, AC initial susceptibility, vibrating samples magnetometer and standard strain technology. The substitution of Al for Fe leads to an expansion of the lattice parameter /spl alpha/ by 5.1%. Curie temperature T/sub c/ is reduced continuously by replacing Fe with Al, which is ascribed to the decrease of the R-T coupling strength. The intersublattice coupling constant J/sub RT/, molecular field coefficient n/sub RT/, and exchange field B/sup ex/, are evaluated by a molecular field model. The decrease of J/sub RT/ with increasing Al content is related to the decrease of the number of (Sm,Pr)-Fe interacting pairs. Both the magnetostriction constant /spl lambda//sub 111/ and the polycrystalline saturation magnetostriction /spl lambda//sub s/ at room temperature decrease with the Al addition, which can be ascribed to the decrease of the coupling strength.

Giant volume magnetostriction in CMR manganites R1−xSrxMnO3(R=Sm, Nd)

Magnetization, ac initial magnetic susceptibility, resistance, magnetoresistance, thermal expansion and magnetostriction measurements are made on R1−xSrxMnO3 (R=Sm, x=0.33, 0.40, 0.45; R=Nd, x=0.33, 0.45) compounds. For all compounds in the TC region we observe a large volume contraction ΔV/V≈0.1% and unusual behavior of the volume magnetostriction ω, namely, a peak of anomalous magnitude of negative volume magnetostriction on the ω(T) curve. We obtain a giant negative volume magnetostriction of ∼5×10−4 at a relatively low magnetic field B=0.9 T and ∼10−3 at a high magnetic field B=13 T for Sm samples. The magnetostriction of Nd compounds is one order of magnitude less. For all compounds the giant magnetostriction is accompanied by colossal negative magnetoresistance equal to 78, 72, and 44% at B=0.9 T for Sm compounds with x=0.33, 0.40, and 0.45, respectively. All of the observed properties are explained in the framework of an electronic phase separation model.