Disappearance Of Ferromagnetism Research Articles

Phase transformations in reduced-activation duplex alloy Fe52Mn30Cr18 under isothermal heat treatment

Reduced-activation Fe-Mn-Cr alloys have been regarded as the candidate structural materials for advanced nuclear systems. The phase stability of one alloy at high temperature is critically required for its application. In this work, a series of phase transformations in a new type reduced-activation duplex alloy Fe52Mn30Cr18 under isothermal heat treatment were investigated. The ferrite phase in the alloy converted to χ phase after the alloy treated at 550°C for 3 h. The cube-on-cube orientation relationship was found between χ phase and the original ferrite phase with the lattice constant ratio of 3:1. The χ phase was eutectoid decomposed into austenite and σ phases with lamellar structures due to the diffusion of Cr atoms at temperatures above 800 °C. According to the evolution of the crystal structure and magnetic properties, the transformation from ferrite to χ phase was considered as atomic rearrangement in the adjacent unit cells, accompanied by the disappearance of ferromagnetism in the alloy. The microhardness results indicated that the χ phase formation could be one of the potential reasons for the intermediate temperature embrittlement in the high Cr Fe-based alloys.

Electrical resistivity across the tricriticality in itinerant ferromagnet

We investigate the discontinuous ferromagnetic phase diagram near tricritical point in UCo1-xRuxAl compounds by electrical resistivity measurements. Separation of phases in UCo0.995Ru0.005Al at ambient pressure and in UCo0.990Ru0.010Al at pressure of 0.2 GPa and disappearance of ferromagnetism at 0.4 GPa is confirmed. The exponent of temperature dependence of electrical resistivity implies change from Fermi liquid-like behavior to non-Fermi liquid at 0.2 GPa and reaches minimum at 0.4 GPa. Our results are compared to results obtained on the pure UCoAl and explanation for different exponents is given.

Effect of the change in the interface structure of Pd(100)/SrTiO3 for quantum-well induced ferromagnetism

Measurements of temperature dependent magnetization of Pd(100) ultrathin films on SrTiO3(100) substrates which shows quantum-well induced ferromagnetism were performed. We observed the jump in magnetization of Pd(100) due to the structural phase transition of SrTiO3, and then, the disappearance of ferromagnetism after temperature-cycle repetition. X-ray reflectivity measurement revealed that the density of a few layers in the Pd film decreased near the Pd/SrTiO3 interface after temperature cycles. This suggests that the structural change affects the quantum-well induced ferromagnetism, and lowering of the crystallinity of Pd at the interface has a negative effect on quantum-well induced ferromagnetism of Pd(100) ultrathin films.

Magnetic, electrical and structural properties of annealed ferromagnetic (Zn,Sn)As2:Mn thin films on InP substrates: comparison with undoped ZnSnAs2

We report the magnetic and electrical properties in Mn-doped and undoped (Zn,Sn)As2 epilayers, that were annealed at slightly higher temperatures than the growth temperature. (Zn,Sn)As2 :Mn were epitaxially grown on InP (001) substrates at 300°C, and showed room-temperature ferromagnetism. The hole concentration, saturation magnetization and Curie temperature were measured and evaluated as a function of annealing temperature. The Curie temperature had a tendency to slightly increase at annealing temperatures up to 340°C, and completely disappeared at 400°C. The ferromagnetism could be attributed to hole-mediated ferromagnetism resulting from Mn ion substitutions at both the II-group Zn and IV-group Sn sites, especially from the large solubility of Mn2+ substitution at Zn sites. The disappearance of ferromagnetism may be explained by several types of mechanisms : migration of mobile interstitial Mn atoms, diffusion of substitutional Mn ions to the surface, substitution of interstitial Mn atoms on Zn vacancies, and formation of MnAs clusters. It is noteworthy that the growth of magnetic semiconductor thin films from substrate lattice matching is essential for avoiding magnetic secondary phases such as MnAs clusters.

Ag-related alloy formation and magnetic phases for Ag/Co/Ir(111) ultrathin films

The Kerr intensity versus the Ag thickness for Ag grown on the top of Co/Ir(111) exhibits an oscillating behavior with a period around one monolayer which should be due to the morphological change related electronic structure differences of the Ag layer. From systematical investigations of Ag/Co/Ir(111) films with the Co layer thinner than 4monolayers at temperatures below 900K, a magnetic phase diagram has been established. As the annealing temperature increases for Ag/Co/Ir(111) films, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface.

Intercalation and Substitution Effects of Si in Weak Itinerant Ferromagnetic Ni3Al

The Si-intercalated compounds Ni 3 AlSi x ( x = 0, 0.01, 0.02, 0.05, 0.07, and 0.1) and Si substituted compounds Ni 3 Al 1- y Si y ( y = 0, 0.01, 0.015, 0.02, 0.04, 0.06, 0.1, and 0.2) were synthesized and investigated by macroscopic and microscopic magnetic measurements. We found that the common features to these two systems are: (i) drastic disappearance of ferromagnetism in Ni 3 Al with only a small Si concentration x c ≃0.01 or y c ≃0.02, (ii) T 4/3 -temperature dependences in both square spontaneous magnetization M (0, T ) 2 and inverse initial susceptibility χ 0 -1 near the Curie temperature T C for ferromagnetic metal. T 4/3 -temperature dependence of χ 0 -1 are found to hold even for the paramagnetic metals that are classified as nearly itinerant ferromagnets very close to the ferromagnetic instability. On the other hand, our 27 Al NMR results show that the Si intercalation leads to the changes of the hyperfine coupling constant A hf , the characteristic parameters of spin fluctuations, T 0 and T ...

Structural defects-mediated room-temperature ferromagnetism in Co-doped SnO 2 insulating films

Co-doped SnO 2 insulating films with different crystal quality were fabricated using magnetron sputtering. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectrum reveal a solid solution of Co dopants in SnO 2 lattice, where Co is in 2+ oxidation state and substitutes for Sn 4+. The as-grown films with different crystallinity exhibit clearly room-temperature ferromagnetism, and the films with more structural defects show higher saturated magnetic moment. With the increase of annealing time, the crystallinity of the films is improved and the defects decrease, leading to the decrease or disappearance of ferromagnetism in the films. The results show that the ferromagnetism in Co-doped SnO 2 films is intrinsic and can be considerably influenced by the concentration of structural defects.

Magnetization study of Fe-doped ZnO co-doped with Cu: Synthesized by wet chemical method

Fe- and Cu-doped ZnO of nominal compositions Zn0.95Fe0.05O and Zn0.94Fe0.05Cu0.01O were synthesized by a wet chemical route. X-ray diffraction analysis of the samples annealed at 575 K showed that they are single phase without any secondary phases. DC magnetization measurements of Cu co-doped samples (Zn0.94Fe0.05Cu0.01O) as a function of field at room temperature showed ferromagnetic signature while the samples without Cu co-doping (Zn0.95Fe0.05O) are paramagnetic in nature. On increasing the temperature of annealing from 575 K to 1,075 K an impurity phase emerges in both the samples, which has been identified as a variant of ZnFe2O4. Both the samples heated at and above 1,075 K are found to be paramagnetic at room temperature. These observations, the absence of room temperature ferromagnetism in Zn0.95Fe0.05O and the disappearance of ferromagnetism in Zn0.94Fe0.05Cu0.01O on raising the temperature of annealing clearly rules out the likelihood of room temperature ferromagnetism arising from the impurity phases like γ-Fe2O3 and/or ZnFe2O4 that might have been formed during the synthesis. Our results strongly suggest that room temperature ferromagnetism in Zn0.94Fe0.05Cu0.01O can be attributed to the formation of a secondary phase of Cu-doped ZnFe2O4.

Ferromagnetism in the Kondo-lattice model

We propose a modified Rudermann, Kittel, Kasuya, Yosida (RKKY) technique to evaluate the magnetic properties of the ferromagnetic Kondo-lattice model. Together with a previously developed self-energy approach to the conduction-electron part of the model, we obtain a closed system of equations which can be solved self-consistently. The results allow us to study the conditions for ferromagnetism with respect to the band occupation n, the interband exchange coupling J, and the temperature T. Ferromagnetism appears for relatively low electron (hole) densities, while it is excluded around half-filling $(n=1).$ For small J the conventional RKKY theory $(\ensuremath{\sim}{J}^{2})$ is reproduced, but with strong deviations for very moderate exchange couplings. For not-too-small n a critical ${J}_{c}$ is needed to produce ferromagnetism with a finite Curie temperature ${T}_{C},$ which increases with J, then running into a kind of saturation, in order to fall off again and disappear above an upper critical exchange J. Spin waves show a uniform softening with rising temperature, and a nonuniform behavior as functions of n and J. The disappearance of ferromagnetism when varying T, J, and n is uniquely connected to the stiffness constant D becoming negative.

Disappearance of Bulk Ferromagnetism of Iron at High Pressures above 9 GPa

At high pressures exceeding 9 GPa, the disappearance of the ferromagnetism of iron has been detected by magnetization measurements. The value of this critical pressure is consistent with the theories in which the Stoner condition for ferromagnetism has been examined along with the martensitic transition path from the bcc to the hcp structure of iron. The sluggish or hysteresis behavior of the compressed iron is discussed.

Pressure-induced structural phase transition in a ferromagnet CrTe

We studied the crystal structure of CrTe at pressures up to 51 GPa in the temperature range of 35∼300 K by in situ angle-dispersive X-ray diffraction using synchrotron radiation. A pressure-induced transition from the NiAs-type structure (space group P6 3/mmc ) to the MnP-type one ( Pbnm) took place at pressures between 13 and 14 GPa. The values of the bulk moduli at room temperature for the low-pressure phase and the high-pressure phase are 43.8(5) and 45.8(5) GPa, respectively. The pressure derivative of the axial ratio c h/ a h is determined to be −2.5×10 −3 GPa −1 at room temperature in the low-pressure phase. The relationship between the disappearance of ferromagnetism and structural phase transition is also discussed.

Magnetic phase diagram of ultrathin Co/Si(111) film studied by surface magneto-optic Kerr effect

A magnetic phase diagram of the ultrathin Co/Si(111) film deposited at 300 K has been established by the surface magneto-optic Kerr effect technique. The temperature, where ferromagnetism vanishes, increases from 375 to 625 K as the coverage of the Co film increases from 3.5 to 16 monolayers. A quantitative calculation of the normalized Auger signal of CoSi2 shows that the calculated values lie between the experimental measured Auger signals before and after ferromagnetism vanishes for films with coverage between 3.5 and 9.1 monolayers. For samples with higher coverage, the experimental data are smaller than that by calculation. This may be qualitatively explained by Co atoms escaping from the CoSi2 phase to diffuse into the Si substrate. The disappearance of ferromagnetism is mainly attributed to silicide formation.

Electronic structure of Ni-Cu alloys: Thed-electron charge distribution

This work investigates charge redistribution in a series of Ni-Cu alloys using x-ray photoemission spectroscopy (XPS) and Ni/Cu ${L}_{3,2}$- and $K$-edge x-ray-absorption near-edge structure (XANES). XPS results show that the constituent $d$ bands are well separated and shifted to a slightly higher binding energy upon dilution into the other host, indicating that the atomic sites in the alloy are not as well screened relative to the pure metal. However, no significant $d$-band narrowing is observed, suggesting that there is modest $d\ensuremath{-}d$ interaction in the alloys. In contrast to the XPS observation, XANES results show a reduction in white-line intensity at both edges relative to the pure metal suggesting that both Ni and Cu sites gain $d$ charge. The unoccupied Ni $d$ band is far from fully occupied even at infinite dilution. The discrepancy between the implications of the XPS and XANES results is dealt with using a charge redistribution model in which $s\ensuremath{-}p\ensuremath{-}d$ rehybridization takes place at both sites within the framework of electroneutrality and electronegativity considerations. It appears that both Ni and Cu gain a small but measurable amount of $d$ charge in alloy formation through rehybridization (loss of non-$d$ conduction charge). Possible connection between these results and the disappearance of ferromagnetism in ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ alloys at $xg0.6$ is discussed.

Structural and Magnetic Properties of Mechanically Deformed Heusler Alloys.

The deformation of Heusler alloys during ball milling resulted in a drastic decrease in the crystallite size to a nanometer level. Nanocrystalline Heusler alloys with a disordered bcc structure were obtained by mechanical milling. This was accompanied by the disappearance of ferromagnetism. These nanocrystalline states were stable up to 523 K and ferromagnetism appeared after annealing at around 413 K. The nanocrystalline state with a L21 type structure was obtained by annealing at around 473 K. The nanocrystalline state lost stability and recrystallized with a D03 type structure when annealed at 573 K. The ferromagnetism almost disappeared in the powders after annealing at 573 K. In the recrystallized state, an increase in magnetization was observed with increasing annealing temperature. This came from the increase in the long range order parameter for the L21 type structure at room temperature.

Existence of the spin-glass state in amorphous Fe.

The volume dependence of magnetic properties in amorphous Fe has been calculated on the basis of the finite-temperature theory of amorphous metallic magnetism which self-consistently determines the distribution of local magnetic moments. The calculated magnetic phase diagram on the T-V plane and magnetization-vs-volume curve show the existence of the spin-glass state in a wide range of volume (10.50 A${\mathrm{\r{}}}^{3}$\ensuremath{\lesssim}V\ensuremath{\lesssim}12.5 A${\mathrm{\r{}}}^{3}$) after disappearance of ferromagnetism. The results verify our previous conclusion of the spin-glass state in amorphous Fe, which is expected from the experimental data of Fe-rich amorphous alloys containing early transition metals and rare-earth metals, but disagree with those obtained from the supercell approaches in the ground-state electronic structure calculations. It is shown that the nonlinear magnetic couplings between the nearest-neighbor Fe local moments and the local environment effects on the amplitude of Fe local moments via structural disorder lead to the spin-glass state in the weak magnetic region, while the volume expansion develops the ferromagnetic couplings and therefore the ferromagnetism.

Finite-temperature magnetism of disordered Fe-Co alloys.

The magnetic properties of disordered Fe-Co alloys have been investigated for both bcc and fcc structures using the finite-temperature theory of local-environment effects. The calculated magnetization, Curie temperature, effective Bohr magneton number, Weiss constant, and high-field susceptibility are shown to explain the concentration dependence of the experimental data. It is found that fcc Fe-Co alloys show ferromagnetic instability at 50 at. % Fe and form a spin-glass state after the disappearance of ferromagnetism at ${c}^{\mathrm{*}}$=78 at. % Fe. This is caused by the nonlinear magnetic coupling between Fe local moments and the local-environment effect on the amplitudes of Fe local moments. Strong local-environment effects around ${c}^{\mathrm{*}}$ are examined in detail with a prediction of the internal-field distribution for $^{57}\mathrm{Fe}$.

Magnetic properties associated with gt-transition and disappearance of ferromagnetism in Pd 1- x-yFe xMn y

Low temperature magnetic behavior of Pd 1- x-y Fe x Mn y was studied by the magnetization and Mössbauer measurements. The magnetic phase diagram and the magnetic properties associated with double and triple transitions are shown.

Magnetic resonance in amorphous Fe<inf>x</inf>B<inf>100-x</inf>sputtered films

We present magnetic resonance data on amorphous Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100-x</inf> sputtered films in the concentration range <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21 \leq x \leq 53</tex> . Simple ferromagnetism prevails for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x \geq 50</tex> and while reentrant magnetism is observed in the range <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40 \leq x \leq 50</tex> , the spin glass state persists down to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x \simeq 26</tex> . A finite moment exists even in a-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">79</inf> which is paramagnetic for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T \geq 4</tex> K. These results are in marked contrast to a recent report on this system which adduced not only the disappearance of ferromagnetism at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x\simeq38</tex> but also vanishing of the Fe moment.

Disappearance of ferromagnetism in amorphous (Fe1−xMnx)75P16B6Al3 ribbons

We present low field dc magnetization (M) measurements on the amorphous alloys (Fe1−xMnx)75P16B6Al3 for x≲0.35. Analysis shows that the spontaneous M goes to zero as T→TC− and Tf+ while the dc susceptibility diverges with T→TC+ and Tf−. It is demonstrated that the ferromagnet–spin glass transition line is significantly different from that obtained by scaling analyses of high field M data.

On the Disappearance of Ferromagnetism in Disordered Fe-Al Alloys

The magnetic properties of disordered Fe rich Fe-Al are investigated theoretically by using the coherent potential approximation (CPA) on the basis of the two different models. The self-consistent CPA calculation combined with the Korringa-Kohn-Rostoker method and the local spin density approximation is shown to reproduce the experimental result of the conceptration dependence of the bulk magnetization only for low Al concentration region. The ferromagnetism-to-spin glass second order transition at the critical Al concentration is obtained by the calculation on the basis of the tight binding model in which Fe atoms with magnetic moment antiparallel to the bulk magnetization are distinguished from Fe atoms with parallel moments. The calculated spin susceptibility well reproduces the experimental result.