Itinerant Electron Ferromagnets Research Articles

Itinerant electron ferromagnetism of Fe xCo 1− xSi ( x=0.3, 0.4, 0.5, 0.7) under high pressure

The intermetallic compound Fe x Co 1− x Si, with a cubic B20-type crystal structure, shows a weak itinerant electron ferromagnetism in the Fe-concentration range of 0.2< x<0.95. We have measured AC susceptibility for Fe x Co 1− x Si ( x=0.3, 0.4, 0.5 and 0.7) under various pressures up to 7.5 GPa using a cubic anvil press in the temperature range from 4.2 to 300 K . The Curie temperature was found to decrease with pressure P proportional to P 3/4 for all the concentrations.

Magnetization Measurements of Weak Itinerant Electron Ferromagnet Ni–Pt Alloy

Disordered Ni–Pt alloys with a face-centered cubic structure can be synthesized in any proportions. With increasing the concentration of Pt, the magnetic moment and the Curie temperature decrease, and then the ferromagnetism vanishes at about 58 at.% Pt. It has been supposed that the alloys in the critical concentration region for the onset of ferromagnetism are good reference materials for studying weak itinerant electron ferromagnets (WIEF). Almost all the magnetic data of Ni–Pt alloys have been discussed using a thermodynamic treatment based on WIEF model with single-particle excitation (so-called Stoner– Wohlfarth model) so far. However, some inconsistencies with experiments remain, and these discrepancies have been considered to be due to the influence of spin fluctuations. Recently, Takahashi pointed out that not only the thermal spin fluctuation but also the ‘‘quantum spin fluctuation’’ plays important roles in the magnetic properties of WIEF and explained some experimental results in the wide temperature range from the ground state to the paramagnetic state. The detailed magnetic properties of Ni–Pt alloys has not been discussed using Takahashi’s theory. In this work, we have performed the precise magnetization measurement and high field magnetization measurements on a Ni0:45Pt0:55 alloy near the critical concentration to study the effect of the quantum spin fluctuation proposed by Takahashi. The polycrystalline sample of Ni0:45Pt0:55 was prepared by repeated arc-melting the elements of 99.99% purity. Since the weight loss after melting was negligible, the nominal composition was accepted as being accurate. To get the homogenized sample, the as-melted ingot was annealed at 1000 C for 3 days, and then quenched in water. By powder x-ray diffraction, the sample was confirmed to be a single phase of the disordered Ni–Pt alloy with the face-centered cubic structure. The determined lattice parameter is a 1⁄4 3:7807 A. Magnetization were measured using a SQUID magnetometer (Quantum Design) and a VSM (Oxford) in magnetic fields H up to 10 kOe and up to 130 kOe, respectively, and in the temperature T range from 5K to 280K at Center for Low Temperature Science, Tohoku University. Figure 1 shows the magnetization curves of Ni0:45Pt0:55 in 100 T 280K. above 200K increases linearly with increasing H. We determined the T-dependence of the susceptibility from the slope of the magnetization curves above 200K. The observed 1= vs. T plot is well expressed by the Cuire–Weiss law, and the paramagnetic Curie temperature p and the effective magnetic moment peff were determined to be 57.6 K and 2:21 B/Ni, respectively. Figure 2 shows the typical magnetization curves in 5 T 60K. The data in lower field region (H 10 kOe) is shown in the inset of Fig. 2. As seen in this figure, the magnetization is not saturated even in the fields up to 130 kOe. The isothermal ðH;TÞ vs. H= ðH;TÞ plot is Fig. 1. Magnetization curves of Ni0:45Pt0:55 at various temperatures from 100 to 280K. The inst shows the temperature dependence of the inverse susceptibility 1= . The straight line in the inset is least-squares fits to the data.

Magnetic Properties in Fe-Ni Invar Alloys Irradiated by High-Energy Ions

ABSTRACTAnomalously large shift of the Curie temperature has recently been observed in Fe-Ni Invar alloys irradiated with high-energy heavy ions. This large effect can be attributed to the large positive magneto-volume effect essentially originated in the itinerant electron ferromagnetism in Fe-Ni Invar alloys. To investigate the mechanism of the large modification of the ferromagnetism and the structure of the modified portion, measurements of the beam energy dependence of AC-susceptibility-temperature curves have been made. It was found that the amount of the shift of the Curie temperature did not change by increasing the beam energy. On the contrary, the intensity of the susceptibility of the modified portion increased with increasing the ion beam energy. The high-density electronic excitation is considered to be responsible for the large modification of the ferromagnetism in Fe-Ni Invar alloys.

A μSR study of the magnetic ground state of Ni 3Al

The magnetic ground state of a well-characterised and highly ordered Ni 3Al sample has been studied by muon spin relaxation. Although it is widely accepted that Ni 3Al is an archetypal homogeneous weak itinerant electron ferromagnet with a Curie temperature of 40 K, our μSR measurements provide no evidence of long-range magnetic order, but instead indicate the evolution of an inhomogeneous magnetic ground state at temperatures below 40 K in which relatively static magnetic clusters coexist with a spin fluctuating state.

Magnetic response ofFe1−xCoxSialloys: A detailed study of magnetization and magnetoresistance

Results of dc magnetization and magnetoresistance measurements are presented for the heliferromagnetic ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ alloys with $x=0.35$ and 0.45. The field and temperature dependence of the magnetization suggests the presence of Stoner-like excitations in these alloys which in turn indicates significant itinerant character of the ferromagnetic state. Similarities with other weak itinerant electron ferromagnets are pointed out. While the relatively large and unusual positive magnetoresistance in the ferromagnetic state of these alloys has earlier been rationalized in terms of quantum interference effect, magnetism probably plays a role in the magnetoresistance of these alloys near the Curie temperature and even at temperatures well inside the paramagnetic regime.

Linear vs. non-linear magnetic and charge relaxation in itinerant ferromagnets: magnetoresistive manganites

An interplay between possible mechanisms of magnetic and charge relaxation in itinerant electron ferromagnets is investigated. It is shown that non-linear ones due to emission (absorption) of spin or charge fluctuations by magnons may dominate giving rise to non-linear quasielastic spin and charge fluctuations observed in the colossal magnetoresistive manganites.

Theory of itinerant-electron ferromagnetism

A theory of Kondo lattices or a $1/d$ expansion theory, with $d$ spatial dimensionality, is applied to studying itinerant-electron ferromagnetism. Two relevant multi-band models are examined: a band-edge model where the chemical potential is at one of band-edges, the top or bottom of bands, and a flat-band model where one of bands is almost flat or dispersionless and the chemical potential is at the flat band. In both the models, a novel ferromagnetic exchange interaction arises from the virtual exchange of pair excitations of quasiparticles; it has two novel properties such as its strength is in proportion to the effective Fermi energy of quasiparticles and its temperature dependence is responsible for the Curie-Weiss law. When the Hund coupling $J$ is strong enough, the superexchange interaction, which arises from the virtual exchange of pair excitations of electrons across the Mott-Hubbard gap, is ferromagnetic. In particular, it is definitely ferromagnetic for any nonzero $J>0$ in the large limit of band multiplicity. Ferromagnetic instability occurs, when the sum of the two exchange interactions is ferromagnetic and it overcomes the quenching of magnetic moments by the Kondo effect or local quantum spin fluctuations and the suppression of magnetic instability by the mode-mode coupling among intersite spin fluctuations.

Determination of susceptibility and specific heat critical exponents for weak itinerant-electron ferromagnets from vibrating reed experiments

We report the observation of a linear relationship between the magnetic contribution to Young's modulus, $\ensuremath{\Delta}{E/E}_{0},$ and inverse magnetic susceptibility ${\ensuremath{\chi}}^{\ensuremath{-}1}$ for amorphous weak itinerant-electron ferromagnets ${\mathrm{Fe}}_{90}{\mathrm{Zr}}_{10}$ and ${\mathrm{Fe}}_{91}{\mathrm{Zr}}_{9}$ in the asymptotic critical region near the ferromagnetic-paramagnetic phase transition. The proportionality $\ensuremath{\Delta}{E(T)/E}_{0}\ensuremath{\propto}{\ensuremath{\chi}}^{\ensuremath{-}1}(T)$ is shown to provide as accurate a means of determining the asymptotic critical exponent $\ensuremath{\gamma}$ and the leading ``correction-to-scaling'' amplitudes for susceptibilty from the $\ensuremath{\Delta}{E/E}_{0}$ data as a direct measurement of magnetic susceptibilty does. Similarly, the well-known relation between the magnetic contributions to sound velocity and specific heat is fully exploited to extract accurate estimates for the universal critical amplitude ratio ${A}^{+}{/A}^{\ensuremath{-}}$ and the asymptotic critical exponents ${\ensuremath{\alpha}}^{\ifmmode\pm\else\textpm\fi{}}$ for the specific heat from the sound velocity data. The presently determined values of ${\ensuremath{\alpha}}^{\ifmmode\pm\else\textpm\fi{}}$ and $\ensuremath{\gamma},$ together with the reported value for spontaneous magnetization critical exponent $\ensuremath{\beta},$ not only obey the scaling equalities ${\ensuremath{\alpha}}^{+}={\ensuremath{\alpha}}^{\ensuremath{-}}$ and $\ensuremath{\alpha}+2\ensuremath{\beta}+\ensuremath{\gamma}=2$ but also assert that the atomic magnetic moments in the alloys in question interact with one another through an attractive interaction which decays faster than ${1/r}^{5}$ with the interatomic spacing, r.

The role of scattering angle in spin-polarised electron energy loss spectroscopy

We study the dependence on scattering angle of the Stoner excitation spectrum of model itinerant-electron ferromagnets in spin-polarised electron energy loss spectroscopy (SPEELS) experiments. It is found that, changing the scattering angle, essentially changes the weight of the contributions to the spectrum of the various possible interband excitations. Thus, high resolution SPEELS measurements may be used to probe the predicted interband density of Stoner states and the underlying band structure of materials exhibiting itinerant magnetism.

金属間化合物の粉末冶金プロセスと諸特性 Ｌ１２型構造を有する新規強磁性金属間化合物Ｍｎ３Ｇｅの合成

A new intermetallic compound, Mn3Ge, has been synthesized from the elemental components under high-pressure condition. The Rietveld analysis of the powder X-ray diffraction data revealed that the compound crystallizes into the cubic L12-type (Cu3Au-type) structure with the lattice parameter of a=0.38019 (3)nm. The result, as well as our former results on high-pressure synthesis of manganese germanides, suggests a remarkable change in binary Mn-Ge phase diagram under high-pressure condition. Mn3Ge exhibits metallic conduction and ferromagnetic behavior with a Curie temperature of 400K. The saturation magnetic moment is 0.87μB/Mn, indicating a possible itinerant electron ferromagnetism. The magnetic properties of Mn3Ge are discussed with isomorphous compounds and structurally related perovskite-type Mn3MC (M=A1, Ga, In, Zn, Sn) compounds.

Itinerant-electron metamagnetism and strong pressure dependence of the Curie temperature

A temperature dependence of the Landau coefficients for an itinerant-electron metamagnet is discussed on the spin fluctuation model, by taking the magnetovolume effect into account. Both of the first- and second-order transitions at the Curie temperature ${T}_{C}$ are possible to take place for an itinerant-electron ferromagnet with a negative mode-mode coupling among spin fluctuations. It is shown that ${T}_{C}$ depends strongly on the pressure near the boundary between the first- and second-order transitions. On the other hand, the spontaneous magnetization at $T=0$ does not depend so much on the pressure. These results are consistent with the observed results for the pyrite compound ${\mathrm{CoS}}_{2},$ the Laves phase compound $\mathrm{Lu}(\mathrm{C}\mathrm{o},\mathrm{A}\mathrm{l}{)}_{2}$ and other ferromagnets which show a metamagnetic transition in the paramagnetic phase.

Non-Fermi-liquid nature of the normal state of itinerant-electron ferromagnets.

A century of research on magnetic phenomena had led to the view that the normal state of itinerant-electron ferromagnets such as Fe, Ni and Co could be described in terms of the standard model of the metallic state or its extension known as the nearly ferromagnetic Fermi liquid theory. In recent years, however, a large body of observations has accumulated from various complex intermetallic systems that raises the possibility that this assumption might be wrong. Here we examine this issue by means of high-precision measurements of the electrical transport and magnetic properties of pure ferromagnets-in particular, MnSi-in which the Curie temperature is tuned towards absolute zero by the application of hydrostatic pressure. With this method, it is possible for us to study the normal state over an extraordinarily large range of temperature of up to five orders of magnitude above the Curie temperature. Our results using MnSi reveal a particularly striking combination of properties-most notably a T3/2 power law for the resistivity-showing clearly that the normal state of this itinerant-electron ferromagnet cannot be described in terms of the standard model of metals.

Layer-dependent magnetization at the surface of a band ferromagnet

The temperature dependence of the magnetization near the surface of a band ferromagnet is measured with monolayer resolution. The simultaneous application of highly surface-sensitive techniques enables one to deduce the layer-dependent magnetization curves at a Fe(100) surface. Analysis of data is based on a simple mean-field approach. Implications for modern theories of itinerant-electron ferromagnetism are discussed.

Magnetic instability of a two-dimensional Anderson non-Fermi liquid

We show that in the Anderson model for a two-dimensional non-Fermi liquid a magnetic instability can lead to the itinerant electron ferromagnetism. The critical temperature and the susceptibility of the paramagnetic phase have been analytically calculated. The usual Fermi behavior is re-obtained by taking the anomalous exponent to be zero.

Quantum spin fluctuation theory of the magnetic equation of state of weak itinerant-electron ferromagnets

On the basis of the spin fluctuation mechanism, magnetic properties of itinerant-electron weak ferromagnets are discussed, for the wide temperature range from the ground state to the paramagnetic state, explicitly taking into account the effects of zero-point quantum spin fluctuations. Particular attention is focused on properties of the ordered phase. The temperature dependence of the spontaneous magnetic moment, for instance, is quantitatively analysed in close comparison with experiments. It is also shown that the fourth-order expansion coefficient of the free energy in powers of the static magnetic moment is temperature dependent, and therefore magnetic isotherms are not so simple as was originally anticipated in the Stoner–Wohlfarth theory.We explicitly examine the effect of the spin-wave mode on the transverse spin fluctuation amplitude, and show that this effect is crucial for the proper theoretical description of magnetic behaviours in the ordered state.

Superconductivity on the border of itinerant electron ferromagnetism in UGe 2

We report on the observation of a superconducting phase in the itinerant electron ferromagnet UGe 2. The superconductivity observed below 1 K and in a limited pressure range immediately below the critical pressure where ferromagnetism is abruptly suppressed, would seem to arise from the same electrons that produce band magnetism. This superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions and via a spin-triplet rather than a spin-singlet pairing normally associated with nearly anti-ferromagnetic metals.

Non-Fermi Behavior of the Itinerant-Electron Ferromagnet near the Quantum Phase Transition Point

We used the Renormalization Group (RG) method in the Hertz–Millis version to study the quantum phase transition of the itinerant-electron ferromagnet. Near the quantum phase transition point the system present a non-Fermi behavior in agreement with the experimental results. The importance of long-range interactions considered by Belitz–Kirkpatrick–Vojta was taken into consideration, showing the importance of the marginal parameters.

High-field and high-pressure effect on the field-induced itinerant metamagnetic transition in YCo 3H 1.8

High-field and high-pressure measurements have been done for the itinerant-electron ferromagnet YCo 3, its hydride YCo 3H 1.8 and Y(Co 0.88Fe 0.12) 3. The critical fields of the Co metamagnetic transition from the low-moment state to the higher ones, initially observed at 60 and 82 T, decrease upon hydrogenation due to the lattice expansion and the change of electronic structure. Using the experimental data on the pressure effect of the critical field, the main contributions to the change of the critical field after hydrogenation can be separated. The results are discussed within an itinerant model of the metamagnetic transition and compared with band structure calculations.

Pseudospin anisotropy classification of quantum Hall ferromagnets

Broken symmetry ground states with uniform electron density are common in quantum Hall systems when two Landau levels simultaneously approach the chemical potential at integer filling factor $\nu$. The close analogy between these two-dimensional electron system states and conventional itinerant electron ferromagnets can be emphasized by using a pseudospin label to distinguish the two Landau levels. As in conventional ferromagnets, the evolution of the system's state as external field parameters are varied is expected to be sensitive to the dependence of ground state energy on pseudospin orientation. We discuss the predictions of Hartree-Fock theory for the dependence of the sign and magnitude of the pseudospin anisotropy energy on the nature of the crossing Landau levels. We build up a classification scheme for quantum Hall ferromagnets that applies for single layer and bilayer systems with two aligned Landau levels distinguished by any combination of real-spin, orbit-radius, or growth direction degree-of-freedom quantum numbers. The possibility of in-situ tuning between easy-axis and easy-plane quantum Hall ferromagnets is discussed for biased bilayer systems with total filling factors $\nu=3$ or $\nu=4$. Detailed predictions are made for the bias dependence of pseudospin reversal properties in $\nu=3$ bilayer systems.

Exchange-enhanced Pauli spin paramagnetism in nanocrystallineNi3Al

The results of a detailed comparative bulk magnetization study of nanocrystalline (average crystallite size $\ensuremath{\simeq}25 \mathrm{nm})$ and polycrystalline samples of ${\mathrm{Ni}}_{3}\mathrm{Al}$ reported here clearly demonstrate that unlike crystalline ${\mathrm{Ni}}_{3}\mathrm{Al},$ which is a well-known weak itinerant-electron ferromagnet, no long-range ferromagnetic order exists in the nanocrystalline counterpart at any temperature $g~5$ K. Instead, nanocrystalline ${\mathrm{Ni}}_{3}\mathrm{Al}$ exhibits exchange-enhanced Pauli spin paramagnetism.