Itinerant Electron Ferromagnets Research Articles

Nonlocal correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets

We present an ab initio method for electronic structure calculations, which accounts for the interaction of electrons and magnons in ferromagnets. While it is based on a many-body perturbation theory we approximate numerically complex quantities with quantities from time-dependent density functional theory. This results in a simple and affordable algorithm which allows us to consider more complex materials than those usually studied in this context ($3d$ ferromagnets) while still being able to account for the nonlocality of the self-energy. Furthermore, our approach allows for a relatively simple way to incorporate self-consistency. Our results are in a good agreement with experimental and theoretical findings for iron and nickel. Especially the experimental exchange splitting of nickel is predicted accurately within our theory. Additionally, we study the half-metallic ferromagnet NiMnSb concerning its nonquasiparticle states appearing in the band gap due to spin-flip excitations.

Spin and Orbital Symmetry Breakings Central to the Laser-Induced Ultrafast Demagnetization of Transition Metals

The role of spin and orbital rotational symmetry on the laser-induced magnetization dynamics of itinerant-electron ferromagnets was theoretically investigated. The ultrafast demagnetization of transition metals is shown to be the direct consequence of the fundamental breaking of these conservation laws in the electronic system, an effect that is inherent to the nature of spin-orbit and electron-lattice interactions. A comprehensive symmetry analysis is complemented by exact numerical calculations of the time evolution of optically excited ferromagnetic ground states in the framework of a many-body electronic Hamiltonian. Thus, quantitative relations are established between the strength of the interactions that break the rotational symmetries and the time scales that are relevant for the magnetization dynamics.

Critical Behavior of the Magnetization in Heusler Alloy Co₂TiGa₀.₈Sn₀.₂

We report the critical behavior of the magnetization in Heusler alloy Co2TiGa0.8Sn0.2 as a potential candidate of high spin polarized materials with the characteristics of topological Weyl fermions. The Weyl semimetals are materials of great interest for applying spintronics devices, due to their unusual magnetoelectronic and magnetothermal properties. The magnetic properties of the synthesized Heusler alloy Co2TiGa0.8Sn0.2 are investigated around the Curie temperature TC. The magnetic isotherm measurements of Co2TiGa0.8Sn0.2 around TC exhibit that the critical exponent δ is deduced to be 4.455(4) in high maximum applied field of 50 kOe and 4.823(9) in low maximum applied field of 10 kOe, respectively. Both δ values are different from 3.0 derived by the conventional molecular field theory, but they are close to 5.0 predicted by the spin fluctuation theory for the itinerant electron ferromagnets. Possible origins for the discrepancy between experimental results and theoretical models are also discussed.

Magnetic Properties of Weak Itinerant Electron Ferromagnet Au<sub>4</sub>V

Precise magnetization measurements have been made on the weak itinerant electron ferromagnet CoVSb. The magnetic moment at 4.2 K and the Curie temperature T C are 0.16 μ B /f.u. and 45 K, respectively. Below 10 K, the decrease in the square of the spontaneous magnetization M s (T) 2 is proportional to T 2 . However, over a wide temperature range from 24 K to the Curie temperature, the decrease in M s (T) 2 is proportional to T 4/3 . The results obtained are analyzed using spin fluctuation theory.

Magnetic properties of ferromagnetic Heusler alloy Co2NbGa

The compound Co2NbGa crystallizes in the Heusler-type structure. Magnetization measurements of Co2NbGa have been made under ambient pressure using a SQUID magnetometer. The spontaneous magnetic moment at 5 K and the Curie temperature are 1.81 μB/f.u. and 351 K, respectively. Many magnetic properties of the itinerant electron ferromagnets are explained by the Takahashi’s spin fluctuation theory. In the Takahashi’s spin fluctuation theory, spectral parameters of spin fluctuations, T0 and TA, are involved in various magnetic properties derived theoretically. In this work, we estimated T0 and TA of Co2NbGa by using the results of magnetization measurements at 5 K: T0 = 2.1 × 103 K and TA = 4.7 × 103 K. This experiment proved that the squared spontaneous magnetization Ms(T)2 is proportional to T2 at low temperature. The TA value estimated from the Ms(T)2 vs. T2 curve is 6.6 × 103 K. Furthermore, the field-induced magnetization M4 around TC is found to be proportional to H/M in the low magnetic field region.

The Characteristic Properties of Magnetostriction and Magneto-Volume Effects of Ni2MnGa-Type Ferromagnetic Heusler Alloys.

In this article, we review the magnetostriction and magneto-volume effects of Ni2MnGa-type ferromagnetic Heusler alloys at the martensitic, premartensitic, and austenitic phases. The correlations of forced magnetostriction (ΔV/V) and magnetization (M), using the self-consistent renormalization (SCR) spin fluctuation theory of an itinerant electron ferromagnet proposed by Takahashi, are evaluated for the ferromagnetic Heusler alloys. The magneto-volume effect occurs due to the interaction between the magnetism and volume change of the magnetic crystals. The magnetic field-induced strain (referred to as forced magnetostriction) and the magnetization are measured, and the correlation of magnetostriction and magnetization is evaluated. The forced volume magnetostriction ΔV/V at the Curie temperature, TC is proportional to M4, and the plots cross the origin point; that is, (M4, ΔV/V) = (0, 0). This consequence is in good agreement with the spin fluctuation theory of Takahashi. An experimental study is carried out and the results of the measurement agree with the theory. The value of forced magnetostriction is proportional to the valence electron concentration per atom (e/a). Therefore, the forced magnetostriction reflects the electronic states of the ferromagnetic alloys. The magnetostriction near the premartensitic transition temperature (TP) induces lattice softening; however, lattice softening is negligible at TC. The forced magnetostriction at TC occurs due to spin fluctuations of the itinerant electrons. In the martensitic and premartensitic phases, softening of the lattice occurs due to the shallow hollow (potential barrier) of the total energy difference between the L21 cubic and modulated 10M or 14M structures. As a result, magnetostriction is increased by the magnetic field.

Weak itinerant-electron ferromagnetism of CrAlGe modified by 3d transition metal

Abstract Substitution effects on weak itinerant electron ferromagnetism in Cr-based ternary alloy CrAlGe were investigated. It was found that substitution of Cr by 3d transition metal with more electrons than Cr enhanced the spontaneous magnetization and Curie temperature. In addition, the itinerancy of CrAlGe was suppressed with the increase of electrons. The ferromagnetism of CrAlGe was related to electron number rather than lattice parameters. The origin of change of itinerancy was suggested to be Coulomb force caused by effective nuclear charge.

Tuning the laser-induced ultrafast demagnetization of transition metals

The ultrafast demagnetization (UFD) dynamics of itinerant ferromagnets is theoretically investigated as a function of the characteristics of the initial laser excitation. A many-body pd-band Hamiltonian is considered which takes into account hybridizations, Coulomb interactions, spin-orbit interactions and the coupling to the laser field on the same electronic level. In this way, a fruitful connection is established between the non-adiabatic quantum dynamics and the well-known equilibrium statistical mechanics of itinerant-electron ferromagnets. The time evolution during and after the pulse absorption is determined exactly by performing numerical Lanczos propagations on a small cluster model with parameters appropriate for Ni. The most relevant laser parameters, namely, the fluence, wave length, polarization and pulse duration are varied systematically. The results show how they allow one to control the total absorbed energy, the spectral distribution of the initial excitation, and the subsequent magnetization dynamics. The calculations show that reasonable changes in these parameters do not affect the UFD dynamics qualitatively and have only a minor influence on the time scale which characterizes the initial demagnetization. In contrast, our model predicts that the degree of demagnetization correlates well with the average number of electrons excited by the laser or average number of absorbed photons. The theoretical results are discussed by comparing them with available experiments. From a fundamental perspective, the robustness of the ultrafast demagnetization effect is theoretically demonstrated, as a phenomenon reflecting the intrinsic dynamics of the metallic 3d valence electrons. A wide variety of well-focused possibilities of tailoring the efficacy of the ultrafast demagnetization process is thereby opened.

Cu-substitution effects on the magnetic properties of weak itinerant electron ferromagnet CrAlGe

Cu-substitution effects on ferromagnetism in Cr-base ternary alloy CrAlGe with an orthorhombic TiSi2-type structure was investigated. The lattice parameters of Cr0.95Cu0.05AlGe were a = 4.748 Å, b = 8.209 Å and c = 8.696 Å at room temperature. The spontaneous and effective moment were determined to be ps = 0.533 μB/f.u at 5 K and peff = 1.89 μB/f.u, respectively. The Curie temperature TC was estimated to be 108 K. These ps and TC of Cr0.95Cu0.05AlGe were larger than those of CrAlGe and Ti-substituted compound. The peff/ps was determined to be 3.55, which was smaller than that of CrAlGe and Ti-substituted compound. Obtained results suggest that the substitution of Cu for Cr in CrAlGe leads suppression of its weak itinerant electron ferromagnetism.

Transitions from a Kondo-like diamagnetic insulator into a modulated ferromagnetic metal in FeGa3−yGey

One initial and essential question of magnetism is whether the magnetic properties of a material are governed by localized moments or itinerant electrons. Here, we expose the case for the weakly ferromagnetic system FeGa3-y Ge y , wherein these two opposite models are reconciled, such that the magnetic susceptibility is quantitatively explained by taking into account the effects of spin-spin correlation. With the electron doping introduced by Ge substitution, the diamagnetic insulating parent compound FeGa3 becomes a paramagnetic metal as early as at y=0.01, and turns into a weakly ferromagnetic metal around the quantum critical point y=0.15. Within the ferromagnetic regime of FeGa3-y Ge y , the magnetic properties are of a weakly itinerant ferromagnetic nature, located in the intermediate regime between the localized and the itinerant dominance. Our analysis implies a potential universality for all itinerant-electron ferromagnets.

Pressure Effect on Magnetic Properties of Weak Itinerant Electron Ferromagnet CrAlGe

The Pressure effect on the magnetic properties of the weak itinerant electron ferromagnet CrAlGe was investigated at pressures up to 1.0 GPa. The spontaneous magnetic moment ps and Curie temperature TC decreased with increasing applied pressure P. The pressure derivatives of ps and TC were dps/dP = −3.98 × 10−5 μB/MPa and dTC/dP = −6.58 × 10−3 K/MPa, respectively. These obtained results are discussed on the basis of Takahashi’s spin fluctuation theory for weak itinerant electron ferromagnets. The characteristic parameters T0 and TA increased with pressure.

Restoration of quantum critical behavior by disorder in pressure-tuned (Mn,Fe)Si

In second-order quantum phase transitions from magnetically ordered to paramagnetic states at T = 0, tuned by pressure or chemical substitution, a quantum critical point is expected to appear with critical behavior manifesting in the slowing down of spin fluctuations in the paramagnetic state and a continuous development of the order parameter in the ordered state. Quantum criticality is discussed widely as a possible driving force for unconventional superconductivity and other exotic phenomena in correlated electron systems. In the real world, however, quantum critical points and quantum criticality are often masked by a preceding first-order transition and/or the development of competing states. Pressure tuning of the itinerant-electron helical magnet MnSi is a well-known example of the suppression of a quantum critical point due to a first-order phase transition and resulting destruction of the ordered state. Utilizing muon spin relaxation experiments, here we report that 15% Fe-substituted (Mn,Fe)Si exhibits completely different behavior with pressure tuning, including the restoration of second-order quantum critical behavior and a quantum critical point at pQPC ~ 21–23 kbar, which coincides with the T = 0 crossing point of the extrapolated phase boundary line of pure MnSi. This result is quantitatively consistent with the recent theory of itinerant-electron ferromagnets by Sang, Belitz, and Kirkpatrick, who argued that disorder would restore a quantum critical point which is otherwise hidden by a first-order transition.

Magnetism and the spin state in cubic perovskite CaCoO3 synthesized under high pressure

Cubic $\mathrm{SrCo}{\mathrm{O}}_{3}$ with an intermediate spin state can only be stabilized by high pressure and high temperature (HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metal perovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubic to orthorhombic as seen in the perovskite family of $\mathrm{Ca}M{\mathrm{O}}_{3}$ ($M={M}^{4+}$ of transition metals, $\mathrm{G}{\mathrm{e}}^{4+}, \mathrm{S}{\mathrm{n}}^{4+}$, and $\mathrm{Z}{\mathrm{r}}^{4+}$) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curie temperature as the crossover to the localized electronic state is approached. We report a successful synthesis of the perovskite $\mathrm{CaCo}{\mathrm{O}}_{3}$ with a HPHT treatment. Surprisingly, $\mathrm{CaCo}{\mathrm{O}}_{3}$ crystallizes in a simple cubic structure that remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has been thoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermal conductivity, and thermoelectric power. Metallic $\mathrm{CaCo}{\mathrm{O}}_{3}$ undergoes two successive magnetic transitions at 86 K and 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state ${t}^{4}{e}^{1}$ of $\mathrm{C}{\mathrm{o}}^{4+}$ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state ${t}^{3}{e}^{2}$ increases for $Tg100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The shortest Co-O bond length in cubic $\mathrm{CaCo}{\mathrm{O}}_{3}$ is responsible for delocalizing electrons in the ${\ensuremath{\pi}}^{*}$-band and itinerant-electron ferromagnetism at $Tl54\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. A comprehensive comparison between $\mathrm{SrCo}{\mathrm{O}}_{3}$ and $\mathrm{CaCo}{\mathrm{O}}_{3}$ and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ bands would make $\mathrm{CaCo}{\mathrm{O}}_{3}$ suitable to study the Hund's coupling effect in a metal.

Quasi-two-dimensional magnetism in Cr-based MAX phases

The MAX phase is one possible candidate for quasi-two-dimensional (Q2D) magnets owing to its layered structure. We report itinerant electron ferromagnetic and antiferromagnetic long-range orders in (Cr1−xMnx)2GeC and Cr2GaN, respectively. Because Cr2GaN orders in a spin-density-wave state, the Q2D character is highly plausible. However, the Q2D nature is not straightforward in (Cr1−xMnx)2GeC because of the absence of useful criteria to judge the dimensionality in the itinerant electron ferromagnet. In this article, we attempt to reveal the Q2D character of ferromagnetic (Cr1−xMnx)2GeC by analyzing magnetic data in terms of the spin fluctuation theory for the Q2D system.

Magnetic properties of weak itinerant electron ferromagnet CrAlGe

Magnetic properties for CrAlGe with an orthorhombic TiSi2-type structure were investigated under ambient pressure and hydrostatic pressure. CrAlGe is ferromagnetic at the temperature below Curie temperature TC of 80 K and has the spontaneous magnetic moment ps of 0.41 µB/f.u. at 5 K and the effective moment peff of 1.89 µB/f.u. under ambient pressure. For the temperature T below 30 K, the decrease in ps(T) 2/ps(5K)2 was proportional to T2. However, the decrease in ps(T) 2/ps(5K)2 was proportional to T4/3 for 30 K < T < TC. The ratio peff/ps was consistent with the theoretical curve of the generalized Rhodes-Wohlfarth plot (Takahashi plot). Obtained results suggest that CrAlGe is a weak itinerant electron ferromagnet. By applying a high pressure of 1 GPa, TC decreased by approximately 10 K.

Pressure Effect on Magnetism of Weak Itinerant Electron Ferromagnet $\mathrm{Ni}_{74.6}\mathrm{Al}_{25.4}$

Pressure effect on the magnetic properties of intermetallic compound Ni 74.6 Al 25.4 was investigated in pressures P up to 1.2 GPa. Spontaneous magnetic moment ps and Curie temperature TC decreased linearly with applying pressure. The dp s /dP and dT C /dP were -0.995 x 10 -5 μB/MPa and -4.57 x 10 -3 K/MPa, respectively. These obtained magnetic properties are analyzed on the basis of Takahashi's spin fluctuation theory for itinerant electron ferromagnets.

Chirality-driven intrinsic spin-glass ordering and field-induced ferromagnetism in Ni3Al nanoparticle aggregates

Weak itinerant-electron ferromagnet Ni3Al is driven to magnetic instability (quantum critical point, QCP, where the long-range ferromagnetic order of the bulk ceases to exist) by reducing the average crystallite size to d=50nm. ‘Zero-field’ (H=0) linear and nonlinear ac-susceptibilities, measured on Ni3Al nanoparticle aggregates, with d=50nm (S1) and d=5nm (S2), provide strong evidence for two spin glass (SG)-like thermodynamic phase transitions: one at Ti(H=0)≃30K (Ti†(H=0)≃230K) and the other at a lower temperature Tp(H=0)≃8K (Th(H=0)≃52K) in S1 (S2). ‘In-field’ (H≠0) linear ac-susceptibility and dc magnetization demonstrate that the thermodynamic nature of these transitions is preserved in finite fields. The presently determined H–T phase diagrams for the samples S1 and S2 are compared with those predicted by the Kotliar–Sompolinsky and Gabay–Toulouse mean-field models and Monte Carlo simulations, based on the chirality-driven spin glass (SG) ordering scenario, for a three-dimensional nearest-neighbor Heisenberg SG system with or without weak random anisotropy. Such a detailed comparison permits us to unambiguously identify various ‘zero-field’ and ‘in-field’ SG phase transitions as: (i) the simultaneous paramagnetic (PM)-chiral glass (CG) and PM-SG phase transitions at Ti(H), (ii) the PM-CG transition at Ti†(H), (iii) the replica symmetry-breaking SG transition at Tp(H), and (iv) the continuous spin-rotation symmetry-breaking SG transition at Th(H). In the presence of random anisotropy, magnetization fails to saturate even at 90kOe in S1 whereas negligibly small anisotropy allows even fields as weak as 1kOe to saturate magnetization and induce ferromagnetism in S2. Due to the proximity to CG/SG-QCP, magnetization and susceptibility both exhibit non-Fermi liquid behavior over a wide range at low temperatures.

Aging and rejuvenation in a ferromagnetic Ni3Al

Aging and rejuvenation phenomena, recently observed in ferromagnetic-like phases of complicated magnets, are studied using a simple itinerant-electron ferromagnet, Ni3Al. Results show that relaxation times of the magnetic response are widely distributed, hence, magnetic domain walls are collectively pinned in a multi-valley structured potential. The absorptive component of the ac-susceptibility gradually decreases with isothermal aging, and hence the domain-wall conformation becomes stabilized in following deeper valleys. If the temperature is shifted slightly after such isothermal aging, the absorptive component temporarily rises as if rejuvenated. This rejuvenation-like phenomenon is always accompanied by acceleration of the magnetic relaxations, regardless of cooling/heating. This thermal perturbation induced de-pinning indicates that the multi-valley structure itself significantly changes with temperature. In other words, the rejuvenation as well as aging can be explained using temperature-sensitive collective pinning of magnetic domain walls. We can hence say that they are intrinsic properties in actual ferromagnets with irregularly distributed pinning centers.

Kondo dynamics in one-dimensional doped ferromagnetic insulators

Some well-established examples of itinerant-electron ferromagnetism in one dimension occur in a Mott-insulating phase. We examine the consequences of doping a ferromagnetic insulator and coupling magnons to gapless charge fluctuations. Using a bosonization scheme for strongly interacting electrons, we derive an effective field theory for the magnon-holon interaction. When the magnon momentum matches the Fermi momentum of the holons, the backscattering of the magnon at low energies gives rise to a Kondo effect of a pseudospin defined from the chirality degree of freedom (right- or left-moving particles). The crossover between weak-coupling and strong-coupling fixed points of the effective mobile-impurity model is then investigated using a numerical renormalization group approach.

Magnetic Properties of Cr-based Ternary Compound CrAlGe

Structural and magnetic properties of Cr-based compound CrAlGe were investigated. The crystal structure was found to be an orthorhombic TiSi2-type with lattice parameters a = 0.4770nm, b = 0.8254nm and c = 0.8725nm at room temperature. Magnetization curve of CrAlGe showed the ferromagnetic behavior. The saturation magnetic moment, spontaneous magnetic moment and Curie temperature of CrAlGe were determined to be 0.45μB/f.u., 0.41μB/f.u. and TC = 80K, respectively. For the temperature T below 30K, the decrease in the square of the spontaneous magnetization M0(T)2 was proportional to T2. However, for 30 <T <TC, the decrease in M0(T)2 was proportional to T4/3. Obtained results suggest that CrAlGe is a weak itinerant electron ferromagnet.