Itinerant Electron Spins Research Articles

Temperature dependence of the spin resistivity in ferromagnetic thin films: Monte Carlo simulations

The magnetic phase transition is experimentally known to give rise to an anomalous temperature-dependence of the electron resistivity in ferromagnetic crystals. Phenomenological theories based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. In this paper, we show by extensive Monte Carlo (MC) simulation the behavior of the resistivity of the spin current calculated as a function of temperature ($T$) from low-$T$ ordered phase to high-$T$ paramagnetic phase in a ferromagnetic film. We analyze in particular effects of film thickness, surface interactions and different kinds of impurities on the spin resistivity across the critical region. The origin of the resistivity peak near the phase transition is shown to stem from the existence of magnetic domains in the critical region. We also formulate in this paper a theory based on the Boltzmann's equation in the relaxation-time approximation. This equation can be solved using numerical data obtained by our simulations. We show that our theory is in a good agreement with our MC results. Comparison with experiments is discussed.

Effects of ferromagnetic ordering and phase transition on the resistivity of spin current

It has been shown experimentally a long time ago that the magnetic ordering causes an anomalous behavior of the electron resistivity in ferromagnetic crystals. Phenomenological explanations based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. We show by extensive Monte Carlo simulation that this behavior is also observed for the resistivity of the spin current calculated as a function of temperature (T) from low-T ordered phase to high-T paramagnetic phase in a ferromagnet. We show, in particular, that across the critical region, the spin resistivity undergoes a huge peak. The origin of this peak is shown to stem from the formation of magnetic domains near the phase transition. The behavior of the resistivity obtained here is compared to experiments and theories. A good agreement is observed.

Misalignment of on-site spins of localized and itinerant electrons in the double-exchangemodel with Coulomb repulsion

We show that the on-site spins of localized and itinerant electrons in the double-exchangemodel are misaligned when the on-site Coulomb repulsion is large enough. To explore thisphenomenon we use the Schwinger-boson representation of the localized spins andintroduce two spin-singlet fermion operators. In terms of the new Fermi fields, the on-siteHund’s interaction is in a diagonal form and the true magnons of the system are identified.The singlet fermions can be understood as electrons dressed by a cloud of repeatedlyemitted and reabsorbed magnons. The quantum phase transition between ferromagnetismand the new phase is studied by varying the Coulomb repulsion for different values ofparameters in the theory such as the Hund’s coupling and chemical potential.

Mean-field theory for double perovskites: Coupling between itinerant electron spins and localized spins

A mean field approximation of a model for double perovskites that takes into account the coupling between itinerant electron spins and localized spins is developed. As in previously reported theoretical results, and contrary to experimental observation, the critical temperature is suppressed for large electron density. An effective Heisenberg model reveals the cause of this discrepancy: the competition between degenerate antiferromagnetic and ferromagnetic channels. This degeneracy can be broken by the inclusion of a Hubbard-type U term. It is therefore suggested that electron correlation effects need to be incorporated in the minimal model of double perovskites in order to explain the experimental observation of increasing ferromagnetic critical temperature with increasing electron doping.

EPR and ferromagnetism in diluted magnetic semiconductor quantum wells.

Motivated by recent measurements of electron paramagnetic resonance spectra in modulation-doped CdMnTe quantum wells [Phys. Rev. Lett. 91, 077201 (2003)]], we develop a theory of collective spin excitations in quasi-two-dimensional diluted magnetic semiconductors. Our theory explains the anomalously large Knight shift found in these experiments as a consequence of collective coupling between Mn-ion local moments and itinerant-electron spins. We use this theory to discuss the physics of ferromagnetism in (II,Mn)VI quantum wells and to speculate on the temperature at which it is likely to be observed in n-type modulation-doped systems.

Magnetic phase diagram of metallic pyrochlore lattice in the double-exchange model

The magnetic phase diagram of metallic kagomeand pyrochlore lattices at the ground state have been studied theoretically in the double-exchange model using a mean-field approximation. The model includes the hopping of itinerant electrons, which is expressed by a single tight-binding band, antiferromagnetic coupling between localized spins, and ferromagnetic exchange interaction between the localized and itinerant electron spins. The phase diagram was found to be rather insensitive to the ferromagnetic exchange interaction but to show a rich variety in magnetic structure with doping. A crossover between ferromagnetic and spin glass phases observed in RE2Mo2O7 pyrochlore lattices, where RE denotes rare-earth ions, is discussed in view of the calculated results.

TRANSPORT PROPERTIES IN A SIMPLIFIED DOUBLE-EXCHANGE MODEL

Transport properties of the manganites by the double-exchange mechanism are considered. The system is modeled by a simplified double-exchange model, i.e. the Hund coupling of the itinerant electron spins and local spins is simplified to the Ising-type one. The transport quantities such as the electronic resistivity, the thermal conductivity, and the thermal power are calculated by using the dynamical mean-field theory. The transport quantities obtained qualitatively reproduce the ones observed in the manganites. The results suggest that the simplified double exchange model underlies the key properties of the manganites.

Geometric frustration in the RMn2 Laves phase compounds

An overview of neutron-scattering investigations of the geometrically frustrated Mn itinerant-electron magnetism in the RMn2 Laves phase compounds is given. Magnetic structures of magnetically ordered compounds are briefly described, in particular the mixed phases where magnetic and nonmagnetic Mn coexist. Magnetic correlations in the paramagnetic phase of YMn2 and in (Y0.97Sc0.03)Mn2, where the substitution of Sc for Y inhibits the magnetic order down to the lowest temperatures, are then discussed. An unusual neutron-scattering function S(Q, ω) is measured, which appear to arise from an itinerant-electron spin liquid. PACS Nos.: 71.27+a, 75.40Gb, 75.50Ee

Novel Mechanism of Biquadratic Coupling in Magnetic Superlattices

The exchange coupling between ferromagnetic layers separated by a thin non-magnetic, metallic spacer, is currently an object 6f intense investigation. The recently discovered, 7r/2 coupling, between magnetizations of the neighbouring magnetic layers [1] added new interest to the problem [2]. Although the dominating mechanism for this exchange interaction has been unambiguously associated with the Ruderman—Kittel—Kasuya—Yoshida (RKKY) interaction [3] the specific mechanisms responsible for the entire coupling are not yet fully understood. There are basically two strategies which have been used theoretically to study the interlayer magnetic coupling: (i) total energy calculations and (ii) perturbative models. Most applications of the latter approach follow earlier studies on the coupling between magnetic impurities in a host metal. The coupling of the ionic spin Sn with the itinerant electron spin o is usually taken as the contact interaction [41:

RKKY-like Biquadratic Exchange in Magnetic Superlattices

Since the discovery of antiferromagnetic coupling of ferromagnetic layers across a nonmagnetic metallic spacer layer, the magnetic superlattices (SL) have become an object of intense interest of both theoretical as well as experimental studies. The problem of RKKY-reminiscent spin polarization oscillations, with a long period and giant interface magnetoresistance attracted the most attention. The recently discovered, π/2 coupling between magnetizations of the neighbouring magnetic layers [1] added new interest to the problem. This type of ordering suggests that along with the bilinear coupling of the type J 1 m1 • m2 between magnetizations of adjacent layers there is an additional multipolar interaction. There were many attempts describing the mechanisms that give rise to biquadratic exchange, being a speciflc form of coupling between magnetic quadupole moments of the ions (for details see Refs. [2, 3] and references therein). The aim of the paper is to study a new intrinsic mechanism that originates in multipolar exchange coupling. In the following we will show that the quadrupole-quadupole coupling can arise in a quite natural way from direct scattering of conduction electrons on magnetic multipole moments in the manner similar to bilinear RKKY interaction. The starting point for any description of metallic magnetic systems is the case of dilute alloys, when a few TM or RE ions are immersed in the sea of conduction electrons. The coupling of the ionic spin Sn with the itinerant electron spin σ is usually taken as the contact interaction [4]: Hex = -2JSn σδ(r). In the case of non-s magnetic ion state along with the scattering due to dipolar contact interaction there appears also scattering of conduction electrons on quadupolar moments. Kondo [5] has proved that the interaction Vqc (k) between conducting electrons and the quadrupoles is given by

Effects of electron-spin exchange interaction on the electronic state of the onedimensional spin-electron-phonon system

A one-dimensional spin-electron-phonon model is proposed for the discovery of new compounds composed of strongly correlated electrons. The total energies and the energy gaps at half filling and less-than half-filling have been computed within a mean-field theory and a periodic boundary condition. As the value of the antiferromagnetic (AF) exchange interaction between nearest-neighbor itinerant-electron spin and localized spin is increased, the energy gaps of 0 and two holes increase and the binding energy of two holes increases. This system with two holes formates a bound state. It turns out that the AF exchange interaction between itinerant-electron spin and localized spin plays the important role in the electronic state of this system.

Magnetism of amorphous transition metals and alloys

Recent development in the magnetism of amorphous transition metals (TMs) and alloys is summarized from the theoretical point of view. The magnetism is shown to be characterized by the itinerant-electron spin glass and re-entrant spin-glass behaviors around amorphous Fe, the enhancement of Curie temperature around amorphous Co, and a weak ferromagnetism around amorphous Ni. The systematic change in magnetism is explained by the main-peak position in the densities of states. The non-linear magnetic couplings and the local environment effects on the local moments are the origin of the itinerant-electron spin glass. Discussions are also given of the magnetism of TM-metalloid amorphous alloys and early TM-TM amorphous alloys. In the former, the hybridization between the metalloid p and TM d orbitals determines their simple magnetism, while in the latter the atomic-size effects play an important role in the concentrated region. The degree of structural and configurational disorder is emphasized as a key factor controlling magnetism in various amorphous alloys.

Finite-temperature theory of amorphous magnetic alloys.

A finite-temperature theory of magnetism in amorphous magnetic alloys with strong environment effects is developed on the basis of the functional integral method for thermal spin fluctuations and the distribution-function method for random distribution of local magnetic moments. The theory drastically simplifies numerical calculations by means of the geometrical-mean model for amorphous structure as well as electronic structure, and allows us to investigate the magnetism in amorphous transition-metal alloys with large difference in atomic size via average coordination number which depends on the type of central atom. Numerical example is given of the amorphous Fe-Zr alloys. It is demonstrated that the theory reproduces quantitatively the local densities of states obtained from the first-principles calculations, and describes the magnetic phase diagram, in particular, the itinerant-electron spin glass. It is also shown that the atomic-size effects play an important role in the formation of ferromagnetism in concentrated Fe-Zr amorphous alloys. The reentrant spin-glass behavior around 90 at. % Fe is shown to be due to the thermal spin fluctuations of amplitudes of local magnetic moments.

Magnetism in amorphous transition metals. II.

Susceptibilities and pressure effects on the magnetic properties of amorphous transition metals have been investigated on the basis of a finite-temperature theory of local-environment effects in amorphous metallic magnetism. Calculated high-field susceptibilities, paramagnetic susceptibilities, effective Bohr-magneton numbers, and the pressure dependence of magnetization, Curie temperature, and spin-glass temperatures are shown to explain various aspects of the magnetism in amorphous and liquid transition metals and alloys as a function of the d electron number. It is demonstrated that these quantities are governed by the electronic structure of amorphous transition metals, in particular, by the main-peak position in the noninteracting density of states for amorphous structure. Moreover, the detailed investigations for susceptibilities, forced-volume magnetostriction, and the T-P phase diagram in the reentrant spin-glass region reveal that the itinerant-electron spin glasses in the Fe-rich amorphous alloys with more than 90 at. % Fe are caused by structural disorder instead of configurational disorder.

On the anomalous forced volume magnetostriction of the re-entrant spin glasses in Fe-rich amorphous alloys

The forced volume magnetostriction in the re-entrant spin-glass regime around amorphous Fe is investigated on the basis of the finite-temperature theory of local environment effects for amorphous metallic magnetism. It is shown that a large peak at the re-entrant spin-glass temperature which has recently been found in Fe-rich Fe-Zr amorphous alloys is a characteristic of the itinerant-electron spin glass since it is mostly due to the change of the amplitudes of local moments.

Finite-temperature theory of local environment effects in amorphous and liquid magnetic alloys.

A finite-temperature theory of magnetism that takes into account the fluctuations of local magnetic moments due to structural disorder is presented on the basis of the functional-integral method and the method of the distribution function. The theory describes qualitatively or semiquantitatively the finite-temperature magnetism of liquid and amorphous metals and alloys as well as the substitutional alloys in a wide range of electron-electron interaction strength from metal to insulator. The results of numerical calculations are presented for amorphous iron. The local environment effects on the density of states, local magnetic moment, susceptibility, and amplitude of local moment are examined. It is found that amorphous iron forms an itinerant-electron spin glass at low temperatures because of the nonlinear magnetic coupling between Fe local moments and the local environment effect on the amplitude of the Fe local moment due to the structural disorder. The calculated spin-glass temperature (100 K) is in good agreement with the value extrapolated from experimental data on Fe-rich amorphous alloys.

Itinerant electron metamagnetism and spin fluctuations in nearly ferromagnetic metals Y(Co1-xAlx)2

High-field magnetisation- and temperature-dependent susceptibility of Laves phase inter-metallic compounds Y(Co1-xAlx)2 are investigated in the strongly exchange-enhanced paramagnetic region (0<or=x<or=0.11) in pulsed magnetic fields up to 100 T. In the whole concentration range, a sharp metamagnetic transition is observed in the low-temperature magnetisation process, while the low-field susceptibility exhibits a maximum at finite temperatures. The transition field Bc and the temperature of the susceptibility maximum Tmax obtained as a function of x indicate a linear relationship, i.e. Bc/Tmax=constant. The result clearly suggests that the susceptibility maximum of the nearly ferromagnetic metals is strongly correlated with itinerant electron metamagnetism.

Ground state of amorphous iron.

On the basis of a new theory of finite-temperature magnetism in amorphous materials and liquids, it is shown that amorphous iron forms an itinerant-electron spin glass at low temperatures because of the nonlinear magnetic couplings between iron local moments and local-environment effects on the amplitude of the local moments. The results explain well the recent experimental data of spin glasses in amorphous ${\mathrm{Fe}}_{93}$${\mathrm{La}}_{7}$ and ${\mathrm{Fe}}_{92}$${\mathrm{Zr}}_{8}$ alloys.

Finite-temperature theory of local-environment effects in Fe-Ni alloys.

Magnetic properties of Fe-Ni alloys have been investigated for both fcc and bcc structures on the basis of the finite-temperature theory of local-environment effects (LEE). The temperature and concentration dependencies of various local moments, susceptibilities, and the internal-field distribution function have been calculated. The results are analyzed from the viewpoint of the LEE. The magnetic phase diagram obtained explains the experimental results well; the bcc alloys show a stable ferromagnetism while the fcc has a ferromagnetic instability at about 65 at. % Fe. In particular, the spin-glass state in the fcc phase is theoretically obtained for the first time on the basis of the itinerant-electron model. It is shown to be a new type of itinerant-electron spin glass which arises from the nonlinearity of the magnetic couplings between Fe local moments. The theory predicts also an asymmetric divergence of the high-field susceptibility around the ferromagnetic instability in the fcc lattice.

Mössbauer Spectra in Fe-Ni Invar Alloys

Mössbauer spectra of Fe-Ni Invar alloys are calculated by means of the mixture model of the ferromagnetic and paramagnetic phases proposed before in the itinerant electron model. Gaussian distribution of the effective fields to Fe 57 nucleus is assumed. It is shown that the calculated spectra agree well with the observed ones. The origin of the distribution of the effective fields is discussed in terms of the environment of Fe 57 atom, spin fluctuations and the relaxation process of itinerant electron spins. It is shown that this model can explain the observed Mössbauer spectra in Fe-Ni Invar alloys.