Band Ferromagnetism Research Articles

Effect of Pressure on Helical Ferromagnetism in MnSi

Within the limits of the Hubbard model supplemented with allowance for relativistic antisymmetric Dzyaloshinskii–Moriya interaction, the spin fluctuation theory of helical band ferromagnetism is developed. The magnetic state equation is derived for the nonuniform order parameter together with expressions for the Neel temperature and the wave vector of a spin structure. The effect of pressure on the helical ferromagnetism is analyzed based on the LDA + U + SO calculations of the MnSi electronic structure at different pressures. It is shown that with increasing pressure, the effect of zero-spin fluctuations that reduce the Neel temperature is amplified, and at a pressure of 15.2 kb a quantum transition takes place accompanied by vanishing of the longrange magnetic order.

Magnetism in Pd: Magnetoconductance and transport spectroscopy of atomic contacts

Since the rapid technological progress demands for ever smaller storage units, the emergence of stable magnetic order in nanomaterials down to the single-atom regime has attracted huge scientific attention to date. Electronic transport spectroscopy has been proven to be a versatile tool for the investigation of electronic, magnetic, and mechanical properties of atomic contacts. Here we report a comprehensive experimental study of the magnetoconductance and electronic properties of Pd atomic contacts at low temperature. The analysis of electronic transport $(dI/dV)$ spectra and the magnetoconductance curves yields a diverse behavior of Pd single-atom contacts, which is attributed to different contact configurations. The magnetoconductance shows a nonmonotonous but mostly continuous behavior, comparable to those found in atomic contacts of band ferromagnets. In the $dI/dV$ spectra, frequently, a pronounced zero-bias anomaly (ZBA) as well as an aperiodic and nonsymmetric fluctuation pattern are observed. While the ZBA can be interpreted as a sign of the Kondo effect, suggesting the presence of magnetic impurity, the fluctuations are evaluated in the framework of conductance fluctuations in relation to the magnetoconductance traces and to previous findings in Au atomic contacts. This thorough analysis reveals that the magnetoconductance and transport spectrum of Au atomic contacts can completely be accounted for by conductance fluctuations, while in Pd contacts the presence of local magnetic order is required.

Local conductance spectra of itinerant ferromagnetic SrRuO3 through break junction

We have measured the local differential conductance spectra (dI/dV–V) of an itinerant ferromagnet composed of polycrystalline SrRuO3 using the mechanically controllable break junction technique. Below the material’s Curie temperature (TC = 160 K), characteristic peak or dip conductance spectra are observed. The characteristic energy scale is comparable to the exchange spin splitting energy that is based on ferromagnetic band calculations. Both the peak and dip spectral shapes are explained based on the itinerant ferromagnetic characteristics of SrRuO3 in terms of spin-dependent transmission, which is similar to the giant magnetoresistance mechanism.

Multiferroic Two-Dimensional Materials

The relation between unusual Mexican-hat band dispersion, ferromagnetism, and ferroelasticity is investigated using a combination of analytical, first-principles, and phenomenological methods. The class of material with Mexican-hat band edge is studied using the α-SnO monolayer as a prototype. Such a band edge causes a van Hove singularity diverging with 1/sqrt[E], and a charge doping in these bands can lead to time-reversal symmetry breaking. Herein, we show that a material with Mexican-hat band dispersion, α-SnO, can be ferroelastic or paraelastic depending on the number of layers. Also, an unexpected multiferroic phase is obtained in a range of hole density for which the material presents ferromagnetism and ferroelasticity simultaneously.

Common effect of chemical and external pressures on the magnetic properties ofRCoPO (R=La,Pr,Nd,Sm). II.

The direct correspondence between Co band ferromagnetism and structural parameters is investigated in the pnictide oxides $R$CoPO for different rare-earth ions ($R$ = La, Pr, Nd, Sm) by means of muon-spin spectroscopy and {\it ab-initio} calculations, complementing our results published previously [G. Prando {\it et al.}, {\it Phys. Rev. B} {\bf 87}, 064401 (2013)]. Both the transition temperature to the ferromagnetic phase $T_{_{\textrm{C}}}$ and the volume of the crystallographic unit cell $V$ are found to be conveniently tuned by the $R$ ionic radius and/or external pressure. A linear correlation between $T_{_{\textrm{C}}}$ and $V$ is reported and {\it ab-initio} calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, $R$ ions are shown to be influencing the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizeable magnetic contribution at much lower temperatures.

Tuning band gap and ferromagnetism in epitaxial Al-doped SnO2 films by defect engineering

The role of acceptor and donor defects in epitaxial Al-doped SnO2 films were systematically investigated. Al doping introduces acceptor defects (AlSn) at low doping concentration while donor ones (Ali) in a concentration range from 8 to 10at%. The band gap is firstly narrowed by hole-doping and then widened by electrons introduced by oxygen vacancies and Ali. Air-annealing absorbs oxygen and makes Al ions transformed from AlSn to Ali, corresponding to a decrease (increase) in the band gap when most Al ions occupy the substitutional (interstitial) sites. The saturation magnetization of the films is enhanced by AlSn doping, with the maximum value in the Sn0.98Al0.02O2 film. The magnetic moment is contributed by the localized holes introduced by AlSn. The existence of the ferromagnetism induced by holes in the film with oxygen vacancy gives a new insight into the behavior of defects in SnO2.

An extension to flat band ferromagnetism

From flat band ferromagnetism, we learned that the lowest energy half-filled flat band gives always ferromagnetism if the localized Wannier states on the flat band satisfy the connectivity condition. If the connectivity conditions are not satisfied, ferromagnetism does not appear. We show that this is not always the case namely, we show that ferromagnetism due to flat bands can appear even if the connectivity condition does not hold due to a peculiar behavior of the band situated just above the flat band.

Ferromagnetism in the one-dimensional Hubbard model with long-range electron hopping and long-range Coulomb interaction

We present a simple, but very realistic, model for a stabilization band ferromagnetism in strongly correlated electron systems. The model is based on a generalized description of electron hopping and electron interactions on a lattice within the frame of the Hubbard Hamiltonian. Instead of the usual nearest-neighbour hopping and on-site Coulomb interaction we consider the long-range electron hopping and the long-range Coulomb interaction both with exponentially decaying amplitudes. It is shown that the simultaneous presence of both long-range mechanisms leads to the stabilization of the ferromagnetic ground state for a wide range of Coulomb interactions and electron concentrations. In particular, it is found that the long-range interaction plays a crucial role in the stabilization of the ferromagnetic state for electron concentrations , while the long-range hopping for n > 1. Thus, one of the possible explanations of the absence of ferromagnetism in the ordinary Hubbard model (with the nearest-neighbour hopping and the on-site Coulomb interaction) could be the oversimplified description of electron hopping and electron interactions on the lattice. This opens a new route towards the understanding of band ferromagnetism in strongly correlated electrons systems.

Flat band ferromagnetism without connectivity conditions in the flat band

It is known that a system which exhibits a half-filled lowest flat band and the localized one-particle Wannier states on the flat band satisfying the connectivity conditions, is always ferromagnetic. Without the connectivity conditions on the flat band, the system is non-magnetic. We show that this is not always true. The reason is connected to a peculiar behavior of the band situated just above the flat band.

Ferromagnetism and quantum anomalous Hall effect in one-side-saturated buckled honeycomb lattices

The recently synthesized silicene as well as theoretically discussed germanene are examples of buckled honeycomb structures. The buckled structures allow one to manipulate asymmetry between two underlying sublattices of honeycomb structures. Here by taking germanene as a prototype of buckled honeycomb lattices, we explore magnetism induced by breaking sublattice symmetry through saturating chemical bonds on one side of the buckled honeycomb lattice. It is shown that when fractions of chemical bonds on one side are saturated, two narrow bands always exist at half filling. Furthermore, the narrow bands generally support flat band ferromagnetism in the presence of the Hubbard $U$ interaction. The induced magnetization is directly related to the saturation fraction and is thus controllable in magnitude through the saturation fraction. Most importantly, we find that depending on the saturation fraction, the ground state of a one-side-saturated germanene may become a quantum anomalous Hall (QAH) insulator characterized by a Chern number that vanishes for larger magnetization. The nonvanishing Chern number for smaller magnetization implies that the associated quantum Hall effect tends to survive at high temperatures. Our findings provide a potential method to engineer buckled honeycomb structures into high-temperature QAH insulators.

Two-step pressure-induced collapse of magnetic order in the MnGe chiral magnet

Cubic helimagnets such as MnSi and FeGe have provided paradigmatic cases of pressure-induced collapse of band ferromagnetism in metals, accompanied with exotic partial order and chiral spin textures. Isostructural MnGe stands out owing to its much shorter helix pitch and high magnetic moment. By combining resistivity, ac susceptibility, and neutron diffraction measurements under very high pressure, we show that the helical order in MnGe transforms around 6 GPa from a high-spin to a low-spin state, recalling the weak ferromagnetism of MnSi at ambient pressure. Helical order collapses only above 10 GPa. The spin-state transition is supported by ab initio calculations.

Dirty limit approach for the proximity effect between a two band superconductor and a ferromagnet

The proximity effect in multilayer structures with dirty two band superconductors and ferromagnets is investigated here. With a homogeneous exchange field and interband scattering in the ferromagnetic layer, the condensate functions that penetrate the ferromagnet are coupled up. This coupling effect is intensively suppressed by the exchange field and modulated by the diffusion coefficients of the two bands. Such coupling also affects the superconductor where the condensate functions of the two bands are also coupled up through the proximity effect. An inhomogeneous exchange field raises the damped oscillation of the condensate functions as the spatial distance from the interface increases. There is a 0−π phase shift when the ferromagnetic layer thickness varies beyond the ferromagnetic coherence length.

Mn-doping-induced itinerant-electron ferromagnetism inCr2GeC

The magnetism of the ${M}_{n+1}A{X}_{n}$ phase, ${\text{Cr}}_{2}$GeC, and its Mn-doped system, (${\text{Cr}}_{1\ensuremath{-}x}$${\text{Mn}}_{x}$)${}_{2}$GeC ($x\ensuremath{\le}0.25$), synthesized via a solid state reaction, was investigated systematically. ${\text{Cr}}_{2}$GeC is in a spin-unpolarized state, but the ferromagnetic band polarization is induced immediately by the Mn doping. The Curie temperature, ${T}_{\mathrm{C}}$, and the spontaneous moment, ${p}_{\mathrm{s}}$, increase almost proportionally to the Mn concentration, strongly suggesting that ${\text{Cr}}_{2}$GeC is located in the vicinity of a ferromagnetic quantum critical point. The strong concentration dependence of ${p}_{\mathrm{eff}}/{p}_{\mathrm{s}}$, where ${p}_{\mathrm{eff}}$ is the effective moment in the paramagnetic state, indicates that the ferromagnetism appearing in the Mn-doped ${\text{Cr}}_{2}$GeC can be classified as a typical itinerant-electron ferromagnetism in a wide range of the degree of electron localization.

A proposal for an alternative class of spin filter materials: Hybridization-induced high-TC ferromagnetic semiconductors CoVXAl (X = Ti, Zr, Hf)

Using ab-initio electronic structure calculations, we propose an alternative class of spin filter materials (SFMs) based on the quaternary Heusler compounds CoVXAl (X = Ti, Zr, Hf). We show that the p-d hybridization leads to the formation of the ferromagnetic band gap with a moderate exchange splitting ΔEex and a Curie temperature TC well above the room temperature. We find that all three compounds are thermodynamically and magnetically stable. Combination of high TC value together with moderate exchange splitting, as well as crystal structures compatible to the existing semiconductors and metals, makes these compounds promising candidates to find applications as SFMs in spintronics devices.

Stability of Ferromagnetic Patterns Inscribed on Arrays of Multisegmented Magnetic Nanocylinders

Magnetic nanocylinders (MNs) can be grown inside of longitudinal nanoporous produced in alumina membranes by means of different techniques. In a similar way, multisegmented MNs (m-MNs) can be obtained by alternating magnetic and nonmagnetic materials during the fabrication process. In both cases, nanocylinders result axially parallel and forming triangular arrays immerse themselves within the membrane used to produce them. Due to shape anisotropy, each magnetic segment presents its magnetization pointing along the cylinder axis either inward or outward of the membrane plane. The as-grown system presents no global magnetization. A strong enough localized magnetic field can revert the magnetization of small groups of neighboring m-MNs. This feature can be used to inscribe ferromagnetic patterns (FP) over either membrane surface to store fixed information (security codes or firmware). Here, we study the total energy per cylinder for the case of a disc shaped FP inscribed over a circular membrane containing a huge amount of m-MNs. To try to prevent spontaneous magnetization reversion due to thermal effects we test for m-MNs a stabilization mechanism proposed previously for homogeneous MNs: an opposite ferromagnetic band (OFB) inscribed inside the FP. We have compared inscription and stabilization of FP over arrays of homogenous and multisegmented MNs.

Ab initio study of electronic structure and magnetic properties in ferromagnetic Be1−xMnxSe and Be1−xMnxTe alloys

Structural, electronic and magnetic properties of Mn-doped BeSe and BeTe have been studied by employing the full-potential linear augmented plane waves plus local orbitals (FP-LAPW+lo) method within the spin-polarized density functional theory (DFT). The investigations are carried out by varying the Mn concentration, “x”, in BeSe and BeTe host matrices for x=0.25, 0.5 and 0.75. The results of spin-polarized calculations manifest the presence of ferromagnetic band structures with both spin-up and spin-down alignments. Our calculated results of band structures reveal that for x=0.25, 0.5 and 0.75, Be1−xMnxSe has a half-metallic (HM) band structure profile showing 100% spin polarization at the Fermi level. On the other hand, the Be1−xMnxTe band structure shows complete 100% spin polarization at the Fermi level only for x=0.25 and 0.5. Spin-dependent charge densities have been calculated to study the bonding nature, and the values of the exchange constants, N0α and N0β, are also determined which are consistent with the values from the typical magneto-optical experiment. In addition, the calculations of spin exchange splitting, ΔX(d), exhibit important indication regarding the attractive effective potential for minority spin rather than majority spin. For each concentration x, the value of total magnetic moment has been estimated to be 5μB, which reveals that addition of Mn impurity does not affect the hole concentration of a perfect BeSe (Te) crystal.

Nano-pattern–induced ferromagnetism in strongly correlated electrons

Band ferromagnetism in strongly correlated electron systems is one of the most challenging issues in today's condensed-matter physics. In this theoretical work, we study the competition between kinetic term, Coulomb repulsion, and on-site correlated disorder for various lattice geometries. Unconventional and complex ferromagnetic phase diagrams are obtained: wide region of stability, cascade of transitions, re-entrance, high sensitivity to the carrier concentration and strongly inhomogeneous ground states for relatively weak on-site potential. The direct and systematic comparison with exact diagonalization shows that the unrestricted Hartree-Fock method is unexpectedly accurate for such systems, which allows large-size cluster calculations. A match of the order of 99.9% for weak and intermediate couplings is found, slightly reduced to about 95% in the large-repulsion regime. Nano-patterned lattices appear to be particularly promising candidates that could, with the tremendous progress in growing and self-organized techniques, be synthesized in the near future.

Enhanced band gap emission and ferromagnetism of Au nanoparticle decorated α-Fe2O3nanowires due to surface plasmon and interfacial effects

Au nanoparticle decorated α-Fe2O3 NWs exhibit enhanced band gap luminescence due to coupling between excitons in α-Fe2O3 NWs and the surface plasmons of Au particles. The enhanced magnetization in α-Fe2O3 NWs with Au coating is attributed to the interfacial effects arising from the trapped metallic electrons at the Au/α-Fe2O3 NWs interface.

Superparamagnetism in iron-doped CeO2−y nanocrystals

We have measured the magnetization of undoped and Fe2+/3+ doped CeO2−y nanocrystals at various temperatures and magnetic fields. The Stoner condition N(0)I > 1 is multiply fulfilled in the case of nano-CeO2−y, favoring the band ferromagnetism approach. In the case of Fe-doped samples, the magnetization versus magnetic field dependence is well fitted with a weighted Langevin function. A blocking temperature of about 20 K is obtained both from the ZFC/FC curves and from the coercive field against temperature dependence. The temperature and field dependence of the magnetization clearly shows the presence of nanosized particles which exhibit superparamagnetic behavior.

Ferromagnetism and nonlocal correlations in the Hubbard model

We study the possibility and stability of band ferromagnetism in the single-band Hubbard model for the simple cubic (sc) lattice. A nonlocal self-energy is derived within a modified perturbation theory. Results for the spectral density and quasiparticle density of states are shown with special attention to the effects of $\mathbf{k}$ dependence. The importance of nonlocal correlations for the fulfillment of the Mermin-Wagner theorem is our main result. A phase diagram showing regions of ferromagnetic order is calculated for the three-dimensional lattice. Besides, we show results for the optical conductivity and prove that the renormalized one-loop contribution to the conductivity already cancels the Drude peak exactly in case of a local self-energy which is not true anymore for a nonlocal self-energy.