Crystalline Ferromagnets Research Articles

Critical behavior at ferromagnetic to paramagnetic phase transition in single crystalline MnNiSi ferromagnet

Ferromagnetic single crystalline MnNiSi samples were first fabricated through a Sn-flux growth technique, followed by measurements of their structural characteristics and intrinsic magnetic properties. Additionally, the critical behavior for second-order ferromagnetic to paramagnetic phase transition was investigated through utilization of techniques such as the modified Arrott plot, the Kouvel–Fisher method, and the magnetocaloric effect scaling law method. Through different methods of analysis, reliable critical exponents were obtained. Renormalization of interactions around the Curie temperature indicates the reliability of the obtained exponents. The obtained critical exponents are close to those theoretically predicted for a three-dimensional isotropic short-range Heisenberg ferromagnet but shift toward the long-range mean-field estimates. This may arise from the coupling of short- and long-range interactions as well as the competition between localized Mn–Mn magnetic interactions and the hybridization between p- and d-type orbitals.

Thresholding of the Elliott-Yafet spin-flip scattering in multi-sublattice magnets by the respective exchange energies

How different microscopic mechanisms of ultrafast spin dynamics coexist and interplay is not only relevant for the development of spintronics but also for the thorough description of physical systems out-of-equilibrium. In pure crystalline ferromagnets, one of the main microscopic mechanism of spin relaxation is the electron-phonon (el-ph) driven spin-flip, or Elliott-Yafet, scattering. Unexpectedly, recent experiments with ferro- and ferrimagnetic alloys have shown different dynamics for the different sublattices. These distinct sublattice dynamics are contradictory to the Elliott-Yafet scenario. In order to rationalize this discrepancy, it has been proposed that the intra- and intersublattice exchange interaction energies must be considered in the microscopic demagnetization mechanism, too. Here, using a temperature-dependent x-ray emission spectroscopy (XES) method, we address experimentally the element specific el-ph angular momentum transfer rates, responsible for the spin-flips in the respective (sub)lattices of Fe_{20}Ni_{80}, Fe_{50}Ni_{50} and pure nickel single crystals. We establish how the deduced rate evolution with the temperature is linked to the exchange coupling constants reported for different alloy stoichiometries and how sublattice exchange energies threshold the related el-ph spin-flip channels. Thus, these results evidence that the Elliott-Yafet spin-flip scattering, thresholded by sublattice exchange energies, is the relevant microscopic process to describe sublattice dynamics in alloys and elemental magnetic systems.

Unexpected dependence of the anomalous Hall angle on the Hall conductivity in amorphous transition metal thin films

The anomalous Hall effect (AHE), and magnetic and electronic transport properties were investigated in a series of amorphous transition metal thin films---${\mathrm{Fe}}_{x}{\mathrm{Si}}_{1--x}, {\mathrm{Fe}}_{x}{\mathrm{Ge}}_{1--x}, {\mathrm{Co}}_{x}{\mathrm{Ge}}_{1--x}, {\mathrm{Co}}_{x}{\mathrm{Si}}_{1--x}$, and ${\mathrm{Fe}}_{1--y}{\mathrm{Co}}_{y}\mathrm{Si}$. The experimental results are compared with density functional theory calculations of the density of Berry curvature and intrinsic anomalous Hall conductivity. In all samples, the longitudinal conductivity (${\ensuremath{\sigma}}_{xx}$), magnetization ($M$), and Hall resistivity (${\ensuremath{\rho}}_{xy}$) increase with increasing transition metal concentration; due to the structural disorder ${\ensuremath{\sigma}}_{xx}$ is lower in all samples than a typical crystalline metal. In the systems with Fe as the transition metal (including ${\mathrm{Fe}}_{1--y}{\mathrm{Co}}_{y}\mathrm{Si}$), the magnetization and AHE are large and in some cases greater than the crystalline analog. In all samples, the AHE is dominated by the intrinsic mechanism, arising from a nonzero, locally derived Berry curvature. The anomalous Hall angle (AHA) $(={\ensuremath{\sigma}}_{xy}/{\ensuremath{\sigma}}_{xx})$ is as large as 5% at low temperature. These results are compared with the AHAs reported in a broad range of crystalline and amorphous materials. Previous work has shown that in a typical crystalline ferromagnet the Hall conductivity (${\ensuremath{\sigma}}_{xy}$) and ${\ensuremath{\sigma}}_{xx}$ are correlated and are usually either both large or both small, resulting in an AHA that decreases with increasing ${\ensuremath{\sigma}}_{xy}$. By contrast, the AHA increases linearly with increasing ${\ensuremath{\sigma}}_{xy}$ in the amorphous systems. This trend is attributed to a generally low ${\ensuremath{\sigma}}_{xx}$, while ${\ensuremath{\sigma}}_{xy}$ varies and can be large. In the amorphous systems, ${\ensuremath{\sigma}}_{xx}$ and ${\ensuremath{\sigma}}_{xy}$ are not coupled, and there may thus exist the potential to further increase the AHA by increasing ${\ensuremath{\sigma}}_{xy}$.

Magnon and phonon dispersion, lifetime, and thermal conductivity of iron from spin-lattice dynamics simulations

In recent years, the fundamental physics of spin-lattice (e.g., magnon-phonon) interaction has attracted significant experimental and theoretical interests given its potential paradigm-shifting impacts in areas like spin-thermoelectrics, spin-caloritronics, and spintronics. Modelling studies of the transport of magnons and phonons in magnetic crystals are very rare. In this paper, we use spin-lattice dynamics (SLD) simulations to model ferromagnetic crystalline iron, where the spin and lattice systems are coupled through the atomic position-dependent exchange function, and thus the interaction between magnons and phonons is naturally considered. We then present a method combining SLD simulations with spectral energy analysis to calculate the magnon and phonon harmonic (e.g., dispersion, specific heat, and group velocity) and anharmonic (e.g., scattering rate) properties, based on which their thermal conductivity values are calculated. This work represents an example of using SLD simulations to understand the transport properties involving coupled magnon and phonon dynamics.

Three-dimensional visualization of dislocations in a ferromagnetic material by magnetic-field-free electron tomography

In conventional transmission electron microscopy, specimens to be observed are placed in between the objective lens pole piece and therefore exposed to a strong magnetic field about 2 T. For a ferromagnetic specimen, magnetization of the specimen causes isotropic and anisotropic defocusing, deflection of the electron beam as well as deformation of the specimen, which all become more severe when the specimen tilted. Therefore electron tomography on a ferromagnetic crystalline specimen is highly challenging because tilt-series data sets must be acquired without changing the excitation condition of a specific diffraction spot. In this study, a scanning transmission electron microscopy (STEM) tomography method without magnetizing a ferromagnetic specimen has been developed for three-dimensional (3D) visualization of dislocations in α-Fe, which is a typical ferromagnetic material. Magnetic-field-free environment down to 0.38 ± 0.07 mT at the specimen position is realized by demagnetizing the objective lens pole piece of a commercial STEM instrument. By using a spherical aberration corrector with the magnetic-field-free environment, an “aberration corrected Low-Mag STEM mode” with no objective lens field reaches a convergence semi angle ∼1 mrad and a spatial resolution ∼5 nm, and shows an adequate performance of imaging dislocations under a two-beam excitation condition for a low-index diffracted beam. The illumination condition for the aberration corrected Low-Mag STEM mode gives no overlap between the direct beam disk (spot) and neighboring diffraction disks. An electron channeling contrast imaging technique, in which an annular detector was located at a doughnut area between the direct beam and the neighboring diffracted beams, was effectively employed with the aberration corrected Low-Mag STEM mode to keep image intensity high enough even at large specimen-tilt angles. The resultant tomographic observation visualized 3D dislocation arrangements and active slip planes in a deformed α-Fe specimen.

Dilatometric analysis of the process of the nanocrystallization of Fe72.5Cu1Nb2Mo1.5Si14B9 soft magnetic alloy

The process of the nanocrystallization of magnetically soft Fe72.5Cu1Nb2Mo1.5Si14B9 alloy has been studied using dilatometry and thermomagnetic analysis, together with structural investigations. It has been shown that the amount of nanocrystalline phase precipitated upon heating of the amorphous precursor is in good agreement with a shortening of the ribbon length in the course of crystallization. Thermal expansion at the different stages of heating and cooling depends on the structural and phase states, as well as on the magnetic state of the alloy. The numerical value of the coefficient of linear thermal expansion decreases with an increase in the fraction of the ferromagnetic crystalline phase.

Out-of-plane magnetoresistance in ferromagnet/graphene/ferromagnet spin-valve junctions

Out-of-plane spin-injection and detection through naturally stacked graphene layers were investigated in ferromagnet/graphene/ferromagnet (FGF) junctions. We obtained a maximum magnetoresistance (MR) of 4.6% at $T=4.2$ K in the junction of a four-layer graphene insertion, having a very small area$\ensuremath{-}$junction-resistance product of 0.2 $\ensuremath{\Omega}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}$m${}^{2}$. According to resistance-temperature and current-voltage characteristics, the graphene layer in the FGF junction acted as a metal-like insertion rather than as an insulating barrier. A lower value for the interfacial spin asymmetry coefficient ($\ensuremath{\gamma}=0.25\ifmmode\pm\else\textpm\fi{}0.05$) obtained from the fitting of variations with interfacial resistance implies that the spin-injection efficiency along the out-of-plane direction was reduced by spin-flip scattering at graphene/ferromagnet interfaces. Our results showed that highly transparent graphene/ferromagnet interfaces with crystalline ferromagnet (FM) electrodes are required to achieve higher spin-injection efficiency through the graphene layer in a FGF junction along the out-of-plane direction.

Concentration of magnetic transitions in dilute magnetic materials

In this paper the two-sublattice crystalline ferromagnet in the approximation of the Ising model was investigated. In each sub-lattice density of magnetic atoms can vary from 0 to 1. The exchange interactions within each sublattice and between the spins of different sublattices are considered as given. The case of direct exchange was investigated and the conditions for the occurrence of ferromagnetism, antiferromagnetism and ferrimagnetism, depending on the concentration of magnetic atoms in each sublattice were established.

Preface: Phys. Status Solidi C 5/2012

AbstractThe 9th International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets (ISCOM 2011) was held from September 25 until September 30, 2011, in the city of Gniezno. Gniezno has a peculiar importance for Polish people – it was the first capital of Poland in the 10th century. ISCOM 2011 follows an already long‐lasting series of previous meetings organized in Mittelberg (Austria, 1995), Sesimbra (Portugal, 1997), Oxford (United Kingdom, 1999), Rusutsu (Japan, 2001), Port‐Bourgenay (France, 2003), Key West (USA, 2005), Peniscola (Spain, 2007), and Niseko (Japan, 2009). ISCOM 2011 was organized by the Institute of Molecular Physics of the Polish Academy of Sciences and the Committee of Physics of the Polish Academy of Sciences.Traditionally, each symposium of the ISCOM series is the preeminent forum for interdisciplinary discussions of chemistry, physics, materials science and technology of crystalline molecular solids, including the following main topics:– Synthesis of New Molecules and Molecular Materials– Physics of Low Dimensional Metals and Superconductors– Electron Correlations in Molecular Materials– Molecular Magnetism– Functional Materials– Organic Thin Films and Devices– Organic Molecular Nanoscience– Field‐ and Photo‐induced Phenomena– Theory, Modeling and ComputationAbout 170 scientists from the following 18 countries: Australia, Brazil, Canada, China, France, Germany, Hungary, Italy, Japan, Korea, Macedonia, Poland, Portugal, Romania, Russia, Spain, UK, USA attended ISCOM 2011. During the symposium 16 invited lectures, 60 oral contributions and about 80 posters were presented. The ISCOM 2011 participants have focused on many hot scientific problems of organic conductors: metal–insulator transitions, nature of the Mott insulating state, charge ordering phenomena, mechanisms of superconductivity, quantum spin liquids, hybrid magnetic conductors, electronic ferroelectricity, and others.We would like to thank all participants and the members of the International Advisory Committee for great contributions and many helpful advices.On behalf of the Organizing Committee (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropy of crystalline ferromagnets with defects

This letter describes a model of the anisotropy of crystalline ferromagnetic alloys containing defect clusters. The focus is on ⟨100⟩-type pairs in bcc lattices each of which is assigned a uniaxial anisotropy of fixed magnitude. The lattice is also characterized by a coherent anisotropy energy due to the transition element host lattice as well as an isotropic exchange and magnetic dipole energy. This model is applied to a study of the concentration dependence of the fourfold magnetic anisotropy Keff(C) of quenched bcc Fe1−CGaC alloys. It yields Keff(C)∝1−κC4 which gives a good account of the dependence of Keff on C. In particular, it accounts for the collapse of Keff at C≈0.2, provided the magnitude of the local anisotropy energy is three orders of magnitude larger than that of the coherent fourfold anisotropy energy.

Silica-based nanospheres, nanowires, nanosubstrates, nanotubes, and nanofiber arrays

Silica-based nanospheres, nanosphere wire-like agglomerates, nanotubes, and nanofiber arrays, bi- and coaxial SiC/SiO2 nanowires, and silica nanosubstrates have been generated and their structural, mechanical, and catalytic properties examined. Nearly monodisperse SiO2 nanospheres of diameter ≤30 nm have been synthesized in gram quantities and used to sequester active copper sites for the selective catalytic conversion of ethanol to acetaldehyde in a process that is at least three times more efficient than that which uses fumed silica produced from the flame hydrolysis of silicon tetrachloride. A Ni(III) solution has been reduced in an electroless process on the surface of the nanospheres, producing a ferromagnetic crystalline nickel coating of variable thickness. Partially agglomerated SiO2 nanospheres of diameter ∼45 nm have been used as nanosubstrates for SnOx crystallites of diameter 3–6 nm. The agglomeration of smaller nanospheres (d≤10 nm) to wire-like configurations has suggested the means to grow silica nanotubes and to produce unique silica nanofiber arrays. Generated silica nanotubes may fill an existing need for the catalytic conversion of high molecular weight hydrocarbons (MW∼1000). Both coaxial and side-by-side biaxially structured silicon carbide-silica nanowires have now also been generated from C/Si/SiO2 mixtures. The structure of these nanowires, their cross-sectional shape, and their structural transformation between a biaxial and coaxial configuration have been studied. The Young's modulus of the biaxially structured nanowires has been measured to be 50–100 GPa, depending on the size of the nanowires.

Thermal evolution of ferromagnetic metallic glasses

A traditional TG apparatus was modified by placing two permanent magnets producing a controlled magnetic field (TG(M): Magneto Thermogravimetry). This technique proved to be useful to study both structural relaxation and crystallisation of ferromagnetic metallic glasses. Results obtained for the amorphous alloys Fe40Ni40P14B6 and Fe62.5Co6Ni7.5Zr6Nb2Cu1B15, are reported in this paper. Structural relaxation can be evaluated by measuring changes in Curie temperature induced by thermal treatments. Crystallisation in TG(M) is detected through a change in the measured apparent mass (difference between the sample mass and magnetic force driving it upward). These results were confirmed by DSC analysis. Whether the obtained crystalline phase is ferromagnetic, it can be identified through its Curie temperature, measured by TG(M). In fact the value of 770°C measured as Curie temperature of crystallised Fe62.5Co6Ni7.5Zr6Nb2Cu1B15led to conclude that the only ferromagnetic crystalline phase is a-Fe. These hypothesis was confirmed by XRD analysis, showing that the first crystallisation yields to a-Fe nanocrystals.

Domain structure of hard magnetic NdAlFeCo bulk metallic glass

Magnetic domain structure of hard magnetic Nd60Al10Fe20Co10 bulk metallic glass (BMG) has been studied by using magnetic force microscopy. In the magnetic force images it is shown that the exchange interaction type magnetic domains with a period of about 360 nm do exist in the BMG, which is believed to be associated with the appearance of hard-magnetic properties in this system. As the scale of the magnetic domain is much larger than the size of the short-range ordered atomic clusters existing in the BMG, it is believed that the large areas of magnetic contrast are actually a collection of a group of clusters aligned in parallel by strong exchange coupling interaction. After fully crystallization, the BMG exhibits paramagnetism. No obvious magnetic contrast is observed in the magnetic force images of fully crystallized samples, except for a small quantity of ferromagnetic crystalline phase with low coercivity and an average size of 900 nm.

Fabrication of Ge1−xMnxTe ferromagnetic fine structure using phase change technology

We present a novel fabrication method of desirable ferromagnetic fine structure patterns in an amorphous GeMnTe film by using phase change optical disk technology. The a-Ge1−xMnxTe films are prepared by the rf sputtering and ionized-cluster beam (ICB) methods. The characteristics of crystalline wire fabricated by irradiating a laser beam in a-Ge1−xMnxTe films are reported. The ferromagnetic crystalline wire pattern of 1 μm in width is successfully formed in this experiment. The magnetic property of crystalline wire for the sputtering methods is similar to that for the ICB method. Magnetoresistance exhibits the negative in both cases of the magnetic fields applied along the longitudinal and transverse directions of crystalline wires. Through the measurement of the magnetic field angular dependence of magnetroresistance, anisotropic magnetoresistance-like behavior is observed in a diluted magnetic semiconductor system.

Pressure effects of genuine organic crystalline ferromagnet possessing intermolecular contacts between nitroxide oxygen and methyl hydrogen atoms

We have found the pressure-induced (1) periodical down–up variation of transition temperature ( T c), (2) ferro- to antiferro-magnetic transition and (3) reduction of the magnetic lattice dimensionality in a genuine organic crystalline bulk-ferromagnet p-Cl–C 6H 4–CHN-TEMPO ( T c=0.28 K) with spin polarizations on intermolecular contacts between the nitroxide oxygen and methyl hydrogen atoms (C–H⋯O–N contacts). These phenomena can be understood by the pressure-induced rotation of the methyl moiety relevant to the interaction-mechanism through C–H⋯O–N contacts.

Low-lying magnetic excitations in amorphous and alloys

Results of high-resolution magnetization (M) measurements performed on amorphous (a-) (x = 0, 1, 2, 4, 6, 8, 10) and (y = 0, 1) alloys over wide ranges of temperature (T) and external magnetic field are presented and discussed in the light of existing theoretical models. The magnetization at 5 K does not saturate even for fields as high as 70 kOe particularly for the alloys with and y =0, 1. The high-field differential susceptibility at , is extremely large for the alloys with x = 0, 1 and y = 0, 1, and decreases rapidly with x for such that it possesses values typical of crystalline ferromagnets such as Fe, Co, Ni for x > 6. The dominant contribution to the thermal demagnetization of the spontaneous as well as `in-field' magnetization comes from spin-wave (SW) excitations at low temperatures and from enhanced local spin-density fluctuations over a wide range of intermediate temperatures and for temperatures close to the Curie point, , for all of the alloys studied. The spin-wave stiffness, D, is independent of for all of the compositions and the ratio possesses a value characteristic of amorphous ferromagnets with competing interactions for the alloys with and y = 0, 1. For these alloys, thermomagnetic and thermoremanent effects generally associated with the cluster spin-glass behaviour have been observed in the re-entrant state which sets in at a temperature . In accordance with the predictions of the spin-fluctuation model, D renormalizes with temperature as and the spin fluctuations get strongly suppressed by Co substitution and . While the spin-fluctuation (SF) model provides a consistent theoretical basis for the observed temperature dependence of the spontaneous and `in-field' magnetization over the entire temperature range , the infinite three-dimensional (FM) matrix plus finite FM spin-clusters model extends the scope of the SF model in that it offers a straightforward explanation for the absence of SW peaks in the inelastic neutron scattering spectra taken over a certain wave-vector-transfer range, the softening of spin-wave modes for , the existence of a significant contribution due to diffusons, in addition to magnons, to the -decrease of the magnetization, and the composition dependence of D(0), M(0,0) and .

MAGNETIC DICHROISM UNDER EXCITATION BY UNPOLARIZED RADIATION

Magnetic dichroism has been observed in the angular distribution of p-core level photoemission spectra excited from crystalline ferromagnets by unpolarized X-rays. The angular dependence of the angular and energy-resolved photoemission intensity was recorded as a function of the emission direction with respect to the crystal, revealing a strong variation of the magnetic dichroism with emission angle due to photoelectron diffraction. This variation is particularly strong around the forward scattering peaks, including sign reversals close to these directions. The results demonstrate that any standard X-ray photoelectron spectroscopy apparatus possesses the potential for combined analysis of surface magnetic structure and geometric structure in a chemically specific way.

Effect of a random crystalline field on the Curie temperature of an anisotropic crystalline ferromagnet

We examine how substitution of nonmagnetic atoms of one species by another that leads to the emergence of a random crystalline field affects the Curie temperature TC of an anisotropic crystalline ferromagnet. We study the case of low concentrations, in which individual substitutional impurities create isolated clusters of perturbed magnetic ions with additional easy-or hard-magnetization axes. Finally, we analyze the various sign relations among the parameters D and d of regular and impurity second-order anisotropies with Kramers (J=1) and non-Kramers (J=3/2) angular momenta, and show that usually the effects of a random crystalline field lower the Curie temperature as the concentration of the nonmagnetic impurity atoms increases.

Theoretical study of the effective exchange interactions between nitroxides via hydrogen atoms

Effective exchange interactions between nitroxides via hydrogen atoms are studied for several model clusters which involve bridge hydrogen atoms at idealized positions. Ab initio post-UHF and UNO CASCI calculations indicate two different mechanisms for ferromagnetic interactions: (1) the spin polarization (SP) effect via a bridge hydrogen and (2) the bridge hydrogen hyperconjugative effect based on the π ∗- like hyperconjugated SOMO. Contributions of the kinetic, potential and SP terms are estimated by UNO CASCI {2,2} and APUHF, and the electron correlation term by MP and CCSD(T). Bridge hydrogen atoms are shown to be important for weak effective exchange interactions between nitroxides. The implications of these findings are discussed in relation to the molecular design of crystalline organic ferromagnets.

Characterization of magnetic materials by Mössbauer spectroscopy

Since its discovery the Mössbauer effect has been a powerful probe of iron in magnetic materials and, together with neutron scattering and magnetic resonance, complements measurements of magnetic susceptibility and magnetization. The magnetic hyperfine splitting of the spectrum gives a measure of the magnetic moment on the iron atoms, and the line intensities enable the orientation of the magnetic moments to be determined. By applying external magnetic fields the local susceptibility of the iron atoms in paramagnets can be determined, and in magnetically ordered crystals magnetic phase changes may be induced and may be used to measure the magnetic anisotropy. In the early days this was used to determine the magnetic structure of crystalline ferromagnets, ferrimagnets and antiferromagnets. More modern applications include the study of amorphous and disordered crystalline materials (speromagnets, spin glasses, cluster glasses), incommensurate spin structures, fine particle magnets (superparamagnets and superferromagnets), magnetic multilayers, high flux density permanent magnet alloys (e.g. ) and high- superconductors.