Generation of an external magnetic field with the spin orientation effect in a single layer Ising nanographene

In this work, the magnetic properties of the single layer Ising nanogaphene (SLING) are investigated by using Kaneyoshi approach (KA) within the effective field theory for different spin orientations of its magnetic atoms. We find that the magnetizations of the SLING has no phase transition, certain Curie temperature and distinct peak of susceptibility at T_c for the some spin orientations at the zero external magnetic field (H=0.0). Because these behaviors occur at H≠0.0, we suggest that the SLING generates an external magnetic field and behaves as an external magnetic field generator for these spin orientations. However, the SLING exhibits ferromagnetic behaviors for only one spin orientations. But, it exhibits antiferromagnetic behaviors for the others. For the AFM cases, diamagnetic susceptibility behaviors and type II superconductivity hysteresis behaviors are obtained. We hope that these results can open a door to obtain new class of single layer graphene and graphene-based magnetic field generator devices with the spin orientation effect.     

Microscopic field in nanocrystalline ferromagnetic metal films and the opportunity for studying it by the μSR method

The relation of the microscopic (local) field in nanocrystalline ferromagnetic metal films to macroscopic characteristics (the external magnetic field, average magnetization, saturation magnetization) is determined for the case where a nanocrystalline ferromagnetic film consists of crystallographically ordered grains separated by disordered regions and where the dimensions of grains along a normal to the film plane are much smaller than those in the film plane. In the case of a strong external field (? ? M), the magnetization direction is determined in grains in the form of oblate ellipsoids for metals with uniaxial or cubic magnetocrystalline anisotropy. Expressions are derived for the spin polarization of an ensemble of rapidly diffusing and nondiffusing muons in nanocrystalline ferromagnetic films. It is shown that experiments with “slow” positive muons make it possible to measure all parameters of such structures and to obtain important information for studying phase transition physics.     

Formation of phase domain structures in thin films under conditions of a first-order magnetic phase transition

This paper reports on the results of a theoretical investigation into the magnetic and resonance properties of thin films in the range of the transition from a paramagnetic state to a ferromagnetic state in the case where the magnetic transition is a first-order phase transformation. It is demonstrated that, in an external magnetic field directed perpendicular to the film plane, the formation of a specific domain structure consisting of domains of the coexisting paramagnetic and ferromagnetic phases can appear to be energetically favorable. The parameters of the equilibrium system of stripe phase domains and their dependences on the temperature, the magnetic field, and the characteristics of the material are calculated. The specific features of the magnetic resonance spectra under the conditions of formed stripe phase domains are considered. A relationship is derived for the dependence of the resonance field of the system of ferromagnetic domains on the magnetization and temperature. It is shown that the alternating external field can fulfill an orientation function in the formation of stripe phase domains.     

Spin and orbital quantization of electronic states as origins of second harmonic generation in semiconductors

Basically different mechanisms of optical second harmonic generation (SHG) in semiconductors, induced by an external magnetic field H, have been identified experimentally by studying the diluted magnetic semiconductor (Cd,Mn)Te. For paramagnetic (Cd,Mn)Te the SHG response is governed by spin quantization of electronic states, in contrast with diamagnetic CdTe with its dominating orbital quantization. The mechanisms can be identified by the distinct magnetic field dependence of the SHG intensity which scales with the spin splitting in the paramagnetic case as compared to the H2 dependence observed for the diamagnetic case.     

Current characteristics of mesoscopic rings in quantum Smoluchowski regime

In normal mesoscopic metals of a ring topology persistent currents can be induced by threading the center of the ring with a magnetic flux. This phenomenon is an example of the famous Aharonov-Bohm effect. In the paper we study the current vs the external constant magnetic flux characteristics of the system driven by both the classical and the quantum thermal fluctuations. The problem is formulated in terms of Langevin equations in classical and quantum Smoluchowski regimes. We analyze the impact of the quantum thermal fluctuations on the current-flux characteristics. We demonstrate that the current response can be changed from paramagnetic to diamagnetic when the quantum nature of the thermal fluctuations increases.     

Magnetocaloric effect and magnetic properties in YMnO3 perovskite

The magnetic properties and magnetocaloric effect in YMnO₃ have been investigated using Monte Carlo simulations. The thermal magnetization, specific heat and magnetic entropy have been obtained for different values of exchange interactions and for a several external magnetic field values. The variation of adiabatic temperature change with the temperatures has been obtained for several values of external magnetic field. It has been found that the sample exhibited a paramagnetic to ferromagnetic phase transition at 30 K. The transition temperature of YMnO₃ has been deduced for different values of size (1/L) and different values of exchange interactions. The relative cooling power with several values of external magnetic field has been established.     

Diamagnetic susceptibility of spin-triplet ferromagnetic superconductors

We calculate the diamagnetic susceptibility in zero external magnetic field above the phase transition from ferromagnetic phase to phase of coexistence of ferromagnetic order and unconventional superconductivity. For this aim we use generalized Ginzburg-Landau free energy of unconventional ferromagnetic superconductor with spin-triplet electron pairing. A possible application of the result to some intermetallic compounds is briefly discussed.     

Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

Defects in graphite may be magnetic and magnetostrictive as revealed by scanning tunneling microscopy

We use scanning tunneling microscopy to measure magnetic field induced strains in highly oriented pyrolytic graphite. This is done by using a scanning tunneling microscope with some magnetic components, which however do not produce an observable response within our resolution in the case of pure (99.999%) paramagnetic or diamagnetic metals (at the low field strengths applied). We study also ferromagnetic metals with this method for comparison. We find a relatively large (similar to that of permalloy) magnetostrictive response of graphite for the low applied field. The data shows saturation of the strain and also that the strain observed is localized and is not the cumulative strain from the mounted edge of the sample to the position of measurement, implying that volume is not conserved with the strains. We believe that the observed strains correspond to a signal of a ferromagnetic material and in this case may be due to the defects observed on the graphite planes.Received: 25 May 2004, Published online: 24 September 2004PACS: 68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM) - 75.80. + q Magnetomechanical and magnetoelectric effects, magnetostriction - 81.05.Uw Carbon, diamond, graphite     

Asymmetrically shaped hysteresis loop in exchange-biased FeNi/FeMn film

The magnetization reversal of the bilayer polycrystalline FeNi(50 Å)/FeMn(50 Å) film sputtered in a magnetic field has been studied by magnetic and magneto-optical techniques. The external magnetic fields were applied along the easy or hard magnetization axis of the ferromagnetic permalloy layer. The asymmetry of hysteresis loop has been found. Appreciable asymmetry and the exchange bias were observed only in the field applied along the easy axis. The specific features of magnetization reversal were explained within the phenomenological model that involves high-order exchange anisotropy and misalignment of the easy axes of the antiferromagnetic and ferromagnetic layers. It has been shown that the film can exist in one of three equilibrium magnetic states in the field applied along the easy axis. The transitions between these states occur as first-order phase transitions. The observed hysteresis loop asymmetry is related to the existence of the metastable state.     

Two-phase paramagnetic-ferromagnetic state of La<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single-crystal lanthanum manganite

Two phases, paramagnetic and ferromagnetic, were shown by the magnetic resonance method to coexist below the temperature T _C in La_0.7Pb_0.3MnO₃ single crystals exhibiting colossal magnetoresistance. The magnetic resonance spectra were studied in the frequency range 10–78 GHz. The specific features in the behavior of the spectral parameters were observed to be the strongest at the temperatures corresponding to the maximum magnetoresistance in the crystals. The concentration ratios of the paramagnetic and ferromagnetic phases in the samples were found to be sensitive to variations in temperature and external magnetic field. This behavior suggests realization of the electronic phase separation mechanism in the system under study.     

Electrical control of valley and spin polarized current and tunneling magnetoresistance in a silicene-based magnetic tunnel junction

We investigate the electronic transport in a silicene-based ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal tunnel junction. The results show that the valley and spin transports are strongly dependent on local application of a vertical electric field and effective magnetization configurations of the ferromagnetic layers. In particular, it is found that the fully valley and spin polarized currents can be realized by tuning the external electric field. Furthermore, we also demonstrate that the tunneling magnetoresistance ratio in such a full magnetic junction of silicene is very sensitive to the electric field modulation.     

Exchange-conductivity shift of ferromagnetic antiresonance (FMAR) and the behaviour of surface impedance in the neighbourhood of FMAR for perpendicular configuration

We have occupied ourselves with studying Larmor and antilarmor ferromagnetic antiresonance (AR) in bulk isotropic and saturated ferromagnetic metals in the region of the normal skin effect, on the basis of a macroscopic theory, for the case of the static external magnetic field,H ₀, perpendicular to the surface of a sample. We have solved Maxwell's equations for the propagation of high-frequency electromagnetic waves and, at the same time, the equation of motion for the magnetization in a Landau-Lifshitz form. On including the spatial dispersion of the permeability, we have found an exchange-conductivity shift of the antiresonance field compared to the AR field derived in the work of Kittel (J. de phys. et rad.12 (1951) 291). In the immediate vicinity of AR we have also derived the surface impedance of the metal and the dispersion relations for all waves produced in it by a linearly polarized high-frequency electromagnetic wave perpendicularly falling on the surface.The author is grateful to Dr. D. Fraitová for useful suggestions and discussions.     

Electron spin polarization in field emission

The field emission microscope can be extended to determine the directionally weighted spin-dependent surface density of states of magnetic metals or to obtain information on the electronic structure of metal to magnetic-semiconductor interfaces. We describe the techniques of spin polarization measurements in field emission. The measurement of Kisker et al. on ferromagnetic EuS evaporated on W are discussed along with recent investigations on Ni.     

Dynamic phase transitions and the effects of frequency of oscillating magnetic field on the dynamic phase diagrams in the bilayer honeycomb lattice with AB stacking geometry

We study some dynamic properties of the bilayer honeycomb lattice with AB stacking geometry in the presence of a time-dependent oscillating external magnetic field. We employ the Glauber transition rates to construct the mean-field dynamical equations. First, we obtain dynamic phases in the system and observe the paramagnetic (p), ferromagnetic (f), compensated (c) antiferromagnetic (af), surface ferromagnetic (sf) and mixed (m) phases. Besides, coexistence phase regions also exist in the system. Second, we investigate the thermal behavior of the dynamic order parameters. From these study, the natures (first- or second-order) of the transitions are characterized and the dynamic phase transition (DPT) points are presented. The DPTs are obtained and the dynamic phase diagrams (DPD) are constructed plane of the temperature versus the amplitude of the magnetic field. We investigate the effect of the frequency of the oscillating external magnetic field on the DPD.     

Magnetoresistive properties of nanostructured magnetic metals,manganites, and magnetic semiconductors

We consider methods for controlling magnetoresistive parameters of magnetic metal superlattices, manganites, and magnetic semiconductors. By reducing the thickness of ferromagnetic layers in superlattices (e.g., Fe layers in Fe/Cr superlattices), it is possible to form superparamagnetic clustered–layered nanostructures with a magnetoresistance weakly depending on the direction of the external magnetic field, which is very important for applications of such type of materials. Producing Mn vacancies and additionally annealing lanthanum manganites in the oxygen atmosphere, it is possible to increase their magnetoresistance by more than four orders of magnitude. By changing the thickness of p–n junction in the structure of ferromagnetic semiconductors, their magnetoresistance can be increased by 2–3 orders of magnitude.     

Magnetic properties of oxides and silicon single crystals

The magnetic properties of single crystals Si, SrTiO₃, LaAlO₃, MgO, and (La,Sr)(Al,Ta)O₃ were investigated systematically. Three origins of the magnetizations of these crystals, namely, an intrinsic diamagnetic, a paramagnetic, and a ferromagnetic contribution, have been found to influence the magnetic signals measured on the crystals, in some important application scenarios such crystals being served as substrates with the magnetic thin film epitaxially grown on. Quantitative analyses methodologies were developed and thorough investigations were performed on the crystals with the intrinsic materials parameters thus revealed, especially that the intrinsic diamagnetic susceptibility differential dχ_dia/dT were identified quantitatively for the first time in SrTiO₃, LaAlO₃, MgO, and (La,Sr)(Al,Ta)O₃. The paramagnetic contribution is found to be the key in terms of the magnetic properties of the crystals, which in turn is in fact a consequence of the 3d impurities doping inside the crystal. All the intrinsic materials parameters are given in this paper as datasets, the datasets are openly available at https://www.doi.org/10.57760/sciencedb.j00113.00028.     

Magnetic phase transition in (Mn0.95Ni0.05)3B4

It was found that the metallic compound (Mn_0.95Ni_0.05)₃B₄ was ferromagnetic below 195 K and antiferromagnetic between 195 and 354 K. The transition temperature from ferromagnetic to antiferromagnetic increases with increasing external magnetic field. On the other hand, the transition temperature from antiferromagnetic to paramagnetic decreases with increasing magnetic field. It is expected that the present results might be explained by the theoretical results on the coexistence of ferro- and antiferromagnetism in the itinerant electron system reported by Moriya and Usami.     

Magnetic measurements of Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B in the vicinity of the Curie temperature

The study of the transition between ferromagnetic and paramagnetic states has been investigated on selected metallic glass systems based on Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B with T_C close to room temperature. Samples in the form of ribbons were prepared by planar flow casting and magnetostriction in parallel and perpendicular directions and saturation magnetostrictions have been determined on these samples in as-cast states together with hysteresis loops. In addition, a magneto-optic device for dynamic domain observation has been used for observation of domain structure. Magnetostriction measurements using direct method of measurement show the decrease of saturation magnetostriction towards zero upon approaching T_C. In paramagnetic state the field dependencies of magnetostriction in parallel and perpendicular configurations exhibit a linear dependence on the external magnetic field. In the transition region of temperatures the dependencies are a combination of ferromagnetic and paramagnetic field dependencies. The coercivity H_C in the materials investigated exhibits values below 20 A/m. The observed magnetic domains are typical for this class of amorphous alloys. The polarization in paramagnetic state increases gradually with increase in magnetizing field, reflecting the increasing amount of polarized regions.     

过渡金属Fe，Co，Ni居里点附近热电势的实验研究

测量了纯金属Fe，Co，Ni的热电势发现，在居里点附近热电势随温度的变化关系曲线均呈现先凹后凸的反常现象.由曲线的转折处可确定三个居里温度，即铁磁态居里点T_f，居里点T_C和顺磁居里点θ_p.由曲线可见，金属由铁磁态到顺磁态的相转变，存在一定温度间隔的转变过程，居里温度是这一过程的中间温度.分析曲线表明，温度在T_f与T_C范围有空穴参与导电，说明磁性负载者是d带中的空穴.对于温度在T_C与θ_p范围可能存在短程有序进行了讨论. 关键词： Fe Co Ni 热电势 居里温度