Effect of the Optical Pumping and Magnetic Field on the States of Phase Separation Domains in Eu0.8Ce0.2Mn2O5

The effect of optical pumping and applied magnetic field on the characteristics of ferromagnetic layers in one-dimensional superlattices is studied. At low enough temperatures, these layers correspond to phase separation domains in RMn₂O₅ and R_0.8Ce_0.2Mn₂O₅ multiferroics. The formation of such domains occurs owing to the charge ordering of Mn³⁺ and Mn⁴⁺ ions and to the finite probability for e_g electrons to tunnel between these pairs of ions. The volume occupied by such superlattices is rather small, and they can be treated as isolated ferromagnetic semiconductor heterostructures, spontaneously formed in the host crystal. The sequences of ferromagnetic resonances related to the superlattice layers in Eu_0.8Ce_0.2Mn₂O₅ are studied. The characteristics of these resonances give information on the properties of such layers. For the first time, it is demonstrated that the optical pumping gives rise to a new metastable state of superlattices, which can be recovered by the magnetic field cycling to the state existing before the optical pumping. It is found that the superlattices recovered by the magnetic field exist up to temperatures higher than those in as-grown crystals.

     

退火对IrMn基磁隧道结多层膜热稳定性的影响

采用磁控溅射方法制备了结构为IrMn/CoFe/AlO_x/CoFe的磁性隧道结多层膜,样品置于真空磁场中进行退火处理. 将在不同温度退火的磁隧道结结构多层膜置于负饱和场中等待,研究退火温度对样品热稳定性的影响. 结果表明:退火提高了多层膜反铁磁层的单轴各向异性能,增加了样品的交换偏置;随着负饱和场等待时间的延长,被钉扎层的磁滞回线向正场偏移,交换偏置单调减小,但退火减弱了这种趋势. 关键词： 磁隧道结 交换偏置 磁化反转     

Effects of copper and oxygen vacancies on the ferromagnetism of Mn- and Co-doped Cu2O

The ferromagnetic properties of Mn- and Co-doped Cu₂O with copper and oxygen vacancies (V_Cu and V_O) are studied by first-principles calculations. The results indicate that Mn-doped Cu₂O has an antiferromagnetic state in the near configuration, while it has a ferromagnetic state for the far configuration. On the contrary, Co-doped Cu₂O possess a ferromagnetic state regardless of the distance between the two Co atoms. The observed ferromagnetism can be attributed to the 90° ferromagnetic super-exchange mechanism. The presence of V_O can enhance the ferromagnetism, whereas V_Cudepresses it.     

Crossover from Coulomb-blockade to ohmic conduction in small tunnel junctions

We discuss the suppression of Coulomb effects in the low temperature conductanceg(T) of small tunnel junctions with increasing dissipation or bare conductanceg. The conductance is expressed in terms of the spin correlation fuction of a classical one dimensionalXY-model with ferromagnetic nearest neighbor plus inverse square interaction. It is shown that at low temperatures the conductance vanishes asymptotically likeT ² instead of exponentially. A Coulomb gap in the sense of a thermally activated contribution tog(T) is present only for bare conductances smaller thang _c 1. A simple model for the spin correlation functions is compared with experiments.     

磁性半导体/磁性d波超导结中的自旋极化输运

通过求解磁性d波超导中的能隙与磁交换能的自恰方程,利用推广的Blonder-Tinkham-Klapwijk 理论研究磁性半导体/磁性d波超导结中自旋极化准粒子输运系数与微分电导. 计算表明： 1) 磁性d波超导结中的磁交换能h₀可导致零偏压电导峰与能隙电导峰劈裂,劈裂的宽度为2h₀;2) 磁性半导体中的磁交换能h_FS可使零偏压电导峰劈裂的峰值变低. 而由能隙电导峰劈裂的两个子峰,当两种磁性材料的磁 关键词： 磁性半导体 磁性d波超导体 自旋极化输运     

Dynamical transport behavior in electron-doped manganites La0.4Ca0.6(Mn1−x Ru x )O3

Measurements of the dynamical electrical transport behavior are performed on electron-doped manganites La_0.4Ca_0.6(Mn_1−x Ru _x )O₃ (x=0 and 0.02). An undoped sample possesses a robust charge-ordered antiferromagnetic ground state, and only a positive resistivity relaxation can be observed. However, a low-temperature negative relaxation behavior arises after inducing a few ferromagnetic orders to the charge-ordered matrix by tiny Ru doping. We assigned this difference to the dynamical competition between ferromagnetic metallic and charge-ordered insulating phases. Consistently, for a doped sample, the crossover from positive to negative resistivity relaxation behavior ensues around T=115 K, which is just below the ferromagnetic Curie temperature.     

Compositional effect of bcc Co100−x Fe x electrodes on magnetoresistance in AlO x -based magnetic tunnel junctions

The effect of the composition of ferromagnetic bcc Co_100−x Fe _x electrodes on tunneling magnetoresistance (TMR) of Co_100−x Fe _x /AlO _x /Co_100−x Fe _x /IrMn magnetic tunnel junctions was studied. The epitaxial growth of the bottom Co_100−x Fe _x electrode leads to a high-quality electrode and interface, which significantly enhances the TMR ratio and the desired effect for study. Other factors that could also affect TMR, such as interface roughness, tunneling barrier properties, and exchange-bias properties, were kept the same within the uncertainty of the experiment in order to minimize their effects. The observed TMR dependence on composition is attributed to the variation of the s-like electron densities of state of the bcc Co_100−x Fe _x electrodes with different compositions.     

Charge-Ordered Antiferromagnetic to Ferromagnetic Phase Transition in Cr-Doped Nd0.5Ca0.5Mn1−x Cr x O3: EMR and Magnetization Study

The Cr-doped rare-earth manganites Nd_0.5Ca_0.5Mn_1−x Cr _x O₃ (x = 0.03, 0.05, 0.10) are studied by electron magnetic resonance (EMR) and magnetization measurements in the paramagnetic as well as in the ferromagnetic phase. The magnetization measurements show that the charge-ordered antiferromagnetic phase decreases at the expense of ferromagnetic metallic phase and for Cr doping of x = 0.1, the charge-ordered phase melts completely. The EMR shows multiple signals for all three compositions in the ferromagnetic phase indicative of an anisotropic ferromagnetic phase. The difference between the shift of the high-field and low-field signals decreases with Cr doping, indicating that the magnetic anisotropy decreases with the Cr doping. In the paramagnetic phase the EMR line width follows Causa's model as observed in other colossal magnetoresistant manganites. Authors' address: Ajay Sharma, Department of Physics, Indian Institute of Science, Bangalore, India     

Quantum modulation effect in a graphene-based magnetic tunnel junction

The electronic (quantum) transport in a NG/F_B/FG tunnel junction (where NG, F_B and FG are a normal graphene layer, a ferromagnetic barrier connected to a gate and a ferromagnetic graphene layer, respectively) is investigated. The motions of the electrons in the graphene layers are taken to be governed by the Dirac Equation. Parallel (P) and antiparallel alignment (AP) of the magnetizations in the barrier and in the ferromagnetic graphene are considered. Our work focuses on the oscillation of the electrical conductance (Gq), of the spin conductance (Gs) and of the tunneling magneto resistance (TMR) of this magnetic tunnel junction. We find that, the quantum modulation due to the effect of the exchange field in F_B will be seen in the plots the conductance and of the TMR as functions of the thickness of ferromagnetic barrier (L). The period of two multiplied sinusoidal terms of the modulation are seen to be controlled by varying the gate potential and the exchange field of the F_B layer. The phenomenon, a quantum beating, is built up with two oscillating spin conductance components which have different periods of oscillation related to the splitting of Dirac's energies in the F_B region. The amplitudes of oscillations of Gq, Gs and TMR are not seen to decrease as the thickness increases. The decaying behaviors seen in the conventional transport through an insulator do not appear.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

Magnetic phase transitions and magnetocaloric effect in the Fe-doped MnNiGe alloys

The magnetic phase transition and magnetocaloric effects in Fe-doped MnNiGe alloys are investigated. The substitution of Fe for Ni decreases the structural transition temperature remarkably, resulting in the magnetostructural transition occurring between antiferromagnetic and ferromagnetic states in MnNi_{1 - x}Fe_xGe alloy. Owing to the enhanced ferromagnetic coupling induced by the substitution of Fe, metamagnetic behaviour is also observed in TiNiSi-type phase of MnNi_{1 - x}Fe_xGe alloys at temperature below the structural transition temperature.     

Room‐Temperature Insulating Ferromagnetic (Ni,Co)1+2xTi1−xO3 Thin Films

Insulating uniaxial room‐temperature ferromagnets are a prerequisite for commonplace spin wave‐based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. It is shown that the uniaxial A_{1 + 2x}Ti⁴⁺_{1 ? x}O₃ (ATO), A = Ni²⁺,Co²⁺, and 0.6 < x ≤ 1, thin films are electrically insulating ferromagnets already at room temperature. The octahedra network of the ATO and the corundum and ilmenite structures are the same yet different octahedra‐filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to x = 1, or vacated (?0.24 < x < 0) in the ATO structure. TiO‐layers, which separate the ferromagnetic (Ni,Co)O‐layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO₃ and CoTiO₃ ilmenites, can continuously be replaced by (Ni,Co)O‐layers to convert the ATO‐films to ferromagnetic insulator with abundant direct cation interactions.     

Spontaneous vortex phase (SVP) of ruthenocuprate highT c magneto-superconductors

The Ru_0.9Sr₂YCu_2.1O_7.9 compound synthesized by HPHT (high pressure high temperature) solid-state reaction route exhibits bulk superconductivity below 30 K. Also the Ru-spins are ordered magnetically above 143 K, with a ferromagnetic component at 5 K. Low field (<1000 Oe)M vs.H plots show that both the superconducting and ferromagnetic components are present in the compound at 5 K. At low temperatures, the compound though remains in spontaneous vortex phase, itsM vs.H hysteresis loop is symmetric instead of the theoretically expected asymmetric one. Our results cast doubts on either theoretical model or the intrinsic nature of ferromagnetic superconductivity in studied ruthenate.     

The quantitative analysis of the temperature dependence of the resistivity in La0.85Ag0.15MnO3 manganite

The quantitative analysis of the temperature dependence of the resistivity in La_0.85Ag_0.15MnO₃ manganite is carried out in the temperature intervals of 4–350 K. The volume concentration of the ferromagnetic phase versus temperature is estimated by means of two different models, which agree well enough. It is shown that the volume concentration of the ferromagnetic phase calculated from ρ(T) data in the first approximation can be considered as the order parameter for this system.     

Sr掺杂Eu1-xSrxMnO3体系的核磁共振和磁结构研究

对Eu_1-xSr_xMnO₃ (x=0.4, 0.5, 0.6, 0.7) 体系的磁结构进行了系统的研究. 通过核磁共振实验, 磁化测量, 并结合电输运测量结果表明, Sr的掺入使得 EuMnO₃反铁磁母相中出现铁磁相. 铁磁相和反铁磁相的竞争导致样品在低温下的自旋玻璃行为.分析认为, Eu_0.4Sr_0.6MnO₃和Eu_0.3Sr_0.7MnO₃的磁结构在低温下呈现更加复杂的特征, 主要源于铁磁团簇的形成以及无序相的存在.     

Properties of ferromagnetic resonance in Fe<Subscript>73.5</Subscript>CuNb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> nanocrystalline alloys

The structural properties and parameters of ferromagnetic resonance have been studied for Fe_73.5CuNb₃Si_13.5B₉ nanocrystalline alloys produced from the initial amorphous state via annealing under different conditions. The dependence of the linewidth of the ferromagnetic resonance on the grain size ΔH ∼ D ⁶ has been found. The result is discussed within the framework of the random magnetic anisotropy model.     

Magnetically driven high-frequency rectification in a cooperative system of magnetic tunnel junctions: Frequency dependence

The effect of magnetically driven high-frequency rectification in a polycrystalline La_0.7Ca_0.3MnO₃ manganite has been measured at different frequencies of microwave radiation. The magnetic field dependence of a rectified voltage has a broad peak resembling an absorption line, whose shape and position are determined by the radiation frequency. The rectification effect in a polycrystalline manganite sample is related to a ramified network of magnetic tunnel junctions, which is formed by ferromagnetic conducting grains with insulator boundaries. The results of measurements are consistent with a model for the magneto-dependent rectification effect based on the interplay between a spin-polarized current through the tunnel junctions and magnetic resonance induced in the grains forming the junctions.     

Magneto-optical properties of amorphous multilayer films

The magneto-optical properties of [{(Co₄₅Fe₄₅Zr₁₀) _Z (Al₂O₃)_100−Z } _X /(α-Si) _Y ]₄₀ multilayer nanoheterostructures with different thicknesses of granular ferromagnetic layers and semiconductor interlayers have been investigated. It is found that the composition layer thickness and the concentration of the ferromagnetic component affect significantly the Si interlayer formation.     

On the magnetically dilute Heisenberg and Ising ferromagnetics

The dependence of Curie point, θ _c, on magnetic concentration, p, is investigated, for the randomly dilute ferromagnetic Heisenberg spin 1/2 problem, by considering successive approximations to 1/χ=0, where χ is the susceptibility. This extends to the next approximation work previously reported by the authors. The simple cubic and body-centred cubic lattices are dealt with in detail. It is concluded that the behaviour of θ _c(p) for small p is not yet fully established: it is possible that the critical concentration p _c, below which there is no ferromagnetic behaviour, is greater for the Heisenberg model than for the corresponding Ising model (where it is the analogous site percolation problem critical probability).     

Spin-flip assisted tunneling through quantum dot based magnetic tunnel junctions

We theoretically study the spin-polarized transport through double barrier magnetic tunnel junction (DBMTJ) consisting of the quantum dot sandwiched by two ferromagnetic (FM) leads. The tunneling current through the DBMTJ is evaluated based on the Keldysh nonequilibrium Green’s function approach. The self-energy and Green’s function of the dot are analytically obtained via the equation of motion method, by systematically incorporating two spin-flip phenomena, namely, intra-dot spin-flip, and spin-flip coupling between the lead and the central dot region. The effects of both spin-flip processes on the spectral functions, tunneling current and tunnel magnetoresistance (TMR) are analyzed. The spin-flip effects result in spin mixing, thus contributing to the spectral function of the off-diagonal Green’s function components ( G_s[`(s)] ^r )\left( {G_{\sigma \bar \sigma }^r } \right). Interestingly, the spin-flip coupling between the lead and dot enhances both the tunneling current and the TMR for applied bias above the threshold voltage V _th . On the other hand, the intra-dot spin-flip results in an additional step in the I-V characteristics near V _th . Additionally, it suppresses the tunneling current but enhances the TMR. The opposing effects of the two types of spin-flip on the tunneling current means that one spin-flip mechanism can be engineered to counteract the other, so as to maintain the tunneling current without reducing the TMR. Their additive effect on the TMR enables the DBMTJ to attain a large tunneling current and high TMR for above threshold bias values.