Ultrathin Co/Cu(110) film as a lattice of ferromagnetic grains with dipolar interaction

The well-known fact of magnetic ordering in ultrathin Co films (with an effective thickness of several monoatomic layers) on a single-crystal Cu(110) substrate is described quantitatively using the model according to which the thin film is a lattice of three-dimensional ferromagnetic grains with dipole-dipole interactions. The critical film thickness corresponding to the ferromagnetic transition and the corresponding Curie temperature were calculated.     

Thickness dependence of the Curie temperature and critical concentration in diluted ferromagnetic thin films

A thickness-dependent critical concentration of an Ising ferromagnetic thin film is predicted. The calculated values of the Curie temperature are bounded by those known for diluted two- and three-dimensional ferromagnets.     

Thickness-dependent Curie temperatures of ultrathin magnetic films: effect of the range of spin-spin interactions

We present a simple model of spin-spin coupling which provides insight into the nature of the rapid decrease in the Curie temperature with decreasing thin film thickness n (number of monolayers). The shift of Curie temperature t(n) = 1-T(c)(n)/T(c)(infinity) follows the usual power law t(n) approximately n(-lambda) in thin films crossing over to linear behavior t(n) approximately n in the ultrathin film thickness limit. Experimental results for ferromagnetic thin films are compared, and shown to follow curves of t(n) with lambda values dependent on the nature of the spin-spin interactions.     

Crystallographic structure of ultrathin Fe films on Cu(100)

We report the observation of bcc-like crystal structures in 2-4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.     

High-temperature dynamics of surface magnetism in iron thin films

Finite-temperature magnetic properties of iron thin films are investigated by computer simulation over a broad range of temperatures up to the point of the ferromagnetic–paramagnetic phase transition. The coupled dynamics of atoms and magnetic moments is treated using the large-scale spin–lattice dynamics (SLD) algorithm. We investigate surface and bulk magnetic properties of iron, and how these properties vary as a function of temperature, film thickness and surface crystallography. We find that magnetization at surfaces is enhanced at low temperatures and suppressed at higher temperatures, in agreement with experimental observations. The effective Curie temperature of a film decreases as a function of thickness. Short-range magnetic order and non-vanishing spin–spin spatial correlations are found above the Curie temperature. The spin autocorrelation functions exhibit slower oscillations with longer decoherence times near the surface. We also find that the directional spin disorder has a significant effect on the surface strain.     

Fabrication of ferromagnetic (Ga,Mn)As by ion irradiation

Using Mn⁺ implantation following ion beam-induced epitaxial crystallization (IBIEC) annealing, high Curie temperature ferromagnetic (Ga,Mn)As thin film was fabricated. The crystalline quality of the Mn⁺ implanted layer was identified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A clear ferromagnetic transition at T_c 253 K was observed by magnetization vs. temperature measurement. We infer that IBIEC treatment is a useful method not only for the low-temperature annealing of (Ga,Mn)As thin films but also for other dilute magnetic semiconductor (DMS) samples.     

采用PVA方法制备ZnO基发光稀磁半导体

不同的制备工艺对ZnO薄膜的微结构和性能有很大的影响,为了得到成本较低,样品具有较好特性的实验方法,对于制备手段进行了探索。使用PVA溶胶-凝胶法制备了Zn_0.88Co_0.12O薄膜,研究了不同退火工艺对其微结构的影响。对于Zn_0.88Co_0.12O样品的微结构和室温下的铁磁性和发光特性,具体比较分析了产生原因。对比了Co掺杂和复合Co、Fe掺杂Zn_0.88(Co_0.5Fe_0.5)_0.12O样品的微结构,采用振动样品磁强计(VSM)测量了样品的磁特性,发现单一掺杂的薄膜具有更好的晶体质量和更强的磁性。     

Microstructure and magnetic properties of Ni-rich Ni54Mn25.7Ga20.3 ferromagnetic shape memory alloy thin film

A Ni₅₄Mn_25.7Ga_20.3 ferromagnetic shape memory alloy thin film has been fabricated by using the RF magnetron-sputtering technique. The structure and magnetic properties of the film were systematically investigated. The results show that the film is in ferromagnetic martensite state at room temperature with the Curie temperature (T_c) of about 370 K. The saturation magnetization (M_s) of the film reaches 45 emu/g at 300 K, which is about 80% as large as that of Ni–Mn–Ga bulk material. The magnetization hysteresis loops significantly depend on temperatures. The residual magnetization (M_r) and the coercive force (H_c) increase with decreasing temperatures. The grains homogeneously distribute in the film. The microstructure of the film consists of martensite plates. The interface between the martensite variants is clear and straight, indicating a good mobility.     

Proximity induced enhancement of the Curie temperature in hybrid spin injection devices

We investigate the increase of the Curie temperature T(C) in a lateral spin injection geometry where the ferromagnetic (Ga,Mn)As injector and detector contacts are capped by a thin iron film. Because of interlayer coupling between Fe and (Ga,Mn)As T(C) gets enhanced by nearly 100% for the thinnest (Ga,Mn)As films. The use of the proximity effect might pave the way for practical implementation of spintronic devices.     

La0.67Sr0.33MnO3薄膜光响应特性研究

实验测量了在不同温度和电场下,超巨磁电阻CMR(colossal magnetoresistance)薄膜材料La_0.67Sr_0.33MnO₃(LSMO)的光响应特性.发现样品被激光照射后,具有光诱导电阻变化的特性.在温度小于居里点Tc时,电阻随温度升高而增大,在Tc以上时,电阻则随温度升高而减小.通过对样品的光脉冲响应和偏置电场的关系分析,可认为其光响应特性的机理与激光激励下引起的自旋系统变化有关.     

Near-Curie magnetic anomaly at the Ni/C interface observed by electron holography

We analyze with electron holography carried out in a transmission electron microscope the near-Curie behavior of magnetism at the edge of a Nickel thin film coated with Carbon. In-situ experiments with finely controlled variations of the sample temperature reveal an anomaly in the ferromagnetic to paramagnetic phase transition when the film temperature is a few degrees above the nominal Curie point. We interpret the anomaly as a strain-induced spin reorientation transition triggered by the differential thermal expansion of Carbon and Nickel. We present a model that quantitatively reproduces the main features of the observed signal. The model is developed in terms of an anisotropic, temperature dependent exchange coupling between the Nickel moments at the Carbon interface that favors their vertical alignment at low temperatures.     

Investigation of the seeding-layer effect for a ferroelectric thin film with the transverse Ising model

The transition feature of a ferroelectric thin film with a seeding layer is studied based on the transverse Ising model. The influence of the seeding layer on the transition behavior of a ferroelectric thin film is investigated systemically, and the effect of the interaction parameters for the seeding layer on the phase diagram is also obtained. Meanwhile, the polarization and Curie temperature of the ferroelectric thin film are calculated for different seeding-layer structures. The results show that the polarization and Curie temperature of the film will be obviously modified on adding a seeding layer.     

Body-centered-cubic Ni and its magnetic properties

The body-centered-cubic (bcc) phase of Ni, which does not exist in nature, has been achieved as a thin film on GaAs(001) at 170 K via molecular beam epitaxy. The bcc Ni is ferromagnetic with a Curie temperature of 456 K and possesses a magnetic moment of 0.52+/-0.08 micro(B)/atom. The cubic magnetocrystalline anisotropy of bcc Ni is determined to be +4.0x10(5) ergs x cm(-3), as opposed to -5.7x10(4) ergs x cm(-3) for the naturally occurring face-centered-cubic (fcc) Ni. This sharp contrast in the magnetic anisotropy is attributed to the different electronic band structures between bcc Ni and fcc Ni, which are determined using angle-resolved photoemission with synchrotron radiation.     

Growth of manganese silicide films by co-deposition of Mn and Si on Si(1 1 1): A spectroscopic and morphological investigation

MnSi is a ferromagnetic compound with a Curie temperature of 29 K. Recent theoretical studies predict that 2 ML of MnSi epitaxially grown on Si show a ferromagnetic metallic ground state with spin polarization of about 50%. This would allow the development of spintronic devices based on the injection of spin-polarized current from a ferromagnetic metal into a semiconductor.In this context the possibility of growing in situ MnSi on the Si(1 1 1) surface has been explored. Thermal reaction, crystalline structure and electronic properties of the grown films have been studied in situ by photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), and low energy electron diffraction (LEED). Depositing a thin film of Mn on Si(1 1 1) the formation of ordered islands (with dimensions dependent on the amount of deposited Mn) is driven by annealing at selected temperatures, as already observed. Our preliminary studies show that by simultaneously depositing Mn and Si in 1:1 stoichiometry on Si(1 1 1) a large improvement in the homogeneity of the MnSi films is achieved.     

Curie temperatures of CoPt ultrathin continuous films

The effects of layer thickness and thermal annealing on Curie temperature have been studied for CoPt ultrathin continuous layers in AlN/CoPt multilayer structures. It is found that there exists a critical thickness below which Curie temperature rapidly decreases due to the loss of spin-spin interactions in the vicinity of interface. After high temperature annealing, the in-plane lattice constant of CoPt film is increased and the exchange coupling parameter is decreased. Consequently, Curie temperatures decrease for some films with large thickness, compared with as-deposited state. Upon annealing at 600?^°C, CoPt undergoes ordering transformation, which also contributes to the degradation of the Curie temperature. However, when the CoPt film thickness is below 2?nm, the Curie temperature is increased after annealing. Especially for 1?nm thick film, the Curie temperature is strikingly increased from 173?^°C to 343?^°C after annealing at 600?^°C. This effect is attributed to the out-of-plane lattice deformation of CoPt thin layers in AlN/CoPt multilayer structures.     

Influence of Magnetization on the Spin Pumping Efficiency in a Ferromagnet–Normal Metal Bilayer Structure

The problem of spin current generation and transformation into electric signals in thin-film ferromagnet/nonmagnetic metal bilayer structures is investigated. This direction is of considerable scientific interest and promising for applications in spintronics. An LSMO/Pt structure consisting of an epitaxial film of ferromagnetic manganite La2/3Sr1/3O3 grown on a single-crystal NdGaO3 substrate and coated with a platinum film has been studied experimentally. The spin current was generated by the spin pumping method upon the excitation of a ferromagnetic resonance in the ferromagnetic layer and was detected by the electric voltage USP arising in the nonmagnetic metal layer due to the inverse spin Hall effect. Owing to its relatively low Curie temperature (~350 K), using LSMO allowed the influence of ferromagnetic-layer magnetization on the spin current generation to be studied in detail in the temperature range 100–350 K. In this case, the influence of the shape of the ferromagnetic resonance line, which is the convolution of homogeneous (Lorentzian) spin packets and inhomogeneous Gaussian broadening (Voigt model), was consistently taken into account. As a result of our analysis of all the parameters defining USP, we have obtained the temperature dependence of the mixed spin conductance, which has turned out to be approximately proportional to the ferromagnet magnetization squared. This result is compared with existing theoretical models.     

多晶Si0.9654Mn0.0346:B薄膜的磁性研究

利用磁控溅射和快速热处理的方法制备了Mn,B共掺的多晶硅薄膜(Si_0.9654Mn_0.0346:B).磁性和结构研究发现薄膜有两个铁磁相.低温铁磁相来源于杂相Mn₄Si₇,高温铁磁相(居里温度T_C～250K)是由Mn原子掺杂进入Si晶格导致.晶化后的薄膜利用射频等离子体增强化学气相沉积系统(PECVD)进行短暂(4min)的氢化处理后发现,薄膜的微结构没有发生变化而饱和磁化强度却随着 关键词： 磁性半导体 硅 氢化     

Photovoltaic effect in thin film of Pb(Zr,Ti)O3 evaporated on stainless steel

A dc photovoltaic effect has been observed in thin films of Pb(Zr, Ti)O₃ evaporated on stainless steel, in the absence of applied field. The spectral photovoltaic response of the film has a peak sensitivity around 4000 Å. The photovoltage exists only below the Curie temperature. The film device becomes semiconductive under illumination.     

Nanoscale multiphase separation at La(2/3)Ca(1/3)MnO3/SrTiO3 interfaces

55Mn nuclear magnetic resonance experiments are reported on a series of fully strained epitaxial La(2/3)Ca(1/3)MnO3 thin films on SrTiO3. We have found evidence of multiple phase segregation into ferromagnetic metallic and nonmetallic regions as well as regions that are nonferromagnetic and insulating. These insulating regions are mainly located close to interfaces and may have a significant impact on the performance of spin-tunnel devices. As a result of phase segregation, the ferromagnetic coupling within the metallic regions is depressed. This accounts for the reduction of the Curie temperature and conductivity in nanometric thin films.     

源于非晶合金的透明磁性半导体

磁性半导体兼具磁性和半导体特性,通过操控电子自旋,有望实现接近完全的电子极化,提供一种全新的导电方式和器件概念.目前磁性半导体的研究对象主要为稀磁半导体,采用在非磁性半导体中添加过渡族磁性元素使半导体获得内禀磁性的方法进行制备.但大部分稀磁半导体仅具有低温磁性,成为限制其在室温可操控电子器件中应用的瓶颈.针对这一关键科学问题,本文提出与传统稀磁半导体制备方法相反的合成思路,在磁性非晶合金中引入非金属元素诱发金属-半导体转变,使磁性非晶获得半导体电性,研制出具有新奇磁、光、电耦合特性的非晶态浓磁半导体,揭示其载流子调制磁性的内禀机理,发展出可在室温下工作的p-n结及电控磁器件.