Evolution of magnetic domain structure of martensite in Ni–Mn–Ga films under the interplay of the temperature and magnetic field

Ferromagnetic shape memory Ni-Mn-Ga films with 7M modulated structure were prepared on MgO （001） substrates by magnetron sputtering. Magnetization process with a typical two-hysteresis loop indicates the occurrence of the reversible magnetic field-induced reorientation. Magnetic domain structure and twin structure of the film were controlled by the in- terplay of the magnetic and temperature field. With cooling under an out-of-plane magnetic field, the evolution of magnetic domain structure reveals that martensitic transformation could be divided into two periods： nucleation and growth. With an in-plane magnetic field applied to a thermomagnetic-treated film, the evolution of magnetic domain structure gives evidence of a reorientation of twin variants of martensite. A microstructural model is described to define the twin structure and to produce the magnetic domain structure at the beginning of martensitic transformation; based on this model, the relationship between the twin structure and the magnetic domain structure for the treated film under an in-plane field is also described.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Effects of Cu on the martensitic transformation and magnetic properties of Mn_(50)Ni_(40)In_(10) alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40-xCuxIn10, Mn50-xCuxNi40In10, and Mn50Ni40In10-xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.     

Temperature dependences of optical properties,chemical composition,structure,and laser damage in Ta2O5 films

Ta2O5 films are prepared by e-beam evaporation with varied deposition temperatures,annealing temperatures,and annealing times.The effects of temperature on the optical properties,chemical composition,structure,and laserinduced damage threshold(LIDT) are systematically investigated.The results show that the increase of deposition temperature decreases the film transmittance slightly,yet annealing below 923 K is beneficial for the transmittance.The XRD analysis reveals that the film is in the amorphous phase when annealed below 873 K and in thehexagonal phase when annealed at 1073 K.While an interesting near-crystalline phase is found when annealed at 923 K.The LIDT increases with the deposition temperature increasing,whereas it increases firstly and then decreases as the annealing temperature increases.In addition,the increase of the annealing time from 4 h to 12 h is favourable to improving the LIDT,which is mainly due to the improvement of the O/Ta ratio.The highest LIDT film is obtained when annealed at 923 K,owing to the lowest density of defect.     

Temperature dependence of multi-jump magnetic switching process in epitaxial Fe/MgO(001) films

Temperature dependence of magnetic switching processes with multiple jumps in Fe/MgO(001) films is investigated by magnetoresistance measurements. When the temperature decreases from 300K to 80K, the measured three-jump hysteresis loops turn into two-jump loops. The temperature dependence of the fourfold in-plane magnetic anisotropy constant K1, domain wall pinning energy, and an additional uniaxial magnetic anisotropy constant KUare responsible for this transformation. The strengths of K1 and domain wall pinning energy increase with decreasing temperature, but KU remains unchanged. Moreover, magnetization reversal mechanisms, with either two successive or two separate 90°domain wall propagation, are introduced to explain the multi-jump magnetic switching process in epitaxial Fe/Mg O(001) films at different temperatures.     

Perpendicularly oriented barium ferrite thin films with low microwave loss,prepared by pulsed laser deposition

Barium ferrite(Ba M) thin films are deposited on platinum coated silicon wafers by pulsed laser deposition(PLD).The effects of deposition substrate temperature on the microstructure,magnetic and microwave properties of Ba M thin films are investigated in detail.It is found that microstructure,magnetic and microwave properties of Ba M thin film are very sensitive to deposition substrate temperature,and excellent Ba M thin film is obtained when deposition temperature is 910℃ and oxygen pressure is 300 m Torr(1 Torr = 1.3332×10~2Pa).X-ray diffraction patterns and atomic force microscopy images show that the best thin film has perpendicular orientation and hexagonal morphology,and the crystallographic alignment degree can be calculated to be 0.94.Hysteresis loops reveal that the squareness ratio(M_r/M_s) is as high as 0.93,the saturated magnetization is 4004 Gs(1 Gs = 10~4T),and the anisotropy field is 16.5 kOe(1 Oe = 79.5775 A·m~(-1)).Ferromagnetic resonance measurements reveal that the gyromagnetic ratio is 2.8 GHz/kOe,and the ferromagnetic resonance linewith is108 Oe at 50 GHz,which means that this thin film has low microwave loss.These properties make the Ba M thin films have potential applications in microwave devices.     

Effect of Different Substrate Temperature on Phosphorus-Doped ZnO Thin Films Prepared by PLD on Sapphire Substrates

Phosphorus-doped ZnO （ZnO：P） thin films are deposited on a c-plane sapphire in oxygen at 350℃, 450℃, 550℃ and 650℃, respectively, by pulsed laser deposition （PLD）, then all the ZnO：P samples are annealed at 650℃ in oxygen with a pressure of 1 × 10^5 Pa. X-ray diffraction measurements indicate that the crystalline quality of the ZnO：P thin films is improved with the increasing substrate temperature from 350℃ to 550℃. With a further increase of the deposition temperature, the crystalline quality of the ZnO：P sample is degraded. The measurements of low-temperature photoluminescence spectra demonstrate that the samples deposited at the substrate temperatures of 350℃ and 450℃ show a strong acceptor-bound exciton （A^0X） emission. The electrical properties of ZnO：P films strongly depend on the deposition temperature. The ZnO：P samples deposited at 350℃ and 450℃ exhibit p-type conductivity. The p-type ZnO：P film deposited at 450℃ shows a resistivity of 1.846Ω·cm and a relatively high hole concentration of 5.100 × 10^17 cm^-3 at room temperature.     

Structural Characterization and Photoluminescent Properties of Zn1—xMgxO Films on Silicon

Zn1-xMgxO films have been grown on silicon at various substrate temperatures by pulsed laser deposition.The structural and photoluminescent properties of films as a function of substrate temperature have been studied.The optimized substrate temperature is 650℃.The x-ray diffraction spectra indicate that the films are highly C-axis oriented,and no phase separation is observed.The crystal grain size of the films is about 100nm as examined by atomic force microscopy.The cross-sectional transmission electron microscopy verified the C-axis orientation of the Zn1-xMgxO.Thesr films showed ultraviolet photoluminescence at room temperature.The near-band-edge emission peak of the Zn1-xMgxO film deposited at 600℃ has a blueshift (0.40eV) larger than that of the film deposited at 500℃ (0.33eV).The ratio of the near-band-edge to defect level peak intensity is as large as 159.     

Effects of Substrate Temperature on Helium Content and Microstructure of Nanocrystalline Titanium Films

Helium-charged nanocrystalline titanium films have been deposited by HeAr magnetron co-sputtering. The effects of substrate temperature on the helium content and microstructure of the nanocrystalline titanium films have been studied. The results indicate that helium atoms with a high concentration are evenly incorporated in the deposited titanium films. When the substrate temperature increases from 60℃ to 350℃ while the other deposition＇parameters are fixed, the helium content decreases gradually from 38.6 at.% to 9.2at.%, which proves that nanocrystalline Ti films have a great helium storage capacity. The 20 angle of the Bragg peak of （002） crystal planes of the He-charged Ti film shifts to a lower angle and that of （100） crystal plane is unchanged as compared with that of the pure Ti film, which indicates that the lattice parameter c increases and a keeps at the primitive value. The grain refining and helium damage result in the diffraction peak broadening.     

Optical and Electrical Properties Evolution of Diamond-Like Carbon Thin Films with Deposition Temperature

Optical and electrical properties of diamond-like carbon （DLC） films deposited by pulsed laser ablation of graphite target at different substrate temperatures are reported. By varying the deposition temperature from 400 to 25℃, the film optical transparency and electrical resistivity increase severely. Most importantly, the transparency and resistivity properties of the DLC films can be tailored to approaching diamond by adjusting the deposition temperature, which is critical to many applications. DLC films deposited at low temperatures show excellent optical transmittance and high resistivity. Over the same temperature regime an increase of the spa bonded C content is observed using visible Raman spectroscopy, which is responsible for the enhanced transparency and resistivity properties.     

The effect of Si content on the martensitic transformation temperature of Ni55.5Fe18Ga26.5-xSix alloys

This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni55.5Fe18Ga26.5-xSix alloys is determined by x-ray diffraction (XRD),and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni55.5Fe18Ga26.5-xSix alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations,size factor,L21 degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.     

Relation between martensitic transformation temperature range and lattice distortion ratio of NiMnGaCoCu Heusler alloys

In order to study the relation between martensitic transformation temperature range AT （where AT is the difference between martensitic transformation start and finish temperature） and lattice distortion ratio （c/a） of martensitic transforma~ tion, a series of Ni46Mnz8_xGa22Co4Cux （x = 2-5） Heusler alloys is prepared by arc melting method. The vibration sample magnetometer （VSM） experiment results show that AT increases when x 〉 4 and decreases when x 〈 4 with x increasing, and the minimal AT （about 1 K） is found at x = 4. Ambient X-ray diffraction （XRD） results show that AT is proportional to c/a for non-modulated Ni46Mn28_xGa22Co4Cux （x = 2-5） martensites. The relation between AT and c/a is in agreement with the analysis result obtained from crystal lattice mismatch model. About 1000-ppm strain is found for the sample at x = 4 when heating temperature increases from 323 K to 324 K. These properties, which allow a modulation of AT and temperature-induced strain during martensitic transformation, suggest Ni46Mn24Ga22Co4Cu4 can be a promising actuator and sensor.     

HEM11 mode magnetically insulated transmission line oscillator： Simulation and experiment

A novel magnetically insulated transmission line oscillator （MILO） in which a modified HEM11 mode is taken as its main interaction mode （HEM11 mode MILO） is simulated and experimented in this paper. The excitation of the oscillation mode is made possible by carefully adjusting the arrangement of each resonant cavity in a two-dimensional slow wave structure. The special feature of such a device is that in the slow-wave-structure region, the interaction mode is HEM11 mode which is a TM-like one that could interact with electron beams effectively; and in the coaxial output region, the microwave mode is TE11 mode which has a favourable field density pattern to be directly radiated. Employing an electron beam of about 441 kV and 39.7 kA, the HEM11 mode MILO generates a high power microwave output of about 1.47 GW at 1.45 GHz in particle-in-cell simulation. The power conversion efficiency is about 8.4 % and the generated microwave is in a TEll-like circular polarization mode. In a preliminary experiment investigation, high power microwave is detected from the device with a frequency of 1.46 GHz, an output energy of 43 J 47 J, and a pulse duration of 44 ns-49 ns when the input voltage is 430 kV450 kV, and the diode current is 37 kA-39 kA.     

Exchange coupling and helical spin order in the triangular lattice antiferromagnet CuCrO2 using first principles

The magnetic and electronic properties of the geometrically frustrated triangular antiferromagnet CuCrO2 are investigated by first principles through density functional theory calculations within the generalized gradient approxi- mations （GGA）＋U scheme. The spin exchange interactions up to the third nearest neighbours in the ab plane as well as the coupling between adjacent layers are calculated to examine the magnetism and spin frustration. It is found that CuCrO2 has a natural two-dimensional characteristic of the magnetic interaction. Using Monte Carlo simulation, we obtain the Neel temperature to be 29.9 K, which accords well with the experimental value of 24 K. Based on non- collinear magnetic structure calculations, we verify that the incommensurate spiral-spin structure with （110） spiral plane is believable for the magnetic ground state, which is consistent with the experimental observations. Due to intra-layer geometric spin frustration, parallel helical-spin chains arise along the a, b, or a＋ b directions, each with a screw-rotation angle of about I20°. Our calculations of the density of states show that the spin frustration plays an important role in the change of d-p hybridization, while the spin-orbit coupling has a very limited influence on the electronic structure.     

Influence of Ni/Mn ratio on magnetostructural transformation and magnetocaloric effect in Ni_(48-x)Co_2Mn_(38+x)Sn_(12)(x = 0, 1.0, 1.5, 2.0,and 2.5) ferromagnetic shape memory alloys

An investigation on the magnetostructural transformation and magnetocaloric properties of Ni_(48-x)Co_2Mn_(38+x)Sn_(12)(x = 0, 1.0, 1.5, 2.0, and 2.5) ferromagnetic shape memory alloys is carried out. With the partial replacement of Ni by Mn in the Ni_(48)Co_2Mn_(38)Sn_(12) alloy, the electron concentration decreases. As a result, the martensitic transformation temperature is decreased into the temperature window between the Curie-temperatures of austenite and martensite. Thus, the samples with x = 1.5 and 2.0 exhibit the magnetostructural transformation between the weak-magnetization martensite and ferromagnetic austenite at room temperature. The structural transformation can be induced not only by the temperature,but also by the magnetic field. Accompanied by the magnetic-field-induced magnetostructural transformation, a considerable magnetocaloric effect is observed. With the increase of x, the maximum entropy change decreases, but the effective magnetic cooling capacity increases.     

Ni4Ti3沉淀相对NiTi形状记忆合金相变行为的影响

采用第二近邻修正型嵌入原子势的分子动力学方法,建立了共格沉淀相与半共格沉淀相块状/柱状模型,模拟了温度诱发相变和应力诱发相变,分析了Ni₄Ti₃沉淀相对Ni Ti形状记忆合金相变行为的影响．结果表明,Ni₄Ti₃沉淀相本征应变诱发的弹性应力场对相变中马氏体变体类型、形核位置、分布等有重要影响.在温度诱发相变时,共格沉淀相促进部分马氏体变体的形核生长,能显著提高Ni Ti超弹性形状记忆合金的马氏体相变开始温度;在应力诱发相变时,Ni₄Ti₃沉淀相使马氏体早于无沉淀相区域形核,导致了相变应力降低、抑制了马氏体解孪,减小了应力-应变曲线的滞回环.     

New aspects of martensite stabilization in Ni–Mn–Ga high-temperature shape memory alloy

We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion.     

Combined experimental and theoretical study on the effect of Nb content on martensitic transformation of NbRu shape memory alloys

The effect of Nb content on the martensitic transformation of NbRu high-temperature shape memory alloys is investigated by experiments and first-principles calculations. We calculate the lattice parameters, density of states, charge density, and heats of formation of Nb_50+xRu_50-x β phase. The results show that an increase in Nb content increases the stability of Nb_50+xRu_50-x β phase, leading to a significant decrease of the β to β' martensitic transformation temperature. In addition, the mechanism of the effects of Nb content on phase stability and martensitic transformation temperature is studied on the basis of electronic structure.