MISM14

Moscow International Symposium on Magnetism 2014

MISM14_photoDate: 29 June – 3 July 2014

Moscow, Russia


 

Abstracts:

Amorphous ferromagnetic glass coated microwires are very interesting materials because of their unusual micromagnetic structure and magnetic properties. Moreover there are many ways to manipulate the magnetic properties and the domain wall dynamics such as annealing, applying of the mechanical stresses, changing of the ρ-ratio of the metallic nucleus diameter, d, to the diameter of the glass coated microwire, D [2] and etc. This makes them very perspective and promising objects for coding system, magnetic memory applications and logical devices [1, 3]. In this work we studied the influence of annealing and applying of mechanical stresses on magnetic properties and the domain wall dynamics of glass-coated Co68.7Fe4Ni1B13Si11Mo2.3 amorphous ferromagnetic microwire (d = 17.0 μm, D = 23.6 μm, ρ = 0.72). The samples were annealed at annealing temperatures 300, 350 and 400 °C without stress and under different applied stresses up to 300 MPa for different time. The magnetic properties were investigated by induction method for as-cast and annealed samples. As-cast microwires had S-shape hysteresis loop, but after annealing the microwires became bistable. The domain wall velocity of bistable microwires was measured using Sixtus-Tonks technique. During the measuring of velocity of the domain wall propagation the axial mechanical stresses were applied. And it is found that the domain wall velocity can enhance with increasing of axial stresses value. Also we measured the magnetostriction coefficient of studied microwires and investigated the influence of magnetostriction coefficient on magnetic properties for as-cast and annealed samples. We obtained the changing of magnetostriction coefficient sign from negative to positive after annealing that explains the changing of micromagnetic structure and hence the shape of hysteresis loop. Resuming, we studied the magnetic properties and domain wall dynamics of Co68.7Fe4Ni1B13Si11Mo2.3 microwire. We showed that there are a lot of ways to tailor the magnetic properties that can be very useful for future applications.


 

[1] A. Zhukov, et al., Encyclopedia of Nanoscience and Nanotechnology, 2004, V.X.– P.23.

[2] K. Chichay, V. Zhukova, V. Rodionova, M. Ipatov, A. Talaat, J. M. Blanco, J. Gonzalez, and A. Zhukov, J. Appl. Phys. 113, 17A318 doi:10.1063/1.4795617 (2013).

[3] M. Vazquez, Handbook of Magnetism and Advanced Magnetic Materials 4: Novel Materials.– John Wiley & Sons, Ltd., 2007.–P.2192-2226.

Ferromagnetic (FM)/nonmagnetic (NM)/FM multilayers and ferromagnetic/antiferromagnetic (AFM) bilayers have been intensively studied because of physical interest and important applications, which use the giant magnetoresistance (GMR) effect of FM/NM/FM multilayers and exchange bias effect of FM/AFM bilayers [1-3]. We have investigated two types of thin film structures: NiFe/IrMn bi-layers and NiFe/Cu/NiFe/IrMn spin-valves. The thickness of antiferromagnetic layer is the same for all samples and equals 15 nm. The thicknesses of the Cu and NiFe layers are from 2 nm to 10 nm and from 5 to 15 nm, respectively. The samples were prepared by DC magnetron sputtering using a magnetron system ATC ORION-5 produced by AJA INTERNATIONAL, at 3×10-3 Torr argon pressure during the deposition with 10-6 Torr base pressure. The compositions of the IrMn and NiFe targets were checked by the Rutherford backscattering and the X-ray energy dispersive analysis. The compositions of the targets are Ni65Fe35 and Ir29Mn71. The deposition times and rates for the layers were preset basing on dummy calibrating samples thicknesses measured by the Rutherford backscattering technique. The magnetic field of 400 Oe was applied in the sample plane during the deposition to induce the uniaxial anisotropy. The magnetic properties of the samples were studied by using two systems: Lake Shore Vibrating Sample Magnetometer (System 7404) in a temperature range of 100-450 K and in magnetic fields up to 12 kOe and Quantum Design Physical Property Measurement System (PPMS) with the 9T superconducting solenoid and EverCool system. The hysteresis loops and temperature dependence of the magnetization, coercive fields were investigated in the temperature range of 10- 400K. Anisotropic properties, exchange interaction between AFM and FM layers have been studied by ferromagnetic resonance. It was found that the effect of exchange bias in NiFe/Cu/NiFe/IrMn depends on the temperature with a noticeable increase below 70 – 80 K. The factors affecting the temperature dependence of the exchange bias in two and three layer structures are considered. The conditions of the functionality of the spin gate for АFM/FМ/NM/FM structures and its temperature dependence were obtained on the basis of proposals for structural stability both in a magnetic field, and without it.

 

[1] A.E. Berkowitz, K. Takano, J. Magn. Magn. Mater, 200 (1999) 552.

[2] M. Ali, C.H. Marrow, B.J. Hickey, Phys. Rev. B,67 (2003) 172405.

[3] R.D. McMichael, M.D. Stiles, P.J. Chen, W.F. Egelhoff, Phys. Rev. B,58 (1998) 8605.

Studies of amorphous magnetically soft glass-coated microwires have attracted considerable interest in the field of applied magnetism because of their reduced dimensions (metallic nucleus diameter ranging between 0.5 and 30 m), cheap and simple fabrication method and outstanding soft magnetic properties, such as giant magnetoimpedance, GMI, effect and the magnetic bistability and related fast domain wall propagation [1]. Magnetic sensors developed using amorphous wires with GMI effect allow achieving pT magnetic field sensitivity. The shape of magnetic field dependence of the GMI effect is intrinsically related to the magnetic anisotropy and peculiar surface domain structure of amorphous wires. The interest on DW propagation in magnetic wires is related with proposals for magnetic logic and memory devices. Naturally the DW speed is one of most important factors affecting the viability of aforementioned potential applications. Generally soft magnetic properties of amorphous ferromagnetic microwires are affected by the magnetoelastic energy related to quenching stresses distribution arising during rapid solidification from the melt. Consequently optimization of the GMI effect and DW dynamics is essential for the applications of glass-coated magnetic microwires. Fast magnetization switching and related fast DW propagation are typically observed microwires with positive magnetostriction coefficient exhibiting rectangular hysteresis loops [1]. But considerable GMI effect is usually observed in microwires with low and negative magnetostriction constant [1]. From the point of view of applications creation of the samples exhibiting both aforementioned properties simultaneously is very attractive. Therefore, we performed studies of the effect of magnetoelastic anisotropy on magnetic properties, GMI effect and DW propagation in amorphous magnetically soft microwires paying attention to find the conditions for observation of these two effects simultaneously. We studied GMI effect and magnetic properties of amorphous Fe-Co rich as-prepared and annealed microwires. We measured the GMI magnetic field and frequency dependences, hysteresis loops and DWs dynamics in Co-rich and observed that these properties can be tailored either controlling magnetoelastic anisotropy of as-prepared CoFeBSiC microwires or controlling their magnetic anisotropy by heat treatment. High GMI effect has been observed in as-prepared Co-rich microwires. High DW velocity and rectangular hysteresis loops we observed in heat treated Co-rich microwires. At certain annealing conditions we observed coexistence of GMI effect and fast DW propagation in the same annealed sample. Support by by EU under FP7 ―EM-safety project, by Spanish and by the Basque Government is acknowledged.

 

[1] A. Zhukov and V. Zhukova, International Frequency Sensor Association Publishing, Barcelona, (2014) 164

Small values of magneto-optical (MO) effects strongly restrict their practical applications and in recent years several topics of photonic researches were targeted on development of new types of small and reliable biosensors. Sensitivity of MO sensors can be enhanced by creating them on base of magnetoplasmonic crystals (MPC). This way can demonstrate that in such systems, it is possible both to enhance the MO activity of the system via surface plasmon excitation, and to modulate the plasmon properties via application of a magnetic field [1]. We investigated magnetic and MO properties of MPC structures, based on embossed substrates with different spatial profiles, and magnetic properties of structures on smooth Si/SiO2 substrates which were created by ion plasma deposition. Spatial profiles of the MPC were obtained by atomic-force microscopy (AFM). The samples of magnetoplasmonic crystals were fabricated by using polymer templates of commercial digital discs. The polymer spiral gratings inside the Blu-ray disc (BD), DVD, and CD have declared periods of 320, 740, and 1600 nm, respectively. First, protective layer of the digital disc‘s surface was mechanically removed. Then, metals (Ag and Ni) were sputtered on the polymer gratings with thicknesses from 80 to 100 nm and from 5 to 100 nm respectively. In the end structures were covered by SiO2 layer to prevent oxidation.

Magnetic properties of all structures were investigated by vibrating sample magnetometer by LakeShore and it appeared that they greatly depend on thickness of ferromagnetic layer. For samples DVD//Ag(80nm)/Ni(10nm)/SiO2(30nm) and BD//Ag(100nm)/Ni(100nm)/SiO2(30nm) a step-like behavior of hysteresis loops in dependence of the magnetic moment versus magnetic field in case of transverse plasmon propagation way (along the direction of substrate battlements) was observed. At the same time, measurements along the plasmon propagation way showed nearrectangular hysteresis loops typical for Ni-based thin films. Thin films of nickel and iron with same thicknesses on Si/SiO2 substrates were sputtered during same sputtering cycles and in-plane anisotropy was found in case with Ni-based thin films. All Fe-based structures had isotropic inplane magnetic properties. AFM images of surface of structures based on digital discs showed that nickel and iron had partly covered sides of the substrate battlements. Thus, the anisotropy of the magnetic properties of MPC can be explained by the magnetostatic interaction between the different factions of nickel. Anisotropy of magnetic properties for nickel thin films on Si/SiO2 substrates can be explained by its magnetostriction properties of Ni. Last result can explain difference in magnetic properties of Fe-based and Ni-based MPC.

MO response of MPC structures was measured by setup which consists of halogen lamp with a monochromator as a light source, and a photomultiplier tube with a lock-in amplifier as detector and enhancement of transversal Kerr effect and magneto-optical activity in the narrow spectral range of the surface plasmon-polaritons excitation was observed. We determined the optimum thickness of ferromagnetic layer (from the submitted samples) for enhancing MO response.

We need to continue investigation of MPC based on digital discs to improve the quality factor of plasmons and to find a way to control properties of MPC for creating sensible biosensors.


[1] A. Grunin, A. Fedyanin, et. al., Appl. Phys. Lett., 97 (2010) 261908 – 261908-3.

Exchange bias phenomenon in ferromagnetic/antiferromagnetic (F/AF) thin film structures is still of a great interest due to its potential applications such as magnetic sensory devises which are based on giant magnetoresistance effect. Along with technological applications there are also interesting fundamental aspects of this phenomenon, like relative orientation of F- and AF- magnetic moments. Here we report on investigations of exchange bias in ferromagnetic/ antiferromagnetic/ ferromagnetic (F/AF/F) trilayer structures with different thickness of F and AF layers. Two sets of experimental samples Si/Ta 30nm/NiFe tF/IrMn 15nm/NiFe tF/Ta 30nm, where tF = 7nm and 10 nm, and Si/Ta 30nm/NiFe 10 nm/IrMn tAF/NiFe 10 nm/Ta 30nm, where tAF = 10, 20, 30, 40 and 50 nm, were deposited by magnetron sputtering in argon at pressure of 3*10-3 Torr with magnetic field of 420 Oe applied in plane of substrate during the deposition. The ferromagnetic resonance (FMR) spectra normally contained two peaks corresponding to upper (AF/F) and lower (F/AF) interfaces and were measured at different angles between directions of external FMR field and magnetic field, Hdep, applied during the samples depositions. Both F/AF and AF/F interfaces in the sample with tF = 7 nm had a non-symmetric FMR field angular distributions. This result is also confirmed by vibrating sample magnetometer measurements, where a shift of hysteresis loop in perpendicular to Hdep direction was observed in case of this sample. The data assume that the exchange bias direction is not parallel to Hdep. The estimated misalignment angles were 20o and 24o for upper and lower interfaces, respectively. The exchange bias values for the interfaces in the structure with tF = 7nm were 67 and 70 Oe, in the case of the structure with tF = 10nm the exchange bias value increased to 110 and 114 Oe for upper and lower F/AF/F interfaces, respectively. For trilayers with 10 nm F-layer thickness and various AF-layer thickness the direction of exchange bias was strongly parallel to Hdepwhile a misalignment between uniaxial magnetic anisotropy and Hdepdirections was observed. Exchange bias magnitude changes non-monotonically in range of 25-58 Oe for left FMR absorption peak and 40-65 Oe for right absorption peak. The behavior of exchange bias thickness dependence is similar for both interfaces but in all thickness range AF/F interface is characterized by larger exchange bias than F/AF one. Along with larger exchange bias the coercivity of AF/F interface along Hdepdirection is averaged two times greater than that of F/AF. In direction perpendicular to Hdepcoercivity of investigated samples was about 3 Oe.
The phenomenon of exchange bias has been known for over 50 years. It is caused by exchange interaction which occurs at the boundary between ferromagnetic (FM) and antiferromagnetic (AFM) materials [1]. This phenomenon manifests itself as a shift of the ferromagnetic hysteresis loop along the field axis. Although many works were aimed to study exchange biased spin-valve like multilayers (for example, [2]), their properties have not been explained completely yet and the topic of active investigation remains. One of very important challenge from both, fundamental and applications, points of veiw is to find the ways of enhancements of exchange bias field. In this work influence of the thickness of antiferromagnetic (AFM) layer on magnetic properties of bilayered structures with two types of deposition order (FM/AFM, AFM/FM) was studied. Angular dependences of the coercive force and exchange bias field were analyzed for structures with different thickness of AFM layer. The layers thicknesses of samples (2 – 40 nm) were determined through deposition rates of calibrating samples obtained by Rutherford backscattering. The hysteresis loops in film plane were obtained at different orientations of the magnetization easy axis of samples relative to the direction of magnetic field: 0, 45, 90, 135, 180, 225, 270, 315 degrees, with help of vibrating sample magnetometer by LakeShore, 7404 System. Magnetic field was varied in the range of -500 – +500 Oe with increment of 5 Oe. In some samples we observed the maximal values of the exchange bias field in the directions different from the magnetic easy axis. It could be explained by the different surfaces conditions of AFM and FM layers and the formation of a granular structure in AFM structures with certain thickness.

The work is supported by RFBR grant № 12-02-31541.

[1] W.H. Meiklejohn and C.P. Bean, Phys. Rev. 102 (1956) 1413.

[2] Sara Laureti, Y.Suck Sarah, Haas Helge, Eric Prestat, Olivier Bourgeois, Dominique Givord, Physical review letters, 108 (2012) 7, 077205.

Among a variety of half metallic ferromagnetic alloys and compounds, Co-based Heusler alloys possess the highest Curie temperature Tc, the highest magnetization saturation and the highest spin polarization at Fermi level. These properties have great importance for spintronic applications.

We investigated magnetic, magnetotransport and structural properties of the Co2(Fe,Ti)Ga thin films with different chemical composition, prepared by a magnetron sputtering technique in Ar atmosphere. Chemical composition was controlled by the changing of the sputtering power of the targets. The measurements were performed on as-prepared and annealed samples. Energy dispersive spectrometer (Oxford X-Act) was used to detect the composition of thin films. The structure of prepared samples was examined by XRD analysis using a Rigaku diffractometer with CuKα radiation. Magnetic measurements were performed by vibration sample magnetometer (Lakeshore 7400) in a temperature range of 100-450K. In-plane hysteresis loops were measured for two different orientations of the samples in relation to the direction of the magnetic field (0, 90 grad) at different temperatures. Magnetoresistance of Co2(Fe, Ti)Ga thin films was obtained in magnetic field up to 10 kOe in a temperature range 100-450 K.

The presence of well-defined (220) and (422) peaks corresponding to the principal reflection of the Heusler structure were revealed by XRD analysis. We found that the magnitude of the Magnetoresistance increases with increasing temperature. Measured in-plane hysteresis loops show anisotropic behavior in some samples and demonstrate step-like shape. Obtained results indicate that magnetic properties of Co2(Fe, Ti)Ga thin films strongly depend on the Fe and Ti concentrations.

The magnetization process of biphase microwires has been investigated along the last few years at room temperature. While most of devices are envisaged to work around room temperature, technological applications can in principle be extended to a much wider range of temperatures, particularly above room temperature [1]. In this regard, it is important to analyze in further detail the high temperature dependence of magnetic properties (saturation magnetization, hysteresis loops shape, susceptibility, coercivity) of biphase magnetic microwires, which has been the objective of the present work. The ferromagnetic biphase amorphous microwires were prepared by combined quenching & drawing and electroplating techniques [2]. Using first method we prepared the glasscoated microwires with compositions: (Co0.94Fe0.06)72.5Si12.5B15 (λS~0, λS – saturation magnetostriction constant) and Fe77.5Si7.5B15 (λS>0). The diameters of the metallic core were d=8µm and d=12µm, respectively. Then a second (external) magnetically soft FeNi phase was electroplated onto the single microwires. The thicknesses of the FeNi shell were tFeNi=1-4 μm (tFeNi was controlled by the electroplating time) [2]. The samples were 4 mm length. The magnetic properties for both types of microwires were analyzed as a function of temperature in the range from 295 K to 1200 K in aVibrating Sample Magnetometer (Lake Shore) under argon atmosphere.

The biphase microwires exhibit two-steps hysteresis loops at room temperature, each of them corresponds to respective remagnetization process of the phases: of the core (take place in small magnetic field) and of the shell (take place in higher magnetic field). When the temperature increase the steps became smaller and disappeared after the reaching of the Curie temperature of the core Tc=700 K. The temperature dependence of magnetic moment under applied magnetic field of 100 Oe for Fe/FeNi microwire with the thicknesses of the shell of 1 µm is depicted in the Figure.

Figure. Temperature dependence for Fe/FeNi microwires with tFeNi= 1 μm in the temperature range from 295K to 1200K.

The magnetic moment decreases with temperature from 300 to 700K (that corresponds to the Curie temperature of the core), after T=775K starts to increase again and after T=800K decreases until T=1200K. Analysis of the magnetization process of each phase in each measuring temperature region has been performed


[1] M. Vazquez, H. Chiriac, A. Zhukov, L. Panina and T. Uchiyama, Phys. Stat. Sol. A, (2010) 1-9.

[2] K. Pirota, M. Hernandez-Velez, D. Navas, A. Zhukov, M. Vazquez, Adv. Funct. Mater. 14 (3) (2004) 266–268.

Heusler alloys attract a lot of attention last decades because of many interesting properties, inherent them. Ni-Mn-In is one of the most studied Heusler alloys, because of the many interesting properties, important for different applications. First of all it is the giant magnetocaloric effect [1], which can be used to create magnetic refrigerator.

Obtaining of the new application opportunities of this exceptional material is possible due to the using of thin films. Therefore, it is very important to know structural and magnetic properties changing upon the decreasing z-dimension of the material from bulk to thin film. We report here on the results of formation and investigation of polycrystalline thin films on MgO and Si substrates by pulsed laser deposition and two-lasers co-deposition. Structural and magnetic properties of these samples were investigated. It was shown, that at same composition (Ni52Mn36In12) polycrystalline samples on oxadized Si and MgO substrates are formed in different structural phases: B2 phase for MgO substrate and L21 for Si/SiO2.

Fig. 1 XRD patterns for Si/SiO2//
Ni51Mn33In16 sample observed at
different temperatures

For both type of samples the structural dynamics of the martensitic transition and magnetic properties, depending on the composition, thickness and deposition parameters were investigated. For several samples near-room tempersture or low temperature martensitic transition has been found (fig. 1). For most of these samples magnetization data depending on temperature in the interval of martensitic transition are strongly differ from bulk samples with similar concentrations.


The work is supported by Ministry of Education and Science of the Russian Federation (Contracts № 14.Y26.31.0002 and 02.G25.31.0086)

[1] J.Liu, T. Gottschall, K.P.Skokov// Nature Materials., 11 (2012)620–626.

The heat pump cycles based on inducement of the AF-F transition in FeRh alloys by tensile stress and magnetic field, were recently discussed [1, 2]. Since this transition can be induced by pressure as well [3], it may be used for construction of combined heat pump cycles with sign-changing mechanical load of the alloy, which may be useful, for example, for broadening of the cycles’ temperature range. Present work is devoted to estimation of the efficiency of the heat pump cycles based on the pressure induced F-AF transition in FeRh alloy.

The efficiency of a heat-pump cycle is expressed in terms of the coefficient of performance, θ, which is defined as the ratio of the amount of heat given by the system undergoing the cycle to the heat receiver, QHR, to the work transfer of energy into the system to accomplish this effect. On the base of the model S–T projection of S–T–P surface for FeRh alloy [4] the 3 types of heat pump cycles are drawn up:

(1) with acquisition by the alloy the heat from external heat source;

(2) partial acquisition by the alloy the heat from external heat source;

(3) and the cycles based on the adiabatic inducement the F-AF transition by pressure.

Fig. 1. Efficiency θ1 of a heat-pump cycles based
Fig. 1. Efficiency θ1 of a heat-pump cycles based on inducement of the F–AF transition in FeRh by pressure. Tc(P’) – the critical temperature of the F-AF transition at pressure P’, Ts – the temperature of the surroundings.

Corresponding efficiencies θ1, θ2, and θ3 of these cycles as functions of temperature at various differences between initial and final temperatures (permanent thermal load ΔT) and as a function of pressure at various differences between initial and final temperatures are analyzed using the certain set of the experimental data [4].

It is found, particularly (Fig. 1), that within the temperature range (320 – 370) K and pressure range (0.4 – 1.0)∙109 Pa efficiency of the heat-pump cycles takes the values from the interval from 30 to 35. These values are comparable to those found in [1] with using the tensile stress up to ψ = 1∙109 Pa and in [2] with using the magnetic field up to H = 2∙106 A/m.


[1] M.P. Annaorazov et al., Int. J. Refrigeration, 25 (2002) 1034-1042.

[2] M.P. Annaorazov et al., J. Magn. Magn. Mater., 251 (2002) 61-73.

[3] L.I. Vinokurova et al., phys. stat. sol. (b), 78 (1976) 353-357.

[4] M.P. Annaorazov, J. Alloys Compd. 354 (2003) 1–5.


Moscow International Symposium on Magnetism (MISM`14)