Typical Ferromagnetic Materials Research Articles

Analysis of the new ternary phase with C6Cr23 structure in Mg-Co-B system by Rietveld method and physical properties of its Ni-substituting effect

• New ternary Mg 1.4 Co 21.6 B 6 compound was firstly reported. • The crystal structure of Mg 1.4 Co 21.6 B 6 was determined by Rietveld XRD method. • The instability of Mg 1.4 Co 21.6 B 6 at elevated temperatures has been discussed. • Physical properties of Ni-substituting Mg 1.4 Co 21.6 B 6 have been investigated. A new ternary Mg 1.4 Co 21.6 B 6 compound in the Mg-Co-B system was synthesized via a conventional solid-state reaction method and the effect of Ni-substitution on its crystal structure, thermal stability, solid solubility and physical properties were systematically investigated. The crystal structure of the Mg 1.4 Co 21.6 B 6 compound was fully determined by the X-ray diffraction technique with Rietveld refinement method. It is found that Mg 1.4 Co 21.6 B 6 crystallizes in the form of C 6 Cr 23 structure type (space group: Fm-3m (No.225), a = 10.5617(2)Å, Z = 4). The results showed that the 4a sites have been occupied completely by Co atoms in present compound which with M 2-x Ni 21+x B 6 form belonging to the W 2 Cr 21 C 6 -type. When Mg 1.4 Co 21.6 B 6 is repeatedly sintered at elevated temperatures, it becomes unstable and decomposes into Co 3 B and Mg. The lattice parameters of the Mg 1.4 Co 21.6 B 6 solid solution alters dramtically with increasing Ni substitution, with no regular trend being observed. The electrical and magnetic performances of the 3.6Mg:3Co:17Ni:6B and 3.6Mg:3Co:18Ni:6B (nominal compositions) samples suggest that both samples are typical ferromagnetic materials. The temperature in the maximum drop of the ρ(T) curve decreases as a function of the Ni content. Base on the correlation between the critical temperature and Ni content, a linear fitting equation is obtained and the critical temperature of Mg 1.4 Co 21.6 B 6 calculated utilizing the linear fitting equation. The findings in this work may provide certain reference values for material science on electrical magnetic properties and other references for researching the material further.

Ultrahigh differential mobility and velocity of Néel domain walls in spin valves with planar-transverse polarizers and perpendicularly injected small currents

Transverse domain wall (TDW) dynamics in long and narrow spin valves with perpendicular current injection is theoretically investigated. We demonstrate that stable traveling-wave motion of TDWs with finite velocity survives for strong enough planar-transverse polarizers. For typical ferromagnetic materials (for example, Co) and achievable spin polarization ($P=0.6$), TDWs acquire a velocity of $10^3$ m/s under a current density below $10^7$ $\mathrm{A/cm^2}$. This efficiency is comparable with that of perpendicular polarizers. More importantly, in this case the wall has ultra-high "differential mobility" around the onset of stable wall excitation. Our results open new possibilities for developing magnetic nanodevices based on TDW propagation with low energy consumption. Also, analytics for parallel and perpendicular polarizers perfectly explains existing simulation findings. Finally, further boosting of TDWs by external uniform transverse magnetic fields is investigated and turns out to be efficient.

나노입자 합성방법에 따른 타이타늄산바륨 나노입자뭉침 현상 연구

타이타늄산바륨(<TEX>$BaTiO_3$</TEX>)은 대표적인 강유전 물질로 유전상수가 200 이상의 값을 나타내는 물질이다. 타이타늄산바륨을 나노입자화하면 나노커패시터(nanocapacitors)와 강유전체 메모리(ferroelectric random access memories)와 같이 여러 용도로 응용 가능하다. 하지만, 나노입자의 합성방법에 따라 나노입자의 분산특성이 달라지며 이에 활용할 수 있는 분야가 달라질 수 있다. 본 연구에서는 타이타늄산바륨 나노입자를 옥살레이트법(oxalate method)과 sol-gel법(ambient condition sol method)으로 합성하고 각 방법에 따른 나노입자의 크기와 분산상태를 확인하였다. 각각의 공정에 사용한 캡핑 에이전트(capping agent)는 poly vinyl pyrrolidone (PVP)을 옥살레이트법에 이용하였고 sol-gel법에는 tetrabutylammonium hydroxide (TBAH)를 이용하였다. 합성된 나노입자의 X-선 회절 분석 패턴을 분석하여 cubic 결정구조를 갖는 타이타늄산바륨을 확인하였다. 푸리에(Fourier) 변환 적외선 분광분석을 이용하여 나노입자의 캡핑 에이전트 결합상태와 시차주사현미경과 입도분석기를 이용한 나노입자의 크기 및 뭉침 변화를 확인하였다. <TEX>$BaTiO_3$</TEX> is typical ferromagnetic materials with dielectric constant of above 200. <TEX>$BaTiO_3$</TEX> nanoparticles applications are available for multiple purposes such as nanocapacitors, ferroelectric random access memories, and so on. Applications are is diverse from the dispersion of nanoparticles depending on the route of synthesis. In this study, <TEX>$BaTiO_3$</TEX> nanoparticles were synthesized by two different methods such as oxalate method and sol-gel process (ambient condition sol method). Particle size and dispersion condition were studied according to the preparation method and capping agent. Poly vinyl pyrrolidone (PVP) was used as a capping agent in oxalate method and tetrabutylammonium hydroxide (TBAH) used as a capping agent in sol-gel process each. Cubic crystal structure of <TEX>$BaTiO_3$</TEX> phase could be confirmed by X-ray diffraction analysis. Fourier transform-infrared spectroscopy was employed for the confirmation of the capping agent and <TEX>$BaTiO_3$</TEX> nanoparticles. The particle size and distribution analysis was also performed by particles size analyzer and scanning electron microscope.

High–Resolution Magnetic Field Measurement Using an STM–SQUID

Abstract We have developed an STM–SQUID microscope that combined a scanning tunneling microscope (STM) and a SQUID probe microscope for the purpose of high–resolution magnetic field measurement. We were able to obtain a spatial resolution of 1 μm or less by polishing the tip of the needle on the SQUID probe microscope to 100 nm or less, and by using a magnetic probe, which was the flux guide, as the STM probe. In this study, we took magnetic images of typical ferromagnetic materials, such as bulk iron and bulk nickel using the STM–SQUID microscope. We successfully observed the magnetic domain structures of these magnetic materials. However, topographic artifacts were also observed in the STM–SQUID images due to the background magnetic field from the sample. The future subject for resolution and quality improvement became clear.

Magnetoresistance due to domain wall bulging

The effect of bulging of domain wall (DW) on the magnetoresistance (MR) is investigated. With taking into account the auto-correlation between the points of the interface, one can formulate the mobility within the relaxation time approximation scheme. The results show that the bulging of DW, evaluated with the commonly accepted magnetic parameters for typical ferromagnetic materials of Co, Fe and Ni, has a countable role into the MR.

Spin glass behavior and magnetocaloric effect in amorphous alloys Ce2Fe23−xMnxB3

The magnetic properties and magnetocaloric effect of amorphous alloys Ce2Fe23−xMnxB3 (1≤x≤6) were investigated. The magnetic properties are sensitive to the composition. For samples with x≤3, the low fraction and isolated Fe–Mn antiferromagnetic (AFM) coupling simply align antiparallel to the majority ferromagnetic (FM) order and reduce the total magnetization but cause no noncollinearity; thus they are typical FM materials with TC decreasing drastically from 336 to 226 K and the magnetization has a sharp drop around respective TC without thermal hysteresis suggesting a second order phase transition resulting from their amorphous nature. For x=4 and 5, amorphous alloys Ce2Fe23−xMnxB3 experience two transitions: first a paramagnetic (PM)-FM second transition at 143 and 81 K, respectively, and then a FM–spin glass (SG) transition. A PM-SG transition occurs at the freezing temperature (Tf) about 28.2 K for x=6. The SG behavior in amorphous alloy Ce2Fe23−xMnxB3 can be attributed to the disordered structure resulting from their amorphous nature and the competition between the Fe–Fe FM coupling and Fe–Mn AFM negative coupling. The magnetic entropy change for alloys Ce2Fe23−xMnxB3 with x≤3 is calculated using the Maxwell relation.

Three-dimensional constitutive equations of ferromagnetic materials with magnetoelastic coupling: Determination of elastic coefficients under magnetic field

This paper determines the elastic coefficients in the constitutive equations of typical ferromagnetic materials. The constitutive equations are given by rate-type equations for the magnetization and the strain expressed in terms of the magnetic field and the stress. The authors have shown that the constitutive equations can describe the stress-dependent hysteretic magnetization curve, the stress magnetization effects, the stress-dependent magnetostriction curve, etc. They have determined the coefficients except the elastic coefficients as functions of the magnetization and the stress for typical ferromagnetic materials. In this paper, we attempt to obtain the elastic coefficients depending on the magnetization and the stress. To do it, applying the magnetoacoustic effect, we measure the stress–strain curves and the speeds of the longitudinal and the transverse ultrasonic waves under the magnetic field.

Magnetic mesa structures fabricated by reactive ion etching withCO/NH3/Xe plasma chemistry for an all-silicon quantum computer

Micromachining of magnetic materials is becoming a crucial technology for futuremagnetic systems/devices such as high-density magnetic recording heads, high-densitypatterned media, magnetic quantum devices, and micro-magnetic smart systems. NiFealloy, one of the typical ferromagnetic materials, can be deposited and fabricatedusing current thin film and micromachining technologies, and it has found variousapplications in microsensors, microactuators, and microsystems. In our recent work,Ni45Fe55 alloy with the highest saturation magnetic-flux density was selected as themicro-magnet for an all-silicon quantum computer. The micro-magnetwas designed to create a large magnetic field gradient and to distinguish29Si nuclear spinsarranged as chains in a 28Si matrix with Larmor frequencies. This paper reports the micromachining of aNi45Fe55 alloy mesa structure for an all-silicon quantum computer and the preliminary evaluation ofits effectiveness using a magnetic force microscope (MFM) and a superconducting quantuminterference device (SQUID).

Solutions for curvilinear cracks in bonded dissimilar materials subject to uniform magnetic induction

The magnetoelastic problem of curvilinear cracks lying along the interface between two dissimilar ferromagnetic materials under remote uniform magnetic induction is considered. On the basis of the Hilbert formulation and a special technique of analytical continuation, the solutions of the magnetic and the magnetoelastic stress fields either in the matrix or in the inclusion are expressed in a general form. Just like the pure elastic problem of curved cracks or the magnetoelastic problem of straight cracks in bonded dissimilar media, the trig-log character for the singularity of magnetoelastic stresses is found in this paper. Both the explicit solutions and the numerical results for the illustrating case of a single circular-arc crack in bi-material plate with typical ferromagnetic materials are given to elucidate the use of the present study. It is noted that the stress intensity factors are obtained and verified by comparison with the existing ones under special cases. The validity of the crack opening condition is also discussed.

Conditions for noncollinear instabilities of ferromagnetic materials.

Two criteria have been identified here which determine whether a magnetic metal orders in a collinear (e.g., ferromagnet) or noncollinear (e.g., spin-spiral) arrangement. These criteria involve the ratio between the strength of the exchange interaction and the width of the electron bands, as well as Fermi-surface nesting between spin-up and spin-down sheets of the Fermi surface. Based on our analysis we predict that even typical ferromagnetic materials (e.g., Fe, Co, and Ni) should be possible to stabilize in a noncollinear magnetic order in, e.g., high pressure experiments.

A magnetization model for computational magnetodynamics

In order to calculate the magnetodynamic fields exactly, it is essential to work out a magnetization model. We have previously proposed a Chua-type magnetization model based on magnetic domain theory. This Chua-type model is now applied to typical ferromagnetic materials, such as iron, ferrite, and amorphous magnetic material. As a result, it is revealed that the typical magnetization characteristics of representative ferromagnetic materials can be satisfactory reproduced by our Chua-type model.

Representation of hysteresis loops by rational fraction approximations

An approximation method of symmetrical loops on the B/H plane is proposed, based on the use of integral functions of rational fraction approximations.The method involves the decomposition of the curves to be approximated into two (odd and even) functions and the preliminary definition of some geometrical parameters identifying the curves themselves, such as, for example, the differential permeability in some characteristic points.By imposing these geometrical constraints, the unknown coefficients can be easily obtained and relevant errors are not introduced by the approximating function into the development of non-linear simulations.The proposed method is applied to some typical ferromagnetic materials.

Acoustic emission characteristics during magnetization of ferromagnetic materials

The characteristics of acoustic emission generated during magnetization of typical ferromagnetic materials (steel, nickel, and cobalt) in alternating magnetic field of 50 Hz were investigated. Acoustic emission parameters measured were root‐mean‐square voltage, frequency spectrum, and waveform. The data were taken as a function of stress, magnetic field strength, and annealing time. It became clear that the magnitudes of acoustic emission during magnetization of steel and nickel were strongly dependent on stress and the features of these stress dependencies changed with magnetic field strength. It was found that the magnitude of acoustic emission during magnetization of cobalt was much smaller than those of steel and nickel. This experimental result and the theoretical calculation of elastic energy emitted by domain wall motion show that the motion of 90° domain wall is the most probable source for the acoustic emission during magnetization of ferromagnetic materials.