Premartensitic and martensitic phase transitions in ferromagnetic Ni2MnGa

We present an experimental study of the premartensitic and martensitic phase transitions in a Ni2MnGa single crystal by using ultrasonic techniques. The effect of applied magnetic field and uniaxial compressive stress has been investigated. It has been found that they substantially modify the elastic and magnetic behavior of the alloy. These experimental findings are a consequence of magnetoelastic effects. The measured magnetic and vibrational behavior agrees with the predictions of a recently proposed Landau-type model [A. Planes et al., Phys. Rev. Lett. 79, 3926 (1997)] that incorporates a magnetoelastic coupling as a key ingredient.

Modeling premartensitic effects in Ni2MnGa: A mean-field and Monte Carlo simulation study

ic first-order transition line ending in a critical point. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling. In particular, the premartensitic transition occurs when such coupling is strong enough to freeze the involved mode phonon. The implication of the results in relation to the available experimental data is discussed.

Temperature and magnetic-field dependence of the elastic constants of Ni-Mn-Al magnetic Heusler alloy

We report on measurements of the adiabatic second-order elastic constants of the off-stoichiometric Ni54Mn23Al23 single-crystalline Heusler alloy. The variation in the temperature dependence of the elastic constants has been investigated across the magnetic transition and over a broad temperature range. Anomalies in the temperature behavior of the elastic constants have been found in the vicinity of the magnetic phase transition. Measurements under applied magnetic field, both isothermal and variable temperature, show that the value of the elastic constants depends on magnetic order, thus giving evidence for magnetoelastic coupling in this alloy system.

Magnetoelasticity and magnetoresistance in Cu-Al-Mn shape-memory alloys

We study the effect of a magnetic field on the martensitic transition of a Cu-Al-Mn shape-memory alloy. The martensitic transition has been studied through resistance measurements under applied magnetic fields ranging from 0 to 50 kOe. Negative magnetoresistance showing an almost linear dependence with the square of the magnetization has been observed. This magnetoresistive effect is associated with the existence of small ferromagnetic Mn-clusters. Its strength and thermal dependence is different in both phases. The martensitic transition temperature is slightly increased and its spread in temperature significantly reduced upon increasing the field. These results show the existence of magnetoelastic coupling, which favors the nucleation of those martensitic variants with the easy magnetization axis aligned with the field.

Magnetoelasticity and magnetoresistance in Cu-Al-Mn shape-memory alloys

We study the effect of a magnetic field on the martensitic transition of a Cu-Al-Mn shape-memory alloy. The martensitic transition has been studied through resistance measurements under applied magnetic fields ranging from 0 to 50 kOe. Negative magnetoresistance showing an almost linear dependence with the square of the magnetization has been observed. This magnetoresistive effect is associated with the existence of small ferromagnetic Mn-clusters. Its strength and thermal dependence is different in both phases. The martensitic transition temperature is slightly increased and its spread in temperature significantly reduced upon increasing the field. These results show the existence of magnetoelastic coupling, which favors the nucleation of those martensitic variants with the easy magnetization axis aligned with the field.

Premartensitic and martensitic phase transition in ferromagnetic Ni2MnGa

We present an experimental study of the premartensitic and martensitic phase transitions in a Ni2MnGa single crystal by using ultrasonic techniques. The effect of applied magnetic field and uniaxial compressive stress has been investigated. It has been found that they substantially modify the elastic and magnetic behavior of the alloy. These experimental findings are a consequence of magnetoelastic effects. The measured magnetic and vibrational behavior agrees with the predictions of a recently proposed Landau-type model [A. Planes et al., Phys. Rev. Lett. 79, 3926 (1997)] that incorporates a magnetoelastic coupling as a key ingredient.

Modelling of iron-filled magneto-active polymers with a dispersed chain-like microstructure

Magneto-active polymers are a class of smart materials commonly manufactured by mixing micron-sized iron particles in a rubber-like matrix. When cured in the presence of an externally applied magnetic field, the iron particles arrange themselves into chain-like structures that lend an overall anisotropy to the material. It has been observed through electron micrographs and X-ray tomographs that these chains are not always perfect in structure, and may have dispersion due to the conditions present during manufacturing or some undesirable material properties. We model the response of these materials to coupled magneto-mechanical loading in this paper using a probability based structure tensor that accounts for this imperfect anisotropy. The response of the matrix material is decoupled from the chain phase, though still being connected through kinematic constraints. The latter is based on the definition of a 'chain deformation gradient' and a 'chain magnetic field'. We conclude with numerical examples that demonstrate the effect of chain dispersion on the response of the material to magnetoelastic loading.

Premartensitic and martensitic phase transitions in ferromagnetic Ni2MnGa

We present an experimental study of the premartensitic and martensitic phase transitions in a Ni2MnGa single crystal by using ultrasonic techniques. The effect of applied magnetic field and uniaxial compressive stress has been investigated. It has been found that they substantially modify the elastic and magnetic behavior of the alloy. These experimental findings are a consequence of magnetoelastic effects. The measured magnetic and vibrational behavior agrees with the predictions of a recently proposed Landau-type model [A. Planes et al., Phys. Rev. Lett. 79, 3926 (1997)] that incorporates a magnetoelastic coupling as a key ingredient.

Modeling premartensitic effects in Ni2MnGa: A mean-field and Monte Carlo simulation study

ic first-order transition line ending in a critical point. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling. In particular, the premartensitic transition occurs when such coupling is strong enough to freeze the involved mode phonon. The implication of the results in relation to the available experimental data is discussed.

Temperature and magnetic-field dependence of the elastic constants of Ni-Mn-Al magnetic Heusler alloy

We report on measurements of the adiabatic second-order elastic constants of the off-stoichiometric Ni54Mn23Al23 single-crystalline Heusler alloy. The variation in the temperature dependence of the elastic constants has been investigated across the magnetic transition and over a broad temperature range. Anomalies in the temperature behavior of the elastic constants have been found in the vicinity of the magnetic phase transition. Measurements under applied magnetic field, both isothermal and variable temperature, show that the value of the elastic constants depends on magnetic order, thus giving evidence for magnetoelastic coupling in this alloy system.

Premartensitic and martensitic phase transition in ferromagnetic Ni2MnGa

We present an experimental study of the premartensitic and martensitic phase transitions in a Ni2MnGa single crystal by using ultrasonic techniques. The effect of applied magnetic field and uniaxial compressive stress has been investigated. It has been found that they substantially modify the elastic and magnetic behavior of the alloy. These experimental findings are a consequence of magnetoelastic effects. The measured magnetic and vibrational behavior agrees with the predictions of a recently proposed Landau-type model [A. Planes et al., Phys. Rev. Lett. 79, 3926 (1997)] that incorporates a magnetoelastic coupling as a key ingredient.

Magnetic field induced lattice effects in a quasi-two-dimensional organic conductor close to the Mott metal-insulator transition

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Fractional order and dynamic simulation of a system involving an elastic wide plate

Numerous researchers have studied about nonlinear dynamics in several areas of science and engineering. However, in most cases, these concepts have been explored mainly from the standpoint of analytical and computational methods involving integer order calculus (IOC). In this paper we have examined the dynamic behavior of an elastic wide plate induced by two electromagnets of a point of view of the fractional order calculus (FOC). The primary focus of this study is on to help gain a better understanding of nonlinear dynamic in fractional order systems. © 2011 American Institute of Physics.

Magnetic properties of selected 3d- and 4f-systems on alpha-Al 2 O 3 substrates

In this work the growth and the magnetic properties of the transition metals molybdenum, niobium, and iron and of the highly-magnetostrictive C15 Laves phases of the RFe2 compounds (R: Rare earth metals: here Tb, Dy, and Tb{0.3}Dy{0.7} deposited on alpha-Al2O3 (sapphire) substrates are analyzed. Next to (11-20) (a-plane) oriented sapphire substrates mainly (10-10) (m-plane) oriented substrates were used. These show a pronounced facetting after high temperature annealing in air. Atomic force microscopy (AFM) measurements reveal a dependence of the height, width, and angle of the facets with the annealing temperature. The observed deviations of the facet angles with respect to the theoretical values of the sapphire (10-1-2) and (10-11) surfaces are explained by cross section high resolution transmission electron microscopy (HR-TEM) measurements. These show the plain formation of the (10-11) surface while the second, energy reduced (10-1-2) facet has a curved shape given by atomic steps of (10-1-2) layers and is formed completely solely at the facet ridges and valleys. Thin films of Mo and Nb, respectively, deposited by means of molecular beam epitaxy (MBE) reveal a non-twinned, (211)-oriented epitaxial growth as well on non-faceted as on faceted sapphire m-plane, as was shown by X-Ray and TEM evaluations. In the case of faceted sapphire the two bcc crystals overgrow the facets homogeneously. Here, the bcc (111) surface is nearly parallel to the sapphire (10-11) facet and the Mo/Nb (100) surface is nearly parallel to the sapphire (10-1-2) surface. (211)-oriented Nb templates on sapphire m-plane can be used for the non-twinned, (211)-oriented growth of RFe2 films by means of MBE. Again, the quality of the RFe2 films grown on faceted sapphire is almost equal to films on the non-faceted substrate. For comparison thin RFe2 films of the established (110) and (111) orientation were prepared. Magnetic and magnetoelastic measurements performed in a self designed setup reveal a high quality of the samples. No difference between samples with undulated and flat morphology can be observed. In addition to the preparation of covering, undulating thin films on faceted sapphire m-plane nanoscopic structures of Nb and Fe were prepared by shallow incidence MBE. The formation of the nanostructures can be explained by a shadowing of the atomic beam due to the facets in addition to de-wetting effects of the metals on the heated sapphire surface. Accordingly, the nanostructures form at the facet ridges and overgrow them. The morphology of the structures can be varied by deposition conditions as was shown for Fe. The shape of the structures vary from pearl-necklet strung spherical nanodots with a diameter of a few 10 nm to oval nanodots of a few 100 nm length to continuous nanowires. Magnetization measurements reveal uniaxial magnetic anisotropy with the easy axis of magnetization parallel to the facet ridges. The shape of the hysteresis is depending on the morphology of the structures. The magnetization reversal processes of the spherical and oval nanodots were simulated by micromagnetic modelling and can be explained by the formation of magnetic vortices.

Strain-Tunable Spin Moment in Ni-Doped Graphene

Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Ni_sub). We have found that, although Ni_sub defects are non-magnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3-0.4 \mu_B per Ni_sub, until a critical strain of ~6.5% is reached. At this point, a sharp transition to a high-spin state (~1.9 \mu_B) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene.