Mn Ga Thin Films Research Articles

Ferromagnetic Shape Memory Microactuators

The technologies for fabrication, micromachining and integration of Ni–Mn–Ga thin films are developed in order to create novel microactuators and sensors. These devices simultaneously make use of the electrical, thermoelastic and ferromagnetic properties of the thin films allowing a new level of multifunctionality and, as a consequence, particularly compact designs. By adjusting the Ni-content of the thin films, the martensitic and ferromagnetic transformation temperatures are tuned close to each other above 373 K, which has important consequences on the device performance such as actuation stroke and response time. This article focuses on the mechanisms, fabrication technologies as well as typical performance characteristics of Ni–Mn–Ga microvalves and microscanners. The present state-of-the-art of FSMA microactuators is highlighted.

Transformation behavior of Ni–Mn–Ga thin films

The transformation behavior of Ni–Mn–Ga submicron polycrystalline thin films wasinvestigated with respect to target composition, different substrates, annealing temperatureand film thickness. Two series of thin films (A and B) with thicknesses ranging from 0.1 to5 µm were deposited onto alumina ceramic, poly-vinyl alcohol (PVA), quartz andglass by the RF magnetron sputtering technique. The use of the targets ofNi49.5Mn28.0Ga22.5 and Ni52Mn24Ga24 facilitated the formation of 10M and 14M martensitic structures for heat treated films Aand B, respectively. Magnetization curves and temperature dependences of resistivitydemonstrated anomalies and features typical for the bulk Heusler alloys exhibiting bothmartensitic and ferromagnetic transformations. The transformation temperatures anddistribution of magnetic moments were found to be dependent on the film thickness. Amagnetoresistance ratio of 1.5% was achieved at a maximum field of 1.5 T at roomtemperature.

SMA microactuators for microvalve applications

This paper presents a study of the mechanical and thermal properties of SMA microvalves, which are driven by a microactuator of a NiTi foil or a Ni 2 MnGa thin film. By coupled finite element simulations, von Mises stress profiles and resulting actuation forces are determined, which are compared with experimental force-displacement characteristics. The relatively high phase transformation temperatures of the used Ni 2 MnGa specimens M f / A f of 92 / 125 °C, give rise to cooling times of about 25 ms compared to 90 ms for NiTi. Thus, Ni 2 MnGa microvalves show a superior switching performance. However, due to brittleness and buckling instability, Ni 2 MnGa microvalves only allow a maximum controllable pressure difference of 70 kPa, which is more than a factor of 7 smaller compared to NiTi microvalves.

Comparison of magnetic properties of GaMnN thin films grown by NH 3 ‐MBE at different growth temperatures

Ga1−xMnxN diluted magnetic semiconductor (DMS) films were grown by NH3-MBE at different growth temperatures. Single phase Ga1−xMnxN films were characterized by RHEED, XRD and AFM. Magnetization measurements showed single paramagnetic phase in GaMnN films grown at 700 °C, while ferromagnetic and paramagnetic phases coexisted in films grown at a lower temperature (600 °C). The paramagnetic behavior is attributed to the intrinsic property of Ga1−xMnxN films, while the origin of ferromagnetism of the sample grown at lower temperature is yet unclear. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical scanner based on a NiMnGa thin film microactuator

An optical scanner of 9 x 3 x 5 mm 3 size is presented, which is driven by a microactuator of Ni 2 MnGa. The microactuator is fabricated by magnetron sputtering of a Ni 2 MnGa thin film and subsequent photochemical micromachining. For operation of the scanner, a novel mechanism is proposed, which is based on the antagonism of magnetic and shape recovery forces. Thus, large bending forces in both actuation directions and low biasing forces can be generated simultaneously in a single microdevice. This mechanism is used to realize a large scanning angle of 50 deg. The dynamics of motion is characterized by heat-transfer times. Typical heating and cooling time constants are 2 and 16 ms, respectively. Below a critical frequency of about 55 Hz, the scanning angle is independent of the actuation frequency.

Structure and thermomagnetic properties of polycrystalline Ni–Mn–Ga thin films

The structure and thermomagnetic properties of pulsed laser deposited polycrystalline Ni–Mn–Ga alloy films, grown onto thermally oxidized Si(100) wafers, have been investigated. The 300-nm-thick films were deposited at substrate temperatures between 500 and 600 °C from targets made from slices of a Ni2MnGa single crystal. The trends in the average composition suggest that samples deposited around 500 °C are composed of the Ni2MnGa (L21) phase as well as a nonmagnetic B2 phase. On increasing the temperature of the substrate, the Mn content as well as the saturation magnetization increase suggesting the appearance of a different magnetic phase that is tentatively ascribed to the Ni3(MnGa) (L11) magnetic phase.

Magnetic properties of n-GaMnN thin films

GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.

Ni–Mn–Ga thin films produced by pulsed laser deposition

Polycrystalline Ni–Mn–Ga thin films have been deposited by the pulsed laser deposition (PLD) technique, using slices of a Ni–Mn–Ga single crystal as targets and onto Si (100) substrates at temperatures ranging from 673 K up to 973 K. Off-stoichiometry thin films were deposited at a base pressure of 1×10−6 Torr or in a 5 mTorr Ar atmosphere. Samples deposited in vacuum and temperatures above 823 K are magnetic at room temperature and show the austenitic {220} reflection in their x-ray diffraction patterns. The temperature dependences of both electrical resistance and magnetic susceptibility suggest that these samples exhibit a structural martensitic transition at around 260 K. The magnetoresistance ratio at low temperature can be as high as 1.3%, suggesting the existence of a granular structure in the films.

Fabrication and Characterization of Sputtered Ni<SUB>2</SUB>MnGa Thin Films

Ni 2 MnGa bulk specimen has a Heusler-type crystal structure and is ferromagnetic. The cubic structure transforms to a tetragonal structure through the martensitic transformation during cooling. In this study, Ni 2 MnGa thin films were deposited on a PVA substrate by the sputtering method using a high frequency magnetron sputtering apparatus. The thickness of the film was 5 μm. The Heusler type cubic crystal structure of the film, which was annealed at 1073 K for 3.6 ks, changed to a tetragonal structure by martensitic transformation during cooling. Temperature dependence of magnetization was observed under the external magnetic field which was applied parallel and perpendicular to the film. The magnetization was saturated under the external magnetic field of 400 kA/m applied parallel to the film while the saturation magnetization was not observed under the external magnetic field applied perpendicular to the film.

Formation and Characterization of Single Crystal Ni2MnGa Thin Films

AbstractIn this paper, molecular beam epitaxial growth of Ni2MnGa single crystal layers on GaAs (001) using a NiGa interlayer is reported. X-ray diffraction and transmission electron microscopy confirmed an epitaxial relationship of Ni2MnGa [100]“010] // GaAs [100] [010] and a tetragonal structure of the film (a = b = 5.79 Å, c = 6.07 Å). Magnetic measurements using vibrating sample and superconducting quantum interference device magnetometers revealed an in-plane magnetization of ∼200 emu/cm3at room temperature and a Curie temperature of ∼350 K. The martensitic phase transformation was observed to occur at ∼250 K

Epitaxial ferromagnetic thin films and superlattices of Mn-based metallic compounds on GaAs

Epitaxial ferromagnetic thin films grown directly on III–V compound semiconductors can lead to the integration of magnetism and high-speed III–V electronics/photonics, offering possibilities of hybrid ferromagnetic-semiconductor devices. Despite stringent material requirements for this purpose, we have successfully grown some of Mn-based metallic compounds sharing common atoms with the underlying III–V using molecular beam epitaxy (MBE). First, it is shown that ferromagnetic MnGa thin films with perpendicular magnetization were obtained on (001) GaAs substrates. The c axis, which is the easy axis for magnetization, of the tetragonal unit cell is found to be desirably aligned perpendicular to the film plane. Second, in order to derive more functionality and to increase the freedom in material design, we have also created a new class of magnetic superlattices (SLs) of metallic compounds, MnGa/NiGa, on GaAs substrates. The MnGa/NiGa SLs show even stronger perpendicular magnetization and evenly-spaced multiple steps in the hysteresis loops in extraordinary Hall effect measurements. Third, ferromagnetic hexagonal MnAs, an As-based compound which is more compatible with existing III–V MBE technology, is shown to be successfully grown on GaAs. It was found that the MBE-grown MnAs/GaAs heterostructures have a unique epitaxial relationship and a mono-atomic template plays a critical role in determining the epitaxial orientation and magnetic properties.

First volatile alkylgallyl manganese complexes; structure of [(CO5)Mn]2Ga[(CH2)3NMe2]. Molecular control of the stoichiometry of Mn–Ga thin films grown by low-pressure MOCVD

The Mn–Ga complexes of the general formula {L(CO)4Mn}a[GaR3–a(Do)](L = CO, R = alkyl; Do =N-Lewis-donor) are obtained in yields 90% and used as volatile single source precursors for the gas-phase deposition of thin Mn–Ga alloy films.

Epitaxial ferromagnetic MnAs thin films grown by molecular-beam epitaxy on GaAs: Structure and magnetic properties

We have studied structural and magnetic properties of epitaxial MnAs thin films with various thicknesses (L=1.0–200 nm) on GaAs substrates. The MnAs thin films were grown at 200–250 °C on an As-rich disordered c(4×4) (001) GaAs surface by molecular-beam epitaxy (MBE). The growth direction of the MnAs was found to be along the [1̄100] axis of the hexagonal unit cell. X-ray spectra of the MnAs at room temperature have two peaks, indicating that the present MBE-grown MnAs films consist of the hexagonal ferromagnetic phase and orthorhombic paramagnetic phase. Magnetization measurements revealed that the MnAs thin films have perfectly square hysteresis characteristics with relatively high remnant magnetization Mr=300–567 emu/cm3 and low coercive field Hc=65–926 Oe, compared with those of epitaxial MnGa and MnAl thin films reported previously.

Magnetotransport properties of MBE-grown magnetic superlattices of Mn-based intermetallics on GaAs heterostructures

We have studied magnetic and magnetotransport properties of two kinds of new epitaxial magnetic superlattices, MnGa/NiGa and MnAl/NiAl, consisting of ferromagnetic tetragonal MnGa (MnAl) and non-magnetic CsCl-type NiGa (NiAl), grown by molecular beam epitaxy (MBE) on (001) GaAs substrates. Strong perpendicular magnetization is evidenced in both types of superlattices, with the values of remanent magnetization higher than those of MBE-grown MnGa and MnAl thin films reported previously. At room temperature, the hysteresis loops in the extraordinary Hall effect (EHE) measurements on the MnGa/NiGa superlattices exhibit evenly-spaced multiple steps, while the MnAl/NiAl superlattices show rectangular single-stepped EHE hysteresis loops with nearly perfect squareness. The capability of growing these kinds of new epitaxial magnetic superlattices gives a new degree of materials design of the magnetic thin films on GaAs substrates, offering the possibility of integrating magnetic devices with III–V semiconductor electronics and photonics.

Epitaxial MnGa/NiGa magnetic multilayers on GaAs

We have grown a new class of epitaxial metallic multilayers consisting of ultrathin ferromagnetic tetragonal MnGa and nonmagnetic (CsCl-type) NiGa on (001) GaAs substrates by molecular beam epitaxy. Reflection high energy electron diffraction and cross-sectional transmission electron microscopy analyses show that MnGa/NiGa multilayers with atomically abrupt interfaces are formed with the expected epitaxial orientations, and, in particular, that the c-axis of the tetragonal structure of the MnGa film is aligned perpendicular to the substrate. Perpendicular magnetization of the MnGa/NiGa multilayers was evidenced by both vibrating sample magnetometer and extraordinary Hall effect measurements at room temperature, with higher values (Mr=267–302 emu/cm3) of remanent magnetization than those of previously reported MnGa and MnAl thin films. The capability of growing this new class of materials will allow a new degree of artificial materials design on semiconductor substrates.