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Heusler-type magnetic shape memory alloys (MSMAs) exhibit a martensitic transformation (MT) accompanied by a complex magnetic reordering, strongly affected by an intricate martensitic microstructure. The hierarchic twin structure of martensite, formed as a result of minimization of elastic energy down to atomic scale, is under intensive study nowadays. On the other hand, the much more sophisticated problem of the relationship between nanoscale twin structure and the magnetism in MSMAs has being tackled only recently. It will be shown in this topical review that the nanotwin structure affects not only the basic magnetic parameters of MSMAs, but also can change qualitatively its magnetic nature and related magnetodynamic and magnetoresistance properties. This will be primarily illustrated, both theoretically and experimentally, on the prototype Ni-Mn-Ga and Ni(Co)-Mn-Sn MSMAs in the form of epitaxial thin films, but the conclusions are also valid for other Heusler-type MSMAs, both in the form of thin films, ribbons and bulk single crystals and polycrystals. The following new and remarkable phenomena will be highlighted. (i) A strong ferromagnetic exchange coupling is observed between the submicron twin components in Ni-Mn-Ga ferromagnetic martensite. It results in the modification of the average magnetic anisotropy and the formation of a non-collinear magnetic structure, whereby a negative magnetoresistance appears in a wide temperature range. (ii) Weak antiferromagnetic coupling occurs between the ferromagnetically ordered twin components in Ni(Co)-Mn-Sn martensite. This coupling enabled to explain the exchange bias and magnetic resonance spectra in the same terms as for artificial antiferromagnetically coupled multilayered structures.

Standing spin waves in triangular dots (truncated pyramids) were investigated both experimentally and theoretically. Arrays of nickel triangular pyramids with the base side of 270 nm and height of 70 nm were deposited on Si (111) substrate. The spectra of ferromagnetic resonance were obtained at room temperature with an external saturating magnetic field directed perpendicular to the array plane. The theoretical approach, which allows to describe standing spin wave modes both in perpendicular magnetized regular prisms and in close to prisms truncated pyramids, was developed. Theoretically calculated resonance fields for the observed modes are in a good correlation with the experiment.

The influence of film thickness on the formation of twinning structure, martensitic transformation and magnetoelastic properties of epitaxial films of Ni(Co)MnSn magnetic shape memory alloy is investigated by means of ferromagnetic resonance spectroscopy, synchrotron X-ray diffraction and standard magnetic measurements. It is found that constraints from the film/substrate interface block the martensitic transformation in the 20 nm thick film. The increase of the film thickness results in a progressive stress relaxation and, as a result, the martensitic transformation becomes possible starting from 50 nm. Twinning of the films is required to conserve the films surface area. The elastic energy balance between the film/substrate interface and the twin boundaries leads to the formation of a submicron wide, stripe like, periodical structure of twins, which is of interest for spintronic or magnonic applications. The width of the twin variants increases with the film thickness growth, resulting in the dramatic modification of magnetic properties.

In this work, a detailed report on the growth of elongated bar-like crystallites with a definite crystallographic orientation, forming a self-organized patterned surface layer, with a thickness of about 100 nm, onto 900 nm thick ferromagnetic shape memory alloy Ni–Mn–Ga films epitaxially grown on MgO(001) is presented. The phenomenon appears above a threshold of lattice mismatch (≈2%) between the film and substrate. The structural, magnetic and transformation properties of such films are described and analyzed. Particularly, the different magnetic states of the two film layers are discerned by ferromagnetic resonance. The feasibility of creating a ferromagnetic shape memory alloy with a nanoscale self-patterned surface, during the very same process of film deposition, can be interesting for potential applications where control of the surface architecture would be needed.

Magnetic shape memory alloys are under intensive investigation due to their unusual physical properties, such as magnetic shape memory effect, magnetic field induced superelasticity, direct and inverse magnetocaloric effect etc., promising for novel applications. One of the intriguing properties of these materials in a single phase state is a giant magnetoresistance. Here we report the remarkable results about the magnetoresistive properties of epitaxial films of Ni52.3Mn26.8Ga20.9 magnetic shape memory alloy in the temperature range of 100–370 K, well below the martensitic transformation temperature. It was found that the formation of non-collinear magnetic structure due to a nanotwinning of the film results in electron scattering on such a structure and noticeable negative magnetoresistance in the entire investigated temperature range.

Highlights: • Effects of substrates ion beam treatment on magnetoptical properties Bi:YIG films. • Substrate surface damage results in sign inversion of the magneto-optical effects. • Atomically smooth films growth takes place on low energy ions treated substrates. • High energy ions treatment results in selective nucleation mechanism of the growth. - Abstract: The effect of a controlled ion beam pre-treatment of (1 1 1)-oriented Gd{sub 3}Ga{sub 5}O{sub 12} substrates on the magneto-optical properties and surface morphology of the ultrathin bismuth-substituted yttrium–iron garnet films with a composition Bi{sub 2.8}Y{sub 0.2}Fe{sub 5}O{sub 12} was studied. It has been shown that the observed sign inversion of magneto-optical effects (Faraday rotation and magnetic circular dichroism) observed in films that were deposited on the GGG substrate pre-treated by 1 keV and 4 keV Ar{sup +} ion beams is a result of the substrate surface amorphization caused by the ion bombardment.

We report magnetic, dynamic and transport properties of discontinuous metal-insulator multilayers Fe/MgO grown on amorphous Corning® glass and single-crystalline MgO (001) substrates. The films of structure Substrate/MgO (3 nm)/[Fe (0.6 nm)/MgO (3.0 nm)]×10 were prepared in ultra-high vacuum conditions using Pulsed Laser Deposition. It was shown that conditions of epitaxial growth are favorable for MgO substrates. As a result a substantial increase of tunneling magnetoresistance caused by spin-filtering effect was observed and reasonably theoretically explained. The value of TMR ̃ 9.2% at room temperature in 18 kOe magnetic field is three times higher comparing to that for the samples grown on Corning® glass substrates. Copyright © 2012 American Scientific Publishers All rights reserved.

The crystallization processes in Bi2.8Y0.2Fe5O12, Bi2.5Gd0.5Fe3.8Al1.2O12, Bi1.5Gd1.5Fe4.5Al0.5O12 and Bi1.0Y0.5Gd1.5Fe4.2Al0.8O12 garnet films deposited by reactive ion beam sputtering on (1 1 1) gadolinium–gallium garnet substrates, optical glass-ceramic and SiO2 films have been studied. Films were annealed at low pressure in oxygen atmosphere and in the air. The possibility of preparation of crystalline garnet films with high concentration of bismuth on the SiO2 films using a buffer layer with low concentration of Bi has been shown. This allows to produce one-dimensional magneto-photonic crystals with high effective Faraday rotation (several tens of°/μm for the visible optical spectrum).

Magnetodynamical properties of systems of magnetic nanodisks at the microwave range and low temperatures were studied experimentally. Several periodic square arrays of Permalloy nanodisks deposited onto a glass substrate have been studied. The interdisk center-to-center distance in each array was equal to double disk diameter . The magnetoresonance studies were performed at 70-80 GHz frequency band and at the temperature of 4.2 K. The static external magnetic field was applied perpendicularly to the disks array. As a result the magnetoresonance absorption spectra have been obtained, containing both fundamental mode and spin wave modes. A comparative analysis of the results is performed with the X-band results obtained at T = 300K.

Recently a new type of magnonic crystal — Ni 80 Fe 20 films deposited on top of periodic 2-D arrays of Ni 80 Fe 20 submicron circular disks — was proposed [1]. In such structures the continuous film can be considered as a magnonic waveguide, when the role of dot array is to create periodic perturbations of internal fields in the neighbor regions of the film. Here we study how the change of two parameters — inter-disk distances and the thickness of the film on top of the array will affect the in-plane anisotropy of the system.

1 µm-thick polycrystalline thin films of the ferromagnetic shape memory alloy, Ni49.7Fe17.4Co4.2Ga28.7 (at.%), were sputter-deposited on MgO(0 0 1) substrates heated at 773 K. The films' structure, magnetic properties and transformation behavior have been characterized. They exhibit both forward and reverse martensitic transformations at 218 K and 232 K, respectively. Ferromagnetic resonance and x-ray diffraction measurements discerned three ferromagnetic phases: cubic L21-ordered austenite, tetragonal martensite and disordered cubic γ-phase. The latter phase showed a unique spin-waves resonance spectrum, enabling estimation of the value of the exchange stiffness constant equal to A = 0.7 × 10−6 erg cm−1.

Ferromagnetic resonance was used to study the influence of temperature and film thickness on magnetic properties of Ni–Mn–Ga films deposited on single crystal MgO(001) substrates. It has been shown that depending on the film thickness and preparation condition, three configurations of magnetic anisotropy can be realized in these films. The temperature dependence of the anisotropy field is determined.

2-D modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using deep ultraviolet lithography. The array pitch for all the samples was fixed at 620 nm, while the dot diameter varied from 300 nm to 550 nm. Four-fold in-plane magnetic anisotropy (FFA) was detected in the samples using ferromagnetic resonance technique. It was found that the deposition of continuous film on top of the array drastically increases the FFA of the system, with a maximum anisotropy field of 120 Oe being observed for the sample with intermediate diameter. This increase is due to the appearance of a field well in the film underneath the dots with a shape that is dependent on magnetic field orientation and to the coupling of magnetization oscillations in dots and film.

Experimental and theoretical studies of the evolution of the ferromagnetic resonance spectrum with film thickness for hexagonal lattice of antidots have been reported. We found three different quasi-uniform modes in the resonance spectrum. Calculations of the local dynamic susceptibility tensor show that these modes are related to the resonances from quasiuniformly magnetized regions of the hexagonal antidot array unit cell. The increase of the film thickness results in the broadening of the resonance lines as well as the appearance of additional resonance lines. Good qualitative correlation of the theory and experiment was obtained. Additional relaxation mechanisms due to magnetic inhomogeneities are also discussed.

Ni-Mn-Ga ferromagnetic shape memory films with similar thickness and chemical composition, deposited onto cold (with a subsequent annealing) and hot MgO(001) substrates exhibit different internal stress and structure giving rise to a different orientation of the magnetic easy axes. A quantitative theoretical analysis of the crystallographic and ferromagnetic resonance (FMR) data shows that the different anisotropies can be caused by the difference in sign between the transformation volume changes in these films, as influenced by the internal stresses. To explain FMR data, the magnetoelastic coupling term of fourth-order in the magnetic vector and linear in the strain tensor components, appearing in the Landau expansion for the free energy, is taken into account. The coefficient of the term, which couples the magnetic vector components with the volume change of the Ni-Mn-Ga alloy, was estimated to be equal to about 107 erg cm−3.

Spin wave spectra of 40-nm thick perpendicularly magnetized circular Permalloy dots of 250 nm radius were measured using ferromagnetic resonance technique in 70-80 GHz range at 4.2 K and in 10 GHz at room temperature. The five sharp resonance peaks were observed for both frequency ranges. The resonance fields can be well described by a magneto-exchange dispersion relation, implying that the observed resonances correspond to circular “drumhead” modes with Bessel-function profiles. The relative distances between neighbor peaks for different frequency ranges were almost the same, while the absolute interpeak distances in millimeter range were ∼30% bigger than at 10 GHz, as predicted by the theory.

By the ferromagnetic resonance and magnetometry measurements in a broad temperature range, it is found that the 220-fiber textured Ni51.4Mn28.3Ga20.3 (at. %) film of 100 nm thickness deposited onto NaCl(001) exhibits in the martensitic state an easy-plane magnetic anisotropy. This is in a drastic difference with the identical film of the same texture deposited onto Si(001) wafer showing out-of-plane oblique type of anisotropy [Golub et al., J. Appl. Phys. 105, 07A942 (2009)]. The results are discussed in terms of the different martensitic variant selections and stress-induced magnetic anisotropy contributions having opposite signs.

Magnetic, resonance and transport properties of Fe(t nm)/MgO(3.0 nm) multilayers prepared by pulsed laser deposition were investigated. Comparison of the data allows conclusions on Fe layers morphology. For t 1.25 nm a continuous coverage of MgO by Fe takes place. However, the morphology of Fe layers is rough. This causes the appearance of magnetostatic resonance modes analogous to those observed for continuous films deposited on embossed surfaces.