FeAlN Films Research Articles

10.1007/s11508-008-2007-3

The microstructure, magnetic properties, and uniaxial magnetic anisotropy (MA) of FeMN thin films with M = Al and Ta have been studied. The films were prepared by rf sputtering in an applied magnetic field and synthesized either in situ (deposition accompanied by crystallization) or by annealing of amorphous alloys (temperature-induced crystallization). The experiments showed that the FeAlN and FeTaN films differ in the soft magnetic characteristics and origin of uniaxial MA. The cause of the differences was found to be due to the different microstructure and opposite (in sign) strains of the crystal lattices of the FeAlN and FeTaN films that are induced by Ta and Al ions.

Mössbauer study of the nature of differences in the uniaxial magnetic anisotropy of FeAlN and FeTaN films

Thin FeAlN and FeTaN films were deposited by RF magnetron sputtering in an external magnetic field with (i) in situ crystal growth and (ii) ex situ controlled thermal crystallization of amorphous deposit, respectively, and their microstructures were studied using Mossbauer spectroscopy. The films obtained by the two methods possess different uniaxial magnetic anisotropy, which is related to the different microstructures of FeAlN and FeTaN systems and the opposite signs of stresses in the crystal lattices: expansion in FeAlN versus contraction in FeTaN.

Studies of thin FeAlN films prepared by different techniques

The microstructure, morphology, and magnetic properties of FeAlN films deposited by reactive rf magnetron sputtering with subsequent treatment by three techniques, namely, in situ, ex situ (with the sputtering and annealing processes separated), and thermal crystallization of amorphous alloys, have been studied. FeAlN films prepared by the ex situ technique exhibit the best soft magnetic characteristics. Thermal crystallization of amorphous alloys produced films with properties having the highest thermal stability. Films 800-to 1000-nm thick were found to have the best soft magnetic properties. The dependences of the properties of FeAlN films on nitrogen content and annealing temperature were established. The conditions favoring the preparation of thin nanostructured FeAlN films featuring the best soft magnetic characteristics (saturation induction BS = 1.8 T, coercivity HC = 1.2 Oe, magnetic susceptibility μ1 (1 MHz) = 3400) were determined.

Effect of thickness on the properties of FeAlN films

The magnetic properties, microstructure, and morphology of thin nanocrystalline FeAlN films synthesized by RF magnetron sputtering were studied as dependent on the film thickness. The best soft magnetic properties were observed for the films with thicknesses above 800 nm. These films consist predominantly of nanocrystalline α-Fe particles with an average size of ∼15 nm and are characterized by crystal lattice expansion in the (110) plane approaching a critical level of 0.28%. FeAlN films with a thickness of 1000 nm are characterized by μ1 ∼ 2000.

Effect of Synthesis Conditions on the Properties of Fe–Al–N Thin Films

The magnetic properties, microstructure, and morphology of Fe-Al-N films that are deposited by reactive rf sputtering and synthesized in situ, ex situ (deposition followed by annealing), and by thermal crystallization of amorphous films are studied. The FeAlN films synthesized ex situ offer the highest soft-magnetic properties. The films produced by thermal crystallization offer the highest thermal stability.

A study of sputtering process for nanocrystalline FeAlN soft magnetic thin films

Nanocrystalline FeAlN thin films were successfully prepared by three ways: the growth of deposited nanocrystalline film by ex-situ process, in-situ process and controlling crystallization of deposited amorphous films. The magnetic properties and microstructure such as aspect and surface morphology were investigated. It is found that the FeAlN films by ex-situ process show the best soft magnetic properties: higher saturation and lower coercivity. The FeAlN films synthesized by controlling the crystallization of as-deposited amorphous films exhibit the highest thermal stability.

Nitrogen-induced local magnetic and structural properties of sputtered FeAlN thin films

Conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction were used to study a series of rf-reactively sputtered FeAlN thin films with the goal of gaining insight into the origins of their uniaxial magnetic anisotropy (UMA) and the effect of sputtering conditions on its thermal stability. At N/Fe=4.6%(±0.5%), N goes into interstitial vacancies without significant reductions in the hyperfine field (Bhf) of the Fe but causing lattice expansions, leading to an increased linewidth in the CEMS spectra of the film while the sample has an in-plane UMA. Reducing the target-to-substrate spacing used in the deposition process does not affect the nitrogen content of the films but yields an increased CEMS linewidth reflecting larger disorder in the local Fe environment. Heavy N doping causes the phase transform into γ′-Fe4N, ε-Fe3N, and ζ-Fe2N phases. A well defined UMA exists in mildly doped FeAlN films when doped N atoms mostly occupy the interstitial sites while FeAl and heavily doped FeAlN films are either isotropic or weakly anisotropic. UMA and its thermal stability are therefore concluded to have a correlation with the local environment of the Fe structure induced by N doping.

Influence of stress and texture on soft magnetic properties of thin films

This paper presents the relationship among stress, crystallographic texture, and soft magnetic properties of thin films. The magnetic properties considered are those affected by the formation of stripe domains and by the formation of magnetization ripple. In practice, one of these two undesirable domain structures is almost always the impediment to improving the soft magnetic properties. The theoretical analysis accounts for the contribution from stress through magnetostrictive anisotropy, and calculates the resulting total, stress-dependent anisotropy of the film. This anisotropy is then analyzed to yield the effective perpendicular and local in-plane anisotropy constants. These constants allow the calculation of the stripe domain onset thickness through Murayama's stripe domain theory, and the ripple coercivity through Hoffmann's ripple theory. The influence of stress and texture on the stripe domain onset thickness and ripple coercivity is theoretically calculated and experimentally verified for the examples of sputtered CoFe and FeAlN films, two of the most promising and widely studied materials for high-density recording head poles. The results indicate that the interactions between stress and soft magnetic properties depend on the details of composition, growth texture, growth morphology (whether the film is grown in columnar fashion or as equiaxial crystallites), grain size, and thickness of the film under consideration. On the basis of these results, the paper offers a systematic approach to choosing the appropriate composition, texture, and thickness so that low stresses simultaneously promote magnetic states in which stripe domains are suppressed and coercivities are low. The calculations presented here can be easily extended to other aspects of soft magnetic properties addressed by the ripple theory, such as initial permeability.

Effect of stress on stripe domain onset in sputtered FeAlN and CoFe films

Stripe domains caused by perpendicular anisotropy degrade soft magnetic properties of thin films used in recording heads. In this work, the relationship between stress and the susceptibility to stripe domains in soft magnetic thin films is studied theoretically and experimentally. The critical stripe domain onset thickness of a polycrystalline film with cubic unit cell under a planar, isotropic stress is calculated as a function stress and texture for sputtered FeAlN and CoFe films. In good agreement with experimental data, the results show that the stripe domain onset thickness depends on stress and texture. In the sputtered Co65Fe35 film, a residual stress close to zero helps eliminate the possibility of stripe domains, while in the sputtered FeAlN film, the role played by stress depends on texture. For the most frequently seen (110)-textured FeAlN film, large compressive stresses help reduce the possibility of stripe domains for low and moderate N concentrations.

Surface nitrogen concentration dependence of the nitrogen incorporation in reactively sputtered FeXN films

N incorporation is critical to soft magnetic properties of the FeXN film. Previous work showed experimentally that the N concentration is linearly proportional to the ratio of the N to Fe atomic fluxes arriving at the surface of the film (flux ratio rule). Recently it was shown that under certain conditions, N incorporation is more efficient than the flux ratio rule would suggest. In this work a simple analytic model based on steady-state N and Fe transport fluxes during the reactive sputtering process is used to understand these two observed phenomena. It is shown that the N concentration in the film depends on the amount of N on the surface of the growing film. In a surface N-depleted state, the flux ratio rule is followed, while in a surface-N rich state, the N incorporation in the film increases with deposition rates even when the ratio of incoming N to Fe fluxes is kept constant. Experimental data from sputtered FeAlN films can be reproduced reasonably well with the current analysis.

Evolutions of magnetic and structural properties of FeAlN thin films via N doping

The effects of N doping on the magnetic and structural properties of a series of 100 nm FeAlN films sputtered in the presence of N/sub 2/ partial pressures were investigated. Increasing N doping increased film resistivity but decreased M/sub s/. Extended X-ray absorption fine structure spectra of the short-range atomic order in the Fe(Al) lattices directly indicated that the N went into octahedral sites of bcc Fe and fcc Fe in /spl alpha/-Fe and /spl gamma/'-Fe/sub 4/N phases, respectively, and triggered the order-disorder phase transition observed in X-ray /spl theta/-2/spl theta/ diffraction spectra. Mild N doping decreased the grain size and reduced H/sub c/. It also increased the local atomic disorder, which coincided with the maximal value of K/sub u/ and the in-plane anisotropic behaviors of as-deposited films.

Residual stress optimization in FeAlN pole materials

RF-diode reactively sputtered FeAlN films, which are good candidates for high-density write head pole materials, usually have very large compressive residual stresses. In this paper we describe our efforts to lower the residual stress, and study the effects of lowered stress on the magnetic properties of the FeAlN films. It is found that increasing the total sputtering pressure while keeping the optimum Fe to N arrival flux ratio constant can lower the residual stress. The films with lowered residual stress, however, are more susceptible to the formation of stripe domains, which are detrimental to the magnetic properties. In order to avoid these stripe domains, the thickness of the film should not exceed the critical stripe domain onset thickness. For applications where thicker films are desired, we demonstrate that laminating single layer films whose thicknesses do not exceed the critical thickness produces films with good magnetic properties and acceptable residual stress.

Growth rate dependence of nitrogen incorporation in reactively sputtered FeXN films for recording head poles

In this paper we show that the efficiency of nitrogen incorporation in reactively sputtered FeAlN and FeN films is strongly dependent on the growth rate of the films. Efficiency is defined as the amount of nitrogen incorporated for a given ratio of arriving iron atoms to arriving nitrogen atoms. These findings are in contrast to previous work, which has suggested the amount of nitrogen incorporated in the films should be determined by this arrival flux ratio alone, and independent of growth rate. This difference may be due to the larger range of deposition rates explored in this work. A simple and phenomenological kinetic model of the deposition process is used to rationalize the observed behavior. This behavior has important consequences for scaling the deposition of FeXN up to commercial deposition rates while maintaining soft magnetic properties. This is because the amount of nitrogen incorporated has a strong effect on the soft properties of these pole materials.

High frequency initial permeability of NiFe and FeAlN

The initial permeability of NiFe single layer films and single and multi-layer FeAlN films up to 500 MHz are presented. The permeability of thin (<1500 /spl Aring/) NiFe and FeAlN films were fit with calculations based on the Landau-Lifshitz/Gilbert model for phenomenological damping, using damping factors of /spl sim/0.016 and /spl sim/0.026, respectively. Transverse bias permeability measurements identified larger ripple fields in FeAlN samples compared to those in thin NiFe samples. The permeability of /spl sim/2 /spl mu/m thick NiFe and FeAlN films were well described by eddy current theory. The permeability roll off frequency of 2 /spl mu/m thick patterned FeAlN films Increased from 20 to 200 MHz when laminated into 32 layers separated by 25 /spl Aring/ thick SiN interlayers.

Magnetic properties of FeAlN films at elevated temperatures

FeAlN films were fabricated by reactive sputtering from an Fe alloy target in a nitrogen containing plasma. By varying the nitrogen content in the films, magnetic moment as high as 20 kG and coercivities less than 1 Oe can be achieved at room temperature. However, it has been observed that the soft properties of these films experience a reversible degradation at elevated measuring temperatures. The critical temperature, T0c, defined as the temperature at which the coercivity doubles, generally decreases as more nitrogen is incorporated into the film. T0c agrees well with the temperature where remanent squareness (Br/Bs) equals 0.83, indicating a reduced intergranular exchange coupling in films. However, T0c is far below either the reported Curie temperature of the γ′-Fe4N phase or that of the α-Fe phase, which are found in these films. It is suggested that an interfacial granular phase between the grain boundaries of α-Fe or γ′-Fe4N crystallites is responsible for the low T0c. When the temperature increases above T0c, the interfacial phase becomes paramagnetic, and the exchange interaction between crystalline grains becomes weak, thereby causing the coercivity increases.

The effect of nitrogen partitioning on the magnetic properties of FeAlN films

The amount of /spl gamma/'-Fe/sub 4/N phase in FeAlN films has been manipulated by varying the substrate temperature during RF diode sputter deposition. It was found that increasing substrate temperature leads to greater amounts of /spl gamma/'-Fe/sub 4/N precipitation under constant nitrogen flow. It is shown that this precipitation correlates with low coercivities at elevated temperatures and leads to a more negative saturation magnetostriction constant in the films. On the other hand, the magnetostriction constant increases with increasing nitrogen flow at the same substrate deposition temperature. This increase is attributed to the increasing amount of nitrogen in the /spl alpha/-Fe phase. It is shown that the amount of nitrogen in the /spl alpha/-Fe phase is controlling the magnetostriction constant of the film, rather than the /spl gamma/'-Fe/sub 4/N phase.

Lattice dilation and local magnetic anisotropy in nitrogenated iron-based soft magnetic films

Soft magnetism in FeN-based films intimately depends on the lattice dilation caused by N incorporation. We suggest that the softest magnetic properties can be predicted in terms of critical lattice dilation. A detailed investigation of the structure constant in FeTaN films is presented and theoretically described. A comparison is made between Takahashi and Shimatsu's model as a function of the lattice dilation and Doyle and Finnegan's model with respect to the film stress. Finally, predictions are made for the behavior of FeSiN and FeAlN films.

The effect of surface topography on the soft magnetic properties of FeAlN films

FeAlN films were deposited on patterned lines of photoresist, hardcured before deposition, as well as on bare, flat substrates inclined at an angle to the sputter source. Both of these geometries were intended to examine the effect of oblique incidence deposition on the anisotropy of reactively sputtered FeAlN films. Films on topographic substrates showed anisotropy fields of up to 75 Oe, while films on bare tilted substrates showed anisotropy fields up to 100 Oe, presenting the possibility of low permeability in the sloping regions of thin film recording heads which utilize these materials for inductive head poles.

The effect of substrate temperature on the magnetic properties of FeAlN thin films for recording heads

The soft magnetic properties of FeAlN films have been shown to be strongly dependent on the temperature of the substrate during deposition. Films of FeAlN were sputtered deposited onto Alumina/Titanium Carbide (ALTIC) ceramic substrates at substrate temperatures between 20 and 200/spl deg/C. In the range of 150 to 200/spl deg/C, the films display a well oriented easy axis with coercivities less than 1.5 Oe, and a hard axis coercivity of less than 1 Oe. Deposition at temperatures below 150/spl deg/C resulted in high coercivities and no in-plane anisotropy. This behavior is explained in terms of the temperature dependence of the formation of the /spl gamma/'-Fe/sub 4/N phase (identified by x-ray diffraction), which is necessary for the soft magnetic properties. A discussion of heating transients due to substrate thermal mass is also included.

FeAlN/SiO2 and FeAlN/Al2O3 multilayers for thin-film recording heads (invited)

FeAlN films and FeAlN/SiO2 and FeAlN/Al2O3 multilayer films have been studied as potential high moment head materials. The magnetic properties of FeAlN films on glass are similar to those of FeN, but the Al decreases the saturation magnetostriction constant and shifts the minimum coercivity point to a higher N content. Compared with the single layers, FeAlN/SiO2 and FeAlN/Al2O3 multilayers have reduced coercivity, and their permeance spectra have increased roll-off frequencies. In the multilayers, a 4πMs of 18–22 kG, a coercivity of 0.3 Oe, and a saturation magnetostriction constant smaller than 2×10−6 are obtained. These magnetic properties are very stable to annealing up to 360 °C, due to the presence of the Al. FeAlN and FeN single and multilayers show localized corrosion at defect sites when subjected to extended periods of a high-humidity/high-temperature test, but the corrosion is slow. Films with comparable properties to those on glass have been obtained on n-Si substrates; films on alumina-TiC substrates have also been studied.