High Moment Materials Research Articles

Imposing long-range ferromagnetic order in rare-earth-doped magnetic topological-insulator heterostructures

The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped three-dimensional topological insulator (TI). The pioneering transition-metal doped $({\mathrm{Sb},\mathrm{Bi})}_{2}({\mathrm{Se},\mathrm{Te})}_{3}$ thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth-doped $({\mathrm{Sb},\mathrm{Bi})}_{2}({\mathrm{Se},\mathrm{Te})}_{3}$ thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allow for the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$/Cr:${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ can introduce long-range magnetic order in high-moment Dy:${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$---up to a temperature of 17 K---in excellent agreement with first-principles calculations, which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.

Recipe for High Moment Materials with Rare-earth and 3d Transition Metal Composites.

Materials with high volume magnetization are perpetually needed for the generation of sufficiently large magnetic fields by writer pole of magnetic hard disks, especially for achieving increased areal density in storage media. In search of suitable materials combinations for this purpose, we have employed density functional theory to predict the magnetic coupling between iron and gadolinium layers separated by one to several monolayers of 3d transition metals (Sc-Zn). We demonstrate that it is possible to find ferromagnetic coupling for many of them and in particular for the early transition metals giving rise to high moment. Cr and Mn are the only elements able to produce a significant ferromagnetic coupling for thicker spacer layers. We also present experimental results on two trilayer systems Fe/Sc/Gd and Fe/Mn/Gd. From the experiments, we confirm a ferromagnetic coupling between Fe and Gd across a 3 monolayers Sc spacer or a Mn spacer thicker than 1 monolayer. In addition, we observe a peculiar dependence of Fe/Gd magnetic coupling on the Mn spacer thickness.

Shield Design for Enhanced Reader Resolution

We propose a shield design that enhances linear resolution at a given shield-to-shield spacing. Patterning tabs in the cross-track direction of the shields at the read gap introduces a transverse anisotropy that can be further enhanced by using a high-moment material in the shield near the sensor. We use a finite element micromagnetic code to explore the performance of a read head embodying this design. We study the dependence of resolution on both the thickness and the moment of the shield tabs. The increased shield rotation improves the sensor's linear resolution by more than 2% in absolute terms.

Fabrication of Magnetic Tips for High-Resolution Magnetic Force Microscopy

Effects of magnetic material, coating thickness, and tip radius on magnetic force microscope (MFM) spatial resolution have been systematically investigated. MFM tips are prepared by using an UHV sputtering system by coating magnetic materials on non-magnetic Si tips employing targets of Ni, Ni-Fe, Co, Fe, Fe-B, and Fe-Pd. MFM spatial resolutions better than 9 nm have been confirmed by employing magnetic tips coated with high magnetic moment materials with optimized thicknesses.

Textured growth of the high moment material Gd(0 0 0 1)/Cr(0 0 1)/Fe(0 0 1)

By magnetic coupling of Fe and Gd via Cr interlayers, the large local moment of Gd can be combined with the high Curie temperature of Fe. The textured growth of a Gd film is studied here by preparing trilayer systems of Fe/Cr/Gd on MgO(1 0 0) substrates by molecular-beam epitaxy (MBE). The thickness of the Cr interlayer was varied between 3 and 5 monolayers. The structural quality of the samples was confirmed by in situ RHEED and ex situ XRD measurements. Epitaxial Cr(0 0 1)/Fe(0 0 1) growth was observed, as expected. By use of 57Fe-CEMS (conversion electron Mössbauer spectroscopy) in combination with the 57Fe tracer layer method the Fe/Cr interface could be examined on an atomic scale and well separated Fe/Gd layers for all Cr thicknesses were confirmed. The unusual Gd/Cr crystallographic relationship of Gd(0 0 0 1)∥Cr(0 0 1), with domains of the hexagonal Gd basal planes randomly oriented in the sample plane and not in registry with the underlying Cr(0 0 1) lattice, was found from combined RHEED and x-ray measurements. Annealing of the samples resulted in a remarkable improvement of the crystalline structure of the Gd layers. On the other hand, the appearance of a single line in the CEM spectrum leads to the conclusion that during annealing a small amount of Fe diffuses into the Cr layer. The electronic structure and magnetism of this system are investigated by first-principles theory.

High Magnetic Saturation Poles for Advanced Perpendicular Writers

Advanced perpendicular writers continue to demand maximum write fields, fast rise times at ever vanishing head-media spacing and strict reliability standards. This means that the asymptotic progression to a 2.4 T pole with nearly ideal magnetic response continues. At the same time we must control critical pole dimensions, fabricate at a reasonable cost while protecting against corrosion and erasure risks. We will review progress made to meet this challenge in a discussion of high moment materials utilizing electroplating and sputter deposition for single layer films and laminates. Concerns for corrosion will be assessed and minimized by controls on key contaminants. Micromagnetic modeling will be used to study the phenomenology and expected performance impacts of these various materials and structures. Results will be used to provide a better insight into the potential materials/design tradeoffs that must be made. In conclusion, performance and reliability will be assessed through electrical testing.

Magnetic moments of Fe clusters embedded in an Fe-Co alloy

Motivated by the search for high moment materials, we present first-principles calculations of the magnetic moments for Fe clusters embedded in an Fe-Co alloy as well as in a pure Co matrix. Both spin and orbital contributions have been calculated with the use of a real-space linear muffin-tin orbital method within the atomic sphere approximation. The cluster sizes considered range from 1 to 51 atoms. We find an enhancement of the spin moments of the atoms close to the interface between the cluster and the embedding matrix. The spin moment enhancement is decreasing with increasing Fe content in the surrounding matrix. The slightly lower magnetic moments for the Fe clusters embedded in the Fe rich matrices can nevertheless result in a material with an enhanced saturation magnetization due to the higher moments in the matrix. We argue that the largest average magnetic moments for embedded Fe clusters are obtained for an ${\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}$ alloy in the matrix surrounding the clusters.

Magnetic interactions of Mn clusters supported on Cu

A first-principles linear scaling real-space method for investigating non-collinear magnetic behaviour of nanostructured materials has been developed. With this method, the magnetic structures of small supported transition metal clusters have been examined. The geometric constraints imposed on the clusters by the underlying surface is found to cause non-collinear behaviour for V, Cr, and Mn clusters on Cu(111). Fe clusters supported on Cu and Ni have been studied and both spin and orbital moments are found to be enhanced for the Fe atoms, which is attributed to the recuced symmetry present at the surface. Atoms in Co clusters have been found to order antiferromagnetically, and some times in a non-collinear fasion, when deposited on a W surface. Small clusters of fcc Fe embedded in Cu have been examined and a new type of ordering, not present in larger fcc Fe systems was found. Several theoretical studies of Fe and Co based nanostructures consisting of multilayers or embedded clusters have been conducted, with the aim of predicting high moment materials for use in data storage applications. In agreement with previous experiments an enhancement of the magnetic moment is found compared to the magnetic moment of bcc Fe. The enhancement has been shown to be caused by increased spin moments for Fe atoms in close proximity with Co atoms, and this enhancement depends on the number of Co neighbours. As a result of these studies, a possible method of increasing the magnetic moment of cluster based materials has been proposed. Fermi surface analysis have been performed both on bulk materials, in order to investigate mechanisms for stabilizing non-collinear magnetic states, and in layered structures where the effect of the Fermi surface on the interlayer exchange coupling has been investigated. In addition to the development of a real-space electronic structure method for non-collinear magnetism, a density matrix purification method has been implemented in the framework of linear muffin-tin orbitals.

A link between the coercivity and microstructure of high moment Fe films and their use in magnetic tunnel junctions

Magnetron sputtered single Fe films have been “softened” magnetically by controlled N-doping during the sputter deposition. This technique allows a reduction in grain size and coercivity of the Fe films, without decreasing the saturation magnetization and without the formation of any crystalline FeN phases. We describe this effect through a modification of the random magnetocrystalline anisotropy model, by taking the film thickness into account. The coercivities calculated in this way are in good agreement with those obtained experimentally. It is demonstrated that N-doping can be applied beneficially to control the switching field of the ‘free’ layer in magnetic trilayer films of the MTJ type. It is thus possible to construct an all Fe-electrode magnetic tunnel junction (MTJ) that displays the tunneling magnetoresistance (TMR) effect by altering the switching field of one Fe layer using N-doping. The ability to control the magnetic softness of high magnetic moment materials is important in regard to their incorporation into TMR devices.

Fabrication of sub-50 nm critical feature for magnetic recording device using electron-beam lithography

We report an electron-beam lithography method for printing and plating sub-50 nm isolated trenches with a high aspect ratio (AR) for the nanofabrication of magnetic thin-film heads. To eliminate the issues of resist footing and resist residue in the narrow trench process, we coated a thin dissolution layer of polymethylglutarimide (PMGI) as an undercoat layer between a seed layer and a resist layer. The undercoat PMGI layer was easily and more quickly dissolved than the top resist layer, so it completely cleared the trench during the develop process. In addition, a vertical sidewall at the bottom of the narrow trench was achieved by controlling the processing conditions, e.g., bake temperature and thickness of the dissolution layer. All of these allowed us to facilitate plating the narrow trench with a high magnetic moment material. In this work, narrow trenches were electroplated with both 1.0 T NiFe and 1.8 T CoNiFe alloys. We demonstrated the capability of fabricating narrow electrodeposited magnetic write top pole structures with a critical dimension (CD) of 30 nm in a 0.24 μm resist (AR=8:1) and a CD of 22 nm in a 0.11 μm resist (AR=5:1).

Inductive write heads using high-moment pole materials for ultrahigh-density demonstrations

In this paper, the high moment materials and their applications in the write heads for these areal density demonstrations will be discussed. Due to high intrinsic magnetic anisotropy and magnetostriction, which is typically about 4.5x10/sup 5/, FeCoN films exhibit typical coercivity in excess of 50 Oe, and isotropic in-plane magnetic properties, under un-optimised process conditions. A typical hysteresis loop of the film is shown.

Flat-top pole write head design

A write head design with pedestal-defined throat and flat-top pole is presented. The flat-top pole design enables critical dimension control and extendibility to narrower feature size. Write head performance improves further by implementing a single-layer six-turn, closely packed coil with yoke length of 14 /spl mu/m and incorporating a sputtered high moment material on both top and bottom poles. With write current of 40 mA, better than 35 dB overwrite and less than 16% (-16dB) nonlinear transition shift can be achieved on the medium with coercivity of 4400 Oe and Mrt of 0.32 memu/cm/sup 2/ at areal densities up to 60 Gb/in/sup 2/.

Transverse field anneal studies of high moment FeCoB and FeCoZr films

Ever increasing areal densities have driven the need for high moment materials with large saturation magnetizations greater than 2 T. In addition, for applications such as write poles and soft underlayers, the high moment materials must exhibit soft and uniaxial magnetic properties which are thermally stable during high temperature processing. In this work, we examine 2.1 T FeCoB and 1.8 T FeCoZr films which are amorphous in the as-deposited state. The films are given longitudinal field anneals (LFAs) at 220 and 250 °C for 4 h. FeCoZr films remain amorphous after the 220 and 250 °C LFAs. FeCoB films remain amorphous after the 220 °C LFA, however, the 250 °C LFA results in crystallization in the FeCoB film. This crystallization is accompanied by a degradation in soft magnetic properties. As-deposited and 220 and 250 °C LFA FeCoB and FeCoZr films are then given transverse field anneals (TFAs) at 150 °C. As-deposited FeCoB and FeCoZr experience a 90° rotation of Hk or switching after less than 2 h of TFA. LFAs result in improved thermal stability in all FeCoB and FeCoZr films. The crystalline FeCoB film given a 250 °C LFA exhibited the best thermal stability, however, the magnetic properties were not optimal due to crystallization.

Intrinsic nonlinear ferromagnetic relaxation in thin metallic films.

We propose an intrinsic mechanism for ferromagnetic relaxation in thin films that can dominate competing linear mechanisms even for rapidly relaxing metals. In particular, we use an analytic theory of four-magnon scattering to demonstrate rapid decay for technologically important systems involving high moment materials subject to large rotations. A micromagnetic simulation is used to verify the results.

High-performance writer using high-moment sputtered films in top and bottom poles

Sputter-deposited high-moment materials were used in top and bottom poles of a writer that has a pedestal defined zero throat. The high moment materials used were FeRhN/CoZrCr laminated films with a B/sub s/ of 20.3 kG. In the top pole, the sputtered materials were used as the flux enhancement layer with a thickness of 0.5 /spl mu/m. In the bottom pole, the sputtered film forms a pedestal, which has a vertical sidewall at the top to define the zero throat, as well as a tapered foot at the bottom to allow a better flux passage. As a result, the overwrite can be improved by about 9 dB, as compared to heads that employ only conventional Ni/sub 45/Fe/sub 55/ electroplated material in top and bottom poles.

Stress dependence of soft, high moment and nanocrystalline FeCoB films

Soft, high moment materials are crucial for the magnetic data recording industry in applications such as write poles and soft underlayers. Ever increasing areal densities have pushed the requirements for materials exhibiting saturation magnetizations above 2 T. In this work we investigate nanocrystalline FeCoB which exhibits a saturation magnetization of ∼2.2 T. FeCoB exhibits a magnetostriction of >50×10−6 which suggests that the magnetoelastic anisotropy is important. Nanocrystalline FeCoB films are rf diode sputtered as a function of Ar pressure. As the pressure is increased, film stress increases from compressive to tensile with a critical pressure of ∼12 mTorr. TEM and XRD show that pressure has no significant effect on microstructure, however, at pressures above a critical pressure, the FeCoB films change from magnetically isotropic to magnetically uniaxial with low coercivity. This is due entirely to the magnetoelastic anisotropy which must be considered as the industry begins to utilize magnetic alloys with nonzero magnetostriction.

High-moment films produced by assembling nanoclusters

We have used X-ray magnetic circular dichroism to determine the orbital and spin moments in exposed and embedded mass-selected Fe nanoclusters (250-550 atoms) deposited in situ using a gas aggregation source. The isolated and exposed particles have a total moment that increases with decreasing cluster size and reaches 2.4 /spl mu//sub B/ in 250-atom clusters. Although this is enhanced relative to the bulk, it is smaller than found in free clusters of the same size. Coating the exposed clusters with Co, however, increases the spin moment to 2.6 /spl mu//sub B/. The prospects for producing high-moment materials by cluster assembly are discussed.

Magnetic and structural properties of FeCoB thin films

The magnetic and structural properties of FeCoB thin films as a function of boron content have been investigated for applications requiring soft, high moment materials. The films were either co-sputtered from separate FeCo-35 and B targets or from a (Fe/sub 65/Co/sub 35/)/sub 90/B/sub 10/ alloy target. A peak in the coercivity was observed for small amounts of boron. Measurements of coercivity with fields applied along the radial and circumferential directions of the Si wafer showed isotropic magnetic behavior. The coercivity and saturation induction decreased with increasing amounts of boron above 2 at.%. High angle X-ray diffraction showed a broadening of the [110] peak with increasing boron, indicative of either increased microstrains or smaller average grain size perpendicular to the film. The [110] peak, however, did not shift with added boron, suggesting that on average the boron does not expand the FeCo lattice. The drop in coercivity around 10 at.% B was identified with a transition to a primarily amorphous state. Since these alloys have a high positive magnetostriction, their magnetic properties are particularly sensitive to film stress/strain. Using sputtering gas pressure as a variable, differences in coercivity were also correlated with changes in film strain.

Noise characteristics in exchange-biased soft underlayers for perpendicular media

The characteristics of exchange-biased soft underlayers (SULs) were investigated using magnetic force microscopy and spin stand measurements with a single-pole (focused ion-beam trimmed gap) writer and magnetoresistive head. The SUL high-moment materials (FeCoB, FeAlN) were exchange biased by using IrMn. Spin stand data on perpendicular media (CoCrPtB) with an exchange-biased FeCoB SUL also showed a low integrated dc erase noise level, demonstrating its potential for use in double-layered perpendicular media.

Demonstration of 1 Gbit/sec magnetic recording using an integrated inductive write/GMR read head

1 Gbit/sec magnetic recording was demonstrated with integrated inductive write/GMR read heads on a thin film disk with a coercivity of 3200 Oe and an Mrt of 0.34 memu/cm/sup 2/ with a spinstand tester. The write heads were made with high moment materials on both P1 and P2 and a densely packed coil using conventional manufacturing processes, and the read heads were made of dual synthetic spin valve sensor used for the 36 Gbit/in/sup 2/ areal density demonstration. The interconnect between the head and pre-amplifier is a pair of twisted wires of about 2 inch length. The demonstration was done at the outer diameter of the disk rotating at 15000 RPM. With a modest 35 mA (0-P) write current, we achieved an overwrite of about 30 dB and an on track error rate of better than 10/sup -7/. BER test on a disk with a coercivity of 4000 Oe also showed very good results.