Yinfan Xu

Magnetic properties of L10 FePt and FePt:Ag nanocluster films

A sputtering gas-aggregation technique has been used to prepare FePt and FePt:Ag nanocluster films. The cluster size was controlled in a range from 3 to 6 nm. FePt cluster films were directly deposited onto Si substrate; FePt:Ag cluster films were fabricated by depositing a FePt cluster layer between a Ag underlayer and overlayer. Nanostructure and magnetic properties of the samples were characterized by x-ray diffraction, transmission electron microscopy, and magnetometry. The high magnetic anisotropy L10 fct phase was realized in the films annealed at a temperature of 550 °C and above. The orientation of clusters is random. The coercivity increases with an increase of annealing temperature; high in-plane and out-of-plane coercivities, exceeding 10 kOe, were achieved in both FePt and FePt:Ag cluster films after annealing. For FePt:Ag films, the coercivity increases with Ag underlayer thickness, tAg, and reaches about 17 kOe at room temperature for tAg=5 nm after annealing at 650 °C for 10 min. The high c...

Nanostructure and magnetic properties of highly (001) oriented L10 (Fe49Pt51)1−xCux films

We report on nonepitaxially grown L10 Cu-alloyed FePt thin films with strong (001) texture. The FePt films with different Cu contents were deposited directly on Si wafers with a Fe49Pt51∕Cu multilayer structure. The Cu content was varied from 0to13at.%. All films were annealed at 600°C for 5min. X-ray-diffraction characterization showed that only one set of L10 diffraction peaks appeared and no elemental Cu diffraction peaks were visible. This result, along with a varying c∕a lattice-parameter ratio, suggests that Cu substitutes Fe or Pt in the L10 lattice and ternary FePtCu alloy films are formed. (001) texture was enhanced with the increase of Cu content. Transmission electron microscope images showed that the grain size of FePtCu was about 10nm. For FePt film with 11at.% Cu substitution, coercivity was about 5kOe, which is suitable for writing in a practical perpendicular-recording film.

In-cluster-structured exchange-coupled magnets with high energy densities

In this letter, the authors demonstrate isotropic Fe–Pt exchange-spring nanocomposite permanent magnets with a soft magnetic phase fraction of greater than 0.5 with a coercivity of 6.5kOe, single-phase-like magnetic behavior, and an energy product of 25.1MGOe. Sub-10-nm Fe–Pt clusters are formed with compositions in the two-phase Fe3Pt and FePt regions. Intracluster structuring on a scale of a few nanometers occurs after appropriate heat treatment. This ensures full exchange coupling between the two phases, allowing greater soft magnetic phase fractions. The results provide insight into developing high energy product nanostructured permanent magnets.

Highly (001)-oriented Ni-doped L10 FePt films and their magnetic properties

We report on Ni-doped nonepitaxial L10 FePt thin films with strong (001) texture. The influences of Ni doping on L10 ordering, orientation, and the magnetic properties of the FePt films have been investigated. In-plane and out-of-plane x-ray diffractions (XRD) were used to analyze the texture of FeNiPt films. For [Fe(0.38nm)∕Ni(0.04nm)∕Pt(0.4)]13 sample, the out-of-plane XRD data showed only (00l) peaks and in-plane data showed (hk0) peaks after annealing, indicating high (001) texture of the FeNiPt films. In comparison with FePt, the (00l) peak positions shifted to higher angle, indicating partial Ni substitution in the L10 lattice. The coercivity, measured at room temperature, decreased as a function of Ni doping. For the film with a Ni layer thickness of 0.06nm, the coercivity is about 6kOe after annealing, which is suitable for the writing performance of high-anisotropy perpendicular recording media.

Cluster-Assembled Nanocomposites

This chapter focuses on a gas-aggregation technique to prepare magnetic nanoclusters with controllable cluster sizes and size distributions. The review includes current research on nanoclusters, such as uncoated and oxide-coated Fe, Co and Fe clusters and clusters made from alloys, but special emphasis is on highly anisotropic Ll0-ordered FePt clusters, which are of potential interest for magnetic recording with ultrahigh areal densities of more than 1 Tera bit/in2. In particular, we discuss magnetic and structural properties of FePt nanoclusters and thin films. Another approach discussed in the chapter is to create cluster nanocomposites by multilayering with post-deposition annealing. The advantage of this method is that the clusters can be oriented along a desired easy axis. One example is L10-(001) oriented FePt nanocomposite films with a FePt cluster size of about 5 nm.

Magnetic properties of dilute FePt:C nanocluster films

Nanocluster-assembled dilute (FePt)xC100−x films with 30⩾x⩾5 (x denotes volume fraction) were produced using a gas-aggregation technique. FePt clusters with an average size of about 4.0nm (with standard deviation σ∕d=0.09) were embedded in high volume fraction of carbon matrix, which is used to isolate the FePt clusters. Postdeposition annealing was used to realize the high-anisotropy L10 phase. Single-crystal features of well-isolated clusters were observed by transmission electron microscope (TEM) in annealed dilute films. The coercivity of the films annealed at 700°C for 5min was a few hundred Oersteds at room temperature, while a coercivity about 4kOe was observed at low temperature 10K for x=5, indicating partial ordering of the clusters. The coercivity strongly depends on annealing temperature and annealing time, and increases with decreasing FePt volume fraction. For a dilute FePt cluster film with x=5 annealed at 700°C for 60min, a room-temperature coercivity of about 30kOe and low-temperature (10...

Magnetism of L10 compounds with the composition MT (M=Rh, Pd, Pt, Ir and T=Mn, Fe, Co, Ni)

The electronic band structure of ordered equiatomic compounds of 3d transition elements (Mn, Fe, Co, Ni) with nonmagnetic 4d and 5d elements (Rh, Pd, Pt, Ir) are investigated by linear muffin-tin orbital calculations. The systematic study considers 3d and 4d/5d spin moments and interatomic exchange interactions, with emphasis on the comparison between ferromagnetic and antiferromagnetic order. Total and site-resolved exchange interactions are calculated from first principles, and the obtained exchange constants are used to estimate ordering temperatures on a mean-field level.

Magnetic properties and L10 phase formation of FePt films prepared by high current-density ion-beam irradiation and rapid thermal annealing methods

We investigated magnetic properties and L10 phase formation of FePt films by rapid thermal annealing (RTA) and high current-density ion-beam irradiation. The sample prepared by RTA at 550°C has (001) texture and strong magnetic perpendicular anisotropy with Hc equal to 6kOe. The sample irradiated at 5.04μA∕cm2 has Hc equal to 10kOe but has isotropic magnetic properties due to the (111) texture. The magnetic correlation length of the ion-irradiated sample was about twice as large as that of the RTA sample. This may be due to the inhomogeneity of the L10 phase formation in the ion-irradiated film.

Cluster-assembled exchange-spring nanocomposite permanent magnets

Isotropic nanocomposite magnetic structures were produced via cluster assembly routes with energy products reaching 18 MG Oe. Gas aggregation produces Fe clusters with an average size less than 10 nm with a very narrow size distribution, and these were embedded in a hard magnetic matrix by cosputtering of FePt. Structures produced were crystallographically isotropic with Fe cluster phase content ranging from 0 to 30 vol %. The coercivity decreased from 13 kOe as Fe cluster content increased. Single-phase-type hysteresis loops indicating excellent exchange coupling at low Fe cluster content give way to a stepped demagnetization loop at an Fe cluster content greater than 20%. The energy product initially increased with Fe cluster content, then decreased.

Nanostructure and magnetic properties of FePt:C cluster films

Magnetic properties and nanostructure of FePt:C cluster-deposited films with C volume fraction of 7%, 14%, 33%, and 45% have been studied. As-deposited FePt:C films were prepared by a multilayer method in which FePt layers were deposited from a cluster source employing a gas-aggregation technique and C layers from a normal sputtering gun. In the as-deposited films, FePt clusters with fcc structure are embedded in the C matrix. The high anisotropy FePt L1/sub 0/ cluster structure was realized in the films via post-deposition annealing and the nanostructure of the films was observed by high-resolution transmission electron microscope (TEM). The results for a film with 45 vol. % C showed that FePt clusters are well separated by C matrix and the cluster diameter is about 4.5 nm. The coercivity increases with increase of annealing temperature; coercivities larger than 9 kOe were achieved in the films after annealing at a temperature of 700/spl deg/C and above. Magnetization reversal of the films was studied by moment-decay measurements and the data were fitted with the Sharrock formula. For the film with 45 vol. % C annealed at 625/spl deg/C, the thermal stability factor K/sub u/V/sup *//k/sub B/,T activation volume V/sup */, and anisotropy constant K/sub u/ are 231, 0.83/spl times/10/sup -18/cm/sup 3/ and 1.2/spl times/10/sup 7/ erg/cm/sup 3/, respectively.