Jonathan J. Mallett

Artifacts in ballistic magnetoresistance measurements (invited)

We have carried out an extensive search for credible evidence to support the existence of a ballistic magnetoresistance (BMR) effect in magnetic nanocontacts. We have investigated both thin-film and thin-wire geometries for both mechanically formed and electrodeposited nanocontacts. We find no systematic differences between mechanically formed and electrodeposited nanocontacts. The samples we have investigated include mechanical contacts between ferromagnetic wires, electrodeposited nanocontacts between ferromagnetic wires, ferromagnetic nanocontacts electrodeposited on Cu wires, nanocontacts electrodeposited between ferromagnetic films anchored on wafers, ferromagnetic nanocontacts electrodeposited on Cu films anchored on wafers, nanocontacts between two ferromagnetic films connected by a pinhole through an insulating film, and nanocontacts formed by focused ion-beam etching. In none of these samples did we find credible evidence for a BMR effect. However, we did find a number of artifacts due to magneto...

Compositional Control in Electrodeposition of FePt Films

Fe-Pt thin-film alloys have been grown by electrodeposition at potentials positive to that required to deposit elemental Fe. X-ray diffraction studies indicate the formation of fine grained face centered cubic alloys, while Rutherford backscattering spectrscopy and energy-dispersive X-ray spectroscopy reveal substantial incorporation of oxygen in the FePt deposits. The Fe-Pt codeposition process is driven by the negative enthalpy associated with alloy formation. The experimentally determined relationship between alloy composition and the iron group underpotential was found to be in reasonable agreement with free energy calculations for the binary alloy system, based on thermochemical data.

Compositional Control in Electrodeposited Pt100 − x Cu x Alloys

The electrodeposition of Pt 100-x Cu x alloys is demonstrated using an electrolyte comprised of 1 mol/L H 2 SO 4 , 0.001 mol/L K 2 ptCl 4 , and 0.1 mol/L CuSO 4 . Film growth proceeds by facile Cu underpotential deposition occurring in parallel with Pt overpotential deposition, with the latter controlling the overall rate of alloy deposition. Pt and depositions are monitored in real time using an electrochemical quartz crystal microbalance. Pt deposition from 1 mol/L H 2 SO 4 + 0.001 mol/L K 2 PtCl 4 is diffusion limited at potentials below -0.150 V saturated sulfate electrode and occurs with close to 100% current efficiency. In the added presence of CuSO 4 , alloy deposition occurs, supported by the exothermic enthalpy of mixing. Monitoring the steady-state mass/charge ratio enables the composition of the growing film to be evaluated with submonolayer precision. For a given potential, the film composition so determined is in excellent agreement with the alloy composition determined for 1 μm thick films using a combination of ex situ methods. The potential dependence of the alloy composition is described by an asymmetric regular solution model with an extremum for alloying enthalpy of -18.9 ± 0.2 kJ/mol.

Recrystallization texture, epitaxy, and magnetic properties of electrodeposited FePt on Cu(001)

A near-equiatomic FePt face-centered-cubic alloy has been grown by electrodeposition onto Cu(001). Annealing at 450°C results in the onset of a A1 fcc to L10 face-centered-tetragonal phase transformation with a dominant FePt(001)⟨100⟩‖Cu(001)⟨100⟩ recrystallization texture, possibly driven by the reduced ordering temperature in an FePtCu intermixed interface region. Further annealing at 650°C leads to a more complete transformation, and magnetic measurements indicate that high perpendicular coercivities, of up to 10kOe, are accessible through the recrystallization reaction. The magnetic and structural properties reported in this work may allow electrodeposited FePt to play a leading role in future ultrahigh-density patterned perpendicular magnetic media.

Compositional Control in Electrodeposited Ni x Pt1 − x Films

Electrochemical codeposition of a series of face-centered cubic Ni x Pt 1-x alloys is demonstrated (0.1 < x < 0.95). The alloy composition is a monotonic function of potential. The Pt-rich Ni x Pt 1-x alloys are formed at potentials positive to that required to deposit elemental Ni. Codeposition is ascribed to the negative enthalpy of Ni x Pt 1-x alloy formation that proceeds via a Ni underpotential deposition reaction in concert with Pt deposition. Interestingly, this process occurs at higher Ni underpotentials than anticipated based on extrapolated literature data from thermochemical measurements and ab initio calculations of alloy formation. In contrast, Ni-rich Ni x Pt 1-x alloys are produced at Ni overpotentials although the films are formed under conditions where pure Ni deposition is otherwise kinetically hindered. The alloy composition corresponding to the transition from underpotential to overpotential deposition is a function of the PtCl 4 /NiCl 2 electrolyte composition. The films were found to be bright and specular over the full range of compositions studied (grain size < 10 nm). Atomic force microscopy yielded root-mean-square roughness values on the order of 5 nm for Ni-rich deposits up to 2.5 μm thick.

Damping at normal metal/permalloy interfaces

We have determined the Gilbert damping parameter as a function of layer thickness in six sets of samples with either a normal metal (NM)/Permalloy (NiFe) or NM/NiFe/NM structure by measuring the ferromagnetic resonance (FMR) linewidth for all series and correcting for extrinsic broadening effects. In the NM/NiFe samples, we find that increasing the interface roughness increases the damping quadratically from 0.01 at 3 nm to 0.11 at 47 nm, and by controlling the roughness of the copper layer, we have found there is no evidence of increased damping due to the thickness of the copper layer between 100-2050 nm of copper.

A Structural Study of Electrodeposited Fe on n-GaAs ( 001 )

The microstructure of Fe films electrodeposited onto n-GaAs(001) from FeCl 2 and FeSO 4 -(NH 4 ) 2 SO 4 electrolytes was examined by X-ray and electron diffraction. Symmetrical θ-2θ X-Ray diffraction from films deposited from chloride solutions indicates a dominant (001) texture with the presence of additional minority orientations. In contrast, deposition from an ammonium sulfate electrolyte yields films that only exhibit (001) scattering in the θ-2θ geometry. Transmission electron microscopy (TEM) cross-sectional images of films grown in chloride solutions reveal intermittent cube-on-cube epitaxy that is also evident in plan view TEM dark field images. Interestingly, X-ray pole figure measurements for all specimens examined reveal an additional (221) orientation that is undetectable by symmetric θ-2θ diffraction. The (221) oriented material most likely derives from twinning of (001) oriented grains during growth.

Resistance changes similar to ballistic magnetoresistance in electrodeposited nanocontacts

We have studied the behavior of electrodeposited Ni and Fe nanocontacts in magnetic fields and the changes in resistivity (ΔR) that occur. Metallic particles suspended in plating solution, created and collected from the electroplating bath of a nanocontact that later exhibited high values of ΔR/R, have been transferred to a second set of electrodes, in which similar high values of ΔR/R were measured without any plating process being performed. We attribute this effect to a mechanical reorientation of magnetic nanoparticles at the junction between the electrodes as the field is close to zero, and relate this work to present work with ballistic magnetoresistance in nanocontacts. We also show that Fe whiskers brought in close contact can produce this effect as well.

Effect of conformal roughness on ferromagnetic resonance linewidth in thin permalloy films

The ferromagnetic resonance linewidth is a result of both intrinsic damping and contributions from inhomogeneities, which in thin films can be dominated by the roughness. Microstructural measurements and magnetization dynamics are reported here for 50-nm films of Permalloy deposited on substrates with controlled roughness. Ferromagnetic resonance measurements reveal a strong linewidth broadening that peaks when the magnetization is approximately 30° from the in-plane direction. These results are compared with a recently developed two-magnon theory of resonance broadening due to inhomogeneous demagnetizing fields in a rough film.

X-ray Diffraction Study of the Optimization of MgO Growth Conditions for Magnetic Tunnel Junctions

We have carried out a systematic study optimizing the MgO growth via preparation and sputtering conditions and underlayer structures. It was found that to prevent water vapor which is detrimental to MgO (200) growth, the chamber pressure needs to be reduced below 10−8Torr. Simple underlayers such as 5nm CoFeB tend to give better MgO, but we have also succeeded in growing MgO on more complicated underlayers such as 1 Ta/20 Au/5 Co40Fe40B20 and 1 Ta/20 conetic (Ni77Fe14Cu5Mo4)∕1.5 Co40Fe40B20 (units in nanometers). We accomplished this by extensive baking of the deposition chamber and use of Ti-getter films. Short sputtering distance and high sputtering power were found to optimize MgO deposition. We found that both preparation and sputtering conditions have important effects on the MgO growth. X-ray diffraction analysis was used as the characterization tool for optimizing the MgO growth conditions.