D. Mauri

Simple model for thin ferromagnetic films exchange coupled to an antiferromagnetic substrate

The exchange field Hex transferred from a thick antiferromagnetic substrate to a thin exchange coupled ferromagnetic film is shown to reach a limiting value no matter how large the exchange coupling is. The limit is due to domain‐wall formation in the antiferromagnet. Numerical results based on a simple model for the interface are presented and compared to experimental results.

MnxPt1−x: A new exchange bias material for Permalloy

We report exchange biasing of Permalloy in as-grown MnxPt1−x/Permalloy structures grown by coevaporation in ultra-high vacuum. Polycrystalline and epitaxial structures with x between 0.35 and 0.75 were studied. X-ray diffraction confirms the presence of chemical ordering in the MnxPt1−x films (grown at 200 °C) in as-grown polycrystalline and epitaxial structures. An interfacial coupling energy of 0.032 erg/cm2 is found for structures where the MnxPt1−x (x=0.56) is grown at 200 °C onto epitaxial Permalloy (001). Polycrystalline structures have a smaller (0.027 erg/cm2) interfacial coupling energy. No exchange bias is observed for films with x≲0.4. The blocking temperature, for a sample with x=0.56, is >500 K.

Magnetic properties of granular Co-polymer thin films

Composite cobalt‐polymer thin films are created by simultaneous metal sputtering and plasma polymerization of propane. The magnetic properties are investigated as a function of Co volume fraction. At low concentration, Co forms very small isolated clusters (∼10 A) which behave superparamagnetically down to <6 K. At 0.22 ±0.05 Co volume fraction an electrical percolation threshold and a transition to ferromagnetic behavior is observed.

High magnetoresistance permalloy films deposited on a thin NiFeCr or NiCr underlayer

Significantly enhanced anisotropic magnetoresistance (MR) in permalloy (Ni0.81Fe0.19) films deposited on a thin (Ni0.81Fe0.19)1−xCrx or Ni1−xCrx underlayer is reported. The maximum ΔR/R enhancement was observed using the underlayer with x≈0.44 at an optimum thickness of ≈30–45 A, depending on the deposition technique. An enhancement of 75%–150% was observed for 45–430 A thick permalloy films, compared to the films deposited without this underlayer. The ΔR/R enhancement is attributed to the decrease in the resistivity ρ and the increase in Δρ of the permalloy film due to the formation of large (111) textured crystal grains in the permalloy films deposited on this underlayer, as revealed by the x-ray diffraction results obtained using synchrotron radiation.Significantly enhanced anisotropic magnetoresistance (MR) in permalloy (Ni0.81Fe0.19) films deposited on a thin (Ni0.81Fe0.19)1−xCrx or Ni1−xCrx underlayer is reported. The maximum ΔR/R enhancement was observed using the underlayer with x≈0.44 at an optimum thickness of ≈30–45 A, depending on the deposition technique. An enhancement of 75%–150% was observed for 45–430 A thick permalloy films, compared to the films deposited without this underlayer. The ΔR/R enhancement is attributed to the decrease in the resistivity ρ and the increase in Δρ of the permalloy film due to the formation of large (111) textured crystal grains in the permalloy films deposited on this underlayer, as revealed by the x-ray diffraction results obtained using synchrotron radiation.

Thermal magnetization fluctuations in CoFe spin-valve devices (invited)

Thermally induced magnetization fluctuations in the Co86Fe14 free (sense) layer of micron-sized, photolithographically defined giant magetoresistive spin-valve devices are measured electrically, by passing a dc current through the devices and measuring the current-dependent part of the voltage noise power spectrum. Using fluctuation–dissipation relations, the effective Gilbert damping parameter α for 1.2, 1.8, and 2.4 nm thick free layers is estimated from either the low-frequency white-noise tail, or independently from the observed thermally excited ferromagnetic resonance peaks in the noise power spectrum, as a function of applied field. The geometry, field, and frequency dependence of the measured noise are found to be reasonably consistent with fluctuation–dissipation predictions based on a quasianalytical eigenmode model to describe the spatial dependence for the magnetization fluctuations. The extracted effective damping constant α≈0.06 found for the 1.2 nm free layer was close to 3× larger than tha...

Oxygen-enhanced IrMn spin valves deposited by ion-beam and magnetron sputtering

Enhancement of giant magnetoresistance properties of single (bottom) and dual IrMn-based spin valves through exposure of part of the CoFe pinned layer to O2 is reported. Under optimal conditions, a ΔR/R of 10.4% [Hua=460 Oe, Hf=5.1 Oe, and Hc=4.7 Oe for a free and pinned layer thickness (permalloy equivalent) of 50 A each] for an ion beam sputtered single spin valve, and a ΔR/R of as high as 20.5% for a magnetron sputtered dual spin valve having a 30 A thick CoFe free layer are observed, compared to a value of 6.5% and 10.6% for the corresponding spin valve without O2 exposure, respectively. Transmission electron microscopy results reveal the presence of a thin (10 A) crystalline oxygen-containing layer near the IrMn–CoFe pinned layer interface as a result of O2 exposure. X-ray reflectivity data show smoother interfaces for the spin valves subjected to O2 exposure, consistent with the lower Hf and smaller sheet resistance observed for these samples. The enhanced ΔR/R thus can be attributed to improved gro...

Design tradeoffs for beyond 20 Gb/in.2: Using a merged notched head on advanced low noise media (invited)

Design considerations for areal densities beyond 20 Gb/in.2 are discussed, and in particular a demonstration at 24.8 Gb/in.2 is shown. The demonstrations used a single combined write and read head (Merge, Notched head). In this article we will discuss the tradeoffs in kilobytes per inch (KBPI) and tracks-per-inch in obtaining areal densities at 25 Gb/in.2 densities. The KBPI is limited by both the total signal-to-noise ratio and the nonlinear-transition shift. Simple estimates of the increase in channel performance from class IV partial response (PR4), extended partial response 4 (EPR4), and modified EEPR4, as well as the addition of codes which eliminate bit shift and tribit errors are discussed as large contributors to the increase in the areal density.

Magnetic switching in ultrashort field pulses (abstract)

The Ginzburg–Landau–Lifshitz (GLL) equation, which describes the time dependence of spin precesssion in an external magnetic field1 relates the minimal field required to reverse the magnetization at fixed pulse length to the anisotropy field of the sample.23 We have systematically varied this parameter between 1.3 and about 5.0 T in a series of perpendicularly magnetized Co/Pt multilayer films and studied the magnetization reversal in picosecond in plane field pulses. Such pulses of several Tesla field strength and ultrashort duration were obtained in the final focus test beam section of the Stanford Linear Accelerator Center. The resulting magnetization pattern, which is reminiscent of the field during exposure, is subsequently analyzed with Kerr microscopy2 (see Fig. 1). As a prominent feature, we observe a beam field related switching radius from up to down magnetization which we compare to the theoretically expected field within the GLL formalism.

Spatially resolved measurement technique of magnetic anisotropy fields using the magneto‐optic Kerr effect

In this paper a novel method that permits spatially resolved measurements of the anisotropy field and easy axis direction is reported. The technique employs a Kerr based sensor that monitors the direction of magnetization φ of a magnetically soft thin film while an applied field is rotated in the plane of the film. The magnitude of the rotating field is set approximately to twice the anisotropy field (Hk), and thus the film remains saturated at all times and the domain wall artifacts reported previously are eliminated. φ and the direction of applied field β are then independently measured. We show that the amplitude and phase of sin(β−φ) are related to Hk and the easy axis orientation (EAO), respectively. The amplitude and phase information can be easily determined by first normalizing and then applying a fast Fourier transform to the digitally captured Kerr signal. Operationally, computer controlled translation stages are used to scan the sample under the Kerr laser beam, which is fixed with respect to t...