J. L. Robertson

Magnetic and structural properties of FePt-FeRh exchange spring films for thermally assisted magnetic recording media

Recently a novel media structure for thermally assisted magnetic recording was proposed consisting of a layer of FePt exchange coupled to a FeRh layer. The FePt forms a high magnetocrystalline anisotropy, high coercivity ferromagnetic layer. The FeRh layer is antiferromagnetic at room temperature, but upon heating above a transition temperature becomes ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy. The coupled ferromagnetic FePt and FeRh layers form an exchange-spring system significantly lowering the coercive field of the composite system compared to a single layer of FePt. This feature opens intriguing possibilities for media applications for thermally assisted magnetic recording where the ferromagnetic phase of FeRh is exploited to help write the media while the low-temperature antiferromagnetic phase supports the long-term stability. Here temperature-dependent structural and magnetic measurements of undoped and doped FeRh single layer and FePt-FeRh bilayer films are presented and the promises and challenges of the exchange spring media structure are discussed.

Diffuse Neutron Scattering Study of a Disordered Complex Perovskite Pb(Zn1/3Nb2/3)O3Crystal

Diffuse scattering around the (110) reciprocal lattice point has been investigated by elastic neutron scattering in the paraelectric and the relaxor phases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The appearance of a diffuse intensity peak indicates the formation of polar nanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of this diffuse scattering indicates that these regions are in the shape of ellipsoids, more extended in the direction than in the direction. The quantitative analysis provides an estimate of the correlation length, \xi, or size of the regions and shows that \xi ~1.2\xi , consistent with the primary or dominant displacement of Pb leading to the low temperature rhombohedral phase. Both the appearance of the polar regions at T*and the structural transition at Tc are marked by kinks in the \xi curve but not in the \xi one, also indicating that the primary changes take place in a direction at both temperatures.

Neutron diffraction and reflectivity studies of the Cr Néel transition in Fe/Cr (001) superlattices

Abstract The effects on the interlayer coupling of the Cr Neel transition is studied in Fe/Cr(001) superlattices. The Neel transition is suppressed for Cr layer thickness 42 A of Cr, the Neel temperature TN initially increases rapidly and then asymptotically approaches its bulk value with a three-dimensional transition-temperature shift exponent value of λ = 1.4 ± 0.3. Neutron diffraction confirms both the Cr antiferromagnetic order and the existence of the incommensurate, transverse spin density wave whose nesting wave vector is the same as that of bulk Cr. The ordering of the Cr dramatically alters the coupling of the Fe layers. The biquadratic Fe interlayer coupling observed for T > TN vanishes below TN as confirmed by polarized neutron reflectivity. The behavior can be understood in terms of finite-size and spin frustration effects at rough Fe-Cr interfaces.

Development of a compact in situ polarized 3He neutron spin filter at Oak Ridge National Laboratory

We constructed a compact in situ polarized (3)He neutron spin filter based on spin-exchange optical pumping which is capable of continuous pumping of the (3)He gas while the system is in place in the neutron beam on an instrument. The compact size and light weight of the system simplifies its utilization on various neutron instruments. The system has been successfully tested as a neutron polarizer on the triple-axis spectrometer (HB3) and the hybrid spectrometer (HYSPEC) at Oak Ridge National Laboratory. Over 70% (3)He polarization was achieved and maintained during the test experiments. Over 90% neutron polarization and an average of 25% transmission for neutrons of 14.7 meV and 15 meV was also obtained.

In-situ Polarized 3He system for the Magnetism Reflectometer at the Spallation Neutron Source

We report on the in situ polarized (3)He neutron polarization analyzer developed for the time-of-flight Magnetism Reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. Using the spin exchange optical pumping method, we achieved a (3)He polarization of 76% ± 1% and maintained it for the entire three-day duration of the test experiment. Based on transmission measurements with unpolarized neutrons, we show that the average analyzing efficiency of the (3)He system is 98% for the neutron wavelength band of 2-5 Å. Using a highly polarized incident neutron beam produced by a supermirror bender polarizer, we obtained a flipping ratio of >100 with a transmission of 25% for polarized neutrons, averaged over the wavelength band of 2-5 Å. After the cell was depolarized for transmission measurements, it was reproducibly polarized and this performance was maintained for three weeks. A high quality polarization analysis experiment was performed on a reference sample of Fe/Cr multilayer with strong spin-flip off-specular scattering. Using a combination of the position sensitive detector, time-of-flight method, and the excellent parameters of the (3)He cell, the polarization analysis of the two-dimensional maps of reflected, refracted, and off-specular scattered intensity above and below the horizon were obtained, simultaneously.

Growth and characterization of epitaxial FexPt100−x films on MgO(111)

FexPt100−x films were grown on MgO(111) by co-sputtering Fe and Pt. Composition of the films was determined by Rutherford backscattering spectrometry with an accuracy of 1%. Epitaxy and alloy ordering were quantified by x-ray diffraction and the order parameter was determined to be 0.97 for a film with x=30 and 0.99 for a film with x=25. Neutron diffraction measurements established the presence of an antiferromagnetic phase at T=100K in 500 nm FePt3 samples grown on MgO(111). Since FePt3 can be grown as an ordered antiferromagnet and a disordered ferromagnet, these films provide a pathway to grow lattice matched interfaces for exchange bias studies.

Phonon densities of states of gamma-cerium and delta-cerium measured by time-of-flight inelastic neutron scattering

Time-of-flight inelastic neutron scattering spectra were measured on cerium metal at temperatures near the fcc (gamma)-to-bcc (delta) transition temperature. Phonon density-of-states (DOS) curves were extracted from data acquired over a wide range of momentum transfers. A large softening of the phonon DOS was found in going from gamma-cerium to delta-cerium, and this accounts for an increase in vibrational entropy of (0.71 +/- 0.05)k B/atom. To be consistent with the latent heat of the gamma-delta transition, this increase in vibrational entropy must be accompanied by a large decrease in electronic entropy. The results not only confirm the recent discovery of a significant electronic contribution to the gamma-delta transition but also suggest that it may be twice that previously reported.

Phonon densities of states of gamma-cerium and delta-cerium measured by inelastic neutron scattering

Inelastic neutron scattering spectra were measured on Ce metal at temperatures near the fcc (gamma) to bcc (delta) transition, and phonon DOS curves were obtained. A large difference in the phonon DOS of gamma-Ce and delta-Ce was found, and this difference accounts for a change in vibrational entropy of (0.51 0.05)kB atom-1 at the gamma-delta transition. To be consistent with the latent heat of the gamma-delta transition, this large change in vibrational entropy should be accompanied by a thermodynamically-significant change in electronic entropy of the opposite sign.

Electrical heating for SEOP-based polarized 3He system

Development of neutron spin filters based on polarized 3He is underway at Spallation Neutron Source (SNS). We report the progress of electrical heating tests in polarized 3He based on Spin-Exchange Optical Pumping (SEOP) method. We first test the system performance based on electrical heating via non-inductance heating pads. We observe a contribution of 955 hours to the relaxation time T1 from the heating pads. We then test the electrical heating SEOP pumping system at the SNS beamline Magnetic Reflectometer. We currently obtain 73% 3He polarization in a cell with 820 cm3 in volume.

Magnetic transitions in lattice-matched, ordered FePt3 based antiferromagnetic/ferromagnetic films

Measurements of lattice-matched antiferromagnetic/ferromagnetic films which are ideal layered systems to study exchange bias are reported. Epitaxial films of FePt3 have two kinds of antiferromagnetic ordering. The spin ordering phase with wave vector Q1=(12120) has a Neel temperature TN=160K and that with wave vector Q2=(1200) has TN=100K. Neutron diffraction confirmed the presence of Q2=(1200) antiferromagnetic ordering in 200nm Fe25Pt75 grown on MgO(100). The loop shift and coercivity of a trilayer film of CoPt3∕FePt3∕CoPt3 decrease with increasing temperature, consistent with the observed Neel temperature of FePt3. The x-ray diffraction rocking curve widths of films grown on MgO(100) and Al2O3(1120) are compared and related to the loop shifts that are observed in the films with lattice-matched antiferromagnetic/ferromagnetic interfaces.