G. Lauhoff

Magnetisation reversal in thin Cu/Ni/Cu/Si(0 0 1) films

Abstract A step wedged structure of the form 30 A Cu/50 A Ni/ h A Cu/Si(0 0 1) ( h = 600, 1000, 1500, 2000 A ) grown by MBE has been characterised by in situ RHEED and ex situ polar MOKE magnetometry and MOKE microscopy. The magnetisation reversal process was investigated through measurements of hysteresis loops, magnetisation relaxation and domain images. The sample exhibits a strong magnetic aftereffect and the macroscopic relaxation measurements were fitted by a model which assumes thermally activated magnetisation reversal through domain nucleation and domain-wall propagation. Magnetic imaging confirms that domain-wall propagation dominates over domain nucleation at all Cu buffer layer thicknesses. For the 600 A Cu buffer layer, strong domain-wall pinning occurs on a micron scale, whereas for the 2000 A Cu a domain-wall can propagate freely over hundreds of microns. The reduced pinning causes the coercive field to decrease as the Cu buffer thickness increases. The Barkhausen length deduced from fits to the relaxation data increases with Cu buffer thickness. The correlation length at the Cu buffer and Ni overlayer surface as obtained from the width of the RHEED streaks also increases, suggesting an interdependence between the crystallographic properties of the buffer and overlayer and the magnetic properties.

Effect of the Cu capping thickness on the magnetic properties of thin Ni/Cu(0 0 1) films

Abstract We have studied the effect of the Cu capping thickness (in the range 20⩽ t Cu ⩽180xa0A) on the magnetic and structural properties of ultrathin FCC 40xa0A Ni/Cu(0xa00xa01) films by means of magneto-optic Kerr effect (MOKE) magnetometry, polarised neutron reflection (PNR) and grazing incidence X-ray surface diffraction (GID) measurements. MOKE measurements show that perpendicular magnetic anisotropy (PMA) is observed for the whole of the Cu thickness range studied, while an increase of the coercive field with t Cu is found. The variation of the in-plane strain in the Ni film is shown to increase monotonously between 20 and 90xa0A Cu capping thickness from −1.60±0.03% to −1.86±0.07% as revealed by GID, while PNR measurements show a marked decrease in the Ni magnetic moment per atom with increasing Cu capping layer thickness, from 0.54±0.03xa0 μ B for t Cu =50xa0A to 0.45±0.03xa0 μ B for t Cu =180xa0A. These results clearly show that a strain-induced reduction in the Ni magnetic moment per atom occurs. The increase in the coercive field is also attributed to the increase in the strain of the Ni film.

A direct test of x-ray magnetic circular dichroism sum rules for strained Ni films using polarized neutron reflection

We have directly compared the values of the magnetic moment obtained from x-ray magnetic circular dichroism (XMCD) measurements with those obtained from polarized neutron reflection (PNR) measurements on strained Ni films grown on Cu(001)/Si(001). The PNR measurements show that the absolute magnetic moments differ from that of bulk Ni. We find agreement within experimental errors between the two magnetometry measurements, confirming that the XMCD sum rules are applicable to this strained low-symmetric system.

Temperature dependence of spin waves in Co/CoO bilayers

Brillouin light scattering measurements of spin-wave frequencies in an exchange coupled ferromagnet/antiferromagnet epitaxial Co/CoO bilayer are reported. A striking temperature dependence of the measured spin-wave frequencies in the cobalt layer in the range 77–300 K was observed which has been demonstrated to be due to exchange coupling to the ultrathin (7 A) CoO layer as antiferromagnetic order develops. The temperature dependence of the spin-wave frequency demonstrates that interface exchange coupling occurs in the absence of the unidirectional anisotropy. A study of the mode line width shows a broadening with reducing temperature which indicates that locally ordered antiferromagnet regions persist above the Neel temperature and play a central role in determining the magnetic behavior of the bilayer system.

Variation of the magnetic moment and strain in epitaxial Cu/Ni/Cu sandwiches

We have studied the variation of the strain and the magnetic moment in epitaxial fct Ni(001) film structures using a combination of reflection high energy electron diffraction (RHEED) and x-ray magnetic circular dichroism. A 30 A Cu/Ni step-wedge (30, 60, 90, and 150 A)/600 A Cu buffer/Si(001) structure has been used to study the Ni thickness-dependent strain and magnetic moment. A decreased magnetic moment with decreasing Ni thickness is observed. A 30 A Cu/50 A Ni/step-wedged (600, 1000, 1500, and 2000 A) Cu buffer/Si(001) was used to vary the degree of strain as determined by in situ RHEED measurements in the same Ni film by varying the Cu buffer layer thickness. Our study shows a possible correlation between the magnetic moment and strain.

Ferromagnetic coupling and anisotropy in epitaxial Cu/Co/Ni/Cu (0 0 1)

Abstract We have studied the effect of Co overlayers (0–12 A) on a Ni (60 A)/Cu (1000 A)/Si (0 0 1) film structure. Epitaxial growth was confirmed by reflection high-energy electron diffraction. Magneto-optic Kerr effect and X-ray magnetic circular dichroism (XMCD) measurements show a transition from an out-of plane to an in-plane easy axis with increasing Co-layer thickness. XMCD confirms that the Co and Ni layers are ferromagnetically coupled and suggests that the magnetic moments in the Ni/Co system are sensitively dependent on the Co overlayer thickness.

Magnetic anisotropy and layer-selective magnetometry of Cu/Co/Ni/Cu/Si (0 0 1)

Abstract Perpendicular magnetic anisotropy (PMA) up to 6.15±1.25xa0A Co thickness is found for ferromagnetically coupled epitaxial Co/60xa0A Ni/Cu/Si(0xa00xa01) structures. Polarized neutron reflection measurements on a Cu/9xa0ACo/Ni/Cu(0xa00xa01) structure yield almost bulk like magnetic moments at room-temperature of 1.70±0.20xa0μB and 0.57±0.05xa0μB for Co and Ni, respectively. Magneto-optic Kerr effect measurements show that the uniaxial Co volume anisotropy is entirely attributed to the Co shape anisotropy due to an absence of strain in the Co film.

Magnetic anisotropy, magnetic moments and coupling of Cu/Co/Cu/Ni/Cu(001) trilayer

Polar magneto-optic Kerr effect (MOKE) measurements on an epitaxial Cu/Co (23 A)/Cu (0-49 A)/Ni (53 A)/Cu/Si(001) structure reveal that the Ni magnetization is aligned in plane for zero Cu spacer layer thickness and becomes increasingly aligned out of plane with increasing Cu spacer layer thickness, whereas an in-plane remanent magnetization for Co is always observed. Layer selective polarized neutron reflection (PNR) measurements yield almost bulk-like magnetic moments of 1.57±0.08 µB for Co and 0.50±0.04 µB for Ni for a Cu/Co (22 A)/Cu (10 A)/Ni (53 A)/Cu/Si(001) structure at room temperature. A reduced remanence is observed for both the out-of plane magnetization by polar MOKE, and the in-plane magnetization for Co and Ni by PNR. This could suggest either a canted magnetization or multidomain state at remanence.

Temperature dependent spin-wave behaviour in Co/CoO bilayers

Abstract We report Brillouin light scattering measurements of spin-wave frequencies in an exchange coupled ferromagnet (FM)/antiferromagnet (AF) epitaxial fcc Co/CoO bilayer structure. A striking temperature dependence of the measured frequencies in the cobalt layer in the range 77 to 300xa0K was observed and has been demonstrated to be due to exchange coupling to the antiferromagnetic layer. The existence of a uniaxial anisotropy field in the range 100–300xa0Oe within the AF layer along the FM magnetisation direction was demonstrated. The ratio of the interface to bulk AF exchange coupling strengths was found to lie in the range 0.75–1.1. The temperature dependence of the spin-wave line widths was also examined and used to show that locally ordered AF regions persist above the Neel temperature and play a central role in determining the magnetic behaviour.

Element-Specific Magnetic Anisotropy Determined by Transverse Magnetic Circular X-Ray Dichroism

The vertical correlation or anti-correlation of magnetic domains from one magnetic layer to the next in a multilayer structure is controlled by interand intra-layer coupling. By using a combination of absorption magnetic circular dichroism (MCD) and x-ray magnetic resonant scattering ( X M R S ) from magnetic multilayers (absorption and reflection of circularly polarized light at the L2.3 edge energies of the transition metals of the multilayer), the vertical correlation of magnetic domains can be statistically quantified. Angle and magnetic field dependent XMRS scans used in conjunction with MCD element-specific magnetic hysteresis loops (ESMH) are used to statistically determine whether magnetic domains in Fe/Cu/Co and Co/Cr/Co trilayer structures are vertically correlated (aligned), uncorrelated, or anti-correlated (anti-aligned). ESMH is used to determine the distribution of magnetic domains for each individual layer and XMRS identifies the vertically localized relative alignment of the magnetic moment of each layer. The data taken at the SLS Magnetic Circular Dichroism Facility (NSLS Beamline U4B) show that the magnetic domains in both structures are highly correlated, indicating the possible presence of interlayer dipole coupling.