W. J. M. de Jonge

Magnetic interface anisotropy in Pd/Co and Pd/Fe multilayers

Pd/Co and Pd/Fe multilayer thin films containing ultrathin (2-12 A) Co and Fe layers were prepared by vapour deposition in ultrahigh vacuum. Their magnetization was measured at room temperature in fields parallel and perpendicular to the film plane. The Pd/Co multilayers show a transition of the preferred direction from lying in the film plane towards lying along the film normal when decreasing the Co layer thickness below 8 A. The Pd/Fe multilayers are preferably magnetized in the film plane, although the anisotropy decreases with lower Fe layer thickness. The magnetic anisotropy of both types of films can be interpreted by assuming an interface contribution which favours a perpendicular magnetization, and a volume contribution which favours an in-plane magnetization. The magnitude of the volume contribution is discussed in terms of magnetostatic, magnetocrystalline and magnetoelastic anisotropies, and equals, surprisingly, the anisotropy of the ferromagnetic elements in bulk thin film form in both cases.

Magnetic properties and structure of Pd/Co and Pd/Fe multilayers

Pd/Co and Pd/Fe multilayer films containing ultrathin Co and Fe layers were prepared by vapor deposition on substrates at room temperature. Their modulated structure, even for films containing 2‐A‐thin Co and Fe layers, was proved by x‐ray diffraction and transmission electron microscopy. Below a Co layer thickness of about 8 A, the Pd/Co multilayers acquire an easy magnetic axis perpendicular to the film, which is mainly caused by magnetic interface anisotropy. This leads for multilayers containing Co monolayers to almost rectangular hysteresis loops, by which these films may be very suitable as a perpendicular magnetic recording medium. Pd/Fe multilayers also have a perpendicular interface anisotropy, but the shape anisotropy dominates. Per unit Co volume the Pd/Co multilayers have a higher saturation magnetization than pure Co, which is attributed to an induced ferromagnetism on Pd interfacial atoms.

Magnetization curves of Pd/Co multilayers with perpendicular anisotropy

In a ferromagnetic thin film with strong perpendicular anisotropy, saturation may be reached at fields lower than the magnetization. This field is calculated for a multilayer with alternating ferromagnetic and nonmagnetic layers assuming that the stripe domains are oriented only up or down along the anisotropy axis. The results are compared with experimental data on Pd/Co multilayers with ultrathin Co layers. The agreement is very good if we take σw=1×10−3 J/m2 as the energy of walls between the domains. It is argued also that with other materials, high perpendicular relative remanence may be achieved when multilayers with suitable parameters are used.

Thermally assisted reversal of exchange biasing in NiO and FeMn based systems

The stability of the exchange bias field Heb has been studied for magnetron sputtered NiO/Ni66Co18Fe16 and Ni66Co18Fe16/FeMn bilayers. A forced antiparallel alignment of the ferromagnetic magnetization to Heb results in a gradual decrease of Heb as a function of time for NiO as well as FeMn based samples. The observed decrease of Heb increases with temperature and is interpreted as a thermally assisted reversal of magnetic domains in the antiferromagnetic layer.

Surface and volume anisotropy from dipole‐dipole interactions in ultrathin ferromagnetic films

At the boundary of a ferromagnetic material, the local change in the surroundings of the atomic magnetic moments induces an additional magnetic anisotropy. The dipole‐dipole interaction, responsible for the form‐dependent demagnetizing field inside the ferromagnet, differs for magnetic moments at the boundary and magnetic moments inside the bulk material. By calculation it is shown that the demagnetization factor for an ultrathin ferromagnetic film is thickness dependent. However, the anisotropy resulting from the dipole‐dipole interaction can be interpreted as a surface and a volume anisotropy which depend on the crystalline structure and orientation of the film, but are independent of the thickness of the film.

An STM study of Fe3O4 (100) grown by Molecular Beam Epitaxy

Abstract STM imaging of MBE-grown pseudomorphic (100) Fe 3 O 4 surfaces reveals terrace widths that are typically a few hundred angstroms broad, and can be as broad as 1000 A. These terraces are separated by steps that are 1 4 of the spinel lattice constant high, corresponding to the distance (2.1 A) between planes of oxygen (or equivalent iron) atoms. The images show that the p(1 × 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a [110] direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 A spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 A step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred angstroms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order.

INTERPLAY BETWEEN EXCHANGE BIASING AND INTERLAYER EXCHANGE COUPLING IN NI80FE20/CU/NI80FE20/FE50MN50 LAYERED SYSTEMS

Magnetoresistance curves have been measured for 80 A Ni80Fe20/tCu Cu/60 A Ni80Fe20/80 A Fe50Mn50 layered systems (tCu=9–93 A). After separating the contributions from anisotropic magnetoresistance and spin valve or ‘‘giant’’ magnetoresistance (GMR), the GMR curves were interpreted in terms of a minimum‐energy model for the magnetization orientation of the Ni80Fe20 layers. The interlayer exchange coupling was found to be purely ferromagnetic, and monotonically decreasing with increasing tCu. The interplay of this coupling with the exchange biasing effect results in a critical interlayer thickness, tcritCu=15 A, below which no perfect antiparallel alignment of the Ni80Fe20 magnetizations can be realized. The GMR, which would otherwise increase with decreasing Cu layer thickness, is therefore expected to reach a maximum at or slightly below tcritCu. Indeed, such a maximum was found (with ΔR/R=4.3%) at tCu=22 A, a value well above tcritCu. This is attributed to the occurence of inhomogeneities in the interlay...

Analysis of breakdown in ferromagnetic tunnel junctions

Due to their very thin tunnel barrier layer, magnetic tunnel junctions show dielectric breakdown at voltages of the order of 1 V. At the moment of breakdown, a highly conductive short is formed in the barrier and is visible as a hot spot. The breakdown effect is investigated by means of voltage ramp experiments on a series of nominally identical Co/Al2O3/Co tunnel junctions. The results are described in terms of a voltage dependent breakdown probability, and are further analyzed within the framework of a general model for the breakdown probability in dielectric materials, within which it is assumed that at any time the breakdown probability is independent of the (possibly time-dependent) voltage that has been previously applied. The experimental data can be described by several specific forms of the voltage breakdown probability function. A comparison with the models commonly used for describing thin film SiO2 breakdown is given, as well as suggestions for future experiments.

Perpendicular anisotropy in Pd/Co multilayers

The anisotropy values in polycrystalline Pd/Co multilayers with [111] texture as determined from magnetization measurements, torque measurements, and ferromagnetic resonance are compared. It is concluded that the most reliable value is obtained from the area between the magnetization curves measured with the field parallel and perpendicular to the plane of the film. The anisotropy is made up of a volume and an interface contribution. In a pair interaction model for the interface anisotropy it is shown that the latter is very sensitive to the precise distribution of Co and Pd at the interfaces. From the experimental data we derive a lower bound value of 0.2 meV per Co atom for the energy change with the direction of the magnetic moment. The temperature dependence of the magnetization indicates a long‐range interaction across the Pd layers up to 15–25 A.

MAGNETORESISTANCE IN NI80FE20/CU/NI80FE20/FE50MN50 SPIN VALVES WITH LOW COERCIVITY AND ULTRAHIGH SENSITIVITY

We present magnetoresistance measurements on Ni80Fe20/Cu/Ni80Fe20/Fe50Mn50 spin valves with crossed anisotropies: the easy magnetization direction of the unbiased Ni80Fe20 layer is perpendicular to the exchange biasing field which effectively works on the second Ni80Fe20 layer due to the interaction with the Fe50Mn50 layer. The hysteresis in the low‐field magnetoresistance is less than 0.03 kA/m, which is ten times smaller than the hysteresis for similar materials with parallel anisotropies. The sensitivity (∂R/R∞)/∂H ranges from 8% to 18%/(kA/m), depending on the preparational method. To our knowledge this combination of very high sensitivity and very low coercivity has not been reported before.