Torsion Oscillation Magnetometry Research Articles

Torsion oscillation magnetometry (TOM) of Fe films on Ni(1 1 1)/W(1 1 0) substrates

Fe films 2–20 atomic monolayers (ML) thick have been deposited on Ni(1 1 1) films (4–60 ML) prepared on W(1 1 0) under UHV conditions. The Ni(1 1 1) films grow in a Nishijama–Wassermann orientation with a 3.6% lattice expansion along Ni[2̄ 1 1] (‖ W[1̄ 1 0]). On top of these slightly distorted Ni films iron is observed to grow preferentially in two mirror orientations. The saturation magnetization of these Fe films and the anisotropies of Fe/Ni bilayers have been studied using torsion oscillation magnetometry. The magnetization of the Fe films of 2.13 μ B per atom is close to the bulk value of Fe. The out-of-plane surface (interface) anisotropy constant K FeNi s=(−0.65±0.15) mJ/m 2 of the bilayers prefers a magnetization perpendicular to the surface. The exceptionally high value of the volume anisotropy constant of the Fe films, K Fe v=(+0.71±0.10) MJ/m 3=(52.2±7.5) μeV/atom, was ascribed to magnetoelastic contributions.

Adsorption-driven spin reorientation transition in sesquilayers of Fe(1 1 0) on W(1 1 0)

Epitaxial sesquilayers (sesqui = one and a half) of Fe(1 1 0), prepared near room temperature on W(1 1 0), are composed of a pseudomorphic monolayer sea which is ferromagnetically magnetized in the plane below its Curie temperature T c(ML) = 222 K, and a system of pseudomorphic double layer islands which just after preparation are perpendicularly magnetized at room temperature. During exposure to residual gas, the double-layer islands switch to in-plane magnetization. This adsorption-driven spin reorientation transition is studied using torsion oscillation magnetometry (TOM), for the case of a sesquilayer consisting of 1.46 pseudomorphic layers, prepared at 325 K.

Magnetic moments and anisotropies in smooth and rough surfaces and interfaces

The relation between the magnetic surface and step moments and anisotropies in free and Pd-covered Fe(110) surfaces is studied experimentally by torsion oscillation magnetometry (TOM) of ultrathin Fe(110) films on W(110) in situ in UHV before, during and after Pd coverage. As a result of Pd coverage, the excess surface moment is enhanced by 0.70μB per surface atom. The enhancement is restricted to the first monolayer of Pd. Near [001] steps, the additional excess step moment is reduced by 0.65μB by Pd. The surface anisotropies of both free and Pd-covered Fe(110) surfaces support perpendicular magnetization. Near [001] steps, they are enhanced by additional step anisotropies, of equal sign and order of magnitude, if expressed by anisotropy energy per atom.

Magnetization of free Fe(110) surfaces from thin film magnetometry

Using torsion oscillation magnetometry of uncovered Fe(110) films on W(110) in situ in UHV, we have determined a surface excess moment in free Fe(110) surfaces of 0.39(16) monolayer equivalents. The results are compared with previous determinations using SPLEED, and with theoretical values.

Magnetic moments and anisotropies in smooth and rough surfaces and interfaces

The relation between the magnetic surface and step moments and anisotropies in free and Pd-covered Fe(110) surfaces is studied experimentally by torsion oscillation magnetometry (TOM) of ultrathin Fe(110) films on W(110) in situ in UHV before, during and after Pd coverage. As a result of Pd coverage, the excess surface moment is enhanced by 0.70μ B per surface atom. The enhancement is restricted to the first monolayer of Pd. Near [001] steps, the additional excess step moment is reduced by 0.65μ B by Pd. The surface anisotropies of both free and Pd-covered Fe(110) surfaces support perpendicular magnetization. Near [001] steps, they are enhanced by additional step anisotropies, of equal sign and order of magnitude, if expressed by anisotropy energy per atom.

Epitaxial strain and magnetic anisotropy in ultrathin Co films on W(110).

The role of epitaxial strain for the anisotropies of ultrathin films is studied experimentally for the case of Co(0001) films on W(110), based on the measurement of anisotropies using torsion oscillation magnetometry combined with measurement of the strain by high-angular-resolution low-energy electron diffraction. Up to a thickness of t=2 nm, the films grow in a state of constant strain, governed by pseudomorphism in the direction ([11\ifmmode\bar\else\textasciimacron\fi{}00]Co\ensuremath{\parallel}[11\ifmmode\bar\else\textasciimacron\fi{}0]W), which results in a true volume-type strain anisotropy. Above 2 nm, a relaxation of strain is observed which scales roughly with 1/t and therefore results in an apparently surface-type contribution to strain anisotropy, superimposed on a reduced volume contribution. In-plane and out-of-plane volume and surface-strain anisotropies are calculated from the observed strain. For the four anisotropy constants, their calculated differences between the two regimes above and below 2 nm agree with their differences as determined by magnetometry.

Magnetic surface anisotropies of Co(0001)-based interfaces from in situ magnetometry of Co films on Pd(111), covered with ultrathin films of Pd and Ag

Magnetic surface anisotropies (MSA) of Co(0001) interfaces with Ag(111) and Pd(111), and of the free Co(0001) surface, were determined as a function of the thickness of Ag or Pd coverage, respectively, by torsion oscillation magnetometry in situ in UHV. Both second- and fourth- order contributions were considered. Whereas the free surface supports in-plane magnetization, the metal interfaces support perpendicular magnetization, with a pronounced maximum in the magnitude of MSA for 1 monolayer Ag, but a monotonic dependence on Pd thickness.

Magnetic anisotropies of Fe(110) interfaces

Magnetic surface and interface anisotropies were determined for interfaces of Fe(110) with UHV, W, Cr, Cu, Ag and Au. This determination of single interface anisotropies is based on torsion oscillation magnetometry in situ in UHV of Fe(110)-films on Cr(110), in extension of previous work on Fe(110)-films on W(110). The out-of-plane anisotropy of the W(110)/Fe(110) interface is easy plane with an extraordinary strength of 1.92 mJ/m2, whereas all other interfaces show perpendicular out-of-plane anisotropy. The in-plane surface anisotropy supports [001] for Fe / Cr and Fe / Au interfaces, it supports [110] for W / Fe, Fe / UHV, Fe / Cu and Fe / Ag interfaces. Higher order terms of angular dependence beyond Néel's quadratic approximation are needed for a full description of magnetic surface anisotropies. A convenient notation is proposed.

Angular dependence of perpendicular magnetic surface anisotropy and the spin-reorientation transition.

For a complete description of out-of-plane magnetic surface anisotropy and its angular dependence, higher-order terms are needed beyond the quadratic N\'eel-type approximation. The spin-reorientation transition between perpendicular and in-plane magnetization is influenced by these higher-order terms in an elementary manner. Second- and fourth-order terms of the magnetic surface anisotropy were determined by torsion-oscillation magnetometry for uncovered Fe(110) films prepared on Cr(110), and for Pd-covered Co films prepared on Pd(111). These samples show different types of the spin-reorientation transition.

Oscillatory Indirect Coupling Between Perpendicularly Magnetized Co Monolayers Through Cu (111)

ABSTRACTCo-Monolayers, prepared by MBE on Cu (111) -surfaces at room temperature and covered by Cu, are ferromagnetic with a Curie-temperature of about 430 K. They are magnetized perpendicularly because of a strong perpendicular magnetic surface anisotropy of the Cu/Co (111) -interface. They provide a remarkably good representation of the 2-dimensional Ising Model. The indirect coupling between these perpendicularly magnetized ferromagnetic monolayers was investigated using samples of type Cu (111) /lCo/DçuCu/lCo/Cu, containing Co/Cu/Co-trilayers composed of Co-Monolayers and a spacer consisting of DCu atomic layers of Cu (111). Torsion oscillation magnetometry of these samples showed clearly a coupling between the monolayers with an oscillatory dependence on DCu. The amplitude of the oscillation is strongly reduced if the coupled Co-films consist of 5 ML instead of 1 M.L. The present controversy on the presence or absence of antiferromagnetic and oscillatory indirect coupling in the Co/Cu (111) -system is discussed in the light of these experiments. The discussion shows that the oscillatory coupling is an intrinsic property of ideal (111)-structures, and can be understood by the RKKY-type theory of indirect coupling between ferromagnetic Monolayers. The usual application of this theory to the coupling between thicker films is justified. However, in the fcc (111) -system there is apparently a specific barrier against complete coalescence, resulting in a tendency to retain holes and channels in the Cu-spacer. This tendency is stronger in flat single-crystal samples than in sputtered films with high densities of atomic steps. Apparently, this results in competing ferromagnetic hole coupling which may more or less completely obscure the intrinsic oscillatory coupling, preferentially in samples grown on extremely flat single crystal surfaces.

Magnetic step moments

Magnetic properties of rough Fe(110) films have been investigated by Torsion Oscillation Magnetometry (TOM). It is found that the magnetic moment of a single Fe-atom on a step edge is increased by (0.7±0.4) μ B to (3.4±0.4) μ b, compared to a surface moment of (2.7±0.2) μ B. The result is roughly in accordance with values obtained from a local moment theory.

Magnetic anisotropies in Fe(110) films on W(110) (abstract)

Magnetic anisotropies in epitaxial Fe(110) films on W(110) were analyzed using torsion oscillation magnetometry (TOM) in situ in UHV. Films with clean surfaces and films coated by Cu, Ag, and Au were analyzed. Out-of-plane anisotropies were determined by standard TOM methods, in-plane anisotropies from hard-axis magnetization loops. All anisotropies showing a clear linear dependence on reciprocal film thickness, a straightforward separation of volume- and surface-type anisotropies could be performed. Volume anisotropies can be explained as a superposition of standard fourth-order magnetocrystalline and of strain anisotropies, caused by residual strain of a Fe coincidence lattice on W. Surface anisotropies, both in-plane and out-of-plane, are connected by Néel’s model with bulk magnetoelastic constants, to a surprisingly good approximation.

Magnetic anisotropies in Fe(110) films on W(110)

Magnetic anisotropies in epitaxial Fe(110) films on W(110) were analyzed using torsion oscillation magnetometry (TOM) in situ in UHV. Films with clean surfaces and films coated by Cu, Ag, and Au were analyzed. Out-of-plane anisotropies were determined by standard TOM methods, in-plane anisotropies from hard-axis magnetization loops. Since all anisotropies showed a clear linear dependence on reciprocal film-thickness, a straightforward separation of volume and surface type anisotropies could be performed. Volume anisotropies can be explained as a superposition of standard fourth-order magnetocrystalline and strain anisotropies, caused by residual strain of the Fe coincidence lattice on W. Surface anisotropies, both in-plane and out-of-plane, are related by Neel's model to the bulk magnetoelastic constants, to a surprisingly good approximation.

The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mossbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μB could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.

Ferromagnetism in the thermodynamically stable monolayer Fe(110) on W(110), coated by Ag

Ferromagnetic order in the thermodynamically stable, pseudomorphic monolayer Fe(110) on W(110), coated by Ag, was studied in situ in UHV using Conversion Electron Mossbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). Films near the monolayer coverage, prepared at 475 K, consist of nearly independent monolayer and double-layer patches. The properties of monolayer patches are nearly independent of the mean film thickness resulting in excellent conditions to determine the true monolayer properties. The Curie temperature is reduced toTc, mono= 282 K = 0.27 Tc,bulk, the ground state hyperfine field is reduced toBhf(0)=11.9 T = 0.35Bhf,bulk(0) and the magnetic moment per atom is enhanced toμ(0) = 2.53 μB=1.14μ(0)bulk. Remanent magnetization is detected forT ≦260 K=0.92Tc, mono, square loop magnetization forT ≦230 K=0.82Tc, mono. Unusual properties of the phase transition are detected by the combination of both experimental techniques.

Magnetism of interfaces between Fe(110) and noble metals

Changes of magnetic surface moment and out-of-plane surface anisotropies of Fe(110) surfaces, caused by coating with Cu, Ag, and Au, are determined by torsion oscillation magnetometry of epitaxial Fe(110) films on W(110) in UHV. Changes of magnetic moment Δm, normalized by the monolayer moment mML, are given by Δm/mML=0.00 for Cu coating, 0.01 for Ag coating, and −0.09 for Au coating, respectively.

Surface magnetism of oxygen and hydrogen adsorption on Ni(111)

The changes of surface magnetization and of surface anisotropies of Ni(111) surfaces during adsorption of oxygen and hydrogen were measured at room temperature using torsion oscillation magnetometry in situ in UHV (UTOM) of ultrathin epitaxial Ni(111) films on Re(0001). In the case of oxygen, a rapid chemisorption process is connected with an initial change in magnetic moment per O atom, Δμox=−4.5μNi (bulk Ni moment μNi), which is reduced to −3.8μNi in the saturated chemisorption state (θ= (1)/(3) ). This chemisorption is followed by a slow oxidation up to three layers of epitaxial NiO, connected with a change of the film moment Δm=−2.7mML (moment mML of bulk Ni monolayer). Surface anisotropy fields are reduced from (10±1) T (free surface) to (0±1) T both for chemisorbed and oxidized surfaces. For the case of hydrogen, saturation adsorption at room temperature (θ= (1)/(2) ) results in ΔμH=−1.6μNi and a reduction of the surface anisotropy field to (3±1) T.

Magnetometric analysis of the interaction of Ni(111) with oxygen

The interaction of oxygen with Ni(111) films on Re(0001), consisting of 18–50 atomic layers, was analyzed using LEED, AES and torsion oscillation magnetometry in situ in UHV (U-TOM) at room temperature. The sequence of rapid chemisorption up to θ = 13, followed by slow oxidation to 3 layers NiO, by lateral growth of islands, known from bulk crystals, has been confirmed. The initial sticking coefficient is determined to be So = 0.52 ± 0.1; it persists up to θ = 0.2. In the chemisorption state θ = 13 a minimum Smin ≤ 0.005 is observed. For the initial chemisorption (θ ≤ 0.2) the reduction of magnetic moment per O atom, ΔμOx, in comparison with the metallic Ni moment μNi, is given by ΔμOx = -4.5μNi. It is reduced to ΔμOx = -3.8μNi for θ = 13. The total reduction Δm of the magnetic moment saturates at ∼ 100 L with Δm = -2.7mML (moment mML of bulk monolayer Ni), corresponding to 3 monolayers of antiferromagnetic NiO. The surface anisotropy field is reduced from 9.9 ± 1 T for the free surface to 0 ± 1 T, both for the chemisorbed and the oxidized surface.