M.D. Wieczorek

Mössbauer studies of spin wave excitations in Fe/Ag multilayers

The magnetic properties of molecular beam epitaxially (MBE) grown FE(110)/Ag(111) heterostructures were investigated with Mössbauer spectroscopy. The Fe bilayers were fixed at 3 ML (monolayer) thickness and the Ag bilayer thickness varied from 4 ML to 20 ML. We found that as the Ag layer became thick enough (>17 ML) to magnetically isolate the Fe layers, a quasi-linear temperature dependence of the hyperfine field results due to the 2-D spin wave excitations. As the Ag layer is reduced, a dimensional crossover in the excitations is induced by the magnetic interaction between Fe layers which makesM(T) change from a two-dimensionalT relation to a three-dimensionalT3/2 dependence. We constructed a simple theoretical model to motivate the explanation for the experimental results and obtained approximate values for the interlayer coupling strength for various Ag bilayer thicknesses.

Effects of interfacial roughness on site-probed multilayers of Fe(100)/Ag(100)

Abstract Mossbauer spectroscopy was performed on three [Fe 9 (100)/Ag 40 (100)] 25 multilayers grown by molecular beam epitaxy (MBE). All of the Fe bilayer components in each sample were selectively loaded with 2 monolayers (ML) of 57 Fe: at the Fe-on-Ag interfaces of one sample, at the Ag-on-Fe interfaces of another, and at the Fe bilayer centers of the third. The hyperfine fields, quadrupole splittings, and isomer shifts of these multilayers were measured at 16, 100 and 300 K. We observed widely different values and trends as the temperature increased, leading us to repeat an earlier claim that three rather than two distinct sites are needed to correctly describe the Fe bilayer components in Fe(100)/Ag(100) multilayered systems.

The Mössbauer properties of epitaxial Fe-bcc Ni multilayers

Abstract Two series of Fe(110)/[bcc Ni] and Fe(100)/[bcc Ni] heterostructures were grown by molecular beam epitaxy. Analysis of the multilayers with 57 Fe Mossbauer spectroscopy indicated that all of the films local magnetization directions were oriented in-plane, and revealed that the spectra of both multilayer series consisted of two slightly broadened sextet components, corresponding to interface and interior Fe bilayer sites. The interior sites had slightly enhanced ground state hyperfine field values compared to bulk, while the interfacial sites had reduced ground state hyperfine field values. SQUID magnetometry revealed that the typical film coercivity of the Fe(100)/Ni was approximately an order of magnitude greater than that observed for the Fe(110)/Ni series.

Mössbauer studies of Fe(100)/Ag(100) multilayers grown on NaCl(100) by molecular beam epitaxy

Abstract Fe(100)/Ag(100) heterostructures, with varying Fe bilayer thicknesses equal to 3, 6 and 9 monolayers (ML) and a constant Ag bilayer thickness equal to 40 ML, were grown on suitably prepared monocrystalline NaCl(100) substrates by molecular beam epitaxy. The multilayer moncrystallinity was verified with in situ reflection high energy electron diffraction. Transmission Mossbauer measurements revealed a perpendicular surface anisotropy for the Fe(100) bilayers at 4.2 K. This behavior is unlike that of Fe(100)/Ag(111) ultrathin films of about the same thickness.

Growth of Fe(110)/bcc Ni(110) superlattices by molecular beam epitaxy

Abstract We have grown a series of Fe(110)/bcc Ni(110) superlattices by MBE. RHEED and X-ray analysis indicated that even for Fe thicknesses of only 2 ML, the subsequent Ni layer could be grown in a pseudomorphic phase with a structure resembling that of the Fe template up to a thickness of 8 ML. Magnetometry determined that this distorted bcc phase is ferromagnetic at room temperature. Transmission Mossbauer spectroscopy implied the existence of two magnetic Fe sites, one we associate with Fe atoms that have diffused into the Ni layers and the other with Fe atoms that mainly coordinate other Fe atoms. The diffused site showed no magnetic splitting at 300 K in an Fe(110)/Ag(111) film that was grown for comparison to the Fe(110)/Ni films, implying that the distorted bcc Ni(110) is magnetic.

The observation of a 3-D to 2-D crossover in the magnetism of epitaxial Fe(110) / Ag(111) multilayers

Abstract Transmission 57 Fe Mossbauer spectroscopy (TMS) was used to determine the temperature dependence of the magnetization of a series of Fe(110)/Ag(111) multilayer films grown by molecular beam epitaxy (MBE). The multilayer series of films had 3 monolayer (ML) thick Fe(110) bilayer components, and Ag(111) bilayer component thicknesses equal to 4, 8, 12 and 20 ML. The TMS spectra of each of these films consisted of a single magnetically-split sextet, with no additional superparamagnetic central features apparent. The multilayer with the 4 ML Ag bilayer component exhibited a T 3 2 hyperfine field temperature dependence. However, a transitional crossover in the Mossbauer hyperfine field temperature dependence with mixed T 3 2 and linear behavior was observed for the multilayers with intermediate Ag bilayer component thicknesses, while the 20 ML Ag bilayer component multilayer exhibited a linear hyperfine field temperature dependence. In the light of the absence of significant superparamagnetism in these films, the linear hyperfine field temperature dependence in the thickest Ag bilayer component multilayer is most likely the result of a genuine quasi-two-dimensional behavior.

Interlayer coupling in epitaxial Fe(110)/Ag(111) multilayers

Abstract We have detected a possible oscillatory interlayer coupling in Fe(110)/Ag(111) multilayers grown by molecular beam epitaxy. The oscillation period is between 5 and 9 ML. The coupling was examined using a site-specific Mossbauer spectroscopy technique to determine the spin-wave spectrum at the first two monolayers of the Fe(110) surface. As the interlayer exchange decreases, the spin-wave spectrum softens, resulting in a faster decrease in the nuclear hyperfine field with temperature. At an interlayer thickness of 6 ML, the coupling is so strong that the spin-wave spectrum appears almost bulk-like.

Genuine-2-dimensional magnetism and interlayer magnetic coupling in Fe(110)/Ag(111) multilayers

Abstract We report the observation of genuine two-dimensional ferromagnetism in MBE-grown Fe(110)/Ag(111) multilayers of fixed 3 monolayer (ML) thick Fe components. A linear temperature dependence of the magnetic hyperfine field was observed with transmission Mossbauer spectroscopy in the film whose Ag spacer layer is 20 ML thick. Comparative Mossbauer studies with and without the application of an external magnetic field ruled out the possibility of magnetic relaxation effects as the cause of the observed linear temperature dependence, and thus firmly established that this linear temperature dependence truly represents two-dimensional ferromagnetism. By reducing the thickness of the Ag spacer layer, a weak interlayer magnetic coupling between the Fe layers can be induced. As a result of this weak coupling, the multilayer system behaves three-dimensionally at temperatures sufficiently below a characteristic temperature determined by the strength of the interlayer coupling.

Two‐dimensional spin‐wave excitations in MBE‐grown Fe(110)/Ag(111) multilayers

It is well known that two‐dimensional spin‐wave excitations result in a linear temperature dependence of the magnetization in a quasi‐two‐dimensional ferromagnetic system. However, it has been shown also that magnetic relaxation from small islands inside a film can also result in a similar linear temperature dependence. In this paper, it is found that comparative Mossbauer measurements with and without a weak magnetic field can clearly distinguish these two different mechanisms: The linear temperature dependence of the magnetization is unaffected by the external field if 2D spin‐wave excitations are responsible for the linear behavior, while the linear slope of the temperature dependence of the magnetization is reduced by the external field if magnetic relaxation is involved.

Dimensional crossover in the magnetism of MBE‐grown Fe(110)/Ag(111) multilayers

Fe(110)/Ag(111) heterostructures, composed of fixed 3 monolayer (ML) Fe bilayers and variable‐thickness Ag bilayers, were grown by molecular‐beam epitaxy and investigated by transmission Mossbauer spectroscopy in the temperature range from 4.2 to 300 K. We found that as the Ag layer is thick enough (≳17 ML) to magnetically isolate the neighboring Fe layers, a quasilinear temperature dependence of the Mossbauer hyperfine field results due to the two‐dimensional spin‐wave excitations. As the Ag layer is reduced to 4 ML, a dimensional crossover in the spin‐wave excitations is induced by the magnetic interaction between neighboring Fe layers which makes the magnetic temperature dependence change from a two‐dimensional T‐linear relation to a three‐dimensional T3/2 dependence.