C. J. Gutierrez

A Mössbauer study of ultrathin 57Fe(110)/Ag(111) heterostructures grown by molecular beam epitaxy

Abstract We have grown good quality single crystal heterostructures of Fe(110) / Ag(111) with a high degree of epitaxy not previously achieved for films this thin. The iron-free mica substrates are transparent to gamma rays and aid in the epitaxial growth process. These films were characterized by reflection high-energy electron diffraction (RHEED), residual gas analysis (RGA), and transmission Mossbauer spectroscopy. The film with the thickest Fe component (8 monolayers) showed a hyperfine field comparable to bulk while the films with the thinnest Fe components (1.3 and 2.0 monolayers) showed an enhanced hyperfine field at 4.2 K. All the films were magnetically ordered at all the temperatures measured. Most importantly, and in contrast to other work, the direction of magnetization was in-plane for all of the films.

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.

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.

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.

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.

Fe(100)/Ag(100) heterostructures grown on NaCl(001) by molecular beam epitaxy (abstract)

Three heterostructures were grown, each with 40 monolayer (ML) Ag bilayer thicknesses. The Fe(100) bilayers had thicknesses of 3, 6, and 9 ML. All growths were performed with a Perkin‐Elmer PHI 430B molecular‐beam‐epitaxy (MBE) system equipped with reflection high‐energy electron diffraction (RHEED) and a quadrupole mass analyzer. The growth region consistently achieved a base pressure of less than 5×10−10 Torr, and a growth pressure of less than 1.5×10−9 Torr. The base for all of our heterostructures consisted of 5 kA Ag(100) grown on polished single‐crystal NaCl(001) substrates. Before the Ag base growth, a 200‐A NaCl epilayer was deposited on the previously out‐gassed NaCl substrate at 250 °C. Excellent Ag(100) RHEED patterns were obtained after a 3‐h post‐growth anneal of the base at 375 °C. Typical heterostructure growth rates were 2 ML/min for Fe and 15 ML/min for Ag. All the heterostructures were capped by a 5‐kA Ag protective cover.Our growths experienced a ramped substrate growth temperature betw...

Measurements of magnetic interaction through silver in epitaxial Fe(110)/Ag(111) superlattices by Mössbauer spectroscopy

Magnetic interactions between Fe layers in Fe(110)/Ag(111) superlattices grown by molecular beam epitaxy have been observed using Mössbauer spectroscopy. By measuring the temperature dependence of the hyperfine field at the Fe layers, characteristics of the spin-wave spectrum can be deduced. As the Ag thickness between layers is increased, the magnetic interaction between Fe layers decreases, and the spin-wave spectrum undergoes a transformation from three-dimensional to quasi-two-dimensional.