Magnetic Properties Of Ultrathin Films Research Articles

Observation of Mermin-Wagner behavior in LaFeO3/SrTiO3 superlattices

Two-dimensional magnetic materials can exhibit new magnetic properties due to the enhanced spin fluctuations that arise in reduced dimension. However, the suppression of the long-range magnetic order in two dimensions due to long-wavelength spin fluctuations, as suggested by the Mermin-Wagner theorem, has been questioned for finite-size laboratory samples. Here we study the magnetic properties of a dimensional crossover in superlattices composed of the antiferromagnetic LaFeO3 and SrTiO3 that, thanks to their large lateral size, allowed examination using a sensitive magnetic probe — muon spin rotation spectroscopy. We show that the iron electronic moments in superlattices with 3 and 2 monolayers of LaFeO3 exhibit a static antiferromagnetic order. In contrast, in the superlattices with single LaFeO3 monolayer, the moments do not order and fluctuate to the lowest measured temperature as expected from the Mermin-Wagner theorem. Our work shows how dimensionality can be used to tune the magnetic properties of ultrathin films.

Novel Tunnel Magnetoresistive Sensor Functionalities via Oblique-Incidence Deposition.

Controlling the magnetic properties of ultrathin films remains one of the main challenges to the further development of tunnel magnetoresistive (TMR) device applications. The magnetic response in such devices is mainly governed by extending the primary TMR trilayer with the use of suitable contact materials. The transfer of magnetic anisotropy to ferromagnetic electrodes consisting of CoFeB layers results in a field-dependent TMR response, which is determined by the magnetic properties of the CoFeB as well as the contact materials. We flexibly apply oblique-incidence deposition (OID) to introduce arbitrary intrinsic in-plane anisotropy profiles into the magnetic layers. The OID-induced anisotropy shapes the magnetic response and eliminates the requirement of additional magnetic contact materials. Functional control is achieved via an adjustable shape anisotropy that is selectively tailored for the ultrathin CoFeB layers. This approach circumvents previous limitations on TMR devices and allows for the design of new sensing functionalities, which can be precisely customized to a specific application, even in the high field regime. The resulting sensors maintain the typical TMR signal strength as well as a superb thermal stability of the tunnel junction, revealing a striking advantage in functional TMR design using anisotropic interfacial roughness.

Narrow-band tunable terahertz emission from ferrimagnetic Mn3-xGa thin films

Narrow-band terahertz emission from coherently excited spin precession in metallic ferrimagnetic Mn3-xGa Heusler alloy nanofilms has been observed. The efficiency of the emission, per nanometer film thickness, is comparable or higher than that of classical laser-driven terahertz sources based on optical rectification. The center frequency of the emission from the films can be tuned precisely via the film composition in the range of 0.20–0.35 THz, making this type of metallic film a candidate for efficient on-chip terahertz emitters. Terahertz emission spectroscopy is furthermore shown to be a sensitive probe of magnetic properties of ultra-thin films.

Structural and magnetic properties of MnPd/Fe grown on MgO(100) substrate: Ab initio studies

Structural and magnetic properties of ultrathin films MnPd/Fe grown on MgO(001) are investigated using a self-consistent pseudopotential plane waves method based on density functional theory in the Perdew–Burke–Ernzerhof generalized gradient approximation. The results obtained reveal the presence of an antiferromagnetic coupling between successive Mn [100] rows, combined with a ripple where Mn outward atoms exhibit a positive magnetic moment, in the case of Mn overlayer on Fe/MgO(001). In the case of MnPd monolayer ordered alloy, the c(2×2) structure formation is more favorable than the p(1×2) one, exhibiting a ferromagnetic coupling between Mn neighbor atoms with a positive induced ferromagnetic moment on Pd atoms. Pd atoms are pushed outward. For 1-ML MnxPd1−x on Fe/MgO, the Mn absolute mean magnetization per atom increases as x coverage increases, whereas the Pd mean induced magnetic moment decreases. For systems alternating Mn and Pd monolayers on Fe/MgO(001), a complex magnetic structure is shown on Mn monolayers: changing from Mn neighboring antiferromagnetic coupling to Mn [010] rows antiferromagnetic behavior.The correlation is made between the electronic structure and the magnetic properties, by comparing filled with partially filled components (Pd, Mn and Fe) d-bands. The magnetization easy-axis changes between the in-plane and the out-of-plane orientations from Fe/MgO to MnPd/Fe/MgO systems.

Broadband ferromagnetic resonance system and methods for ultrathin magnetic films

Spintronics requires the development of magnetic thin film structures having a wide range of magnetic properties. Ferromagnetic resonance (FMR) is a well understood experimental technique that has proven to be an invaluable tool to probe the static and dynamic magnetic properties of ultrathin films, multilayer nanostructures, and superlattices. In order to achieve a full characterization of thin film materials, one needs to carry out FMR measurements at a wide range of microwave frequencies. In this paper, we show that one does not have to use a broadband vector network analyzer; similar performance can be achieved by using a broadband microwave signal generator, a coplanar waveguide, and a broadband microwave detector. To obtain a good signal to noise ratio, one needs to employ a modulation technique in order to use lock-in detection; in this paper, we use low frequency external field modulation (105Hz) and microwave power amplitude pulse modulation (10kHz). The sensitivity and the performance of this broadband microwave system is demonstrated on two types of samples: molecular beam epitaxy grown single crystal GaAs(001)/Fe/Au and sputter deposited textured Si(111)/Ta/Ru/Co/Ru superlattice structures. The samples were mounted on a coplanar waveguide, allowing one a broadband measurement, ~0.1–50GHz, of DC field swept FMR signals. The results are compared to traditional field swept, field modulated measurements in microwave cavity resonators. Despite the fact that the FMR signal can be very different from that obtained by standard microwave cavities, we show that the analysis of the FMR signal is fairly simple using an admixture of the in-phase and out-of-phase components of rf susceptibility and that the resulting fitted magnetic parameters are in excellent agreement. Additionally, we demonstrate that microwave power amplitude pulse modulation can be used to greatly speed up data collection times, especially for very weak and broad FMR signals.

Effects of nitrogen annealing on surface structure, silicide formation and magnetic properties of ultrathin films of Co on Si(100)

Effects of nitrogen annealing on structural and magnetic properties of Co/Si (100) up to 700°C has been studied in this paper. Ultrathin Co films having a constant thickness of 50 Å were grown on Si (100) substrates using electron-beam evaporation under very high vacuum conditions at room temperature. Subsequently, the samples were annealed at temperatures ranging from 100–700°C in a nitrogen environment at atmospheric pressure. Sample quality and surface morphology were examined using atomic force microscopy. Silicide formation and the resultant variation in crystallographic arrangement were studied using X-ray diffractometer. The magnetization measurements done using a vibrating sample magnetometer indicate a decrease in coercivity and retentivity values with increase in annealing temperature. Resistivity of the samples measured using a four-point probe set up shows a decrease in resistivity with increase in annealing temperature. Formation of various silicide phases at different annealing temperatures and the resultant variation in the magnetic susceptibility has been thoroughly studied and quantified in this work.

Organic-inorganic magnetic tunnel heterojunctions based on dithiapyrannylidene ultrathin films grown on Fe3O4(111)

We report on the morphology, transport, and magnetic properties of ultrathin films of 4,4′-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph4), a planar quinoïd organic compound with potential applications in electronics. Atomic force microscopy (AFM) shows that highly homogeneous and continuous DITPY-Ph4 ultrathin films with thicknesses as low as 2 nm can be grown on epitaxial Fe3O4(111). Current-sensing AFM performed on ultrathin layers evidences an insulating behavior with a nonresonant tunneling conduction regime. Vibrating sample magnetometry achieved on Fe3O4/DITPY-Ph4/Co organic-inorganic heterojunctions reveals the decoupling between the two magnetic electrodes and demonstrates that DITPY-Ph4 can be used to build magnetic tunnel junctions.

Thickness stabilization of ferromagnetic order of [formula omitted] compounds

Electronic and magnetic properties of ultra-thin films of FeGe 2 compounds have been investigated using ab initio density functional theory (DFT). In the bulk environment we obtained anti-ferromagnetic coupling between Fe atoms and no magnetic polarization on Ge atoms. Besides this ground state configuration a metastable solution is obtained with a ferromagnetic coupling between Fe atoms and anti-ferromagnetic coupling between Fe and Ge. Concerning the thin films, the magnetic coupling between Fe atoms depends on the film thickness. Ferromagnetic order is obtained in the case of an ultra-thin film and a transition to anti-ferromagnetic coupling is observed when we increase the film thickness.

Interface-driven manipulation of the magnetic anisotropy of ultrathin Co films on Pt(111): Substrate deposition of hydrogen and model calculations

The magnetic anisotropy energy (MAE) and the resulting spin-reorientation transitions of Co/Pt(111) ultrathin films are investigated by manipulating the Co/Pt interface with controlled adsorption of hydrogen prior to Co deposition. In situ low-energy electron-diffraction and surface magneto-optical Kerr-effect measurements are performed on Co films grown at low temperatures. The results show that interface H deposition leads to a remarkable change in the magnetization direction from perpendicular to in-plane, even for the thinnest Co films, which is accompanied by an important increase in the coercive force. Layer-resolved self-consistent electronic calculations of the MAE are performed in order to identify the interface contributions responsible for perpendicular magnetic anisotropy (PMA) and to quantify how the MAE depends on various possible adsorbate-induced modifications in the local magnetic moments. The results show that the PMA is quite insensitive to changes in the local magnetic moments at the Co film surface even if they are relatively large. However, the PMA depends crucially on the Co-interface moments, on the Pt-interface moments induced by the proximity to Co, and on the resulting spin-orbit interactions at the Pt atoms. The observed suppression of PMA by interface H adsorption is interpreted as the consequence of the reduction in the interface-Pt moments which, originates either a reduction in the Co-interface moments or at a decoupling of the Pt substrate from the magnetic film. Experiment and theory thus prove the dominant role of the Co-Pt interface MAE on the development of the relative stability of perpendicular and in-plane magnetization directions. The magnetic properties of ultrathin films may thus be tailored to a large extent by adsorbates trapped at $3d\text{\ensuremath{-}}4d$ or $3d\text{\ensuremath{-}}5d$ film-substrate interfaces.

Element specific investigation of ultrathin Co2MnGa∕GaAs heterostructures

We have used x-ray magnetic circular dichroism to study the element specific magnetic properties of ultrathin films of the Heusler alloy Co2MnGa at room temperature. Nine films were grown by molecular beam epitaxy on GaAs substrates and engineered to vary in stoichiometry as Co1.86Mn0.99Ga1, Co1.95Mn0.98Ga1, and Co1.97Mn0.96Ga1, with thicknesses of 5.3, 7.6, and 9.7nm, and were capped with Al to prevent oxidization. Sum rule analysis revealed magnetic moments significantly lower than the values predicted theoretically, especially in the case of the Mn total magnetic moment. The results do not support the hypothesis that a large magnetic moment is transferred from Co to Mn, and may suggest that diffusion and reacted layers at ultrathin thicknesses can effect the establishment of a full Heusler structure.

Ion sculpting: A tool for tuning magnetic anisotropy in ultrathin films

The possibility of modifying the magnetic properties of ultrathin films by means of ion sculpting has been verified for Co/Cu(001) and Fe/Ag(001). For both these systems, ion sputtering at grazing incidence performed in a proper, material-dependent, temperature range induces the formation of ripples at the surface of the films. Following ripple formation, the magnetic anisotropy in the plane of the films undergoes dramatic changes brought about by the out-of-equilibrium distribution of defect sites induced by the ripple structure itself.

The magnetic and magnetoresistance properties of ultrathin magnetite films grown on MgO substrate

We report on the properties of ultrathin (2, 4, 6, and 8nm) epitaxial films of magnetite, Fe3O4, grown on MgO (100). Atomic force microscopy image and V-I curves suggest that the films at this thickness are still continuous. The resistivity versus temperature results imply that the conductivity mechanism in all these films is similar. The resistivity of 4nm thick film is much greater than that of 6 and 8nm films. The films show ferrimagnetic instead of reported superparamagnetic behavior. The dead layer formed by Mg diffusion between MgO substrate and magnetite films and also the dead layer on the top uncapped film could be the possible reasons for the anomalous resistivity and magnetic properties of the ultrathin films. The effect of the “dead layer” in the thinner film is relatively greater than the one in the thicker film and should lead to a lower magnetoresistance.

Developments in surface magneto-optical Kerr effect setup for ultrahigh vacuum analysis of magnetic ultrathin films

We describe a surface magneto-optical Kerr effect (SMOKE) setup designed to study the magnetic properties of ultrathin films in ultrahigh vacuum (UHV). As in earlier systems, a crossed-poles electromagnet is used, which allows measurements in the longitudinal or polar configurations to be carried out without displacing the sample or changing the optical path. The poles are mounted in-vacuum, while the coils and yoke are mounted externally to obtain a higher magnetic field (∼2.8kOe). The width of the gap between the poles can be adjusted depending on the size of the sample. The performance of the instrument is demonstrated by measurements of the paramagnetic-ferromagnetic phase transition in a 2 ML Fe film grown on the Mo(110) surface.

Electronic and magnetic properties of ultrathin films and interfaces investigated by dichroic core-level photoelectron spectroscopy

The electronic and magnetic structures of ultrathin films made of a ferromagnetic and a nonmagnetic material are theoretically investigated by means of magnetic dichroism in spin- and angle-resolved core-level photoelectron spectroscopy. How these properties manifest themselves in the photoemission intensities is analyzed with a focus on the interface between film and substrate. The dependence on both exchange and spin–orbit splitting, magnetic ordering, core-level shift and on the thickness of the covering layer are investigated in detail. Ultrathin films of Fe and Pd serve as prototypical systems because of their large exchange and spin–orbit splittings, respectively.

A direct two-dimensional comparison of magnetic circular dichroism and magnetic linear dichroism in ultraviolet photoemission spectroscopy

The angular dependence of magnetic dichroism in photoemission (MDAD) from valence bands is more complicated than that from core levels. It gives access to spin-resolved information about the bands involved in the photoexcitation process. By using a unique ellipsoidal-mirror analyzer, we are able to observe MDAD from valence bands as a two-dimensional image, which gives a direct view of their angular dependence over a large slice through the Brillouin zone. In this work we have investigated Co films grown on a Cu(001) substrate. By using either circularly polarized light or linearly polarized light at a photon energy of 21 eV, magnetic circular dichroism and magnetic linear dichroism (MLD) from the Fermi edge shows different angular dependencies. This is due to the different excitation processes involved with the different light polarizations and it reflects the symmetry of the 3d bands of the Co. Furthermore, we also find that the sign of the MLD can change if differing initial states are involved. First principles calculations of Co bands and their photoelectron angular distributions with the different light polarizations will be presented. This shows that dichroism in valence band photoemission can provide valuable additional information on the electronic and magnetic properties of ultrathin films.

Magnetization reversal in weakly coupled magnetic patterns

Ion irradiation is an original process to pattern the structural and as a consequence the magnetic properties of ultra-thin films, down to the nanometer scale. Patterns of dots and tracks have been fabricated by focused Ga + ion beam scanned onto a Co layer with perpendicular magnetic anisotropy. Depending on the dose, the magnetic behaviour of the nanometric irradiated lines can be tuned from the ferromagnetic with reduced coercivity to paramagnetic. The larger the fluence, the smaller is the exchange between dots or tracks. These systems enabled investigations of the competition between exchange and dipolar interactions. For arrays designed with high irradiation doses and only coupled by dipolar interactions, the magnetic relaxation proceeds by the magnetization reversal of individual dots and follows a power-law time decay. Monte Carlo simulations reproduce this time dependence.

Layeredmagnetic structures: facts, figures, future

An overview is given of the most important properties of layeredmagnetic structures. This includes the effects of interfaceanisotropy and exchange bias as well as the magnetic properties ofultrathin films in the monolayer range. The current understandingof interlayer exchange coupling, giant magnetoresistance andtunnel magnetoresistance will also be indicated. Futureefforts will most likely include interlayer coupling induced by spininjection. Typical and record values for the strengths of theobserved effects are compiled from the literature.

Surface segregation and restructuring of colossal-magnetoresistant manganese perovskitesLa0.65Sr0.35MnO3

We have investigated the surface chemical composition of crystalline films of ${\mathrm{La}}_{0.65}{\mathrm{Sr}}_{0.35}{\mathrm{MnO}}_{3}$ by angle-resolved x-ray photoelectron spectroscopy. The surface composition was found to be significantly different from that of the bulk because of an appreciable Sr segregation. Furthermore, our study suggests that this Sr segregation has caused a major restructuring of the surface region characterized by the formation of a Ruddlesden-Popper phase $({\mathrm{L}\mathrm{a},\mathrm{S}\mathrm{r})}_{n+1}{\mathrm{Mn}}_{n}{\mathrm{O}}_{3n+1}$ with $n=1.$ Segregation and restructuring in the surface region should be common in these doped perovskites and will have a profound impact on the electronic and magnetic properties of ultrathin films and magnetic tunneling junctions made of these materials.

A setup combining four-point probe and surface magneto-optical Kerr effect for measurements of magnetotransport and magnetic properties of ultrathin films in ultrahigh vacuum

A four-point probe, suitable for in situ surface resistance and magnetoresistance (MR) measurements of ultrathin films, is described. The four-point probe head is mounted and fixed at the center of an electromagnet with the probes perpendicular to the applied magnetic field direction. The ultrathin magnetic film sample can be rotated to face the probe and moved to make contact with the probe using the sample manipulator. The resistance and MR of the magnetic ultrathin film can be measured. At the same sample position, the hysteresis loops can be measured using the surface magneto-optical Kerr effect technique.

The correlation between mechanical stress and magnetic properties of ultrathin films

The cantilever bending beam technique was applied to measure film stress, film magnetization and magneto-elastic coupling in nanometre Fe films grown epitaxially on W substrates. A simple optical deflection technique yielded sub-monolayer sensitivity for stress measurements and was used to determine magnetization and magnetostrictive properties of nanometre Fe films in situ. The combination of an electromagnet inside an ultra-high-vacuum chamber with a rotatable external magnet was employed to perform magneto-optical Kerr-effect measurements in the transversal, longitudinal and polar geometry in fields of up to 0.4 T. Examples for stress-driven structural changes in monolayer Fe films are discussed with respect to the unusual high coercivity found for sesquilayer Fe films and the re-orientation of the easy axis of magnetization in Stranski-Krastanov Fe films. The direct correlation between strain and magnetism was exploited to measure the magnetostrictive bending of the film-substrate composite. The magnitude and sign of the magneto-elastic coupling coefficient were found to depend on the film thickness, in contrast to the respective bulk values.