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Tracking the evolution of magnetic ordering in Co/Ru multilayers with inhomogeneous interlayer coupling using polarised neutron reflectometry
Abstract
We report the measurement of reflected neutron intensity "hysteresis loops" from Co/Ru multilayers that have both antiferromagnetically and ferromagnetically coupled regions. We show that by measuring the four neutron spin-resolved reflectivities at a particular value of wavevector transfer, the normalised value of the relevant magnetic order parameter may be determined. The response of that order parameter to an applied magnetic field may hence be tracked. We have benchmarked our results against conventional magnetometry and magnetotransport measurements. (C) 2011 Elsevier B.V. All rights reserved.
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Magnetic materials have become controllable on the nanometre scale. Such fine structures exhibit a wide range of fascinating phenomena, such as lowdimensional magnetism, induced magnetization in noble metals, electron interference patterns, oscillatory magnetic coupling and 'giant' magnetoresistance. Magnetic multilayers with nanometre spacings are among the first metallic quantum structures to become incorporated into electronic devices, such as reading heads for hard discs. This article is intended to familiarize the reader with the physics and technology of magnetic nanostructures. It starts out with recent progress in nanofabrication, gives a tutorial on the connection between electronic states and magnetic properties, surveys the state of the art in characterization techniques, explains unique phenomena in two-, one- and zero-dimensional structures, points out applications in magnetic storage technology and considers fundamental limits to storage density. Particular emphasis is placed on the connection between magnetism and the underlying electronic states, such as the spin-split energy bands, s, p versus d states, surface states, and quantum well states.
The vertical correlation of magnetic domains in a Co/Cr/Co trilayer is statistically quantified as a function of an applied magnetic field. These measurements, used with determinations of the individual layer magnetometry curves, identify the presence of both antiferromagnetic exchange coupling and ferromagnetic dipolar coupling for different regions within the trilayer.
An in-plane vectorial analysis of the magnetization of thin magnetic films is presented. Longitudinal soft x-ray resonant magnetic reflectivity curves display characteristic nodes where the longitudinal scattering component is suppressed by x-ray interference. The transverse magnetic component can be effectively retrieved at these nodal points, despite the use of circular polarization and longitudinal scattering geometry. Using a single geometric configuration, transverse and longitudinal magnetic hysteresis loops can be clearly separated. Calculations based on a Stoner-Wohlfarth model satisfactorily describe both loops. Therefore, this method presents a viable alternative to standard vectorial analysis techniques, with the additional benefit of element specificity.
Areal density progress in magnetic recording is largely determined by the ability to fabricate low-noise, granular thin lm media with sufficient stability against thermal agitation to warrant long-term data storage. A key requirement is a medium microstructure with small, magnetically isolated grains to establish optimal macro- and micro-magnetic properties. A lower bound for the minimal average grain diameter, compatible with thermal stability, is imposed by the write field capability of the recording head. It is 10-12 nm assuming maximal writeable coercivities of 400 kA/m (5000 Oe). These are already achieved in today's state-of-the-art CoCr-based thin lm alloy media, leaving little room for further improvements and density gains based on continued grain size reduction. A threefold reduction in grain diameter, however, translating into a tenfold increase in areal density is theoretically possible if write field constraints can be overcome, allowing utilization of magnetically harder alloys. This review emphasizes materials and fabrication aspects behind media for extremely high-density longitudinal magnetic recording. Special attention is paid to thermal stability and write coercivity constraints. Various alternative media designs for extremely high-density recording beyond 40-100 Gbits/inch2 are reviewed.
We have used resonant scattering of polarized soft x rays as a direct probe of the magnetic order in a weakly coupled Co/Cu multilayer. Our field dependent results, combined with in situ resistance measurements, show a direct correlation between magnetoresistance and antiparallel magnetic ordering in reversible and irreversible processes. © 2002 American Institute of Physics.
The current interest in the magnetism of ultrathin films and multilayers is driven by their manifold applications in the magneto-and spin-electronic areas, for instance as magnetic field sensors or as information storage devices. In this regard, there is a large interest in exploring spin structures and spin disorder at the interface of magnetic heterostructures, to investigate magnetic domains in thin films and superlattices, and to understand remagnetization processes of various laterally shaped magnetic nanostructures. Traditionally neutron scattering has played a dominant role in the determination of spin structures, phase transitions and magnetic excitations in bulk materials. Today, its potential for the investigation of thin magnetic films has to be redefined. Polarized neutron reflectivity (PNR) at small wavevectors can provide precise information on the magnetic field distribution parallel to the film plane and on layer resolved magnetization vectors. In addition, PNR is not only sensitive to structural interface roughness but also to the magnetic roughness. Furthermore, magnetic hysteresis measurements from polarized small angle Bragg reflections allows us to filter out correlation effects during magnetization reversals of magnetic stripes and islands. An overview is provided on most recent PNR investigations of magnetic heterostructures.
The understanding of electronic behaviour in systems with reduced dimensionality and length scale is a central theme of contemporary condensed matter physics. The unique capabilities of neutron scattering make it an ideal method to study the atomic and molecular, chemical and magnetic structure of a wide class of materials. In this review we highlight recent studies where neutron techniques have been applied to emergent materials and look forward to the possibilities enabled by instrumentation on the ISIS Second Target Station.
By separately identifying magnetic and charge scatter, we find conclusive evidence for conformality in magnetic roughness in {Co (8 Angstrom) Cu (9 Angstrom)} multilayers. For layers magnetized in the easy direction, the magnetic roughness equals the structural roughness but increases when magnetized in the hard direction. The in-plane magnetic correlation length, which changes on magnetization is several orders of magnitude larger than the structural roughness length scales. The magnetic length scale is bf the same order as;magnetic ripple observed in Lorentz microscopy and is not associated with domains.
Oscillations of ferromagnetic to antiferromagnetic exchange coupling between two Fe layers separated by a Cr spacer of linearly increasing thickness were investigated by imaging the magnetic domains with scanning electron microscopy with polarization analysis (SEMPA). Up to six oscillations in the coupling with a period of 10-12 Cr layers were observed, and, in the case of an extremely well ordered Cr interlayer, additional oscillations with a period of 2 Cr layers were observed.
Exchange-biased spin-valve layered structures consist (in their simplest form) of a magnetically soft ferromagnetic layer, separated by a nonferromagnetic layer from a second ferromagnetic layer, which has its magnetization pinned by the exchange biasing interaction with a third magnetic (usually antiferromagnetic or ferrimagnetic) layer. For suitable layer compositions and nanometer-scale layer thicknesses these materials combine a fair Giant Magnetoresistance (GMR) effect with a very small field interval close to zero field in which the resistance change takes place. The resulting high sensitivity makes these materials attractive for sensor applications and other magnetoelectronic devices. In this overview, I first review experimental results on GMR of these materials, their theoretical interpretation, and the magnetic interactions which determine the magnetization reversal process. Subsequently, trends in magnetic recording and the potential benefit of exchange-biased spin-valves in yoke-type read heads for high-density digital magnetic recording are discussed. The chapter closes with an overview of unresolved issues, and with a note added in proof on developments that have taken place after the completion of the manuscript.
Since the advent of the nuclear reactor, thermal neutron scattering has proved a valuable tool for studying many properties of solids and liquids, and research workers are active in the field at reactor centres and universities throughout the world. This classic text provides the basic quantum theory of thermal neutron scattering and applies the concepts to scattering by crystals, liquids and magnetic systems. Other topics discussed are the relation of the scattering to correlation functions in the scattering system, the dynamical theory of scattering and polarisation analysis. No previous knowledge of the theory of thermal neutron scattering is assumed, but basic knowledge of quantum mechanics and solid state physics is required. The book is intended for experimenters rather than theoreticians, and the discussion is kept as informal as possible. A number of examples, with worked solutions, are included as an aid to the understanding of the text. © Cambridge University Press 1978 and Dover Publications Inc. 1996 and G. L. Squires 2012.
We have measured field-dependent resonant magnetic scattering of soft x rays from a Co/Cu multilayer exhibiting giant magnetoresistance. We show that, choosing the appropriate experimental geometries, one can draw hysteresis loops of antiferromagnetic as well as ferromagnetic order.
It is of interest to determine the characteristic length scale that determines giant magnetoresistance (GMR). In order to understand this behavior, GMR multilayers of Co/Ru and Co/Cu have been studied at a temperature of 4.2 K. The total thickness of Co/Ru multilayers has been varied from 96 to 1654 Å and Co/Cu from 77 to 2712 Å by increasing the number of bilayers (N). It has been observed that GMR increases with the number of bilayers and more than 20 bilayers for Co/Ru and 50 for Co/Cu are needed to reach the saturation value.
We have developed a procedure for fitting experimental and simulated X–ray reflectivity and diffraction data in order to automat and to quantify the characterization of thin–film structures. The optimization method employed is a type of genetic algorith called‘differential evolution’. The method is capable of rapid convergence to the global minimum of an error function in paramete space even when there are many local minima in addition to the global minimum. We show how to estimate the pointwise error of the optimized parameters, and how to determine whether the model adequately represents the structure. The procedure i capable of fitting some tens of adjustable parameters, given suitable data.
Recent low-angle neutron scattering results from magnetic multilayers are reviewed. After a recall of the theory, three sorts of studies are examined: the determination of the magnetization profiles in layers, the magnetic structures resulting from the coupling between magnetic layers through nonmagnetic ones and finally the particular case of rare earth/iron systems.
The oscillatory indirect exchange coupling exhibited by magnetic/nonmagnetic metal multilayers is known to be highly sensitive to structural defects. We have measured the effects on coupling of residual gas atoms in sputtered Co/Cu multilayers. We have used a simple technique to selectively probe particular parts of the multilayer stack. A large reduction in the giant magnetoresistance has been observed when gas impurity atoms are introduced into the middle of the Cu spacer layers, with a corresponding increase in the remanence of the sample. These changes are shown to be consistent with overwhelmingly strong biquadratic coupling between the Co layers. This leads to the moments in adjacent layers being no longer collinear in zero applied field, which we have confirmed by other measurements. We discuss the applicability of the various theoretical models of biquadratic coupling to our observations.
Soft x-ray resonant magnetic scattering (SXRMS) has been used to probe the interlayer coupling in amorphous ferromagnetic/semiconductor multilayers. It is shown that the [Co73Si27 (50 Å)/Si (30 Å)] system exhibits an antiferromagnetic (AF) coupling at low fields. Moreover, another aspect of SXRMS effect is reported. Using circularly polarized photons, a shift in the AF order Bragg peaks’ position is observed and related to two opposite AF states with the spin direction longitudinally aligned. As a consequence, the sensitivity of SXRMS to AF domains having the same spin axis but opposite senses is shown. A physical explanation for the origin of this effect is provided in terms of magnetic-resonant-refraction corrections to Bragg’s angle, taking into account the phase shifts between layers with opposite magnetization directions at different in-depth positions. Numerical simulations are performed that reproduced the experimental observations.
The hysteretic behavior of interfacial magnetic moments for CoFe thin films with varying roughness is determined in an element specific manner by monitoring the applied magnetic field dependence of the specular and off-specular (diffuse) contributions to the x-ray resonant magnetic scattering signal. Increasing the interfacial roughness generates a larger variation of the relative coercive field associated with the interfacial moment in comparison to the bulk. © 1998 American Institute of Physics.
Co1−xCux/Cu multilayers have been made by sputtering using codeposition of Co and Cu to obtain Co1−xCux alloy layers that are separated by 20 Å Cu spacers. As with Co/Cu multilayers, this Cu spacer thickness corresponds to the second antiferromagnetic maximum. At ambient temperatures, the Co1−xCux/Cu multilayers with x ≈ 0.5 exhibit an absence of magnetoresistive hysteresis resembling that reported previously for Co/Cu multilayers at the second antiferromagnetic maximum when the Co layers are very thin (∼3 Å). The multilayers with Co1−xCux alloys differ significantly from the low-hysteresis Co/Cu multilayers by exhibiting low hysteresis over a larger range of ferromagnetic layer thickness. This is practically significant because it reduces the demands for thickness control during manufacturing. © 1997 American Institute of Physics.
We have studied the reversal of the bulk and interfacial magnetizations of the free layer of a spin valve using soft x-ray resonant magnetic scattering. By dusting the interface of the NiFe free layer with a few angströms of Co, we were able to distinguish between the interfacial and bulk magnetisms by tuning the x-ray photon energy. We measured hysteresis loops of reflected x-ray intensity at selected points in reciprocal space. We find no difference in the switching fields, showing that in transition metal ferromagnets, the exchange interactions are sufficiently strong to prevent a separate interfacial coercivity from arising.
Interfacial roughness in multilayer films may be random or correlated, i.e., replicated from layer to layer. It is shown that these can be separated and quantified using x‐ray diffraction rocking curves and a straightforward analysis. The lateral correlation length along the interfaces can additionally be determined. A quantitative evaluation for W/C multilayers shows that correlated roughness contributes significantly to the total roughness, even at length scales that are surprisingly short, of the order 2–6 nm.
Polarized neutron reflectometry (PNR) has an important role in solving non-collinear magnetic structures in thin films. In principle PNR provides the unambiguous determination of multi-axial magnetic depth profiles in systems of scientific and technological importance. However, only a limited number of problems have been solved unimpeachably, because the magnetic depth profiles were obtained by fitting procedures yielding solutions whose main features were not evident in the raw data. A recent formulation of reflectometry, and the concurrent development of “total polarimetry” are now stimulating the design of neutron polarization and polarization analysis geometries capable of determining directly if and how a magnetic system contains a non-collinear structure in the film (x–y) plane. In the simplest of these geometries the neutrons are polarized and analyzed along the z-axis perpendicular to the film plane.
We demonstrate that polarised neutron reflection (PNR) sensitively probes the in-plane layer-dependent magnetisation vector profile in multilayer structures on nanometre lengthscales. We derive the spin-dependent reflectivity using an optical method for a general multilayer, and discuss two regimes: first, the kinematical diffraction limit; second, just above critical reflection, where multiple reflections and refraction dominate.
Specular reflectometry of polarized-neutrons was developed in the 1980s as a tool for measuring magnetic depth profiles in flat films, which were laterally uniform. When the lateral uniformity breaks down in an assembly of domains, off-specular grazing incidence scattering takes place. This review discusses this new frontier of reflectometry, describing the advances that are taking place in linking the observations of the scattering at grazing incidence with the size, the statistics, and the magnetic orientation of the domains. The article discusses also the progress made in linking the domain distribution thus found with the transport properties of these nanomagnetic systems.
The in-plane correlation lengths and angular dispersion of magnetic domains in a transition metal multilayer have been studied using off-specular neutron reflectometry techniques. A theoretical framework considering both structural and magnetic disorder has been developed, quantitatively connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The antiferromagnetic domain structure is highly vertically correlated throughout the multilayer. We are easily able to relate the neutron determined magnetic domain dispersion to magnetization and magnetoresistance experiments.
We have performed polarized neutron reflectometry (PNR) on Co/Cu multilayers grown by sputter deposition at the first antiferromagnetic (AF) maximum of the coupling oscillation. The growth of the Cu spacer layers was paused halfway through each layer for a variable amount of time to allow residual gases to be adsorbed onto the surface. A sample with clean Cu spacers shows good AF coupling, with low remanence and high saturation field. The PNR spectra show a strong 12-order Bragg peak and little splitting between the reflectivities for incident ↑ and ↓ spin neutrons at zero field, characteristic of AF ordering. Meanwhile, a more heavily gas-damaged sample with a remanent fraction of ~2/2 has strongly spin-split PNR spectra at the critical edge and nuclear Bragg peak, showing a significant ferromagnetic component. A strong 12-order Bragg peak is still present. We are able to fit accurately the magnetization and PNR data by assuming that such a sample shows considerable biquadratic coupling, with moments coupled close to 90° at zero field.
We have carried out resonant magnetic x-ray scattering from Co/Ru multilayers with weak antiferromagnetic coupling. We have measured hysteresis loops at different points in reciprocal space (specular or diffuse, integer or half-order peaks), which reveal a rich variety of different shapes. These arise from different degrees of mixing of the scattering arising from the field dependence of the order parameters describing the degree of ferromagnetic, antiferromagnetic correlations in the sample, with the off-specular measurements giving information about their lateral extent. We make a comparison with macroscopic measurements that give information about the degree of different forms of magnetic order averaged over the whole sample in real space.
By combining parallel and transverse magnetoresistance measurements on thin films of Co and Ni, the contribution of spin scattering at the domain walls is separated from the anisotropic magnetoresistance (AMR). A model, based on the Larmor-precession-induced deviation of the conduction electron spin direction during domain-wall traversal is developed. By using a scattering probability which varies with the cosine of the angle between the carrier spin and the local exchange field (as used for giant magnetoresistance systems) it is possible to account for the amplitude of the measured magnetoresistive effect.
Magnetoresistance, magnetization, and polarized neutron experiments have been performed on Ni81Fe19/Ag multilayers. The results give evidence for both biquadratic and bilinear contributions to the total coupling energy. The strong temperature dependence of the biquadratic term leads to a crossover from a canted state at low temperature to an antiferromagnetic one beyond 100 K. We suggest that the biquadratic term could arise from local concentration fluctuations in the chemically disordered NiFe layers.
We report the discovery of antiferromagnetic interlayer exchange coupling and enhanced saturation magnetoresistance in two new metallic superlattice systems, Co/Cr and Co/Ru. In these systems and in Fe/Cr superlattices both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance are found to oscillate with the Cr or Ru spacer layer thickness with a period ranging from 12 \AA{} in Co/Ru to \ensuremath{\simeq}18\char21{}21 \AA{} in the Fe/Cr and Co/Cr systems.
Exchange bias has been observed in sputtered magnetic double superlattices which consist of a ferromagnetically coupled superlattice grown on an antiferromagnetically (AF) coupled superlattice. This system exhibits a parallel domain wall, a spin flop transition, and exchange bias when the anisotropy is large in the AF block. This work shows that neither the domain wall nor the spin flop are directly related to exchange bias but that the anisotropy is essential.
Magnetoresistive random access memory (MRAM) technology combines a spintronic device with standard silicon-based microelectronics to obtain a combination of attributes not found in any other memory technology. Key attributes of MRAM technology are nonvolatility and unlimited read and write endurance. Magnetic tunnel junction (MTJ) devices have several advantages over other magnetoresistive devices for use in MRAM cells, such as a large signal for the read operation and a resistance that can be tailored to the circuit. Due to these attributes, MTJ MRAM can operate at high speed and is expected to have competitive densities when commercialized. In this paper, we review our recent progress in the development of MTJ-MRAM technology. We describe how the memory operates, including significant aspects of reading, writing, and integration of the magnetic material with CMOS, which enabled our recent demonstration of a 1-Mbit memory chip. Important memory attributes are compared between MRAM and other memory technologies.
Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics. Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published version
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