Interface Anisotropy Constant Research Articles

Correlation between interface perpendicular magnetic anisotropy and interfacial Dzyaloshinskii–Moriya interactions in Pt/Pd(t Pd)/Co(t Co)/Au

The interfacial Dzyaloshinskii–Moriya interaction (iDMI) and perpendicular magnetic anisotropy (PMA) can be manipulated via interface engineering. Their strength determines the generation and the size of skyrmions, and the correlation between the iDMI and PMA allows them to be controlled simultaneously, thus choosing the material parameters for which skyrmions can be formed. This opens up the possibility of synthesizing suitable magnetic multilayers that are needed for low-power high-density memory and in logic applications. Vibrating sample magnetometry (VSM), Brillouin light scattering (BLS), and microstrip ferromagnetic resonance (MS-FMR) were used to investigate the correlation between the iDMI and the interface PMA. For this, Pt/Pd(t Pd)Co(t Co)/Au thin film structures with 0 ⩽ t Pd⩽1.2 nm and 1.2 nm ⩽ t Co⩽3 nm were grown by combined sputtering and e-beam evaporation on Si/SiO2 substrates. VSM measurements revealed a negligible magnetic dead layer and a Pd thickness-independent magnetization at saturation of around 1200 emu cm−3. MS-FMR and BLS allowed us to conclue that PMA results from weak volume and interfacial contributions induced by Pd/Co or Pt/Co and Co/Au interfaces. The interface anisotropy constants are found to be 0.65 mJ m−2 and 0.85 mJ m−2 for Pd/Co and for Pt/Co interfaces, respectively. The Pd thickness-dependence of surface iDMI (D s) and PMA (K s) constants follow an exponential decay with a characteristic thickness higher for PMA than the iDMI. The slower decrease of K s vs Pd thickness suggests a shorter range of the iDMI with respect to PMA and points out a strongly localized origin for the iDMI. This difference between the iDMI and PMA is most likely responsible for the nonlinear correlation between PMA and iDMI constants. The investigation of the Co thickness-dependence of the iDMI and PMA in Pd/Co/MgO/Ta systems allows us to conclude a zero iDMI constant of Co/Au and to determine the iDMI constants of Pd/Co and Co/MgO, estimated to be D s Pd/Co = −0.096 pJ m−1 and D s Co/MgO = −0.15 pJ m−1, respectively. The criterion of skyrmions stability, applied for our samples, revealed the possibility of the existence of stable skyrmions in some samples.

The Correlation Between Magnetic Dead Layer and Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta Top Structure

MgO-based CoFeB structure is one of the typical layered structures with strong perpendicular magnetic anisotropy (PMA). However, the physical properties depend critically upon the sequence of the metal protection layer, and the top structure is especially sensitive to the thickness of the metal layer. In this paper, the cap layer effect on PMA in top structures of Si(substrate)/MgO/CoFeB (1-1.7 nm)/Ta (1-5 nm) is systematically investigated. An inverse correlation between the interface anisotropy constant (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> ) and the thickness of the magnetic dead layer (MDL) is identified. K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> decreases from 2.2 to 1.4 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while MDL increases from 0.1 to 0.6 nm, both showing a nonlinear dependence upon the Ta thickness with an abrupt change at Ta ~ 1.5 nm. These observations indicate that the cap layer effect on both PMA and MDL originates from the chemical deterioration at MgO/CoFeB and CoFeB/Ta interfaces by Ta diffusion.

The influence of interface anisotropy on demagnetization progress and magnetic properties in parallelly oriented hard/soft exchange-coupled multilayers

Abstract The effect of the interface anisotropy on the demagnetization progress and magnetic properties in Nd2Fe14B/α-Fe exchange-coupled multilayers with parallel crystalline anisotropy has been investigated within a micromagnetic framework. A set of equations satisfied by the angle between the magnetization and the applied field, as well as the formula for the nucleation field, have been derived analytically. The nucleation field goes up linearly with the interface anisotropy constants varying from −1 erg/cm2 to 1 erg/cm2 in a wide thickness range, with an amplitude up to 13%. The microscopic hysteresis loops, the evolution of the domain walls, the macroscopic demagnetization curves and the energy products are given by numerical calculations. The results indicate that the interface anisotropy affects not only the nucleation field and the maximal magnetic energy product, but also the pinning field or the coercivity. Positive interface anisotropy evidently enhances the pinning field or the coercivity, however, negative interface anisotropy obviously deteriorates the maximal energy product. These results suggest that a positive interface anisotropy should be preferred experimentally.

Thickness Dependence of the Dzyaloshinskii-Moriya Interaction in Co2FeAl Ultrathin Films: Effects of Annealing Temperature and Heavy-Metal Material

Interfacial Dzyaloshinskii-Moriya interaction (iDMI) has been investigated in Co2FeAl (CFA) ultrathin films of various thicknesses (0.8 nm<tCFA<2 nm) grown on Si substrates, using Pt, W, Ir and MgO buffer or/and capping layers. Vibrating sample magnetometry revealed that magnetization at saturation (Ms) for the Pt and Ir buffered films is higher than the usual Ms of CFA due to the proximity induced magnetization (PIM) in Ir and Pt, estimated to be 19% and 27%, respectively. The presence of PIM in these materials is confirmed using x-ray resonant magnetic reflectivity. Moreover, while no PIM is induced in W, higher PIM is obtained with Pt when it is used as buffer layer rather than capping layer. Brillouin light scattering (BLS) in the Damon-Eshbach geometry has been used to investigate iDMI constants and the perpendicular anisotropy field versus the annealing temperature. The DMI sign has been found to be negative for Pt/CFA and Ir/CFA while it is positive for W/CFA. The thickness dependence of the effective iDMI constant for stacks involving Pt and W shows the existence of two regimes similarly to that of the perpendicular anisotropy constant due to the degradation of the interfaces as the CFA thickness approaches the nanometer. The surface iDMI and anisotropy constants of each stack have been determined for the thickest samples where a linear thickness dependence of the effective iDMI constant and the effective magnetization has been observed. The interface anisotropy and iDMI constants, investigated for Pt/CFA/MgO system, showed different trends with the annealing temperature. The decrease of iDMI constant with increasing annealing temperature is probably due to the electronic structure changes at the interfaces, while the increase of the interface anisotropy constant is coherent the interface quality and disorder enhancement.

Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co2FeAl/MgO interface upon annealing

We investigate a series of films with different thickness of the Heusler alloy Co2FeAl in order to study the effect of annealing on the interface with a MgO layer and on the bulk magnetic properties. Our results reveal that while the perpendicular interface anisotropy constant is zero for the as-deposited samples, its value increases with annealing up to a value of mJ m−2 for the series annealed at 320 °C and of mJ m−2 for the 450 °C annealed series owing to a strong modification of the interface during the thermal treatment. This large value ensures a stabilization of a perpendicular magnetization orientation for an extrapolated thickness below 1.7 nm. The data additionally shows that the in-plane biaxial anisotropy constant has a different evolution with thickness in as-deposited and annealed systems. The Gilbert damping parameter α shows minima for all series for a thickness of 40 nm and an absolute minimum value of . The thickness dependence is explained in terms of an inhomogeneous magnetization state generated by the interplay between the different anisotropies of the system and by the crystalline disorder.

Anisotropy constant and exchange coupling strength of perpendicularly magnetized CoFeB/Pd multilayers and exchange springs

A model describing the ferromagnetic resonance of multilayer structures is used to characterize the interface anisotropy constant and interlayer exchange coupling strength associated to individual ...

Effect of Mo capping layers thickness on the perpendicular magnetic anisotropy in MgO/CoFeB based top magnetic tunnel junction structure**Project supported by the National Fundamental Research Program of China (Grant No. 2011CB921804) and Beijing Key Subject Foundation of Condensed Matter Physics, China (Grant No. 0114023).

A detailed study of the magnetic characterizations of the top structure MgO/CoFeB/Mo is presented. The samples show strong perpendicular magnetic anisotropy (PMA) when the thickness of CoFeB is 0.9 nm and 1.1 nm. The saturation magnetic moment and interface anisotropy constant are 1566 emu/cm3 and 3.75 erg/cm2, respectively. The magnetic dead layer (MDL) is about 0.23 nm in this system. Furthermore, strong capping layer thickness dependence is also observed. The strong PMA of 1.1 nm CoFeB only exists in a Mo cap layer thickness window of 1.2–2 nm. To maintain PMA, the metal layer could not be too thin or thick in these multilayers. The oxidation and diffusion of the metal capping layer should be respectively responsibility for the degradation of PMA in these thin or thick metal capping layer samples.

Annealing temperature and thickness dependencies of structural and magnetic properties ofCo2FeAlthin films

${\mathrm{Co}}_{2}\mathrm{FeAl}$ (CFA) thin films, of various thicknesses ($3\phantom{\rule{0.28em}{0ex}}\text{nm}\ensuremath{\le}t\ensuremath{\le}50\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$), have been grown by sputtering on (001) MgO single-crystal substrates and annealed at different temperatures ($\text{RT}\ensuremath{\le}{T}_{a}\ensuremath{\le}600\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$, where RT is the room temperature). The influence of the CFA thickness ($t$), as well as ex situ annealing temperature (${T}_{a}$), on the magnetic and structural properties has been investigated by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed an epitaxial growth of the films with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the $B2$ phase to the $A2$ phase when decreasing $t$ or ${T}_{a}$. The deduced lattice parameters showed an in-plane tetragonal distortion and in-plane and out-plane strains that increase with ${T}_{a}$ and $1/t$. For all ${T}_{a}$ values, the variation of the effective magnetization, deduced from the fit of MS-FMR measurements, shows two different regimes separated by a critical thickness, which is ${T}_{a}$ dependent. It decreases (increases) linearly with the inverse thickness ($1/t$) in the first (second) regime due to the contribution of the magnetoelastic anisotropy to surface (to volume) anisotropy. The observed behavior has been analyzed through a model allowing for the separation of the magnetocrystalline, magnetoelastic, and N\'eel-type interface anisotropy constants to the surface and the volume anisotropies. Similar behavior has been observed for the effective fourfold anisotropy field which governs the in-plane anisotropy present in all the samples. Finally, the MS-FMR data also allow one to conclude that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with ${T}_{a}$.

Perpendicular Magnetic Anisotropy in MgO/CoFeB/Nb and a Comparison of the Cap Layer Effect

High perpendicular magnetic anisotropy (PMA) has been recently revealed in CoFeB/MgO/CoFeB tunnel junctions if the thickness of CoFeB is <;1.5 nm. However, PMA has been observed in MgO/CoFeB/Ta but not in MgO/CoFeB/Ru, indicating a metallic cap layer effect. In this paper, we extend the study to MgO/CoFeB(0.8-1.8 nm)/Nb(1.3 nm) by sputtering and find that the magnetic behavior of MgO/CoFeB/Nb resembles the magnetic behavior of MgO/CoFeB/Ta. The interface anisotropy constant of both MgO/CoFeB/Ta and MgO/CoFeB/Nb is ~2.2 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while MgO/CoFeB/Ru has a much weaker interface anisotropy constant ~0.68 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In addition, the magnetic dead layer (MDL) of MgO/CoFeB/cap (cap = Ta and Nb) is ~0.1 nm while MgO/CoFeB/Ru has a much larger MDL ~0.5 nm. The metallic layer effect on the PMA in CoFeB/MgO-based perpendicular structure is attributed to the interdiffusion during the post-annealing process. The possible correlation between PMA and MDL provides a possible mechanism for the metal layer effect in the CoFeB/MgO-based structures.

Magnetocrystalline volume and interface anisotropies in epitaxial films: Universal relation and Néel’s model (invited)

The thickness dependence of the fourfold in-plane magnetic anisotropy was first observed in epitaxial Fe(001) films and described by the volume anisotropy of bcc Fe with a positive anisotropy constant (K1vol&amp;gt;0) superimposed by a negative interface term (K1int&amp;lt;0). This results in a spin reorientation of the easy axis from [100] in thick films to [110] for thicknesses tFe&amp;lt;tcrit=6 ML. To clarify the origin of the interface term Fe and bcc Fe1−xCox alloy films (x&amp;lt;0.8) epitaxially grown on different substrates—Ag(001), Au(001), GaAs(001)—were studied. As a result, magnetoelastic interactions can be ruled out because exactly the same anisotropies are found for tensile strain [Fe on Ag(001) and Au(001)] and for compressive strain [Fe on GaAs(001)]. Also, K1int is not affected by the particular interface material or overlayer, e.g., vacuum or Au. A universal correlation was found between the volume and interface anisotropy constants: −K1int/K1vol=tcrit=6 ML. It is shown that this is indeed expected within Néel's phenomenological theory of magnetic anisotropy if contributions from nearest and next-nearest neighbors are taken into account. Electronic hybridization effects in this case seem to play a minor role compared to the local symmetry. It is suggested that observed effects of overlayers on perpendicular interface anisotropies might be related to surface relaxations which have not been sufficiently studied so far and which, on the other hand, would not affect the fourth-order in-plane anisotropy according to Néel's model in agreement with present experimental results.

Magnetic studies in Fe/Zn multilayers

The structural and magnetic properties of Fe/Zn films prepared by thermal evaporation have been studied by means of X-ray diffraction, vibrating-sample magnetometry and ferromagnetic resonance (FMR). For Fe layer thickness smaller than 20 Å the saturation magnetization decreases with decreasing Fe thickness, which is an indication of the island growth of Zn and Fe–Zn interdiffusion at the layer interfaces. The effective field magnetization 4 πM eff of the Fe/Zn multilayers was determined from the FMR data in a rotating external magnetic field. The interface anisotropy constant of the Fe/Zn multilayers, K S, is found to be 1.0 erg/cm 2 at 300 K. This indicates the presence of a large perpendicular interface anisotropy and this may suggest that the largest part of K S originates from lattice misfit strain.

Volume and interface magnetic anisotropy of Fe1−xCox thin films on GaAs(001)

Epitaxial Fe1−xCox alloy films with x=0.22 and x=0.33 were grown on GaAs(001) by molecular-beam epitaxy in a thickness range of 3 to 80 monolayers (ML). The magnetic properties were investigated by magneto-optic Kerr effect. Ferromagnetic order at room temperature was observed for thicknesses above 4 ML. The in-plane magnetic anisotropy of all films is a superposition of a uniaxial component with the easy axis along [110], which is not discussed here, and a four-fold contribution. The effective uniaxial and four-fold anisotropy constants, KUeff and K1eff, were determined by fits to the hard axis magnetization loops. K1eff contains an interface and a volume term which lead to a linear variation of K1eff with the inverse film thickness. It turns out that the surface and volume anisotropy constants are of opposite sign for all alloy compositions causing a sign reversal at a critical thickness, tcrit. This critical thickness seems to be a universal value, which is caused by a general proportionality between the volume and the interface anisotropy constants with the same negative constant of proportionality for Fe1−xCox/GaAs(001), Fe/Au(001), and Fe/Ag(001). This behavior of the four-fold anisotropy constants is consistently explained within Néel’s pair energy model for a body-centered-cubic ferromagnet.

Magnetic anisotropy in Ce/Fe and Ce/FeCoV multilayers

Multilayer samples of Ce/Fe and Ce/FeCoV, fabricated by sputteringand characterized by means of x-ray diffraction and reflectivity, werestudied by 57Fe Mössbauer spectroscopy and polarizedneutron reflectivity. In the Ce/Fe system a paramagnetic Fe phaseis observed at room temperature for Fe thicknesses below 20 Å.This phase is ordered at 4.2 K, and is attributed to amorphous Fe atthe interface with Ce. In contrast, the Ce/FeCoV system isferromagnetic at room temperature for all layer thicknesses studied.From the relative intensity of the Mössbauer lines we show thatboth systems exhibit a perpendicular magnetic anisotropy formagnetic layers of less than 17 Å thickness, indicating apositive interface anisotropy constant, KS. For a selection ofCe/Fe samples, the reorientation of the magnetic moments in magneticfields applied normal to the layers enabled the magnetic anisotropyenergy Keff of each sample to be evaluated. The trend of thevalues of Keff gave volume and interface anisotropy constantsKV = 1.1 (±1.2)×105 J m-3 andKS = 0.98 (±0.15)×10-3 J m-2 for theCe/Fe system, where the positive values indicate a preference for anout-of-plane orientation of the magnetization.

Equilibrium at stationary solid-liquid interface during phase-field modeling of alloy solidification

The phase-field parameters (coefficient of gradient energy and double-well potential height) were obtained for a binary solid-liquid (SL) interface with a finite thickness (thin interface). The parameters were compared with those obtained at a sharp interface condition in order to test the maintenance of equilibrium condition at a stationary SL interface. The phase-field parameters determined at the thin interface condition account for the chemical energy contribution to the interface energy. They reproduce the interface energy, interface thickness, and capillary effect during phase-field calculation at a wide interface thickness range, even though there still exists interface thickness limitation. The phase-field parameters at the sharp interface ignore the chemical energy contribution to the interface energy. When the interface thickness is small enough, the parameters generate the same interface energy and interface thickness as the corresponding input values since the chemical energy contribution increases with increasing interface thickness. However, regardless of interface models, the equilibrium shape of solid particles embedded in a liquid, obtained from the phase-field equation with different interface anisotropy constants, is in good agreement with theoretical two-dimensional (2-D) prediction. This is due to the equilibrium shape of solid particles being dependent only on anisotropy constant.

Effect of annealing on the magnetic properties of sputtered Co/Cu multilayers

Alternating-gradient field magnetometry (AGFM) and surface magneto-optical Kerr effect (SMOKE) have been exploited in order to study the magnetic properties of both as-prepared and annealed Co/Cu multilayers. It is shown that the effect of annealing is to enhance the net saturation magnetization and to modify the bulk and interface anisotropy constants. This is also confirmed by detection of both spin waves and acoustic phonons by Brillouin light scattering (BLS). The modification of the saturation magnetization and of the anisotropy constants can be correlated to the structural modifications caused by the thermal treatment.

Temperature-driven in-plane anisotropy reorientation transition in Fe(110) films

The competition between surface and volume magnetic anisotropy in ultra-thin films may induce a dramatic thickness and temperature dependence of the anisotropy. Whereas the former has been studied extensively, only a few reports have appeared on the latter for in-plane anisotropy. We have analysed such an in-plane reorientation in a W(110)/Fe(110)(60Å)/W(110) film, which occurs because of the differences in the temperature dependencies of the various contributions involved in the total anisotropy. The second-order interface anisotropy constant is deduced for this system (100 to 300 K) from total anisotropy measurements and subtraction of bulk contributions. The thermal variation of the interface anisotropy constant with the reduced spontaneous magnetization m s is found to follow an m s 13/2 law. Phase diagrams are derived.

Transitions in the direction of magnetism in Ni/Cu(001) ultrathin films and the effects of capping layers.

Ultrathin films of nickel exhibit an unusual sequence of transitions from in-plane to perpendicular magnetization as a function of film thickness. A sharp transition from in-plane to perpendicular magnetization is found near 7 ML thickness, followed by a gradual transition back to in-plane magnetization beginning at 37 ML. This sequence of transitions cannot be explained by the surface or shape anisotropies, both of which favor in-plane magnetization in the thickness range where perpendicular anisotropy is found. We have measured the thickness dependence of these transitions for nickel film wedges, and films capped by nonmagnetic and magnetic overlayers, to experimentally determine the surface, interface, and magnetoelastic anisotropies. We find that both the surface and interface anisotropy constants are negative (favoring in-plane magnetization), with the magnitude of the surface term being larger than that of the interface. A correlation is found between the critical thickness for misfit dislocation formation in the nickel film and a sharp transition in the coercive field. This transition is used to accurately determine the onset of a thickness dependence in the bulk magnetoelastic energy, which causes the magnetization to rotate back into the film plane. This model gives a complete description of the mechanism for the easy-axis changes at both the 7 and 37 ML thicknesses. \textcopyright{} 1996 The American Physical Society.

Reorientational transition of the magnetic anisotropy in Co/Cr(001) superlattices.

We have measured the magnetic anisotropy of Co/Cr(001) superlattices as a function of the Co thickness using magneto-optical methods and a torsion magnetometer. The samples grown by molecular-beam epitaxy exhibit a reorientational transition of the easy axis from in-plane for large Co thicknesses (${\mathit{t}}_{\mathrm{Co}}$ \ensuremath{\ge} 15 \AA{}) to out-of-plane for 10 \AA{} \ensuremath{\le}${\mathit{t}}_{\mathrm{Co}}$ \ensuremath{\le} 15 \AA{} and back again to the in-plane orientation for ${\mathrm{t}}_{\mathrm{Co}}$ \ensuremath{\le} 10 \AA{} . We provide evidence that this reorientational transition of the magnetization direction is due to a sign change of the interface anisotropy constant induced by a concomitant structural phase transition of the Co layers from hcp to bcc with decreasing Co layer thickness. \textcopyright{} 1995 The American Physical Society.

Magnetic anisotropy of sputtered Ni/Ag multilayers

We have studied the magnetic properties and structure of Ni/Ag multilayers, fabricated by the dc magnetron sputtering method. The in-plane uniaxial anisotropy, which is closely related to the sample preparation, has been detected by both vibrating magnetometer (VSM) and ferromagnetic resonance (FMR) measurements where the FMR spectra were obtained as a function of the applied magnetic field orientation in the film plane. The variation of the resonance field with the angle of the applied magnetic field can be well explained by a theoretical model including the in-plane uniaxial anisotropy up to the second-order term. Moreover, both in-plane and perpendicular anisotropy parameters have been deduced by a data-fitting analysis and a negative interface anisotropy constant has been determined.

The interface anisotropy of multilayers prepared by dual-facing-target sputtering

Abstract We have fabricated Fe Ti multilayers by the dual-facing-target sputtering (DFTS) method. The total interaction effective field Heff of Fe Ti multilayers were determined from the ferromagnetic resonance data in a rotating external magnetic field. The interface contribution to the magnetic anisotropy was extracted from the anisotropy field HA that was obtained from Heff. The interface anisotropy constant of Fe Ti multilayers Ks is 0.18 erg/cm2. This indicates the presence of a perpendicular interface anisotropy.