Probing the timescale of the exchange interaction in a ferromagnetic alloy

Abstract

The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.

Progress in magnetic information storage and processing technology is intimately associated with complex materials that are engineered at the nanometer scale. Heat-assisted magnetic recording (1), bit-patterned data storage media (2), all-optical magnetization reversal (3), and giant tunneling magnetoresistive disk drive read sensors are examples of such technologies (4). Next-generation devices will require that the magnetic state of materials be manipulated on fast timescales and at the nanometer level. However, a complete microscopic understanding of magnetization dynamics that involves the correlated interactions of spins, electrons, photons, and phonons on femtosecond timescales has yet to be developed. Two reasons for this lack of fundamental understanding of ultrafast magnetism at the microscopic scale are the complexity of the problem itself, as well as the experimental challenge of accessing ultrafast and element-specific magnetization dynamics. One approach for addressing the experimental challenge is to use X-ray magnetic circular dichroism (XMCD) employing X-rays generated by a synchrotron light source. XMCD has the inherent advantage of element-specific detection, and “sliced” synchrotron pulses are already used for ultrafast studies (5–9). In an alternative approach, we recently demonstrated that coherent extreme ultraviolet (XUV) beams from a tabletop high-harmonic source (10, 11) can also be used to probe ultrafast element-specific magnetization dynamics in permalloy (Ni_0.8Fe_0.2) (12). For that demonstration, we took advantage of magnetic birefringence at the M-edge in transition metals to independently follow dynamics for Ni and Fe. However, the time resolution available in that initial experiment was insufficient to observe any differences in the response of the constituent elements on very short timescales.

In this work, we experimentally answer the fundamental question of whether the magnetization dynamics of individual elements in a ferromagnetic alloy can differ on ultrafast timescales. This is a very important fundamental question that has not been addressed either theoretically or experimentally to date, the answer to which reveals how the exchange interaction can control the ultrafast dynamics of elemental spin subsystems in complex materials. To answer this question, we rapidly excite permalloy with an ultrashort (≈25 fs) laser pulse and probe the element-specific demagnetization dynamics using < 10 fs high-harmonic pulses. The superior time resolution of our experiment allows us to observe that the magnetization dynamics of Fe and Ni are transiently delayed with respect to each other—by about 18 fs in permalloy and 76 fs in Cu-diluted permalloy ((Ni_0.8Fe_0.2)_1-xCu_x). We ascribe this transient decoupling in the magnetic behavior to the finite strength of the fundamental quantum exchange interaction between Fe and Ni atoms in the material. Specifically, for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then, as the Fe moments start to randomize, the strong ferromagnetic interatomic exchange interaction between Fe and Ni induces further demagnetization in Ni, with a characteristic delay determined by the strength of the Fe-Ni exchange interaction. Interatomic exchange energies of transition metal alloys are in the 10–100 meV range, yielding characteristic exchange times in the femtosecond range which corresponds to finite spin-flip scattering times of 10–100 fs (9). Our findings provide crucial information for open questions in femtosecond magnetization dynamics in the case of metallic, multispecies, exchange-coupled systems.

Results

In our experiment, sub-10 fs XUV light pulses from high-harmonic generation (HHG) are produced by focusing 2 mJ femtosecond laser pulses into a Ne-filled waveguide. The harmonic photon energy range of 35 to 72 eV spans the M absorption edges of Fe and Ni at ≈54 eV and ≈67 eV, respectively (see Fig. 2B). In the transverse magneto-optical Kerr-effect (T-MOKE) geometry used for these measurements, the intensity of the reflected HHG light is proportional to the magnetization transverse to the plane of incidence (12). We probe the magnetization by reflecting the XUV beam from a magnetic diffraction grating structure, as shown in Fig. 1A. We used gratings with 1 μm lines and a 2 μm period patterned in three different ways: (i) alternating elemental Fe and Ni stripes to probe the behavior of the pure materials; (ii) permalloy (Ni_0.8Fe_0.2); and (iii) permalloy-Cu ((Ni_0.8Fe_0.2)_0.6Cu_0.4). The Curie temperature T_c for permalloy is 850 K, while for permalloy-Cu, T_c ≈ 400 K. The HHG spectrum diffracted from the grating sample is focused onto an X-ray CCD camera. In order to determine the T-MOKE asymmetry, the change in reflected HHG intensity at the M-absorption edges is monitored while the magnetization direction of the sample is switched. The T-MOKE asymmetry parameter A is calculated from the experimental data as

where I₊ and I_- denote the reflected XUV intensities for the two magnetization directions. More details of the measurement method can be found in Refs. (12–14).

Fig. 2.

XUV spectra and magnetic asymmetry. (A) Magnitude of the asymmetry, coded in color, as a function of photon energy and angle of incidence, measured using synchrotron radiation. The asymmetry signal of Fe (≈54 eV) is clearly separated from Ni (≈67 eV). (B) HHG XUV spectra reflected from the permalloy grating sample at an angle of incidence of 45°, shown as green solid and dotted lines for the two different magnetization directions. The blue line is the calculated asymmetry from the HHG spectra, and the black line the asymmetry from synchrotron data that corresponds to the spectral cut shown as a black dashed line in (A).

Fig. 1.

Schematic of the physics and experiment. (Top) Ultrafast XUV pulses (A) are reflected from a permalloy grating sample, which spatially separates the harmonics to form a spectrum on a CCD camera. The reflected HHG intensity at the Fe and Ni M-shell absorption edges (red and blue) depends on the magnetization transverse to the optical plane of incidence that is periodically reversed by transverse-mounted Helmholtz coils. Exciting the sample with an infrared laser pulse (red) causes the material to demagnetize on femtosecond timescales. (B) After rapid excitation of the electron system by a femtosecond laser pulse, various scattering processes between electrons and phonons (with and without spin-flips) determine the dynamical response of the system. First, the strongly excited electron gas thermalizes by predominantly electron-electron scattering to a Fermi-Dirac distribution. The ferromagnet starts to demagnetize because of spin-flip scattering events during this thermalization process. Electron-phonon scattering processes transfer energy from the excited electron gas to the lattice, and thermal equilibrium is typically reached on picosecond timescales. Finally, on nanosecond timescales, the material cools by thermal diffusion. The red and blue arrows in the lower boxes show the observed distinct demagnetization dynamics of Fe and Ni in permalloy.

The asymmetry for the permalloy sample was measured using XUV radiation from both the HHG source and a synchrotron source. Fig. 2A shows the dependence of the magnetic asymmetry on the angle of incidence and photon energy in the form of a color-coded contour plot. Fe and Ni are easily distinguished by strong, element-specific T-MOKE asymmetry peaks that correspond to excitation of the localized M-shell electrons into unoccupied states above the Fermi energy. XUV T-MOKE is therefore similar to XMCD, providing a localized probe of magnetic moments. Moreover, the magnetic dynamics in pure Ni measured by XUV T-MOKE are in excellent agreement with visible MOKE probes (14, 15). Note that both peaks for the two elements have widths of several eV. The magnetic asymmetry signal shows bipolar contributions over an extended energy range for Fe and Ni, below and above an energy of about 60 eV (white line in Fig. 2A), respectively (13). The detailed peak structure is made complicated by the convolution of the finite lifetime of the p-orbital holes and the weak splitting of the shallow M₂ and M₃ levels. The splitting is largest for Ni.

The largest magnetic asymmetry occurs at an angle of incidence of 45° (black dashed line in Fig. 2A, which corresponds to the geometry used in the HHG setup). Fig. 2B shows the measured magnetic asymmetries using synchrotron and HHG light at a 45° angle of incidence. The spectra are in good agreement with each other. We attribute the minor discrepancies to the qualitatively different spectra for HHG and synchrotron radiation, which is composed of discrete harmonic lines for HHG and is a quasicontinuum for synchrotron radiation. The good agreement in the asymmetry spectra between the HHG and synchrotron sources validates our approach for measuring ultrafast demagnetization dynamics using HHG radiation.

For these measurements, the sample is transiently demagnetized using a focused ultrafast laser pump pulse (25 fs duration, 780 nm wavelength) that rapidly excites the electronic system. After the excitation of the electron system in the material, various scattering processes between electrons and phonons (with and without spin-flips) determine the dynamical response of the system on femtosecond to nanosecond timescales (see Fig. 1B). In our experiment, the demagnetization is captured by measuring the asymmetry A as a function of time delay between the infrared pump and the XUV probe pulses (see Movie S1). We start with a simultaneous measurement of the demagnetization dynamics of elemental Fe and Ni using a sample with interleaved stripes of Fe and Ni (Fig. 3A). After excitation at a laser fluence of ≈2 mJ/cm², the magnetization decreases rapidly and is quenched by about 19% for Fe and 45% for Ni. Using a double exponential fitting function (16) given by m(t) = 1 - Δm[1 - exp(-t/τ_m)] exp(-t/τ_r), we measure demagnetization times of τ_m = 98 ± 26 fs for Fe and 157 ± 9 fs for Ni (with recovery time constants τ_r = 11 ± 7 ps for Fe, and τ_r = 9 ± 1 ps for Ni, respectively), in agreement with earlier studies (17, 18).

Fig. 3.

Ultrafast demagnetization of Fe (red dots) and Ni (blue dots) for elemental Fe and Ni (A), in permalloy (B), and in permalloy-Cu (C). Simple exponential decay fits yield the demagnetization constants of (A) elemental Fe and Ni, and “effective” demagnetization constants τ_Eff for Fe and Ni in (B) permalloy, and (C) permalloy-Cu, data set (see text). Fits to the model (solid lines) are used to extract the intrinsic demagnetization times for Fe and Ni in the alloys, τ_Fe and τ_Ni, as well as the exchange time τ_Ex, after which the Fe and Ni spin baths return to equilibrium with respect to each other with an effective demagnetization time constant of τ_Eff. The data for permalloy-Cu (C) is also shown in log-scale as a function of the normalized asymmetry changes ΔA = (A-A_min)/(A₀-A_min), where A₀ the total asymmetry and A_min the minimum asymmetry reached in the demagnetization process. We stress that the demagnetization data for Fe and Ni are collected at the same time in this measurement, precluding any mismatch between the two elements in the determination of time-zero between pump and probe laser pulses.

Now, moving from single-species metals to the more complex binary alloy permalloy, where the constituents Fe and Ni are miscible and strongly exchange coupled—one might expect identical demagnetization dynamics for the two elements if one assumes a completely delocalized, itinerant spin-polarized band structure i.e. if the Fe and Ni contributions to the magnetization are indistinguishable. If this were the case, even though T-MOKE probes the local magnetic signal in the vicinity of the Fe and Ni atoms, one would expect identical demagnetization timescales at the two different sites. Note here the inherent difference between our measurements in a strongly coupled 3d ferromagnetic system and a recent study by Radu et al. of demagnetization dynamics in the 3d-4f ferrimagnet GdFeCo (9). In that work, distinctly different dynamics of the weakly exchange-coupled elements arranged in sublattices were observed, a natural consequence of the different temperature-dependent properties of the localized 4f Gd moment and less localized 3d Fe moment when in thermodynamic equilibrium (a property that gives rise to a magnetic compensation point whereby the rare earth and transition metal sublattices are of equal but opposite magnetic moment).

Fig. 3B shows the measured element-specific demagnetization of Fe and Ni in permalloy following excitation by a pump pulse with fluence of ≈2 mJ/cm². As expected in a strongly exchange coupled 3d alloy, the magnetization decreases rapidly for both elements to a common minimum of about 70% of the total magnetization. Somewhat surprisingly, however, an inspection of the data on short timescales clearly shows that the demagnetization of Fe precedes that of Ni by approximately 10–20 fs (Inset, Fig. 3B). This relative difference between Fe and Ni in permalloy was not previously observed in Ref. (12), because the temporal resolution in that experiment was insufficient to resolve such a small shift in the onset of demagnetization. We stress that the demagnetization data for Fe and Ni are collected at the same time in this measurement, precluding any mismatch between the two elements in the determination of the arrival time for the pump and probe pulses.

The experimental results of Fig. 3B directly demonstrate that the spin-dependent part of the electronic wave functions in the itinerant 3d bands must also exhibit a local character. Differing dynamics in the vicinity of the Fe and Ni atoms shows that contributions of Fe and Ni to the total magnetic moment can be clearly distinguished. This is a very surprising result, and since we focus in the following discussion on the origin of these distinguishable parts of the Fe and Ni magnetic contributions, we for simplicity denote them as demagnetization dynamics of Fe and Ni, respectively.

The degree to which demagnetization dynamics can be different for Fe and Ni in permalloy necessarily depends on the strength of the Fe-Ni interatomic exchange coupling between neighboring magnetic moments: the weaker the Fe-Ni exchange coupling, the more the dynamics can differ without incurring too large of an energy cost. In the particular case of permalloy, the interatomic exchange coupling is substantial, as indicated by the Curie temperature T_C of 850 K. Motivated by this line of reasoning, we repeated our measurements with the tertiary alloys of permalloy diluted by Cu (permalloy-Cu). Fe, Ni, and Cu are all miscible at room temperature when one dilutes permalloy with Cu (19, 20). The alloying of Cu with permalloy results in a continuous reduction of the volume-averaged exchange parameter through the reduction of the number of ferromagnetic nearest-neighbor atoms. Such alloys also retain the high permeability associated with pure permalloy and avoid any discontinuous crystallographic phase transitions with varying Cu content. This, in turn, provides us with the ability to tune T_C (see Supporting Information) over a broad temperature range. For fixed temperature measurements, the exchange coupling is further reduced by the concomitant renormalization of the effective exchange integral near T_C (21, 22).

We prepared a sample of (Ni_0.8Fe_0.2)_0.6Cu_0.4 by cosputtering from permalloy and Cu targets. X-ray diffraction verified that our samples are a solid solution (i.e., random placement of the Fe, Ni, and Cu atoms in the crystal lattice) fcc phase (see Methods and Materials and Supporting Information). Fig. 3C shows a plot of the element-selective, time-resolved T-MOKE signal for a permalloy-Cu sample with T_C = 406 ± 3 K. We unambiguously observe a significant demagnetization delay for Ni of approximately 76 fs relative to Fe, as indicated by the arrows. Interestingly, after accounting for the delay in the demagnetization, the exponential decay of the magnetization for each of the elements is identical within our error bars, yielding fitted values for the effective demagnetization time τ_Eff of 242 ± 12 fs for Fe and 236 ± 13 fs for Ni. (Note the difference between the effective demagnetization time of the respective element in the alloy and the intrinsic elemental demagnetization times used in the model presented below).

Discussion

The dynamics of ultrafast demagnetization are complex. A proven theory that completely describes all the interactions between photons, electrons, spins, and phonons at a microscopic level does not yet exist. However, it is known that femtosecond infrared pulses coherently interact with the electric charges and spins in the material within ≈0–50 fs (23). Subsequently, the highly excited electrons relax to a thermalized population, accompanied by spin-flip scattering processes that lead to ultrafast demagnetization on timescales of ≈100–1,000 fs (Fig. 1B) (18, 24–27). These details of the scattering processes remain the subject of intense debate in ultrafast magnetism (7, 24, 25, 28–33). Moreover, nonadiabatic heating processes of the electron, spin, and lattice subsystems on such ultrafast timescales, together with strongly nonequilibrium transient phase states, necessarily complicate our understanding of the underlying physics for ultrafast demagnetization. It is therefore important to include the laser-induced hot electrons in a discussion of magnetic dynamics on such ultrashort < 100 fs timescales.

Hot electrons can induce demagnetization by superdiffusive spin transport (33), and also by screening the Coulomb potentials on femtosecond timescales (34, 35). While superdiffusive spin transport leads to a direct demagnetization process, screening might indirectly act on the magnetization of the material by transiently modifying the exchange interaction in ferromagnetic conductors (36) during the 100–500 fs needed for the relaxation of the pump-induced highly excited electrons. Such a modification of the exchange interaction then has been shown to directly influence the ultrafast magnetization dynamics (37). Note that the screening process itself evolves on attosecond timescales in metals, but is active until the highly excited electrons relax their energy. If superdiffusive spin transport or any hot-electron induced modification of the exchange coupling contributed significantly to the observed delay of the demagnetization dynamics of Fe and Ni in permalloy, then we would expect a strong dependence of the delay times on the pump fluence, since the pump fluence controls the number of excited hot electrons. However, the demagnetization delays for permalloy-Cu do not change when the pump fluence is varied between 250 and 360 mW, which corresponds to a variation in the relative change in magnetization, ΔM/M, between about 50% and 80%, respectively (data shown in Supporting Information). We therefore conclude that neither superdiffusive spin transport nor a transient modification of the exchange interaction (e.g. due to hot electrons) are the dominant processes causing the demagnetization delay of Ni. Rather, the demagnetization delay between Ni and Fe in permalloy and permalloy-Cu is an intrinsic property that depends upon the strength of the interatomic Fe–Ni exchange interaction, since the demagnetization delay is increased when the exchange interaction is reduced in permalloy-Cu.

To gain more physical insight into demagnetization dynamics in ferromagnetic systems, we need to take the interatomic Fe–Ni exchange coupling into account. Our data clearly shows that using a double exponential fitting function for elemental Fe and Ni is not sufficient to describe the coupled dynamics in the alloyed systems. Therefore, to extract quantitative timescales for the demagnetization process, we modeled our experimental data using the following first-order coupled rate equations:

[1a]

[1b]

where m_Fe and m_Ni are the normalized Fe and Ni magnetizations, τ_Fe and τ_Ni are the intrinsic decay times for Fe and Ni in the absence of exchange coupling between them, and τ_Ex the characteristic “exchange time” that describes the thermodynamic coupling of spins in the Fe and Ni systems. Solving the equations for the limit where τ_Fe ≪ τ_Ni , τ_Ex emerges as the delay time between Fe and Ni, where both species have the same effective time constant τ_Eff (See Fig. 3B); i.e., the exchange time and the measured delay time are equivalent quantities. The solution for the initial condition m_Fe(t = 0) = m_Ni(t = 0) = 1 is:

[2a]

[2b]

where

[3a]

[3b]

and

[4]

Fitting our permalloy-Cu data to this model reproduces the distinct demagnetization dynamics of Fe and Ni on timescales shorter than the exchange time τ_Ex, and also the observed delay of Ni with respect to Fe at times larger than τ_Ex. It additionally yields a smaller intrinsic demagnetization constant (i.e., the “virtual” constant in the absence of Fe–Ni interatomic exchange coupling) for Fe in comparison to Ni; i.e., τ_Ni > τ_Fe, qualitatively consistent with our observations for elemental Fe and Ni (see Fig. 3). For permalloy-Cu, a reasonable fit requires τ_Ni > 500 fs, an indication that the Ni itself is barely affected by the pump pulse (see Supporting Information, where as expected, the demagnetization times are somewhat different in the alloy and the pure material). The fit to our model thus uncovers a picture validated by measurements: after a characteristic exchange time τ_Ex, the Ni magnetization is essentially “dragged down” by the strong Fe–Ni exchange coupling to the rapidly demagnetizing Fe moments. At this point both moments decay at the same effective time scale τ_Eff ∼ 2τ_Fe. We note that the Ni magnetization, when alloyed with Fe, is only weakly affected by the pump pulse immediately after excitation—leading to the very large intrinsic demagnetization constants of τ_Ni when the data are fitted to the model. For the case of permalloy with stronger interatomic Fe–Ni exchange coupling, a smaller exchange time τ_Ex is expected. Indeed, using the same rate equations, we can reproduce the demagnetization dynamics of permalloy, which is not possible with the usual double exponential decay function. Fitting all our data yields mean values of τ_Ex = 18 ± 10 fs for permalloy, and 76 ± 9 fs for permalloy-Cu (see Supporting Information).

Additional support for our interpretation can be found by considering the Landau–Lifshitz equation for magnetization dynamics, where spin relaxation in ferromagnets proceeds at a rate proportional to the gyromagnetic precession frequency. In the present case of disproportionate demagnetization between the Ni and Fe components, we expect local gyromagnetic dynamics to be dominated by interatomic exchange coupling. Based on the values of T_c, the average exchange energy for permalloy-Cu is approximately 3.3 times less than that of pure permalloy (see Supporting Information). The ratio of τ_Ex extracted from our data for permalloy-Cu relative to permalloy is 4.2 ± 2.8 (corresponding to characteristic exchange times of τ_Ex = 18 ± 10 fs and 76 ± 9 fs, respectively). Thus, the scaling of exchange energy and τ_Ex between permalloy and permalloy-Cu are comparable, supporting our interpretation.

The significantly higher intrinsic demagnetization times extracted for Ni, τ_Ni > 500 fs, compared to Fe, (τ_Fe ≈ 89 ± 8 fs in permalloy and τ_Fe ≈ 126 ± 9 fs in permalloy-Cu) indicate that the uniformity of the Ni spins in the alloy are most strongly influenced by the exchange coupling to the Fe, and much less influenced by the laser excitation in comparison to the pure material (τ_Ni,elemental ≈ 157 fs). In contrast, our data indicate that the laser excitation induces demagnetization for Fe on a comparable timescale to that for an elementally pure material (τ_Fe,pure ≈ 98 ± 26 fs). Because of interatomic Fe–Ni exchange coupling, the Ni spins eventually demagnetize with the same time-constant as Fe in the alloys via the thermodynamic contact to the Fe spin bath—but with an apparent delay that is given by τ_Ex. Our data indicate that this delay is larger in permalloy-Cu than in permalloy due to the reduced exchange energy. To our knowledge, such a delayed behavior of magnetization dynamics in metallic alloys has not been previously predicted or observed.

Current macroscopic and microscopic models that explain demagnetization dynamics for pure materials need to be extended to alloyed magnetic materials. The absence of such microscopic models for multicomponent systems prevents us from addressing why Ni intrinsically reacts slower in the specific alloys in comparison to the pure Ni material. However, our experiment provides a clear observation of how the strength of the exchange coupling between the constitutive atomic components can influence magnetization dynamics in alloys on ultrafast timescales. As such, our data help elucidate the microscopic role of the fundamental quantum mechanical exchange interaction in the ultrafast demagnetization process.

In summary, we explore the consequences of the fundamental quantum exchange interaction in strongly coupled ferromagnetic systems, showing that quantitatively different magnetization dynamics of the individual elements can be observed on timescales shorter than the characteristic exchange timescale. On longer timescales, the dynamics are dominated by the faster of the two species. Analysis of our data indicates that the observed differences in demagnetization rate are primarily determined by intrinsic properties of the material rather than the result of photo-induced ultrafast transient changes in the material, e.g., hot-electron-gas screening or nonequilibrium phases. This fact has significant impact for fundamental models of ultrafast magnetism, and for the dynamical magnetic behavior for all types of exchange-coupled materials, including both the alloys and multilayer structures that are widely used for data storage.

Materials and Methods

Experimental Setup.

We generate coherent high-harmonics (HHG) by focusing 25 fs laser pulses (780 nm central wavelength) into a neon-filled hollow waveguide. The laser operates at 2 kHz repetition rate with the pulse energy of approximately 2.2 mJ/pulse. Ninety percent of the laser power is used for HHG, while the remaining light is used to excite the sample. The waveguide is filled with neon gas that is pressure tuned to approximately 400 torr in order to phase-match a broad range of harmonics in the range of extreme ultraviolet (XUV) from 35 to 70 eV (21st–43rd harmonic), covering the region of the spectrum where the M-edge resonances of 3d ferromagnetic metals are located. A 200 nm thick Al filter blocks the fundamental laser light. The Al filter limits our highest energy HHG photons to 72 eV as a result of strong absorption above the Al L_2,3 edges. The HHG beam is refocused onto the grating sample using a grazing incident toroidal mirror. The HHG spot size on the sample is less than 500 μm, which is smaller than the pump laser spot size of approximately 1–2 mm to ensure good spatial overlap and uniform demagnetization. Water-cooling stabilizes the sample temperature at 293 K.

Sample Fabrication.

A 10 nm thick permalloy-Cu ((Ni_0.8Fe_0.2)_0.6Cu_0.4) alloy thin film was grown by cosputter deposition with permalloy (Ni_0.8Fe_0.2) and pure Cu targets. The rates from a permalloy target and a pure Cu target were calibrated using a quartz crystal monitor and a profilometer. A thin 3 nm Ta seed layer was first sputter deposited onto a thermally oxidized Si(100) wafer to provide a strong (111)-texture and good adhesion prior to depositing the permalloy-Cu alloy. Diffraction gratings were patterned from the film via optical lithography and a subsequent Ar ion milling at 300 eV. The grating consisted of an array of 1 μm wide stripes with a center-to-center spacing of 2 μm. The 10 nm thick permalloy (Ni_0.8Fe_0.2) diffraction grating was fabricated by a direct liftoff process from a film grown by ion beam deposition with a target made from the same source material that was used for sputtering of the permalloy-Cu film. A 3 nm Ta seed layer was also used for adhesion to promote strong (111)-texture prior to depositing the permalloy layer. X-ray diffraction data, magnetometry measurements, ferromagnetic resonance measurements, and static element-specific magnetization measurements presented in the SI verify a random placement of the Fe, Ni, and Cu atoms in single fcc-phase crystal lattice.