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Magnetic-field-dependent THz emission of spintronic TbFe/Pt layers Robert Schneider, Mario Fix, Richard Heming, Steffen Michaelis de Vasconcellos, Manfred Albrecht, and Rudolf Bratschitsch ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00839 • Publication Date (Web): 09 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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Magnetic-field-dependent THz emission of spintronic TbFe/Pt layers Robert Schneider†, Mario Fix‡, Richard Heming†, Steffen Michaelis de Vasconcellos†, Manfred Albrecht‡, and Rudolf Bratschitsch†* †

Institute of Physics and Center for Nanotechnology, University of Münster, Wilhelm-KlemmStr. 10, 48149 Münster, Germany ‡

Institute of Physics, University of Augsburg, Universitätsstr. 1 Nord, 86159 Augsburg, Germany

KEYWORDS spintronic THz emitters, THz spectroscopy, ferrimagnets, terbium-iron thin films

ABSTRACT

We measure the THz emission of a layered spintronic system based on platinum (Pt) and terbium-iron (TbxFe1-x) alloys for the entire range of Tb content (0 ≤ x ≤ 1) under different external applied magnetic fields. We find that the THz emission amplitude closely follows the in-plane magnetization. Deviations occur when the ferrimagnetic TbFe layer changes from an inplane to an out-of-plane easy axis at x = 0.2, and in the medium composition range x = 0.45 – 0.55, where Tb magnetic moments dominate the total magnetic moment. The increasing

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influence of Tb also leads to an inverted THz amplitude for samples with comparable Fe and Tb contents. The THz emission is highest for TbxFe1-x/Pt samples with small amounts of Tb (x = 0.03 - 0.15) due their reduced electrical conductivity compared to pure Fe/Pt and strongly decreases with increasing Tb content by two orders of magnitude. Our systematic study paves the way for designing optimized spintronic THz emitters and demonstrates that transient THz spectroscopy is a powerful tool to gain insight into complex magnetic systems.

Introduction THz radiation in the frequency range from 0.3 to 30 THz bridges the gap between electronic and optical frequencies. It has been used for probing and driving fundamental resonances in gaseous, liquid, and solid materials.1–3 However, it is still challenging to generate broadband THz radiation with sufficient power in a convenient way. Recently, a new type of THz emitter has been discovered, which is based on the inverse spin Hall effect (ISHE).4,5 These “spintronic” THz emitters can generate a high THz intensity, are broadband up to 30 THz and easy to fabricate.4–6 For example, Seifert et al. measured single-cycle THz pulses from a W/CoFeB/Pt trilayer with an electrical field strength of up to 300 kV∙cm-1.6 Previous works have been devoted to finding the optimum materials, layer thicknesses, layer numbers, and sample geometries.4–11 In all these measurements, the magnetization of the samples was kept constant and almost completely saturated by an external magnetic field. In this work, we show how the THz amplitude, generated in spintronic emitters based on ferrimagnetic TbxFe1-x alloy thin films, depends on the sample magnetization. We measure THz waveforms for varying external in-plane magnetic fields for Tb contents 0 ≤ x ≤ 1 and relate the observed THz emission to the in-plane magnetization of the samples.


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Samples and Experimental Setup TbxFe1-x(20 nm)/Pt(5 nm) bilayer samples with different Tb concentrations x are fabricated by magnetron sputtering at room temperature. To avoid oxidation of the spintronic emitter, the layers are covered by either 2 nm Co, which partially oxidizes in ambient atmosphere, or 5 nm Si3N4. For the magnetic measurements, depositions are performed on 375 µm thick p-doped Si(100) substrates with a 100 nm thick thermally oxidized SiO2 layer on top. For the THz measurements, spintronic emitters are sputtered on 500 µm thick Al203(0001) substrates. A sketch of the layer stacking is depicted in Figure 1 a (see methods for more details). In the amorphous TbxFe1-x films, the magnetic moments of Fe are aligned almost collinearly due to the strong interatomic coupling, whereas the Tb moments have a broad orientational distribution, because of the weak exchange coupling and the strong magnetic anisotropy. The magnetic exchange of the Fe and the Tb moments, based on the hybridization of the 3d, 5d and 4f electrons along with spin-orbit coupling, results in an antiparallel alignment of the total magnetic moments of Fe and Tb, generating the so called fanning cone.12 Due to this sperimagnetism, thin films of amorphous TbFe alloys have a rich diversity of magnetic properties, depending on composition, thickness, and temperature.13–15 For example, a compensation point exists, where the magnetization of the Fe and Tb sublattices cancels, resulting in a zero net magnetization.13,16 Another interesting property of TbFe alloys is the occurrence of all-optical magnetic switching.17–19 The magnetic properties of the samples are characterized by superconducting quantum interference device measurements - vibrating sample magnetometry (SQUID-VSM). The effective magnetic anisotropy Keff at 300 K is estimated using the in-plane and out-of-plane magnetization loops (Figure 1 b and S1 supporting information). Below a Tb concentration of 0.15, the system has an in-plane easy axis of


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magnetization. For Tb contents between 0.15 and 0.5, the system exhibits an out-of-plane easy axis. The minimum in Keff at Tb concentrations around 0.3 can be explained by the low moment near the compensation point. For the samples with Si3N4 capping layer, the Curie temperature TC of the TbxFe1-x layer for x = 0.6 is below 300 K. Therefore, the system is in the paramagnetic state at room temperature.

Figure 1: (a) Sample and excitation geometry. The sample consists of a 20 nm TbxFe1-x layer, a 5 nm Pt layer and is capped with a 2 nm film of CoO. An external magnetic field is applied in the x direction. The pump pulse is incident on the CoO capping layer. (b) Effective magnetic anisotropy Keff of the samples, measured at room temperature. Between Tb concentrations of x = 0.15 and x = 0.5 the system has an out-of-plane (oop) easy axis (EA).

The samples with CoO capping layer have a small remaining magnetic moment, which can be attributed to the ferromagnetic contribution due to incomplete oxidation. We determine the room-temperature compensation point by measuring the remanent magnetization versus temperature (see Figure S1). For the samples with CoO capping, the room temperature compensation point occurs at a Tb concentration between 0.28 and 0.3, which is slightly higher compared to the samples with Si3N4 capping (x = 0.26) and values from the literature.20


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For the THz measurements, samples with a lateral size of 10 x 5 mm2 are placed centered between two pole shoes of an electromagnet, with a maximum in-plane magnetic field of 544 mT at the sample position. The ultrafast laser pulses used for excitation of the sample as well as probing of the transient THz electric field are generated by a 1 kHz Ti:Sapphire laser amplifier system, have a center wavelength of 800 nm, a pulse duration of 60 fs and are linearly polarized. The sample is illuminated by a collinear beam with a full width at half maximum (FWHM) diameter of 4.5 mm and a fluence of 0.75 mJ∙cm-2 (see methods, S2 and S3 for more details). 30 % of the pump light is absorbed by the sample (Figure S4) and heats the electronic system in the magnetic TbxFe1-x layer as well as in the Pt layer, generating a non-equilibrium electron distribution. For the sample consisting of pure Fe, the majority of electron spins is aligned parallel to the applied in-plane magnetic field in the magnetic layer. After the ultrashort laser pulse has hit the sample, the electrons in Fe and the Pt layers start to diffuse21–24, whereas the high spin polarization in the Fe layer and the random orientation of spins in the Pt layer result in a net spin current from the Fe into the Pt layer (z-direction in Figure 1 a). There, the ISHE transforms the transient spin current into an ultrafast charge current25,26 perpendicular to the applied magnetic field (-x-direction) and the optical axis (z-direction), resulting in the emission of radiation in the THz range. In the samples containing Tb, the generation of the spin current and hence also the THz generation is more complex and depends on the prevailing spin orientation of the non-equilibrium electron distribution heated by the optical pump pulse. The spin-polarized electrons in the 3d shell of Fe can be directly heated by the 1.55 eV pump pulse, because they are energetically close to the Fermi level.5 In contrast, for Tb the electron spin polarization arises from the more than half-filled 4f shell. These electrons are about 2.23 eV below the Fermi energy.27 Therefore, direct heating of the electrons with the 1.55 eV pump pulse


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is unlikely. However, due to the hybridization of 3d, 5d, and 4f electrons the situation becomes even more complex. As a consequence, both Fe as well as Tb are expected to contribute to the spin current, while Fe should be dominant due to the excitation energy of the laser. There are other possible mechanisms, which could also generate THz radiation in magnetic systems, such as the inverse Rashba-Edelstein or the inverse Faraday effect. However, both effects are assumed to be small compared to the ISHE for the samples investigated here.28 The emitted THz pulses are measured by electro-optical sampling with a 1 mm thick (110)-oriented zinc telluride crystal, allowing us to detect frequencies up to 3 THz (see methods). Results Figure 2 a presents the measured normalized transient THz electric fields for selected Tb concentrations at a constant applied in-plane magnetic field of 200 mT. We observe a prominent signal at zero time delay followed by small oscillations caused by the electro-optical crystal used for detection.29 In Figure 2 b we plot the peak-to-peak THz amplitude and magnetization as a function of the applied in-plane magnetic field. All THz measurements are carried out stepwise for increasing external magnetic fields, unless otherwise noted. For every magnetic field, the transient THz electric field is averaged over 10 to 100 single measurements. The error bars correspond to the standard deviation of the peak-to-peak THz amplitude. In the case of x < 0.3, x = 0.45, and x = 0.5 the size of the error bars is smaller than the symbols representing the data points. For the Fe/Pt sample (x = 0), we observe a steep rise of THz emission as well as of the magnetization with increasing external magnetic field, followed by saturation. The THz amplitude closely follows the in-plane magnetization of the sample. Saturation of the THz emission is already reached at (7±2) mT. Samples with small Tb concentrations in the range of 0.015 – 0.15 exhibit a


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Figure 2: (a) Measured transient THz electric fields for TbxFe1-x/Pt layers with CoO capping with different Tb concentrations x. For Tb concentrations in the composition range from 0 to 0.3 and higher than 0.6 the THz signals are comparable and only vary in amplitude, whereas for samples between 0.45 and 0.55 Tb an inverted THz signal is observed. (b) THz amplitude and magnetization as a function of the applied in-plane magnetic field. For samples with either low or high Tb content, the THz amplitude closely follows the in-plane magnetization.


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similar behavior, but with a saturation at higher magnetic fields with increasing Tb contents (see Figure S5 for samples with x = 0.015, 0.03, and 0.28). Also for this composition range, the correlation between THz amplitude and magnetization is excellent. This effect can be directly linked to the number of heated spin-polarized electrons in the magnetic layer. If the external magnetic field becomes higher, the magnetization increases and also the number of spinpolarized electrons. As a consequence, a higher spin current flows from the magnetic layer into the Pt layer, causing a higher THz amplitude. This process is however limited by the saturation magnetization of the sample, because the maximum electron spin polarization is reached at this point and as a consequence, the THz emission also saturates. Therefore, higher external magnetic fields do not cause a stronger THz emission, which can be nicely seen for the Fe/Pt sample with x = 0 (Figure 2b). For Tb concentrations higher than 0.15, the functional dependence of THz emission and inplane magnetization deviates (see Figure S5 for all Tb concentrations and Figure S6.2 for samples with Si3N4 capping layer), while both values are still increasing with rising magnetic field. A possible reason for this effect is the transition from an in-plane (x < 0.15) to an out-ofplane (0.15 ≤ x ≤ 0.5) easy axis of the magnetic layer, as shown in Figure 1 b. Interestingly, at a Tb content of 0.3 the perfect correlation between THz amplitude and in-plane magnetization is restored. However, at the same time, the THz waveform changes significantly (Figure 2 a). The prominent minimum observed in the THz signal at zero time delay for samples with a Tb concentration up to 0.3 morphs into a dispersive shape at x = 0.4, until one finds an inverted THz signal for 0.45 – 0.55 compared to the samples with low Tb contents. A possible explanation of this behavior is discussed further below. For samples with x > 0.6, the THz emission again closely follows the in-plane magnetization until the Tb concentration finally reaches x = 1.


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Figure 3 a presents the THz peak-to-peak amplitude and sample magnetization as a function of Tb content for a constant applied magnetic field of 544 mT (see Figure S6.1 for samples with Si3N4 capping layer).

Figure 3: (a) THz peak-to-peak amplitude (measured | | (blue dots) and modelled | | (red stars)), together with the in-plane magnetization || (red circles), as a function of Tb content x for a constant applied in-plane magnetic field of 544 mT. A prominent decrease of the THz signal and the magnetization by two orders of magnitude is observed with increasing Tb content. The model shows a good agreement with the measured THz amplitude for the entire Tb range with smaller deviations for 0.15 ≤ x ≤ 0.4. (b) Schematic drawing indicating the alignment of the Tb and Fe magnetic moments and corresponding THz waveforms for selected Tb contents x. In the case 1, 4 and 5 both magnetic moments are aligned in-plane. In 2 and 3 the magnetic moments are aligned out-of-plane and tilted in the direction of the external applied magnetic field, resulting in in-plane components.


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The THz emission intensity strongly decreases with increasing Tb concentration by about two orders of magnitude. We believe that this effect is related to the reducing amount of Fe in the magnetic layer. The smaller amount of Fe reduces the number of spin-polarized non-equilibrium electrons generated by the pump pulse, because the photon energy of 1.55 eV is too low to efficiently heat the spin-polarized 4f electrons in Tb. The overall drop in THz amplitude with increasing Tb content follows the decreasing in-plane magnetization (Figure 3 a). However, the highest THz amplitude is not obtained for the Fe/Pt sample, but for samples with a small amount of Tb up to 0.15. This result could be of high interest for the design of THz emitters with high efficiency. A possible explanation for this effect is the influence of the electrical conductivity. A reduced electrical conductivity should increase the emitted THz radiation.4,5 For a rough estimation, we have measured the DC electrical conductivity of all our samples with collinear four-point probe measurements (see Figure S7). For low Tb contents x < 0.2, we find a fast decrease of the DC electrical conductivity with increasing Tb content. This trend is continued for higher Tb concentrations, but with a much smaller slope. If we assume that the electrical conductivity for low THz frequencies is comparable to the DC case (see S7 for more details), these results explain the increase of the THz amplitude for samples with a small amount of Tb, despite the lower Fe content.

To model the measured THz emission amplitude | | in the entire range of Tb contents based on the mechanisms discussed above, we use:

| | = ∙ 1 − ∙ ∗ ∙ ,

1

with a proportionality constant , the Fe content represented by 1 − , the conductivity ∗ (see S7 for details), and the in-plane magnetization at 544 mT. The modelled THz peak-to-


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peak amplitude | | in the entire range of Tb contents is plotted in Figure 3 a (red stars) and agrees very well with the measurement. Small deviations occur in the composition range of 0.15 ≤ x ≤ 0.4 due to the alignment of the Tb and Fe sublattices, which is also important for the amplitude and sign of the THz emission. The specific alignment of the Tb and Fe moments depends strongly on the local magnetic anisotropy and exchange interaction, which results in a non-collinear distribution of the magnetic moments (sperimagnetic), still having on average a preferred antiparallel alignment of the two sublattices.12,30 By applying an external magnetic field, this alignment is further changed due to the Zeeman contribution, acting differently on both sublattices. For a Tb concentration 0.15 ≤ x ≤ 0.5, the magnetic system changes from a preferred in-plane to an out-of-plane easy axis. If we apply an external in-plane magnetic field to samples of this composition range, the initially antiparallelly aligned sublattices of both Tb and Fe tilt in the direction of the external magnetic field by a small angle (Figure 3 b, case 2 and 3), causing also an in-plane component of the magnetization, which is responsible for the generated THz emission.31 Therefore, and because mainly the Fe contributes to the spin polarization of the electrons heated by the pump pulse, THz radiation is also emitted for samples with a preferred out-of-plane orientation of the magnetic moments. This mechanism also applies for samples in vicinity of the magnetic compensation point (x = 0.28 - 0.3). The THz waveform, however, remains almost unchanged up to x = 0.3 (Figure 2 a). The effect of tilted magnetic moments in the external magnetic field can also be witnessed in Figure 2 b. The measured THz emission amplitude almost linearly increases for rising magnetic fields in the composition range 0.2 ≤ x ≤ 0.45, because the in-plane component is proportional to the tilting angle for small deflections. However, the exact angle of the Fe and Tb moments with respect to the external field is not known. As a consequence, the in-plane component of the Fe moments can be only roughly


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approximated by the measured in-plane magnetization, which is an average of both the Fe and Tb magnetic moments. Therefore, element-resolved magnetization measurements will be needed to fully model the measured THz amplitude. For increasing Tb contents (x > 0.45) the magnetic system turns back in-plane. In this composition range, the Tb moments are dominant and aligned parallel to the external applied magnetic field. In contrast, the Fe moments are now aligned antiparallel to the external field and therefore also the spin polarization of the optically excited non-equilibrium electrons. As a consequence, the spin-dependent splitting in the Pt layer due to the ISHE takes place in opposite direction and the THz signal changes sign (Figure 3 b, case 4). The composition range, in which we observe the inverted THz signal, corresponds to a Tb concentration of 0.45 – 0.55 (see also Figure 2 a). For even higher Tb concentrations (x > 0.55) the influence of Fe further decreases. The small THz emission can be understood by the low Curie temperature of these samples, which is close (x = 0.6) or even below (x > 0.6) room temperature. Only the magnetic moment induced by the external applied magnetic field gives rise to a small amount of spin-polarized electrons heated by the pump pulse. The spin-polarization of these electrons is aligned parallel to the external magnetic field and the THz signal flips back to the original orientation (Figure 3 b, case 5). We also observe a strong THz emission for our TbFe samples without an external applied magnetic field, if they have been magnetized before (see S9).9 This result demonstrates that for future applications, spintronic THz emitters could be either magnetized by a magnetic field pulse or already during manufacturing to avoid space and mass restrictions of a bulky external magnet.

Conclusion


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In conclusion, we have demonstrated that the THz emission of the ferrimagnetic TbxFe1-x/Pt system is governed by the in-plane magnetization of the samples. However, both the change from in-plane to out-of-plane easy axis at x = 0.2 and the increasing importance of Tb in the medium composition range cause deviations. The increasing Tb content in the range x = 0.45 – 0.55 also lead to an inverted THz amplitude. The THz emission was the highest for samples with small amounts of Tb (x = 0.03 - 0.15). The Tb0.05Fe0.95/Pt emitter yielded the strongest THz signal, which was more than twice higher compared to pure Fe/Pt, due to the decreased conductivity of the sample. Overall, the THz amplitude was strongly decreased by two orders of magnitude for decreasing Fe content. We attributed this effect to the reducing amount of Fe and the reducing in-plane magnetization. Fe dominates the THz generation process due to the photon energy of the laser pulses used for excitation, which predominantly heat spin-polarized Fe and unpolarized Tb electrons. Our systematic study is important for the design of optimized and easy-to-use spintronic THz emitters and demonstrates that transient THz spectroscopy provides comprehensive insights into complex magnetic systems. Methods Sample preparation: TbxFe1-x(20 nm)/Pt(5 nm) bilayers with varying Tb content x between 0 and 1 were prepared by magnetron sputtering at room temperature. The layers were covered by either 2 nm Co or 5 nm Si3N4 to avoid oxidation of the underlying TbxFe1-x. For all films, except for the Si3N4 layer, Argon (5.0) gas with a partial pressure pAr = 3.5 µbar was used for the sputtering process. For the deposition of the Si3N4, a mixture consisting of 96 % Argon (5.0) and 4 % Nitrogen (5.0) gas with a partial pressure pAr/N = 1.5 µbar served as sputter gas. A quartz balance was used to adjust the sputter rate and time in order to achieve the desired film thickness and composition. For the co-sputtering of Fe and Tb, the deposition rate of the Tb target was set


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to values between 0.02 and 2.00 Å∙s-1, while the Fe rate was adjusted to values between 0.07 and 0.53 Å∙s-1. To ensure the homogeneity of the films, the substrates were rotated during the sputtering process. All depositions were made on 375 µm thick p-doped Si(100) substrates with a 100 nm thick thermally oxidized SiO2 layer on top as well as on 500 µm thick Al203(0001) substrates. Rutherford backscattering measurements were performed to confirm the stoichiometry and film thickness of the samples. The deviation from the nominal composition of the TbxFe1-x layer was found to be less than 1 %, the variation of the film thickness less than 15 %. Magnetization measurements: The magnetic properties of the samples were characterized by SQUID-VSM. All SQUID-VSM measurements were performed on samples grown on Si/SiO2(100 nm) substrates. An exemplary comparison between the magnetic properties of films grown on the two different substrates performed on the sample with x = 0.2 and the Si3N4 capping layer showed only a small dependency of the magnetic properties on the substrate type. THz measurements: For the THz measurements, the investigated sample was placed centered in an electromagnet on a custom made sample holder, which allowed for an easy exchange of the samples without the need for realigning the experimental setup (see Figure S3). The ultrafast laser pulses used for excitation of the sample as well as probing of the transient THz electric field were generated by a 1kHz Ti:Sapphire laser amplifier system (Newport-Spectra Physics, Spitfire Ace), had a center wavelength of 800 nm, a pulse duration of 60 fs and were linearly polarized. The sample was illuminated by a collinear beam with a diameter of 4.5 mm and a fluence of 0.75 mJ∙cm-2. The samples were optically excited from the side of the capping layer (see Figure 1 a) to easily compare the different samples. For excitation through the Al2O3 substrate, the THz amplitudes were up to a factor of three higher (data not shown), but the THz


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radiation generated in the Pt film is transmitted through the TbxFe1-x layer in this case, which complicates the analysis due to the varying composition of the TbxFe1-x layer. The created THz and the pump light passed through a beam expander (PM1 and PM2). No plasma generation between PM1 and PM2 was observed and also no plasma-induced THz radiation was measureable. Residual pump light was removed by a 1 mm thick silicon wafer with a resistivity higher than 10000 Ω∙cm and stopped with a beam block. The emitted THz pulses were measured by electro-optic sampling with a 1 mm thick (110)-oriented ZnTe crystal using 0.05% of the pump intensity to probe.32 The polarization rotation of the probe beam was measured by a temperature stabilized photodiode detector connected to a lock-In amplifier, which was referenced to the 1 kHz repetition rate of the laser. Due to the electro-optic crystal, we could detect THz radiation up to 3 THz. All measurements were carried out at room temperature in a dry nitrogen atmosphere.

ASSOCIATED CONTENT Supporting Information Details of the temperature dependent magnetic properties of the TbxFe1-x samples, additional information to the experimental setup, THz peak-to-peak amplitudes as a function of the magnetic field for samples with Si3N4 capping layer, DC conductivity measurements and a THz as well as a magnetic hysteresis measurements on the samples Tb0.1Fe0.9/Pt and Tb0.55Fe0.45/Pt AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank Tobias Kampfrath for fruitful discussions concerning the THz setup.


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SYNOPSIS The THz emission of TbxFe1-x/Pt layers is measured under different externally applied magnetic fields. The THz intensity is mainly governed by the in-plane magnetization of the ferrimagnet TbFe and decreases by two orders of magnitude with increasing Tb content. The spintronic THz emitters are also operational without an external applied magnetic field due to the remanence of the magnetic layer.

Toc figure


21

a

b

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0.4

Hext

/CoO/TbxFe1-x/Pt Si3N4//TbxFe1-x/Pt

0.2

800nmK/60fs

-3

Keff//MJ//m r

Al2O3

Pump

Pt

0.0

CoO TbxFe1-x

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

THz

MTb

-0.2 -0.4

ip/EA

Curie/temperature TC

Pt layers

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