Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

Journal of Electron Spectroscopy and Related Phenomena 185 (2012) 356–364

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Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope Takeshi Nakagawa a,b,∗ , Toshihiko Yokoyama a,b a b

Institute for Molecular Science, Myodaiji-cho, Okazaki, 444-8585, Japan The Graduate University for Advanced Studies (Sokendai), Myodaiji-cho, Okazaki, 444-8585, Japan

a r t i c l e

i n f o

Article history: Available online 7 March 2012 PACS: 73.20At 68.35.Bs 71.45.Lr Keywords: Magnetic circular dichroism Photoemission Magnetic thin film

a b s t r a c t This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (h≤ 6 eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(0 0 1), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Magnetic domain imaging using magnetic circular dichroism (MCD) has been a common technique, which covers magneto-optic Kerr effect (MOKE) microscope, photoemission electron microscope (PEEM), and transmission microscopy using X-ray and visible light (Faraday effect) [1]. Although MCD does not directly offer information about the magnetic moment except for few cases like X-ray MCD (XMCD) [2], it is widely used due to its high efficiency, which is one of the most important factors for the imaging. The high throughput comes from the fact that MCD measurements do not require any spin analyzers like a Mott detector, which is low efficient [3]. MCD is derived from both spin–orbit coupling and exchange interaction in magnetic materials. Due to the strong spin–orbit coupling in core levels, XMCD in core states has a strong

∗ Corresponding author at: Institute for Molecular Science, Myodaiji-cho, Okazaki, 444-8585, Japan. E-mail addresses: [email protected] (T. Nakagawa), [email protected] (T. Yokoyama). 0368-2048/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2012.02.009

merit to measure element specific magnetization with high sensitivity [2] and is used widely in synchrotron radiation facilities. MCD effects in valence states are usually measured in transmission or reflection. Among them, MOKE is of importance for the investigation of magnetism of thin films in laboratories, because feasibly available light sources, like diode lasers or discharge lamps, make the MOKE measurement possible and its experimental setup is quite simple [4]. However the spatial resolution of MOKE is limited by the diffraction limit of light. With the photoelectron emission, the valence band MCD combined with electron microscope can give better spatial resolution. Its sensitivity to the magnetization is usually so low due to the weak spin–orbit coupling in the valence band that it is difficult to investigate the microscopic magnetic structure of thin films. Valence electrons have weak spin–orbit coupling, but they are highly spin polarized in magnetic materials. It is well known that MCD asymmetry in MOKE is less than 10−3 , due to its energy and momentum averaged measurements of valence band states. On the other hand, angle and energy resolved photoemission in valence bands shows relatively large asymmetry [5,6]. Its asymmetry can be an order of 10% on Ni/Cu(0 0 1), but this is not directly

T. Nakagawa, T. Yokoyama / Journal of Electron Spectroscopy and Related Phenomena 185 (2012) 356–364

Polar MOKE

(a)

Probe beam

(b)

M

M Delay line

Photodiode Polarizer

PEEM

Quater waveplate Circular polarization

BBO

UHV

357

Lens

H Anode

Lens

Ti-Sapphire

e

Sample UHV

Pump beam

Sample

Electromagnet

Cs source

Quartz viewing port Photoelastic modulator or Quater waveplate Lens Polarizer λ/2 plate

A

MCD measurement

Laser

Fig. 1. (a) Experimental setup for threshold photoelectron measurements. Light is circularly polarized using a quater wave plate or a photoelastic modulator. The sample is placed between the electromagnet. A Cs source is installed close to the sample in order to change the workfunction. (b) A custom PEEM setup for conventional as well as time resolved measurements. Figures adapted from Ref. [9] with permission.

applicable to PEEM measurements since installation of the angle and energy resolved spectrometer with PEEM decreases the transmission. With the valence band photoemission, one can find only very few examples for the PEEM measurements of magnetic domains using ultraviolet magnetic dichroism. First PEEM observation of magnetic domain with magnetic dichroism in valence band was reported for polycrystalline Fe films of 100 nm thickness using a Hg lump (h ∼ 5 eV) [7]. The measurement employed magnetic liner dichroism. The obtained dichroic asymmetry is 0.2%, which does not compete with the asymmetry with XMCD (∼10%). Because of its inefficient sensitivity to the magnetism, dedicated measurements are needed to image magnetic domains. Recently it has been reported that the total electron yield in valence band near the photoemission threshold gives high asymmetry [8]. On perpendicularly magnetized Ni/Cu(0 0 1), the asymmetry near the threshold amounts to more than 10%, which is close to the value obtained by angle resolved photoemission. The threshold measurement of photoelectron in a total electron yield can take advantage of the measurement of photoelectrons with limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic properties and makes it feasible to observe magnetic domains by PEEM [9]. PEEM experiments with pulse lasers have been used for investigations of multiphoton photoemission processes and time resolved electron motions. Recently the two-photon photoemission (2PPE) experiment on nanostructures has been attracting great interests because the strong laser field enhances photoemission probabilities from nanostructure due to the localized surface plasmon and modification of the local electromagnetic field [10]. 2PPE PEEM gives fruitful information about the nanostructures. Laser MCD PEEM can offer to explore multiphoton photoemission imaging of magnetic domains, and furthermore time resolved measurements in nanometer scale. In this article, threshold photoemission MCD results and its application to PEEM are presented. Threshold MCD measurements concerning energy dependence, incident angle dependence, magnetization axis, and probing depth are shown. It is demonstrated that high brilliance of lasers enables MCD PEEM to acquire magnetic domain images in a video rate. Also shown are the results

for 2PPE MCD PEEM, which has a possibility to enhance the MCD asymmetry using resonant excitation. 2. Experimental 2.1. Threshold photoelectron and angle resolved measurements Threshold photoelectron measurements are performed either by tuning the photon energy or the work function of samples with fixed photon energy [8,11,12]. Energy tunable photon sources (e.g. a Ti:Sapphire laser, synchrotron radiation, etc.) are desirable since the manipulation of the work function is not feasible. Ultrashort pulse and broad band lasers (<100 fs, 80 MHz, 1.2–1.8 eV) are used to generate their second and fourth-order harmonics (2.4–3.1 eV and 4.9–6.0 eV, respectively) using BBO crystals. Also used are continuous wave lasers with photon energies of 3.81 eV, 3.05 eV, 1.95 eV. The incident light is circularly polarized by a quarter-wave plate or an electro-optic modulator together with a polarizer, which can manipulate the polarization of the light with a high repetition rate (∼100 kHz). A laser power of a few mW is sufficient for one photon photoemission even around the photoemission threshold. The MCD asymmetry is defined here by A = (I+ − I− )/(I+ + I− ), where I+ (I− ) is sample current for parallel (antiparallel) orientation between the angular momenta of photons and electrons. Threshold photoelectron MCD measurements are performed with an electromagnet (maximum field ∼ 3000 Oe) and a positively biased anode to extract photoelectrons (Fig. 1(a)). The anode is important mainly for two reasons. First the electrode ensures that the photoelectron emitted from samples can reach lower potential destinations, which is critical for the low work function samples like alkaline metal covered ones. Second, the electrode gives photoelectrons high kinetic energy. Otherwise slow photoelectrons could go back to the sample under the magnetic field. MCD angle resolved photoelectron spectroscopy (ARPES) is done with a hemispherical electron analyzer [6,13]. The light incidence angle is 0◦ or 45◦ , and the photoelectron emission angle is 0◦ with the angle resolution of ±1◦ . The measurements are done with samples in remanece magnetization. A pulse coil behind the samples is used to generate a magnetic field normal to the sample surface.

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In order to manipulate the sample work function, Cs is deposited from commercial dispensers (SAES getters) at a rate of 0.01 monolayer (ML)/min. When ∼0.15 ML Cs adsorbs on the sample, the work function decreases down to ∼1.5 eV. 2.2. PEEM measurements PEEM experiments are done with an electrostatic microscope with a resolution of better than 100 nm. The prepared Cs/Ni/Cu(0 0 1) showed a time dependent work function change in the case of extended measurement period. To prevent the work function change during the measurements, the sample was intensionally left in the 10−10 Torr vacuum overnight. After this procedure the work function is stable over several days. MCD-PEEM images are measured by the difference between successive images from right and left circularly polarized beams. The incident angle of the laser beam is 60◦ from the surface normal.

(a) 2 MCD Asymmetry (%)

358

0 -2 -4

-10

Fig. 2 shows the photon energy dependence of the 1PPE MCD asymmetry in a total electron yield. The work function of the Ni film studied is determined to be 5.15 ± 0.1 eV from the onset of the photocurrent. The 1PPE MCD shows a negative asymmetry with the minimum of −12% around the photoemission threshold. With increasing the photon energy, the asymmetry is suppressed and approaches zero around the photon energy of ∼0.75 eV away from the threshold, while the electron emission increases. Note that the asymmetry slightly above the threshold originates from the thermal broadening at 300 K and energy spread of the pulse laser. The energy dependence of the 1PPE MCD is similar to other results with different photon energies (3.8 and 1.95 eV) [8], where the photon energy is fixed and the work function is changed with the deposition of Cs. The rapid suppression of the MCD asymmetry with the increase in the photon energy demonstrates that the threshold photoemission is crucial for large MCD asymmetry. When the photon energy increases, the total electron yield measures the integrated photoelectron intensity over a wider energy range towards a higher binding energy, which could cancel the MCD effect due to the integration of different spin bands and opposite spin–orbit splitting bands [8,6]. This would require higher photon flux for threshold MCD PEEM measurements compared to that for ordinary laser based PEEM. MCD asymmetry in valence band photoemission depends on the magnetization axis. Fig. 3(a) plots MCD asymmetry at the photoemission threshold on Cs/Ni/Cu(0 0 1) as a function of Ni thickness. Ni/Cu(0 0 1) is known to exhibits a spin reorientation transition around 7.5 ML with increasing Ni thickness, where the magnetization easy axis changes from the in-plane to the perpendicular direction [14]. Coincident with the spin reorientation, the MCD asymmetry changes by one order of magnitude. The MCD asymmetry is higher for the perpendicular magnetized Ni film. A similar behavior is found for MOKE measurements [15]. The MCD asymmetry difference between the perpendicular and in-plane magnetized films originates from the light reflection and subsequent change of polarization, which is less important for XMCD. Fig. 3(b) shows the degree of the circular polarization (Pc ) on reflection as a function of the incident angle using Fresnel equations. The photon energy used is h = 1.95 eV with refractive index of 1.8–3.6i. The incident light is circularly polarized (Pc = 1.0), but the reflected one is elliptically polarized. Furthermore, on reflection the degree of the circular polarization changes its sign except below the Brewster angle of 75◦ . This sign change has a crucial effect on the MCD asymmetry

A

e-

-12 5.0

5.2

5.4

5.6

5.8

6.0

Photon Energy (eV)

Normalized Sample Current

3.1. Magnetic circular dichroism in valence band

hv

-8

(b) 3. Results

M

-6

1.2

1.1

1.0

0.9

0.8

-200

-100

0

100

200

Magnetic Field(Oe) Fig. 2. (a) MCD asymmetry from a perpendicularly magnetized Ni(12 ML)/Cu(0 0 1) as a function of the photon energy. The MCD asymmetry is measured by the total electron yield as shown in the inset. The light incidence angle is 0◦ . The vertical dashed line indicates the work function (5.15 eV) estimated from the sample current against the photon energy. (b) Magnetization cuve measured at photoemission threshold. Figures adapted from Ref. [12] with permission from APS.

since the asymmetry is proportional to the product of the projected magnetization on the light direction and the degree of the circular polarization. The MCD effect is enhanced on reflection for the perpendicularly magnetized films, but canceled for the in-plane magnetized one due to the reflected light. This explains higher MCD asymmetry for the perpendicularly magnetized films. 3.2. Application of threshold MCD to PEEM The high asymmetry in the total electron yield naturally promises MCD PEEM observation of magnetic domains. MCD asymmetry more than 1% is sufficient to measure MCD PEEM images in a good signal to noise ratio and a reasonable accumulation time. Fig. 4 shows the MCD-PEEM results for Cs/Ni(12 ML)/Cu(0 0 1) [9]. The light source is a CW laser of 1 mW with a photon energy of 3.05 eV (2 × 1015 photons/s). Fig. 4(a) shows the MCD image with 4 s accumulation, giving 4% asymmetry. With this accumulation time, magnetic domains are clearly observed. In this image, stripes across the surface correspond to the up and down magnetized domains. Fig. 4(b) shows the MCD image taken in the same area as Fig. 4(a) with 1/30 s accumulation time for each circularly polarized light.

T. Nakagawa, T. Yokoyama / Journal of Electron Spectroscopy and Related Phenomena 185 (2012) 356–364

MCD Asymmetry(x10-2)

-18

in-plane

-16

out-of-plane M

M

spin reorientation

-14 -12 -10 -8 -6

x10

-4

Cs/Ni/Cu(001)

-2

Ts = 90 K

0 3

(a)

4

5

6

7

8

9 10 11 12 13

Pc circular polarization factor

Ni thickness (ML) 1.0 Ni

N = 1.8 - 3.6 i

k

0.5 0.0

in

Pc = 1.0 in Pc = 1.0

θi

ref

-0.5

ref

Pc

k in

(b)

20

40

60

80

θi, Incident angle (degree)

Pc = 1.0

k' ref

Pc = -0.941

M

in-plane

-1.0 0

45 45

45 45

out-of-plane

359

boundary. With increasing the photon flux, the domain structure begins to distort. With the input power of 7.8 mW, the MCD image is obscure compared with that obtained with 0.2 mW. Fig. 5(b) plots the MCD asymmetry across the images. Note that the asymmetry (∼4%) is almost constant regardless of the incident power used, indicating that the diffused domain image at high laser power does not originate from the heating by laser. Fig. 5(c) shows the domain boundary width as a function of the incident laser power. A repulsion between neighboring emitted electrons shifts the electron trajectory when the electron density is high. This space charge affects the spatial resolution and linearly depends on emitted electron current density [16]. The linear dependence of boundary broadening for higher laser power in Fig. 5(c) is consistent with this concept. In this experiment, a typical emitted electron current is 1 nA for 1 mW laser power. Considering the pulse width (100 fs) and the beam diameter (∼ 200 ␮m), the electron current density corresponds to 0.4 A/cm2 . However the theory with this current density indicates that the deterioration of the resolution is 0.3 ␮m [16], while it is 2 ␮m in our experiment. Although its linear dependence on laser power is observed, other factors, like an electron density at apertures, should be taken into account [17].

k' ref

Pc = -0.941

M

Fig. 3. (a) Ni thickness and magnetization axis dependence of MCD asymmetry from Ref. [8]. The sample is a Ni wedge film/Cu(0 0 1), which is covered by Cs. With increasing Ni thickness, Ni/Cu(0 0 1) shows spin reorientation around 7.5 ML, above which the magnetization easy axis is perpendicular to the surface. The MCD asymmetry for perpendicular magnetized films is one order of magnitude higher than that for in plane magnetized one. Figure from Ref. [8] with permission from APS. (b) A change of the circular polarization factor of light (h = 1.95 eV) by reflection for Ni film. The incident beam is assumed as fully circular polarized. Figure adapted from Ref. [36] with permission.

The magnetic domain is still visible. The rapid acquisition of MCD images is achieved owing to large photon flux of the laser as well as the high MCD asymmetry. Laser induced MCD-PEEM has a possibility to measure magnetic domains in a video rate by its high brilliance using CW lasers. With pulse light sources, the situation is somewhat different. A MCD-PEEM measurement with a pulse laser is shown in Fig. 5 [9]. Here shown is an example of the space charge effect due to high photoelectron flux in a short time. The laser used is frequency-doubled light (h = 3.1 eV, pulse width = 100 fs, 80 MHz repetition) from a Ti:Sapphire laser. The beam is focused on the sample with ∼ 200 ␮m. Fig. 5(a) shows the MCD-PEEM images for different laser power. With low incident laser power (0.2 mW), the MCD image shows clear domain structure with a sharp domain

3.3. MCD angle resolved photoelectron measurements It is interesting to estimate how large the asymmetry by ARPES using a photon energy close to the threshold (h ≤ 6 eV) is. Fig. 6 shows a set of angle resolved photoemission spectra for Ni(12 ML)/Cu(0 0 1) with opposite magnetization directions, while the helicity of the circular polarization is fixed. These experimental configurations for APRES are similar to those for the total electron yield measurement. The photon energy used is 5.28 eV and the light incidence angle is 0◦ . In order to lower the sample work function and investigate the MCD asymmetry around the Fermi level, 0.15 ML Cs is deposited onto the sample, which results in the work function of 1.5 eV. This expands the width of photoemission spectra noticeably for the low photon energy excitation. These spectra are measured in the normal emission and in remanence magnetization states after pulse magnetization at H ∼ 200 Oe normal to the surface. This measures the electronic structure along the  − X direction in fcc structure, neglecting the distortion of the Ni lattice on Cu(0 0 1). A peak around the binding energy (EB ) of 0.2 eV in Fig. 6 is derived from Ni 3d–4sp bands, which are split due to the spin–orbit coupling. The peak shift observed for the different magnetization directions is estimated to be 30 meV, which corresponds to the size of spin–orbit coupling strength and is similar to the observation (50 meV at the maximum point along  − X direction) by Kuch et al. [5]. According to their structure band calculation, the peak is constituted of by 57 and 56 bands. Fig. 6(b) shows MCD asymmetry obtained from the spectra in Fig. 6(a). The MCD asymmetry is evaluated according to (I+ (E) − I− (E))/(I+ (E) + I− (E)). The definition of I+(−) is the same as that for the total electron yield, while the integrated MCD asymmetry is evaluated according to (S+ (E) − S− (E))/(S+ (E) + S− (E)), where the

E

Fig. 4. PEEM of magnetic domain imaging using 1PPE-MCD on Cs/Ni(12 ML)/Cu(0 0 1). The light source is a continuous wave (CW) laser of h = 3.05 eV. (a) MCD-PEEM image for 4 s integration time for each circularly polarized light source. (b) The same as (b), but the integration time is 1/30 s. The field of view is 100 ␮m. Figures from Ref. [9] with permission.



S+(−) (E) is an integrated intensity, S +(−) (E) = −0.2 eV I +(−) (E  )dE . The integrated MCD asymmetry can be compared with that obtained by the total electron yield. The MCD-ARPES spectrum shows a minimum around the Fermi level, and its magnitude is −6%. For the higher binding energy, the MCD-ARPES asymmetry goes to zero and changes its sign at 0.13 eV from the Fermi level. Similarly the integrated MCD-ARPES changes slowly and crosses the zero at 0.26 eV from the Fermi level. This behavior is similar to the result by the total electron measurements in Fig. 1.

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Fig. 5. Effect of space charge by a ultra short pulse laser. MCD-PEEM images are taken by a fs laser with a photon energy of 3.1 eV on Cs/Ni(12 ML)/Cu(0 0 1). (a) MCD images for increasing laser power. With increasing the laser power, the domain structure becomes blurred. The beam spot used is ∼200 ␮m. The field of view is 25 ␮m. (b) Cross section line profiles along A–B in (a) across a domain boundary. (c) Observed domain boundary width as a function of incident laser power. Figures from Ref. [9] with permission.

3.4. Electron energy and momentum limitation in threshold photoemission The total electron yield measurement shows high MCD asymmetry in spite of the angle and energy integrated nature. The MCD asymmetry by the total electron yield can be compared with the integrated MCD asymmetry by ARPES. These results by the total electron yield and ARPES are taken under the similar experimental conditions where the incidence angles are 0◦ both for the total electron yield and ARPES, and the photon energy is ∼ 5.3 eV. The MCD asymmetry is −10% by the total electron yield measurement, while the integrated MCD asymmetry is −5% by ARPES. The MCD asymmetry is slightly smaller for the ARPES experiment. The smaller MCD asymmetry for ARPES might be caused by the Cs deposition. The similarity between ARPES and the total electron yield can be explained by two aspects. First, as shown in Fig. 7(a), the total electron yield measurement near the threshold is somehow an energy filtered measurement by the narrow energy window between the photon energy and the vacuum level (the sample work function, ), E = h − . Thus it is similar to the energy resolved measurement near the photoemission threshold. Second, as shown in Fig. 7(b), photoelectrons coming to the crystal surface are diffracted by the potential difference. The maximum incidence angle of the photoelectrons passing through the crystal surface, imax , is expressed as sin2 imax = (h − )/(h − E0 ) [18], where E0 is the bottom of the valence band (the inner potential). This diffraction effect is significant for low kinetic energy electrons excited by low energy photons. With h = 5.3 eV,  = 5.1 eV, and E0 = −10 eV, imax is estimated as 6.6◦ . Thus the photoelectrons inside the crystal with larger angle than 6.6◦ are reflected and cannot escape from the surface. This corresponds to the maximum projected momentum of 0.18 A˚ −1 in the surface Brillouin zone. Since the surface Brillouin zone boundary of Ni(0 0 1)/Cu(0 0 1) is located at 1.23 A˚ −1 , only 14% of the zone width is detected. Therefore the spontaneous angle selection in threshold photoemission limits the transition from other Ni electronic bands than 57 and 56 bands along X direction, which might give opposite sign of MCD. With increasing photon energy or lowering the work function, these two conditions are getting blurred. These two arguments are the main reasons for the enhanced MCD asymmetry in the total electron yield. Therefore high asymmetry

in magnetic dichroism can be expected, if one can measure large magnetic dichroism by ARPES around the Fermi level in the normal emission. 3.5. Probing depth of threshold photoemission MCD The threshold photoemission MCD method employs low energy electrons with kinetic energy (Ekin ) of less than 6 eV. Low kinetic energy photoelectrons, Ekin ≤ 20 eV, show long inelastic mean free path (IMFP). Literally IMFP for photoelectron with the kinetic energy of 10 eV is 0.5–5 nm, wide variation depending on elements [19–21]. However, as mentioned above, the dominant factor for the high MCD asymmetry in threshold photoemission is the spontaneous limitation of kinetic energy as well as momentum of emitted photoelectrons. Therefore elastic scattering inside the crystal which changes the electron momentum decreases the asymmetry, but it does not change the inelastic mean free path. The depth information for MCD measurement could be different from that for conventional IMFP. Overlayer capping effects on the MCD asymmetry are measured on Cu/Ni/Cu(0 0 1). Cu is chosen as the cupping layer since it can pseudomorphically grow on Ni/Cu(0 0 1), which ensures uniform wetting of the overlayer [22]. Here 1 ML Cu capping corresponds to 0.18 nm. The photon energy used is 5.8 eV, thus the IMFP for the threshold electrons is expected to be larger than ∼1 nm. Fig. 8 plots the asymmetry reduction as a function of Cu overlayer thickness. The asymmetry is normalized to that of the uncovered 15 ML Ni film on Cu(0 0 1). The asymmetry monotonically decreases with increasing the thickness of Cu overlayer. Cu cupping of 1 ML reduces the asymmetry by half, but Cu cupping of 20 ML does not extinguish the asymmetry. The remaining asymmetry at 20 ML cupping is 1/10 of the uncupped Ni film. The behavior of the asymmetry as a function of overlayer is fitted using an IMFP. The fitting assumes an exponential decay law, exp (− t/), with t and  being Cu thickness and probing length, respectively. However it turns out to be inappropriate because of the two contradicting behavior in thin and thick overlayer regimes. The rapid reduction up to 4 ML Cu cupping can be fitted using  = 0.5 nm, while the gradual reduction for thick overlayer can be fitted using  = 1.4 nm IMFP, which is close to the value in literatures

T. Nakagawa, T. Yokoyama / Journal of Electron Spectroscopy and Related Phenomena 185 (2012) 356–364

I+ I−

Photoelectron intensity (a)

361

M

en hv

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

Energy relative to EF (eV)

MCD asymmetry (%)

10

(b)

Fig. 7. (a) Schematic description of threshold photoemission. The photoelectron excited by the photon energy of h is filtered by the vacuum level, resulting in narrow energy distribution. (b) Diffraction of photoelectrons at the crystal surface. This diffraction effect limits the maximum angle,  i , for the photoelectron to escape from the crystal surface. (c) Brillouin zone for fcc structure. For a (0 0 1) surface, the  − X direction is normal to the surface, while for a (1 1 1) surface, the  − L direction.

5

Figures adapted from Ref. [13] with permission.

0

thickness shows that the overlayer scattering process is of importance for threshold photoemission MCD.

-5 3.6. Two photon photoemission

-10 1.0

0.8

0.6

0.4

0.2

0.0

-0.2

Energy relative to EF (eV) Fig. 6. (a) Energy distribution curves by ARPES for perpendicularly magnetized Ni(12 ML)/Cu(0 0 1). The spectra taken with parallel and anti-parallel alignments of the angular momentum of the photon relative to that of majority electron spin are shown. The photon energy used is 5.28 eV. A small amount of Cs (0.15 ML) is deposited in order to decrease the work function down to 1.5 eV. The light incidence angle is 0◦ and the electron emission angle is 0◦ . The inset shows the measurement settings. (b) MCD asymmetry (line) and integrated MCD asymmetry (dashed line) evaluated from the spectra set in (a). See the text for the definition of MCD asymmetry.

A possible application of pulsed laser MCD is a study of multiphoton processes [12]. Fig. 9(a) shows the 2PPE MCD asymmetry measured on the same sample as the 1PPE MCD one (Fig. 2). The 2PPE asymmetry is again large near the threshold, giving ∼6% asymmetry. With increasing the photon energy, the asymmetry decreases and inverts its sign around 0.45 eV above the threshold, at 2h = 5.6 eV. Although the sign inversion is not observed for the 1PPE on the same sample, it is reported that for h = 3.8 eV and the work function less than 2.8 eV, the asymmetry changes from a negative to positive value. The sign inversion is also observed for the

Normalized MCD Asymmetry

[20]. The drastic decrease of the asymmetry by a few monolayer deposition of Cu suggests that the momentum change by elastic scattering in Cu overlayers should be involved. After some overlayers the scattering effect is saturated and the momentum restriction due to the refraction at the surface is much less effective. The thick region above 6 ML cupping is fitted by rather long probing length of 4 nm. The electrons along the  − X direction dominantly contribute to the asymmetry for the uncovered Ni film. On the other hand, by Cu capping, the electrons along different directions from  − X begin to be involved due to the scattering by the Cu overlayer, which would cancel the MCD asymmetry. The saturated asymmetry reduction above 6 ML is explained by the averaged MCD asymmetry over the Brillouin zone. The gradual reduction for the thick overlayer region above 6 ML capping, fitted using IMFP (4 nm), is due to the conventional inelastic scattering process. It is reported that the probing depth of threshold PEEM for polycrystalline Fe films covered by silver overlayer amounts to as large as 16.2 nm [23]. Such an extremely large probing depth is not found on Cu/Ni/Cu(0 0 1). The drastic reduction of the MCD asymmetry for small Cu

Cu Cap Thickness (nm)

0

Figures from Ref. [13] with permission.

1

2

3

4

5

1.0 0.8 0.6 0.4 0.2 0.0 0

5

10

15

20

25

30

Cu Cap Thickness (ML) Fig. 8. Effect of overlayer (Cu) on the MCD asymmetry of Ni(15 ML)/Cu(0 0 1). The asymmetry is measured by ARPES, and the maximum asymmetry near the Fermi level is plotted as a function of Cu overlayer thickness. The fitting with the probing depth () of 0.5 nm (solid line) is done for the thickness below 5 ML, while that with  = 1.4 nm (dashed line) for more than 10 ML. Also shown is the fitting result (dot line) above 5 ML with  = 4.0 nm.

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(a)

2

Φ

MCD Asymmetry (%)

1 0 -1 -2 -3 -4 -5 -6 -7 5.0

5.2

5.4

5.6

5.8

6.0

-0.35

-25

-0.30 -0.25

-20

-0.20 -15 -0.15

Calc. x1/4

-10 2PPE MCD 1PPE MCD

-5 0

(b)

-0.10

0

10

20

30

40

-0.05 50

60

70

80

Calculated MCD (%)

-30

Normalized Sample Current

MCD asymmetry (%)

Excitation Energy (eV)

0.00 90

Incident Angle (deg)

1.3

2PPE MCD

1.2 1.1 1.0 0.9 0.8 0.7 -200

(c)

-100

0

100

200

Magnetic Field(Oe)

Fig. 9. (a) Energy dependence of 2PPE MCD asymmetry on Ni(12 ML)/Cu(0 0 1) as a function of excitation energy, 2h. The work function is 5.15 eV as indicated by a dashed line. (b) Photon incident angle dependence of 2PPE MCD asymmetry (2h = 5.3 eV), together with the 1PPE result (h = 5.3 eV) with the same final state. Solid and dashed lines are calculated results by the magneto-optical theory. (c) 2PPE MCD measurement of the magnetization curve on Ni/Cu(0 0 1) at h = 2.65 eV, showing huge magnetic contrast of 70%. Figures adapted from Ref. [12] with permission from APS

2PPE MCD with h = 1.55 eV [9]. The rapid decrease in the 2PPE MCD asymmetry with the photon energy resembles the 1PPE MCD, and the asymmetries for 1PPE and 2PPE are of the same order of magnitude for the present Ni film. Similarities of the 1PPE and 2PPE MCD will be discussed later.

3.6.1. Angle dependence Light incident angle dependence of MCD asymmetry gives us hints about the physics behind MCD photoemission and the relationship between 1PPE and 2PPE. Fig. 9(b) shows incident angle dependence of the MCD asymmetry near the threshold for the 1PPE and 2PPE on Ni(12 ML)/Cu(0 0 1), and displays the comparison of magnetization curves at the incident angle of 33◦ . The photon energy for the 1PPE and 2PPE are 5.3 and 2.65 eV, respectively, while the work function is fixed at 5.15 eV. The 1PPE MCD asymmetry is nearly constant up to 60◦ , and decreases slightly above 60◦ . On the contrary, the 2PPE MCD asymmetry shows drastic increases with an inclination of the incidence angle and gives a maximum of as much as 28% at 45◦ , exceeding the 1PPE MCD asymmetry. The observed MCD is compared with the calculated results based on the magneto-optical theory [24]. The present MCD asymmetry, AM , can be expressed as AM ∼ − 2(εK R + εF T)/(1 − R − T), where εK , εF , R, and T, respectively, represent the Kerr and Faraday ellipticities, reflectivity, and transmittance [24,25]. Although usually the transmission term does not appear in absorption MCD of a magnetic film on a substrate, it should be here included since the present photoemission MCD is surface sensitive and detects dominantly the Ni 3d electrons only. Instead of using an empirical dielectric tensor of Ni to evaluate the MCD asymmetry, density functional calculations [26,27] for bulk Ni are performed, where electronic excitations below the vacuum level are omitted to account for the limited transitions in energy. The empirical dielectric constants

of Cu are used without modification. The angle dependence of the 1PPE MCD shows a typical trend of the Kerr effect, and is in good agreement with the theory except its magnitude. The angle dependence of the 2PPE MCD resembles more closely the calcula¯  = 2.6 eV rather than with h tion with h ¯  = 5.3 eV, because the MCD maximum at ∼45◦ is well reproduced. This agreement between the 2PPE results of incident angle dependence and corresponding 1PPE calculations by the magneto-optical theory suggests that the first photoexcitation process is important for the 2PPE MCD asymmetry. The treatment of 2PPE MCD in the framework of 1PPE is also successful for Co/Pt(1 1 1) [28] and Co/Cu(0 0 1) [29].

3.6.2. 2PPE MCD PEEM An application of 2PPE to MCD-PEEM is demonstrated in Fig. 10. Fig. 10(a) shows a 2PPE-PEEM image by a circularly polarized laser. A spot on the right side of the image is extremely bright, which is known as a hot spot in a strong photon radiation [30]. These hot spots are scattered on the surface with a substantial density, hindering the magnetic domain observation. The hot spot follows intensity oscillation, which also interferes the magnetic domains. This kind of the hot spots is always seen in a strong laser field, but it is missing in the one-photon process. The exclusive response to the strong laser rejects the possibility that the hot spot come from spatially distributed places with a lower work function. The hot spot originates from a structural effect with localized electromagnetic field enhancement. A MCD-PEEM image by 2PPE is shown in Fig. 10(b), which is measured at the same position as Fig. 10(a). The hot spot and oscillation as well as domains are observed in the differential image. The hot spot and oscillation depends on In , where I is laser power and n > 2. Thus the slight instability of the laser pointing or the laser power changes the intensity of the hot spot and oscillation in the

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Fig. 10. 2PPE MCD PEEM measurements on Cs/Ni(12 ML)/Cu(0 0 1) surface. (a) 2PPE microscope image of Cs/Ni/Cu(0 0 1) showing a hot spot and intensity oscillation. The beam is circularly polarized, and the photon energy is 1.55 eV. The arrow indicates the beam impact direction. The field of view is 25 ␮m. (b) A 2PPE MCD image in the same region as (a). Behind the strong oscillation and the hot spot, magnetic domains are visible. The inset shows the MCD PEEM image by one-photon photoemission (h = 3.1 eV) in the same region. (c) Contrasted 2PPE MCD image away from the hot spot. (d)1PPE MCD image on the same area as (c). Figures adapted from Ref. [9] with permission.

PEEM image much more than that of the normal 2PPE process. This makes it difficult to cancel out the hot spot and oscillation between the right and left circularly polarized beams. The differential intensity of the hot spot and oscillation in Fig. 10(b) is not an intrinsic but artificial one due to the intensity fluctuation between the images for the right and left circular polarized beams. Behind the large intensity difference, a magnetic domain structure by 2PPE-MCD are observed. This domain structure is confirmed by one photon MCD-PEEM as shown in the inset in Fig. 10(b). The one photon MCD image is free from the hot spot and its accompanying oscillation, giving a clear magnetic domain structure. The 2PPE-MCD is observable, but it is overlapped with strong hot spots. Fig. 10(c) is a contrasted image, measured away from the hot spot, which shows the magnetic domain clearly but with slight oscillating background. This background may be reduced with a better quality sample free from nanostructures or impurities. Note that the domain boundary widths for the 1PPE and 2PPE MCD PEEM are the same within the experimental error. 4. Discussion Measurements of MCD in photoelectrons using low photon energy close to the work function have recently been performed for several systems [8,28,31–34]. 1PPE measurements of MCD asymmetry are performed for Co and Fe films grown on Cu(0 0 1). The MCD asymmetries observed are at most 0.12% in in-plane magnetized 15 ML Fe/Cu(0 0 1) and 0.5% in in-plane magnetized 15 ML Co/Cu(0 0 1). The asymmetries in the in-plane magnetized films are found to be much smaller than the perpendicular magnetized ones in spite of their large thicknesses. On the other hand, a perpendicularly magnetized 3 ML Fe film on Cu(0 0 1) shows 4% asymmetry, much larger than that for the in-plane magnetized 15 ML Fe film. This observation shows again the higher asymmetry for the perpendicular magnetized film, in agreement with the magneto-optical concept.

Co/Pt(1 1 1) with perpendicular magnetization has been known to show a large magneto-optical effect in a reflection mode using visible light sources, which indicates strong spin–orbit coupling. The threshold photoemission MCD effect is measured on 5 ML Co/Pt(1 1 1) with a strained fcc structure [28]. The obtained MCD asymmetry is 10% for 2PPE, but it shows relatively reduced asymmetry, 2%, for 1PPE. Note that the electronic band structure in Co/Pt(1 1 1) does not have any real states in the conduction band along the  − L direction (normal to the (1 1 1) surface) below 6 eV. Thus, according to the ordinary photoemission theory, the photoelectrons occurs via evanescent states localized at the surface, which does not have large photoemission probability both for 1PPE and 2PPE. For the direct transition model, a theoretical band structure calculation shows much smaller asymmetry. On the other hand, the optical transition along the  − X direction gives 1 and 5% asymmetry for 1PPE and 2PPE, respectively, which is in rough agreement with the experimental results and the factor between the 2PPE and 1PPE asymmetries is well reproduced. This scenario needs additional momentum transfer for threshold electrons to escape from the surface. The indirect transition scenario may be explained by electron scattering by phonon or magnon in the near threshold photoemission. Further theoretical calculations including the scattering effects on photoelectrons are demanded to reveal the threshold photoemission and its MCD. It is reported that 2PPE studies of Co/Cu(0 0 1) show enhanced magnetic linear dichroism (MLD) as well as MCD in the valence band [32,29,35]. Strong MLD with 10% asymmetry derived from quantum well states can be observed only in two photon photoemission experiments, while MCD in 2PPE gives 4% asymmetry near the Fermi level. On the other hand, the 1PPE on Co/Cu(0 0 1) is reported to show 0.5% asymmetry, [8] one order of magnitude smaller than that by 2PPE. In 2PPE MLD, spin–orbit coupling in the initial states as well as the quantum well state plays an important role. The 2PPE MCD using intermediate states such as quantum well states, conventional band states, would enhance the dichroic

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asymmetry. Using 2PPE MCD PEEM, Tusche et al. observed magnetic domain on Co/Cu(0 0 1) at h = 3 eV. [35] They also observed magnetic domain using a spin filter by the scattering at the surface of W(0 0 1) with PEEM. This spin filtering PEEM with lasers would be of great possibilities. 5. Conclusion Near the photoemission threshold, the MCD asymmetry is large for the total electron yield without an electron energy or emission angle analyzer because of the spontaneous limitation of energy and angle (momentum) for photoelectrons. Combined with PEEM, threshold photoemission MCD enables one to observe magnetic domains in laboratories using lasers. This method has an advantage over XMCD-PEEM since one does not need third generation synchrotron radiation and can use ultra short pulse lasers for multiphoton excitation and time resolved measurements. Characteristics of threshold photoemission is its sensitivity on the magnetization axis and its probing depth. The threshold photoemission MCD is rather enhanced in perpendicularly magnetized films compared with in-plane ones, which is well explained by the conventional magneto-optic theory. Its probing depth is at least a few nm due to the long mean free path of low energy electrons. However scattering by impurities and phonon introduced by overlayers decreases the asymmetry much because the condition for the spontaneous limitation of electron momentum is broken. The application for the time resolved measurements is a next step for laser MCD PEEM, which would open a new way of spatio-temporal measurements into a femto second region. References [1] A. Hubert, R. Schäfer, Magnetic Domains, Springer, Berlin, 1998. [2] J. Stöhr, J. Magn. Magn. Mater. 200 (1999) 470. [3] H. Hopster, H. Oepen (Eds.), Magnetic microscopy of nanostructures, Springer, Berlin, 2003. [4] Z.Q. Qiu, S.D. Bader, Rev. Sci. Instrum. 71 (2000) 1243. [5] W. Kuch, A. Dittschar, K. Meinel, M. Zharnikov, C.M. Schneider, J. Kirschner, J. Henk, R. Feder, Phys. Rev. B 53 (1996) 11621. [6] W. Kuch, C.M. Schneider, Rep. Prog. Phys. 64 (2) (2001) 147, URLhttp://stacks.iop.org/0034-4885/64/i=2/a=201. [7] G.K.L. Marx, H.J. Elmers, G. Schönhense, Phys. Rev. Lett. 84 (2000) 5888–5891, doi:10.1103/PhysRevLett.84.5888, URL http://link.aps.org/ doi/10.1103/PhysRevLett.84.5888. [8] T. Nakagawa, T. Yokoyama, Phys. Rev. Lett. 96 (2006) 237402, doi:10.1103/PhysRevLett.96.237402, URL http://link.aps.org/doi/10.1103/ PhysRevLett.96.237402. [9] T. Nakagawa, T. Yokoyama, K. Watanabe, Y. Matsumoto, J. Phys.: Condens. Matter 21 (2009) 314010. [10] L.I. Chelaru, M. Horn-von Hoegen, D. Thien, F.-J. Meyer zu Heringdorf, Phys. Rev. B 73 (2006) 115416, doi:10.1103/PhysRevB.73.115416, URLhttp://link.aps.org/doi/10.1103/PhysRevB.73.115416.

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