Fe

A study on exchange coupled structures of Fe/NiO and NiO/Fe interfaced with n- and p-silicon substrates Neelabh Srivastava and P. C. Srivastava Citation: J. Appl. Phys. 111, 123909 (2012); doi: 10.1063/1.4729857 View online: http://dx.doi.org/10.1063/1.4729857 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i12 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 111, 123909 (2012)

A study on exchange coupled structures of Fe/NiO and NiO/Fe interfaced with n- and p-silicon substrates Neelabh Srivastava and P. C. Srivastavaa) Department of Physics, Banaras Hindu University, Varanasi-221005 (U.P.), India

(Received 20 January 2012; accepted 21 May 2012; published online 21 June 2012) Interfacial structures of ferromagnetic (FM)/antiferromagnetic (AF) (Fe/NiO) and AF/FM (NiO/Fe) on n- and p-Si substrates have been realized by sequential deposition of FM and AF layers on the silicon substrates by electron beam evaporation technique. The structures have been characterized from x-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM), and M-H characteristics. It has been found that there is a strong interfacial intermixing to form the various oxide and silicide phases of Fe2O3, b-Fe2O3, b00 -Fe2O3, NiSi, Ni3Si, and Fe5Si3. AFM micrographs show the granular morphology of the top layer of the structure, with a large grain size of 400 nm, however, the XRD data show the crystallite size of 20 to 70 nm. It seems that the crystallites are clustered to form larger grains. MFM features show a large domain size corresponding to AFM grain size for Fe/NiO/Si structure and very small domain of nanometer size for NiO/Fe/Si structure (having NiO as a top layer). M-H characteristics show that the magnetic behavior is only significant for Fe/NiO/nSi structure with a significant coercivity and exchange bias than for all other interfacial structures of Fe/NiO/pSi, NiO/Fe/pSi, and NiO/Fe/nSi. Thus, it has been found that Fe/NiO/nSi structure can be used in magneto-electronic device applications. It seems that the observed result of significant exchange bias and coercivity is due to the microstructural and chemical structure changes in the antiferromagnetic layer along with the roughness (data as obtained C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729857] from AFM). V

I. INTRODUCTION

Ferromagnetic (FM) films grown on antiferromagnetic (AF) oxides or vice versa (i.e., AF on FM) having a semiconductor substrate is of significant interest for fundamental studies of exchange biased systems along with their possible technological applications in “Spintronics” (magneto-electronics). The arrangement of spins at the interface in layered magnetic materials often play a very crucial role for the understanding of their magnetic properties and has profound consequences for practical applications. Particularly important is the unidirectional exchange coupling between the spins in an antiferromagnet (AF) and ferromagnet (FM), referred as exchange bias.1 Exchange bias phenomenon, i.e., shift of the hysteresis loop along the field axis can be described in terms of an alignment of the AF spins at the FM–AF interface parallel to the FM spins. This shift, known as the exchange bias (EB) effect, has been linked to uncompensated interfacial spins that are pinned in the AF and are not affected by the external field. The coupling between the AF and FM spins at the interface exerts an additional torque on the FM spins, which the external field has to overcome. The exchange bias effect represents an important aspect of modern magnetism and presents interesting applications.2,3 Despite extensive studies,4–6 the microscopic understanding of exchange bias has been hindered up to now by the lack of information about the structural and chemical rearrangements

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: þ91 9839234757. Fax: þ91 542 2369889.

0021-8979/2012/111(12)/123909/9/$30.00

that may occur at the interface and which may influence the interface magnetic structure. NiO can be selected as a suitable antiferromagnetic material due to its several advantages such as, no current shunting, excellent thermal stability, strong resistance to corrosion, and high Neel temperature (TN 523 K, for bulk NiO, but lower for thin films).7 Moreover, NiO is an antiferromagnetic material with a density of 6.67 g/cm3 and having a cubic structure. Bulk nickel oxide (NiO) has a very high resistivity and high melting point (2000 C) suggesting that it can be used in high temperature applications. In this study, we have taken Fe and NiO as a FM and AF materials, respectively, which has been interfaced with n- and p-Si substrates. Among the several exchange biased interfacial structures composed by a FM metal and AF oxides, the Fe/NiO interface has been one of the most studied system. In exchange coupled structure (i.e., FM/AF or AF/FM), NiO plays a major role as the AF partner due to the larger anisotropy (k 2.8 105 J/m3) and the low lattice mismatch with respect to Fe and Fe alloys.8 In the present investigation, we have used x-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM), and magnetization (i.e., M-H) characteristics to investigate the properties of Fe/NiO/Si and NiO/Fe/Si interfacial structures on both n- and p-Si substrates. MFM and M-H characteristics have shown the significant magnetic behavior for Fe/NiO/nSi interfacial structure than for its interface on p-Si substrate (i.e., Fe/NiO/pSi structure). Moreover, the reverse interfaces, i.e., NiO/Fe on n- and p-silicon substrates also do not show any perceptible coercivity and exchange bias. The observed magnetic behavior of FM/AF, i.e., Fe/NiO and of the reverse interface, i.e., NiO/Fe on

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C 2012 American Institute of Physics V

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n- and p-silicon substrates is quite intriguing and interesting. From the present study, it has been found that the bilayer of Fe/NiO interfaced with n-Si substrate has significant magnetic property for its application in magneto-electronics. The results have been understood in the realm of the interfacial chemistry of Fe/NiO (and NiO/Fe) and with n- and p-silicon substrates to result in interfacial microstructural and chemical structure changes. A full understanding of the coupling requires consideration of the true interface structure including interface mixing, roughness and structural relaxation, and how these properties affect the exchange and magnetostatic interactions across the interface. II. EXPERIMENTAL DETAILS

The n-Si and p-Si h100i wafers of resistivity 8–10 X-cm have been used for the realization of the Fe/NiO/Si (FM/AF/ Si) and NiO/Fe/Si (AF/FM/Si) interfacial structures. Bilayers of Fe/NiO and NiO/Fe films have been deposited on chemically etched and cleaned Si wafers by sequential deposition of NiO (thickness of 50 nm) and Fe (thickness of 50 nm) using electron beam evaporation technique. These realized interfacial structures have been characterized from x-ray diffraction (XRD, Philips PW-1710 diffractometer), atomic force microscopy (AFM, Digital Instruments Nanoscope IIIa), magnetic force microscopy (MFM, Digital Instruments Nanoscope IIIa), and vibrating sample magnetometer (VSM, EV7 ADE technologies) facilities. The magnetization (M) versus magnetic field (H) measurement was performed for both the field orientations, i.e., for in plane field (magnetic field applied along the plane of the interface, k) and out of plane magnetic field applied perpendicular to the plane of the interface, \. III. RESULTS AND DISCUSSION A. XRD study

To investigate the chemical phases formed as a result of interfacial intermixing due to the chemical reactivity of constituent layers, XRD pattern have been recorded. Figures 1 and 2

FIG. 2. XRD pattern of Fe/NiO and NiO/Fe interfacial structure on p-Si substrate.

show the XRD pattern of the interfacial structure of Fe/NiO and NiO/Fe, realized on n-Si and p-Si substrates, respectively. The observed diffraction peaks have been identified from JCPDS card data (as tabulated in Tables I and II). From Figures 1 and 2, it is clear that the observed diffraction peak at 2h 69 can be assigned to the h400i diffraction plane of Si substrate, which has been observed for all the structures. The other observed diffraction peaks at 2h 38 , 44 and 79 are also common to all the structures, and these diffraction peaks correspond to various phases of oxides and silicides of Fe2O3, b-Fe2O3, NiSi, Ni3Si, and Fe5Si3 (Tables I and II). Moreover, there is an additional peak at 2h 33 (for the interfaces on n-Si substrate, Figure 1 and Table I), which corresponds to the b00 -Fe2O3 h1011i/Fe5Si3 h111i phases. These peaks have not been observed for p-Si substrates (Figure 2 and Table II). The formation of the observed phases of oxides (i.e., Fe2O3 phase) in case of Fe/NiO/Si and NiO/Fe/Si interfacial structures can be understood due to the diffusion of oxygen atoms from the NiO towards Fe. Similar effects have also been earlier reported by Luches et al.,9 where they suggested that Fe-oxidation and NiO reduction at the interface are expected because of the larger Gibbs free energy of formation of FeO (58.1 kJ/mol) with respect to NiO (50.6 kJ/ mol). The formation of FeO-like phase on NiO is also favored by their similar lattice parameters of 2.14 and ˚ , respectively, which allows the epitaxial arrangement 2.08 A of the observed bct Fe-Ni metallic phase. The presence of such oxide phases has also been confirmed from our x-ray photoelectron spectroscopy (XPS) data (not reported here). The observation of silicide phases can be understood due to the room temperature chemical reactivity to result in interfacial intermixing of the constituent layers. Formation of nickel silicide phases can also due to the fact that nickel (Ni) is an extremely fast diffusing10 species in silicon. B. MFM/AFM study

FIG. 1. XRD pattern of Fe/NiO and NiO/Fe interfacial structure on n-Si substrate.

Magnetic force microscopy (MFM) has become a standard diagnostic tool in understanding surface magnetism. In

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TABLE I. XRD data of the Fe/NiO/nSi and NiO/Fe/nSi interfacial structure. Angle (2h)

˚) d-value (A

Peak width (2h)

Identified possible phases

1. Fe/NiO/nSi

32.6811 38.3308 44.5853 68.8499 69.0750 78.1318

2.74017 2.34830 2.03233 1.36257 1.36206 1.22228

0.1476 0.2952 0.2460 0.1800 0.1200 0.7200

b00 -Fe2O3h1011i/Fe5Si3h111i b-Fe2O3h400i/Fe5Si3h002i/SiO2h213i 220i/NiSih210i/FeNi3h111i Fe2O3h410i/Nih111i/Ni3Sih 512i/Fe5Si3h224i NiSih122i/b-Fe2O3h444i/Ni3Sih NiSih122i/Sih400i Ni3Sih420i/NiSih410i

2. NiO/Fe/nSi

32.6851 38.3576 44.5958 68.8720 78.1432

2.73985 2.34672 2.03187 1.36332 1.22213

0.0984 0.1968 0.2460 0.1476 0.7200

b00 -Fe2O3h1011i/Fe5Si3h111i b-Fe2O3h400i/Fe5Si3h002i/SiO2h213i 220i/NiSih210i/FeNi3h111i Fe2O3h410i/Nih111i/Ni3Sih NiSih122i/Sih400i Ni3Sih420i/NiSih410i

Sample description

its basic implementation, technique maps an image which is proportional to the local magnetostatic force gradient between a ferromagnetic sample and a magnetic probe. In an ideal case of a magnetic dipole probe, the force gradient can be expressed as:11 3 X @ m @2 F ðx; yÞ ¼ l0 mi 2 Hi ðx; yÞ; @z @z i¼1

(1)

BÞ. where Fm is the gradient of the magnetostatic energy,ðm: The image thus depends upon the direction of the probe’s magnetic moment and contains the contribution of the different components of the surface stray field. It allows the topographic and magnetic force gradient images to be collected separately and simultaneously in the same area of the sample by using tapping/lift modes. The magnetic tips used were fabricated with a pyramidal tip coated with magnetic thin film. In practice, it is customary to premagnetize the probe along the surface normal direction, ^z, which makes the contrast proportional to the second derivative of the normal magnetic field component. For the mode of dynamic detection, the cantilever is vibrated at its resonant frequency, f0 , and phase, /, which will be modulated by the magnetic forces exerted on the tip from the stray field, H, emerged from the magnetic structures

in the sample surface layer while the tip is scanning. In fact, for the phase detection, the phase shift, D/, can be written as: 0

D/ ¼

Q:F ; k

(2)

where F0 is the vertical (z) component of the force gradient, k and Q are the spring constant and quality factor of the cantilever, respectively. Imaging was performed with Digital Instruments Nanoscope IIIa. The MFM tips were of the commercially available MESP type coated with 50 nm of ferromagnetic CoCr alloy and magnetized in the z-direction parallel to the tip axis. MFM measurements were taken in the lift height mode, where topography and magnetic field are probed in alternating passes of the tip. During a MFM pass, the tip-to-sample’s surface distance is held constant at a predetermined lift height to minimize influence from topography. The tip responds to changes in the magnetic force with a phase shift in the cantilever oscillation,12 where a positive shift results from a repulsive interaction and negative shift from attractive interaction. The lift height was kept at 50 nm during MFM scans. The magnetic force gradient was measured in the frequency shift mode. In order to analyse the microscopic details of the magnetic structure, such as the size and the arrangement of magnetic

TABLE II. XRD data of the Fe/NiO/pSi and NiO/Fe/pSi interfacial structure. Angle (2h)

˚) d-value (A

Peak width (2h)

Identified possible phases

1. Fe/NiO/pSi

38.3300 44.5837 68.8903 69.1250 78.1047

2.34834 2.03239 1.36187 1.36120 1.22264

0.2460 0.2460 0.1800 0.1200 0.6000

b-Fe2O3h400i/Fe5Si3h002i/SiO2h213i Fe2O3h410i/Nih111i/Ni3Sih 220i/NiSih210i/FeNi3h111i 512i/Fe5Si3h224i NiSih122i/b-Fe2O3h444i/Ni3Sih Sih400i/NiSih122i Ni3Sih420i/NiSih410i

2. NiO/Fe/pSi

16.3788 24.9402 38.3369 44.5986 68.6157 68.8250 78.1099

5.41214 3.57031 2.34794 2.03175 1.36665 1.36639 1.22257

0.5904 0.7872 0.2460 0.2460 0.1800 0.1200 0.4800

Cello tape Cello tape b-Fe2O3h400i/Fe5Si3h002i/SiO2h213i 220i/NiSih210i/FeNi3h111i Fe2O3h410i/Nih111i/Ni3Sih 512i/b-Fe2O3h444i NiSih104i/NiSih122i/Ni3Sih NiSih104i/Sih400i Ni3Sih420i/NiSih410i

Sample description

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domains, AFM/MFM measurements have been applied. All AFM and MFM images taken simultaneously on the same area are obtained in a consecutive scans. Figures 3(a) and 3(b) show the AFM and the corresponding MFM image of the Fe/NiO bilayer interfaced with n-type silicon substrate. The images were scanned over a scan area of 2 2 lm2. Surface topography (i.e., AFM image) shows a grainy structure having grains of 400 nm. The line scan sectional analysis of the AFM image is shown in Figure 3(a0 ), which also shows the grain size of 453 nm. The bigger grains seem to be due to the clustering of grains. The rms roughness (RMS) data were estimated from the software attached with the AFM facility and were found to be of 48 nm (Figure 3(a)). The corresponding MFM image is shown in Figure 3(b), which also shows similar clusters. The resemblance between topography and MFM image is very close. In order to rule out the possibility of topographical interference in MFM images, lift height was further increased from 50 nm to 80 nm, and the corresponding MFM image is shown in Figure 3(c). It has been observed that the features are still

J. Appl. Phys. 111, 123909 (2012)

similar for the increased lift height of 80 nm, which confirms that the feature observed in MFM is exclusive of AFM image. The line scan sectional analysis, which is a line scan of the magnetic image is shown in Figures 3(b0 ) and 3(c0 ) for the lift height of 50 nm and 80 nm, respectively. The line scan of the magnetic image illustrates the magnetic contrast in a better way. It shows the positive and negative phase shift along the scanned line on the image. The magnitude of phase change shows the strength of the magnetic signals. For a lift height of 50 nm, the sectional analysis shows a phase shift of 0.63 (shown in Figure 3(b0 )) as compared to the image for the lift height of 80 nm, which shows a phase shift of 0.22 (shown in Figure 3(c0 )). This can be easily understood due to the reduction of the magnetic signal on increasing surface-to-tip distance, i.e., lift height. Moreover, the domain size measured from the line scan analysis (from the positive and negative phase shift) is in range of 200-400 nm, which is similar to the size of the grains observed in AFM image (Figure 3(a)). Thus, it seems that the observed magnetic structure consist of single-domain particles or it looks like

FIG. 3. (a) Surface morphology AFM image of Fe/NiO/nSi interfacial structure and (b) corresponding MFM image of (a) for a lift height 50 nm. (a0 ) The line scan sectional analysis of the AFM image (a). (b0 ) The line scan sectional analysis of the magnetic image (b). (c) MFM image of (a) for a lift height 80 nm. (c0 ) The line scan sectional analysis of the magnetic image (c).

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that as if the grains were of single-domain in nature. However, the magnetization data (discussed in Sec. III C) show that the domains can be classified as pseudo-single domain (PSD), which is a mixture of single domain (SD) like and multi-domain (MD) like behavior. Similar study has also been performed for the reverse structure, i.e., NiO/Fe bilayer interfaced with n-type silicon substrate. Figures 4(a) and 4(b) show its AFM and the corresponding MFM image, respectively. The line scan sectional analysis of the images is shown in Figures 4(a0 ) and 4(b0 ), respectively. In this case, there is no resemblance between AFM and MFM images. AFM image shows the feature of aligned grains having a size distribution from 50 nm to 200 nm. The grains are closely spaced with their boundary resolved. The line scan sectional analysis of the AFM image is shown in Figure 4(a0 ), which also shows the size distribution in the range of 50 nm to 250 nm. The rms roughness was estimated from AFM data, and it was found to be 24 nm (Figure 4(a)). The MFM image (see Figure 4(b)) shows a fine domain structure and its corresponding line scan analysis (Figure 4(b0 )) also shows the similar feature. From the smallest period of positive to negative phase shift, the estimated domain size is of 10 nm. Moreover, the observed magnetic signal from the magnetic structure is weak and shows a phase shift of 0.12 .

J. Appl. Phys. 111, 123909 (2012)

Thus, it is observed that the Fe/NiO/nSi structure shows a stronger magnetic signal as compared to NiO/Fe/nSi structure. Moreover, it is also found that there are larger magnetic regions having single domain configuration for Fe/NiO/nSi structure, whereas there is no such feature for NiO/Fe/nSi structure. The surface roughness has been observed to decrease for NiO/Fe/nSi than for Fe/NiO/nSi structure. The features have been further investigated from M-H characteristics of these structures in next section. C. Magnetization study

Magnetic hysteresis loops (i.e., M-H characteristics) were measured for the interfacial structures from the vibrating sample magnetometer facility at room temperature by sweeping the applied field from 1.75 kOe to 1.75 kOe and back to 1.75 kOe. Figures 5 and 6 show the M-H characteristics for Fe/NiO/nSi and Fe/NiO/pSi interfacial structures, respectively, which has been corrected for the diamagnetic contribution of the silicon (Si) substrate. The magnetic behavior of these structures is observed to be significantly different. The M-H characteristics have been measured for both the applied magnetic fields, i.e., along the plane of the interface and across the interface. The magnetic parameters have been estimated for both the magnetization directions

FIG. 4. (a) Surface morphology AFM image of NiO/Fe/nSi interfacial structure and (b) corresponding MFM image of (a) for a lift height 50 nm. (a0 ) The line scan sectional analysis of the AFM image (a). (b0 ) The line scan sectional analysis of the magnetic image (b).

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FIG. 5. M-H characteristics of Fe/NiO/nSi interfacial structure for both applied magnetic field orientations (i.e., parallel and perpendicular to the interfacial plane).

(i.e., for in plane and out of plane) from the M-H characteristics and are tabulated in Table III. The data (Table III) clearly show that the coercive field (Hc of 90 Oe) is significantly higher for Fe/NiO/nSi interfacial structure than for Fe/NiO/pSi structure (for which Hc is 40 Oe). Moreover, the FM/AF bilayer of Fe/NiO interface with n-Si substrate shows a significant negative exchange bias field (Hex) of 38 Oe with an upward vertical shift on the magnetization axis as compared to its interface on p-Si substrate, which practically do not show any exchange bias for in plane magnetization. The out of plane magnetization data show the same coercivity (Hc) of 40 Oe as for in plane magnetization and a weak exchange bias field (Hex) 10 Oe for Fe/NiO/pSi interfacial structure, whereas the Hc and Hex are practically negligible for out of plane magnetization for Fe/NiO/nSi interfacial structure. Thus, it is observed that the magnetic property of FM/AF (i.e., Fe/NiO) interface with n-Si substrate has a significant in plane magnetic behavior, which can be used for magneto-electronic devices. The

FIG. 6. M-H characteristics of Fe/NiO/pSi interfacial structure for both applied magnetic field orientations (i.e., parallel and perpendicular to the interfacial plane).

J. Appl. Phys. 111, 123909 (2012)

observed magnetic behavior with a significant exchange bias (Hex) is useful for magnetic recording devices, where it is used to pin the state of readback heads of hard disk drives. The exchange bias phenomenon is currently being employed in giant magnetoresistive spin valves.13 It is also useful for magnetic random access memories (MRAMs) devices in which a magnetoresistive device is integrated with a silicon based selection matrix.14 Thus, it seems that n-Si substrate for the FM/AF bilayers should be preferable for such applications. However, it is also found that the interfacial structure of Fe/NiO (i.e., FM/AF) on p-Si substrate is not so useful for such applications. To investigate the role of reversing FM/AF layer, i.e., AF/FM layer to be interfaced with n-type and p-type silicon substrate, the magnetic properties of NiO/Fe/Si structures have also been studied. Figures 7 and 8 show the M-H characteristics of NiO/Fe/nSi and NiO/Fe/pSi interfacial structures, respectively. The coercivity and exchange bias for NiO/Fe/nSi interfacial structure are very small (Table III), whereas there is not so perceptible coercivity and exchange bias observed for NiO/Fe/pSi structure. It is interesting and intriguing to observe that on reversing the FM and AF layer, i.e., from Fe/NiO to NiO/Fe on n-Si and p-Si substrates, it does not show any significant coercivity and exchange bias. Thus, the observed magnetic behavior of the reverse interfacial structure on Si substrate (i.e., AF/FM/Si structure) has been found insignificant. The reverse interface is characterized by a different dynamics of the oxidation/reduction chemical processes occurring at the oxide/metal interface and/or also by different physical properties. The XRD results of the structures have shown the presence of oxides and silicide phases. In Fe/NiO system, it has been shown from the ab-initio study15 that Fe atoms adsorb preferentially on O sites and that a chemical reduction of NiO occurs, giving rise to adsorption of more than one monolayer. Moreover, it has also been shown that an intermediate interface region is formed where different reactions take place, leading to partially oxidized and reduced layers in the metal and the oxide sublattice, respectively. These phenomena are now widely recognized to be one of the main sources of the uncompensated magnetic moments in this system. It has also been shown from the experimental studies16 that the coupling of Fe on top of NiO layer is collinear, i.e., the Fe moments are either parallel or antiparallel to the Ni moments, whereas for the reverse system, i.e., NiO on top of Fe layer, there is a perpendicular coupling.17 Thus, the observed magnetic property for the interfacial structures (i.e., Fe/NiO and NiO/Fe) on Si substrates can be understood as the contribution received from the exchange coupled bilayers of Fe/NiO, NiO/Fe and the interfacial chemistry of Fe/NiO, NiO/Si (for Fe/NiO/Si), and Fe/Si (for NiO/Fe/Si) layers. The observed large coercive field along with a significant exchange bias field for Fe/NiO/nSi interfacial structure as compared to Fe/NiO/pSi structure may also be due to the electron carriers of n-type as compared to the hole carriers of p-type semiconductor. It has already been shown by Dung et al.,18 from first principles calculations using the allelectron full-potential linearized augmented plane wave

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TABLE III. Magnetic property data of Fe/NiO and NiO/Fe interfacial structures on n-Si and p-Si substrates. Magnetic field orientation In plane field (0 )

Out of plane field (90 )

Magnetic parameters

Fe/NiO/nSi

Fe/NiO/pSi

4

4

NiO/Fe/nSi 4

Ms Hc Hex Mr Vertical shift

1.1 10 emu 90 Oe 38 Oe 4.7 106 emu 2.5 106 emu

3.4 10 emu 41.5 Oe 1.5 Oe 2.2 105 emu 2.0 106 emu

2.7 10 emu 12.5 Oe 7.5 Oe 4.0 106 emu 4.5 106 emu

Squareness

0.042

0.065

0.015

4

4

Ms Hc Hex Mr Vertical shift

1.2 10

emu

Not perceptible

Squareness

2.8 10 emu 39 Oe 11 Oe 3.8 106 emu 0.3 106 emu

2.4 104 emu 14 Oe 2 Oe 3.1 106 emu 2.3 106 emu

0.014

0.013

NiO/Fe/pSi 0.6 104 emu

Not perceptible

3.5 105 emu

Not perceptible

method, that the addition of an electron carrier strengthens the ferromagnetic coupling while the hole carrier causes to weaken it. Moreover, since, n-type silicon is more reactive than p-type Si substrate19 which shall also cause a different interfacial chemical structure and microstructure to influence the magnetic behavior for n-type Si substrate than for p-type Si substrate. The roughness data (as obtained from AFM) can also be correlated with the magnetic property data, since exchange bias (Hex) depends strongly on the spin structure at the interface between FM and AF materials. The magnitude of the Hex is a combination of spin orientation, roughness, and crystallinity. It is observed from the AFM data that the rms value of roughness (48 nm) for Fe/NiO/nSi interfacial structure (Figure 3(a)) is larger than (12 nm) for Fe/NiO/ pSi structure. The increased roughness for the interface of Fe/NiO (i.e., FM/AF) on n-Si substrate may also explain the increased coercivity (Hc) and exchange bias (Hex). Moreover, it has also been observed that the rms value of roughness is larger for Fe/NiO/Si interfacial structure as compared to NiO/Fe/Si interfacial structure for which a small coercivity has been observed. It is believed that the roughness data

might influence the interfacial interaction by causing local spin dispersion that results in additional local anisotropies. This competes with the demagnetization field to serve as an energy barrier to domain wall motion leading to the increased coercivity.20 Apart from this, spin arrangement at the interface and higher order anisotropies which come from the exchange interaction between AF and FM layers are also found to have a strong influence on coercivity.21 It has also been suggested earlier22 that coercivity enhancement can be due to the domain wall pinning at centers created by interface roughness in the ferromagnet. Interestingly, the largest exchange bias effects at room temperature has been observed for polycrystalline granular films having significant roughness which lead to both structural and magnetic disorder.23 The hysteresis loop also shows a vertical shift on the magnetization axis for both the orientations. If the magnetization of the antiferromagnet is irreversible under field reversal, it can be identified as a vertical shift of the measured hysteresis loop. The presence of vertical shift is related to the presence of pinned interfacial spins.24 The number of pinned spins is a measure of the vertical shift. A positive

FIG. 7. M-H characteristics of NiO/Fe/nSi interfacial structure for both applied magnetic field orientations (i.e., parallel and perpendicular to the interfacial plane).

FIG. 8. M-H characteristics of NiO/Fe/pSi interfacial structure for both applied magnetic field orientations (i.e., parallel and perpendicular to the interfacial plane).

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(upward) vertical shift can be attributed to a ferromagnetic (FM) interface coupling, which indicates a positive interaction between the two types of spins across the interface and a negative (downward) vertical shift for an antiferromagnetic (AF) interface coupling.25 Mr/Ms data (or squareness of the M-H characteristics, Table III) show the formation of pseudo-single domain (PSD) and multi-domain (MD) like features. It has been shown by Day et al.26 that the Mr/Ms 0.5 represents pseudo-single domain configuration, whereas Mr/Ms < 0.05 represent a multidomain behavior. For nanogranular system, the suppression of Mr/Ms ratio is directly proportional to the volume fraction of the superparamagnetic (SP) particles because Mr for SP grains is nearly zero. Moreover, the coercivity remains unchanged which is entirely due to the non-SP fractions. PSD (pseudosingle domain) is understood to be due to the SD (single domain) and MD (multi-domain) particles. In the situation, when the population of SD particles is so large as to influence one another, there will be an effect of particle interaction to suppress Mr/Ms ratio to the PSD limit. Moreover, from Table III, it can also be inferred that the exchange bias (Hex) and coercivity (Hc) decrease inversely with the saturation magnetization (Ms) of the interfacial structure (of Fe/NiO/Si); Hc or Hex a ðMs Þ1 : This observation has been suitably explained4 by the partial domain wall (PDW) model. In the case of Fe/NiO/nSi interfacial structure, NiO layer reacts strongly with silicon substrate to result in microstructural and chemical changes as compared to its interface on pSi substrate (because p-type Si is less reactive than n-type Si) (Ref. 19) and the reverse interface of NiO/Fe/Si, where NiO is not in direct contact to silicon substrates. On reacting with silicon substrate, there is a intermixing of silicon and antiferromagnetic layer of NiO, which shall result in incorporation of silicon atoms and structural defects in the NiO layer. It has been shown from Monte-Carlo simulations27 that the presence of structural defects and non-magnetic defects in volume part of the antiferromagnetic layer can enhance the exchange bias coupling. Thus, from our observation for the Fe/NiO/Si and NiO/Fe/Si interfacial structures, it seems that the observed significant exchange bias and coercivity is influenced by the microstructural and chemical structure changes in the antiferromagnetic layer. Similar feature has been discussed in a model proposed by Beschoten et al.27 for the exchange bias phenomenon in FM/AF structures. Thus, our study shows the significant role of the interfacial structural modifications in the antiferromagnetic layer for the exchange bias phenomenon. O’Grady et al. have recently reviewed the models for the exchange bias in FM/AF bilayer system.23 The study shall be in progress for investigating the effect of temperature, role of defects (by intentionally introducing the defects), and ageing effect on exchange bias of FM/AF bilayer on Si substrates. IV. CONCLUSIONS

In summary, we have studied the interfacial structures of Fe/NiO (FM/AF) and its reverse interface i.e., NiO/Fe

J. Appl. Phys. 111, 123909 (2012)

(AF/FM) on p- and n-silicon substrates. XRD patterns have shown the formation of various magnetic phases of oxides and silicides. Morphological studies have shown the granular like morphology and larger roughness for Fe/NiO/nSi interfacial structure. MFM features have shown the singleparticle domain like configuration and larger magnetic signal for Fe/NiO/nSi interfacial structure, whereas for NiO/Fe/Si interfacial structure, very fine domain structure (of few nanometer sized) has been observed. Magnetic property measurement has shown the significant magnetic behaviour, such as coercivity (Hc) and exchange bias (Hex) for Fe/NiO/nSi structure as compared to all other structures. It has been found from the study that the interfacial structure of Fe/NiO on n-Si substrate is more relevant for the application in “magneto-electronics” (Spintronics). The roughness, interfacial microstructure, and chemical structure seem to play significant role to control the magnetic behavior of the interfaces. ACKNOWLEDGMENTS

The authors are thankful to Professor O. N. Srivastava (Emeritus Professor, Department of Physics, Banaras Hindu University, Varanasi, India) for providing the XRD facility. The authors are also grateful to Dr. Indra Sulania (Scientist ‘D,’ Inter University Accelerator centre, New Delhi, India) for performing the AFM/MFM measurements. Thanks are also due to Mr. M. Siva Kumar (Advanced Center for Materials Science, IIT Kanpur, India) for performing the magnetization measurements. One of the authors (N.S.) also wants to acknowledge the financial support received from University Grants Commission (UGC), New Delhi, India in the form of meritorious (UGC-RFSMS) fellowship. 1

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