AHE measurements of very thin films and nanosized dots

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Journal of Magnetism and Magnetic Materials 287 (2005) 320–324 www.elsevier.com/locate/jmmm

AHE measurements of very thin films and nanosized dots N. Kikuchi, R. Murillo, J.C. Lodder Systems and Materials for Information storage, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Available online 30 October 2004

Abstract In this paper we present anomalous Hall effect analysis from very thin Co (0.5 nm) film, Co/Pt multilayers and large areas of nanosized dots as well as from a few magnetic dots having a diameter of 120 nm. The dot array is prepared from Co/Pt multilayer by using laser interference lithography (LIL) while the Hall crosses for measuring a few dots are prepared in combination with focussed ion beam (FIB). The hysteresis loops from a few dots are showing significant Hall voltage jumps corresponding to magnetic response due to an inhomogeneous reversal mechanism because the intensity of the jumps is smaller than the expected value from a total magnetization reversal of one dot. r 2004 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 75.75.+a; 75.60.Jk Keywords: Anomalous Hall effect; Co/Pt multilayer; Patterned media

1. Introduction It is a challenging and important issue to understand the behaviour of nanoscale magnetic structures from both practical and scientific points of view. Arrays of nanoscale magnetic structures, so called patterned media, have been proposed as one of the solutions to overcome the superparamagnetic effect and large transition noise of thin film media. In order to realize patterned media, it is essential to investigate the switching behaviour Corresponding

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of individual dot-structure quantitatively. However, the sensitivity of standard magnetometers such as vibrating sample magnetometers (VSM) and superconducting quantum interference devices (SQUID) is not high enough to investigate details of magnetization reversal behaviour of small volumes. The resolution of magnetic force microscopy (MFM) reaches below 50 nm and allows investigation of switching behaviour of individual structures [1]. But the drawback of MFM is that information of MFM is qualitative because the signal is affected by the very complex magnetization state of the MFM tip. The sensitivity of Micro-SQUID [2] and Hall magnetometer made of semiconductor [3] is high enough to investigate the

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.052

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switching behaviour of nanoscale particles. However, there are some difficulties in the study of magnetic recording media because of the restriction of fabrication process and the working temperature. In case we have to deal with films/ dots with perpendicular anisotropy, anomalous Hall effect (AHE) measurement is one of the most suitable methods. In this paper, we will show the sensitivity and various measurements on investigation of Co/Pt multilayers and dots prepared from the continuous multilayer structure.

magnetization reversal of volumes smaller than one column/crystallite of 50 nm in diameter were observed. Recently it has been reported that AHE measurement can be adopted for the measurement of a single isolated magnetic dot [5]. AHE voltage curves of a L10–FePt dot with a diameter of 60 nm were measured in a wide range of temperature from 10 to 300 K. In this paper we will discuss AHE measurements from Co/Pt multilayers as well as small dots prepared from those multilayers by laser interference lithography (LIL).

2. Anomalous Hall effect

3. Experimental

If the magnetization is isotropic in the film plane, the Hall effects of magnetic thin films can be expressed by two terms which have different origin, normal Hall effect (NHE) and AHE. If the film plane is assumed as the x–y plane and current is applied along x-axis (I x ), Hall voltage of the film (V y ) can be written as

Co/Pt multilayers were deposited using DC magnetron sputtering with a 4 nm thick Pt buffer layer on a Si substrate with a 1000 nm thick oxidized layer. The composition of multilayers are [Pt (2 nm)/Co (0.5 nm)]n/Pt (4 nm). For n we prepared samples with n ¼ 1; 5; 20: The Ar pressure during sputtering process was kept at 1  102 mbar, and the deposition rates of Co and Pt were 0.05 and 0.13 nm/min, respectively. Dot patterns of 200 nm thick positive resist for etching mask were made on the films by using LIL [6] and the patterns were transferred into the Co/Pt multilayers by using Ar+ ion beam etching (IBE) (see also Fig. 1(a)). The etch-rate is about 3 nm/ min. The diameter and the periodicity of the dot arrays are 120 and 300 nm, respectively. After

V y ðHÞ ¼ ðRNHE H z þ RAHE M z =m0 ÞI x =t;

(1)

where RNHE and RAHE are Hall resistance of NHE and AHE, H z and M z are perpendicular components of external field and magnetization, respectively, m0 is permeability and t is film thickness. If the Hall voltage is plotted as a function of applied field, the contribution of the NHE can be easily subtracted from the AHE curve as a linear function of applied field. Therefore, relevant parameters such as coercivity (H c ), squareness ratio (S ¼ M r =M s ), anisotropy field (H k ), etc. can be obtained. One of the advantages of AHE measurement is that signal intensities are independent of film thickness if current density is kept at constant values because AHE voltage is proportional to current per thickness as written in Eq. (1). It should be also noted that AHE measurement is a powerful method not only for homogeneous films but also inhomogeneous structures, for example, segregated polycrystalline films and patterned media. One of the examples of AHE measurement of polycrystalline structure is the study on a Co–Cr 200 nm thick film patterned into a cross shape of 280 nm width [4]. In this sample, small steps of the AHE voltages corresponding to

Fig. 1. Schematic sketch of sample fabrication process. (a) The dot array is fabricated by using LIL. (b) A 4 nm thick Pt layer is deposited on the array of dots. (c) The Pt layer is patterned into cross shape by photolithography. (d) SEM picture of a dot array with a 4 mm width Pt cross structure.

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(b) Fig. 2. (a) Virgin curves and full hysteresis curves and (b) MFM images of [Co(0.5 nm)/Pt (2 nm)]n (n ¼ 1; 5; 20) continuous films. Films are demagnetized in an alternating perpendicular field before measurements.

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removal of the resist mask, a 4 nm thick Pt layer was deposited on the dot arrays by sputtering (Fig. 1(b)). After this Pt deposition, magnetically isolated dots have conductivity through the platinum layer. There are two requirements for this Pt deposition process from different points of view. One is that the platinum layer has to make good electric contacts with dots. This requirement can be realized by depositing a relatively thick platinum layer. The other is that the conductivity of platinum part should be kept as low as possible to increase current density in magnetic dots. The intensity of the anomalous Hall voltage originated by magnetic dots depends on the relative conductivity of electrode and magnetic dots [5] and lower conductivity materials are desirable to increase the intensity. After deposition of the Pt layer, cross patterns are fabricated by means of photolithography and IBE. Because the substrate is SiO2, the applied current can only flow through the Pt cross and the most of AHE signals are originated by the dots inside the cross area. In one of our previous studies on CoCr AHE structure it was reported that at least 70% of the signal is due to the material in the cross-centre area [4]. Fig. 1(d) is a scanning electron microscope (SEM) picture of a dot array of 120 nm diameter and a 4 mm wide Pt cross structure. This sample has about 200 dots, which contribute to AHE signal. All AHE measurements are carried out with 833 Hz AC current of 10 mA.

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4. Results and discussion 4.1. Continuous film In Fig. 2(a) an AHE virgin curve and full hysteresis curve are shown and in Fig. 2(b) an MFM image of a demagnetized state of Co/Pt multilayers is shown. Before the virgin curve and the MFM measurements are obtained, the film has been demagnetized in an alternating perpendicular field. Even the film with 0.5 nm thickness of Co shows a clear hysteresis with high accuracy due to large AHE signal in Co/Pt multilayer. From the MFM figures, it can be seen that the domain size decreases rapidly with increasing the number of

Fig. 3. Initial and hysteresis AHE voltage curves of a dot array of [Co(0.5 nm)/Pt (2 nm)]5 with 4 mm width Pt Hall cross. The sample is demagnetized in an alternating perpendicular field for initial curve measurements.

bilayers. Moreover, the domain size becomes uniform in the thicker films. 4.2. Patterned film (LIL) Fig. 3(a) shows the virgin curve and full hysteresis curve of a patterned film (n ¼ 5) obtained by AHE measurement. The sample has dots of 120 nm in diameter and a 4 mm wide cross electrode and about 200 dots in the cross area. In

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4.3. AHE crosses made with focussed ion beam (FIB) In order to reduce the number of dots contributing to the AHE signal, we made smaller Hall cross in combination with FIB. Before the FIB process, a dot array of [Co (0.5 nm)/Pt (2 nm)]5 with 4 mm width Pt Hall cross was made by using the same procedure mentioned above. The schematic of FIB process were shown in Fig. 4(a). Four grooves of 50 nm width were made from the corners of the cross toward the centre and the grooves were stopped just before that point. Fig. 4(b) shows a SEM picture around the centre of cross structure made by FIB. We confirmed that the dots in the grooves are also cut and disconnected. It can be seen that there are only six dots in the cross centre. Fig. 5(a) is a Hall voltage hysteresis loop of the sample shown in Fig. 4(b).

Fig. 4. (a) Schematic figure of FIB process and (b) SEM picture of FIB sample. There are six dots in the cross area.

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this geometry, the magnetostatic field from neighbour dots is at most 1 kA/m and can be negligible. The grain size of the film is evaluated as 10 nm from AFM measurement. The fact means that each dot consists of a few hundreds of grains. The sample was demagnetized in an alternating perpendicular field before measurements. It should be noted that a hysteresis curve of such a small number of dots is measured with high accuracy. After patterning, the coercivity increases from 40 to 105 kA/m (see also Fig. 2(a)). But the value of H c is only 10% of anisotropy field H k determined from in-plane VSM measurement. The switching field is dispersed in a wide range from 70 to 140 kA/m even from such a small number of dots.

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Fig. 5. (a) Major hysteresis loop and (b) minor loop of the FIB sample. The reproducible large steps of Hall voltage are observed around 20 and 75 kA/m.

Several significant steps of AHE voltage are found in the loop. The inserted figure of Fig. 5(a) is a part of the loop and two large steps are clearly observed. Fig. 5(b) is a minor loop of the same sample in field range from 45 to 45 kA/m. Around 20 kA/m, significant large steps are observed in both upward and downward parts of the loop. We found that the steps are reproducible and it proves that they are not related to noise but originated by magnetic responses. Moreover, the steps are observed at the same field in major and minor loops. From our studies on CoCr Hall bar, it was shown that 70% of the AH voltage is originated from the cross area [4]. If we assume that in the case of our dot sample (see Fig. 4(b)) is 50%, then the expected AH voltage obtained from a magnetization reversal of a whole dot becomes more than 8% of full scale. The largest steps

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observed in Fig. 5(a), however, is less than 5% of full scale. From these results, we conclude that the inhomogeneous magnetization reversal occurs in the dots. From micro-magnetic simulation, it is predicted that the critical diameter of between single- and multidomain states is around 120 nm for the case of a Co/Pt dot. Moreover, inhomogeneity of the multilayer and dot structure during deposition and etching may cause such gradual magnetization process in a dot.

tained Hall voltage steps, we conclude that the inhomogeneous magnetization reversal occurs in the dots. For further investigation it would be necessary to reduce the number of dots in cross centre as well as the effects from surrounding area. We are also planning to measure one dot of the presented material with different diameters.

References 5. Summary In summary, we have fabricated Co/Pt multilayers and dot arrays with 120 nm diameter of those films. Their magnetic properties were investigated by measuring AHE. The hysteresis of very thin films and a small number of dots were presented with very high accuracy. By using focused ion beam the number of dots in the Hall cross were reduced and significant AH voltage steps corresponding to magnetic response were successfully detected. From the comparison of expected and experimentally ob-

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