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1. Introduction&lt;br/&gt;Ultra-high sensitivity and micro-sized magnetic sensors are required in biomagnetic measuring devices. The GigaHertz spin rotation sensor, which is an evolution of the magneto-impedance sensor, has shown high sensitivity while transmitting GHz pulse currents [1]. Detection magnetic fields are sensed via a pickup coil wrapped around the wire when the GHz pulse currents are applied. However, the micro-mechanism in the amorphous wire, which exhibits high sensitivity, has not been characterized. Thus, we investigated the relationship between peak voltages and magnetic fields and the magnetization behavior inside the amorphous wire.&lt;br/&gt;2. Calculation method and conditions&lt;br/&gt;The magnetization behavior was calculated using the Landau–Lifshitz–Gilbert equation, which considers eddy currents. The calculation model of the amorphous wire is shown in Fig. 1. The magnetic properties included a saturation magnetization of 1.0 T, a magnetic anisotropy constant of 250 J/m&lt;sup&gt;3&lt;/sup&gt;, and exchange constants between cells in the plane and the axial direction of 1.0 × 10&lt;sup&gt;−11&lt;/sup&gt; J/m and 2.0 × 10&lt;sup&gt;−11&lt;/sup&gt; J/m, respectively. The magnetization easy axis of the amorphous wire from the outer circumference to the inside by 0.75 µm was assumed to be oriented in the circumferential direction because compressive stress is generated in the FeCoSiB amorphous wire, which is induced via rapid quenching during the fabrication processes [2].&lt;br/&gt;3. Results&lt;br/&gt;Fig. 2 shows the relationship between the peak voltages at the fall times of the pulse currents and magnetic fields. When the fall time was 1.0 ns, the calculated and experimental results showed the same tendency. A shorter fall time meant higher absolute values of the peak output voltages and stronger magnetic fields as shown in Fig. 2. At a fall time of 0.385 ns, only the rotations of magnetizations in the outer circumference of the amorphous wire contributed to the output voltages, whereas at a fall time of 5.0 ns, the domain wall motions inside the wire contributed to the output voltages. This was the reason why the output peak voltages increased as the fall time was shortened.

![](https://assets.underline.io/uploads/markdown_image/1/image/b0cfc2b624142dd7949f1e74c66d616b.jpg)

Fig. 1 Model of a magnetic sensor with an amorphous wire

![](https://assets.underline.io/uploads/markdown_image/1/image/fee592d4ca3a224b0b9da7393a6acba5.jpg)

Fig. 2 Relationship between peak voltages and magnetic fields

2022 Joint MMM-INTERMAG

Effects of fall times of pulse currents on output voltages for amorphous-wire-based magnetic sensors

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INTERMAG 2021

1. Introduction Ultra-high sensitivity and micro-sized magnetic sensors are required in biomagnetic measuring devices. The GigaHertz spin rotation sensor, which is an evolution of the magneto-impedance sensor, has shown high sensitivity while transmitting GHz pulse currents [1]. Detection magnetic fields are sensed via a pickup coil wrapped around the wire when the GHz pulse currents are applied. However, the micro-mechanism in the amorphous wire, which exhibits high sensitivity, has not been characterized. Thus, we investigated the relationship between peak voltages and magnetic fields and the magnetization behavior inside the amorphous wire. 2. Calculation method and conditions The magnetization behavior was calculated using the Landau–Lifshitz–Gilbert equation, which considers eddy currents. The calculation model of the amorphous wire is shown in Fig. 1. The magnetic properties included a saturation magnetization of 1.0 T, a magnetic anisotropy constant of 250 J/m3, and exchange constants between cells in the plane and the axial direction of 1.0 × 10−11 J/m and 2.0 × 10−11 J/m, respectively. The magnetization easy axis of the amorphous wire from the outer circumference to the inside by 0.75 µm was assumed to be oriented in the circumferential direction because compressive stress is generated in the FeCoSiB amorphous wire, which is induced via rapid quenching during the fabrication processes [2]. 3. Results Fig. 2 shows the relationship between the peak voltages at the fall times of the pulse currents and magnetic fields. When the fall time was 1.0 ns, the calculated and experimental results showed the same tendency. A shorter fall time meant higher absolute values of the peak output voltages and stronger magnetic fields as shown in Fig. 2. At a fall time of 0.385 ns, only the rotations of magnetizations in the outer circumference of the amorphous wire contributed to the output voltages, whereas at a fall time of 5.0 ns, the domain wall motions inside the wire contributed to the output voltages. This was the reason why the output peak voltages increased as the fall time was shortened.

![](https://assets.underline.io/uploads/markdown_image/1/image/b0cfc2b624142dd7949f1e74c66d616b.jpg)

Fig. 1 Model of a magnetic sensor with an amorphous wire

![](https://assets.underline.io/uploads/markdown_image/1/image/fee592d4ca3a224b0b9da7393a6acba5.jpg)

Fig. 2 Relationship between peak voltages and magnetic fields

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The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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In bit patterned media recording (BPMR), to increase the areal density (AD), we must reduce the distance between the magnetic islands. This leads to severe two-dimensional (2D) interference. The 2D interference includes inter-symbol interference (ISI) and inter-track interference (ITI) from the down- and across-track directions, respectively. To combat the 2D interference, we can decompose the ISI and ITI into the serial model [1]. However, in [1], the authors just use the Viterbi algorithm for detection, the detection lacks prior information from the inner detector to the outer detector. In this article, we use the maximum a posteriori (MAP) algorithm to supply the prior information from the inner detector to the outer detector. Our proposed model is presented in Fig. 1. We use the serial GPR target in [1]. The received signal is feed to the horizontal detection (using MAP algorithm). Each output data of the horizontal detection r[j,k] includes six probability values of 6 symbols, respectively. We use the argmax(r[j,k]) to convert the signal r[j,k] into the hard output b[j,k]. b[j,k] is used to extract the noise information and create the soft output signal [2] (i.e., soft(b[j,k])). The signal soft(b[j,k]) is detected by the MAP algorithm in the vertical detection and creates the probability of -1 or +1 in s[j,k]. Then, s[j,k] and r[j,k] are entered to the log detection. We calculate the product of the probability of +1 from s[j,k] and the sum of the probability from the symbols, which contains the input +1, in r[j,k]. This product is assigned to a numerator. Then, we calculate the product of the probability of -1 from s[j,k] and the sum of the probability from the symbols, which contains the input -1, in r[j,k]. This product is assigned to a denominator. Finally, we calculate d = log(numerator/denominator). If d ≥ 0, â[j,k] = 1 and if d &lt; 0, â[j,k] = 0. The results of our proposed model are presented in Fig. 2. We compare our model with the serial detection in [2]. Our proposed model improved the bit error rate (BER) performance of the serial detection.

![](https://assets.underline.io/uploads/markdown_image/1/image/31b45afd062763fbaee54fb1167b8174.jpg)

Fig.1 Our proposed model.

![](https://assets.underline.io/uploads/markdown_image/1/image/0ef7ea1ba565b056af38a507b0793812.jpg)

Fig. 2 BER performance of the proposed model.

Improving Serial Detection with MAP Algorithm for Bit Patterned Media Recording

Since conventional HDDs that use perpendicular magnetic recording suffer from the problems of thermal stability, superparamagnetic limit, and so on, bit-patterned media recording (BPMR) system is a promising candidate for the next generation of data-storage system to achieve areal density (AD) beyond one terabit per square inch (Tb/in2) [1]. To increase the AD of the BPMR, the space between islands in both the down- and cross-track directions must be reduced. Consequently, data detection is interrupted by inter-symbol interference (ISI) and inter-track interference (ITI). In this paper, we propose a bit-flipping (BF) scheme using k-means algorithm for BPMR system to improve a bit error rate (BER) performance. A partial response maximum likelihood (PRML) detector which consists of a two-dimensional minimum mean square error (MMSE) equalizer and a one-dimensional PR target soft-output Viterbi algorithm (SOVA) detector is used for equalization and detection, respectively. Fig. 1 shows a block diagram of the proposed BF scheme. We use sj,k = [sj-2, sj-1, sj, sj+1, sj+2] = [vj-1,k, vj,k-1, vj,k, vj,k+1, vj+1,k] as the input sequence of k-means algorithm in SOVA output sequence vj,k = [vj-1,k-1, vj-1,k, ..., vj+1,k, vj+1,k+1] to consider the effect of ISI and ITI and decrease the number of clusters. After k-means algorithm, we compare a sign between pj of the predicted value in predicted sequence pj,k = [pj-2, pj-1, pj, pj+1, pj+2] and vj,k in in SOVA output sequence vj,k. When the signs are different, we change the sign of vj,k. In the simulation, we compare the PRML detector with BF and no BF schemes on the BPMR channel [2]. Fig. 2 shows the BER performance of the PRML detector with BF scheme according to SNR. When the ADs are 2.5Tb/in2 and 3Tb/in2, the performance of the proposed scheme is 1.1dB and 2.8dB better than that of one LDPC code at 10-4 BER, respectively.

![](https://assets.underline.io/uploads/markdown_image/1/image/cac33bc2d083c848fe5e6236fb342fe3.jpg)

Fig. 1. Block diagram of the proposed bit-flipping scheme.

![](https://assets.underline.io/uploads/markdown_image/1/image/fd9bb39b0553ae861daced494f20aa6e.jpg)

Fig. 2. BER performance of the PRML detector with BF scheme according to SNR.

Bit-Flipping Scheme Using K-Means Algorithm for Bit-Patterned Media Recording

If hard disc drives were able to store information in more than one recording layer, or structure, their storage capacity should be greatly increased. In recent years microwave-assisted and heat-assisted magnetic recording (HAMR) on dual structure media have been investigated [1][2]. Techniques for deconvoluting the readback signals from two recording structures have also been developed [3]. Here we focus on dual structure, bit patterned media for use with HAMR. Each recording medium dot contained two, discrete storage structures with different Curie temperatures, Tc, allowing data to be written to one, or both structures depending on the maximum temperature to which the dots were heated. The effect of the dot diameter, D, and the thickness of the recording structures, z, was investigated, with the aim of maximising the user areal density (UAD). The Curie temperatures were optimised to maximise the joint switching probability (i.e. P = P1 × P2) for D = 6 nm and z1 = z2 = 5 nm. The switching probability peaked when (Tc1, Tc2) were around (460 K , 550 K), or (530 K, 460 K). Next, the magnetostatic field distributions from surrounding dots were calculated for a dot pitch of 10 nm. The switching probability as a function of magnetostatic field, Hd, and D is shown in fig. 1. The switching probability increased with D (volume). The switching probability was also calculated as a function of recording structure thickness, z. Fig. 2 shows P versus KuV, where KuV was evaluated at the point where the coercivity, Hc, was equal to the head field Hhead. In fig. 2 both D and z were varied in each data set. Hc was calculated from M-H loops, whilst Hhead depended on the distance from the write head to the recording structure. Higher KuV resulted in higher switching probabilities for both recording structures, but RL1 showed a much greater variation than RL2. To maximise the switching probabilities of both recording structures RL1 should have a larger volume than RL2. The UAD was calculated using Shannon's formula for channel capacity. Values of around 8.6 Tbits/in2 were predicted for dot areal densities of about 4.5 Tdots/in2.

![](https://assets.underline.io/uploads/markdown_image/1/image/2a94c3c7e71339febbff789a66d45499.jpg)

Fig 1 P vs. magnetostatic field from surrounding dots and D

![](https://assets.underline.io/uploads/markdown_image/1/image/bd241e8cdc22543bc4f245ec032c65c8.jpg)

Fig 2 P vs. KuV when Hc = Hhead. Inset: Hc vs. T

Optimising Dual Structure Patterned Media for Heat Assisted Magnetic Recording

Spin-torque oscillators (STO) with two field generation layers (dual FGL), have superior characteristics compared with conventional STOs with a single FGL used for microwave-assisted magnetic recording (MAMR) [1], [2]. As shown in Fig. 1, we used an integrated model with a STO inserted into the gap between the main pole (MP) and the trailing shield (TS) of a write head. The dual FGL STO was composed of two FGLs and one spin injection layer with negative polarization (nSIL). The thicknesses were FGL1 = FGL2 = 5 nm, IL1 = 2 nm, IL2 = IL3 = 1 nm, and nSIL = 3 nm. The STO was tilted by 25° with respect to the x axis. The MP – TS gap = 18.8 nm at the air bearing surface (ABS). We assumed an injected current from MP to TS, and no spin-torque between FGL1 and FGL2. Antiferromagnetic coupling, Jex, was assumed between FGL1 and FGL2. Fig. 2 shows the in-plane component of the magnetization of FGL1 and FGL2 (Min-plane/Ms) averaged over the FGL volume, relative to a unit vector lying in the plane of the FGL. When Min-plane/Ms = 1 the FGL rotates perfectly in the FGL plane. Fig. 2(a) shows Min-plane/Ms for Jex = -1.33 erg/cm2 [2] and (b) Jex = -10 erg/cm2. The average values of Min-plane/Ms between zero and 16.0 ns were 0.653 for (a) and 0.808 for (b), demonstrating a remarkable improvement in FGL rotation. We also performed calculations for Jex = +1.33, 0, -5, -50, and -100 erg/cm2. We confirmed that Jex = +1.33 did not work; there was a significant improvement between Jex = 0 and -10; while there was little change from -10 erg/cm2 for Jex = -50 and -100 erg/cm2. We also varied some other parameters, e.g., FGL thickness, exchange coupling between nSIL - TS, however, no distinct effect was found. The most dominant factor affecting the FGL rotation was the strength of the antiferromagnetic coupling between FGL1 - FGL2. We acknowledge financial support from ASRC.

![](https://assets.underline.io/uploads/markdown_image/1/image/f66b1d4affcc87fdccb31a87caeec52a.jpg)

Fig. 1 Schematic of dual FGL STO.

![](https://assets.underline.io/uploads/markdown_image/1/image/46ec27ce246774c0fd194facedc6a8ce.jpg)

Fig. 2 In-plane component of volume-averaged FGL rotation vs. time. nSIL Po = -0.3, J = 4.0×108 A/cm2, coil current = 0.2 ATpp. (a): Jex = -1.33 erg/cm2, (b): Jex = -10.0 erg/cm2. Degauss of write head was carefully done, then STO calculations were performed. Data from 14 – 16 ns are shown.

Micromagnetic Analysis of Dual FGL STO

Racetrack memory (RM) is a promising device for a new class of non-volatile memory because of its large storage capacity, fast data access, and low power consumption. Although many studies have been focused on improving the domain wall motion (DWM), only a few studies have been considered the effect of Joule heating on DWM. We first measured the domain wall velocity in the GdFeCo wires by applying single voltage pulses (Picosecond) and then observed the DWM by using a Kerr microscope. Figure 1. (a) shows the DWM in the GdFeCo wire as a function of current density, where the pulse widths of the applied current are 3 ns and 30 ns, respectively. By increasing the current density, DW velocity increases. It is clearly observed that the DW velocity becomes faster as the pulse duration becomes shorter [1][2][3]. In order to clarify the effect of Joule heating on the magnetic wire, we performed a numerical simulation based on the heat diffusion on the surface of the GdFeCo wire when both short and long pulse width currents were applied to the wire. Besides, we found that the temperature of the magnetic wire is higher when the pulse width was longer, as shown in Fig. 1(b). Therefore, the slower DWM in the magnetic wire was observed for a long pulse current of 30 ns due to the higher temperature of the magnetic wire. Consequently, we have experimentally demonstrated and theoretically explained that a short pulse current of 3 ns is faster than a long pulse width of 30 ns. ACKNOWLEDGEMENT This work was supported by the JSPS KAKENHI (grants 918023, 918043,918055)

![](https://assets.underline.io/uploads/markdown_image/1/image/3326716efb14c75c90173084fc43ba5c.jpg)

Figure 1.(a) The velocity of the magnetic domain wall in the Pt(5)/Gd24(Fe88Co22)76(20)/SiN(10) sample as a function of current density

![](https://assets.underline.io/uploads/markdown_image/1/image/9dd719a62293a5500fcf19a587c87e9f.jpg)

Figure 1.(b) Temperature distribution of GdFeCo magnetic wire in the short hand direction when pulsed current of 3ns and 30ns widths is applied

Influence of pulse width and Joule heating on current-induced domain wall motion

Microwave-assisted magnetic recording (MAMR) [1] is a technology which can potentially enable the areal recoding density to further increase. Notched media have been proposed [2] for improving MAMR writability as well as MAMR recording characteristics. In the present study, dependence of microwave assisted magnetization switching (MAS) and magnetic recording characteristics on layer anisotropy structure were investigated by micromagnetic simulations regarding the interlayer exchange constants for notched media. Fig. 1(a) illustrates the layer structure assumed in this study. Recording characteristics were evaluated in terms of signal-to-noise ratios (SNR) derived from the signal power for 200 kbpi and noise power for 1600 kbpi, where a 30-nm-wide single-pole-type head with a spin torque oscillator with the frequency at 22 GHz [3] was assumed in recording process. Fig. 1(b) presents the MAS plots where Aint12, Aint23, and Aint34 are 0.5, 0.5, and 0.7 μerg/cm and Hk4 is 35 kOe, which were estimated under the continuous AC with pulse DC fields. The figure shows that higher switchable max. averaged Hk (Hk,ave) is obtained when only one of the three layers has large Hk and the other two layers have low Hk. This study found that the MAS properties are sensitive to field durations of DC and AC. In the case when a continuous DC and pulse AC fields, higher switchable max. Hk,ave was obtained only when media structure has large Hk1, low Hk2 and low Hk3. The calculation also indicated that relatively high Aint34 (&gt; 0.5 μerg/cm) was preferable. Fig. 2 compares SNRs for three the type of Hk layer structure media where Aint12, Aint23, and Aint34 were fixed to 0.5, 0.5, and 0.7 μerg/cm. The figure suggests that SNR for large Hk1 structure media reaches 16.7 dB around 25 kOe of Hk,ave. On the other hand, SNRs for large Hk2 and large Hk3 structure media decrease with increase in Hk,ave. The poor SNRs attribute to low signal power for 200kbpi. This may be translated by the MAS properties for multilayer films sensitive to the field durations.

![](https://assets.underline.io/uploads/markdown_image/1/image/f69d815b065f51bebe061d7e60d1fa7e.jpg)

Fig.1. (a) Schematic of medium layer structure and anisotropy fields and (b) MAS plots for a multilayer.

![](https://assets.underline.io/uploads/markdown_image/1/image/fd78099fea71c0873d44bece0fddeedf.jpg)

Fig.2. SNRs comparison among three type of layer Hk structure media with r = 0.4.

Dependence of microwave assisted magnetization switching and magnetic recording characteristics on layer anisotropy structure for multilayer media

Recently, it was discovered that without any external magnetic field, only by a single laser pulse, selective and reversible photo-magnetic switching in Co-doped yttrium iron garnet films (YIG:Co) can be obtained. This recording was characterized by an extremely short switching time of about 20 ps at room temperature. Moreover, because of its resonant character the write–read events are accompanied by an unprecedentedly low heat load [1]. Microscopically, in this mechanism incident linearly polarized pump pulse excites strongly anisotropic garnet ions, generating an effective field of photo-induced magnetic anisotropy [2],[3]. Here, we demonstrate the tunability of both magnetocrystalline and photo-induced anisotropies by the engineering of Co-doped thin iron garnet films and temperature variation. All experimental results were obtained using the time-resolved two-color pump and probe setup at Faraday geometry. The setup was tailored to perform the time-resolved femtosecond single-shot imaging on a CCD camera assuring obtain high temporal and spatial sensitivity. Through modifying the pump light fluence we were able to introduce a change in the light-induced anisotropy field. It allowed us to observe the change of switching times of magnetization and decrease it even more by two times but at the expense of the greater energy required to achieve the switching threshold using a single pump laser pulse. On the other hand, by steering the sample temperatures in the 100-450 K, the range we triggered a change in the magnetocrystalline anisotropy field. Increasing it allowed us to decrease the energy dissipation threshold required for switching over five times. However, decreasing the switching threshold comes at the expense of a longer switching time. Moreover, we demonstrate that the photo-magnetic recording-based garnet medium operates within an extremely large temperature range between -100°C to 100°C. This finding reveals the principles to be employed in achieving cold and ultrafast magnetic recording in dielectrics far beyond today’s state-of-the-art.

Control of switching time and energy dissipation for ultrafast photo-magnetic recording in dielectrics

A magnetic field sensor using magnetoimpedance (MI) has a higher sensitivity [1] and MI properties strongly depend on a magnetic anisotropy of a material [2]. Thus, controlling anisotropy is quite important to obtain large impedance changes. In general, an anisotropy can be controlled by a field annealing in vacuum using a furnace. The furnace is a large-scale, and consumes a lot electric power for heating and generating strong field. On the other hand, Joule-heating, a current is directly applied to an element, is a simple way and therefore achieves a compact equipment with a low power consumption. However, there are few reports that Joule-heating applies to ferromagnetic thin-film [3] and no reports that studies controlling anisotropy itself by Joule-heating. Thus, we investigated to controlling magnetoimpedance properties by Joule-heating on a thin-film element as a feasibility study. The amorphous CoNbZr thin-film element with Cu electrode was prepared. Then, a direct DC current of 80-120 mA was applied to the elements using a wafer probe with 1 minute in the atmosphere. In some cases, the element was applied to an external field parallel to the width direction of the elements by magnets during heating. Fig. 1 shows the field dependence of impedance at 100 MHz. Before heating, impedance has maximum at zero field and then decrease with increasing field intensity, which is typical profile when an element has the easy axis along with the longitudinal direction of the element. On the other hand, a double peak appears and its peak height slightly increases with increasing intensity of DC current after Joule-heating with magnets. The field intensity where the impedance has a peak, shifts to higher field with increasing intensity of DC current. Fig. 2 shows the domain structures of the element observed by a Kerr-effect microscopy before and after Joule-heating. Although the element has domains parallel to the length direction of the element before heating, 180° domains parallel to almost width direction appear after heating. This means the easy axis of the thin-film element can be controlled by Joule-heating.

![](https://assets.underline.io/uploads/markdown_image/1/image/81a717f414b1d4e3f236a72ceb55f496.jpg)

Fig. 1 Field dependence of impedance.

![](https://assets.underline.io/uploads/markdown_image/1/image/183d6970dffe317aed0c4c460999ad0a.jpg)

Fig. 2 Changes in domain structures.

Controlling of magnetoimpedance property of thin-film element using Joule-heating

Magnetic Tunnel Junctions (MTJ) are widely used as magnetic sensors but their major disadvantage is their high noise level at low frequency. Some methods to overcome 1/f noise involve adding magnetic modulation [1] or increasing the MTJ area [2]. In order to get a linear response, in-plane MTJ are usually designed with the free layer (FL) set perpendicularly to the reference. However, for a specific application [3], a symmetric R-H characteristic is more suitable to drive a modulation and should be easily obtained by pinning the FL in the same direction as the reference. The aim of this study is to process MTJs with symmetric R-H curve and high sensitivity. The magnetic stack is composed of a pinned synthetic antiferromagnet, MgO tunnel barrier and CoFeB/NiFe free layer. After patterning micron-size junctions of different shapes, we perform a post-process annealing under magnetic field. In this work, we study different methods to achieve a coherent rotation of the FL magnetization towards the hard axis: i) applying a small external field along the easy axis; ii) using shape anisotropy; iii) using soft pinning by adding an antiferromagnetic layer on the FL separated by a thin Ta spacer [4]. Our measurements highlight the importance of FL pinning on the reversibility and smoothness of the R-H curves. Nevertheless, since pinning reduces sensitivity, an optimum trade-off must be found. We observed that shape anisotropy alone is not sufficient to pin the FL (fig.1), but a coherent rotation is achieved by applying an external field. Moreover, our results show the importance of a good alignment between the reference and the easy axis. Compensating for the process misalignement by tilting the applied field allows to reduce the pinning field to 5 Oe (fig.2) with a sensitivity of 3.5%/mT. On the other hand, soft pinning can provide internal pinning ranging from 120 to 0 Oe by increasing the Ta spacer up to 0.25 nm. Choice of the optimum spacer thickness and further process optimizations should reduce hysteresis and improve the sensitivity.

![](https://assets.underline.io/uploads/markdown_image/1/image/73f780f33019b4c19ae68487962887c0.jpg)

Fig.1: R-H curves measured along easy axis Hy and hard axis Hx

![](https://assets.underline.io/uploads/markdown_image/1/image/b9125ad521ed2eba162c211ac36f36e5.jpg)

Fig. 2: R-H curves measured with respect to the reference misalignment (1.6°) and pinning fields Hpin of 5 and -5 Oe

Development of magnetic tunnel junctions with symmetric R-H curve for specific sensor application

In recent years, giant magnetoresistance (GMR) devices have been used in many applications due to their small size, high sensitivity, low power consumption, and low cost[1, 2]. For a high sensitive magnetic sensor application, the device resistance is required to be varied linearly over a wide magnetic field range. A high aspect ratio of the GMR line device is useful for the wide range high linearity of the MR curve. However, the high aspect ratio reduces MR ratio and increase signal to noise ratio (SNR) because the exchange bias for minor axis direction in the GMR line is suppressed due to the strong shape anisotropy in the pinned layer. In this study, we investigated the correlation between SNR and MR properties in GMR line devices for magnetic sensor applications. The GMR stack is composed of Si/SiO sub.//seed layer/IrMn(6)/CoFe(1.6)/ Ru(0.85)/CoFe(1.6)/Cu(2.5)/CoFe(1)/NiFe(3.5)/capping layer (all thickness in nm). The pinned layers were magnetized for minor axis direction in GMR lines. Figures 1(a)-(c) shows a schematic, cross sectional and top view of scanning electron microscope (SEM) images of the GMR device, respectively. The GMR device has ten GMR lines in the parallel configuration. The line length and space width were fixed at 100 μm and 600 nm, respectively. The line width (W) was varied from 300 nm to 2 μm. Figure 2(a)-(e) show MR curves of the GMR lines with various W with external field of ±20 Oe. The MR curves were measured 100 round trips for evaluating SNR. MR ratio increased as increasing W due to the stabilization of antiparallel configuration of free and pinned layers. On the other hand, the linearity of the MR curve decreased at W of 2 μm because of the reducton of shape anisotropy. The reduction of MR curve linearity increased device resistance variation. Figure 2(f) shows SNR values calculated from the MR ratio and coefficient of the resistance variatoin. SNR increased as increasing W. These results mean that it is important to increase W within the range that maintains the linearity of the MR curve.

![](https://assets.underline.io/uploads/markdown_image/1/image/b522fc897ee5005e5c29521ea007053a.jpg)

Fig. 1 (a) A schematic, (b) cross-sectional and (c) top view of SEM images of the GMR device

![](https://assets.underline.io/uploads/markdown_image/1/image/4efd064f0bca7a055f9527d6a114c070.jpg)

Fig. 2 (a)-(e) MR curves and (f) signal to noise ratios for the various W of GMR devices.

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