Group 83 Lab Report:

Contents

Tunneling Magneto-Resistance................................2

1. Introduction...........................................................2

2. Experimental Work...............................................3

3. Results & Discussion............................................4

4. References...........................................................11

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Group 83 Lab Report:

Tunneling Magneto-Resistance

1. Introduction1.1. Basic Principle:Tunneling magneto-resistance (TMR) is a dramatic change of the tunneling current in magnetic tunnel junctions (MJT) when relative magnetizations of the two ferromagnetic layers change their alignment. [1]

1.2. Theory:To study TMR effect, between the two ferromagnetic layers (sense layer and pinned layer (artificial antiferromagnetic)) there is a barrier or insulating layer as shown in figure 1. Then magnetic field of known value is applied. The resistance between ferromagnetic layers is dependent on the orientation of the layers. As an assumption, in TMR effect the electrons originating from one spin state of the first ferromagnetic layer are accepted by unfilled states of the same spin of the second layer. If the two ferromagnetic films are magnetized parallel, the minority electron tunnel to the minority states and the majority spins tunnel to the majority states figure 2-a. If, however, the two films are magnetized anti-parallel, the effect is reversed, so the majority spins of the first layer tunnel to the minority states in the second layer and vice versa figure 2-b.

Figure 1: Sample arrangement for TMR. [1]

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Group 83 Lab Report:

Figure 2: Working principle of tunneling magneto-resistance (TMR effect), (a) Parallel state, (b)Antiparallel state [2]

TMR coefficient can be determined by using the following formula:

TMR Ratio = (Rap –Rp)/Rp (1)

Where

Rap = Antiparallel resistance

Rp = Parallel resistance

TMR effect is applied in the read-heads of modern hard disk drives which work on the basis of magnetic tunnel junctions. A new non-volatile memory, the so-called MRAM, is being developed on the basis of the TMR. Further, magnetic tunnel junctions are also used for sensing applications (e.g. ABS sensors in cars)

2. Experimental Work2.1. Equipment and Auxiliaries

TMR device

Power supply

TMR samples

Tweezers, etc.

2.2. Experimental Procedure

TMR samples were placed on the stage of TMR device, at the center of magnetic solenoid poles and then two precision needles are used to make electric connection between two ferromagnetic layers. Source meter is used to supply voltage and measure the current. To study magnetic behavior of TMR samples, Hysteresis loops (resistance vs applied field) of these samples were recorded using magnetic fields switching from negative to positive field values and vice versa. Resistance (R) vs. applied field (H) plots (0.02-0.5T) for five

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Group 83 Lab Report:

different samples were measured. Measurement of IV curve was performed by keeping field constant, for positive and negative field. Magnetic behavior of these samples was recorded and studied.

3. Results & Discussion

The table shows the stray resistance of the different samples, which is actually the combination of resistance of the gold pad, contact and other series resistor

Table 1: Series resistance for different samples dimension.

Samples dimensionsµm2

Stray resistanceΩ

5 x 5 394

10 x 10 449

20 x 20 443

5 x 10 432

5 x 20 433

3.1. Graphs between resistances vs. field for different samples at

saturation field of 0.02 Tesla

Figure 3 to 7 shows the resistance (R) vs magnetic field (H) plot of different sample size of MTJ. In the figures 3 and 4, arrows indicate the direction of magnetic field, also same magnetic direction for figure 5 to 7. The sample size is mentioned in the table 1 and also in the caption below every figure 3 to 7.

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Group 83 Lab Report:

-0.025 -0.015 -0.005 0.005 0.015 0.02530000

40000

50000

60000

70000

80000

Resistance Vs Field

Field [T]

Resi

stan

ce [O

hm]

Figure 3: R-H Curve for sample 5x5 µm2

-0.025 -0.015 -0.005 0.005 0.015 0.02514000

19000

24000

29000

34000

39000

Resistance Vs Field

Field [T]

Resi

stan

ce [O

hm]

Figure 4: R-H Curve for sample 5 x 10 µm2

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Group 83 Lab Report:

-0.025 -0.015 -0.005 0.005 0.015 0.0256000

8500

11000

13500

16000Resistance Vs Field

Field [T]

Resi

stan

ce

[Ohm

]

Figure 5: R-H Curve for sample 5 x 20 µm2

-0.025 -0.015 -0.005 0.005 0.015 0.0255000

7000

9000

11000

13000Resistance Vs Field

Field [T]

Resi

stan

ce

[Ohm

]

a

Figure 6: R-H Curve for sample 10 x 10 µm2

-0.025 -0.015 -0.005 0.005 0.015 0.0251000

1500

2000

2500

3000Resistance Vs Field

Field [T]

Resi

stan

ce

[Ohm

]

b

Figure 7: R-H Curve for sample 20 x 20 µm2

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Group 83 Lab Report:

Table 2: Calculation of TMR effect for different samples dimensions.

No

Sample

sizes (µm2)

Series resistance Rs

(Ω)

Graph

Rmin

(Ω)

Graph

Rmax

(Ω)

Antiparallel

resistance Rap (Ω)

Parallel

resitance

Rp (Ω)

R’ap

(Ω/µm2)

R’p

(Ω/µm2)

TMR

TMR in %

1 5 x 5 394 34344

76809 76415 33950 3056.

6 1358 1.25 125

2 5 x 10 432 15572

35394 34962 15140 699.2 303 1.31 131

3 5 x 20 433 6827 15400 14967 6394 149.7 63.9 1.34 134

4 10 x 10 449 5486 1190

5 11456 5037 114.5 50.4 1.27 127

5 20 x 20 443 1393 2842 2399 950 6 2.4 1.5 150

Rp = (Rmin –Rs)

Rap = (Rmax –Rs)

R’p = Rp / Sample area

R’ap = Rap / Sample area

3.1.1 Discussion:For sample having size of 5x5 µm2, the value of resistance is highest as compared to other samples, as observed in figure 3. Resistance shows a decreasing behavior with increasing sample area, shows a minimum for sample 20x20 µm2, which is also justified by observing figure 3 to 7 respectively.

The difference in the resistance behavior is due to the area of the samples and the orientation of sense layers with the field. As the area of the sample is increased, the value of resistance decreased, as shown in the table 2, because more electrons can tunnel through barrier due to increase in area. The increase in magnetic field in positive direction cause the domains in the sense layers to align themselves in the direction of applied field, which leads to anti-parallel state between sense and pinned layer, therefore resulting in a high value of resistance. The resistance at this point called the anti-parallel resistance (Rap) When the magnetic field applied in opposite direction, resistance starts decreasing because the domains in sense layers again align themselves in the direction of applied magnetic field which leads to the parallel state between two ferromagnetic layer (sense and pinned layer).

The orientation in the direction of magnetic field of sense layers can easily be seen in figure 6 and 7. Region ‘a’ in figure 6 is result of the orientation of

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Group 83 Lab Report:

domains, indicates that magnetic domains in sense layer were not completely aligned in the direction of the magnetic field, still it require more field (H) to completely align in the direction of applied field. Region ‘b’ in figure 7 has the same explanation as mention above for region a

3.2. Graphs between resistances vs. field at saturation field

0.5T:

-0.55 -0.44 -0.33 -0.22 -0.11 0 0.11 0.22 0.33 0.44 0.556000

8000

10000

12000

14000

16000

Resistance Vs Field

Field [T]

Resi

stan

ce [

Ohm

]

2

Figure 8: R-H Curve of sample having size of 5x20 µm2 at 0.5T

3.2.1 Discussion:

Figure 5 and 8 show the same value of maximum (Rmax) and minimum resistance (Rmin), in figure 8 it is clearly seen that as the applied magnetic field (above 0 to 0.52 T) is increased resistance, tunneling current decreased.

Firstly when field is applied in the positive direction sense layer start aligning in the direction of applied field, it shows highest resistance due to antiparallel state between sense and pinned layers (Point 1 in figure 8). Now magnetic field increases so that it reaches to the point domain in the pinned layer start align in the direction of magnetic field, increasing magnetic field increases the number of domain in the field direction and come to the point where saturation in the pinned layer take place, at this point both sense and pinned layer in the parallel state (point 2 in figure 8) and the tunneling current increases in parallel state (point 2) as compared to antiparallel state (point 1). As the direction of field changes (in opposite direction), at first pinned layer (artificial hard magnetic

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Group 83 Lab Report:

layer) change its direction towards negative direction and takes the same path from which it became positive. Further from maximum resistance, same effect can be seen as shown in figure 5.

3.3. Graph between Current vs. Voltage.-1

.1 -1-0

.9-0

.8-0

.7-0

.6-0

.5-0

.4-0

.3-0

.2-0

.1 00.

10.

20.

30.

40.

50.

60.

70.

80.

9 11.

1

-0.0004

-0.0003

-0.0002

-0.0001

0

0.0001

0.0002

0.0003

0.0004

IV curve

-0.02T+0.02T

Voltage [Volt]

Curr

ent [

Amp]

Figure 9: I-V Curve for sample 5x20 µm2

3.3.1 Discussion:

Figure 9 is the current vs voltage characteristics at constant magnetic field i.e. ±0.02T. Both I-V curves (in constant negative and positive magnetic field) show the same behavior but I-V curve in case of negative magnetic field shows less resistance (due to high value of current) as compared to I-V curve in constant positive field. Reason behind is the parallel state in the negative field and anti-parallel state in the positive field between the two layers.

The overall behavior of I-V curves of magnetic tunneling junction (MTJ) is non-linear in comparison to the linear behavior observe in I-V characteristics of MTJ. The reason that experimental results deviate from theoretical is due to the series resistance, which is between 394 to 433 Ω for different size of MTJ. This series resistance can’t be neglected which lead to the voltage drop and reduces the value of voltage across the end of the MTJ, hence it gives the lower current. [3]

However, it is found in the article that the voltage drop due to series resistance will effectively disappear when the area of the MTJ is reduced to submicron

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Group 83 Lab Report:

size because the resistance of the MTJ cell is inversely proportional to the area of the MTJ cell. [3]

3.4. Graph between TMR vs. Voltage.

-1.5 -1 -0.5 0 0.5 1 1.50.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40TMR Vs Voltage

Voltage [Volt]

TMR

Rati

o

Figure 10: TMR vs voltage curve for sample 5x20 µm2 at 0.02T

3.4.1 Discussion:

Figure 10 shows the TMR vs voltage, TMR value is calculated by using equation (1). This effect was first observed by Julleire and then conformation and observation of this effect by many researchers. A conclusion came after these studies that the bias dependence of TMR is an intrinsic effect [4], although its magnitude may vary considerably and it depend mainly on the area of the MTJ.

The experimental results shows that the TMR ratio is maximum at ±0.02 then it decreases as the voltage increase (both in the positive and negative direction) in figure 10. Furthermore, the figure 10 shows symmetry along y axis which is a characteristic of TMR vs voltage plot. [3, 4]

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In literature it is found that TMR shows a roughly parabolic voltage dependence at intermediate bias (0.5V) and then tends smoothly to zero at higher biases, with a half voltage (V1/2) of typically 0.3 to 0.5 V, where V1/2 is the voltage at which TMR reduces by a factor of 2. [4]

4. References1. http://physics.unl.edu/~tsymbal/reference/spindependent_tunneling/tunnel-

ing_magnetoresistance.shtml2. Lab manual3. The electrical characterization of magnetic tunneling junction cells using

conductive atomic force microscopy with an external magnetic field gener-ator by Jinhee Heo (Korea Institute of Materials Science), published in transactions on electrical and electronic materials on 25 Dec 2010

4. Fundamental Aspects of spin polarized tunneling by Patrick Royce Leclair http://alexandria.tue.nl/extra2/200211695.pdf

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