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The planar Hall effect: sensor and memory applications
Lior Klein
Department of Physics, Bar-Ilan University
The Itinerant Magnetism Laboratory – Department of Physics – Bar-Ilan University
Magnetoresistance of magnetic films – angular dependence
MJ
VL
VT
Anisotropic magnetoresistance (AMR)
Planar Hall effect (PHE)
Extraordinary Hall effect (EHE)jjjE H
))(( ||
AMR and PHE in non-crystalline magnetic conductors
jjjE H
))(( ||
I
A
B
Ma
cossin/
cos/
||
2||
xyxy
xxxx
jE
jE
AMR and PHE in non-crystalline magnetic conductors
I
A
B
Ma
cossin/
cos/
||
2||
xyxy
xxxx
jE
jE
PHE as a probe for non-magnetic resistivity anisotropy
SrRuO3
45O
[100]
[001]
[010]a
c
[110]
b
J
CE
G F
D
AB
Genish et al PRB 2007
PHE as a probe for crystal symmetry effects
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
T=5 K
-2
-1.5
-1
-0.5
0
0.5
1
1.5
=0o
=15o
=30o
=45o
=60o
=75o
=90o
T=125 K
-30
-20
-10
0
10
20
0 45 90 135 180
T=300 K
3
1,,
mlk
mlkklmijlkklijkkijijij aaaa
22sin22sin
4cos22cos22cos
BA
DCBA
trans
long
Bason et al PRB 2009
Advantages of PHE for Applications
I
A
B
Ma
Maximum slope at q=0
Zero baseline
Single-layer PHE-based MRAM
Objective:
Development of a new type of magnetic random access memory
(MRAM) that will be based on the planar Hall effect (PHE)
The MTJ memory cell in MRAM
•The MTJ is the heart of the MRAM memory cell.
•The read current flows between the top and bottom electrode.
•The writing operation is performed by a grid of write lines (word lines
and bit lines). The currents that flow in these lines generate at the
intersection of “word line” and “bit line” magnetic fields that are large
enough to determine the orientation of the free ferromagnetic layers in the
selected MTJ.
The complexity of MTJ-MRAM
In addition to the multilayer structure one needs very tight control on the filmthickness; particularly of the tunnel barrier whose thickness is ~ 1.5 nm
The future of MTJ-MRAM
Despite the apparent success of prototypes of MTJ-MRAM the issue of cost may eventually become a critical consideration
There is need for simpler and cheaper MRAM
Our proposal
Planar Hall Effect MRAM (PHE-MRAM)
A US patent together with Yale collaborators
Why PHE-MRAM is better than AMR-MRAM
Less sensitivity to resistance variations
Less resistance
A
C
B
The PHE-MRAM
The operation of a single PHE-based memory is tested by aligning the magnetizationin the middle of the cross along two different axes and measuring the resultingtransverse voltage. Typical line width in patterns we have used so far is 1 micron.
I
A
B
H1H2
We define the PHE resistance as
Rxy=VAB/I
Two states of Rxy are observed:1. After H1 is applied and then set to zero2. After H2 is applied and then set to zero
Rxy reverses its sign between the two states
-0.2
-0.1
0
0.1
0.2T=300 K
-0.4
-0.2
0
0.2
0.4T=310 K
-1
-0.5
0
0.5
1T=320 K
0
15
30
45
60
0 50 100 150Time [min]
Demonstration of PHE-MRAM operation with manganite films
Field pulses along EA1 (blue) andEA2 (red) give PHE signals withOpposite signs
The results indicate the feasibility of PHE-MRAM
Bason et al JAP 2006
The PHE resistivity of a 50 nm thick permalloy film (NiFe) grown on Si(100) switches between two opposite values as pulses of small magnetic fields are applied at 45 degrees (H1) or at 135 degrees (H2). RA and RB refer to PHE resistivities of two different patterns on the same film. Reducing film thickness and fine tuning of the film composition are expected to increase the signal by more than an order of magnitude.
Permalloy PHE-MRAM (Room Temperature)
0
40
80
120
H1 (
Oe)
0
40
80
120
0 1500 3000
H2 (
Oe)
time (seconds)
-0.2
0
0.2
RA (
Ohm
)
-0.2
0
0.2
RB (
Ohm
)
I
B
H1H2
Write line x
Writ
e lin
e y
I
One cell architecture (induced magnetism)
V
Multi-cell architecture (induced magnetism)
V
V
V
Where are we now?
Shape induced shape anisotropy
Reducing the size of the memory cell
Looking for industrial partner
Magnetic sensors based on the planar Hall effect
What are magnetic sensors?
Magnetic sensor
B
Input output
current or voltage voltage
Magnetic field
output
Transfer function for a given input
BinVoutV
Span – operational field range
Sensitivity -
Types of magnetic sensors
Magnetoresistive sensors
Change in resistance due to change in the state of a magnetic metal – spin polarized current
I
A
B
M
GMR-CPP
GMR-CIP
AMR-PHE
AMR and PHE
I
A
B
M
x
y
2||
||
( ) cos
( ) sin cos
x x x
y x
E j j
E j
q
AMR
PHE
The magnetization prefers to be along the long axis – therefore small rotationof the magnetization leads to linear PHE response and quadratic AMR response. This is a very big advantage for PHE-sensors.
AMR sensors
To overcome the problem of non-linear AMR response shorting bars are deposited in order to change the current flow direction in the magnetic film.
PHE sensors
A
B
B
VAB
B
I
M
PHE sensors are simpler than AMR sensorsand can be made more sensitive – no needfor shorting bars.
Hall effect vs Magnetoresistive sensors
MR sensors are 3 orders of magnitude more sensitive – therefore they can be used without amplifiers
Effect of size on performance
Small enough - single domain particles
Using nano-lithography tools, it is possible to fabricating sub-micron devices to ensure that the magnetic sensor will not be able to divide into magnetic domains – this will enhance the performance of the sensors in terms of sensitivity and operational field range.
Effect of shape and thickness on performance
By changing the shape of the sensor we will be able to determinethe operational field range according to the required application.
Theoretical models – numerical simulations – experiments
Stoner Wohlfarth
OOMMF
Sputtering and nano-litography
)cos(sin2 MHKH u
Genish et al JAP 2010
Experimental results
Stoner Wohlfarth
)cos(sin2 MHKH u
Genish et al JAP 2010
PHE-sensors – sensitivity
Permalloy on silicon
Demonstrated Sensitivity:
40 mW/gauss
4 mV/(V gauss) = 50 mV/(V kA/m)
Expected: on the order of 100 mV/(V gauss)
Sensitivity of the most sensitive
Honeywell MR sensor (HMC1001/2)
is 2.5-4 mV/(V gauss)
PHE-sensors – sensitivity
Permalloy on silicon
PHE-sensors – applications