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