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 M J VLVL VTVT Anisotropic magnetoresistance (AMR) Planar Hall effect (PHE) Extraordinary Hall effect (EHE)

AMR and PHE in non-crystalline magnetic conductors I A B M 

I A B M 

PHE as a probe for non-magnetic resistivity anisotropy SrRuO 3 45 O [100] [001] [010] a c [110] b J C E GF D A B Genish et al PRB 2007

PHE as a probe for crystal symmetry effects Bason et al PRB 2009

Advantages of PHE for Applications I A B M  Maximum slope at  =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 film thickness; 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 magnetization in the middle of the cross along two different axes and measuring the resulting transverse voltage. Typical line width in patterns we have used so far is 1 micron. I A B H1H1 H2H2 We define the PHE resistance as R xy =V AB /I Two states of R xy are observed: 1.After H 1 is applied and then set to zero 2.After H 2 is applied and then set to zero R xy reverses its sign between the two states

Demonstration of PHE-MRAM operation with manganite films Field pulses along EA1 (blue) and EA2 (red) give PHE signals with Opposite 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 (H 1 ) or at 135 degrees (H 2 ). R A and R B 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) I B H1H1 H2H2

Write line x Write line 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 Inputoutput current or voltagevoltage Magnetic field output Transfer function for a given input 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  AMR PHE The magnetization prefers to be along the long axis – therefore small rotation of 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 V AB B I M PHE sensors are simpler than AMR sensors and can be made more sensitive – no need for 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 determine the operational field range according to the required application.

Theoretical models – numerical simulations – experiments Stoner Wohlfarth OOMMF Sputtering and nano-litography Genish et al JAP 2010

Experimental results Stoner Wohlfarth Genish et al JAP 2010

PHE-sensors – sensitivity Permalloy on silicon Demonstrated Sensitivity: 40 m  /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 mV/(V gauss)

PHE-sensors – sensitivity Permalloy on silicon

PHE-sensors – applications