C-X3 · Turn a magnet, measure a voltage

GMR Spin Valve

How does rotating one magnetic layer change the resistance of a spin valve? The two magnetic layers scatter spin channels differently. Parallel magnetizations give a lower resistance, and antiparallel magnetizations give a higher resistance. At constant current, the changing resistance appears as a changing voltage.

Rotate a free magnetic layer above a pinned reference and watch the carriers cross the metallic stack. Acquire a full angle scan and recover the parallel and antiparallel resistances from its voltage samples.

Present resistance $0$

Present voltage $0$

Fitted parallel resistance $0$

Fitted antiparallel resistance $0$

A metallic magnetic sandwich

Rotate the free layer

Relative magnetization angle

\theta/\mathrm{deg}

125

Sense current

I/\mathrm{mA}

1

A default acquisition is ready.

Noise and material parameters

Antiparallel resistance increase

100\Delta R/R_{\rm P}

18

Parallel resistance

R_{\rm P}/\Omega

100

Voltage read noise

\sigma_V/\mathrm{mV}

0.25

Sampling seed

s_0

23

Model, scale and sources

This synthetic current-perpendicular-to-plane metallic spin valve uses a phenomenological cosine interpolation between specified parallel and antiparallel resistances. The free-layer angle is imposed in the film plane; the pinned layer points along the positive horizontal axis. A constant sense current converts resistance to voltage, with seeded independent Gaussian voltage noise. The fit uses stored samples. Carrier dots illustrate transport direction; their paths and spin colors do not solve microscopic scattering. The model omits magnetic hysteresis, domains, interlayer coupling, spin diffusion, spin torque, heating, anisotropic magnetoresistance and tunneling magnetoresistance. The real films are contiguous. Display gaps separate the enlarged layers for visibility and add no resistance. It is not a reconstruction of the original 1988 in-plane-current experiments.

Baibich et al. · 1988 IBM · Spin valve angular response

Synthetic acquisition

A full turn in voltage samples

Default record

Saved amplifier samples

Fit to saved samples

The scan acquires the entire rotation. The apparatus above shows the current chosen angle.

Start with the essentials

Focus question: How does rotating one magnetic layer change the resistance of a spin valve?
One-sentence intuition: The relative magnetization angle changes the resistance, which a constant current converts into a voltage.

Core mathematical model

Synthetic angular response

R(\theta)=R_{\rm P}+\frac{R_{\rm AP}-R_{\rm P}}2(1-\cos\theta)

The angle is between the two in-plane magnetizations. This phenomenological interpolation fixes the parallel and antiparallel limits.

Voltage at constant current

V_k=I R(\theta_k)+\epsilon_k

The stored voltage samples include independent Gaussian amplifier noise. The graph divides those samples by the known sense current.

Magnetoresistance contrast

\mathrm{MR}=\frac{R_{\rm AP}-R_{\rm P}}{R_{\rm P}}

The reported resistance limits come from fitting the saved scan to a constant plus a cosine.

Common difficulties

Angle is not applied field

Typical misconceptionThe resistance curve predicts a magnetic switching loop.

Better mental modelThis Lab imposes the angle. It does not calculate hysteresis, domains or the relation between field and magnetization.

A metallic spacer conducts

Typical misconceptionAll spin valves work by tunneling through an insulating barrier.

Better mental modelThis GMR model uses a conducting spacer. An insulating tunnel junction has tunneling magnetoresistance and needs a different transport model.

Run the experiment

01
Compare aligned and opposed magnets
Select Easy passage and Opposed layers. Watch the free-layer arrow and compare resistance.
What to observe: The pinned arrow stays fixed. Rotating the free arrow by half a turn changes the model from its lower to its higher resistance.
02
Record a full rotation
Set a relative angle, then record an angle scan and export its voltage samples.
What to observe: The live apparatus shows the chosen angle. The separate graph contains a complete acquired rotation, and its fit uses those saved readings.
03
Make the signal harder to read
Choose Small magnetic contrast and compare scans at lower and higher sense current.
What to observe: The resistance contrast shrinks. With fixed voltage noise, a larger current reduces the resistance uncertainty, although heating is outside this model.