# Image of dynamic local exchange interactions in the dc magnetoresistance of spin-polarized current through a dopant

## TL;DR

This paper predicts and analyzes strong dynamical effects in the dc magnetoresistance caused by exchange interactions in spin-polarized current through dopants, enabling precise measurement of local spin couplings.

## Contribution

It introduces a theoretical framework to detect exchange and hyperfine interactions via dc magnetoresistance resonances without external ac fields.

## Key findings

- Resonances in dc magnetoresistance occur at specific magnetic fields matching exchange interactions.
- Spin precession is canceled at these resonances, indicating a dynamic bottleneck.
- The method allows measuring small spin couplings near surfaces using STM.

## Abstract

We predict strong, dynamical effects in the dc magnetoresistance of current flowing from a spin-polarized electrical contact through a magnetic dopant in a nonmagnetic host. Using the stochastic Liouville formalism we calculate clearly-defined resonances in the dc magnetoresistance when the applied magnetic field matches the exchange interaction with a nearby spin. At these resonances spin precession in the applied magnetic field is canceled by spin evolution in the exchange field, preserving a dynamic bottleneck for spin transport through the dopant. Similar features emerge when the dopant spin is coupled to nearby nuclei through the hyperfine interaction. These features provide a precise means of measuring exchange or hyperfine couplings between localized spins near a surface using spin-polarized scanning tunneling microscopy, without any ac electric or magnetic fields, even when the exchange or hyperfine energy is orders of magnitude smaller than the thermal energy.