---
title: LLG
status: implemented
doc_kind: reference
audience: user
owner: fullmag-public-docs
---

(public-docs-python-api-dynamics-llg)=
# LLG

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## Contract
This page records the current public Python authoring contract and canonical lowering; it does not redefine solver physics.

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## Governing equations
This API page introduces no independent governing equation. Physical equations belong to interaction and solver-lane pages.

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## Symbols and SI units
Every owned input has its SI unit below; $1$ denotes dimensionless data.

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## Assumptions and validity
Constructor checks run immediately. Lowering and planning additionally check mesh cardinality, capability, and backend legality.

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## Python API
| Python | Type | Default | SI unit | Validation | Meaning | Backend support | ProblemIR |
|---|---|---|---|---|---|---|---|
| `LLG.gamma` | `float` | `221100.0` | $\mathrm{m\,A^{-1}\,s^{-1}}$ | Positive finite gyromagnetic ratio used by the H-field LLG convention. | Positive finite gyromagnetic ratio used by the H-field LLG convention. | FEM/FDM CPU/GPU; planner checks combinations | `study.dynamics.gamma` |
| `LLG.integrator` | `str` | `"auto"` | $1$ | Canonical supported integrator identifier or `auto`; planner and runtime legality are validated explicitly. | Canonical supported integrator identifier or `auto`; planner and runtime legality are validated explicitly. | FEM/FDM CPU/GPU; planner checks combinations | `study.dynamics.integrator` |
| `LLG.fixed_timestep` | `float \| None` | `None` | $\mathrm{s}$ | Positive fixed step when supplied; mutually constrained with adaptive stepping. | Positive fixed step when supplied; mutually constrained with adaptive stepping. | FEM/FDM CPU/GPU; planner checks combinations | `study.dynamics.fixed_timestep` |
| `LLG.adaptive_timestep` | `AdaptiveTimestep \| None` | `None` | $1$ | Optional adaptive-step contract. | Optional adaptive-step contract. | FEM/FDM CPU/GPU; planner checks combinations | `study.dynamics.adaptive_timestep` |
| `LLG.field_refresh` | `FieldRefreshPolicy \| None` | `None` | $1$ | Optional field-refresh policy. | Optional field-refresh policy. | FEM/FDM CPU/GPU; planner checks combinations | `study.dynamics.field_refresh` |


### Complete stage-first example

The executable public form configures the LLG policy on `study.solver(...)`. The constructor
`LLG(...)` is not a standalone simulation workflow.

```python
# %% LLG policy in a complete stage-first study
import fullmag as fm

nm = 1.0e-9
study = fm.study("llg_api_example")
study.engine("fdm")
study.device("cpu", precision="double")
study.mode("strict")
study.exchange()
study.objects.mesh.defaults(cell_size=(2 * nm, 2 * nm, 5 * nm))
film = study.geometry(fm.Box(100 * nm, 20 * nm, 5 * nm), name="film")
film.Ms = 800.0e3
film.Aex = 13.0e-12
film.alpha = 0.02
film.m = fm.init.UniformMagnetization((1.0, 0.0, 0.0))
study.solver(
    integrator="rk45",
    adaptive_timestep=fm.AdaptiveTimestep(
        atol=1.0e-6,
        rtol=1.0e-3,
        dt_min=1.0e-15,
        dt_max=1.0e-12,
    ),
    gamma=2.211e5,
)
study.stages.add_run(stage_id="run", until=1.0e-9)
```

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## ProblemIR
The final column gives the serialized destination owned by the current lowering implementation.

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## Round-trip and failure semantics
Requested intent is preserved in Python and IR. Resolved execution is selected by the planner. Validation errors reject malformed values; unsupported combinations fail capability checks without silent fallback.

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## Discrete realization
This page owns authoring and lowering only; numerical realization belongs to solver-lane documentation.

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## Implementation mapping
The adjacent map anchors claims to `packages/fullmag-py/src/fullmag/model/dynamics.py` and `class LLG`.

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## Validation
Tests compare this inventory with live signatures and validate its source map.

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## Limitations
Representability does not prove every backend combination executable; planner capabilities are authoritative.

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## Scientific bibliography
No physical model is introduced. Primary references belong to consuming interaction pages.

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## Control Room crosswalk

Status: Common solver/stage fields are partial; backend-specific options without editor fields remain not implemented.

| Python/API surface | Control Room path | Status | Transaction |
|---|---|---|---|
| Parameters documented on this page | `Model Explorer -> Stages -> <stage> -> Solver` | `partial` | Apply stage draft; solver request and result resources become stale |
| Parameters without a named UI field | `Model Explorer -> Stages -> <stage> -> Solver` | `not implemented` | Python-only until implemented |

frontend support is not implemented for dynamics parameters not rendered by the stage editor.
See [Control Room capability register](/frontend/capability-register) for the support matrix and not implemented policy.
Frontend source owner: `apps/control-room/src/modules/inspector/panels/StudyStageDraftEditor.tsx (StudyStageDraftEditor)`.

## Source-code index
| Claim | Path | Stable symbol | Responsibility | Evidence |
|---|---|---|---|---|
| Constructor, validation, lowering | `packages/fullmag-py/src/fullmag/model/dynamics.py` | `class LLG` | Canonical Python API behavior | Ownership test and source-map validator |