Nanometers-Thick Ferromagnetic Surface Produced by Laser Cutting of Diamond

At a Glance

Metadata	Details
Publication Date	2022-01-28
Journal	Materials
Authors	A. Setzer, P. Esquinazi, С.Г. Буга, M. Georgieva, T. Reinert
Institutions	Technological Institute for Superhard and Novel Carbon Materials, Leipzig University
Citations	5
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel method for creating robust, room-temperature ferromagnetism (FM) on diamond surfaces using standard laser cutting techniques.

Core Achievement: A nanometers-thick surface layer exhibiting stable ferromagnetism at 300 K was produced on both natural and CVD diamond crystals via pulsed laser cutting.
Mechanism and Thickness: The FM originates from a disordered, graphitic-like layer created by the laser treatment, a phenomenon known as Defect-Induced Magnetism (DIM). The magnetic layer thickness is estimated to be ≤20 nm.
Critical Anisotropy: Robust FM was observed only when the laser cut was performed along the diamond (100) surface orientation. Signals were significantly weaker or negligible for (110) and (111) orientations.
High Curie Temperature (T_c): The ferromagnetic order is highly stable, with an estimated Curie temperature ranging between 500 K and 750 K.
Magnetic Properties: Saturation magnetization (M_sat) at 300 K was measured between 10 and 20 emu/g (assuming the 20 nm layer thickness).
Verification: The magnetic signal is confirmed to be surface-related, vanishing completely after chemical etching with strong oxidizing acids or moderate thermal annealing (T < 650 °C).

Technical Specifications

Parameter	Value	Unit	Context
Laser Wavelength	532	nm	Cutting process
Laser Pulse Energy	0.5	mJ	Cutting process
Laser Pulse Duration	200	ns	Cutting process
Laser Repetition Rate	15	kHz	Cutting process
Laser Energy Density	300	J/cm²	Cutting process
Laser Focus Spot Diameter	15	µm	Cutting process
Optimal Crystal Orientation	(100)	N/A	Required for robust room-temperature FM
Ferromagnetic Layer Thickness	≤20	nm	Estimated thickness of the magnetic graphite layer
Saturation Magnetization (M_sat)	10 to 20	emu/g	At 300 K, assuming 20 nm layer thickness
Coercive Field (H_c)	~80	Oe	Observed for CVD samples #1a and #1b at 300 K
Curie Temperature (T_c)	500 to 750	K	Estimated range for the defect-induced FM
Magnetic Impurity Concentration (CVD)	≤2	ppm	Total Fe, Co, Ni concentration in bulk CVD samples
Raman G-band Peak	1580	cm^-1	Characteristic of disordered graphite on cut surface
Raman D-band Peak	1350	cm^-1	Characteristic of disordered graphite on cut surface
Chemical Etching Temperature	120	°C	Used for graphite removal (H₂SO₄/HNO₃/HClO₄)
Thermal Annealing Temperature	< 650	°C	Used for graphite removal in air

Key Methodologies

The experiment involved precise laser processing, chemical and thermal post-treatment, and advanced characterization to isolate the magnetic signal.

Sample Preparation and Alignment:
- Diamond crystals (natural and CVD) were fixed to a mandrel.
- The face to be cut was oriented perpendicular to the laser axis to minimize cut length.
- Samples were prepared with three orientations: (100), (110), and (111).
Laser Cutting and Polishing:
- A 532 nm pulsed laser was used with high energy density (300 J/cm²) and a 200 ns pulse duration.
- The laser beam was moved along the cut line, burning material and creating a graphitic-like layer.
- Some samples were subsequently “laser polished” (state “b”) by focusing the laser directly on the cut face to burn off surface material.
Ferromagnetic Layer Removal (Verification):
- Chemical Etching: Samples were treated with a strong oxidizing acid mixture (H₂SO₄, HNO₃, HClO₄) heated to 120 °C for 4 hours to remove the graphitic residue (estimated removal depth: ≥20 nm).
- Thermal Annealing: Samples were annealed in air at temperatures T < 650 °C (e.g., 550 °C, 600 °C) to oxidize and remove the graphitic surface layer.
Magnetic Characterization (SQUID):
- Magnetic moment measurements were performed using a SQUID magnetometer at 300 K.
- Field hysteresis loops and temperature dependence (Field-Cooled, FC, and Zero-Field-Cooled, ZFC) were recorded after demagnetizing the samples at 380 K.
Structural and Impurity Analysis:
- Raman Spectroscopy: Confocal Raman measurements (532 nm laser) confirmed the presence of disordered graphite (D-band and G-band peaks) on the laser-cut surfaces, contrasting with the sharp diamond peak (1332 cm^-1) on virgin surfaces.
- PIXE (Particle-Induced X-ray Emission): Used 2.0 MeV protons to quantify magnetic impurity concentrations (Fe, Co, Ni) deep within the bulk material, confirming that bulk impurities were too low to account for the observed FM signal.

Commercial Applications

The ability to create localized, high-temperature ferromagnetic spots on diamond substrates opens avenues for advanced device integration, particularly in quantum and high-performance electronics.

Quantum Sensing and Metrology:
- NV Center Manipulation: Creating localized magnetic spots (µm scale) adjacent to Nitrogen-Vacancy (NV) centers in diamond. This localized field can be used to tune or enhance the NV center’s magneto-optical response, improving field-sensitivity for specific applications.
High-Density Magnetic Storage:
- Diamond’s stability and thermal conductivity, combined with the high T_c (500-750 K) of the surface FM, make this technology suitable for developing robust, high-temperature magnetic memory elements.
Diamond Machining and Fabrication:
- The strong orientation dependence of graphitization during laser cutting provides critical feedback for optimizing industrial diamond processing, ensuring minimal magnetic contamination or controlled functionalization of specific surfaces.
Spintronics on Wide Bandgap Semiconductors:
- Integrating spin functionality directly onto a diamond substrate (a wide bandgap semiconductor) is crucial for developing next-generation spintronic devices that require extreme thermal and chemical stability.

View Original Abstract

In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15031014

References

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