Single crystal diamond was processed by machining with an abrasive diamond grain and cleaning with acetone. The exposed face of diamond sample was (110) plane. Surface modification was processed as following: sputtering of gold coating (99.9% purity) by an argon ion beam on the diamond was conducted to prevent the reaction with manganese by smoothing a gold film of 60 nm in thickness. At six specific spots, as shown in Figure 1, the gold film was peeled off the diamond (110) surface under an optical microscope. Manganese powder with an average diameter of less 100 μm ground by ceramic ball milling to have a diameter less than 45 μm.

A bulk sample of diamond (natural type Ia, dimensions: 1.4 × 5.0 × 2.0 mm) was placed in a platinum cell, and a small amount of manganese powder was mounted on diamond (110). The specimens were irradiated with near-infrared radiation using an elliptic mirror with a quartz viewing port, as shown in Figure 2. The spot diameter was 18 mm, and it was focused on the manganese powder. Diamond is transparent to near-infrared radiation, so the input heat was restricted to the surface of the diamond-mounted manganese powder. An ultra- high vacuum chamber was used to prevent the oxidation of manganese, which was tracked by the differential thermal analysis under argon gas and X-ray diffraction patterns to detect thermal increases after heating over 670 K and the formation of Mn₂O₄ and MnO, based on the results of previous study. A ther-

mocouple was fixed on a quartz stage to make contact to the bottom of the platinum cell to measure its temperature avoiding direct infrared radiation. The mixture of diamond powder and manganese powder exothermically reacted at 693 K, as detected the differential thermal and gravitational analyses. The modification process was conducted under vacuum at 8 × 10⁻⁴ Pa at 623 K.

X-ray diffraction analysis was performed on the modified diamond surface to detect the carbide reaction products.

Magnetic flux measurement was performed on the modified diamond surface and non-modified diamond surface to assess spontaneous magnetization using a Hall-effect sensor at room temperature and low temperature. The specifications of the Hall-effect sensor specification are listed in Table 1. The sensor was applied with 1.78 V and 250 mA, as shown in the measurement circuit. The output signal from the sensor was amplified by a thousand times.

The modified diamond (110) specimen is shown in Figure 3. It became black in color after modification, and hexagonal formations were observed. The spots are numbered clock wise from 1 to 6. The hexagons with metallic luster were observed. This reveals that the manganese powder directly contacted with diamond was different from the one contacted with the gold film. The X-ray diffraction pattern of the modified diamond (110) is shown in Figure 4. The peak of diamond (110) at 75˚ is observed. The peaks of pure manganese at 42.9˚ and 48˚ are observed, and pure gold peaks at 39˚ and 65˚ are also observed. Peaks of manganese carbides, Mn₅C₂ at 44.8˚, 45.4˚ and 49.7˚are observed. These peaks

are overlapped with gold-manganese peaks. Peaks of Mn₇C₃ at 39.5˚, 42.5˚ and 44.6˚ are observed. A peak of manganese oxide cannot be observed.

Figure 5 shows the results of the measured magnetic flux densities on spot A

(number 3), where manganese carbides formed, and on spot B, which was covered with gold to distinguish the carbide magnet formation with varied temperature. Spot B was outside the hexagon near the diamond edge. The difference of the measured magnetic flux density at the two locations was apparent. The magnetic flux density on gold covered area was negligibly small (5 μT), and a temperature dependence was not observed. On the other hand, the modified (110) surface with manganese powder exhibited the magnet properties. The magnetic flux density increased greater than that of geomagnetism, as shown in the broken line upon lowering the temperature.

In addition, using the active area of the Hall-effect sensor (1.02 mm in diameter), measurements of the magnetic flux densities on the hexagonal spots numbered from 1 to 6 were conducted at a low temperature of 262 K. The results of the measurements at 262 K are presented in Table 2. The amount of formed manganese carbide was different in each spots, and therefore, the measurement values were also different. The direction of the spontaneous magnetism was aligned. The temperature of 262 K was detected by the thermocouple on the chilled copper foil dipped into liquid nitrogen on the other side.

Since diamond was covered with gold, the modified diamond was moved on top of the Hall device, which was near the manganese carbide for the Hall sensor measurement. The magnetic flux densities were measured along diagonal lines. The maximum magnetic flux density was measured near the center of the hexagonal spots, near spot 3. The measured magnetic flux density was 73 μT.