Figure 1

Cathodoluminescence spectra of sample A (grown with N getter) and sample B (grown without N getter) recorded on (100) and (111) growth sectors under similar conditions.Reuse & Permissions

Figure 2

(a) SQUID measurement of sample B. The symbols show a fit using a Brillouin function as described in the text. (b) SQUID measurement of sample A at temperatures 300, 200, 50, 10, 4, and 2 K. The broken lines indicate the method used to determine the spontaneous magnetization $M\left(T\right)$.Reuse & Permissions

Figure 3

Reduced magnetic moment $M\left(T\right)/M_S$ versus reduced temperature for bulk nickel (line) and sample A (symbols), drawn for $T_c=631$ K for nickel and $T_c=500$ K for sample A.Reuse & Permissions

Figure 4

Simplified schematic of the HTHP synthetic diamond crystals labeling the principal crystallographic axes. The small (113) and (101) faces of our cubo-octahedral crystal have been omitted for clarity.Reuse & Permissions

Figure 5

Typical $\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}$-photoluminescence spectra (zero-phonon doublet) of Ni incorporated in HPHT synthetic diamond for samples A (at two different positions) and B at $T=4.2$ K and $B=0$ T. All spectra are taken on the (111) face. A linear background has been subtracted from the rather weak PL signal of sample B and the curves are offset vertically for clarity. Position 2 is close to the bottom of the $\left(111\right)$ face, i.e., near the seed crystal. See $\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}$-PL map in Fig. 11.Reuse & Permissions

Figure 6

Representative $T=4.2$ K magnetophotoluminescence spectra in the Faraday configuration of the $1.4$-eV Ni color center in HPHT synthetic diamond. (a) Macro-PL with $B\parallel \left[111\right]$ collected from the $\left(111\right)$ face. (b) $\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}$-PL with $B\parallel \left[001\right]$ collected from the $\left(001\right)$ face. Please note the different magnetic field ranges.Reuse & Permissions

Figure 7

Energy of the transitions (symbols) for photoluminescence collected from the $\left(111\right)$ face of the diamond crystal (sample A) as a function of the magnetic field applied parallel to the $\left[111\right]$, $\left[1\overline{1}0\right],$ and $\left[11\overline{2}\right]$ directions. The transition energies calculated with the effective spin Hamiltonian are shown as red solid lines for a defect axis aligned along the $\left[111\right]$ growth direction of the face. The green broken lines are the calculated energies for defects aligned along the other $\langle 111\rangle$ directions. The legend labels the $Z$ axis of the defect as in Table .Reuse & Permissions

Figure 8

Energy of the transitions (symbols) for photoluminescence collected from the $\left(001\right)$ face of the diamond crystal (sample A) as a function of the magnetic field applied parallel to the $\left[001\right]$, $\left[1\overline{1}0\right],$ and $\left[110\right]$ directions. The transition energies calculated with the effective spin Hamiltonian are shown as red solid lines for a defect axis aligned along the $\left[111\right]$ growth direction of the face. The green broken lines are the calculated energies for defects aligned along the other $\langle 111\rangle$ directions. The legend labels the $Z$ axis of the defect as in Table .Reuse & Permissions

Figure 9

Energy of the observed transitions (symbols) for $T=4.2$ K $\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}$-photoluminescence collected on the $\left(001\right)$ face with $B\parallel \left[001\right]$ on samples A and B as a function of the magnetic field. The red solid lines are the predictions of the NND effective spin Hamiltonian (all defect alignments are equivalent for $B\parallel \left[001\right]$). The legend labels the $Z$ axis of the defect according to Table .Reuse & Permissions

Figure 10

(a), (b) Energy of the observed transitions (symbols) for $T=4.2$ K $\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}$-photoluminescence collected at two different positions on the $\left(111\right)$ face with $B\parallel \left[111\right]$ on sample A as a function of the magnetic field. The predictions of the NND effective spin Hamiltonian are shown for a defect preferentially aligned along $\left[111\right]$ (red solid lines) and the other three equivalent $\langle 111\rangle$ directions (green broken lines). Note that at position 2, the defects are not preferentially aligned along the growth direction and fitting requires a radically different set of parameters. (c) Splitting of the excited state; the high- (circles) and low- (triangles) energy transitions in (a), from which the average energy of the transition has been subtracted, are plotted versus magnetic field. The solid lines are a least-squares fit to extract $g_3^e=0.16\pm 0.03$.Reuse & Permissions

Figure 11

Spatial PL map of the $\left(111\right)$ face measured at $T=4.2$ K and $B=0$ T showing the integrated intensity of the high-energy component of the $1.4$-eV doublet. The dashed line indicates the bottom of the (111) face. Intensity decreases rapidly below this line due to the loss of focus. The positions at which the $\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}$-PL spectra in Fig. 5 were taken are indicated.Reuse & Permissions

Figure 12

The circles show the angular dependence of the line position (effective $g$ factor) of the NIRIM-2 center, taken from Ref. 13, with the magnetic field rotated about the $\left[\overline{1}10\right]$ axis at $T=4$ K and $B\simeq 0.5$ T. The triangles are the splitting of the excited state measured using MCDA taken from Ref. 17. Also shown is the predicted angular dependence of the ground-state splitting (solid lines) and excited-state splitting (broken lines), calculated using the effective spin Hamiltonian with the parameters found in this work, and given in Table . The curves are labeled according to the alignment of the defect $Z$ quantization axis.Reuse & Permissions

Figure 13

Energy levels of the ${}^{2}D$ state of $3d^9$ interstitial Ni${}^{+}$ showing the subsequent effects of the cubic ($T_d$) and trigonal ($C_{3v}$) crystals fields and spin-orbit interaction (Ref. 17). The broken arrows indicate the observed zero-field optical transitions.Reuse & Permissions