Influence of the crystalline microstructure on the magnetic ordering of nanocrystalline chromium

The magnetic and structural properties of nanocrystalline chromium (n-Cr) were studied by neutron powder diffraction and by synchrotron radiation diffraction techniques. The nanocrystalline Cr is composed of small-sized particles with antiferromagnetic ordering, medium-sized particles with transverse spin density wave (SDW) ordering, and large-sized particles with the same magnetic ordering as bulk Cr. The critical size $D_C$ between the small and medium-sized crystallites is $18\pm 2$ nm. The alteration of the magnetic properties of n-Cr is due to microstrain fluctuations which are correlated with the crystallite sizes rather than to the crystallite size itself. The microstrain fluctuations increase the SDW modulation length up to large values (10.5 nm for n-Cr vs 7.8 nm for bulk Cr).

Figure 1

Dependence between the microstrain fluctuations $\Delta d/d$, the average crystallite size $D$, and the annealing temperature for electrodeposited nanocrystalline chromium. The solid and empty symbols are taken from Refs. 15 and 21, respectively, while the star symbols denote the samples studied in the present paper. The dotted line represents the microstrain fluctuations observed in standard poly-Cr. The samples n-Cr2 and n-Cr3 were used in “as prepared” (a.p.) form while n-Cr4 was annealed at 500 ${}^{\circ }$C.

Figure 2

Temperature changes of the relative volume $m_{{\mathrm{AF}}_1}$ occupied by the AF${}_1$ magnetic phase for n-Cr and poly-Cr samples. The arrows indicate possible spin-flip $({\mathrm{AF}}_1\to {\mathrm{AF}}_2)$ transitions. Vertical dotted lines indicate the spin-flip temperature $T_{\mathrm{SF}}=122$ K observed for bulk Cr.[1]

Figure 3

(a) Temperature changes of the modulation length ${\Lambda }_{\mathrm{SDW}}$ for n-Cr, poly-Cr, and reference bulk Cr.[2] (b) Temperature changes of the modulation length ${\Lambda }_{\mathrm{SDW}}$ for n-Cr, Cr thin film (100 nm),[12] and annealed n-Cr (100 nm).[7]

Figure 4

Log-normal distribution function $g\left(V\right)$ (a) and the cumulative distribution function $G\left(V\right)$ (b) for the n-Cr samples. The vertical dotted lines (b) indicate the critical volumes $V_C$ while the horizontal dotted lines denote the relative volume $m_{{\mathrm{AF}}_0}$ of the ${\mathrm{AF}}_0$ magnetic phase determined for n-Cr at 130 K (see Table ).