Evolution of magnetic phases in ${\mathrm{SmCrO}}_3$: A neutron diffraction and magnetometric study

The classical belief about the mechanism of spin reorientation phase transition (SRPT) and ground-state magnetic structure in ${\mathrm{SmCrO}}_3$ has become intriguing because of inconsistent bulk magnetization observations. The presence of highly neutron-absorbing Sm atom has so far evaded the determination of microscopic magnetic structure. In the present report, we have utilized very high-energy “hot neutrons” to overcome the Sm absorption and to determine the thermal evolution of magnetic configurations. Unambiguously, three distinct phases are observed: the uncompensated canted antiferromagnetic structure ${\mathrm{\Gamma }}_4(G_x,A_y,F_z;F_z^R)$ occurring below the Néel temperature ($T_N=191$ K), the collinear antiferromagnetic structure ${\mathrm{\Gamma }}_1(A_x,G_y,C_z;C_z^R)$ occurring below 10 K, and a nonequilibrium configuration with cooccurring ${\mathrm{\Gamma }}_1$ and ${\mathrm{\Gamma }}_4$ phases in the neighborhood of the SRPT (10 K $\le T\le$ 40 K). In differing to the earlier predictions, we divulge the SRPT to be a discontinuous transition where chromium spins switch from the $a-b$ crystallographic plane to the $b-c$ crystallographic plane in a discrete manner with no allowed intermediate configuration. The canting angle of chromium ions in the $a-b$ plane is unusually not a thermal constant, rather it is empirically discerned to follow exponential behavior. The competition between magnetocrystalline anisotropy and free energy derived by isotropic and antisymmetric exchange interactions between different pairs of magnetic ions is observed to govern the mechanism of SRPT.

Figure 1

(a) $M\left(T\right)$ curves following ZFC, FCC, and FCW modes in the presence of the applied field ${\mu }_{0}H=0.05$ T. (b) Crystallographic unit cell of ${\mathrm{SmCrO}}_3$.

Figure 2

(a) Thermal evolution of neutron diffraction patterns. (b) The difference in diffraction patterns at 2 K with respect to room temperature. (c) Deconvolution of the magnetic peak at $2\theta =6.{44}^{\circ }$. The subscripts $m$ and $n$ denote magnetic and nuclear planes, respectively. (d) Neutron diffraction pattern at 300 K along with Rietveld-refined pattern of nuclear structure. Hollow circles and solid circles represent the experimentally observed and calculated data points, respectively. The vertical bars denote the Bragg's positions. Solid lines connecting the data points are guides to the eyes.

Figure 3

(a, c, e) Neutron powder diffraction profiles at 180 K, 40 K, and 5 K, respectively, refined with the model pattern generated from ${\mathrm{\Gamma }}_4, {\mathrm{\Gamma }}_4+{\mathrm{\Gamma }}_1$, and ${\mathrm{\Gamma }}_1$ magnetic configurations respectively. (b) Comparative view of the magnetic peak fitted with ${\mathrm{\Gamma }}_4, {\mathrm{\Gamma }}_2, {\mathrm{\Gamma }}_1$, and mixed phases ${\mathrm{\Gamma }}_4+{\mathrm{\Gamma }}_1$ at 40 K. (d) At $T=5$ K, the fitted magnetic peak with model ${\mathrm{\Gamma }}_2, {\mathrm{\Gamma }}_4$, and ${\mathrm{\Gamma }}_1$ configurations. (f) Modification in intensity corresponding to different magnetic Bragg's planes across $T_{\mathrm{SRPT}}$. Hollow circles and solid circles represent the experimentally observed and calculated data points, respectively. The vertical bars denote the Bragg's positions. Solid lines connecting the data points are guides to the eyes.

Figure 4

Schematic of the ${\mathrm{\Gamma }}_4$ and ${\mathrm{\Gamma }}_1$ spin configurations.

Figure 5

(a) Magnetic isotherms measured at various temperatures. (b) Variation of magnetic moments with temperature and phase diagram in temperature space.

Figure 6

(a) Variation of integrated intensities corresponding to Feature 2: ${\left(011\right)}_m+{\left(101\right)}_m$. The integrated intensity profile is fitted with the power law behavior $I=I_0{(1-T/Tc)}^{2\beta }$ in the vicinity of $T_N$. The inset shows the integrated intensity variation in the neighborhood of the SRPT. (b) Evolution of canting angles with respect to temperature in the ${\mathrm{\Gamma }}_4$ and ${\mathrm{\Gamma }}_1$ phases. The inset illustrates the nomenclature of angles with respect to the crystallographic axes.

Figure 7

Schematic illustration of isotropic exchange interactions between chromium ions and effective fields corresponding to antisymmetric exchange interactions caused by ${\mathrm{Sm}}^{3+}$ ions acting on chromium moments.