Complex ferromagnetic state and magnetocaloric effect in single crystalline ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$

The magnetocaloric effect in single crystalline ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ is investigated by measuring the field-induced adiabatic change in temperature $\left(\mathrm{\Delta }{T}_{\mathrm{ad}}\right)$ which reveals a single negative peak around $130\mathrm{K}$ well below the Curie temperature $(T_C=203\mathrm{K})$. In order to understand this unusual magnetocaloric effect, we invoke the reported ${}^{55}\mathrm{Mn}$ spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$. We show that this effect is a manifestation of a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom which results in a complex ferromagnetic state. The critical behavior of ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ near Curie temperature is investigated to study the influence of the coupled degrees of freedom. We find a complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields.

Figure 1

(Color online) (a) Structural diagram of ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ showing the various atomic coordinates. The radius of the circle varies according to the atomic mass. (b) Temperature dependent resistivity and real part of ac susceptibility of NSMO-0.3 measured at frequency of $313\mathrm{Hz}$ and an applied ac field of $0.1\mathrm{Oe}$.

Figure 2

(Color online) (a) $M$ vs $T$ at different applied magnetic fields, (b) Temperature dependent entropy change of NSMO-0.3.

Figure 3

Temperature dependent adiabatic change in temperature; Inset: Temperature dependent zero-field heat capacity of NSMO-0.3.

Figure 4

Temperature dependence of adiabatic change in temperature of NSMO-0.3 [$\mathrm{\Delta }{T}_{\mathrm{ad}}$ (direct)] in an applied field of $1\mathrm{T}$. Inset shows the behavior observed about $T_C$.

Figure 5

(Color online) Results of ${}^{55}\mathrm{Mn}$ spin echo (a) NMR, (b) Raman scattering, (c) ESR, and (d) MCE showing changes in the small window from $130–150\mathrm{K}$ (refer to text for details).

Figure 6

(Color online) A comparison of ESR spectrum with the structurally correlated synchrotron peak intensity in NSMO-0.3 sample (Ref. 10) (refer to text for details).

Figure 7

(Color online) (a) Field dependence of magnetization of NSMO-0.3 at different temperatures; (b) Arrott plot; (c) $H∕M$ vs T (open circles) in an applied field of $1\mathrm{T}$. The filled circles denote the temperature corresponding to the $M\text{−}H$ isotherms which exhibit a $dM∕dH$ peak. The open squares and the filled squares denote the temperatures at which the slope of the Arrott plot is negative and positive respectively. The line is a fit to the power law behavior expected for a compound with a Griffiths phase.

Figure 8

(a) Modified Arrott plot; (b) high field portion of the modified Arrott plot; (c), (d) $\ln \text{−}\ln$ plot used to obtain the final values of $\beta$ and $\gamma$.

Figure 9

Critical isothermal curve of $M\left(H\right)$, taken from at $T_C=203\mathrm{K}$. Inset shows $\ln \left(M\right)$ vs $\ln \left(H\right)$ plot at $T_C$; slope gives $\delta =3.03\left(2\right)$.

Figure 10

Modified Kouvel Fischer plots of the real part of the first order susceptibility for the sample ${\mathrm{Nd}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ inverse of slope is $\gamma =1.17\left(4\right)$ and $T_C=203.3\mathrm{K}$.

Figure 11

(Color online) (a) Scaling plot of $M{t}^{-\beta }$ vs $H{t}^{-(\gamma +\beta )}$ for NSMO-0.3 single crystal using $\gamma =1.16$ and $\beta =0.57$. (b) $M{t}^{-\beta }$ vs $H{t}^{-(\gamma +\beta )}$ plotted in $\ln \text{−}\ln$ scale.

Figure 12

(Color online) Open circles denote measured $T_C$ values and the solid line guides the eye (adapted from Ref. 23; filled circles denote computed $T_C$ in absence of $A$-site disorder and the dotted line guides the eye (adapted from Ref. 26). The hashed box demarcates the two regions above and below $t=0.92$ (adapted from Ref. 24 refer to text for details). The solid line across the hashed box is a schematic boundary between orthorhombic and Rhombohedral phases (adapted from Ref. 25). PMI: paramagnetic insulator; PMM: paramagnetic metal; FMM: ferromagnetic metal; CO: charge ordered.