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Large out-of-equilibrium magnetocaloric effect in rare-earth zirconate pyrochlores

O. Benton, Y. Skourski, D. Gorbunov, A. Miyata, S. Chattopadhyay, J. Wosnitza, M. Ciomaga Hatnean, G. Balakrishnan, S. Zherlitsyn, O. A. Petrenko

Abstract

We explore the magnetic properties of Nd$_2$Zr$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$ single crystals subjected to pulsed magnetic fields up to 60 T using magnetization and magnetocaloric-effect (MCE) measurements, with initial temperatures ranging from 2 to 31K. The MCE data exhibit pronounced and unconventional hysteresis loops, in which the sample temperature increases during both the up-sweep and down-sweep of the field. In Nd$_2$Zr$_2$O$_7$, the MCE further displays a striking plateau as a function of time, followed by a rapid temperature rise that begins at the maximum applied field, across pulses with differing peak-field strengths. Our magnetization measurements reveal an inferred temperature of the magnetic subsystem that differs significantly from the directly measured sample temperature and exhibits opposite hysteresis: the temperature is higher on the up-sweep than the down-sweep, unlike the direct measurements. These observations indicate a breakdown of thermal equilibrium between magnetic and lattice degrees of freedom on the timescale of the pulse ($\sim 10^{-1}$s). We interpret the results using a phenomenological model involving two thermally coupled subsystems - the magnetic ions and phonons, and a thermal reservoir, which accounts well for the behavior of Pr$_2$Zr$_2$O$_7$. However, it fails to reproduce the plateau seen in Nd$_2$Zr$_2$O$_7$. Agreement with Nd$_2$Zr$_2$O$_7$ data is improved substantially if we allow the thermal coupling between the magnetic and the lattice subsystems to depend on the product $\frac{HdH}{dt}$. Our results reveal anomalously slow heat transfer between magnetic and lattice subsystems and point toward a novel mechanism for dynamically controlling the heat flow in Nd$_2$Zr$_2$O$_7$ via the rate of magnetic field variation.

Large out-of-equilibrium magnetocaloric effect in rare-earth zirconate pyrochlores

Abstract

We explore the magnetic properties of NdZrO and PrZrO single crystals subjected to pulsed magnetic fields up to 60 T using magnetization and magnetocaloric-effect (MCE) measurements, with initial temperatures ranging from 2 to 31K. The MCE data exhibit pronounced and unconventional hysteresis loops, in which the sample temperature increases during both the up-sweep and down-sweep of the field. In NdZrO, the MCE further displays a striking plateau as a function of time, followed by a rapid temperature rise that begins at the maximum applied field, across pulses with differing peak-field strengths. Our magnetization measurements reveal an inferred temperature of the magnetic subsystem that differs significantly from the directly measured sample temperature and exhibits opposite hysteresis: the temperature is higher on the up-sweep than the down-sweep, unlike the direct measurements. These observations indicate a breakdown of thermal equilibrium between magnetic and lattice degrees of freedom on the timescale of the pulse (s). We interpret the results using a phenomenological model involving two thermally coupled subsystems - the magnetic ions and phonons, and a thermal reservoir, which accounts well for the behavior of PrZrO. However, it fails to reproduce the plateau seen in NdZrO. Agreement with NdZrO data is improved substantially if we allow the thermal coupling between the magnetic and the lattice subsystems to depend on the product . Our results reveal anomalously slow heat transfer between magnetic and lattice subsystems and point toward a novel mechanism for dynamically controlling the heat flow in NdZrO via the rate of magnetic field variation.

Paper Structure

This paper contains 15 sections, 23 equations, 18 figures.

Table of Contents

  1. Introduction
  2. Experimental details
  3. Experimental Results: $\rm Pr_2Zr_2O_7$
  4. Magnetization measurements
  5. Magnetocaloric effect measurements
  6. Experimental Results: $\rm Nd_2Zr_2O_7$
  7. Magnetization measurements
  8. Magnetocaloric effect measurements
  9. Theoretical modeling
  10. Application to $\rm Pr_2Zr_2O_7$
  11. Application to $\rm Nd_2Zr_2O_7$
  12. Refinement to the model: field and sweep-rate dependence of $\lambda_1$
  13. Summary
  14. Calculation of thermodynamics from CEF spectrum
  15. Details of crystal field model

Figures (18)

  • Figure 1: Representative magnetic-field pulse profiles used in magnetization $M(H)$ and MCE $T(H)$ experiments.
  • Figure 2: Field dependence of the magnetization of $\rm Pr_2Zr_2O_7$ for (a) $H \! \parallel \! [110]$ and (b) $H \! \parallel \! [100]$ at $T=1.5$ and 10 K.
  • Figure 3: Variation of the sample temperature due to the MCE in $\rm Pr_2Zr_2O_7$ for the initial temperature of 4 K (solid lines) and 10 K (dotted line). The colors differentiate the $T(H)$ curves obtained by pulsing the applied field to different maximum values.
  • Figure 4: Time dependence of the sample temperature of $\rm Pr_2Zr_2O_7$ measured while pulsing magnetic field applied along the [110] direction. The sample temperature and the applied field are presented on the left and right axes, respectively. The orange arrows indicate the times when the maximum $T$ is recorded, while the field maximum is reached at 33 ms for any value of $H_{\rm max}$. The initial sample temperature is 4 K (solid orange lines) and 10 K (dashed orange line).
  • Figure 5: Field dependence of the magnetization of $\rm Nd_2Zr_2O_7$ for $H \! \parallel \! [100]$ measured at (a) $T=2.0$ K and (b) $T=4.2$ K. The colors of the magnetization curves distinguish data collected for magnetic fields pulsed to different maximum values.
  • ...and 13 more figures