Metamagnetic Behavior and Kondo Breakdown in Heavy-Fermion CeFePO

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional $XY$-type anisotropy shows a metamagnetic behavior at the metamagnetic field $H_M\simeq 4\mathrm{T}$ perpendicular to the $c$ axis and that a critical behavior is observed around $H_M$. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, $H_M$ in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that $H_M$ is a magnetic field breaking the local Kondo singlet, and the critical behavior around $H_M$ is driven by the Kondo breakdown accompanied by the Fermi-surface instability.

Figure 1

(Main panel) $T$ dependence of $H$-swept NMR spectra at 31.7 MHz for $H\perp c$ (solid line) and $H\parallel c$ (broken line). $K=0$ was determined by reference material ${\mathrm{H}}_3{\mathrm{PO}}_4$. (Inset) Angle dependence of $H$-swept NMR spectra at 31.7 MHz measured at 20 K. $\theta$ is the angle between magnetic field and $c$ axis. Solid line is corresponding to fitting line.

Figure 2

(a) $T$ dependence of the Knight shift determined at the peaks of $H\perp c$ and $H\parallel c$ spectra obtained at various $H$. Strong anisotropy of Knight shift suggests that static spin properties possess $XY$-type spin anisotropy. (b) $H$ dependence of magnetization $M_i(H)$ ($i=\perp$ and $\parallel$) using the relation of $M_i(H)=K_i(H)H_{\mathrm{res}}/A_{\mathrm{hf}}$. Solid and broken lines are guide to eyes. $M_{\perp }(H)$ suddenly increases with increasing $H$ and deviates from linear relation in the field range of 3–5 T, which is a definition of a metamagnetic behavior, while such a behavior was not observed in $M_{\parallel }(H)$ up to 6.2 T and down to 1.5 K.

Figure 3

(Main panel) $T$ dependence of low-energy spin fluctuations parallel and perpendicular to the $c$ axis at $\simeq 0.6\mathrm{T}$ evaluated with $1/T_{1}T$ measured in $H\perp c$ and $H\parallel c$ [see Eq. (2)]. The in-plane spin fluctuations are dominant at low $T$, suggesting that the spin dynamics also possess $XY$-type anisotropy. (Inset) $T$ dependence of $1/T_{1}T$ at 10.3 MHz ($\simeq 0.6\mathrm{T}$) for $H\perp c$ and at 10.3 MHz ($\simeq 0.6\mathrm{T}$), 31.7 MHz ($\simeq 1.8\mathrm{T}$), and 107.2 MHz ($\simeq 6.2\mathrm{T}$) for $H\parallel c$. $(1/T_{1}T{)}_{H\parallel c}$ is independent of $H$ up to 6.2 T.

Figure 4

$T$ dependence of $(1/T_{1}T{)}_{H\perp c}$ below 4 T (a), and above 4 T (b). (c) $H$ dependence of $(1/T_{1}T{)}_{H\perp c}$ at low $T$. $(1/T_{1}T{)}_{H\perp c}$ shows a distinct maximum at around $H_{\mathrm{M}}$, indicating the enhancement of $N(E_F)$ related to metamagnetic anomaly. (d) $H\mathrm{\text{−}}T$ phase diagram defined by $T_{\max }$ where $(1/T_{1}T{)}_{H\perp c}$ shows a maximum. Non-FL behavior characterized by continuous increase in $(1/T_{1}T{)}_{H\perp c}$ with decreasing $T$ [broken lines are shown in (b)] was observed in a narrow field region intervening between low-field paramagnetic (PM) state and high-field polarized PM state above 6 T.

Figure 5

Fermi surfaces (FSs) calculated with the ab initio band-structure calculation as Ce-$4f$ electrons are itinerant (a), or localized (b), where the Fermi velocity is mapped on the FSs. (c) shows the FS colored by the $j_z$ character in the $j=5/2$ multiplet of $4f$ orbitals, where $j$ is the total angular momentum. (d) the partial density of states. The Fermi level corresponds to 0 eV.

Figure 6

(a) Values of metamagnetic fields are plotted against $T_{\max }$ determined from a maximum in static susceptibility (a) or inverse of Sommerfeld coefficient $\gamma$ at $H=0$ (b) in CeFePO, ${\mathrm{CeCu}}_6$, and ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$ with pressurized and La-doped system. Broken lines are a guide to eyes. A linear relation holds between two quantities, although three compounds possess quite different crystal structures and magnetic properties, indicating that $H_M$ is linked with the local Kondo singlet energy $T_{\max }$.