Magnetocrystalline interactions in spinel MnCr₂O₄ single crystal probed by electron spin resonance

Highlights

• The field-induced effect of the magnetization indicates the strong spin-lattice coupling.

• Below 35 K, the spiral spin order becomes obvious.

• Gilbert damping parameter α shows an abnormal behavior due to the local lattice distortion.

Abstract

In this work we present a detailed magnetization, electron spin resonance (ESR) and specific capacity study on spinel MnCr₂O₄ single crystals in an extended temperature range of 5–300 K. The field-induced effect of the magnetization indicates the strong spin-lattice coupling in MnCr₂O₄ single crystal. The following results are obtained by the ESR investigation: (i) Below 35 K, since the spiral spin order, a driving force of the multiferroic (MF) effect, becomes obvious, the line shape of ESR shows asymmetric characters and the ferromagnetic resonance (FMR) model of Smit and Beljers formulation is invalid; (ii) The temperature dependence of Gilbert damping parameter α, reflecting the strength of the spin-lattice coupling, shows an abnormal behavior due to the local lattice distortion in MnCr₂O₄ single crystal. And the distortion is a triggering force for the formation of spiral spin order. Moreover, our research helps to understand the origin of the MF effect in MnCr₂O₄.

Introduction

The chalcogenide spinel compounds AB₂X₄ (A and B = 3d transitional metals, Cd and Hg, X = O, S, and Se) have been attracting a lot of interest in the past ten years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. It is because a variety of important physical effects have been found in these compounds, such as magnetostriction, colossal magnetoresistance, multiferroic, spin frustration and so on [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. These compounds are also important on electronic materials industries, i.e., computer memory cores, transformer core, and microwave devices [11], [12], [13], [14], [15]. Among AB₂X₄ compounds, cubic spinel ACr₂O₄ oxides have special characters. The representative examples are ACr₂O₄ chromite spinels with nonmagnetic A-site ions observed the spin Jahn-Teller (JT) effect, which is similar to the ordinary JT effect when the orbital degeneracy of an ion is lifted by the lowering of the crystal symmetry [1]. In these spinels, the frustration of J_CrCr antiferromagnetic (AFM) nearest neighbor interaction on the corner sharing network of Cr³⁺ tetrahedra, the so-called pyrochlore structure, is eliminated by a tetragonal distortion of the lattice. This releases the frustration and removes the spin degeneracy of the ground state, and consequently a long-range magnetic order can develop. These interesting phenomena have been observed at low temperatures in the case of ZnCr₂O₄ and CdCr₂O₄ [16], [17], [18]. In addition, the presence of magnetic moment on the A-site ions can definitely change the magnetic and structural properties in chromium spinel oxides as compared to those members of the ACr₂O₄ family where Cr³⁺ is the only magnetic ion [10], [19], [20], [21], [22], [23].

When A = Fe and Ni, the remained orbital degeneracy on A²⁺ ions is lifted by a cooperative JT distortion, resulting in structural transitions from cubic symmetry to tetragonal symmetry. Due to the spin-lattice coupling, the further distortion from tetragonal phase to orthorhombic one occurs around the magnetic phase transition [19], [20], [21]. The magnetic ground states of NiCr₂O₄ and FeCr₂O₄ exhibit a spiral spin structure in which the ordering of a ferrimagnetic (FIM) (longitudinal) component and that of an AFM (transverse) one occur at T_C = 74 and 80 K and T_S = 31 and 35 K, respectively [21], [22].

On the other hand, when the A (A = Co²⁺ and Mn²⁺) sites occupied by ions without JT active, the cubic structure are still remained at the low temperature [10], [23]. However, recently, the anomaly of the specific heat has been reported, which is regarded as a signature of the first-order phase transition around T_S $\cong$ 24–27 K in the CoCr₂O₄ single crystal and polycrystalline one [4], [24], [25], [26]. A similar specific heat anomaly is also observed in the polycrystalline MnCr₂O₄ at T_S $\cong$ 18 K [10]. In addition, both MnCr₂O₄ and CoCr₂O₄ show multiferroic (MF) effect below T_S, which is attributed to spin-lattice interaction arising from the inverse Dzyaloshinsky-Moriya (DM) effect [10], [24], [25], [27], [28]. Incomprehensibly, the magnetoelasticity is weak in these compounds [1], [29]. Hence, here we focus on the case of MnCr₂O₄ crystal as a typical example, in order to investigate the relationship between the lattice distortion and the formation of spiral spin order.

For MnCr₂O₄, recent experiments have shown that the thermomagnetic irreversibility between field-cooled cooling (FCC) and field-cooled warming (FCW) curves and the anomaly in the FCC curve at T_S strongly depend on magnetic field H. With increasing H both of them gradually disappear until H $\le$ 10 kOe [10]. By means of neutron scattering measurements, it has been confirmed that the reentrant-spin-glass-like (RSG-like) behavior of the FIM domains is due to the spiral component and not to the long-range FIM (LFIM) component. The growth of the spiral component makes the freezing of the FIM domains harder; and it is the freezing and fluctuation of the spiral component that cause the RSG-like behavior of the FIM domains [23]. X-ray powder diffraction studies with a high flux synchrotron source observe an apparent signature of anomaly at T_C, a change of slope at T_S in the thermal variation of lattice constant a(T) and a steplike change of O²⁻ coordinates close to T_C and T_S. This clearly reveals a strong magnetoelastic coupling at T_C and T_S [10]. In addition, MnCr₂O₄ shows MF effect below T_S, which is attributed to spin-lattice interaction arising from the inverse DM effect. The MF effect is driven by spiral spin order and the modulation of oxygen atom coordinating is correlated to the inverse DM interaction [10]. The above results demonstrate the local lattice distortion plays a key role in the formation of spiral spin order, but an experimental confirmation is still lacking.

It is well known that electron spin resonance (ESR) is certainly one of the most powerful tools to investigate the intrinsic magnetic behavior of magnetic materials [8], [30], [31], [32]. For example, a trigonal distortion is confirmed by ESR in CdCr₂S₄ [31]. Resonance occurs when the magnetized material is subject to microwave radiation with a frequency near the precessional frequency of the magnetization. Due to dissipation, the magnetization quickly relaxes back into static alignment with the magnetic field, which leads to linewidth broadening. The physical origin of the relaxation is quite complicated. In general, there are a variety of loss mechanisms which cause magnetization to relax. The mechanism of particular interest in this work is intrinsic process. An intrinsic mechanism is the one which is inherent in the material itself and does not require the presence of a defect or inhomogeneity. The magnon-phonon interaction, where the precessional energy of the magnetization is coupled directly to the lattice vibrations, is an example of such a mechanism. Intrinsic mechanisms can be described by the Landau-Lifshitz-Gilbert equation of motion [32].

$$\frac{\partial \text{M}}{\partial t}=-\left|\gamma \right|\text{M}\times {\text{H}}_{\text{eff}}-\alpha \frac{\text{M}}{M_S^2}\times \frac{\partial \text{M}}{\partial t}$$

Here, γ is the gyromagnetic ratio, M is the instantaneous magnetization vector, H_eff is the instantaneous effective field, α is the Gilbert damping parameter characterizing the intrinsic loss, and M_S is the saturation magnetization of the material. The first term in Eq. (1) describes the torque driving the magnetization precessing, and the second term describes the damping.

Although, the ESR experiment has been performed on the polycrystalline MnCr₂O₄, the detailed investigation below the T_C is still missing [8], [33]. It is important to do the comprehensive ESR experiments to investigate the relationship between the local lattice distortion and the spiral spin order in MnCr₂O₄ single crystals. Here we present the results of magnetization, ESR, and specific capacity studies of spinel MnCr₂O₄ single crystals. Our results show that the local lattice distortion may be a triggering force for the formation of spiral spin order which is related to the MF effect and perhaps help to understand the origin of the MF effect in MnCr₂O₄. The experimental details are given in Sec. II. Section III is devoted to results and analyses on MnCr₂O₄ single crystal, and a summary is given in Sec. IV.