Study of metamagnetism in Sm(Ni<inf>0.5</inf>Fe<inf>0.4</inf>Cu<inf>0.1</inf>)<inf>7</inf>

Abstract

Metamagnetism, a sudden increase in the magnetization of a material with a small change of external magnetic field, are calling more attentions because of their rich magnetic phenomena and scientific significance [1][2]. There are quite different physical causes for different types of metamag-nets. In this work, metamagnetism has been found in arc-melting Sm(Ni 0.5 Fe 0.4 Cu 0.1 ) 7 . The physical property measurement system (PPMS) is used to measure the magnetic properties. The hysteresis loops at different temperatures (T>5K) are shown in Fig.1. Both sides of the hysteresis loops exhibit obvious metamagnetic behaviors. The hysteresis loops show wasp-waisted character. At the beginning of the curves the magnetization increases rapidly and then gets saturation at about 1T. When field increases continuously to the critical magnetic fields (H cm ) metamagnetic behavior appears. The lower the temperature is, the higher the H cm is. The magnetization appears to saturate when field increases to a higher value. The magnetization stays at high magnetization state until the field decreases to about 1T. The magnetizatic behaviors at another side of the hysteresis loops are the same. The critical magnetic fields (H cm ) corresponding to the metamagnetic points increases with the temperature decreasing by an exponential dependence (see Fig.1 inset). The spin reverse model with thermal activation (TA) is used to explain the relation of the critical field to temperature. The expression can be written as H cm (T)=H cm (0)exp(−kT/U), where Hcm(T) is the critical field at T, H cm (0) is the calculated value at 0K, k is the Boltzmann constant, U is the energy needed to turnover one spin [3]. The fitting results show U≈6.6×10−15 erg. At temperatures, below 5K, the smooth jumps turn into step-like jumps (see Fig.2). The number of the steps and the values of critical fields vary with different samples. The inset in Fig.2 is the enlargement of one step with a field interval of 0.05T. When repeating the magnetization process, the hysteresis loops can coincide compactly, which is different from the usual Barkhausen jumps [4]. The XRD results show that the main phase is hexagonal P6/mmm structure and the easy magnetization direction at room temperature is along c axis, which may show high anisotropy constants, where macroscopic quantum tunneling (MQT) may happen [5][6][7]. As temperature gets extremely low, the thermal activation can be ignored and the quantum behavior becomes obvious. One possible explanation is as follows: the MQT happens first, which leads to a release of thermal energy and increase of sample temperature, followed by a huge magnetization reversal due to the external magnetic field. A step-like magnetic jump appears. The differences of step numbers and H cm values between different polycrystalline samples are thought to be related to the relative orientation of the crystalline grain and the field direction. It seems that the MQT and TA models have solved the problem well. Another possible explanation is the narrow domain-wall pinning, but such a mechanism would have difficulty accounting for the presence of multistep jumps and the transition from one smooth jump to several sharp jumps just by changing few kelvins.