Pressure-induced enhancement of the magnetic anisotropy in $\text{Mn}{\left(\text{N}{\left(\text{CN}\right)}_2\right)}_2$

Using dc and ac magnetometry, the pressure dependence of the magnetization of the three-dimensional antiferromagnetic coordination polymer $\text{Mn}{\left(\text{N}{\left(\text{CN}\right)}_2\right)}_2$ was studied up to 12 kbar and down to 8 K. The antiferromagnetic transition temperature, $T_{\mathrm{N}}$, increases dramatically with applied pressure $\left(P\right)$, where a change from $T_{\mathrm{N}}(P=\text{ambient})=16.0\mathrm{K}$ to $T_{\mathrm{N}}(P=12.1\text{kbar})=23.5\mathrm{K}$ was observed. In addition, a marked difference in the magnetic behavior is observed above and below 7.1 kbar. Specifically, for $P<7.1$ kbar, the differences between the field-cooled and zero-field-cooled magnetizations, the coercive field, and the remanent magnetization decrease with increasing pressure. However, for $P>7.1$ kbar, the behavior is inverted. Additionally, for $P>8.6$ kbar, minor hysteresis loops are observed. All of these effects are evidence of the increase of the superexchange interaction and the appearance of an enhanced exchange anisotropy with applied pressure.

Figure 1

Isobaric field-cooled (fc) magnetizations as a function of temperature and at the pressures given in the legend. The cooling and measuring fields are both 100 Oe.

Figure 2

Isobaric fc-zfc magnetizations as a function of temperature are shown at several pressures. The symbols and colors designating each pressure are the same as those used in Fig. 1. For $P<7.1$ kbar, the fc-zfc magnetization decreases with pressure and the behavior is inverted for larger pressures. The dotted line serves as a guide for the eyes and represents the trend of $T_{\mathrm{N}}\left(P\right)$. A detailed $T_{\mathrm{N}}\left(P\right)$ plot is given as Fig. 3.

Figure 3

Transition temperatures of the long-range canted-antiferromagnetic order of $\text{Mn}{\left(\text{N}{\left(\text{CN}\right)}_2\right)}_2$ as a function of pressure, extracted from the fc data; see Fig. 1.

Figure 4

Coercive fields and remanent magnetization values extracted from the field sweeps at $T=8$ K at different pressures; see Fig. S7. The positive and negative coercive fields are defined as the crossing of the hysteresis loop with the positive and negative $x$ axis, and similarly, the positive and negative remanent magnetization are the crossings with the $y$ axis.

Figure 5

Expanded view of the low-field regions of the experimentally accessible ($\pm 70$ kOe) hysteresis loops after cooling in the fields indicated in the legend. The lines are a guide for the eye. The asymmetries are minor loop effects, due to the fact that the maximum applied field $H_{\max }=70$ kOe is lower than the saturation field $H_{\mathrm{S}}=304$ kOe [23]. The existence of the minor loop effects at high pressures is a fingerprint of the large pressure-induced magnetic anisotropy in $\text{Mn}{\left(\text{N}{\left(\text{CN}\right)}_2\right)}_2$; see text.

Figure 6

Spin-flop fields for different pressures. The closed circle (red) data points are extracted from the location of the peak in the derivative of the dc magnetic hysteresis loops, and the closed square (blue) data points from the location of the peak in the real component of the ac magnetization; see Figs. S8 and S9.

Figure 7

$(P,T,H)$ phase diagram of $\text{Mn}{\left(\text{N}{\left(\text{CN}\right)}_2\right)}_2$. The black spheres are the data points from Figs. 3 and 6, and those reported by Manson et al. [18]. Four regions are identified by shading and correspond to the paramagnetic (PM) phase (black), the canted-antiferromagnetic (C-AFM) phase with low anisotropy (blue) and high anisotropy (red), and the spin-flop (SF) phase (green).