Substrate-controlled ferromagnetism in iron phthalocyanine films due to one-dimensional iron chains

Using the magneto-optical Kerr effect, superconducting quantum interference device magnetometry, and magnetic circular dichroism spectroscopy, ferromagnetism was found below 4.5 K in iron phthalocyanine thin films grown by molecular-beam epitaxy. The stacking of the molecules can be controlled via deposition temperature and substrate choice. The molecules self-assemble such that quasi-one-dimensional iron chains form. The chains are limited in length by the grain size or film thickness, which are manipulated using judicious growth methods. Magnetic hysteresis loops show distinct features in the saturation magnetization and susceptibility that depend on the thin-film structure and/or substrate. We find two regimes for the magnetic behavior: below temperatures of $\sim$25 K, intrachain interactions couple the Fe ions and produce nontraditional paramagnetic behavior, while at low temperatures below $\sim$$4.5$ K interactions between the one-dimensional chains produce hysteresis, magnetic order, and slow magnetization dynamics.

Figure 1

Due to substrate surface-molecule interactions phthalocyanine thin films stack differently on (a) sapphire and (b) smooth gold surfaces. The arrows point along the crystalline $b$ axis, which correspond to the direction of the one-dimensional magnetic chains. The circle represents the metal ion and the line represents the organic part of the molecule.

Figure 2

X-ray diffraction spectrum of two codeposited 60-nm-thin FePc films on two different surfaces. The iron phthalocyanine film on sapphire shows prominent (200) and (400) peaks, which are absent from the FePc film deposited on gold. However, a prominent $2\theta$ peak at 28${}^{\circ }$ indicates that the FePc molecule lies flat on the gold surface. The larger background response on gold at low angles ($2\theta <{10}^{\circ }$) is due to the higher electron density of gold as compared to sapphire. The inset shows the chemical structure of iron phthalocyanine.

Figure 3

Temperature dependence of longitudinal MOKE magnetization loops for (a) FePc on gold and (b) FePc deposited on a Si(110) surface. The sample is 50 nm thick and deposited at 160 ${}^{\circ }$C . No hysteresis is observed above 8 K. A linear background is subtracted to illustrate the temperature dependence.

Figure 4

SQUID measurements at 2 K show differences in the hysteresis for a sample with 132 nm of iron phthalocyanine deposited on sapphire and gold. The coercivities for both samples are $250\pm 20$ Oe, but they have different anisotropy fields and remanent magnetizations. Both samples were deposited side by side, and the applied field is in the plane of the thin film. Inset: Thickness dependence of areal saturation moment for a series of FePc samples deposited on sapphire and gold. Lines are linear fits and suggest a higher saturation moment per volume for FePc/Au samples as compared with FePc/sapphire samples.

Figure 5

Isothermal hysteresis loops show that hysteretic effects disappear above 4.5 K as measured with a vibrating-sample magnetometer. Low magnetic fields align the magnetic moments of the iron phthalocyanine film below $\sim$25 K. The magnetization is corrected for a diamagnetic background from the substrate.

Figure 6

The spectrum of FePc/sapphire as a function of the wavelength exhibits strong MCD response. Each peak in the MCD spectrum corresponds to one of the absorption bands of FePc. The magnetic field curve on the right exemplifies the saturation behavior.

Figure 7

MCD absorption signal as a function of the magnetic field for (a) FePc/Au and (b) FePc/sapphire thin films. The MCD absorption is measured at $\lambda =588$ and 585 nm for FePc/Au and FePc/sapphire, respectively. The magnetic field is applied perpendicular to the sample plane.

Figure 8

The inverse MCD signal of the FePc/Au sample measured at 0.25 T as a function of temperature is analogous to the inverse susceptibility. The line is a fit to the Curie-Weiss equation with $\Theta =41$ K.

Figure 9

Magnetic anisotropy measured in an FePc/Si sample as measured in a vibrating-sample magnetometer at $T=2$ K. The magnetic field is applied either parallel or perpendicular to the plane of the substrate. The slow saturation of the magnetic moment, when the magnetic field is applied parallel to the substrate plane (i.e., the magnetic field is perpendicular to the molecule's plane), supports previous results (Ref. 16) that the molecule's magnetic moment is slightly tilted away from the molecular plane.