Operating electronic devices in a cryogenic environment helps to obtain excellent performance like reliable stability, low noise and high efficiency^{1⇓⇓⇓⇓⇓⇓-8}. Compared with the RAM operating at room temperature, recent studies have demonstrated that cryogenic random access memory (RAM) is endowed with prominent advantages in read-write capability, reducing noise and power consumption^9⇓-11. However, scaling a cryogenic memory storage unit down to the nanoscale and further reducing the power consumption still remain challenging at present¹². One of the potential solutions is to construct cryogenic memory chips with novel materials¹³.

Organic molecules are attracting extensive attention due to their structural stability, robust and tunable magnetic properties^14⇓⇓-17. Moreover, their ability of self-assemble to well-ordered two-dimensional structures through hydrogen bonds¹⁸, coordination bonds and van der Waals force^19⇓⇓-22. enables the creation of ultrahigh-density magnetic arrays. The combination of organic materials and cryogenic technology further brings promising opportunities to construct novel cryogenic magnetic memory chips with ultrahigh density and low power consumption. Scanning tunneling microscope (STM) and spectroscopy (STS) performed at low temperatures are powerful tools to investigate the structural and magnetic properties of organic molecules on the surface, facilitating the exploration of employing organic molecules in cryogenic storage devices. The two-dimensional molecular arrays formed by all-trans retinoic acid (ReA) on Au(111) have been reported previously by Wang et al.^23⇓⇓-26, in which each ReA molecule can be transformed into a spin-carrying radical by applying a large voltage with the tip. Although the ReA molecule can behave as a promising candidate to construct magnetic memory arrays, its long polyene chain limits its potential to achieve ultrahigh storage density.

In the current work, (2E,4E)-3-methyl-5-(2,6,6-trimethylcyclohex-1-enyl)penta-2,4-dienoic acid (denoted as DiA) was chosen to construct close-packed molecular arrays on Au(111) due to its relatively shorter polyene chain. The topographic and spectroscopic features of DiA were investigated vis STM at a temperature of 4 K. Through the dehydrogenation process induced by an electric voltage pulse, a DiA which was originally in its nonmagnetic state could be switched to a spin-carrying state. The spin states are confirmed by STS and density functional theory (DFT) calculations and demonstrate a representative example of converting sequential molecules in the arrays into magnetic memory units, which implies the potential applications of organic molecules in constructing cryogenic information storage chips.

DiA molecules were thermally evaporated onto atomically flat Au(111) substrate with coverage of one monolayer at room temperature and conducted STM measurements at 4 K. A DiA molecule is composed of methyl-decorated six-membered ring and polyene chain and a terminal carboxy group, as illustrated in Fig. 1a. It was observed large-scale close-packed molecular arrays formed by DiA which were stabilized through intermolecular van der Waals force, as shown in Fig. 1b. In addition, from the close-up STM image in Fig. 1c, it is found that the basic periodic unit of the molecular arrays was a hydrogen-bonded DiA dimer in which two DiA molecules interacted with each other via the terminal carboxy groups²³ as indicated by Fig. 1a and the dashed contours in Fig. 1c. The head part of the molecule appears as a lighter protrusion because of the two steric methyl groups. Compared to other organic molecules like metalloporphyrin and metal phthalocyanines whose potential in spintronics has been frequently reported^27⇓-29. DiA has a relatively smaller molecular size and is free of metal centers, benefiting the construction of high-density information arrays with the ease of manipulation. The ability of self-assembly via hydrogen bonding interactions also promotes its potential applications in spintronics. Moreover, a single DiA molecule in the arrays occupies less than 0.25 nm², providing a nice candidate to perform magnetic state transversion and construct ultrahigh-density magnetic molecular arrays.

To convert the nonmagnetic molecular state of DiA to a spin-carrying state, the tip right above the six-membered ring part was located and applied a large negative voltage pulse at about −2.1 V. This could induce dehydrogenation and generate radical according to previous studies^23,26. Fig. 2a shows a DiA dimer and its neighboring molecules in a molecular array and the voltage pulse imposed to the upper left DiA molecule. After the electrical operation, it is found the target molecule became much brighter than the others (Fig. 2b). This change in molecular topography is attributed to an electric field-induced dehydrogenation since a drastic current change was obscrved at around −2.07 V when the voltage was ramped from −2.05 V to −2.20 V (Fig. 2c), indicating a change in the molecular state. Notably, the pulsed electric field imposed to the molecule depends on the actual geometry of the STM tip, so the switching voltage can be slightly different from tip to tip. Differential conductance (dI/dV) measurements on the target molecule was performed to verify its spin state before and after the electrical manipulation and the results are exhibited in Fig. 2d. While the dI/dV spectrum of an unoperated DiA is featureless (black line), that of a switched DiA presents a profound peak near zero bias (green line), reminiscent of the Kondo resonance which was reported in earlier work^23,24.

DFT calculations were further performed to better understand the switching process of the DiA molecule. Fig. 3a shows the optimized adsorption configuration of an individual DiA molecule on a Au(111) substrate consisting of three atomic layers, showing adsorption energy of −106.0 meV. The hydrogen atoms that are supposed to allow the dehydrogenation process are marked by blue arrows. By applying a voltage pulse atop the six-membered ring of DiA, one of the two hydrogen atoms can fall off and a neutral radical is formed, as shown in Fig. 3b. The adsorption energy of a dehydrogenated DiA molecule is calculated as −206.6 meV. The spin-resolved charge density distributions of a DiA radical in vacuum (Fig. 3c) and on Au(111) surface (Fig. 3d) reveal that the spin is delocalized and extends along the molecular skeleton. In addition, the polarity of the spin density of two adjacent carbon atoms tends to be opposite. The calculated spin magnetic moment of a DiA radical in vacuum is 1 μ_B, implying the total spin of the whole molecule is s = 1/2. In comparison, the calculated spin magnetic moment of a DiA radical on Au(111) is 0.67 μ_B, resulting from charge transfer between the molecular radical and the metal substrate.

To demonstrate the prospective of utilizing DiA molecules to build magnetic storage device, voltage manipulations were consecutively performed on sequential molecules in an array, as shown in Fig. 4a and 4b. Each molecule in Fig. 4a is marked by a grey circle with two arrows pointing in opposite directions to represent the nonmagnetic molecular state. Then the molecules in the left column were switched into magnetic radical state one by one using voltage pulses as introduced above, leading to a unified change in the topography (Fig. 4b), consistent with what was observed in Fig. 2b. With the aid of dI/dV measurements, it was verified that those molecules were successfully switched to spin-carrying states. Each switched molecule is marked by a blue circle with a single up arrow, implying the unpaired electron of the molecule.

Considering each molecule in Fig. 4a as a storage bit, logic numbers ‘0’ and ‘1’ can be used to represent the nonmagnetic and magnetic state of the molecule, respectively. Then the eight molecules compose a byte of a memory chip. The bits are all initialized to ‘0’ states (left panel of Fig. 4c), referring to the ‘00000000’ state of the byte without any interference. By applying electrical voltage pulses to the bits in the left column one by one, each bit can be converted from ‘0’ to ‘1’ and the byte is written as ‘10101010’ (right panel of Fig. 4c). Notably, two representative cases are only exemplified in Fig. 4c while all the 256 states of the byte can be ideally realized by performing controllable electrical manipulations with atomic resolution. Moreover, up to 3 × 10¹⁴ bytes are contained in a DiA molecular array of one square inch. In other words, a memory chip constructed by DiA can reach an ultrahigh density of ∼320 terabytes (Tb) (or ∼2500 terabits) per square inch. Notably, the information written into the molecule can be reversibly erased via electrical methods as previously reported^23,24.

In summary, we successfully prepared large-scale two-dimensional organic molecular arrays using self-assembly strategies and investigated their potential in constructing cryogenic magnetic memory devices of ultrahigh density. The dehydrogenation can be induced and the nonmagnetic molecular state be switched to a spin-carrying radical state by applying a voltage pulse to the molecule. dI/dV measurements and DFT calculations further verify the magnetic molecular state after switching. By considering each molecule as a storage bit occupying a physical space of only 0.25 nm², a DiA array composed of eight molecules forms a byte and a memory chip based on DiA arrays can achieve an ultrahigh density of ∼320 Tb per square inch. This work sheds light on the potential of employing organic molecules to construct ultrahigh-density magnetic memory arrays and provides useful fundamental reference for future study of novel molecule-based spintronics and RAM devices in cryogenic environments.

Acknowledgments This work is supported by the Ministry of Science and Technology (2018YFA0306003) and the National Natural Science Foundation of China (22225202, 22132007, 21991132, 21972002, 22172002, 21972067). DFT calculations are carried out on TianHe-1A at the National Supercomputer Center in Tianjin and supported by the High-performance Computing Platform of Peking University and Beijing Super Cloud Computing Center (BSCC) (URL: http://www.blsc.cn/).

Declaration of Competing Interest The authors declare no competing interests.